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State selective laser photodissociation of Tl2 at 337.1 nm by optogalvanic spectroscopy
Volume 80, number 5,6
OPTICS COMMUNICATIONS
15 January 1991
State selective laser photodissociation of T12 at 337.1 nm by optogalvanic spectroscopy Athar S. Naqvi, K. Naveed Ullah and M.I. Rehmatullah Physics Department, Collegeof Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
Received 1 June 1990; revised manuscript received 19 September 1990
This paper reports the first time achievement of laser photolysis of thallium dimers by single photon absorption of N2-1aserlight at 337.1 nm resulting in a large population inversion of thallium 72S~/2excited state with respect to 62P3/2 state. The photodissociation nitrogen laser pulse is spatially and temporally overlapped with the tunable dye laser pulse that is used for confirming the production of selectively excited thallium atoms. The dye laser excites the thallium atoms from 72Sl/2 state to high lying Rydberg states that collisionally ionize giving an ion-current signal which is subsequently processed by a box-car average/integrator and recorded on the chart recorder. The photodissociation of Tl2 to Tl ( 72S1/2) state demonstrates the existence of the molecular dissociative state lg that is correlated with Tl 72S~/2+ 62p1/2 states. A complete absence of 6 2p3/2 state population among the photolysis products indicates a 100% prompt population inversion between 72S~/2and 62P3/2 atomic states.
I. Introduction In our previous paper [ l ] , we reported the achievement o f large population inversion between thallium 7 2Sl/2 and 6 2p3/2 states by photodissociation o f pure thallium dimers through two-photon absorption o f the green 532 nm second harmonic radiation o f a Nd-YAG laser. That was the only study thus far on the photolysis o f pure thallium metal. Almost all of the work reported in the literature [ 2 - 7 ] used various monohalides o f thallium to achieve the required population inversion for operating a thallium photodissociation laser. Moreover the photolysis was achieved by those authors mostly through single photon absorption o f UV radiation from various excimer lasers. In our work [ 1 ] two-photon absorption o f green 532 nm laser radiation from the T12 ground state 0~- to the b o u n d 0~- (or 2u) state resulted in predissociation to T1 (7 2S~/2 ) through the repulsive molecular state l u (or 0~-). An important result of our work [ 1 ] was the identification o f bound molecular states 0~- (or 2u) at 37594 c m energy and the l~ state at about 38444 c m - t energy. Moreover, the existence o f a repulsive state lu (or 0~- ) at a large internuclear separation greater than 8.5 bohr was also deduced.
In this paper we report an extension o f the above work [ l] where experiments were conducted with photodissociation laser being the nitrogen laser operating at 337.1 nm. The only previous work on thallium c o m p o u n d photodissociation using an N2 laser was for operation of a T1 Br laser [8] at 535 nm. Thus, this paper reports the first time achievement of population inversion in Tl (7 2S1/2) and (6 2p3/2) states using pure thallium metal with N2-1aser light for photodissociation o f the dimers. The details of the optogalvanic spectroscopic technique used in this work have been described previously [ l ] . Here it suffices to briefly mention that the ion-detection technique used here, in addition to offering high sensitivity o f detection, allows precise determination o f the excitation levels o f various product species. This is done by using a simultaneous pulse from the second dye laser that is spatially and temporally overlapped with the photodissociating pulse. The products o f the photolysis are promptly excited (with a very small delay o f a few nanoseconds) to high lying Rydberg states o f the atom, that subsequently ionize due to collisions with other atoms. Moreover, the approximate slope o f the molecular repulsive state can be determined by this technique by tuning the energy o f the photodissociating dye laser and moni-
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toring the ionization signal of the particular atomic transition from the product atomic excited state to a particular Rydberg state. This is done by realizing that the cross-section for photolysis of the dimer molecules into the product atoms depend on such factors as the matrix elements for the transition between the molecular electronic states, the FranckCondon factors of the initial and final molecular states and upon the probabilities for spontaneous dissociation or predissociation of the excited molecular state. For metals that require high temperatures for obtaining working vapor pressure, for example, 700 °C for producing 0.1 Tort of thallium vapors, one of the complications that arise in the construction of the repulsive state potential curves is the excitation of most of the vibrational levels of the molecular ground state. This paper reports the identification of one more T12 repulsive state correlated with the T1 (7 S~/2) and (6 P~/2) states. In contrast to the results in our previous paper [ 1 ] where two-photon photolysis of T12 took place by predissociation of the molecular through a repulsive state of very steep slope (lu or 0~ ) we obtain in this work dissociation of the TI2 repulsive state l g which is concluded to have a small slope.
15 January 1991
2. Experimental The optogalvanic spectroscopic technique used in this work essentially relies on the demonstration by Lee and Mahan [9 ] that if the atoms are excited close to the ionization limit, the excited atom will ionize with almost unit probability upon undergoing collisions with other atoms in the metallic vapor. These collisions produce excited molecules that decay to either molecular ions and electrons or to positive and negative atomic ions. The ion-ceU in our experiment was a tee-shaped stainless steel heatpipe oven with 1" internal diameter. It contained thallium metal heated to about 700°C to produce a vapor pressure of 0.1 Torr [ 10 ]. At this temperature, collisions of the fast moving atoms produce about 0.1% concentration of the dimers ( 11 ). About 10 Tort of buffer gas such as argon or helium was introduced into the ion-cell to prevent the thallium vapor drifting to the cold pyrex windows. A tungsten rod electrode 2 m m diameter entering from the bottom of the tee extended into the circular cross-section of the pipe. The tungsten electrode was biased at + 9 V relative to the heat pipe. The schematic layout of the experimental setup is shown in fig. 1. A series 1 M ohm resistor converts the ion-current pulse into a voltage pulse that is monitored on the oscilloscope and fed to the box-car integrator. The latter receives a synchronous trigger pulse 70 ns prior to the arrival of each ion-
PTP M
CRO
PTP-'~N2
LASER
1
"%s
Fig. 1. Schematic layout of the photodissociation experiment using nitrogenlaser. PTP: pretriggerpulse, IC: ion cell, BC: box-car averager, CR: strip chart recorder. 332
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current pulse at the box-car integrator. The signal to noise ratio is optimized by choosing 5 ns gate at the box-car. As the dye laser is tuned, the resulting ioncurrent signal is processed by the box-car and plotted on a chart recorder. A quarter of the nitrogen-laser beam energy (4 mJ energy per pulse and 10 ns pulse width) was obtained for photodissociation of the dimers, by inserting a beam splitter prior to its entrance into the dye laser while the remaining portion pumped a dye laser containing coumarin 480 and operating in 452 to 500 nm range, for achieving absorption transitions from TI 7 2Sl/2 to n 2p1/2,3/2, where n = I 1 to 17. The three two-photon absorption transitions in T1, 6 2P~/2-8 2p1/2,3/2 and 5F also occur in this range of dye laser wavelength and are used as marker for setting up the ion-cell as regards the ideal distance of the ion-producing region from the central tungsten electrode. This is done by observing the ion-current pulses on the oscilloscope as the dye laser is tuned to obtain these transitions. The nitrogen laser beam was of rectangular shape. It was focused using a cylindrical lens into a fine filament (1 cm long and 0.02 cm wide) parallel and near to the horizontal tungsten electrode extending along the side arm of the tee cell. The dye laser beam (0.8 m J) was gaussian and was focused by a cylindrical lens as a filament to tightly overlap with the focused nitrogen laser beam.
3. Results and discussion
Fig. 2a shows the result of the dye laser scan between 452 nm and 500 nm in the absence of the N2laser beam. It shows three two-photon absorption transitions in thallium namely 6 2P1/2 to 8 2P1/2.3/2 and 5F. Fig. 2b shows a portion the dye laser scan with the dye laser pulse overlapping spatially but temporally slightly delayed by 3 ns with respect to the N2-1aser pulse. The two tall peaks are the twophoton transitions 6 2P~/2 to 8 2p3/2 and 5F atomic states. Also present are the peaks corresponding to the single photon absorption transitions T1 7 2S,/2 to 12 2p1/2,3/2 and 13 2p1/2,3/2 [ 12]. The single photon absorption was confirmed by using neutral density filters in the UV beam, producing a linear change in the intensity of the dye laser transition, while main-
15 January 1991
taining a constant intensity of the dye laser output. In our present experiment, the ratio of the intensity of the 7 2S1/2 to 12 2p1/2,3/2 transition and the accompanying two-photon absorption transitions is much smaller than in our previous experiment [ 1 ] with a Nd-YAG laser. This reduction in the relative intensity is primarily due to two factors. Firstly, the degree of overlap at the focused region of the initially rectangular N2-1aser beam and the gaussian dye laser beam is not as good as in the case of two gaussian beams in our previous experiments. Secondly, the energy per pulse of the N2-1aser beam used for photodissociation was less than 1 mJ compared to about 50 mJ of the green SHG of Nd-YAG laser. In spite of the above difficulties it is apparent from fig. 2b that the use of a box-car integration technique allows a rather good recovery of the signal. Although, at this stage, quantitative measurements of the relative cross-sections of photodissociation through single and two-photon absorptions could not be performed, the presence of T1 72S~/2--, 12, 13 2p1/2,3/2 transitions in our present experiment quite unambiguously demonstrates the occurrence of single photon dissociation of TI> To achieve a population inversion between T1 75t/2 and 62p3/2 it is necessary to ascertain about the population of the 62p3/2 state. At the operating temperature of 700°C, the Boltzmann thermal population of 6 2P3/2 (term energy 7792 c m - ' ) is much less than 1% of that of the ground state 62P~/2. The dye laser photons have much greater energy than 7792 c m - l . Thus the only means by which 62P3/2 can be expected to be populated is by photodissociation of any repulsive molecular state that may be correlated with atomic 6 2P3/2 state. The method of verifying the population of 62p3/2 or lack of it is by monitoring the possible absorption transitions from 6 P3/2 within the range of wavelengths of the dye laser i.e. from 452 nm to 500 nm for coumarin 480 being used. Two-photon absorption transitions from 6P3/2 to n2p states with n = 13 to 17 lie within the above wavelength range. None of these transition was observed in the dye laser scan thus proving the absence of population of the 6 2p3/2 state. Thus we have a complete population inversion between the TI 7 S,/2 state produced as a result of photodissociation o f TI2 and the unpopulated T1 6 2p3/2 state. Due to a great interest in achieving photodisso333
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OPTICS COMMUNICATIONS
15January1991
5F
/ Z Q Z 9
BP3/2
(a)~ 8PI/2
N Z O
I
J
502
492
. . . . . .
I
482
LASER
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I
472
462
I 452
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Fig. 2. (a) Dye laser scan between 452 and 500 nm showing two-photon absorption transitions 8 :pi/3...~ 8 2p1/2,3/2 and 5 2F in atomic thallium in the absence of the nitrogen-laser beam. (b) A portion of the dye laser scan (with increased sensitivity) in the presence of the nitrogen-laser beam spatially and temporally overlapping with the dye laser beam.
ciation lasers, there is a need to study the manifold o f molecular states correlated with T1 (7 2S~/2) and 6 2P~/2 atomic states. So far only Christiansen et al. [ 13 ] have performed calculations for the dissociation curves o f only nine low lying states o f TI2 correlated with 2p i/2 -t- 2p i/2 and 2P I/2 -I-2p3/2 states. N o theoretical calculations have so far been performed on higher Tl2 states. Experimental evidence regard334
ing the existence of a repulsive state lu (or 0Z ) correlated with 7 2Si/2 + 6 2p i/2 atomic states and lying at larger internuclear separation, was presented in our previous work [ 1 ]. The selection rules for transitions in TI2 further help to bring forth additional evidence as to the existence of a shallow repulsive state correlated with 7 2S1/2 + 6 2pl/2. For heavy elements such as thallium ( Z = 8 1 ) , ( c o - c o ) coupling
Volume 80, number 5,6
OPTICS COMMUNICATIONS
prevails and only the component of the angular momentum along the internuclear axis of the molecule is defined. Thus the manifold of molecular states resuiting from 72S1/2+62P~/2 comprise to 1,, 0u, 0u+, l g, 0~- and 0~-. The selection rules for Hund's case (c) will apply, where for one-photon absorption A.Q= + 1. Also for molecules with nuclei of equal charge as for T12, even electronic states will combine only with odd, for one-photon absorption. The additional rule that also applies is 0 +,--*0÷ and 0-,--,0-. Thus one-photon absorption transition from T12 ground state 0 ; terminates on the I s state which is a repulsive state as it dissociates to T1 72S~/2. The TI2 ground state 0~- has dissociation energy of 1291 c m - ~ and the minimum of the potential curve lies at 6.5 bohr [ 13]. The atomic state T1 7 2Si/2 lies at 27768 cm-~ above the bottom of the T12 ground state potential energy curve. Thus with an N2-1aser photon energy of 29665 c m - l , it becomes possible to access the repulsive state 1s by a single photon absorption even from the bottom of the TI2 ground state. At the operating temperature of 700°C, the turning points of the vibrational level that is populated by 20% as compared to the lowest vibrational level of this state lie at 5.75 and 8.5 bohr. The effect on the signal of the vertical transitions (those allowed by the Franck-Condon principle [ 14 ] ) from the lowest vibrational level will be five times more than the transitions from the turning point of the vibrational level with 20% relative population. In fig. 3, these transitions are marked as (I), (II) and (III), and originate from internuclear separations 6.5 bohr, 5.75 bohr and 8.5 bohr respectively. Also shown are the possible locations of the corresponding repulsive states that may be responsible for producing the experimental photodissociation-ionization signal of fig. 2b. For transition (III) in fig. 3, a rather steep repulsive state would be required, which obviously could not be accessed by transitions (I) and (II). In this case the ion-current signal would be a sensitive function of the operating temperature that determines the population of the vibrational levels according to the Boltzmann distribution. The temperature reduction affecting the dimer concentration equally reduces the ionization signals for the absorption transitions I, II and III. There is also a reduction of the ionization signal for the transition III as a result of relative decrease of the ther-
15 January 1991
+49266cm
///f, rf//f///ff//a'
26477
-i
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7S1,,2+ 6Pi/2
I i i TL 2
= 337.1 n~ p
i
1
(I]
,, I !El i
I
,,
Ii
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i i
~
i ;
7792
I,
1 6
8 R ( bohr )
6 P~t- 6Pin
Ocm" ee,,2,66,, ~ I 9
I IO
I II
- 1291
Fig. 3. Tl2-potential energy curves showing one-photon dissociation at 337.1 nm to 72SI/2 atomic state. Also shown is two-photon absorption transition at 532 nm resulting in predissociation ofTl (ref. [ 1 ] ). The ground state potential curves are taken from
ref. [13].
mal population of the vibrational level near the dissociation limit. The equilibrium constant K, defined as the ratio N d i m e r / N 2 monomer, N being the number density, depends through an exponential factor on the dissociation energy of the dimer and the temperature [ 11 ]. When the temperature is decreased from 1000 K to 900 K, the ionization signal for the transition I will reduce by about 19%. If the transition III was responsible for the ionization current, a reduction of about 36% will be expected. Experimentally the first case prevailed thus ruling-out the configuration III for the repulsive state that might have been involved in the photodissociation process. Regarding the two possible shapes (I) and (II) of the repulsive state 1s, it is noted that, at the operating temperature of 700 ° C, the contribution of the transition (II) is only about 20% of the contribution 335
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OPTICS COMMUNICATIONS
for the transition (I). Operating at a reduced temperature (about 6 5 0 ° C ) produced only a slight reduction in the photodissociation-ionization signal. In the presence o f contributory factors such as the reduction in dimer density, background ion-current noise and the slope of repulsive state, it is difficult to distinguish between the shapes (I) and (II) o f the state lg. However, the conclusion is nevertheless the same, that the repulsive state lg is rather shallow as drawn in fig. 3. The likelihood of accessing a repulsive state from two other states l u and 0 s that are correlated with ground state T1 atoms 6 2p i/2-1- 6 2p 1/2 which are populated by 30% at the operating temperature, was eliminated by conducting the experiment at a reduced temperature of 650 ° C. The population of these two states becomes almost negligible thus leaving only the transition form the ground state 0~-. Also shown in fig. 3 is the two-photon absorption transition ( 1 ) resulting in pre-dissociation to T17 2S l/2 state via the repulsive state lu (or 0~-) that is correlated with 725t/2-1-6 2Pl/2. As discussed in ( I ) , this repulsive state lies at large internuclear separations. As a conclusion it is remarked that double resonance technique using a dye laser to monitor, by optogalvanic spectroscopy, the selectively excited photodissociation products allows identification o f various repulsive states o f the dimers. We have so far succeeded in identifying two repulsive states, one odd and the other even, that are correlated with the T1 72Sl/2q-62pl/2 states. Further experiments are planned using a tunable dye laser for photodissociation o f the dimer to determine the slopes o f these re-
336
15 January 1991
pulsive states. It is worthwhile to point out the usefulness o f studying thallium, as the large gap o f 7792 cm-~ between 6 2p3/2 and 62p1/2 ground state doublet does not allow the 62P3/2 ground state to be thermally populated even at high operating temperatures. Further, a large gap between 7 2S1/2 and 6 2p3/2 precludes the interference o f relevant molec# ular states correlated with these atomic energy levels. Thus it becomes certain to attain a complete prompt population inversion between 7 2S~/2 and 6 2p3/2.
References [ 1] A.S. Naqvi, J. Chem. Phys. 82 (1985) 2217. [2] D.A. Scott and J.A. Piper, J. Appl. Phys. 52 ( 1981 ) 5426. [3] K. Ludewigt, S. Shahdin and B. Wellegehausen, Optics Comm. 42 (1982) 143. [4] P. Burkhard, W. Luthy and T. Gerber, Optics Comm. 34 (1980) 451. [5] D.J. Ehrlich, J. Maya and R.M. Osgood, Appl. Phys. Lett. 33 (1978) 931. [ 6 ] P. Burkhard, W. Luthy and T. Gerber, Optics Lett. 5 (1980) 522. [7] J.C. White and D. Hendersen, Phys. Rev. A 25 (1982) 1226. [ 8 ] W. Luthy, B. Burkhard, T.E. Gerber and H.P. Weber, Optics Comm. 38 (1981) 413. [ 9 ] Y.T. Lee and B.H. Mahan, J. Chem. Phys. 42 ( 1965) 2893. [10] J.L. Margrave, The characterization of high temperature vapors (Wiley, New York, 1967) p. 480. [11 ] M. Lapp and L.P. Harris, J. Quant. Spectrosc. Radiat. Transfer 6 (1966) 169. [ 12] C.E. Moore, Atomic energy levels, Vol. 3, Natl. Bur. Stand. (US GPO, Washington, 1958) pp. 202, 203. [ 13] P.A. Christiansen et al., J. Chem. Phys. 79 (1983) 2928. [ 14] G. Herzberg, Spectra of diatomic molecules, 2nd Ed. (Van Nostrand Reinholt, New York, 1955).
OPTICS COMMUNICATIONS
15 January 1991
State selective laser photodissociation of T12 at 337.1 nm by optogalvanic spectroscopy Athar S. Naqvi, K. Naveed Ullah and M.I. Rehmatullah Physics Department, Collegeof Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
Received 1 June 1990; revised manuscript received 19 September 1990
This paper reports the first time achievement of laser photolysis of thallium dimers by single photon absorption of N2-1aserlight at 337.1 nm resulting in a large population inversion of thallium 72S~/2excited state with respect to 62P3/2 state. The photodissociation nitrogen laser pulse is spatially and temporally overlapped with the tunable dye laser pulse that is used for confirming the production of selectively excited thallium atoms. The dye laser excites the thallium atoms from 72Sl/2 state to high lying Rydberg states that collisionally ionize giving an ion-current signal which is subsequently processed by a box-car average/integrator and recorded on the chart recorder. The photodissociation of Tl2 to Tl ( 72S1/2) state demonstrates the existence of the molecular dissociative state lg that is correlated with Tl 72S~/2+ 62p1/2 states. A complete absence of 6 2p3/2 state population among the photolysis products indicates a 100% prompt population inversion between 72S~/2and 62P3/2 atomic states.
I. Introduction In our previous paper [ l ] , we reported the achievement o f large population inversion between thallium 7 2Sl/2 and 6 2p3/2 states by photodissociation o f pure thallium dimers through two-photon absorption o f the green 532 nm second harmonic radiation o f a Nd-YAG laser. That was the only study thus far on the photolysis o f pure thallium metal. Almost all of the work reported in the literature [ 2 - 7 ] used various monohalides o f thallium to achieve the required population inversion for operating a thallium photodissociation laser. Moreover the photolysis was achieved by those authors mostly through single photon absorption o f UV radiation from various excimer lasers. In our work [ 1 ] two-photon absorption o f green 532 nm laser radiation from the T12 ground state 0~- to the b o u n d 0~- (or 2u) state resulted in predissociation to T1 (7 2S~/2 ) through the repulsive molecular state l u (or 0~-). An important result of our work [ 1 ] was the identification o f bound molecular states 0~- (or 2u) at 37594 c m energy and the l~ state at about 38444 c m - t energy. Moreover, the existence o f a repulsive state lu (or 0~- ) at a large internuclear separation greater than 8.5 bohr was also deduced.
In this paper we report an extension o f the above work [ l] where experiments were conducted with photodissociation laser being the nitrogen laser operating at 337.1 nm. The only previous work on thallium c o m p o u n d photodissociation using an N2 laser was for operation of a T1 Br laser [8] at 535 nm. Thus, this paper reports the first time achievement of population inversion in Tl (7 2S1/2) and (6 2p3/2) states using pure thallium metal with N2-1aser light for photodissociation o f the dimers. The details of the optogalvanic spectroscopic technique used in this work have been described previously [ l ] . Here it suffices to briefly mention that the ion-detection technique used here, in addition to offering high sensitivity o f detection, allows precise determination o f the excitation levels o f various product species. This is done by using a simultaneous pulse from the second dye laser that is spatially and temporally overlapped with the photodissociating pulse. The products o f the photolysis are promptly excited (with a very small delay o f a few nanoseconds) to high lying Rydberg states o f the atom, that subsequently ionize due to collisions with other atoms. Moreover, the approximate slope o f the molecular repulsive state can be determined by this technique by tuning the energy o f the photodissociating dye laser and moni-
0030-4018/91/$03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )
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toring the ionization signal of the particular atomic transition from the product atomic excited state to a particular Rydberg state. This is done by realizing that the cross-section for photolysis of the dimer molecules into the product atoms depend on such factors as the matrix elements for the transition between the molecular electronic states, the FranckCondon factors of the initial and final molecular states and upon the probabilities for spontaneous dissociation or predissociation of the excited molecular state. For metals that require high temperatures for obtaining working vapor pressure, for example, 700 °C for producing 0.1 Tort of thallium vapors, one of the complications that arise in the construction of the repulsive state potential curves is the excitation of most of the vibrational levels of the molecular ground state. This paper reports the identification of one more T12 repulsive state correlated with the T1 (7 S~/2) and (6 P~/2) states. In contrast to the results in our previous paper [ 1 ] where two-photon photolysis of T12 took place by predissociation of the molecular through a repulsive state of very steep slope (lu or 0~ ) we obtain in this work dissociation of the TI2 repulsive state l g which is concluded to have a small slope.
15 January 1991
2. Experimental The optogalvanic spectroscopic technique used in this work essentially relies on the demonstration by Lee and Mahan [9 ] that if the atoms are excited close to the ionization limit, the excited atom will ionize with almost unit probability upon undergoing collisions with other atoms in the metallic vapor. These collisions produce excited molecules that decay to either molecular ions and electrons or to positive and negative atomic ions. The ion-ceU in our experiment was a tee-shaped stainless steel heatpipe oven with 1" internal diameter. It contained thallium metal heated to about 700°C to produce a vapor pressure of 0.1 Torr [ 10 ]. At this temperature, collisions of the fast moving atoms produce about 0.1% concentration of the dimers ( 11 ). About 10 Tort of buffer gas such as argon or helium was introduced into the ion-cell to prevent the thallium vapor drifting to the cold pyrex windows. A tungsten rod electrode 2 m m diameter entering from the bottom of the tee extended into the circular cross-section of the pipe. The tungsten electrode was biased at + 9 V relative to the heat pipe. The schematic layout of the experimental setup is shown in fig. 1. A series 1 M ohm resistor converts the ion-current pulse into a voltage pulse that is monitored on the oscilloscope and fed to the box-car integrator. The latter receives a synchronous trigger pulse 70 ns prior to the arrival of each ion-
PTP M
CRO
PTP-'~N2
LASER
1
"%s
Fig. 1. Schematic layout of the photodissociation experiment using nitrogenlaser. PTP: pretriggerpulse, IC: ion cell, BC: box-car averager, CR: strip chart recorder. 332
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current pulse at the box-car integrator. The signal to noise ratio is optimized by choosing 5 ns gate at the box-car. As the dye laser is tuned, the resulting ioncurrent signal is processed by the box-car and plotted on a chart recorder. A quarter of the nitrogen-laser beam energy (4 mJ energy per pulse and 10 ns pulse width) was obtained for photodissociation of the dimers, by inserting a beam splitter prior to its entrance into the dye laser while the remaining portion pumped a dye laser containing coumarin 480 and operating in 452 to 500 nm range, for achieving absorption transitions from TI 7 2Sl/2 to n 2p1/2,3/2, where n = I 1 to 17. The three two-photon absorption transitions in T1, 6 2P~/2-8 2p1/2,3/2 and 5F also occur in this range of dye laser wavelength and are used as marker for setting up the ion-cell as regards the ideal distance of the ion-producing region from the central tungsten electrode. This is done by observing the ion-current pulses on the oscilloscope as the dye laser is tuned to obtain these transitions. The nitrogen laser beam was of rectangular shape. It was focused using a cylindrical lens into a fine filament (1 cm long and 0.02 cm wide) parallel and near to the horizontal tungsten electrode extending along the side arm of the tee cell. The dye laser beam (0.8 m J) was gaussian and was focused by a cylindrical lens as a filament to tightly overlap with the focused nitrogen laser beam.
3. Results and discussion
Fig. 2a shows the result of the dye laser scan between 452 nm and 500 nm in the absence of the N2laser beam. It shows three two-photon absorption transitions in thallium namely 6 2P1/2 to 8 2P1/2.3/2 and 5F. Fig. 2b shows a portion the dye laser scan with the dye laser pulse overlapping spatially but temporally slightly delayed by 3 ns with respect to the N2-1aser pulse. The two tall peaks are the twophoton transitions 6 2P~/2 to 8 2p3/2 and 5F atomic states. Also present are the peaks corresponding to the single photon absorption transitions T1 7 2S,/2 to 12 2p1/2,3/2 and 13 2p1/2,3/2 [ 12]. The single photon absorption was confirmed by using neutral density filters in the UV beam, producing a linear change in the intensity of the dye laser transition, while main-
15 January 1991
taining a constant intensity of the dye laser output. In our present experiment, the ratio of the intensity of the 7 2S1/2 to 12 2p1/2,3/2 transition and the accompanying two-photon absorption transitions is much smaller than in our previous experiment [ 1 ] with a Nd-YAG laser. This reduction in the relative intensity is primarily due to two factors. Firstly, the degree of overlap at the focused region of the initially rectangular N2-1aser beam and the gaussian dye laser beam is not as good as in the case of two gaussian beams in our previous experiments. Secondly, the energy per pulse of the N2-1aser beam used for photodissociation was less than 1 mJ compared to about 50 mJ of the green SHG of Nd-YAG laser. In spite of the above difficulties it is apparent from fig. 2b that the use of a box-car integration technique allows a rather good recovery of the signal. Although, at this stage, quantitative measurements of the relative cross-sections of photodissociation through single and two-photon absorptions could not be performed, the presence of T1 72S~/2--, 12, 13 2p1/2,3/2 transitions in our present experiment quite unambiguously demonstrates the occurrence of single photon dissociation of TI> To achieve a population inversion between T1 75t/2 and 62p3/2 it is necessary to ascertain about the population of the 62p3/2 state. At the operating temperature of 700°C, the Boltzmann thermal population of 6 2P3/2 (term energy 7792 c m - ' ) is much less than 1% of that of the ground state 62P~/2. The dye laser photons have much greater energy than 7792 c m - l . Thus the only means by which 62P3/2 can be expected to be populated is by photodissociation of any repulsive molecular state that may be correlated with atomic 6 2P3/2 state. The method of verifying the population of 62p3/2 or lack of it is by monitoring the possible absorption transitions from 6 P3/2 within the range of wavelengths of the dye laser i.e. from 452 nm to 500 nm for coumarin 480 being used. Two-photon absorption transitions from 6P3/2 to n2p states with n = 13 to 17 lie within the above wavelength range. None of these transition was observed in the dye laser scan thus proving the absence of population of the 6 2p3/2 state. Thus we have a complete population inversion between the TI 7 S,/2 state produced as a result of photodissociation o f TI2 and the unpopulated T1 6 2p3/2 state. Due to a great interest in achieving photodisso333
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5F
/ Z Q Z 9
BP3/2
(a)~ 8PI/2
N Z O
I
J
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492
. . . . . .
I
482
LASER
]
I
472
462
I 452
WAVELENGTH
I
Fig. 2. (a) Dye laser scan between 452 and 500 nm showing two-photon absorption transitions 8 :pi/3...~ 8 2p1/2,3/2 and 5 2F in atomic thallium in the absence of the nitrogen-laser beam. (b) A portion of the dye laser scan (with increased sensitivity) in the presence of the nitrogen-laser beam spatially and temporally overlapping with the dye laser beam.
ciation lasers, there is a need to study the manifold o f molecular states correlated with T1 (7 2S~/2) and 6 2P~/2 atomic states. So far only Christiansen et al. [ 13 ] have performed calculations for the dissociation curves o f only nine low lying states o f TI2 correlated with 2p i/2 -t- 2p i/2 and 2P I/2 -I-2p3/2 states. N o theoretical calculations have so far been performed on higher Tl2 states. Experimental evidence regard334
ing the existence of a repulsive state lu (or 0Z ) correlated with 7 2Si/2 + 6 2p i/2 atomic states and lying at larger internuclear separation, was presented in our previous work [ 1 ]. The selection rules for transitions in TI2 further help to bring forth additional evidence as to the existence of a shallow repulsive state correlated with 7 2S1/2 + 6 2pl/2. For heavy elements such as thallium ( Z = 8 1 ) , ( c o - c o ) coupling
Volume 80, number 5,6
OPTICS COMMUNICATIONS
prevails and only the component of the angular momentum along the internuclear axis of the molecule is defined. Thus the manifold of molecular states resuiting from 72S1/2+62P~/2 comprise to 1,, 0u, 0u+, l g, 0~- and 0~-. The selection rules for Hund's case (c) will apply, where for one-photon absorption A.Q= + 1. Also for molecules with nuclei of equal charge as for T12, even electronic states will combine only with odd, for one-photon absorption. The additional rule that also applies is 0 +,--*0÷ and 0-,--,0-. Thus one-photon absorption transition from T12 ground state 0 ; terminates on the I s state which is a repulsive state as it dissociates to T1 72S~/2. The TI2 ground state 0~- has dissociation energy of 1291 c m - ~ and the minimum of the potential curve lies at 6.5 bohr [ 13]. The atomic state T1 7 2Si/2 lies at 27768 cm-~ above the bottom of the T12 ground state potential energy curve. Thus with an N2-1aser photon energy of 29665 c m - l , it becomes possible to access the repulsive state 1s by a single photon absorption even from the bottom of the TI2 ground state. At the operating temperature of 700°C, the turning points of the vibrational level that is populated by 20% as compared to the lowest vibrational level of this state lie at 5.75 and 8.5 bohr. The effect on the signal of the vertical transitions (those allowed by the Franck-Condon principle [ 14 ] ) from the lowest vibrational level will be five times more than the transitions from the turning point of the vibrational level with 20% relative population. In fig. 3, these transitions are marked as (I), (II) and (III), and originate from internuclear separations 6.5 bohr, 5.75 bohr and 8.5 bohr respectively. Also shown are the possible locations of the corresponding repulsive states that may be responsible for producing the experimental photodissociation-ionization signal of fig. 2b. For transition (III) in fig. 3, a rather steep repulsive state would be required, which obviously could not be accessed by transitions (I) and (II). In this case the ion-current signal would be a sensitive function of the operating temperature that determines the population of the vibrational levels according to the Boltzmann distribution. The temperature reduction affecting the dimer concentration equally reduces the ionization signals for the absorption transitions I, II and III. There is also a reduction of the ionization signal for the transition III as a result of relative decrease of the ther-
15 January 1991
+49266cm
///f, rf//f///ff//a'
26477
-i
ATOMIC IP
7S1,,2+ 6Pi/2
I i i TL 2
= 337.1 n~ p
i
1
(I]
,, I !El i
I
,,
Ii
G(532nm):
i i
~
i ;
7792
I,
1 6
8 R ( bohr )
6 P~t- 6Pin
Ocm" ee,,2,66,, ~ I 9
I IO
I II
- 1291
Fig. 3. Tl2-potential energy curves showing one-photon dissociation at 337.1 nm to 72SI/2 atomic state. Also shown is two-photon absorption transition at 532 nm resulting in predissociation ofTl (ref. [ 1 ] ). The ground state potential curves are taken from
ref. [13].
mal population of the vibrational level near the dissociation limit. The equilibrium constant K, defined as the ratio N d i m e r / N 2 monomer, N being the number density, depends through an exponential factor on the dissociation energy of the dimer and the temperature [ 11 ]. When the temperature is decreased from 1000 K to 900 K, the ionization signal for the transition I will reduce by about 19%. If the transition III was responsible for the ionization current, a reduction of about 36% will be expected. Experimentally the first case prevailed thus ruling-out the configuration III for the repulsive state that might have been involved in the photodissociation process. Regarding the two possible shapes (I) and (II) of the repulsive state 1s, it is noted that, at the operating temperature of 700 ° C, the contribution of the transition (II) is only about 20% of the contribution 335
Volume 80, number 5,6
OPTICS COMMUNICATIONS
for the transition (I). Operating at a reduced temperature (about 6 5 0 ° C ) produced only a slight reduction in the photodissociation-ionization signal. In the presence o f contributory factors such as the reduction in dimer density, background ion-current noise and the slope of repulsive state, it is difficult to distinguish between the shapes (I) and (II) o f the state lg. However, the conclusion is nevertheless the same, that the repulsive state lg is rather shallow as drawn in fig. 3. The likelihood of accessing a repulsive state from two other states l u and 0 s that are correlated with ground state T1 atoms 6 2p i/2-1- 6 2p 1/2 which are populated by 30% at the operating temperature, was eliminated by conducting the experiment at a reduced temperature of 650 ° C. The population of these two states becomes almost negligible thus leaving only the transition form the ground state 0~-. Also shown in fig. 3 is the two-photon absorption transition ( 1 ) resulting in pre-dissociation to T17 2S l/2 state via the repulsive state lu (or 0~-) that is correlated with 725t/2-1-6 2Pl/2. As discussed in ( I ) , this repulsive state lies at large internuclear separations. As a conclusion it is remarked that double resonance technique using a dye laser to monitor, by optogalvanic spectroscopy, the selectively excited photodissociation products allows identification o f various repulsive states o f the dimers. We have so far succeeded in identifying two repulsive states, one odd and the other even, that are correlated with the T1 72Sl/2q-62pl/2 states. Further experiments are planned using a tunable dye laser for photodissociation o f the dimer to determine the slopes o f these re-
336
15 January 1991
pulsive states. It is worthwhile to point out the usefulness o f studying thallium, as the large gap o f 7792 cm-~ between 6 2p3/2 and 62p1/2 ground state doublet does not allow the 62P3/2 ground state to be thermally populated even at high operating temperatures. Further, a large gap between 7 2S1/2 and 6 2p3/2 precludes the interference o f relevant molec# ular states correlated with these atomic energy levels. Thus it becomes certain to attain a complete prompt population inversion between 7 2S~/2 and 6 2p3/2.
References [ 1] A.S. Naqvi, J. Chem. Phys. 82 (1985) 2217. [2] D.A. Scott and J.A. Piper, J. Appl. Phys. 52 ( 1981 ) 5426. [3] K. Ludewigt, S. Shahdin and B. Wellegehausen, Optics Comm. 42 (1982) 143. [4] P. Burkhard, W. Luthy and T. Gerber, Optics Comm. 34 (1980) 451. [5] D.J. Ehrlich, J. Maya and R.M. Osgood, Appl. Phys. Lett. 33 (1978) 931. [ 6 ] P. Burkhard, W. Luthy and T. Gerber, Optics Lett. 5 (1980) 522. [7] J.C. White and D. Hendersen, Phys. Rev. A 25 (1982) 1226. [ 8 ] W. Luthy, B. Burkhard, T.E. Gerber and H.P. Weber, Optics Comm. 38 (1981) 413. [ 9 ] Y.T. Lee and B.H. Mahan, J. Chem. Phys. 42 ( 1965) 2893. [10] J.L. Margrave, The characterization of high temperature vapors (Wiley, New York, 1967) p. 480. [11 ] M. Lapp and L.P. Harris, J. Quant. Spectrosc. Radiat. Transfer 6 (1966) 169. [ 12] C.E. Moore, Atomic energy levels, Vol. 3, Natl. Bur. Stand. (US GPO, Washington, 1958) pp. 202, 203. [ 13] P.A. Christiansen et al., J. Chem. Phys. 79 (1983) 2928. [ 14] G. Herzberg, Spectra of diatomic molecules, 2nd Ed. (Van Nostrand Reinholt, New York, 1955).