Z. Phys. D - Atoms, Moleculesand Clusters4, 3-14 (1986)

Atoms,Molecules and Clusters f~r Physik D

@ Springer-Verlag1986

High Resolution Synchrotron Spectroscopy With and Without Magnetic Fields J.-P. Connerade and J. Hormes Physikalisches Institut Universit/it Bonn, Federal Republic of Germany and Blackett Laboratory, Imperial College, London, United Kingdom

Received March 21, 1986 We present recent experimental work on atomic and molecular spectra in the vacuum ultraviolet and soft X-ray ranges, including magneto-rotation spectra of atoms in high fields and MCD spectra of matrix isolated species. New studies with a double crystal monochromator recently installed in Bonn are described. PACS: 32.00; 33.00; 78.00

1. Introduction

The Synchrotron Radiation Laboratory in Bonn has depended over recent years on two electron synchrotrons: a 500 MeV machine (just recently dismantled) and a 2.5 GeV machine, which will be used to inject a new accelerator, the 3.5 GeV electron stretcher ring (ELSA [41]) which will operate as a storage ring up to 2.5 GeV. In the near future, we will thus have access to stable synchrotron radiation, and at least one undulator (already built and tested) will be provided. For nearly fifteen years now, researchers from the Physikalisches Institut Bonn and the Blackett Laboratory, Imperial College have collaborated in the investigation of atomic and molecular spectra and the study of high magnetic fields effects. The synchrotron radiation facilities in Bonn are set up for the exploitation of (/) very high spectral resolution over a very wide wavelength range, using windowless absorption cells and (i0 the polarisation characteristics of the radiation, using superconducting magnets for Zeeman and Faraday rotation studies in the vacuum ultraviolet. The present review is a summary of the most recent work, with emphasis on current and unpublished data.

2. High Resolution Spectroscopy of Atoms and Molecules in the Normal Incidence Range

We adopt here a working definition of high resolution given by Connerade and Baig [21] namely the resolu-

tion provided by an instrumental resolving power better than 100 x 2, where 2 is measured in angstroms. This definition matches the instrumentation well right down into the range of crystal optics. By the normal incidence range, we understand the coverage of a grating spectrograph of conventional Eagle mount installed at a synchrotron radiation source, which usually extends from around 2,500 A to at least 300 ~. Currently, the highest resolving power available to us is provided by a 6,000 l i n e / ~ grating operated in the second order. Our studies of atomic spectra in this wavelength range have concentrated on two aspects for which the highest available resolving power is desirable, namely the unravelling of series perturbations by the now standard technique of Lu-Fano graphical analysis [48], complemented where necessary by computerised fitting [55], and the related investigation of autoionising line profiles. Both themes are of course united within the general framework of multichannel quantum defect theory (MQDT [561). For an impression of what high resolution means in the context of synchrotron radiation studies at the present date, we show, in Fig. 1, some high series members recorded in a typical experiment. The resolution we possess is not, of course, as high as can be achieved in the best CW-laser experiments, especially when the latter are rendered Doppler-free by the use of counter-propagating beams. However, many UV lasers are not CW but pulsed at high repetition rates, and single photon excitation is also practiced with UV lasers. Under these less sophisticated conditions, the resolution available by synchrotron

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spectroscopy is not much worse - indeed often equal to - the resolution available in laser experiments, and one can point to the important advantage of combining high resolution with an extremely high wavelength coverage (much higher indeed than available with any laser system, since it extends to the soft X-ray range). Thus, the synchrotron - classical spectrograph combination provides us with high performance in a very simple experimental arrangement. By exploiting such techniques, we have been able

to perform useful analyses of series perturbations in a large number of spectra. A typical example is shown in Fig. 2. Currently, we are studying the p-subshell excitation spectra of Ca and Sr, for which we have recorded new, very high resolution spectra containing much new detail. A portion of the Ca spectrum is shown in Fig. 3. These spectra are of great importance to the further understanding of the two step autoionisation process [22, 23] and are complementary to photoionisation studies recently completed at Photon Factory [42]. In our studies of autoionising line profiles, we have exploited a variety of theoretical approaches. The two channel QDT formula of Dubau and Seaton [26] initially used in theoretical interpolations was adapted [16, 17] for the analysis of experimental line profiles - a good example being the 5p-nd excitation series of Xe I (Fig. 4a). An equivalent formula was given by Giusti-Suzor and Fano [32]. More recently, we have made use of two channel QDT as a tool for the interpretation of molecular spectra, also in the autoionising range. In cases where a strong quasiatomic series can be separated out by comparison with a Dubau-Seaton fit, the remaining structure is then more readily recognised. This approach is illustrated in Fig. 4b, which shows an autoionising series in C2HsI. The series arises by excitation of a z~ lonepair electron, i.e. an electron which, although in the valence shell of the chromophore, does not participate in the bonding of the molecule. Its excitation spectrum is therefore built on a quasi-atomic vacancy, and exhibits well-developed Rydberg series.

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Fig. 5. A Lu-Fano graph for a Rydberg series in a rather complex molecule: C2HsI. The series studied arise by excitation of a lone pair electron, and therefore possess some quasi-atomic character, Nevertheless, it is a remarkable fact that the principles of MQDT can be transposed to such a complex situation

Clearly, a two channel analysis is a drastic simplification which requires rather favourable circumstances to be successful in a molecule. Nevertheless, it can provide a useful insight in some cases. The study of long Rydberg series in molecules is one of our current themes of interest. It opens up prospects of adapting the more familiar atomic MQDT to the analysis of molecular spectra. There exists already a considerable literature on MQDT in molecules (see eg. Fano [30] and refs. therein). Our approach [24] exploits the united atom analogies hand in hand with the graphical techniques of Lu and Fano [48]. It yields a remarkable new insight into the spectra of molecules as complex as C2HsI, for which we present a Lu-Fano plot in Fig. 5. Obviously, the technique is limited to Rydberg

series in molecular spectra, and one must proceed with due caution in the interpretation since the situation is vastly more complicated than in an atom. Our assertion is that the correspondence between a molectflar spectrum and that of a properly chosen atom expresses itself experimentally by a topological correspondence between Lu-Fano graphs and not necessarily by a numerical correspondence between quantum defects, as had been the assumption in the pioneering analyses of the selfsame spectra by Price [54]. Our new approach thus lends far greater flexibility to the interpretation. We require only that the branch structure of the diagrams be preserved, and we are able to retain desirable atomic labelling, which is a simple descriptive framework. In addition to MQDT formulae, we have also, in collaboration with Dr. A.M. Lane F.R.S. of the Harwell laboratory, made good use of the K-Matrix formulation originated for nuclear scattering theory (see eg. the review by Lane & Thomas; see also Lane [45] for connection with MQDT) which provides an alternative and powerful approach in atomic physics. For coulomb potentials, there is the advantage that analytic expressions can be derived, which encompass such effects as q-reversals [13, 21] and the vanishing width effect [20]. Figure 6 shows an example of a 'q-reversal' computed from theory, and Fig. 7, an experimental example recorded at the 500 MeV synchrotron laboratory in Bonn. Of course, fine structure within a broader perturbing feature is by no means exclusive to atomic spectra. Indeed, it is much more common in molecules. In a recent experiment, in collaboration with Dr. P. Brint of University College Cork, we have observed the absorption spectrum of formaldehyde in the vacuum ultraviolet, and found that it contains a broad feature (see Fig. 8) which we identify as a giant d - ~ f resonance. This feature does contain

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sharper structure but, so far, we have not detected any change of symmetry due to coupling with a 'giant resonance'. This would be of some theoretical interest in the context of controlled collapse experiments [14, 15] since it would allow one to 'track' more accurately any displacement in the maximum of the density of states. Although the present review is primarily aimed at our own synchrotron radiation studies, we must mention here two very interesting examples of the 'q-reversal' phenomenon in molecular spectra, which have recently been uncovered. The first concerns the predissociating levels of NO, which have been studied by optical-optical two-colour double-resonance techniques [2]: a dissociative perturbation of a discrete molecular level has been shown to result in profiles of changing asymmetry similar in their properties to the atomic series of Tl discussed above. In another recent experiment, also laser-based, a prominent 'qreversal' has been detected in the spectrum of molecu-

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lar hydrogen [44]. These two experiments constitute the first observations of q-reversals in diatomic molecules.

3. Magnetic Field Effects In conjunction with the high resolution facilities described in the previous section, we have also installed a superconducting magnet which offers the opportunity of extending studies of Zeeman structure and high field effects into the vacuum ultraviolet. Potentially, the fact that synchrotron radiation is linearly potarised in the plane of the orbit is helpful, although it must be said that an intense source of circularly polarised continuum radiation would be even more advantageous. Using this facility, we have observed the first resolved Zeeman structures in the vacuum ultraviolet, and we have begun to explore the/-mixing range in several atomic spectra Also, we have been

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able to develop a novel technique, based on Faraday rotation, to measure the f-values of atomic transitions in the presence of a strong magnetic field. In Fig. 9, we show some typical experimental magneto-optical patterns recorded at the 500 MeV electron synchrotron in Bonn, together with some calculated patterns, which demonstrate the form of the spectra. By analysing the detailed structure of these patterns, one can achieve accuracies at the percent level or better in the determination of relative fvalues. The method does not require intensity measurements, but depends only on measuring rotation angles, so it is independent of optical depth, and indeed of most kinds of detection non-linearities. This turns out to be an attractive application of synchrotron radiation, since it exploits both its high integrated intensity (to achieve high resolution) and its polarisation properties (via the Faraday effect).

4. Magnetic Circular Dichroism of Matrix Isolated Species The Faraday effect alluded to in the previous section is otherwise referred to as magnetic circular birefringence, and is therefore complementary to the effect known as magnetic circular dichroism (MCD). Often, one separates one from the other, which is readily

achieved for MCD spectroscopy by a modulation technique in which the differential absorption between right- and left-hand circularly polarised radiation is measured as a function of wavelength. We refer to this technique as MCD-spectroscopy, which we have also applied using Synchrotron radiation. It is profitable to combine MCD with matrix isolation, in which a specimen is trapped in a solid rare gas matrix. Matrix isolation is a commonly used technique for the study of reactive species, such as free radicals and reaction intermediates [35, 51, 52]. Although a basic premise of the method is that the embedded species are weakly perturbed, matrix influences manifest themselves in all absorption spectra by line broadening, level shifts and additional level splittings. These effects can be systematically investigated in the spectra of atoms or molecules whenever the gas-phase spectra are well known. Even in such cases, unambiguous assignments of absorption features are hardly possible because of matrix influences. So MCD is very helpful: it assists making band assignments more definite, and allows one to investigate the guest-host interaction more fully, as recently demonstrated [1, 40, 49 61]. In the present section, we will not pursue this discussion further, but merely wish to point out that MCD+matrix-isolation spectroscopy can also be helpful in interpreting complex atomic gas phase spectra for which no Zeeman spectra are available or even possible. The physical basis for the method is the difference in the influence of the matrix according to the form of the ground and excited state functions: states of large radial extension are perturbed in energy and intensity differently from more compact states, e.g. those involed in the valence transitions of a molecule. In some cases, Rydberg states are effectively quenched, and one can actually separate out the valence transitions from higher excitations which overlap them in the gas phase, so that the matrix perturbation actually 'cleans up' the valence transition. A famous example of this effect is the b 1~ ~- X 12;"transition of N2 [8]. Similar situations are encountered in the inner shell excitation of atoms, which tend to be overlapped by Rydberg series of the resonance channel, as we will discuss below. As previously mentioned, the MCD technique measures a differential absorption, and the signal is typically one thousandth or even less than the corresponding absorption signal. Three distinct mechanisms give rise to MCD and are denoted A, B and C respectively [59]. The contributions of the three types are additive. The A-type, or Zeeman MCD arises when the upper states of a transition are degenerate. The degeneracy is lifted by an external magnetic field, and Zeeman components are separated in

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energy, so that each one absorbs a circularly polarised component of different frequency. The C-type MCD results from the corresponding mechanism operating amongst degenerate lower states. Here, the population of the Zeeman levels is determined by a Boltzmann distribution, and the resulting MCD spectrum is therefore temperature-dependent. Besides the splitting of energy-degeneracies by external fields, there is also the possibility that external fields will mix close-lying electronic states in varying proportion so as to produce MCD. This is referred to as a B-type spectrum. These differences of type already provide information on the degeneracy of upper or lower states. In suitable cases, about the same level of information can be extracted as would be available from resolved Zeeman spectra if they were observable (i.e. g-factors, spin-orbit parameters, etc.). Then, the analysis of higher moments in our combination with matrix isolation also yields information on the type and strength of the guest-host interaction. We now turn to examples. Figure 10 shows the absorption and MCD spectra of Ag isolated in a Kr matrix, in the energy range between 6.8 and 9.0 e¥. All observed bands are atomic in origin as can be concluded from the temperature-dependence of the MCD signals. In the gas phase, the first ionisation potential of

A g I is at 7.576 eV [12] so that only the highest members of the resonance Rydberg series overlap with the 4p states of the d-*p transition. Thus, there is no ambiguity in the assignments of the gas phase transition. This is nicely illustrated in Fig. 1 a of the paper by Connerade and Baig [18]. Consequently, all the lines in the matrix isolated spectrum can also be assigned to d-*p transitions, and this detailed interpretation is strongly supported by calculated spectra (Grinter [34] - s e e Fig. 11), who used [he parameters obtained by Martin and Sugar [50] from SlaterCondon theory applied to the data of Shenstone [58]. For convenience, Grinter assumed a Gaussian form tbr the calculated lines, with a half-width of about 50 meV, corresponding to the resolution of the matrix spectra. When Figs. 10 and 11 are intercompared, very good agreement is obtained, although crystal field effects which lift the degeneracy of some of the atomic transitions were not included. Similar agreement is obtained when the simulation is based on ab initio integrals derived from a multiconfigurational Hartree-Fock approach, so the case of Ag I can be regarded as well understood. The situation is quite different in Au I. Here, the ionisation potential determined from the np 2p series is 9.226 eV [12] and there is a severe overlap of d-*p transitions with members of the np Rydberg series, so that a reliable analysis of the gas phase spectrum

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is, in our view, much more difficult than for Ag I. We have performed Hartree-Fock and even DiracFock calculations with up to four interacting configurations to confirm an assignment proposed by Brown and Ginter [12]. F r o m the Slater-Condon integrals, an ab initio simulated spectrum was then calculated [34], as well as an 'experimental simulated spectrum' obtained from energy positions and relative intensities given by Brown and Ginter [12]. Figure 12 displays these simulated spectra which, however do not compare well between themselves, even if we bear in mind that the one based on the data o f Brown and Ginter [12] includes np Rydberg lines. The same is true of calculations based on the data of Jannitti et al. [43]. The agreement is also p o o r when compared to copper [36] and to silver. In Figs. 13 and 14, we show some matrix isolated data for Au. These differ appreciably from theory. Finally, we comment briefly on some gas phase studies we embarked on to clarify this unsatisfactory situation. We have investigated Au I in photoabsorption with the aim o f improving the resolution and extending the observations further into the VUV. Our

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first result is that a strong line reported in the literature is probably due to an impurity. In Fig. 15, we show a comparison between our data and that of Tegeder and Lincke [60] for the lowest autoionising lines. Our analysis, is still in progress, but we stress that the intercomparison between MCD and photoabsorption spectra is proving a most useful double check on the quality of the calculations.

5. High Resolution X-Ray Spectroscopy The Bonn 2.5 GeV synchrotron is a very powerful source of X-rays, having a characteristic wavelength of about 3.5 ~ at 2.3 GeV, thus enabling measure-

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merits up to about 20 KeV to be performed. The present section illustrates the advantage of wide wavelength coverage referred to in tile introduction by presenting some recent extensions of our measurements on atoms and small molecules into the vacuum ultraviolet. It is worth noting that a resolving power 2 ~ > 5 , 0 0 0 suffices in the X-ray range to attain the natural linewidth. We achieve an experimental resolving power superior to the latter figure by using a double-crystal vacuum monochromator similar in its design to the one described by Lemonier et al. [47]. From a comparison of the K-shell absorption of atomic Ar with previous data [9, 25], and from a study of the rocking curve of our crystal, the instrumental resolution is estimated as ~0.5 eV at about 3 KeV. For this measurement, the monochromator was equipped with a pair of Si (111) crystals, effectively suppressing second orders, so that the sum of all higher orders contributes less than 0.5% to the total intensity. The spectra we report were obtained by standard X-ray photoabsorption techniques. The monochromatised radiation is monitored by an ionisation chamber before passing through the sample cell. A second ionisation chamber is used as the detector. Preamplified currents detected by the ionisation chambers are converted by ADC's and read by a Commodore 3016 microcomputer at least ten times before the crystals are stepped to the next wavelength under computer control. This simple setup is very effective in averaging intensity fluctuations which are frequent when using a synchrotron as a radiation source. The high quality of the spectra can be judged from Fig. 15 showing the K-shell spectrum of Ar I. The inset in the Figure shows the double excitation region. The spectrum we obtain is comparable to the best results published to date with storage ring sources [25]. The problems of interest in the X-ray range are of course rather different from those in the VUV. At present, we see two main areas of investigation, namely (/) double excitation structures in atoms and molecules and (i0 the systematic investigation of near-edge absorption in molecules of well-known structure. Double excitations are of great importance as probes of correlation and many-body effects, since they are rigorously excluded within an independent particle theory. In Fig. 16, we show new high resolution measurements of the Sulphur K-photoabsorption edge in H2S. This molecule has been investigated by several groups [7]. It is, however, only by using synchrotron radiation from a high energy machine, lead-

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ENERGY [eV]

Fig. 18. EXAFS of the K-Shellabsorption of sulphur in SCI2

has been well-known for some 60 years, but it is only over the past 15 years or so that structure more than 50 eV from threshold, the Extended X-ray Absorption Fine Structure or EXAFS - has been exploited for the determination o f local geometry. Progress on the understanding o f near-edge structure - now named X A N E S or N E X A F S - was even slower because of the complex physical phenomena occurring near the ionisation threshold. In molecular spectra, for example Rydberg and valence states, shape resonances due to multiple scattering and 'shake up' peaks have been found to be significant [5]. Only recently has it been demonstrated that information on local structure can be extracted from X A N E S which is not available from EXAFS, e.g. the coordination geometry around the excited atom, and the effective atomic charge on the absorbing atom [6, 27, 28]. By a systematic investigation o f K-shell excitation spectra of gas phase molecules containing B, C, N, O and F atoms, Sette et al. [57] found a linear dependence between the energetic position o f the shape resonance relative to the ionisation potential and the intramolecular bond length. This empirical finding can also be justified by multiple scattering theory [531. Although the aforementioned results hold out the promise that X A N E S will become a new tool to determine structures, it must be said that several questions remain open. A final p r o o f is still required that complete structure can be determined from a X A N E S spectrum. The best candidates for such a demonstration experiment are stable molecular gases, for which coordination geometry is well known from other experiments, and for which external perturbations by non next-neighbour shells (a problem in solids) can be ruled out. In order to facilitate the interpretation, we have concentrated on K shell photoabsorption near-shell structure in small molecules containing just

13

one sulfur atom, and we have tried to study 'similar' molecules, so as to reveal systematic trends. Such sequences are e.g. H2S, SOz and SC12, which all possess C2~ symmetry, or SO2, SO3 and HzSO~, which possess the same nearest neighbour atoms. Some of the molecules listed had already been studied before, but a reinvestigation with our improved facility seems desirable, since the kind of systematic intercomparisons o f structure we describe has not previously been reported. An example from our study was shown in Fig. 16. The spectrum of HzS does not reveal molecular structure above the ionisation potential as hydrogen is a poor back-scatterer. Thus, as noted above, all the structure above the ionisation potential in Fig. 16 is confidently attributed to quasi-atomic excitation. Chlorine, with Z = 17 is already a good backscatterer for the outgoing electronic wave, and the expected molecular effects are clearly visible in Fig. 17, which shows the near-edge structure of SC12 at the sulphur K-edge. Here, a pronounced shape resonance is apparent at around 2,481 eV. Some structure occurs on both sides o f this resonance which has yet to be analysed in detail. Because of the good statistics of our data, it is even possible to observe well resolved EXAFS structure, as shown in Fig. 18. A more detailed description o f our new results will be published in the near future.

6. Conclusion We have tried to illustrate in a brief overview the wide variety of new Physics which is accessible by high resolution spectroscopy using synchrotron radiation in the Vacuum Ultraviolet and soft X-ray ranges, and by exploiting the polarised nature of the radiation. Much remains to be done in this rapidly growing field o f research, and we confidently predict that the present report will soon be overtaken by the profusion of new data which make the field we describe such an exciting one to work in. We are grateful to numerous colleagues, both present and former members of Imperial College and the University of Bonn, who have either participated in the work presented here or facilitated oar experiments. Our research is supported both by the B.M.F.T. under special funds for Synchrotron Radiation (West Germany) and by the Scienceand EngineeringResearch Council (UK). References 1. Armstrong, S., Grinter, R., McCombie,J.J. : J. Chem. Soc. Faraday Trans. 77, 123 (1981) 2. AshfoId, M.N.R., Dixon, R.N., Prince, J.D., Tutcher, B., Western, C.M. : (to be published) 3. Baig, M.A., Connerade, J.P.: J. Phys. B. 17, 1785 (1984a)

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J.-P. Connerade and J. Hormes: High Resolution Synchrotron Spectroscopy

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J.-P. Connerade J. Hormes Physikalisches Institut Universit/it Bonn D-5300 Bonn Federal Republic of Germany and Blackett Laboratory Imperial College London SW7 2AZ United Kingdom