Two-photon spectroscopy of thallium

Volume 25, number 2

OPTICS COMMUNICATIONS

May 1978

TWO-PHOTON SPECTROSCOPY OF THALLIUM M.Y. MIRZA and W.W. DULEY Physics Department, York University, Downsview, Ontario, Canada M3J 1_,°3 Received 26 January 1978 2 0 The states n 2P°/2,. n 2p°/2, and n Fs/2,7/2 have been observed in T1 to n = 18, 21 and 26 respectively, using two p h o t o n laser spectroscopy and an ionization detector. An analysis o f the quantum defects for these states and the spin-orbit splitting o f the n 2p0 states is given.

Two and three-photon spectroscopy using tunable laser sources has been shown [1-6] to be invaluable in the characterization of the spectra of the alkali [ 1 4] metals and the alkaline earths [5,6] ; when multi -~ photon spectroscopy is combined with electrical detection of synchronous ionization in a space-charge limited diode [7] one has a powerful spectroscopic technique for the identification of highly excited atomic states. The development of this method has proceeded to the point [8] where spectra can now be obtained of refractory elements that require temperatures in excess of 1000 K to yield vapour pressures in the range 1 0 100/~m. As part of a study of the spectra of the Group IlIB elements we have examined the two-photon spectrum of thallium. This paper reports the results of this investigation. The oven and ionization detector were constructed from a stainless steel pipe 2.5 cm in diameter. Pyrex windows at each end allowed the laser beam to enter and to leave the cell. A tungsten wire extended along the axis of the tube and was biased at - 8 V relative to the tube. A regulated DC power supply was used to drive two heating elements that were wrapped around the stainless steel tube for 15 cm at the centre of the tube. The temperature at tile centre of this section was usually ~ 980 K corresponding to a T1 vapour pressure o f ~ 0.1 torr [9]. Helium gas at a pressure of 1 0 - 2 0 tort was used to inhibit diffusion of T1 from this central region to the windows. The dye laser was of the Hgnsch type and was pumped by an N 2 laser. The peak output power was 15 kW at 5 Hz with a linewidth of 0.4 A in the 4100 A region.

This laser was scanned at a rate of 4 A/min. The output from the ionization cell was coupled to a boxcar integrator set for optimum signal to noise ratio [5]. Part of the laser beam was passed through an air-spaced etalon with a separation of 0.3418 mm and detected with a photomultiplier. The resulting fringes were used for wavelength calibration of spectral features. The 6s26p configuration in T1 produces two electronic states, 6p 2P~/2 and 6p 2p0/2, split by spin orbit coupling so that the 2p0/2 state lies 7793 c m - " above the 2p0/2 state [10]. In view of this large splitting and the temperature of our T1 cell (~1000 K) transitions from 6p 2p0/2 are expected to dominate. In indium this splitting is reduced to 2212 cm -1 and transitions from the 2p~/ state are seen [11]. Two photon transitions from @2p°/2 to np 2p0 and nf 2F° states are permitted and should lle in the region X > 4060 A. We have observed 45 of these transitions in the wavelength region between 3900 and 6400 )~. A portion of this data showing the convergence of the 6s26p 2p0/7_._~ 6s2np 2P9_2,3_2/ / and 6s2nf 2F0tv 7t~ series to the two photon limit at 24633.35 cmZ]-ls's']lown in fig. 1. The 6s26p 2P°/2 -+ 6s2np 2p0/2,3/2 series has been observed for 7 ~
Volume 25, number 2

OPTICS COMMUNICATIONS

May 1978

6s ~ 6p zP,g e - 6 s * o,9 eg{,~

I/o

[11 2elIIITIE! I

4060

5,4

4100

13

,2

/~--~o

4150

4 s t nf atF~z.t.z

I/0

le

F~

17

4200 4250 LASER WAVELENGTH(A)

4300

4350

Fig. 1. Photoionization signal versus wavelength showing spectral features due to two photon transitions in TI. our measured energies for the n 2p0/2 levels are larger b y 4 - 5 cm -1 than those given b y Moore. Using these data and 49266.7 cm -1 for the ionization limit o f T1 [12], q u a n t u m defect plots for the n 2P°/2 and 2P°/2 levels are given in fig. 2. Fig. 3 shows a similar plot for n 2 F. For comparison, values for the q u a n t u m defects of the levels given b y Moore are also shown. With the Table 1 Energy levels for 1, 2 P 10 / 2 , 3 / 2 and 2 F5/2 states in T1. Energies are in vacuum wavenumbers n

2P~/2

2P~/2

Fine-structure

2F~/2

exception of the 15p state, which seems to be shifted 4 cm -1 to higher energy, these plots show little deviation as the limit is approached. Reeves et al. [12] have found little evidence for p e r t u r b a t i o n s to n 281/2 and It 2D5/2 states ofT1 for n > 12. The Land~ theory for the fine-structure splitting, AT, of states of a one-electron system that correspond to electrons in penetrating orbits gives the following expression [ 13 I 2 2 2 3/2 1(I + AT= a Z i Zoi / 1)X/~,, where A T = doublet splitting energy, a = fine structure 1

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

186

44381.0 45939.7 46854.4 47439,2 47832.6 48112,2 48320.4 48471.4 48590.7 48685.5

44567.5 46047.6 46920.9 47481.2 47862.7 48134.9 48337.4 48484.0 48600.6 48693.4 48768.9 48829.6 48880.4

186.5 107.9 66.5 42.0 30.1 22.7 17,0 12,6 9.9 7.9

44823.5 46185.7 47004.6 47539.1 47900.4 48161.8 48354.4 48501.0 48614.4 48703.4 48777.4 48835.4 48886.1 48927,1 48962,3

41,40

®

I

I

MOORE'S TABLE (NBS) THIS W O R K

1

~ 4.30

4.20

416

~

I

7

-

I

14 n~

L

21

Fig. 2. Quantum defect plot for n 2p°/2 and 2p°/2 states in Tl.

Volume 25, number 2 --[--

OPTICS COMMUNICATIONS I

I

A MOORE'.9 TABLE (NBS) ® THIS WORK

1.10

1.05

2

I

&

Q

m

IlO0

0.95

I 5

I lO

l

I 15

I 20

n~

t:ig. 3. Quantum defect plot for n 2Fs/2 states in TI. constant, Z i and Z o are effective inner and outer nuclear charges, l = orbital angular m o m e n t u m quantum number, R = = Rydberg constant and T = term value. If the values o f Z i and Z o appearing in this formula are independent o f n then A T e: T 3/2 so that a plot o f ATversus T 3/2 should be linear. This relation has been shown to 125

I

I

May 1978

hold well for n 2D states in Rb[3] where A T cx T 1.53. It is of interest to see if the predictions o f this simple theory hold for T1 since T1 is also a 'one-electron' system. Fig. 4 gives a plot of AxT for the n2p states ofT1 versus T 3/2 for these states. It can be seen that a linear relation is obtained; a fit o f a straight line to these data gives A T ec T 1.51 for T1. Garton et al. [15] have obtained absorption spectra of shock-heated T1 vapour and have identified several transitions to the 4D, 2D and 2p states of the 6s6p 2 configuration. Components o f these states which have energies in excess o f 49267 cm-1 show the effects o f interaction with continuum states. In particular, the transition 6s26p 2P1°/2 ~ 6s6p 2 4P3/2 at 2007 ~ shows a well developed Beutler-Fano resonance [16,17] with an emission wing to the long wavelength side. We have observed this line as a three p h o t o n transition at 6021 A but see no evidence for an emission wing as observed in absorption. We are reexamining this feature at higher resolution to determine if this effect is real or is an arTWO PHOTON TRANSITION

I

MOORE'S TABLE (NBS) ® THIS WORK

IO0

ZIT= T(n 2p~/2)_ T(n2pT/2 )

d~0-

75 THREE PHOTON TRANSITION 6 $ 2 6 P 2~/?2 ~ 6 $ 6o 2 4P3/2 'q

I

5O

25

¢

0

I

....

0

____2

/ / 4 13

1~

5

I

I0 T 3 / 2 x 10-4 (crn-3/2)

20

Fig. 4. Spin-orbit splitting AT of n 2pO states in TI versus term

values, T ~12 .

6021

I

I

LASER WAVELENGTH ( 7 )

I

15

5854.8

Fig. 5. Ionization signal observed on scan through the line at 6021 A that corresponds to the three photon transition 6s26p 2pO/~ ~ 6s6pZ 4p3/2" A two photon transition is also shown for comparison. 187

Volume 25, number 2

OPTICS COMMUNICATIONS

tifact o f the m e a s u r e m e n t s y s t e m w h i c h involves det e c t i o n o f TI+ ions. T h e i o n i z a t i o n signal o b t a i n e d on s c a n n i n g t h r o u g h 6021 ~ is s h o w n in fig. 5. This research was s u p p o r t e d b y a grant from the N a t i o n a l R e s e a r c h Council o f Canada. We t h a n k D.M. Bruce a n d W.A. Y o u n g for assistance with t h e s e experiments.

References [1 [ C.D. Ifarper, S.E. Wheatly and M.D. Levenson, J. Opt. Soc. Am. 67 (1977) 579. [2] N.M. Shen and S.M. Curry, Opt. Comm. 20 (1977) 392. [3] C.B. Collins, S.M. Curry, B.W. Johnson, M.Y. Mirza, M.A. Chellehmalzadeh, J.A. Anderson, D. Popescu and I. Popescu, Phys. Rev. A14 (1976) 1442. [4] M.Y. Mirza and W.W. Duley, J, Phys. B (1978), in press. [5] P.E. Sherick, J.A. Armstrong, R.W. Dreyfus and J.J. Wynne, Phys. Rev. Lett. 36 (1976) 1296.

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May 1978

[6] P. Esherick, J.J. Wynne and J.A. Armstrong, Opt. Lett. 1 (1977) 19. [71 M.Y. Mirza, Ph.D. Thesis, Muhiphoton Ionization of Cesium through Resonant Dissociative States of Cs2, Int. Diss. Abstr. B36 (1975) 289. [8] M.Y. Mirza and W.W. Duley, J. Opt. Soc. Am. 67 (1977) 1417. [9] J.L. Margrave, The characterization of high-temperature vapors (John Wiley & Sons, Inc., New York 1967) p. 480. [10] C.E. Moore, Atomic energy levels, Natl. Bur. Std. (USGPO. Washington, D.C. 1958) Vol. II1, pp. 202-203. [1 l ] M.Y. Mirza and W.W, Duley (unpublished work). [12] E.M. Reeves, W.R.S. Garton and A. Bass, Proc. Phys. Soc. 86 (1965) 1077. [13] H.G. Kuhn, Atonric spectra (Acad. Press, New York, 1962) p. 166. [14] S.M. Curry, C.B. Collins, M.Y. Mirza, D. Popescu and 1. Popescu, Opt. Comm. 16 (1976) 251. [15I W.R.S. Garton, W.H. Parkinson and E.M. Reeves, Can. J. Phys. 44 (1966) 1745. [16] It. Beutler and W. Demeter, Z. Physik 91 (1934) 143, 202. [171 C.V. Marr and R. tteppinstall, Proc. Phys. Soc. 87 (1966) 293,547.