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Infrared Diode Laser Spectroscopy of the ν1Fundamental and ν1+ ν5− ν5Bands of the C3S Molecule
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
178, 194–198 (1996)
0174
Infrared Diode Laser Spectroscopy of the n1 Fundamental and n1 / n5 0 n5 Bands of the C3S Molecule Shuro Takano, Jian Tang, 1 and Shuji Saito Institute for Molecular Science and The Graduate University for Advanced Studies, Myodaiji, Okazaki 444, Japan Received September 18, 1995; in revised form April 2, 1996
The vibration–rotation absorption spectra of the C3S molecule were studied using a tunable infrared diode laser spectrometer. The n1 fundamental and n1 / n5 0 n5 hot bands were measured between 2046 and 2067 cm01 . The C3S molecule was produced by a glow discharge in a flowing mixture of CS2 , C2H2 , and He. The analysis of the measured spectral lines yielded the band origins, the rotational and centrifugal distortion constants in the upper vibrational states, and the l-type doubling constant for the n1 / n5 state. The band origins were determined to be 2058.21875(47) and 2053.03769(45) cm01 for the n1 fundamental and n1 / n5 0 n5 bands, respectively. The values in the parentheses correspond to three standard deviations. q 1996 Academic Press, Inc.
laser spectrometer. In addition, we observed a hot band, n1 / n5 0 n5 , in nearly the same wavenumber region.
INTRODUCTION
The C3S molecule, a transient species in the laboratory, has been known to be one of the fundamental interstellar carbon-chain molecules. The rotational spectral lines of this molecule were first observed with the 45 m radio telescope at the Nobeyama Radio Observatory as unidentified lines in the 20–50 GHz region in a dark cloud TMC-1 (1), and the carrier of these lines was definitely identified as C3S by microwave spectroscopy in the laboratory (2). Subsequently, the four isotopic species of C3S were studied by Fourier-transform microwave spectroscopy, and its rs structure was determined (3). In addition, the dipole moment of C3S was determined to be 3.704(9) D (4). Recently Tang and Saito (5) studied the rotational spectra of C3S in the £5 Å 1 to 4 and £4 Å 1 bending vibrational states. The infrared spectroscopy of C3S is limited to a matrix isolation study. Maier et al. (6) reported the infrared spectrum of C3S isolated in Ar matrix. They reported the band origins and their relative intensity. According to their results the intensity of the n1 stretching band is much (more than 10 times) stronger than those of other bands, and the band origin of the n1 band is 2046.2 cm01 in Ar matrix. Several ab initio calculations have been reported for C3S (7–10). Recently Seeger et al. (10) carried out extensive calculations reporting the band origin of the n1 band to be 2071.0 cm01 by the CCSD(T) method. Since gaseous free carbon-chain molecules have rarely been studied by high-resolution infrared spectroscopy, we measured the vibration–rotation absorption spectra of C3S in the n1 fundamental band with a tunable infrared diode 1
Research fellow of the Japan Society for the Promotion of Science.
EXPERIMENTAL
The experiments were performed using a tunable infrared diode laser spectrometer (Analytics Division, Laser Photonics) at the Institute for Molecular Science (11). The absorption cell used is a White-type multireflection one of 2 m long and 10 cm in diameter. The path length used was about 40 m. The C3S molecule was produced by a glow discharge in a flowing mixture of CS2 (45 mTorr), C2H2 (55 mTorr), and He (10 mTorr). The temperature of the cell was maintained at 10–207C. We tried much lower temperature, but there was no drastic change for the production of C3S until 01007C: the spectrum became weaker below 01007C, perhaps due to the freezing of CS2 on the wall of the cell. Initially, the discharge modulation method with a half cycle alternating current (AC) of 160 mA was employed for the phase-sensitive detection of C3S. The signal-to-noise ratio (S/N) was best at the modulation frequency of 2.0 kHz, but the absorption signal itself was strongest at 1.5 kHz. This result indicates that the life time of C3S is the order of 10 03 s in our experimental conditions. The wavenumbers of the lines were measured based on the known wavenumbers of absorption lines of OCS (reference gas) (12), and on the fringe by a Fabry–Perot type e´talon as an interpolation and extrapolation device. The frequency modulation (10 kHz) was applied to the diode laser, and the absorption signal of the reference gas and the fringe of the e´talon were phase-sensitively detected and recorded simultaneously with the spectrum of C3S. After the initial detection of the series of lines between 2060 and 2062 cm01 , we started to measure each line by averaging the wavenum-
194 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 7040
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06-26-96 15:11:52
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AP: Mol Spec
INFRARED DIODE LASER SPECTROSCOPY OF C3S
195
bers obtained in one upward scan and one downward scan. The lock-in amplifier (PSD) was used in the 1f detection mode, and the time constant employed was 1 s. During the measurement we found a problem in the accuracy of the wavenumber measurement. Namely, the diode lasers were unexpectedly frequency modulated also by the discharge frequency due to a leakage pickup of the discharge noise. This pickup distorted the line shape of the signal, and the deviation of the wavenumber was 0.003 cm01 (in maximum) which is much larger than the errors of the measurements. Therefore, the frequency modulation (10 kHz) was employed for the wavenumber measurements of C3S, which was produced in this case by a 120 mA direct current (DC) glow discharge instead of the AC discharge for the discharge modulation. FIG. 1. The vibration–rotation spectra of the n1 fundamental and n1 / n5 0 n5 hot bands of C3S. The hot band shows l-type doublet structure. The time constant of the lock-in amplifier was 300 ms. The spectra were recorded with source frequency modulation in the 2f detection mode of the lock-in amplifier.
RESULTS AND ANALYSIS
The series of lines of the n1 fundamental band was measured between 2046 and 2067 cm01 , and another series of
TABLE 1 Observed Wavenumbers for the n1 Fundamental Band of C3S (in cm01 )
Copyright q 1996 by Academic Press, Inc.
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196
TAKANO, TANG, AND SAITO
TABLE 2 Observed Wavenumbers for the n1 / n5 0 n5 Band of C3S (in cm01 )
doublet lines which have about half intensity of the n1 fundamental band was measured between 2047 and 2064 cm01 . A part of the spectrum is shown in Fig. 1. The series of doublet lines was considered to show the l-type doubling of the n5 bending states, and this series was assigned to a hot band n1 / n5 0 n5 . The harmonic vibrational wavenumber of the n5 mode was reported to be 155.2 cm01 by an ab initio calculation using the CCSD(T) method (10). Fifty-eight lines were measured for the n1 fundamental band, and 54 lines for the hot band n1 / n5 0 n5 . The measured wavenumbers were listed in Tables 1 and 2 for the n1 fundamental and hot bands, respectively. The assignment of the quantum number J of the n1 fundamental band was definitely confirmed by using combination differences employing the microwave spectroscopic data (2). But the combination differences could not be applied to the hot band, because the spectral lines in both P and R branches with a common upper state could not be measured. The assignment of J for the hot band is determined to obtain reasonable molecular constants. The standard expressions of energy levels for the 1S linear molecule in the ground and bending excited states (5, 13, 14) were used for the analyses of the n1 fundamental and
hot bands, respectively. The measured wavenumbers were least-squares analyzed to obtain the molecular constants. In the least-squares analyses, the constants for the lower states were fixed to those obtained by Tang and Saito (5). When the l-type doublets were not resolved in the low-J lines of the hot band, the l-type doubling terms were not included in the analyses. The obtained molecular constants were listed in Table 3 with those of the lower states reported by Tang and Saito (5). In the n1 / n5 state, the values for g *ll and d *ll were fixed to zero, because these constants could not be determined from our measurements. The small effect of g *ll and d *ll is, therefore, included in the B * constant. The standard deviations of the least-squares fits are 0.00045 and 0.00054 cm01 for the n1 fundamental and hot bands, respectively. These values are comparable to the experimental errors of the measurements. DISCUSSION
The obtained band origin in the n1 fundamental band (2058.21875 cm01 ) can be compared with the values obtained by the Ar matrix experiment (2046.2 cm01 ) (6) and
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7040
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06-26-96 15:11:52
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AP: Mol Spec
INFRARED DIODE LASER SPECTROSCOPY OF C3S
197
TABLE 3 Molecular Constants of C3S in the n1 and n1 / n5 0 n5 Bands (in cm01 )
by the ab initio calculation (2071.0 cm01 ) (10). The value obtained by the ab initio calculation agrees with the present value within 1%. The shift of the band origin in the Ar matrix from that in gas-phase is 12.0 cm01 . This value can be compared with the corresponding shift, 14.3 cm01 , of the n1 fundamental band of C3O (15, 16). The a1 value obtained in our study is 14.828(22) MHz. The a1 values were reported by the ab initio calculation to be 15.29 (CEPA-1) and 15.15 MHz (CCSD(T)) (10), which are in good agreement with the experimental value. The vibrational anharmonicity constant, x51 , is derived to be 05.18106(66) cm01 from the difference of the band origins between the n1 and n1 / n5 0 n5 bands. Sensitivity of the present spectrometer was estimated from the observed signals of C3S. The ratio ( DI/I) of the absorbed infrared power ( DI) and the incident total infrared power (I) was 6.6 1 10 04 for the R(22) transition in the n1 fundamental band. This line was observed with the signal-to-noise ratio (S/N) of about 5 as shown in Fig. 1. Therefore, the limit for the detection is DI/I Ç 10 04 in our system. The
detection limit expressed by the absorption coefficient ( a ) is 2.5 1 10 08 cm01 using the path length of 40 m. The number of the C3S molecule per cm3 in the cell was estimated using the formulae in references (17) and (18) under the assumptions that the temperature is 300 K and that the transition dipole moment is 0.1 D. The ground state and the bending excited states of the n4 and n5 modes were considered for the calculation of the partition function. The path length of 40 m was employed in our experiment, but the actual path length seems shorter in case of C3S, because C3S is probably produced in the central region of the cell. Thus, 20 m was assumed to be the actual path length. The calculated value is 8 1 10 10 molecules cm03 : 20 ppm of the total gas in the cell. If 200 or 400 K is assumed as rotational and vibrational temperature, the concentration of C3S changes to 3 1 10 10 or 1 1 10 11 molecules cm03 , respectively. ACKNOWLEDGMENT S. T. is grateful to Drs. Kentarou Kawaguchi, Misaki Okunishi, and Toshinori Suzuki for their kind help in the operation of the diode laser
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7040
/
6t0c$$$182
06-26-96 15:11:52
mspa
AP: Mol Spec
198
TAKANO, TANG, AND SAITO
spectrometer. The calculations in the analyses were carried out at the Computer Center of the Institute for Molecular Science.
REFERENCES 1. N. Kaifu, H. Suzuki, M. Ohishi, T. Miyaji, S. Ishikawa, T. Kasuga, M. Morimoto, and S. Saito, Astrophys. J. 317, L111–L114 (1987). 2. S. Yamamoto, S. Saito, K. Kawaguchi, N. Kaifu, H. Suzuki, and M. Ohishi, Astrophys. J. 317, L119–L121 (1987). 3. Y. Ohshima and Y. Endo, J. Mol. Spectrosc. 153, 627–634 (1992). 4. R. D. Suenram and F. J. Lovas, Astrophys. J. 429, L89–L90 (1994). 5. J. Tang and S. Saito, J. Mol. Spectrosc. 169, 92–107 (1995). 6. G. Maier, J. Schrot, H. P. Reisenauer, and R. Janoschek, Chem. Ber. 124, 2617–2622 (1991). 7. D. J. Peeso, D. W. Ewing, and T. T. Curtis, Chem. Phys. Lett. 166, 307–310 (1990). 8. A. Murakami, Astrophys. J. 357, 288–290 (1990). 9. R. G. A. R. Maclagan and P. Sudkeaw, Chem. Phys. Lett. 194, 147– 151 (1992).
10. S. Seeger, P. Botschwina, J. Flu¨gge, H. P. Reisenauer, and G. Maier, J. Mol. Struct. (THEOCHEM) 303, 213–225 (1994). 11. C. Yamada, K. Nagai, and E. Hirota, J. Mol. Spectrosc. 85, 416–426 (1981). 12. A. G. Maki and J. S. Wells, ‘‘Wavenumber Calibration Tables From Heterodyne Frequency Measurements,’’ U.S. Government Printing Office, Washington, DC, 1991. 13. G. Amat and H. H. Nielsen, J. Mol. Spectrosc. 2, 152–162 (1958). 14. K. M. T. Yamada, F. W. Birss, and M. R. Aliev, J. Mol. Spectrosc. 112, 347–356 (1985). 15. D. McNaughton, D. McGilvery, and F. Shanks, J. Mol. Spectrosc. 149, 458–473 (1991). 16. P. Botschwina and H. P. Reisenauer, Chem. Phys. Lett. 183, 217–222 (1991). 17. L. A. Pugh and K. N. Rao, in ‘‘Molecular Spectroscopy: Modern Research’’ (K. N. Rao, Ed.), Vol. II, Chap. 4, Academic Press, London, 1976. 18. D. Papousek and M. R. Aliev, ‘‘Molecular Vibrational–Rotational Spectra,’’ Elsevier, Amsterdam, 1982.
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7040
/
6t0c$$$182
06-26-96 15:11:52
mspa
AP: Mol Spec
178, 194–198 (1996)
0174
Infrared Diode Laser Spectroscopy of the n1 Fundamental and n1 / n5 0 n5 Bands of the C3S Molecule Shuro Takano, Jian Tang, 1 and Shuji Saito Institute for Molecular Science and The Graduate University for Advanced Studies, Myodaiji, Okazaki 444, Japan Received September 18, 1995; in revised form April 2, 1996
The vibration–rotation absorption spectra of the C3S molecule were studied using a tunable infrared diode laser spectrometer. The n1 fundamental and n1 / n5 0 n5 hot bands were measured between 2046 and 2067 cm01 . The C3S molecule was produced by a glow discharge in a flowing mixture of CS2 , C2H2 , and He. The analysis of the measured spectral lines yielded the band origins, the rotational and centrifugal distortion constants in the upper vibrational states, and the l-type doubling constant for the n1 / n5 state. The band origins were determined to be 2058.21875(47) and 2053.03769(45) cm01 for the n1 fundamental and n1 / n5 0 n5 bands, respectively. The values in the parentheses correspond to three standard deviations. q 1996 Academic Press, Inc.
laser spectrometer. In addition, we observed a hot band, n1 / n5 0 n5 , in nearly the same wavenumber region.
INTRODUCTION
The C3S molecule, a transient species in the laboratory, has been known to be one of the fundamental interstellar carbon-chain molecules. The rotational spectral lines of this molecule were first observed with the 45 m radio telescope at the Nobeyama Radio Observatory as unidentified lines in the 20–50 GHz region in a dark cloud TMC-1 (1), and the carrier of these lines was definitely identified as C3S by microwave spectroscopy in the laboratory (2). Subsequently, the four isotopic species of C3S were studied by Fourier-transform microwave spectroscopy, and its rs structure was determined (3). In addition, the dipole moment of C3S was determined to be 3.704(9) D (4). Recently Tang and Saito (5) studied the rotational spectra of C3S in the £5 Å 1 to 4 and £4 Å 1 bending vibrational states. The infrared spectroscopy of C3S is limited to a matrix isolation study. Maier et al. (6) reported the infrared spectrum of C3S isolated in Ar matrix. They reported the band origins and their relative intensity. According to their results the intensity of the n1 stretching band is much (more than 10 times) stronger than those of other bands, and the band origin of the n1 band is 2046.2 cm01 in Ar matrix. Several ab initio calculations have been reported for C3S (7–10). Recently Seeger et al. (10) carried out extensive calculations reporting the band origin of the n1 band to be 2071.0 cm01 by the CCSD(T) method. Since gaseous free carbon-chain molecules have rarely been studied by high-resolution infrared spectroscopy, we measured the vibration–rotation absorption spectra of C3S in the n1 fundamental band with a tunable infrared diode 1
Research fellow of the Japan Society for the Promotion of Science.
EXPERIMENTAL
The experiments were performed using a tunable infrared diode laser spectrometer (Analytics Division, Laser Photonics) at the Institute for Molecular Science (11). The absorption cell used is a White-type multireflection one of 2 m long and 10 cm in diameter. The path length used was about 40 m. The C3S molecule was produced by a glow discharge in a flowing mixture of CS2 (45 mTorr), C2H2 (55 mTorr), and He (10 mTorr). The temperature of the cell was maintained at 10–207C. We tried much lower temperature, but there was no drastic change for the production of C3S until 01007C: the spectrum became weaker below 01007C, perhaps due to the freezing of CS2 on the wall of the cell. Initially, the discharge modulation method with a half cycle alternating current (AC) of 160 mA was employed for the phase-sensitive detection of C3S. The signal-to-noise ratio (S/N) was best at the modulation frequency of 2.0 kHz, but the absorption signal itself was strongest at 1.5 kHz. This result indicates that the life time of C3S is the order of 10 03 s in our experimental conditions. The wavenumbers of the lines were measured based on the known wavenumbers of absorption lines of OCS (reference gas) (12), and on the fringe by a Fabry–Perot type e´talon as an interpolation and extrapolation device. The frequency modulation (10 kHz) was applied to the diode laser, and the absorption signal of the reference gas and the fringe of the e´talon were phase-sensitively detected and recorded simultaneously with the spectrum of C3S. After the initial detection of the series of lines between 2060 and 2062 cm01 , we started to measure each line by averaging the wavenum-
194 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 7040
/
6t0c$$$181
06-26-96 15:11:52
mspa
AP: Mol Spec
INFRARED DIODE LASER SPECTROSCOPY OF C3S
195
bers obtained in one upward scan and one downward scan. The lock-in amplifier (PSD) was used in the 1f detection mode, and the time constant employed was 1 s. During the measurement we found a problem in the accuracy of the wavenumber measurement. Namely, the diode lasers were unexpectedly frequency modulated also by the discharge frequency due to a leakage pickup of the discharge noise. This pickup distorted the line shape of the signal, and the deviation of the wavenumber was 0.003 cm01 (in maximum) which is much larger than the errors of the measurements. Therefore, the frequency modulation (10 kHz) was employed for the wavenumber measurements of C3S, which was produced in this case by a 120 mA direct current (DC) glow discharge instead of the AC discharge for the discharge modulation. FIG. 1. The vibration–rotation spectra of the n1 fundamental and n1 / n5 0 n5 hot bands of C3S. The hot band shows l-type doublet structure. The time constant of the lock-in amplifier was 300 ms. The spectra were recorded with source frequency modulation in the 2f detection mode of the lock-in amplifier.
RESULTS AND ANALYSIS
The series of lines of the n1 fundamental band was measured between 2046 and 2067 cm01 , and another series of
TABLE 1 Observed Wavenumbers for the n1 Fundamental Band of C3S (in cm01 )
Copyright q 1996 by Academic Press, Inc.
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JMS 7040
/
6t0c$$$182
06-26-96 15:11:52
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AP: Mol Spec
196
TAKANO, TANG, AND SAITO
TABLE 2 Observed Wavenumbers for the n1 / n5 0 n5 Band of C3S (in cm01 )
doublet lines which have about half intensity of the n1 fundamental band was measured between 2047 and 2064 cm01 . A part of the spectrum is shown in Fig. 1. The series of doublet lines was considered to show the l-type doubling of the n5 bending states, and this series was assigned to a hot band n1 / n5 0 n5 . The harmonic vibrational wavenumber of the n5 mode was reported to be 155.2 cm01 by an ab initio calculation using the CCSD(T) method (10). Fifty-eight lines were measured for the n1 fundamental band, and 54 lines for the hot band n1 / n5 0 n5 . The measured wavenumbers were listed in Tables 1 and 2 for the n1 fundamental and hot bands, respectively. The assignment of the quantum number J of the n1 fundamental band was definitely confirmed by using combination differences employing the microwave spectroscopic data (2). But the combination differences could not be applied to the hot band, because the spectral lines in both P and R branches with a common upper state could not be measured. The assignment of J for the hot band is determined to obtain reasonable molecular constants. The standard expressions of energy levels for the 1S linear molecule in the ground and bending excited states (5, 13, 14) were used for the analyses of the n1 fundamental and
hot bands, respectively. The measured wavenumbers were least-squares analyzed to obtain the molecular constants. In the least-squares analyses, the constants for the lower states were fixed to those obtained by Tang and Saito (5). When the l-type doublets were not resolved in the low-J lines of the hot band, the l-type doubling terms were not included in the analyses. The obtained molecular constants were listed in Table 3 with those of the lower states reported by Tang and Saito (5). In the n1 / n5 state, the values for g *ll and d *ll were fixed to zero, because these constants could not be determined from our measurements. The small effect of g *ll and d *ll is, therefore, included in the B * constant. The standard deviations of the least-squares fits are 0.00045 and 0.00054 cm01 for the n1 fundamental and hot bands, respectively. These values are comparable to the experimental errors of the measurements. DISCUSSION
The obtained band origin in the n1 fundamental band (2058.21875 cm01 ) can be compared with the values obtained by the Ar matrix experiment (2046.2 cm01 ) (6) and
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7040
/
6t0c$$$182
06-26-96 15:11:52
mspa
AP: Mol Spec
INFRARED DIODE LASER SPECTROSCOPY OF C3S
197
TABLE 3 Molecular Constants of C3S in the n1 and n1 / n5 0 n5 Bands (in cm01 )
by the ab initio calculation (2071.0 cm01 ) (10). The value obtained by the ab initio calculation agrees with the present value within 1%. The shift of the band origin in the Ar matrix from that in gas-phase is 12.0 cm01 . This value can be compared with the corresponding shift, 14.3 cm01 , of the n1 fundamental band of C3O (15, 16). The a1 value obtained in our study is 14.828(22) MHz. The a1 values were reported by the ab initio calculation to be 15.29 (CEPA-1) and 15.15 MHz (CCSD(T)) (10), which are in good agreement with the experimental value. The vibrational anharmonicity constant, x51 , is derived to be 05.18106(66) cm01 from the difference of the band origins between the n1 and n1 / n5 0 n5 bands. Sensitivity of the present spectrometer was estimated from the observed signals of C3S. The ratio ( DI/I) of the absorbed infrared power ( DI) and the incident total infrared power (I) was 6.6 1 10 04 for the R(22) transition in the n1 fundamental band. This line was observed with the signal-to-noise ratio (S/N) of about 5 as shown in Fig. 1. Therefore, the limit for the detection is DI/I Ç 10 04 in our system. The
detection limit expressed by the absorption coefficient ( a ) is 2.5 1 10 08 cm01 using the path length of 40 m. The number of the C3S molecule per cm3 in the cell was estimated using the formulae in references (17) and (18) under the assumptions that the temperature is 300 K and that the transition dipole moment is 0.1 D. The ground state and the bending excited states of the n4 and n5 modes were considered for the calculation of the partition function. The path length of 40 m was employed in our experiment, but the actual path length seems shorter in case of C3S, because C3S is probably produced in the central region of the cell. Thus, 20 m was assumed to be the actual path length. The calculated value is 8 1 10 10 molecules cm03 : 20 ppm of the total gas in the cell. If 200 or 400 K is assumed as rotational and vibrational temperature, the concentration of C3S changes to 3 1 10 10 or 1 1 10 11 molecules cm03 , respectively. ACKNOWLEDGMENT S. T. is grateful to Drs. Kentarou Kawaguchi, Misaki Okunishi, and Toshinori Suzuki for their kind help in the operation of the diode laser
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7040
/
6t0c$$$182
06-26-96 15:11:52
mspa
AP: Mol Spec
198
TAKANO, TANG, AND SAITO
spectrometer. The calculations in the analyses were carried out at the Computer Center of the Institute for Molecular Science.
REFERENCES 1. N. Kaifu, H. Suzuki, M. Ohishi, T. Miyaji, S. Ishikawa, T. Kasuga, M. Morimoto, and S. Saito, Astrophys. J. 317, L111–L114 (1987). 2. S. Yamamoto, S. Saito, K. Kawaguchi, N. Kaifu, H. Suzuki, and M. Ohishi, Astrophys. J. 317, L119–L121 (1987). 3. Y. Ohshima and Y. Endo, J. Mol. Spectrosc. 153, 627–634 (1992). 4. R. D. Suenram and F. J. Lovas, Astrophys. J. 429, L89–L90 (1994). 5. J. Tang and S. Saito, J. Mol. Spectrosc. 169, 92–107 (1995). 6. G. Maier, J. Schrot, H. P. Reisenauer, and R. Janoschek, Chem. Ber. 124, 2617–2622 (1991). 7. D. J. Peeso, D. W. Ewing, and T. T. Curtis, Chem. Phys. Lett. 166, 307–310 (1990). 8. A. Murakami, Astrophys. J. 357, 288–290 (1990). 9. R. G. A. R. Maclagan and P. Sudkeaw, Chem. Phys. Lett. 194, 147– 151 (1992).
10. S. Seeger, P. Botschwina, J. Flu¨gge, H. P. Reisenauer, and G. Maier, J. Mol. Struct. (THEOCHEM) 303, 213–225 (1994). 11. C. Yamada, K. Nagai, and E. Hirota, J. Mol. Spectrosc. 85, 416–426 (1981). 12. A. G. Maki and J. S. Wells, ‘‘Wavenumber Calibration Tables From Heterodyne Frequency Measurements,’’ U.S. Government Printing Office, Washington, DC, 1991. 13. G. Amat and H. H. Nielsen, J. Mol. Spectrosc. 2, 152–162 (1958). 14. K. M. T. Yamada, F. W. Birss, and M. R. Aliev, J. Mol. Spectrosc. 112, 347–356 (1985). 15. D. McNaughton, D. McGilvery, and F. Shanks, J. Mol. Spectrosc. 149, 458–473 (1991). 16. P. Botschwina and H. P. Reisenauer, Chem. Phys. Lett. 183, 217–222 (1991). 17. L. A. Pugh and K. N. Rao, in ‘‘Molecular Spectroscopy: Modern Research’’ (K. N. Rao, Ed.), Vol. II, Chap. 4, Academic Press, London, 1976. 18. D. Papousek and M. R. Aliev, ‘‘Molecular Vibrational–Rotational Spectra,’’ Elsevier, Amsterdam, 1982.
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7040
/
6t0c$$$182
06-26-96 15:11:52
mspa
AP: Mol Spec