ISSN 0030400X, Optics and Spectroscopy, 2010, Vol. 108, No. 6, pp. 899–914. © Pleiades Publishing, Ltd., 2010. Original Russian Text © M.A. Khodorkovskiі, S. V. Murashov, T.O. Artamonova, A.A. Belyaeva, L.P. Rakcheeva, A.A. Pastor, P.Yu. Serdobintsev, N.A. Timofeev, I.A. Shevkunov, I.A. Dement’ev, R. Hallin, J. Nordgren, 2010, published in Optika i Spektroskopiya, 2010, Vol. 108, No. 6, pp. 947–962.

SPECTROSCOPY OF ATOMS AND MOLECULES

Electronic Spectra of XeNe Molecules in the Range 77100–90100 cm–1 near Xe* (6p, 5d, 6p', 7s, 7p, 6d) Obtained by the (3 + 1) REMPI and (2 + 1) REMPI Methods M. A. Khodorkovskiіa, S. V. Murashovb, T. O. Artamonovaa, A. A. Belyaevaa, L. P. Rakcheevaa, A. A. Pastorc, P. Yu. Serdobintsevc, N. A. Timofeevc, I. A. Shevkunovc, I. A. Dement’evc, R. Hallind, and J. Nordgrend a

Russian Scientific Center Applied Chemistry, St. Petersburg, 197198 Russia St. Petersburg State Polytechnic University, St. Petersburg, 195251 Russia c St. Petersburg State University, St. Petersburg, 198904 Russia d Uppsala Universitet, Fysiska Institutionen, Department of Physics, Box 530, SE751 21 Uppsala, Sweden b

Received October 6, 2009

Abstract—The electronic spectra of XeNe molecules in the range of 77100–90100 cm–1 are measured by the method of laser resonance multiphoton ionization in a supersonic jet. The photoionization spectra are obtained upon two and threephoton excitations of molecules and their ionization by the next photon. In the range of 80300–90100 cm–1 near Xe*(5d, 6p', 6d, 7s, and 7p), the spectra are obtained for the first time. A whole number of vibrational systems are measured in this range. The majority of vibrational systems near Xe* (5d, 6d, 7p, and 7s) are located in the red range with respect to their dissociation limits. In the blue range with respect to the dissociation limits, continua corresponding to transitions of molecules from the ground state to repulsive potential curves of excited states are detected. For a number of excited states of XeNe mol ecules, the vibrational analysis is performed and molecular constants are estimated. DOI: 10.1134/S0030400X10060123

INTRODUCTION Laser and incoherent sources of VUV radiation have been widely used in microelectronics. In [1–3], electronic transitions of heteronuclear molecules of inert gases have been proposed for use in creating effi cient narrowband sources of VUV radiation near reso nance transitions of atoms. Data on the potential curves of electronically excited states of these mole cules are highly incomplete, which impedes the cre ation and optimization of radiation sources. This work continues the cycle of our investigations of electroni cally excited states of inert gases by the method of laser resonance multiphoton ionization in a supersonic jet [4–6] and presents the results of the study of states of electronically excited XeNe molecules. Among diatomic molecules of inert gases, the elec tronic structure of XeNe molecules has been the least studied. The majority of investigations dealt with the ground state of XeNe molecules [7–9]. In theoretical work [7], vibrational levels of Rg1Rg2 molecules in the ground state were calculated and values of the dissoci ation energy De were estimated. Based on these calcu lations, the authors of [10] obtained and presented the following molecular constants for the ground state of XeNe molecules: De'' = 52.12 cm–1, ω''e = 22.23 cm–1, ωe x e'' = 2.371 cm–1, re'' = 3.879 Å. Excited states of

XeNe molecules were investigated by the methods of laser resonance multiphoton ionization [10, 11] and laser fluorescence [12, 13]. Photoelectronic spectra of XeNe molecules near excited states 6p[5/2]2, 6р[5/2]3, 6р[3/2]2, and 6р[1/2]0 of Xe* atoms were measured in [10, 11] by the (2 + 1) REMPI method; electronic spectra of these molecules near the state 6s'[1 / 2]1 of Xe* atoms were obtained in [12] by the LIF and (1 + 1') REMPI methods, while those near the state 6s [3 / 2]1 of Xe* were measured in [13] by the LIF method. It was concluded from these experimental data that potential curves of XeNe* molecules (6р) have a shallow well and hump located above their dis sociation limits. This was confirmed by calculations in [14], where the potential curves of XeNe molecules for the excited states NeXe* (np, n = 6–9) were calcu lated. It was found that, for NeХе*(6p), the potential curves lie above the dissociation limits and are charac terized by a shallow well and a hump, whereas the potential curves for NeXe* (np, n = 7–9) can lie below the dissociation limits. In this work, we study excited states of XeNe mol ecules by the multiphoton mass spectrometry method in a broad energy range of 77100–90100 cm–1. The photoionization spectra that we measured in the range of 77100–80200 cm–1 generally coincide with the

899

KHODORKOVSKIІ et al.

900

spectra reported in [10–12]. As far as we know, data on the electronic states of XeNe molecules in the range of 80300–90100 cm–1 near the excited states Xe* (5d, 7s, 7p, and 6d) are unavailable. Singlephoton absorption spectra of NeXe molecules exhibited certain bands near the excited atomic states Xe* 5d and 6р [9]; how ever, their analysis is very difficult. In this work, we used two methods, (2 + 1) REMPI and (3 + 1) REMPI, for measuring the spectra. EXPERIMENT Measurements were performed using the same experimental setup as we previously used in our studies of excited states of XeRg molecules (Rg = Xe, Kr, Ar) [4–6]. It involves a pulsed supersonic nozzle, an exci mer pump laser, a dye laser, a BBO frequencydou bling crystal, and a timeofflight mass spectrometer. Dimeric molecules were formed upon expansion of a mixture of inert gases from a region of a high pressure (5 atm) into a vacuum (10–6 Torr) through the pulsed nozzle 0.1 mm in diameter. In this case, the gas was cooled to very low temperatures (~10 K). Working mixtures were composed of xenon and krypton in a proportion such that it ensured the highest concentra tion of XeNe molecules. The pulse duration of the molecular beam was 300 ns. The central part of the beam was separated by a skimmer and was irradiated by a focused beam from an FL 3002 dye laser pumped by an EMG103 XeCl excimer laser (both lasers are from Lambda Physik, Germany). The duration of the laser pulse was 15 ns. Positive ions formed were elected by the electric field toward an MKP detector, which was positioned at a distance of ~8 cm; so that the res olution of the timeofflight mass spectrometer was M/ΔM = 50. Spectra of multiphoton ionization of XeNe mole cules were measured in the range of the dissociation limits XeNe* → Xe* + Ne 1S0 (77 100–90 100 cm–1). In the case of the twophoton resonance, (2 + 1) REMPI, molecules were excited by radiation from the second harmonic of the tunable laser that operated using coumarins 2, 102, and 307 (λ = 220–260 nm, up to 0.1 mJ per pulse). In the case of the threephoton resonance, (3 + 1) REMPI, molecules were excited by radiation of the tunable laser that operated on pTer phenul, DUI, and QUI dyes (λ = 335–390 nm, up to 10 mJ per pulse). The radiation wavelength was cali brated based on the optogalvanic effect using a hol lowcathode neon lamp; the calibration error was 0.5 cm–1. The spectral width of radiation bands was 0.3 cm–1. Molecular and atomic ions XeNe+ and Xe+ of the natural composition were simultaneously detected, since the resolution of the mass spectrometer was too low to detect ions of isotopomers.

EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 presents outline photoionization spectra of XeNe molecules obtained by the (2 + 1) REMPI (2Phspectrum) and (3 + 1) REMPI (3Phspectrum) methods in the range of 77100–90100 cm–1 upon detection of XeNe+ ions. It can be seen from this figure that the spectra of molecules measured under the two photon and threephoton excitations are very differ ent, which is caused by different selection rules. Excited electronic states of dimeric molecules of noble gases are described in terms of Hund’s case С. In this case, the projection of the total angular momen tum of electrons onto the axis of the molecule (Ω) is preserved, and molecular states are denoted by Ω. The rotational angular momentum N is summed with the momentum of electrons Ω and yields the total momentum J. The combination of the ground state 1S0 of the Ne atom with the excited state of the Xe* atom with the momentum JXe yields electronic state, for which 0 ≤ Ω ≤ JXe. In the dipole approximation, tran sitions with |ΔΩ| ≤ 2 and with |ΔΩ| ≤ 3 are allowed for two and threephoton transitions of molecules, respectively. For transitions of both types, the transi tion 0+ ↔ 0+ is allowed, while the transition 0+ ↔ 0– is forbidden. Table 1 presents electronic states of XeNe molecules that arise as combinations of the 1S0 ground state of Ne with the excited states Xe* (6р, 6р', 6d, 7p, and 7s) [15]. To determine the symmetry of the excited state of molecules in the case of (2 + 1) REMPI, the spectra were recorded with circularly polarized light. It was shown theoretically [16] and experimentally [10, 17] that the twophoton transitions 1 ← 0+ or 2 ← 0+ recorded with circularly polarized light are more intense than upon their excitation by linearly polar ized light (by a digit of approximately 3/2), whereas the intensity of the twophoton transitions 0+ ← 0+ excited by circularly polarized light is markedly decreased. It is known that the most intense molecular transi tions are those at which one of the atoms passes from the ground state to a state that is allowed by the dipole selection rules. For singlephoton dipole transitions atoms, there are two strict selection rules, ΔJ = 0, ±1, with the transition J'' = 0 → J' = 0 being forbidden, and Laport’s rule, which states that even terms can only be combined with odd terms (even ↔ odd). For twophoton transitions of atoms, dipole transitions with ΔJ = 0, ±2 and even ↔ even and odd ↔ odd tran sitions are allowed. For threephoton transitions of atoms, dipole transitions with ΔJ = ±1, ±3 and even ↔ odd transitions are allowed. Since, under our experimental conditions, the concentration of xenon atoms in the supersonic molecular beam was considerably higher than the con centration of XeNe molecules, XeNe+ ions of the cor OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES

901

III I

II 3Ph IV V

78000

82000

86000

90000 ν, сm−1 2Ph

×5

78000

82000

86000

90000 ν, сm−1

Fig. 1. Outline (3 + 1) and (2 + 1) photoionization spectra of XeNe molecules (3Ph and 2Ph, respectively) obtained upon detec tion of molecular ions XeNe+. Roman numerals I–V indicate spectral ranges discussed in the text.

responding mass are also detected in the case where the frequency of the excitation radiation is tuned to the twophoton dipole transition of atomic xenon. Therefore, upon the detection of molecular ions, the atomic line can be observed in the spectrum of laser excitation if the concentration of atoms is high and if the corresponding electronic transition is allowed. We took into account this effect in interpretation of pho toionization of molecular spectra. For the conve nience of consideration, the entire spectrum was divided into several ranges and, below, each range will be discussed separately. Range of 77000–79200 cm–1 Figure 1 shows that, in this spectral range, bands were observed in the spectral intervals 77000–77600 and 78100–79100 cm–1. In the interval 77000– 77600 cm–1, the spectrum was only measured under the threephoton excitation of molecules. Upon the detection of XeNe+ molecular ions, we observed two broad bands, whose maxima were at 77267.8 and 77327.8 cm–1 and whose halfwidths were about 20 and 60 cm–1, respectively, and a continuum with a maxi mum at 77470 cm–1. The bands at 77267.8 and 77327.8 cm–1 coincide with the bands that were previ OPTICS AND SPECTROSCOPY

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ously obtained in [12] using the laser fluorescence and (1 + 1') REMPI methods. In that work, these bands were attributed to bound–free transitions of XeNe molecules from the ground state X0+ to the repulsive parts of two excited states of the symmetry Ω = 0+, 1 with the dissociation limit XeNe → Хe* 6s'[1 / 2]1 + Ne 1S0 at 77226 cm–1. In the dipole approximation, the transition Хе1S0 → 6s'[1 / 2]1 is allowed with respect to parity and ΔJ. The dissociation limit is equal to the sum of the energy of the excited atom and the depth of the potential well of the ground state of the XeNe molecule counted off from the zeroth vibra tional level, Еlim = Е(Хe*) + D0'' (XeNe). A very weak continuum with a maximum at 77470 cm–1, which has not been previously observed, is tentatively attributed to the transition of the mole cule from the ground state to the repulsive part of the excited state with the nearest dissociation limit Хe*6р[1/2]1 + Ne 1S0 at 77310.649 cm–1. In the dipole approximation, the transition Xe 1S0 → 6р[1/2]1 is for bidden with respect to parity and is allowed with respect to ΔJ. Since the symmetry of molecular states that arise as combinations of atoms Xe* 6р[1/2]1 and

KHODORKOVSKIІ et al.

902

Table 1. Electronic states of XeNe molecules from combination of atoms Xe*(6p, 6p', 6d, 7p, 7s) + Ne 1S0 studied in this work by the (2 + 1) and (3 + 1) REMPI methods Range, cm–1

Dissociation limit Хe* + Ne 1S0 states of Хе*

77000–79200

79200–80300

Energy of Xe*, cm–1

Molecular states Ω

6s' [ 1/2 ] 1

77185.04

0+, 1

6p[1/2]1 6p[5/2]1 6p[5/2]3 6p[3/2]1 6p[3/2]2

77269.14 78119.8 78403.06 78956.538 79212.04

0–, 1 0 , 1, 2 – 0 , 1, 2, 3 0–, 1 + 0 , 1, 2

o

o

80300–85450

5d [ 1/2 ] 1

79986.6

0+, 1

6p[1/2]0

80118.9

0+

5d [ 7/2 ] 3

o

80970

0–, 1, 2, 3

o

82430

0+, 1, 2, 3

o

83889.9

0+, 1

7s [ 3/2 ] 1

o

85440

0+, 1

7p[5/2]2

88351.7

0+, 1, 2

6d [ 1/2 ] 1

88549.8

0+, 1

7p[3/2]2 7p[1/2]0

88686.5 88842.3

0+, 1, 2 0+

6d [ 7/2 ] 3

o

89024.9

0+, 1, 2, 3

6p'[3/2]2 6p'[1/2]1

89162.4 89278.7

0+, 1, 2 0–, 1

6d [ 5/2 ] 3

o

89534.6

0+, 1, 2, 3

6p'[1/2]0

89860

0+

o

90032

0, 1

5d [ 5/2 ] 3 5d [ 3/2 ] 1

88200–89000

o

89050–91000

+

6d [ 3/2 ] 1

Ne 1S0 is Ω = 0–, 1; then, taking into account the selection rules, it is necessary to attribute Ω = 1 to the excited state. In the range 78 100–79 100 cm–1, spectra were only observed under twophoton excitation. Figure 2 pre sents the 2Ph photoionization spectrum of XeNe mol ecules upon the detection of XeNe+ ions in the range of 78100–79100 cm–1. This range exhibits four sys tems of electronic–vibrational bands, I, II, III, and IV, which coincide with band systems that were previ ously investigated in [10] by the (2 + 1) REMPI method, and two new systems of bands, V and VI. Each system contains two terms of the v'progression. In [10], systems I, II, and III in the range 78100– 78350 cm–1 were attributed to transitions from the ground state X0+ of the molecule to excited states with

the dissociation limit XeNe → Хe*6р[5/2]2 + Ne 1S0 at 78 160.8 cm–1. Based on polarization measure ments, vibrational system I was attributed to the elec tronic transition Х0+ → Ω = 0+, and vibrational sys tems II and III were attributed to the electronic tran sitions Х0+ → Ω = 1, 2. Consequently, we obtained data for all possible excited states of the molecule for this dissociation limit. It should be noted that the most intense bands belong to the transition 0+ → 0+. In [10], band system IV was attributed to the transition of the molecule from the ground to the excited state with the dissociation limit XeNe → Хe*6р[5/2]3 + Ne 1S0. In view of the selection rules, the symmetry of the excited state for this limit upon twophoton excitation can be Ω = 1 or 2. OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES

903

I

6p[3/2]1

6p[5/2]3

6p[5/2]2

(0, 0) (1, 0)

II (1, 0)

×10

VI (0, 0)

III

IV

(0, 0)

V

(1, 0) (0, 0)

(0, 0) (1, 0)

(0, 0)

(1, 0)

(1, 0)

78200

78600

79000 ν, cm−1

Fig. 2. (2 + 1) photoionization spectrum of XeNe molecules (2Ph) in the range 78 050–79 100 cm–1 obtained upon detection of molecular ions XeNe+. Dashed vertical lines indicate the positions of excited states of Xe* atoms. Roman numerals I–VI indicate systems of electronic–vibrational bands (see the text).

We will attribute new band system V to the dissoci ation limit XeNe → Хe* 6р[5/2]3 + Ne 1S0 at 78444.06 cm–1 and refer it to the following two terms of the vibrational progression ν(v', v''): ν(0, 0) = 78656.3 cm–1 and ν(1, 0) = 78 693.5 cm–1. The sym metry of the excited state can be either Ω = 1 or Ω = 2. We attribute the second new band system VI to the transition of the molecule from the ground to the excited state with the dissociation limit XeNe → Хe* 6р[3/2]1 + Ne 1S0 at 78995.5 cm–1 and refer it to the following two terms of the vibrational progression: ν(0, 0) = 79049.3 cm–1 and ν(1, 0) = 79081 cm–1. Taking into account the selection rules, the observed state should be attributed to the symmetry Ω = 1. Both new vibrational systems V and VI are weaker by an order of magnitude compared to remaining I–IV found in the considered range. This is quite natural because, in this case, molecular bands are observed near atomic transitions that are forbidden with respect to ΔJ. Upon monitoring Хе+ ions, the intensity of band (1, 0) in systems I–VI increases compared to band (0, 0), which indicates that the predissociation increases with increasing v'. In all these systems, both terms (0, 0) and (1, 0) of the vibrational progression lie higher than the photodissociation limit. Based on the OPTICS AND SPECTROSCOPY

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obtained two terms in each of systems I–IV, the authors of [10] estimated molecular constants for excited states of molecules. Similar estimates can be made for our new systems V and VI. In the harmonic approximation, the dissociation energy of the molecule in the excited state can be esti mated by the formula D e' = ν(Xe*) + De'' − Te',

(1)

where ν(Xe*) is the wave number of the resonance line of the Xe atom, De'' is the dissociation energy of the XeNe molecule in the ground state, and Te' is the min imal energy of the excited state. Without taking into account the anharmonicity, Te' = ν(0, 0) + (1/2)ω''e − (1 / 2) ω'e ,

(2)

where ν(0, 0) is the wave number of the observed tran sition, and ω''e, ω'e are the wave numbers of harmonic vibrations in the ground and excited states, respec tively. We assume in this case that ω'e = Δ G1'/ 2 . Since band (2, 0) is not observed in the spectra, the upper limit of the barrier height (BH) of the potential curve can be estimated by the formula BH < ν(2, 0) – ν(Xe*) – D0'' ,

(3)

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where ν(2, 0) = ν(1, 0) + ΔG1'/2 and ΔG1'/2 = ν(1, 0) – ν(0, 0). Using formulas (1)–(3), we obtained the following values of the molecular constants for systems V and VI. For the symmetry state Ω = 1 or 2 with the asymp tote XeNe → Хe* 6р[5/2]3 + Ne 1S0 (system V), ω'e = 37.2 cm–1, Te' = 78648.8 cm–1, D e' = –193.7 cm–1, and BH < 286.6 cm–1. For the symmetry state Ω = 1 with the asymptote Xe* 6р[3/2]1 + Ne 1S0 (system VI), ω'e = 31.7 cm–1, Te' = 79033.5 cm–1, D e' = –36 cm–1, and BH < 105.1 cm–1. The negative dissociation energy of the molecule in the excited state means that the potential curve that corresponds to this state is located in a blue range with respect to the dissociation limit. Therefore, the char acter of potential curves of XeNe* molecules in the energy range of excited Хе*(6р) atoms is consistent with the shape of potential curves calculated in [14]. Range of 79200–80300 cm–1 It can be seen from Fig. 1 that, in the range of 79200–80300 cm–1, molecular bands were observed upon both two and threephoton excitation. Figure 3 presents photoionization spectra of XeNe molecules upon detection of XeNe+ ions in this range. The spec tra that were obtained upon twophoton excitation of molecules, coincide with those reported in [10]. Two band systems I and II in the range of 79 200– 70500 cm–1 belong to the molecular transitions Х0+ → Ω = 1 or 2 with the dissociation limit XeNe → Хе*6р[3/2]2 + Ne 1S0, and band system III in the range of 80150–80300 cm–1 was attributed to the transition of the molecule from the ground state to the excited state Ω = 0+ with the dissociation limit Хе*6р[1/2]0 + Ne 1S0. All observed progressions are located in a blue range with respect to the dissociation limits, and the potential curves of the corresponding excited states are characterized by shallow wells and by barriers. As can be seen from Fig. 3, all bands in the 3Ph spectrum are broad and their intensities are approxi mately two orders of magnitude lower compared to bands in the 2Phspectrum. In vibrational system I, bands (0, 0) and (1, 0) broadened and were slightly blueshifted. Previously, we observed this effect for XeKr* molecules [6]. In vibrational sequence II, bands (0, 0) and (1, 0) have not been detected, and the band with a maximum at 79 440.9 cm–1, which coin cides with band (2, 0), increased. In vibrational sequence III, the bands at 80228.4 and 80258.2 cm–1 are slightly blueshifted with respect to bands (0, 0) and (1, 0) and have equal intensities. It is difficult to explain this behavior of bands in systems II and III by the field effect. We have to assume that bands in these

systems correspond to transitions of molecules to an excited state with a different dissociation limit, e.g., Xe 1S0 → Хе*5d[1 / 2]1 + Ne 1S0 at 80 127.6 cm–1, all the more so because the corresponding atomic transi tion upon threephoton excitation is allowed with respect to both parity and ΔJ. Range of 80300–85500 cm–1 As can be seen from Fig. 1, bands in the range 80300–85500 cm–1 are observed only upon three photon excitation of molecules. This range contains the atomic transitions Xe 1S0 → Хе*5d[7 / 2]3 , Хе 1S0 → Хе*5d[5 / 2]3 , Хе 1S0 → Хе*5d[3 / 2]1 , and Xe 1S0 → Хе*7s[3 / 2]1 , which, in the case of threephoton exci tation, are allowed with respect to both parity and ΔJ and are forbidden upon twophoton excitation. Figure 4 shows the 3Ph spectrum of XeNe mole cules in the range 80300–84800 cm–1 obtained upon detection of molecular ions XeNe+. It can be seen from this figure that narrow bands are observed in the intervals 80900–81020 (I) and 82310–82460 cm–1 (II) near Хе*5d[7 / 2]3 and Хе*5d[5 / 2]3 , which we attribute to two terms of the vibrational progression ν(v', v''). Each of sequences I and II exhibits only two vibrational bands (0, 0) and (1, 0), which are red shifted with respect to the dissociation limits. In the figure, intervals I and II are shown in the insets. We attribute the longestwavelength band in each of the systems to the (0, 0) transition. In vibrational system I with the dissociation limits Хе*5d[7 / 2]3 + Ne 1S0 at 81011 cm–1, we have ν(0', 0) = 80933.9 cm–1, ν(1, 0) = 80958 cm–1, and, in vibrational system II with the dissociation limit Хе 1S0 → Хе*5d [5 / 2]3 + Ne 1S0 at 82471 cm–1, we have ν(0', 0) = 82369.7 cm–1 and ν(1, 0) = 82398.3 cm–1. In system I, band (1, 0) has an appreciable intensity compared to band (0, 0), and, in system II, its intensity is even greater than the intensity of band (0, 0). Because the next terms of the vibra tional sequences are not observed, we can assume that the molecule dissociates upon the transition to the vibrational level v' = 3. We can also assume that the equilibrium distances between atoms in corresponding excited states of molecules are smaller compared to the, ground state, re' < re'' . Unfortunately, the symme try of the observed molecular state has not been deter mined, and the excited states can have Ω = 1, 2, or 3. For molecules in these excited states, certain molecu lar constants can be estimated by formulas (1) and (2). For the excited state with the dissociation limit Хе*5d[7 / 2]3 + Ne 1S0 of system I, ω'e = 24 cm–1, Te' = 80 933 cm–1, and D e' = 89 cm–1. For the excited state with the dissociation limit Хе*5d[5 / 2]3 + Ne 1S0 of OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES

905

~ ~

3Ph

6p[1/2]0

6p[3/2]2

2Ph III (0, 0)

I (0, 0)

II (0, 0)

(2, 0) (1, 0)

(0, 1)

(1, 0)

~ ~

(1, 0)

79300

79200

79400

79500

80100

80200

80300 ν, сm−1

Fig. 3. (2 + 1) and (3 + 1) photoionization spectra of XeNe molecules (2Ph and 3Ph, respectively) in the range 79200– 80300 cm ⎯1 obtained upon detection of molecular ions XeNe+. Dashed vertical lines indicate the positions of excited states of Xe* atoms. Roman numerals I–III indicate systems of electronic–vibrational bands (see the text).

system II, ω'e = 28.6 cm–1, Te' = 82366.5 cm–1, and D e' = 85.6 cm–1. In a blue range with respect to the dissociation limit Хе*5d[7 / 2]3 + Ne 1S0, two broad bands are observed; one of them is at 81 270 cm–1 and its halfwidth is 120 cm–1 and the other has a maximum at 82 010 cm–1 and a halfwidth of 40 cm–1. These bands seemingly correspond to the transitions to repulsive potential curves with the same dissociation limit.

In the range 83500–84800 cm–1, two broad bands at 84050 and 84480 cm–1 are observed, with their half width approximately being 100 cm–1 (Fig. 4). This range contains only one atomic transition Xe 1S0 → OPTICS AND SPECTROSCOPY

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Хе*5d[3 / 2]1 that is allowed upon threephoton exci tation. We attribute both observed bands to transitions to the excited states with the dissociation limit Хе*5d[3 / 2]1 + Ne 1S0 at 83 931 cm–1. Since molecular states that arise from combinations of atoms Хе*5d[3 / 2]1 and Ne 1S0 are of the symmetry Ω = 0+, 1, we can assume that both transitions occur to the repulsive curves of these excited states of the XeNe molecule. Figure 5 presents the spectra of XeNe molecules in the range 85330–85450 cm–1 that were obtained upon detection of molecular ions XeNe+ created under the threephoton excitation by laser pulses with different energies, 5 mJ (~109 mW) and 10 mJ. As the pulse

KHODORKOVSKIІ et al.

906 I

(0, 0)

(1, 0)

II

5d[7/2]о3

5d[5/2]о3

(0, 0)

5d[3/2]о1

(1, 0)

80900 80950 81000

81000

82000

82350 82400 82450

83000

84000

ν, сm−1

Fig. 4. (3 + 1) photoionization spectrum of XeNe molecules (3Ph) in the range 80 400–84 800 cm–1 obtained upon detection of molecular ions XeNe+. Dashed vertical lines indicate the positions of excited states of Xe* atoms. Roman numerals I and II indi cate systems of electronic–vibrational bands (see the text).

energy of the excitation light was increased two times, all observed bands broaden by approximately a factor of two and slightly shift toward the blue range. We pre viously observed this effect upon threephoton excita tion of XeKr molecules [6]. The spectrum is a compli cated system of bands that can be represented by two vibrational sequences that are superimposed on one another. One of these sequences consists of four com ponents (system I), and the other consists of two com ponents (system II). Since this range contains only one atomic transition Xe 1S0 → Хе*7s[3 / 2]1 that is allowed upon threephoton excitation, we attribute the two sequences to transitions of XeNe molecules in the excited state with the dissociation limit Хе*7s[3 / 2]1 + Ne 1S0 and Ω = 0 +, 1. The vibrational quantum number v' = 0 will be attributed to the long estwavelength component in systems I and II. Exper imentally determined wave numbers, their interpreta tion, and Δ G1' / 2 for these systems are presented in Table 2.

For system II, we constructed the potential curve in the approximation of the Morse function. It is shown in the inset of Fig. 5. The potential energy curve was described using the expression for the wave number of the transition from the zeroth level of the ground state to the excited state [18] ν v' = Te + G ( v' ) = Te + ω'e( v ' + 1 / 2) − ωe x e'( v ' + 1 / 2) , (4) 2

where Те is the minimal energy of the excited state, ω'e is the wave number of the harmonic vibration, and ωе x e' is the anharmonicity constant. The dissociation energy is given by De = ω2e /4ωехе.

(5)

The parameters in Eq. (4), i.e., the molecular con stants of the excited state, were found by the least squares method based on the experimentally deter mined wave numbers. For system I with the dissocia tion limit Хе*7s[3 / 2]1 + Ne 1S0, we obtained the fol OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES

907

сm−1 85500

(0, 0) I

85450 85400 II 85350

(0, 0)'

4

6

8

r, Å

7s[3/2]о1

2

(1, 0)

(3, 0)

(2, 0) (1, 0)'

85350

85450 ν, сm−1

85400

Fig. 5. (3 + 1) photoionization spectrum of XeNe molecules (3Ph) in the range 85 340–85 460 cm–1 obtained upon detection of molecular ions XeNe+. Solid curve shows the spectrum obtained upon laser excitation with a pulse energy of 10 mJ. Dashed curve shows the spectrum obtained upon laser excitation with a pulse energy of 5 mJ. Dashed vertical line indicates the position of the excited state of Xe* atoms. Roman numerals I and II indicate systems of electronic–vibrational bands (see the text). The inset shows the potential curve for vibrational system I.

lowing molecular constants: Te' = 85343.6 cm–1, ω'e = 26.6 cm–1, ωе x e' = 0.93 cm–1, and D e' = 190.2 cm–1. Since the dissociation limit to which this excited state is attributed, Е(Хе*) + De'' = 85481 cm–1, is smaller than the experimentally obtained limit Te' + D e' = 85547 cm–1, the potential curve in the figure has a hump. The height of the hump is approximately 66 cm–1. The equilibrium distance, which was deter mined taking into account the Franck–Condon rule, is re = 3.8 Å. System II consists of two bands, strong band (0, 0)' at 85 393.1 cm–1 and very weak band (1, 0)' at 85408.9 cm–1 (Table 2). Both bands lie in the range that is redshifted relative to the dissociation limit Хе*7s[3 / 2]1 + Ne 1S0. For molecules in this excited state with the symmetry Ω = 0+ or 1, we estimated cer tain molecular constants using formulas (1) and (2), ω'e = 16 cm–1, Te' = 85 390 cm–1, the D e' = 102 cm–1. OPTICS AND SPECTROSCOPY

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Range of 88200–89100 cm–1 In the range of 88200–89100 cm–1, photoioniza tion spectra were obtained both upon threephoton and upon twophoton excitation of XeNe molecules. Table 2. Wave numbers of observed transitions of vibrational sequences (v', 0) in the range of threephoton resonance Xe 1S Хе*7s [ 3/2 ] 1° 0 v' 0 1 2 3 0' 1'

ν, cm–1 System I 85356.7 85381.3 85404.3 85425.2 System II 85393.0 85408.9

ΔGv' + 1/2 , cm–1 24.6 23.0 20.9

15.9

KHODORKOVSKIІ et al.

908 v'

0

1

v'

0

1

2

3

I

3Ph 2

3

II 0

1

2

III

6d[1/2]o1

6d[7/2]o3

V'

* *

2

4

5

6

v'

0

I

88200

88400

1 23

II

2Ph

*

7p[3/2]2

1

7p[5/2]2

v'

0

88600

88900 ν, сm−1

88800

Fig. 6. (2 + 1) and (3 + 1) photoionization spectra of XeNe molecules (2Ph and 3Ph, respectively) in the range 88200– 89050 cm ⎯1 obtained upon detection of molecular ions XeNe+. Dashed vertical lines indicate the positions of excited states of Xe* atoms. Roman numerals I–III indicate systems of electronic–vibrational bands (see the text). Unattributed bands are marked with an asterisk.

In this range, excited state of atoms of different parity are located, Xe* (7р and 6d). As was noted above, upon the threephoton excitation of atoms, transitions between states of the same parity are forbidden, whereas, upon twophoton excitation, they are allowed. This means that the transitions Хе(5р6) → Хe*(7р) are allowed upon twophoton excitation, while the transitions Хе(5р6) → Хe*(6d) are allowed upon threephoton excitation. In the considered range, one should expect that transitions of molecules from the ground state to the excited states with the dissociation limits Хе*6d[1 / 2]1 + Ne 1S0 and Хе*6d[7 / 2]3 + Ne 1S0 will be the most intense in the photoionization 3Phspectra. In the same range, tran sitions of molecules from the ground state to the excited states with the dissociation limits Хе*7р[5/2]2 + Ne 1S0 and Хе*7р[3/2]2 + Ne 1S0 should be the most intense in the 2Phspectrum. Figure 6 presents the spectra of molecules obtained upon detection of XeNe+ ions created as a result of the threephoton and twophoton excitations. Upon threephoton excitation, two groups of bands were observed in the ranges 88 350–88 520 and

88900–89000 cm–1 (3Phspectrum). The system of bands in the range 88 350–88 520 cm–1 can be attrib uted to the transition of the XeNe molecule from the ground to the excited state with the dissociation limit Хе*6d[1 / 2]1 + Ne 1S0, while the system of bands in the range 88900–89000 cm–1 can be associated with the transition to the excited state with the dissociation limit Хе*6d[7 / 2]3 + Ne 1S0. From the analysis of the group of bands in the range 88350–88520 cm–1, we assumed that it consists of two superimposed vibra tional systems (I and II), each of which consists of four bands with the wave numbers of the first two bands coinciding. To the longestwavelength band, the vibra tional quantum number v' = 0 is attributed. The fre quencies of the bands of these systems, their interpre tation, and values of Δ Gv' +1 / 2 are presented in Table 3. Using the obtained wave numbers, we determined numerical expressions for the potential curves of vibrational sequences I and II by the least squares method and, correspondingly, determined the molec ular constants for the excited states. The molecular constants for the excited state of system I with the dis OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES

sociation limit Хе*6d[1 / 2]1 + Ne 1S0 at 88 590.8 cm–1 with the symmetry Ω = 0+ or 1 are as follows: Те = 88328.8 cm–1, ωе = 48.4 cm–1, ωехе = 1.6 cm–1, and De = 309.8 cm–1. The molecular constants for the excited state of system II with the same dissociation limit are Те = 88 330.7 cm–1, ωе = 46.1 cm–1, ωехе = 1.9 cm–1, and De = 274.7 cm–1. Among bands in the range 88350–88 520 cm–1, two bands located at 88421 and at 88449 cm–1 remain unattributed; they are aster isked in the figure. In the range of 88850–89100 cm–1, we observed a vibrational system of three bands located at 88 918.5, 88950.9 and 88 978.3 cm–1, which we attributed to the dissociation limit Хе*6d[7 / 2]3 + Ne 1S0 at 88951.7 cm–1. The vibrational quantum number v' = 0 was attributed to the longestwavelength band. In the harmonic approximation, the molecular constants for the excited state of system III of the symmetry Ω = 0+ or 1, 2, 3 were estimated to be Те = 88900.4 cm–1, ωе = 37.5 cm–1, ωехе = 2.5 cm–1, and De = 139.2 cm–1. It should be noted that the systems of bands observed in this range are redshifted from their disso ciation limits Хе*(6d). However, the dissociation lim its Е(Хе*) + De'' , to which vibrational systems I, II, and III were attributed, are smaller than the experi mentally determined limits Te' + De' . This can mean that the potential curves have humps. In the considered range, the 2Phspectrum exhibits two groups of molecular bands, which are located in the ranges 88200–88450 and 88600–88700 cm–1. We suggest that the system of bands in the range of 88200–88450 cm–1 is a vibrational sequence and attribute it to transitions of molecules from the ground state to excited states with the dissociation limit Хе*7р[5/2]2 + Ne 1S0. Since the observed bands increase in intensity upon excitation of molecules by circularly polarized light, the symmetry of the corre sponding excited state of molecules can be Ω = 1 or 2. To the longestwavelength band at 88 264.9 cm–1, we attribute the vibrational quantum number v' = 0. This sequence consists of seven bands, among which band (3, 0) is not observed and band (4, 0) is very weak. We assume that the vanishing of band (3, 0) and very low intensity of band (4, 0) are caused by the fact that the range of these vibrational levels intersects with other potential curve, along which dissociation of molecules occurs. This potential curve can be, e.g., the potential curve of the excited state with the dissociation limit Хе*6р[3/2]1 + Ne 1S0 at 88 379.1 cm–1, for which the corresponding atomic transition is forbidden upon twophoton excitation. The group of bands in the range 88550–88700 cm–1 can be attributed to the transition to the excited state with the dissociation limit Хе*7р[3/2]2 + Ne 1S0. In OPTICS AND SPECTROSCOPY

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909

Table 3. Wave numbers of observed transitions of vibrational sequences (v', 0) in the range of threephoton resonance Хе 1S0 Хе*6d [ 1/2 ] 1° v' 0 1 2 3 0 1 2 3

ν, cm–1 System I 88397.7 88437.0 88475.7 88508.0 System II 88397.7 88432.3 88468.1 88499.2

ΔGv' + 1/2 , cm–1 39.3 38.7 32.3

34.6 35.8 31.1

spectra of molecules excited by circularly polarized light, the intensity of all bands proved to be higher compared to their intensity obtained with a linear polarization of light. Consequently, the symmetry of the excited state can be either Ω = 1, or Ω = 2. It is dif ficult to represent the spectrum observed in the range 88550–88 700 cm–1 by a simple vibrational sequence. The vibrational sequence can be represented as a set of four bands located at 88618, 88642, 88663, and 88678 cm–1. We attribute the vibrational quantum number v' = 0 to the longestwavelength band at 88618 cm–1. The intense band (3, 0) at 88678 cm–1 is the terminal in this sequence. This sharp termination can indicate that this potential curve intersects another potential curve, along which the dissociation of molecules occurs, e.g., the potential curve with the dissociation limit Хе*7р[1/2]0 + Ne 1S0. We can assume that this potential curve is repulsive because the corresponding spectrum is not observed. The question of the most intense band at 88 653 cm–1 remains open. We can assume that band (2, 0) is split into two components at 88 653 and 88 663 cm–1. This situation can occur if the potential curve of the excited state consists of two wells. One of them is compara tively deep, while the other well is shallow and con tains only one level near the dissociation limit. This form of the potential curve for the excited state of XeKr* molecules was discussed in [19]. One can also propose a different interpretation of the group of bands in the range 88 550–88 700 cm–1. Assume that this group consists of two vibrational sequences. One of them contains four components, while the other has only one intense band at 88 653 cm–1, which can be attributed to the (0, 0) transition of the molecule to the excited state with the same dissociation limit Хе*7р[3/2]2 + Ne 1S0. Experimentally determined fre quencies, their interpretation, and values of Δ G1' / 2 for systems I and II are presented in Table 4.

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Table 4. Wave numbers of observed transitions of vibrational sequences (v', 0) in the range of twophoton resonances Xe 1 S0 Хе*7p[5/2]2 (system I) and Xe 1S0 Хе*7р[3/2]2 (system II) v' 0 1 2 3 4 5 6 0 1 2 3

ν, cm–1

ΔGv' + 1/2 , cm–1

System I 88264.9 88295.5 88321.2 – 88368.7 88394.5 88417.1 System II 88617.4 88645.3 88663.5 88676.6

30.6 25.7 23.8 – 25.8 22.6

27.9 18.2 13.1

Range of 89000–90100 cm–1 The spectrum of molecules in the range of 89 000– 90 100 cm–1 was only obtained by the (2 + 1) REMPI method. This range contains excited states (6p', 6d) of the Xe* atom. Figure 7 presents the 2Phspectrum of molecules obtained upon detection of XeNe+ ions in the range 89050–89270 cm–1. In this range, we observed two groups of bands that were located in the intervals 89080–89160 (I) and 89 180–89 250 cm–1 (II). Since the considered part of the spectrum has only one atomic transition that is allowed in the dipole approx imation, Xe 1S0 → Хе*6p'[3/2]2, the observed systems can naturally be attributed to transitions of the mole cule to the excited state with the dissociation limit Хе*6p'[3/2]2 + Ne 1S0 at 89 213 cm–1. Upon excitation by circularly polarized light, the intensity of all bands proved to be higher than their intensity upon excita tion by linearly polarized light by a digit of 1.5–2. For this reason, we can assume that both sequences belong to the transitions Х0+ → Ω = 1, 2. In system I, we attribute the bands at 89097.7, 89126.1, and 89 151.6 cm–1 to three terms of vibra tional progression (v', 0). We attribute the vibrational quantum number v' = 0 to the longestwavelength band. In the range that is redshifted with respect to band (0, 0), a weak band at 89 080.2 cm–1 is observed at a distance of 17.5 cm–1 from this band, which coin cides with the vibrational quantum of the ground state, Δ G1''/ 2 = 17.4 cm–1. This gives grounds to attribute this band to hot transition (0, 1). It can be seen from Fig. 7 that two weak bands at 89132.0 and 89 136.2 cm–1 are

imposed on band (1, 0) in the blue range. Unfortu nately, we failed to explain the occurrence of these bands. In Fig. 7, they are denoted by asterisks. Based on the presented wave numbers and their interpretation, we estimated the molecular constants for the observed excited states of molecules using potential curves approximated by the Morse function. For sequence I with the dissociation limit Хе*6p'[3/2]2 + Ne 1S0 at 89203.4 cm–1, we obtained ω'e = 31.5 cm–1, ωе x e' = 1.6 cm–1, D e' = 155 cm–1, and Te' = 89093.1 cm–1. It should be noted that all observed bands of vibrational progression I are redshifted with respect to the dissociation limit. For sequence II, we only observed the following two bands: (0, 0) at 89 202.5 cm–1 and (1, 0) at 89226.1 cm–1. Both bands are blueshifted with respect to the dissociation limit Хе*6p'[3/2]2 + Ne 1S0. Using formula (2), we find that the minimum of the potential energy almost coincides with the atomic dissociation limit, ω'e = 23.6 cm–1, Te' = 89 201.8 cm–1. Because band (2, 0) is not observed, the height of the barrier can be estimated by formula (3), BH < 46.3 cm–1. Figure 8 presents 2Phspectra of molecules obtained upon detection of molecular (XeNe+) and atomic (Хе+) ions in the range 89250–89650 cm–1. Three systems of bands, I, II, and III, are observed in the spectrum, which we attribute to different elec tronic–vibrational transitions of the XeNe molecule. The energy interval 89250–89650 cm–1 contains the excited atomic states Хе*6d[5 / 2]3 at 89534.568 cm–1 and Хе*6p'[1/2]1 at 89278.7 cm–1. According to the selection rules, in the dipole approximation, both res onance transitions are forbidden upon twophoton excitation; therefore, it is no surprising that the observed spectrum is rather weak. Upon the detection of molecular ions, system I is represented by a single band at 89 381.6 cm–1, which can tentatively be attributed to transition (0, 0) to the excited state of the molecule with the symmetry Ω = 1 and the dissociation limit Хе*6p'[1/2]1 + Ne 1S0 at 89278.706 cm–1. Upon the detection of atomic ions, in addition to band (0, 0), a band occurs at 89 401 cm–1, which we attribute to transition (1, 0). The wave num bers of both bands exceed the dissociation limit by 100 cm–1 more than; the level v' = 1 is the predissoci ation level and the potential function is a shallow well with a hump. The height of the barrier can be esti mated by formula (3), BH < 110 cm–1. We tentatively attribute systems II and III to the transitions of the molecule to excited states with the nearest dissociation limit Хе*6d[5 / 2]3 + Ne 1S0 at 89574.6 cm–1. In the twophoton approximation, according to the selection rules, the atomic transition Xe 1S0–Хе*6d[1/2]3 is completely forbidden; how OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES I

911

II 6p'[3/2]2

(1, 0)

(0, 0) (0, 0) (2, 0) *

(1, 0)

*

(0, 1)

89050

89150

89250 ν, сm−1

Fig. 7. (2 + 1) photoionization spectrum of XeNe molecules (2Ph) in the range 89 050–89 270 cm–1 obtained upon detection of molecular ions XeNe+. Dashed vertical line indicates the position of the excited state of Xe* atoms. Roman numerals I and II indicate systems of electronic–vibrational bands (see the text). Unattributed bands are marked with an asterisk.

ever, this forbidding is partially removed due to the 6р–6d configurational interaction, and, although molecular transitions near Хе*6d[5 / 2]3 are observed, the intensity of observed bands is not high. System II obtained upon detection of molecular ions is represented by a single band with a maximum at 89462.9 cm–1. We attribute it to transition (0, 0). This band lies 112.7 cm–1 lower than the dissociation limit. Because it has a phonon wing in the blue range, we can conclude that the potential curve of the excited state is strongly anharmonic and re' < re'' . System III contains four very weak components, which we attribute to four terms of vibrational progres sion (v', 0). We attribute the vibrational quantum number v' = 0 to the longestwavelength band, and the bands at 89570.8, 89581.8, 89590.5, and 89598.7 cm–1 are attributed to transitions (0, 0), (1, 0), (2, 0), and (3, 0), respectively. As can be seen from Fig. 8, the ter minal component ν'(3, 0) is clearly pronounced only following the detection of atomic Хе+ ions, which indicates that, after the transition to the third vibra tional level, molecules dissociate. All bands of this sys tem are blueshifted with respect to the dissociation limit. Based on the determined wave numbers, we esti mated the following molecular constants of the OPTICS AND SPECTROSCOPY

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excited state with the dissociation limit Хе*6d[5 / 2]3 + Ne 1S0 in the harmonic approximation using the potential curve described by the Morse function: ωе x e' = 0.95 cm–1, ω'e = 12.9 cm–1, Te' = 89575.4 cm–1, and D e' = 43.8 cm–1. Figure 9 presents the 2Phspectrum of molecules obtained upon detection of molecular (XeNe+) and atomic (Хе+) ions in the range of 89900–90100 cm–1. The spectrum exhibits two systems of bands, I and II, which we attribute to different electronic–vibrational transitions of XeNe molecules. The energy interval of 89800–90100 cm–1 contains two excited states of the Xe* atom, 6d[3 / 2]1 at 90032.155 cm–1 and 6p'[1/2]0 at 89860.015 cm–1. Upon twophoton excitation, the atomic transition Xe 1S0 → Xe*6d[3 / 2]1 is forbidden with respect to both parity and ΔJ, while the transition Xe 1S0 → Xe*6p'[1/2]0 is allowed with respect to both parity and ΔJ. System I contains two bands, a strong band at 89969.3 cm–1 and a weak band at 89987.8 cm–1, which we attribute to two terms (0, 0) and (1, 0) of vibrational progression. Upon detecting atomic ions, the intensity of band (0, 0) sharply decreases, while the intensity of band (1, 0) increases. Band (2, 0) is absent

KHODORKOVSKIІ et al.

912 I

III

(0, 0) 0 1 23

6p'[1/2]1

6d[5/2]o3

ν'

(1, 0)

II (0, 0) Xe+

XeNe+

89300

89400

89500

89600

ν, сm−1

Fig. 8. (2 + 1) photoionization spectrum XeNe molecules (2Ph) in the range 89 250–89 670 cm–1 obtained upon detection of molecular ions XeNe+ and atomic ions Xe+. Dashed vertical lines indicate the positions of excited states of Xe* atoms. Roman numerals I–III indicate systems of electronic–vibrational bands (see the text).

in both cases. We attribute this system to the transition of the molecule from the ground state to the excited state Ω = 0+ with the dissociation limit Xe* 6p'[1/2]0 + Ne 1S0 at 89901 cm–1. Polarization measurements confirm this interpretation. In the range that is red shifted with respect to band (0, 0), a very weak band is observed at 89 952.9 cm–1, which can be attributed to hot transition (0, 1) because the distance between this band and band (0, 0) is 16.4 cm–1, which approxi mately coincides with the vibrational quantum of the ground state of NeXe. The shape of the spectrum indi cates that, upon transition of the molecule to the first vibrational level of the excited state, the molecule dis sociates; i.e., the vibrational level v' = 1 is the predis sociation level. From the available experimental data for the excited state of the XeNe molecule with the symmetry Ω = 0+ and dissociation limit Хе*6p'[1/2]0 + Ne 1S0, we can estimate the molecular constants by formulas (1) and (2) as follows: ω'e = Δ G1' / 2 = 18.5 cm–1, Te' = 89971 cm–1, and D e' = –59 cm–1. The negative dissociation energy of the molecule in the excited state means that the potential curve is blueshifted with respect to the dissociation limit. The potential curve of

this state is characterized by a shallow well and a hump. According to estimates, the height of the bar rier is no more than 105 cm–1. Group of bands II in the range of 90040– 90100 cm–1 consists of seven components, which we attribute to terms of vibrational progression (v', 0). Vibrational sequence II will be attributed to the transi tion of the molecule to the excited state with the near est dissociation limit Хе*6d[3 / 2]1 + Ne 1S0 at 90073 cm–1. Because the corresponding atomic tran sition is forbidden with respect to both parity and ΔJ, it is natural that all observed bands of this system are very weak. Our measurements showed that, upon the excitation of molecules by circularly polarized light, the intensity of all bands decreases; therefore, band system II should be attributed to the transition Х0+ → Ω = 0+. Experimentally determined wave numbers, their interpretation, and values of Δ G1' / 2 for system II are presented in Table 5. The vibrational quantum number v' = 0 was attributed to the longestwave length band at 90048.8 cm–1. Upon detecting Хе+ atomic ions, an intensity redistribution between com ponents is observed. The intensity of vibrational bands OPTICS AND SPECTROSCOPY

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ELECTRONIC SPECTRA OF XeNe MOLECULES v'

(1, 0)

0

1

2 3 4 567

6d[3/2]o1

6p'[1/2]0

(0, 0)

913

Xe+

(0, 1) XeNe+ 89850

89950

90050

ν, сm−1

Fig. 9. (2 + 1) photoionization spectrum of XeNe molecules (2Ph) in the range 89 850–90 110 cm–1 obtained upon detection of molecular ions XeNe+ and atomic ions Xe+. Dashed vertical lines indicate the positions of excited states of Xe* atoms. Roman numerals I and II indicate systems of electronic–vibrational bands (see the text).

with large quantum numbers (v' = 4, 5, 6) increases compared to first terms of vibrational sequences, and the terminal term of the progression with v' = 7 is observed only in this case. First levels of the vibrational progression v' = 0–3 lie below the dissociation limit, while the latter are located above it. Based on the determined wave numbers, using the harmonic approximation and describing the potential curve by the Morse function, we estimated the molec ular constants for the excited state of the XeNe mole cule with the symmetry Ω = 0+ and dissociation limit Table 5. Wave numbers of observed transitions of vibrational sequence (v', 0) in the range of twophoton resonance Xe 1S Хе*6d [ 3/2 ] 1° (system II) 0 v'

ν, cm–1

ΔGv' + 1/2 , cm–1

0 1 2 3 4 5 6 7

90048.8 90058.1 90067.9 90076.0 90082.5 90088.4 90093.0 90096.1

9.3 9.1 8.1 6.5 5.9 4.6 3.1

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Хе*6d[3 / 2]1 + Ne 1S0 as follows: ωe x e' = 1.3 cm–1, ωe = 10.5 cm–1, De = 42.4 cm–1, and Te' = 90 054.6 cm–1. CONCLUSIONS We studied states of electronically excited XeNe molecules. Contrary to the widespread notion, according to which heteronuclear molecules formed by atoms of heavy and light inert gases are very weakly bound, we observed clearly pronounced sequences of electronic–vibrational transitions, which belong to electronically excited states of XeNe molecules. We determined the molecular constants of studied states. Based on the entire body of data, we can draw cer tain conclusions. Molecular spectra are mainly observed near dipoleallowed atomic transitions. Upon twophoton excitation, transitions between configurations of the same parity Xe 1S0 → Хе*(6р, 6p') are allowed, whereas, upon threephoton excita tion, interconfigurational transitions Xe 1S0 → Хе*(6s', 5d, 7s, 6d) are allowed. In cases where this rule is violated, a weak spectrum is usually observed. Molecular bands that correlate with the dissociation limits Хе*(6р, 6p') + Ne 1S0 are, as a rule, located in a range that is blueshifted with respect to the limits, whereas bands that correlate with the dissociation lim its Хе*(5d, 6d) + Ne 1S0 are mainly located in a range

KHODORKOVSKIІ et al.

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redshifted with respect to the limits. In all cases, the depth of potential wells in excited states is about 50– 150 cm–1, almost all potential curves have a hump, and a part of potential curves are repulsive. This investigation can serve as a starting point for theoretical calculations and the creation of VUV light sources based on the studied transitions. ACKNOWLEDGMENTS This work was supported by the Federal Agency for Science and Innovations of the Russian Federation (state contract no. 02.513.12.3089) and by the Russian Foundation for Basic Research (project no. 0802 00966a). REFERENCES 1. G. N. Gerasimov, Usp. Fiz. Nauk 174 (2), 155 (2004). 2. B. Krylov, G. Gerasimov, A. Morozov, A. Arnesen, R. Hallin, and F. Heijkenskjold, Eur. Phys. J. D 8, 227 (2000). 3. A. Morozov, B. Krylov, G. Gerasimov, R. Hallin, and A. Arnesen, Eur. Phys. J. 11, 379 (2000). 4. M. A. Khodorkovskiі, A. A. Belyaeva, L. P. Rakcheeva, T. O. Artamonova, P. Yu. Serdobintsev, A. A. Pastor, A. S. Kozlov, S. V. Murashov, A. Z. Devdariani, and K. Siegbahn, Opt. Spektrosk. 100 (4), 549 (2006) [Opt. Spectrosc. 100 (4), 497 (2006)]. 5. M. A. Khodorkovskiі, A. A. Belyaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, R. Hallin, and K. Siegbahn, Opt. Spe ktrosk. 102 (6), 908 (2007) [Opt. Spectrosc. 102 (6), 834 (2007)].

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Translated by V. Rogovoi

OPTICS AND SPECTROSCOPY

Vol. 108

No. 6

2010