Journal of Applied Spectroscopy, VoL 63, No. 2, 1996

RATIO

OF

42D3,~ A N D

OSCILLATOR

STRENGTHS

OF

42pa,,'2 -* 42Ds/2 T R A N S I T I O N S

42pt/2 OF

--,

ATOMIC

POTASSIUM V. F. Gamalii

UDC 543.42.535.34

Key words: oscillator strength, modulation, intracavity spectroscopy, multirnode laser, two-phonon absorption, total absorption. When the intensity of the intracavity field of a broad-band laser with an analyte medium inserted into its cavity is modulated by high-power external narrow-band radiation, the lasing spectrum essentially changes through the appearance of additional absorption lines. This investigative technique is known as modulational intracavity laser spectroscopy (MICLS). The appearance of new absorption lines is duc to the presence of narrow-band losses in the cavity of a broad-band laser because of the nonlinear process of two-photon absorption (TPA). In TPA, an atom of the material being investigated absorbs at once one photon of the narrow-band radiation and one photon of the broad-band laser radiation. At a certain frequency the number of photons decreases I1 I and gaps arise in the lasing spectrum of the broad-band laser. In this paper we give the results of measurement of the ratio of the oscillator strengths and of the oscillator strengths themselves the 42p1/2 -'- 42D3,2 a n d 42p3.,2 --,. 42D/2 electric dipole transitions between the excited electron levels in atomic potassium by MICLS. A heated cell filled with potassium vapor was placed in the cavity of a broad-band laser based on a solution of the organic dye DOTC (3,3'-diethyl oxatricarbocyaniniodite) in dimethyl sulfoxide. The narrow-band radiation of a giant-pulse ruby laser (694.3 nm, 40 nsec, 0.14 J) passed through the cell filled with potassium vapor, modulating the intracavity field, and pumped the dye (lasing region 755-775 nm). In the lasing spectrum of the broad-band laser in parallel with absorption lines of the potassium resonance doublet 766.49 and 769.9 nm there arise absorption of lines 769.11 and 769.1 nm (Fig. 1), which are due to the two-photon 42St,,2 -,. 42D3,7 and 42Sb,2 ~ 42D5/-2 transitions between levels of the same parity, respectively. The lasing spectrum of the broad-band laser was investigated with the help of a spectrograph of high resolving power (140,000) and recorded on photographic film which was subsequently scanned photometrically. The concentration of the potassium vapor was =1014 cm -3. The relation between total absorption .4ik (equivalent width) in the line corresponding to TPA and lhe value of the oscillator strength fgk of the electron transition between the excited levels is shown in 12, 3 1:

,-lik-

9~e WdNFl 128

m2Eo L

1 ,

o,.i d

1

+ ~sj~,

U~R

f,g f~k ~~

~

,

( I)

. (Jig ~'d~,,k

where rn and e are the electron mass and charge; e0 is the dielectric constant of vacuum; Wig and COgk are the frequencies of the resonance transitions and the transitions between the excited electron levels; -h is Planck's constant; ~d is the TPA line frequency; wR is the rubs-laser radiation frequency; N is the concentration of the potassium atoms in the ground state; F is the ruby-laser pulse energy relative to the beam cross-section area; IlL is the resonator duty factor; fig is the oscillator strength of the resonance transition in potassium; Oig and 0gk are Kirovograd State Pedagogic Institute, 1, Shcvchcnko Sir., Kirovograd, 315050, Ukraine. Translated from Zhurnal Pnkladnoi Spektroskopii, Vol. 63, No. 2, pp. 334-336, March-April, 1996. Original article submitted March 31, 1995.

276

0021-9037/96/6302-0276515.00

9

Plenum Publishing Corporation

Fig. 1. Lasing spectrum of dye laser with potassium vapor in its cavity when the intracavity field is m o d u l a t e d bv high-power r u b y - l a s e r radiation; J-1 = 769.11 nm, 22 = 769 nm are the TPA lines.

Fig. 2. Diagram of the energy levels of atomic potassium. D a s h - d o t lines d e n o t e the virtual levels. n u m b e r s d e p e n d e n t on the combination of the quantum numbers J of the total orbital moment of transitions; Z is the initial level; k is the final level; g are all the i n t e r m e d i a t e real tcvels of the potassium atom. The summation is over all the real i n t e r m e d i a t e levels. As can be seen from the d i a g r a m of the electron levels in atomic potassium (Fig. 2), only one m e m b e r connected with the level 42p2~2 makcs a contribution to the total absorption of the two-photon line c o r r e s p o n d i n g 1o the 42St.2 + 42D5.2 transition. Accordingly, for the two-photon 42S1.2 -+ 42D32 transilion only the m e m b e r connected with the level 42p12 makes a substantial contribution to the sum. Taking into account these peculiarities a n d a p p l y i n g the values of the oscillator strengths of transitions from (4) we derived a relationship for e x p e r i m e n t a l d e t e r m i n a t i o n of the ratio of the oscillator strengths of the electric dipole transitions between the excited levels in atomic potassium B = fPI ,- 2-'1)3 ~ 2 i f P3 2-'D5.'2 = 0"28"1~'1/A'a'2 "

(2)

In the method proposed the quantity B is not connected with the concentration of atoms a n d the energy in the ruby laser pulse. The corresponding values of total absorption A;,i,;~2 were d e t e r m i n e d in e x p e r i m e n t s . T h e s e values are d e p e n d e n t on the area of the T P A line profile. The decomposition of the spectrum into components of known form in the region of the line overlap was performed by the method of least squares.

277

The experimentally derived ratio of the oscillator strengths was B = 1.1. The accuracy of measurement was ~0%. The ratio of the same oscillator strengths 0.37.10-3/0.34 . 10 -3 from the data of 14 ] was equal to 1.088. The radiation energy of a single ruby-laser pulse was measured by a calorimeter. The area of the beam cross-section was determined by a photographic method. The ruby-laser radiation had a multimode structure and, after passage through a diaphragm and a telescope forming a cylindrical beam with a diameter of 0.3 cm, its intensity was practically constant over the cross-section in the region of interaction with potassium vapor. Applying the experimentally measured values of the quantities A, N, F, l/L,and Wd and equation (I) we

F42pl/2_,4203/.2

derived the oscillator strengths 0.27.10 -3 and f42r,:t/2.,42oS/2 = 0.25.10 -3. The accuracy of the measurement of these values was 40~o and was dependent for the most part on the accuracy of determining the concentration N and the energy F. The derived values of the oscillator strengths are in good agreement with the =

data of [4 1. The proposed measunng technique offers a few advantages over the traditional methods. For example, TPA originates from the ground level of the material being investigated instead of from the excited level, and the concentration of the absorbing atoms can be determined with a high degree of accuracy. The work was partly supported by the State Committee for Science and Technics of the Ukraine (grant 43/189).

REFERENCES I.

2. 3. 4,

278

V. M. Baev, T. P. Belikova, V. F. Gamalii, E. A. Sviridenkov, and A. F. Suchkov, Kvant. Elektron., !1, 2413 (1984). V. B. Baev, V. F. Gamalii, E. A. Sviridenkov, D. D. Toptygin, and O. I. Yushchuk, Zh. Prikl. Spektr., 46,573 (1987). USSR Author's Certificate No. 1332197 (1987). G. A. Kasabov and V. V. Eliscev, Spectroscopic Tables for Low-Temperature Plasma lin Russian I, Moscow (1973), p.94.