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Production and laser-induced fluorescence spectrum of aluminum sulfide
Volume 204, number 5,6
CHEMICALPHYSICSLETTERS
26 March 1993
Production and laser-induced fluorescence spectrum of aluminum sulfide Min He, Hongxin Wang and Brad R. Weiner Department of Chemislry and Chemical Physics Program, UniversityofPuerto Rico, Box 23346, UPR Station, 00931 Rio Piedras, Puerto Rico
Received 8 October 1992;in final form 20 December 1992
The first laser-iiduced fluorescenceexcitation spectrum of the AIS radical has been observed in the wavelengthregion of 418430 nm follawing excimer laser ablation of aluminum metal in the presenceof carbonyl sulfide. The spectrum has been assigned to the ( 1,O) and (0,O) bands of the A2Z+-X*E+transition by using existing spectroscopicconstants to simulate the observed rovibronic structure. The zero-pressurefluorescencelifetimes of AlS(A 2Z+) were determined to be 880f 366 ns and 433f 150 ns for u’=O and v’= 1, respectively.
1. Introdoction Gas-phase metal sulfides have been predicted to be in abundance in stellar environments [ 11, and may also play an important role in homogeneous catalysis [ 2 ] and in the production of semiconductor materials by molecular beam epitaxy and chemical vapor deposition [ 31. Information about the structure and reactivity of these types of species may be useful in gaining a more thorough understanding of the chemistry of these complex systems. This type of information has been difftcult to obtain, primarily because of the difficulty in generating gas-phase metal sulfides cleanly. McKinney and Innes reported the first observation of the gas-phase aluminum sulfide radical, AlS, by studying the emission spectrum of the A-X system [ 41. Absorption spectra of this transition, as well as the B-X and the C-X, have been measured by several other groups [ 5-71. Recently, the first pure rotational spectrum of the aluminum monosulfide radical was reported by Takano et al., and yielded precise molecular constants of the ground state species [ 8 1. To our knowledge, no laser spectroscopic studies of the radical exist. Due to the unique spatial and temporal advantages of lasers, particularly in their application to the complex chemical environElsevierSciencePublishersB.V.
ments described above, a laser spectroscopic probe of AlS is desired. We report here the first laser-induced fluorescence (LIF) spectrum of the AlS radical on the ( 1,O) and (0,O) bands of the A *x+-X ‘C+ transition. The radical was produced for these studies by pulsed laser vaporization of an aluminum disk to generate Al atoms in the presence of carbonyl sulfide. The subsequent gas-phase chemical reaction of the metal atom with the sulfide source is believed to be the principal source of the aluminum monosulfide radical.
2. Experimental The experimental apparatus used for these studies has been described in detail [ 9 ] . Briefly, AlS was produced by the reaction of aluminum atoms with carbonyl sulfide in a vacuum chamber. Al atoms were generated by laser ablation of a 1 inch diameter aluminum disk, which has been machined flat and is mounted on the shaft of a rotary motion feedthrougb with an adjustable nylon coupler. The disk is slowly rotated (Z 5 rpm) to avoid drilling a hole. An extimer laser (Lambda Physik, LPX 205i), operating at 248 nm (KrF transition z 100 mJ/cmZ prior to focusing) is focused to a point ( l-4 mm2 spot area) onto the aluminum surface to initiate the ablation. 563
Volume 204, number 5,6
26 March 1993
CHEMICAL PHYSICS LETTERS
The copious production of Al atoms has been confirmed by resonance fluorescence [ IO]. AIS is produced when the laser vaporization is carried out in the presence of OCS (0. I-5.0 Torr). Fluorescence of the aluminum monosulfide is induced by a Lambda Physik FL 3002 dye laser (bandwidth x0.25 cm-‘) pumped by a XeCl excimer laser (Lambda Physik LPX 205i; pulsewidth ~20 ns) operating at 308 nm. LIF excitation spectra are recorded in the region of 418-430 nm by using a Stilbene 3 dye (Exciton, Inc.). The dye laser beam (fluence is l-5 mJ/cm*) is directed down the length of the cell and passes within 2-3 mm of the rotating flat, perpendicularly intersecting the focused vaporization laser. The fluorescence is observed with a photomultiplier tube (Hamamatsu R329-02) at 90” relative to both the probe and the vaporization lasers. A series of cutoff filters are placed in front of the photomultiplier tube to minimize the scattered light of the vaporization laser. The emission is also passed through a bandpass filter centered on the (A-+X, Av= - 1) transition of AIS at z 440 nm, to reduce dye laser scatter. For the spectroscopic data, the output of the phdtomultiplier tube is signal averaged with a gated integrator/microcomputer system. The dye laser is stepped across the wavelength region at intervals of z 0.1 cm-‘. Fluorescence lifetime measurements are obtained by sending the output of the photomultiplier to a 350 MHz digital storage oscilloscope (LeCroy model 9420). For these experiments, the temporal resolution of the oscilloscope is IOns. 200 transients are averaged in the oscilloscope, and then transferred to the microcomputer for data analysis and storage. Logarithms of the decays are taken and fit by a linear least-squares routine. The dye laser system can be fired at variable delay times relative to the vaporization laser by using a digital delay generator ( Stanford Research Systems DG5 35 ) .
3. Results and discussion
E
z c) T LI:
5
0
428.0
564
429.5
(nm)
Fig. 1. (a) LIF excitation spectrum of the (0,O) band of the A%+-X %+ electronic system of AIS obtained by 248 nm ablation of an aluminum disk in I Torr of OCS, and 5 Torr of He. The probe laser is tired 10 p following the ablation laser. (b) Simulated spectrum of the (0,O) band of the A 2Z+-X ‘Z+ spectrum using the constants of refs. [4,8]. The spectrum was simulated assuming a temperature of 300 K.
._*_._.-.-._.
P**(R)
. . . . . _ ._.... _..___
418.5
___I
-
___.
-._-
418.0 Wavelength
Ablation of aluminum metal in the presence of carbonyl sulfide leads to laser-induced fluorescence excitation spectra in the regions of 418-422 nm and 427-430 nm. Typical experimental spectra amshown in figs. 1 and 2. These excitation spectra were obtained with 1 Torr of OCS and 5 Torr of He at a de-
429.0
428.5 Wavalength
-.--.P*,(“)
__.I__.
419.5
420.0
(nm)
Fig. 2. LIF excitation spectrum of the (1,O) band of the A ‘IFX 2C+ electronic system of AIS obtaiued by 248 nm ablation of an aluminum disk in 1 Torr of OCS, and 5 Torr of He. The probe laser is fired 10 ps following the iblation laser. (b) Simulated spectrum of the ( 1,O) band of the A 22+-X 2Z+ spectrum using the constants of refs. [ 4,8]. The spectrum was simulated assuming a temperature of 300 K.
Volume204, number$6
CHEMICAL PHYSICS LETTERS
lay of 10 ps between the ablation and probe lasers. We have assigned these spectra to Au= + 1 and Au=0 bands originating out of the vibrational ground state of AIS( A 21+-X *Z+) transitions. Rotational structures within these vibrational bands have also been assigned (see figs. 1 and 2 ) . Assignments were made by calculating the transition frequencies based on the constants of iefs. [ 4,8]. The agreement between calculated rotational line positions and the experimental positions is within 1 cm-‘. The spectra were calibrated by adjusting the bandheads to the literature values. Due to the spectral resolution ( x 0.25 cm-’ ) limitations of our dye laser, the rotational structure of the vibrational bands could not be completely resolved. Thus, we have generated a spectral simulation of the experimental data (see figs. 1 and 2). Fre quencies were calculated from the literature value constants and the relative spectral intensities were derived from the standard equations [ 111. HonlLondon factors for a ‘&*I; system were obtained using the relationships of Mulliken [ 12 ] . A Boltzmann distribution of rotational states was assumed with a temperature of 300 K. A Gaussian profile with a laser-limited bandwidth of 0.25 cm-’ was superimposed on the stick spectrum. As can be seen in figs. 1 and 2, a strong resemblance between the experimental and simulated spectra exists, but several features merit discussion. The spectra displayed in figs. 1 and 2 (both experimental and stimulated) appear to have different temperature profilesThe 1,O band (fig. 2) seems to be “hotter” due to a more fortuitous overlap of the P and R branches. The lesser overlap of these branches is observed in the 0,O band (fig. 1), and causes the P branch transitions to appear to have “shoulders”. These overlap features may also contribute to the P-branch irregularities seen in the experimental spectra. Shot-to-shot fluctuations in the ablation process, observed to be about lOI20% in previous studies [ lo], also contribute to irregular intensities. A better understanding of these rotational features can be gained by using a higher resolution probe laser. No signal is detected in the absence of carbonyl sulfide in the cell, even for the case where OCS has remained in the cell for a period of time, and is subsequently pumped out. This result is suggestive of a gas-phase production mechanism’for AlS, rather than
26 March 1993
aluminum sulfide formed on the surface and then ablated. In the region that we observed LIF excitation spectra (i.e. 418-430 nm), several other A-X vibronic bands for Au= + 1 and Au=0 (but not originating out of the ground vibrational state) exist. We could not observe any of these hot bands, even at lower pressures and shorter delay times. Under similar conditions, hotter rotational distributions were observed. This seems to indicate that the AlS formed in the Al t OCS reaction is vibrationally cold and rotationally hot, suggesting some sort of bent transition state. These mechanisms need to be studied in greater experimental detail however, because of the possible presence of higher order metal clusters, both neutral and ionic, that may be contributing to the production of AlS. One of the advantages of having a pulsed source of AIS is that it now allows us to measure some of the dynamic properties of the radical. We have measured the fluorescence lifetime of AlS (A ‘Z+, u’ =O,l ). This was done by measuring the radiative decay of the fluorescence signal as a function of added OCS (no added buffer gas), fogowing excitation at the bandhead of each of the respective bands. The decays were fit over 2-3 radiative lifetimes with uncertainties of less than 10%. The data were then treated by a standard Stern-Volmer analysis, yielding radiative lifetimes of 880f 366 ns and 433? 150 ns (errors are 20) for v’ = 0 and v’ = 1, respectively. No additional quenching was observed for added helium, argon or nitrogen up to 2 Torr. A small amount of fluorescence quenching of AlO(B ‘C+ ) has been reported previously for He, Ar and N,, possibly due to interactions with an overlapping electronic state, e.g. A10 (A ‘l7 ) [ 9 1. No such state has been reported in this region for aluminum sulfide, and may explain the lack of quenching. A more recent measurement of the fluorescence quenching of the B state of Al0 claims that no quenching occurs with helium and nitrogen [ 131.
4. Summary We have demonstrated here a proficient method for the production of AlS, by using pulsed laser vaporization followed by chemical reaction. The eficient production method has allowed us to measure
Volume 204, number $6
CHEMICAL PHYSICS LETI’ERS
and report the first LIF excitation spectra of this radical species. The combination of these two techniques, pulsed laser vaporization and laser-induced fluorescence now permits the study of the chemical reactivity and other dynamical properties of this important radical. Investigation of these properties, as well as higher resolution laser-induced fluorescence spectroscopic studies of the A-X, B-X and C-X transitions, are in progress in our laboratory.
Acknowledgement The experiments performed here were done in conjunction with the Puerto Rico Laser and Spectroscopy Facility and the UPR Materials Research Center, both sponsored by NSF-EPSCoR. We would also like to kindly acknowledge the partial support of the Ofice of Naval Research.
References [ 1] T. Tsuji, Astron. Astrophys. 23 ( 1973 ) 441.
566
26 March 1993
[ 21 E.L. Muetterties and M.J. Krause, Angew. Chem. 95 (1983) 135. [ 31 N.G. Pate1 and A.G. Fischer, Thin Solid Films 162 ( 1988) 263. [ 41 C.N. McKinney and K.K. Innes, J. Mol. Spectry. 3 ( 1959) 235. [ 51 A.A. Maltscv, V.F. Shevelkov and E.D. Krupnikov, Opt. Spectry Suppl. 2 ( 1966) 4. [ 61 M. Kronekvist and A. Lagerqvist, Arkiv. Fysik 39 ( 1969) 133. [ 71 H. Iavendy, J.M. Mahieu and M. Becart, Can. J. Spectry. 18 (1973) 13; H. Lavendy and D. Jacquinot, Can. J. Spectry. 20 (1975) 141; H.Lavendy,J.Phys.B 13 (1980) 1151. [ 81 S. Takano, S. Yamamoto and S. Saito, J. Chem. Phys. 94 (1991) 3355. [ 91A.P. Salzberg, D.I. Santiago, F. Asmac,D.N. Sandoval and B.R. Weiner, Chem. Phys. Letters 180 ( 1991) 161. [ lo] H. Wang, A.P. Sal&erg and B.R. Weiner, Appl. Phys. Letters 59 (1991) 935. [ 111 G. Henberg, Molecular spectra and molecular structure, Vol. 1. Spectraofdiatomic molecules (Krieger, New York, 1989) p. 204. [ 121R.S. Mulliken, Phys. Rev. 30 (1927) 138. [ 131 M.L. Campbell, R.E. McLean, N.L. Garland and H.H. Nelson, Chem. Phys. Letters 194 ( 1992) 187.
CHEMICALPHYSICSLETTERS
26 March 1993
Production and laser-induced fluorescence spectrum of aluminum sulfide Min He, Hongxin Wang and Brad R. Weiner Department of Chemislry and Chemical Physics Program, UniversityofPuerto Rico, Box 23346, UPR Station, 00931 Rio Piedras, Puerto Rico
Received 8 October 1992;in final form 20 December 1992
The first laser-iiduced fluorescenceexcitation spectrum of the AIS radical has been observed in the wavelengthregion of 418430 nm follawing excimer laser ablation of aluminum metal in the presenceof carbonyl sulfide. The spectrum has been assigned to the ( 1,O) and (0,O) bands of the A2Z+-X*E+transition by using existing spectroscopicconstants to simulate the observed rovibronic structure. The zero-pressurefluorescencelifetimes of AlS(A 2Z+) were determined to be 880f 366 ns and 433f 150 ns for u’=O and v’= 1, respectively.
1. Introdoction Gas-phase metal sulfides have been predicted to be in abundance in stellar environments [ 11, and may also play an important role in homogeneous catalysis [ 2 ] and in the production of semiconductor materials by molecular beam epitaxy and chemical vapor deposition [ 31. Information about the structure and reactivity of these types of species may be useful in gaining a more thorough understanding of the chemistry of these complex systems. This type of information has been difftcult to obtain, primarily because of the difficulty in generating gas-phase metal sulfides cleanly. McKinney and Innes reported the first observation of the gas-phase aluminum sulfide radical, AlS, by studying the emission spectrum of the A-X system [ 41. Absorption spectra of this transition, as well as the B-X and the C-X, have been measured by several other groups [ 5-71. Recently, the first pure rotational spectrum of the aluminum monosulfide radical was reported by Takano et al., and yielded precise molecular constants of the ground state species [ 8 1. To our knowledge, no laser spectroscopic studies of the radical exist. Due to the unique spatial and temporal advantages of lasers, particularly in their application to the complex chemical environElsevierSciencePublishersB.V.
ments described above, a laser spectroscopic probe of AlS is desired. We report here the first laser-induced fluorescence (LIF) spectrum of the AlS radical on the ( 1,O) and (0,O) bands of the A *x+-X ‘C+ transition. The radical was produced for these studies by pulsed laser vaporization of an aluminum disk to generate Al atoms in the presence of carbonyl sulfide. The subsequent gas-phase chemical reaction of the metal atom with the sulfide source is believed to be the principal source of the aluminum monosulfide radical.
2. Experimental The experimental apparatus used for these studies has been described in detail [ 9 ] . Briefly, AlS was produced by the reaction of aluminum atoms with carbonyl sulfide in a vacuum chamber. Al atoms were generated by laser ablation of a 1 inch diameter aluminum disk, which has been machined flat and is mounted on the shaft of a rotary motion feedthrougb with an adjustable nylon coupler. The disk is slowly rotated (Z 5 rpm) to avoid drilling a hole. An extimer laser (Lambda Physik, LPX 205i), operating at 248 nm (KrF transition z 100 mJ/cmZ prior to focusing) is focused to a point ( l-4 mm2 spot area) onto the aluminum surface to initiate the ablation. 563
Volume 204, number 5,6
26 March 1993
CHEMICAL PHYSICS LETTERS
The copious production of Al atoms has been confirmed by resonance fluorescence [ IO]. AIS is produced when the laser vaporization is carried out in the presence of OCS (0. I-5.0 Torr). Fluorescence of the aluminum monosulfide is induced by a Lambda Physik FL 3002 dye laser (bandwidth x0.25 cm-‘) pumped by a XeCl excimer laser (Lambda Physik LPX 205i; pulsewidth ~20 ns) operating at 308 nm. LIF excitation spectra are recorded in the region of 418-430 nm by using a Stilbene 3 dye (Exciton, Inc.). The dye laser beam (fluence is l-5 mJ/cm*) is directed down the length of the cell and passes within 2-3 mm of the rotating flat, perpendicularly intersecting the focused vaporization laser. The fluorescence is observed with a photomultiplier tube (Hamamatsu R329-02) at 90” relative to both the probe and the vaporization lasers. A series of cutoff filters are placed in front of the photomultiplier tube to minimize the scattered light of the vaporization laser. The emission is also passed through a bandpass filter centered on the (A-+X, Av= - 1) transition of AIS at z 440 nm, to reduce dye laser scatter. For the spectroscopic data, the output of the phdtomultiplier tube is signal averaged with a gated integrator/microcomputer system. The dye laser is stepped across the wavelength region at intervals of z 0.1 cm-‘. Fluorescence lifetime measurements are obtained by sending the output of the photomultiplier to a 350 MHz digital storage oscilloscope (LeCroy model 9420). For these experiments, the temporal resolution of the oscilloscope is IOns. 200 transients are averaged in the oscilloscope, and then transferred to the microcomputer for data analysis and storage. Logarithms of the decays are taken and fit by a linear least-squares routine. The dye laser system can be fired at variable delay times relative to the vaporization laser by using a digital delay generator ( Stanford Research Systems DG5 35 ) .
3. Results and discussion
E
z c) T LI:
5
0
428.0
564
429.5
(nm)
Fig. 1. (a) LIF excitation spectrum of the (0,O) band of the A%+-X %+ electronic system of AIS obtained by 248 nm ablation of an aluminum disk in I Torr of OCS, and 5 Torr of He. The probe laser is tired 10 p following the ablation laser. (b) Simulated spectrum of the (0,O) band of the A 2Z+-X ‘Z+ spectrum using the constants of refs. [4,8]. The spectrum was simulated assuming a temperature of 300 K.
._*_._.-.-._.
P**(R)
. . . . . _ ._.... _..___
418.5
___I
-
___.
-._-
418.0 Wavelength
Ablation of aluminum metal in the presence of carbonyl sulfide leads to laser-induced fluorescence excitation spectra in the regions of 418-422 nm and 427-430 nm. Typical experimental spectra amshown in figs. 1 and 2. These excitation spectra were obtained with 1 Torr of OCS and 5 Torr of He at a de-
429.0
428.5 Wavalength
-.--.P*,(“)
__.I__.
419.5
420.0
(nm)
Fig. 2. LIF excitation spectrum of the (1,O) band of the A ‘IFX 2C+ electronic system of AIS obtaiued by 248 nm ablation of an aluminum disk in 1 Torr of OCS, and 5 Torr of He. The probe laser is fired 10 ps following the iblation laser. (b) Simulated spectrum of the ( 1,O) band of the A 22+-X 2Z+ spectrum using the constants of refs. [ 4,8]. The spectrum was simulated assuming a temperature of 300 K.
Volume204, number$6
CHEMICAL PHYSICS LETTERS
lay of 10 ps between the ablation and probe lasers. We have assigned these spectra to Au= + 1 and Au=0 bands originating out of the vibrational ground state of AIS( A 21+-X *Z+) transitions. Rotational structures within these vibrational bands have also been assigned (see figs. 1 and 2 ) . Assignments were made by calculating the transition frequencies based on the constants of iefs. [ 4,8]. The agreement between calculated rotational line positions and the experimental positions is within 1 cm-‘. The spectra were calibrated by adjusting the bandheads to the literature values. Due to the spectral resolution ( x 0.25 cm-’ ) limitations of our dye laser, the rotational structure of the vibrational bands could not be completely resolved. Thus, we have generated a spectral simulation of the experimental data (see figs. 1 and 2). Fre quencies were calculated from the literature value constants and the relative spectral intensities were derived from the standard equations [ 111. HonlLondon factors for a ‘&*I; system were obtained using the relationships of Mulliken [ 12 ] . A Boltzmann distribution of rotational states was assumed with a temperature of 300 K. A Gaussian profile with a laser-limited bandwidth of 0.25 cm-’ was superimposed on the stick spectrum. As can be seen in figs. 1 and 2, a strong resemblance between the experimental and simulated spectra exists, but several features merit discussion. The spectra displayed in figs. 1 and 2 (both experimental and stimulated) appear to have different temperature profilesThe 1,O band (fig. 2) seems to be “hotter” due to a more fortuitous overlap of the P and R branches. The lesser overlap of these branches is observed in the 0,O band (fig. 1), and causes the P branch transitions to appear to have “shoulders”. These overlap features may also contribute to the P-branch irregularities seen in the experimental spectra. Shot-to-shot fluctuations in the ablation process, observed to be about lOI20% in previous studies [ lo], also contribute to irregular intensities. A better understanding of these rotational features can be gained by using a higher resolution probe laser. No signal is detected in the absence of carbonyl sulfide in the cell, even for the case where OCS has remained in the cell for a period of time, and is subsequently pumped out. This result is suggestive of a gas-phase production mechanism’for AlS, rather than
26 March 1993
aluminum sulfide formed on the surface and then ablated. In the region that we observed LIF excitation spectra (i.e. 418-430 nm), several other A-X vibronic bands for Au= + 1 and Au=0 (but not originating out of the ground vibrational state) exist. We could not observe any of these hot bands, even at lower pressures and shorter delay times. Under similar conditions, hotter rotational distributions were observed. This seems to indicate that the AlS formed in the Al t OCS reaction is vibrationally cold and rotationally hot, suggesting some sort of bent transition state. These mechanisms need to be studied in greater experimental detail however, because of the possible presence of higher order metal clusters, both neutral and ionic, that may be contributing to the production of AlS. One of the advantages of having a pulsed source of AIS is that it now allows us to measure some of the dynamic properties of the radical. We have measured the fluorescence lifetime of AlS (A ‘Z+, u’ =O,l ). This was done by measuring the radiative decay of the fluorescence signal as a function of added OCS (no added buffer gas), fogowing excitation at the bandhead of each of the respective bands. The decays were fit over 2-3 radiative lifetimes with uncertainties of less than 10%. The data were then treated by a standard Stern-Volmer analysis, yielding radiative lifetimes of 880f 366 ns and 433? 150 ns (errors are 20) for v’ = 0 and v’ = 1, respectively. No additional quenching was observed for added helium, argon or nitrogen up to 2 Torr. A small amount of fluorescence quenching of AlO(B ‘C+ ) has been reported previously for He, Ar and N,, possibly due to interactions with an overlapping electronic state, e.g. A10 (A ‘l7 ) [ 9 1. No such state has been reported in this region for aluminum sulfide, and may explain the lack of quenching. A more recent measurement of the fluorescence quenching of the B state of Al0 claims that no quenching occurs with helium and nitrogen [ 131.
4. Summary We have demonstrated here a proficient method for the production of AlS, by using pulsed laser vaporization followed by chemical reaction. The eficient production method has allowed us to measure
Volume 204, number $6
CHEMICAL PHYSICS LETI’ERS
and report the first LIF excitation spectra of this radical species. The combination of these two techniques, pulsed laser vaporization and laser-induced fluorescence now permits the study of the chemical reactivity and other dynamical properties of this important radical. Investigation of these properties, as well as higher resolution laser-induced fluorescence spectroscopic studies of the A-X, B-X and C-X transitions, are in progress in our laboratory.
Acknowledgement The experiments performed here were done in conjunction with the Puerto Rico Laser and Spectroscopy Facility and the UPR Materials Research Center, both sponsored by NSF-EPSCoR. We would also like to kindly acknowledge the partial support of the Ofice of Naval Research.
References [ 1] T. Tsuji, Astron. Astrophys. 23 ( 1973 ) 441.
566
26 March 1993
[ 21 E.L. Muetterties and M.J. Krause, Angew. Chem. 95 (1983) 135. [ 31 N.G. Pate1 and A.G. Fischer, Thin Solid Films 162 ( 1988) 263. [ 41 C.N. McKinney and K.K. Innes, J. Mol. Spectry. 3 ( 1959) 235. [ 51 A.A. Maltscv, V.F. Shevelkov and E.D. Krupnikov, Opt. Spectry Suppl. 2 ( 1966) 4. [ 61 M. Kronekvist and A. Lagerqvist, Arkiv. Fysik 39 ( 1969) 133. [ 71 H. Iavendy, J.M. Mahieu and M. Becart, Can. J. Spectry. 18 (1973) 13; H. Lavendy and D. Jacquinot, Can. J. Spectry. 20 (1975) 141; H.Lavendy,J.Phys.B 13 (1980) 1151. [ 81 S. Takano, S. Yamamoto and S. Saito, J. Chem. Phys. 94 (1991) 3355. [ 91A.P. Salzberg, D.I. Santiago, F. Asmac,D.N. Sandoval and B.R. Weiner, Chem. Phys. Letters 180 ( 1991) 161. [ lo] H. Wang, A.P. Sal&erg and B.R. Weiner, Appl. Phys. Letters 59 (1991) 935. [ 111 G. Henberg, Molecular spectra and molecular structure, Vol. 1. Spectraofdiatomic molecules (Krieger, New York, 1989) p. 204. [ 121R.S. Mulliken, Phys. Rev. 30 (1927) 138. [ 131 M.L. Campbell, R.E. McLean, N.L. Garland and H.H. Nelson, Chem. Phys. Letters 194 ( 1992) 187.