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Radiative lifetime of Li2 A 1Σ+u
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
1 February
1977
RADIATIVE LIFETIME OF Liz A ‘2: Paul H. WINE” and L.A. MELTON Chemistry Department, The University of Texas at Dallas, Richardson, Texas 75080, USA Recclved 23 September
1976
Collision free liietimes and total quenching cross sections have been measured for Liz A ‘C”, as a function of the degree of vibrational excitation. The lifetimes are mdependent of u and average 18.0 t 1.9 ns. The quenching cross sections range from 185 A’ to 406 A’.
Because of their potential as laser systems [l] and their rather unique electronic properties, the Iowlying excited states of the alkali dimers have been of interest to both theorists and experimental&s for some time. The inherent simplicity which results from the hydrogen-like character of these molecules makes them a favorite “proving ground” for ab initio calculaticn techniques. Accurate calculations of excited state properties has made it possible to obtain electronic transition moments, and hence, radiative lifetimes. Accurate experimental data is needed to test the validity of such calculations. Experimental lifetimes have been reported for Na, B ‘II, [2,3], K, B ‘II, [3,4], and Na, A ‘Xi IS]. We now report experimental values for collision free radiative lifetimes and total quenching rates of Li, A ‘2: as a function of the degree of vibrational and rotational excitation. We are particularly interested in Liz A ‘ZZt as a possible product of the very efficient quenching of LiH A ‘IZ+ by Li ?S [6J f. U it ium dimers were prepared in a heat pipe oven 171 by heating metallic lithium in the presence of helium buffer gas (helium pressure O-1-0.65 torr) until the temperature was reached at which the vapor pressure of lithium in the center of the heat pipe was equal to the helium pressure in the water cooled outer * Robert A. Welch postdoctoral fellow. ’ Recent work in our faboratory suggests that onIy a smaU fraction of the LIH A lX+v, J collisional loss rate is ae counted for by transitions to other v, J levels of the same electronic state.
regions. A capacitance manometer measured PHe, while a chromel-alumel thermocouple inserted into the center of the heat pipe measured the temperature of the lithium vapor. Proper heat pipe operation was confirmed by comparing PLi f P,b (obtained from vapour pressure data [8] ) with pHz_ All experiments were performed at temperatures in the range of9001000 K. The lithium vapor composition in this temperature region is 2-3% dimer and 97-98% monomer [9] ; hence, the observed decrease in lifetime with increasing pressure almost certainly results from coliisions with atomic lithium. Excitation to the A 1X: state was achieved with a nitrogen laser pumped tunable dye laser. Fluorescence was observed at right angles to the laser beam by a detection system consisting of a SPEX 1 m monochromator, a Hamamatsu R446 photomultiplier tube, and a PAR model 1621164 boxcar integrator. The boxcar gate width was set at 5 ns and scanned in time, allowing the time profile of both the laser and the fluorescence to be recorded_ The lifetime was then obtained by least squares fitting the fluorescence profiIe to a convolution of the laser profile with a single exponential decay. Cahbration of the time base was achieved by scanning the boxcar gate through the output from a crystal controlled 50 MHz oscillator. Typicat lifetime data is displayed in fig. I_ In fig. 2, all of the measured lifetimes are presented in Stern-VoImer type plots. The straightforward procedure used to extract collision free lifetimes and quenching cross sections from the 1/r versus pressure data has been discussed ekewhere 161. 509
CHEMICAL PHYSICS LE-lTERS
Volume 45, number 3
Fig. 1. Typical lifetime data; excitation pressure 0.400 torr.
wavelength:
6046 A.
-Jf&&zr i
.l
.2
3
p;-,,,
i
.6
.i
Fig. 2. Determination of TO by extrapolation of l/r versus pressure data. o experimental data, linear least squares fits. Excitation wavelength: (A) 5847 A, (B) 5949 A, (c) 6046 A, (D) 6315 A, (E) 6513 A. Error bars are 20.
Because the ground state vibrational frequency of Li2 is only ~350 cm-l, absorption from high u” levels is observed. This results in a very dense absorption spectrum. With our laser bandwidth of eO.5 A, several 510
1 February
1977
u’, J’ levels were excited simultaneously. Attempts to isolate single fluorescence lines were frustrated due to diminishing intensity. Therefore, all lifetime measurements were carried out with 24 A spectral resolution. The monochromator served to filter out laser scattered light and provided some additional state selection, but the observed fluorescence always emanated from more than a single u’, J’ level. Resolved fluorescence spectra were obtained at each excitation wavelength and assigzments were made based on spectroscopic constants Teported by Hsu [9]. Concurrent agreement of line positions, P-R doublet splittings, and splittings of equivalent lines in successive bands (which serves to identify u”) was taken as proof that the assignments are correct. The relative contribution of each u’, J’ to the fluorescence observed in each lifetime measurement was computed assuming a triangular slit function and is tabulated with some important experimental parameters in table 1. Collision free lifetimes (TV) and quenching cross sections (uQ) are tabulated in table 2. Within experimental error, the lifetimes are independent of vibrational-rotational quantum number and yield an average value of 18.0 i 1.9 11s.Tango and Zare [4] have calculated alkali dimer lifetimes from a simple model based on the following properties of this group of molecules: (1) A-X and B-X transitions remain strong even as the internuclear separation approaches infinity and (2) binding results almost entirely from a sharing of valence electrons with no distortion of the atomic shell. They predict that the Li2 A ‘2: lifetime increases monotonically from 17.3 ns to 18.4 us as u increases from 7 to 17*-While our measurements confirm the theoretical magnitude of 70, our experimental uncertainties make it impossible to observe the small predicted u dependence. It is worth noting that a much larger u dependence predictcd for Na, A ‘2: was not observed by Ducas et al. [5]. Although the agreement of our results with the calculations of Tango and Zare is gratifying, its significance is limited by the fact that their method cannot be extended to molecules other than the alkali dimers. Olson and Konowalow [lo] have carried out computationally more involved, but more generally applicable, multiconfigurational self-consistent field (MC SCF) * The theoretical predictions for A-state lifetimes reported by Tango and Zare must be divided by two, as noted in ref. [5].
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
1 February 1977
Table 1 Experimental parameters and states observed in Liz A ‘Xi lifetime measurements Excitation wavelength (A)
Observation wavelength (A)
Laser dye
u’. J’ levels observed u’
J 8 9 13
5847
rhodamine 6G in ethanol + HFP b,
5956
20 20 20
5949
rhodamine 6G in ethanol
6066
14 24 16 30 18 34 19 18 21 46 22 6 22 IL 22 34 23 25 23 30 24 49 unidentified
0.04 0.07 0.06 0.33 0.04 0.08 0.10 0.05 0.03 0.09 0.04 0.07
6046
rhodamine 6G in ethanol
6170
23 24 14 12 L6 19 22 a unidentified
0.32 0.22 0.26 0.16 0.04
6315
rhodamine B in acidic ethanol
6458
9 5 14 16 unidentifZed
0.79 0.12 0.09
6513
rhodamine 6G + cresyl violet in ethanol
6669
7 7 9
8 15 29
0.72 O.LS 0.L3
0.12 Ct.76 0.0
a) f= fraction of observed fluorescence emanating from u’, J’. b, HFP = 1,1,1,3,3,3,-hexafluoro-‘l-propanol. Table 2 Summary of results: collision free radiative lifetimes (70) and cross sections (UQ) for quenching of Li:! A ’ Z: observed fiuorescence by lithium atoms. Reported uncertainties are 20 Excitation wavelength (A)
70 (ns)
5847 5949 6046 6315 6513
17.7 17.5 17.7 17.6 19.3
f t f f t
0~ (A*>
2.1 2.0 1.5 1.5 2.3
254 f 45 406 + 59 313237 185 zt 32 369 * 55
calculations to obtain accurate potential energy curves for the A and X states of Liz. Calculation of a reasonably accurate electronic transition moment from their
MC SCF wavefunctions should be straightforward [1 1J_ Lifetimes couid then be calculated and compared with our results. Of interest in this regard is that Stevens et al. [12] have calculated the Na, A-X transition moment from MC SCF wavefunctions and, using experimental potential energy curves, obtained radiative lifetimes which are in good agreement with experiment PI. The most striking feature of the quenching cross’ sections is that they are very large. Although a factor of two variance in quenching rates is observed, there is no apparent trend as a function of either vibrationa or rotational quantum number. Such effects would be somewhat masked in our experiments because we observed the decay of several u’, J’ levels simultaneously. None the less, the work reported here lays the ground511
Volume 45,
number 3
CHEMICAL PHYSICS LETTERS
work for more detailed photochemical studies of such simple systams as Li2H and Li3. Because the reactive triatomic system Li2H, with relatively few electrons, is computationally tractable, its dynamics are currently receiving much attention, both experimental [6,‘7]? and theoretical [14,151. This work was supported by the Robert A. Welch Foundation under Grant AT-562 and by the Faculty Research Fund at the University of Texas at Dallas. We would like to thank Professor P. Kusch for helpful discussions concerning spectroscopic assignments and Mr. R. Rhoden for technical assistance. .I.
1 Transbtionally hot H atom + Liz crossed molecular experiments are in progress [ 13 j.
beam
References [ 11 G. York and A. Gallagher, JlLA report # 114, University of Colorado (1974).
512
1 February
1977
[2] M. McClintock, W. DemtrZider and R.N. Zarc, J. Chem. Phys. 51(1969) 5509. [3] G. Baumgartner. W. Demtrcder and M. Stock, Z. Physik 232 (1970) 462. (41 W.J. Tango and R.N. Zarc. J. Chem. Phys. 53 (1970) 3094. 151 T.W. Ducas, M-G. Littman, M.L. Zimmerman and D. Kleppner, J. Cbem. Phys 65 (1976) 842. (61 P.H. Wine and L.A. hfelton, J. Chem. Phys. 64 (1976) 2692. (71 C.R. Vidal and J. Cooper, J. Appl. Phys. 40 (1969) 3370. [*I D.R. Stull and H. Prophet, JANAr Thcrmocbemical Tables, 2nd Ed., NSRDS-NBS 37 (U.S. Govt. Printing Office, Washington, 1971). D.K. Hsu, dissertation, Fordham University (1974). M.L. Olson and D.D. Konowalow, Thirty First Symposium on Molecular Spectroscopy, paper :: MNS, Columbus, Ohio (1976). D.D. Konowalow, private communication. W.J. Stevens, P.J. Bertoncini and A.C. Wahl, Thirty First Symposium on Molecular Spectroscopy, paper :: MN7, Columbus, Ohio (1976). WC. Stwalley, private communication. W.B. England, N-H. Sabelli and A.C. Wahl, J. Chem. Phys. 63 (1975) 4596. W.3. England, N.H. Sabelli, A.C. Wahl and A. Karo, to be published.
Volume 45, number 3
1 February
1977
RADIATIVE LIFETIME OF Liz A ‘2: Paul H. WINE” and L.A. MELTON Chemistry Department, The University of Texas at Dallas, Richardson, Texas 75080, USA Recclved 23 September
1976
Collision free liietimes and total quenching cross sections have been measured for Liz A ‘C”, as a function of the degree of vibrational excitation. The lifetimes are mdependent of u and average 18.0 t 1.9 ns. The quenching cross sections range from 185 A’ to 406 A’.
Because of their potential as laser systems [l] and their rather unique electronic properties, the Iowlying excited states of the alkali dimers have been of interest to both theorists and experimental&s for some time. The inherent simplicity which results from the hydrogen-like character of these molecules makes them a favorite “proving ground” for ab initio calculaticn techniques. Accurate calculations of excited state properties has made it possible to obtain electronic transition moments, and hence, radiative lifetimes. Accurate experimental data is needed to test the validity of such calculations. Experimental lifetimes have been reported for Na, B ‘II, [2,3], K, B ‘II, [3,4], and Na, A ‘Xi IS]. We now report experimental values for collision free radiative lifetimes and total quenching rates of Li, A ‘2: as a function of the degree of vibrational and rotational excitation. We are particularly interested in Liz A ‘ZZt as a possible product of the very efficient quenching of LiH A ‘IZ+ by Li ?S [6J f. U it ium dimers were prepared in a heat pipe oven 171 by heating metallic lithium in the presence of helium buffer gas (helium pressure O-1-0.65 torr) until the temperature was reached at which the vapor pressure of lithium in the center of the heat pipe was equal to the helium pressure in the water cooled outer * Robert A. Welch postdoctoral fellow. ’ Recent work in our faboratory suggests that onIy a smaU fraction of the LIH A lX+v, J collisional loss rate is ae counted for by transitions to other v, J levels of the same electronic state.
regions. A capacitance manometer measured PHe, while a chromel-alumel thermocouple inserted into the center of the heat pipe measured the temperature of the lithium vapor. Proper heat pipe operation was confirmed by comparing PLi f P,b (obtained from vapour pressure data [8] ) with pHz_ All experiments were performed at temperatures in the range of9001000 K. The lithium vapor composition in this temperature region is 2-3% dimer and 97-98% monomer [9] ; hence, the observed decrease in lifetime with increasing pressure almost certainly results from coliisions with atomic lithium. Excitation to the A 1X: state was achieved with a nitrogen laser pumped tunable dye laser. Fluorescence was observed at right angles to the laser beam by a detection system consisting of a SPEX 1 m monochromator, a Hamamatsu R446 photomultiplier tube, and a PAR model 1621164 boxcar integrator. The boxcar gate width was set at 5 ns and scanned in time, allowing the time profile of both the laser and the fluorescence to be recorded_ The lifetime was then obtained by least squares fitting the fluorescence profiIe to a convolution of the laser profile with a single exponential decay. Cahbration of the time base was achieved by scanning the boxcar gate through the output from a crystal controlled 50 MHz oscillator. Typicat lifetime data is displayed in fig. I_ In fig. 2, all of the measured lifetimes are presented in Stern-VoImer type plots. The straightforward procedure used to extract collision free lifetimes and quenching cross sections from the 1/r versus pressure data has been discussed ekewhere 161. 509
CHEMICAL PHYSICS LE-lTERS
Volume 45, number 3
Fig. 1. Typical lifetime data; excitation pressure 0.400 torr.
wavelength:
6046 A.
-Jf&&zr i
.l
.2
3
p;-,,,
i
.6
.i
Fig. 2. Determination of TO by extrapolation of l/r versus pressure data. o experimental data, linear least squares fits. Excitation wavelength: (A) 5847 A, (B) 5949 A, (c) 6046 A, (D) 6315 A, (E) 6513 A. Error bars are 20.
Because the ground state vibrational frequency of Li2 is only ~350 cm-l, absorption from high u” levels is observed. This results in a very dense absorption spectrum. With our laser bandwidth of eO.5 A, several 510
1 February
1977
u’, J’ levels were excited simultaneously. Attempts to isolate single fluorescence lines were frustrated due to diminishing intensity. Therefore, all lifetime measurements were carried out with 24 A spectral resolution. The monochromator served to filter out laser scattered light and provided some additional state selection, but the observed fluorescence always emanated from more than a single u’, J’ level. Resolved fluorescence spectra were obtained at each excitation wavelength and assigzments were made based on spectroscopic constants Teported by Hsu [9]. Concurrent agreement of line positions, P-R doublet splittings, and splittings of equivalent lines in successive bands (which serves to identify u”) was taken as proof that the assignments are correct. The relative contribution of each u’, J’ to the fluorescence observed in each lifetime measurement was computed assuming a triangular slit function and is tabulated with some important experimental parameters in table 1. Collision free lifetimes (TV) and quenching cross sections (uQ) are tabulated in table 2. Within experimental error, the lifetimes are independent of vibrational-rotational quantum number and yield an average value of 18.0 i 1.9 11s.Tango and Zare [4] have calculated alkali dimer lifetimes from a simple model based on the following properties of this group of molecules: (1) A-X and B-X transitions remain strong even as the internuclear separation approaches infinity and (2) binding results almost entirely from a sharing of valence electrons with no distortion of the atomic shell. They predict that the Li2 A ‘2: lifetime increases monotonically from 17.3 ns to 18.4 us as u increases from 7 to 17*-While our measurements confirm the theoretical magnitude of 70, our experimental uncertainties make it impossible to observe the small predicted u dependence. It is worth noting that a much larger u dependence predictcd for Na, A ‘2: was not observed by Ducas et al. [5]. Although the agreement of our results with the calculations of Tango and Zare is gratifying, its significance is limited by the fact that their method cannot be extended to molecules other than the alkali dimers. Olson and Konowalow [lo] have carried out computationally more involved, but more generally applicable, multiconfigurational self-consistent field (MC SCF) * The theoretical predictions for A-state lifetimes reported by Tango and Zare must be divided by two, as noted in ref. [5].
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
1 February 1977
Table 1 Experimental parameters and states observed in Liz A ‘Xi lifetime measurements Excitation wavelength (A)
Observation wavelength (A)
Laser dye
u’. J’ levels observed u’
J 8 9 13
5847
rhodamine 6G in ethanol + HFP b,
5956
20 20 20
5949
rhodamine 6G in ethanol
6066
14 24 16 30 18 34 19 18 21 46 22 6 22 IL 22 34 23 25 23 30 24 49 unidentified
0.04 0.07 0.06 0.33 0.04 0.08 0.10 0.05 0.03 0.09 0.04 0.07
6046
rhodamine 6G in ethanol
6170
23 24 14 12 L6 19 22 a unidentified
0.32 0.22 0.26 0.16 0.04
6315
rhodamine B in acidic ethanol
6458
9 5 14 16 unidentifZed
0.79 0.12 0.09
6513
rhodamine 6G + cresyl violet in ethanol
6669
7 7 9
8 15 29
0.72 O.LS 0.L3
0.12 Ct.76 0.0
a) f= fraction of observed fluorescence emanating from u’, J’. b, HFP = 1,1,1,3,3,3,-hexafluoro-‘l-propanol. Table 2 Summary of results: collision free radiative lifetimes (70) and cross sections (UQ) for quenching of Li:! A ’ Z: observed fiuorescence by lithium atoms. Reported uncertainties are 20 Excitation wavelength (A)
70 (ns)
5847 5949 6046 6315 6513
17.7 17.5 17.7 17.6 19.3
f t f f t
0~ (A*>
2.1 2.0 1.5 1.5 2.3
254 f 45 406 + 59 313237 185 zt 32 369 * 55
calculations to obtain accurate potential energy curves for the A and X states of Liz. Calculation of a reasonably accurate electronic transition moment from their
MC SCF wavefunctions should be straightforward [1 1J_ Lifetimes couid then be calculated and compared with our results. Of interest in this regard is that Stevens et al. [12] have calculated the Na, A-X transition moment from MC SCF wavefunctions and, using experimental potential energy curves, obtained radiative lifetimes which are in good agreement with experiment PI. The most striking feature of the quenching cross’ sections is that they are very large. Although a factor of two variance in quenching rates is observed, there is no apparent trend as a function of either vibrationa or rotational quantum number. Such effects would be somewhat masked in our experiments because we observed the decay of several u’, J’ levels simultaneously. None the less, the work reported here lays the ground511
Volume 45,
number 3
CHEMICAL PHYSICS LETTERS
work for more detailed photochemical studies of such simple systams as Li2H and Li3. Because the reactive triatomic system Li2H, with relatively few electrons, is computationally tractable, its dynamics are currently receiving much attention, both experimental [6,‘7]? and theoretical [14,151. This work was supported by the Robert A. Welch Foundation under Grant AT-562 and by the Faculty Research Fund at the University of Texas at Dallas. We would like to thank Professor P. Kusch for helpful discussions concerning spectroscopic assignments and Mr. R. Rhoden for technical assistance. .I.
1 Transbtionally hot H atom + Liz crossed molecular experiments are in progress [ 13 j.
beam
References [ 11 G. York and A. Gallagher, JlLA report # 114, University of Colorado (1974).
512
1 February
1977
[2] M. McClintock, W. DemtrZider and R.N. Zarc, J. Chem. Phys. 51(1969) 5509. [3] G. Baumgartner. W. Demtrcder and M. Stock, Z. Physik 232 (1970) 462. (41 W.J. Tango and R.N. Zarc. J. Chem. Phys. 53 (1970) 3094. 151 T.W. Ducas, M-G. Littman, M.L. Zimmerman and D. Kleppner, J. Cbem. Phys 65 (1976) 842. (61 P.H. Wine and L.A. hfelton, J. Chem. Phys. 64 (1976) 2692. (71 C.R. Vidal and J. Cooper, J. Appl. Phys. 40 (1969) 3370. [*I D.R. Stull and H. Prophet, JANAr Thcrmocbemical Tables, 2nd Ed., NSRDS-NBS 37 (U.S. Govt. Printing Office, Washington, 1971). D.K. Hsu, dissertation, Fordham University (1974). M.L. Olson and D.D. Konowalow, Thirty First Symposium on Molecular Spectroscopy, paper :: MNS, Columbus, Ohio (1976). D.D. Konowalow, private communication. W.J. Stevens, P.J. Bertoncini and A.C. Wahl, Thirty First Symposium on Molecular Spectroscopy, paper :: MN7, Columbus, Ohio (1976). WC. Stwalley, private communication. W.B. England, N-H. Sabelli and A.C. Wahl, J. Chem. Phys. 63 (1975) 4596. W.3. England, N.H. Sabelli, A.C. Wahl and A. Karo, to be published.