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Millimeter- and Submillimeter-Wave Spectrum of Methylene Fluoride-d2
Journal of Molecular Spectroscopy 196, 212–219 (1999) Article ID jmsp.1999.7864, available online at http://www.idealibrary.com on
Millimeter- and Submillimeter-Wave Spectrum of Methylene Fluoride-d 2 M. N. Deo and K. Kawaguchi Spectroscopy Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India; and Nobeyama Radio Observatory, Nobeyama, Minamimaki, Minamisaku, Nagano-ken 384-1305, Japan E-mail: [email protected] Received January 5, 1999; in revised form March 26, 1999
The millimeter- and submillimeter-wave spectrum of methylene fluoride-d 2 (CD 2F 2) has been observed in the region between 200 and 415 GHz. The spectrum was recorded using a frequency-modulated millimeter- and submillimeter-wave spectrometer. More than 400 rotational transitions up to J # 60 and K a # 21 in the ground state and more than 200 rotational transitions up to J # 48 and K a # 9 in the v 4 5 1 vibrational state have been assigned. The lines assigned are simultaneously least-squares fitted along with the microwave data in the Watson’s A- and S-reduced Hamiltonian. Improved values have been obtained for the rotational constants and quartic centrifugal distortion constants, including for the first time, sextic distortion constants of the ground state as well as the v 4 5 1 state. © 1999 Academic Press
in that four (n 3, n 5, n 7, n 9) of the nine modes have their fundamental frequencies in this narrow region (14). As a result, the infrared spectrum is extremely crowded and complexity is further enhanced by strong Coriolis interactions between the various modes. As CD 2F 2 is a relatively light molecule (A 5 34.7 GHz, B 5 10.2 GHz, and C 5 8.8 GHz), the greater part of its pure rotational spectrum lies above 200 GHz. Therefore, in order to obtain the higher order centrifugal distortion constants by including high J and high K a transitions, it is necessary to measure the mm/submm-wave spectra. We have undertaken a systematic study of the mm/submm spectra of the ground and v 4 5 1 state, and the high-resolution FTIR spectra in the 9 –10 mm region (n 3, n 5, n 7, n 9, and 2n 4). Although the main infrared absorption of the pump laser is expected to be associated with transitions in the n 3, n 7, and n 9 bands, some absorption due to hot band transitions is to be expected from the low-lying n 4 vibrational state which has about 8% population at room temperature. Therefore, as a first step, in the present paper we report the results of the mm/submm-wave measurements and analysis of the rotational spectra in the ground state and v 4 5 1 state.
I. INTRODUCTION
CH 2F 2 and its isotopic species, in particular CD 2F 2, are among the most efficient laser medium for the generation of strong far-IR laser emission optically pumped by CO 2 lasers (1– 6). The practical importance of these molecules is because of their large permanent dipole moment (;2 D), dense energy levels introduced by asymmetry (k 5 ;– 0.9), and very strong absorption in the 9 –10 mm region of CO 2 lasers. Numerous submillimeter-wave emissions from CH 2F 2 and CD 2F 2 pumped by CO 2 lasers have been reported (1– 6). However, to assign the energy levels involved in the pumping and emission process without ambiguity, it is necessary to know the accurate ground and upper state rotational energies. The ground state of CH 2F 2 has been extensively studied by microwave (7), mm/submm-wave (8), and far-IR (9) techniques, and accurate molecular parameters of the ground state have been reported. The 9 –10 mm region (n 3, n 9, n 7 bands) of CH 2F 2 has also been studied to some extent at high resolution (3, 10, 11). Deroche et al. (3) have reported the assignment of many laser lines. The situation is much less satisfactory for CD 2F 2, for which only microwave spectra (12, 13) in 8 – 68 GHz region and low-resolution infrared data (14) have been reported. The microwave spectrum of CD 2F 2 was first reported by Hirota et al. (12). Hirota and Sahara (13) reinvestigated the microwave spectra in detail up to J 5 19 and determined the rotational constants and quartic centrifugal distortion constants of the ground states and limited number of excited vibrational states. CD 2F 2 has an unusual spectral feature in the 9 –10 mm region 212 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
II. EXPERIMENTAL DETAILS
The mm/submm-wave spectra of CD 2F 2 were measured at Nobeyama Radio Observatory, Japan with a source modulated mm/submm-wave spectrometer (15), equipped with phaselocked Gunn oscillators combined with Schottky diode multipliers as tunable radiation sources. The radiation from the
213
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2
ment by symmetry lies along the b axis, the pure rotational spectrum has b-type transitions that follow the selection rules DK a 5 61, 63, . . . ;
DK c 5 61, 63, . . . ; and
DJ 5 0, 61.
Assignments of the observed transitions were initiated on the basis of the line positions predicted from the reported ground state parameters of Hirota and Sahara (13). The centrifugal distortion constants in Ref. (13) which are in an alternate but equivalent form were first converted to the more commonly reported form in A as well as in S reduction (16). For the two D nuclei in CD 2F 2, each has a nuclear spin of 1 while each F has a nuclear spin of 21. These can combine to give a nuclear statistical weight of 21 for K a, K c 5 (even, odd) or (odd, even) levels and a weight of 15 for
FIG. 1. Typical spectrum observed for the ground state of CD 2F 2: A: Low J transitions with asymmetry splitting, B: High J and high K a transitions with asymmetry splitting, C: High J transition.
multiplier was detected by a liquid-helium-cooled InSb bolometer through a 1.2 m free-space absorption cell (10 cm in diameter). The measurements were performed at room temperature and pressures below 8 mTorr. The spectrum was measured in the 208 – 411 GHz frequency range. The transitions are rather strong and thus more that 600 lines with good signalto-noise ratios have been observed. Each line frequency was determined by averaging one or two pairs of upward and downward frequency sweep measurements. Figures 1(A–C) and 2(A–C) show examples of the observed spectral lines. III. ANALYSIS, RESULTS, AND DISCUSSION
CD 2F 2 is a near-prolate asymmetric rotor (k 5 20.89) with C 2v symmetry point group. Since the permanent dipole mo-
FIG. 2. Typical spectrum observed for the v 4 5 1 state of CD 2F 2: A: Low J transitions with asymmetry splitting, B: Medium J and K a transitions with asymmetry splitting, C: High J transition.
Copyright © 1999 by Academic Press
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DEO AND KAWAGUCHI
TABLE 1 Observed Millimeter-Wave and Submillimeter-Wave Transitions (in MHz) for the Ground State of CD 2F 2
Copyright © 1999 by Academic Press
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2
215
TABLE 1—Continued
K a, K c 5 (even, even) or (odd, odd) levels. For a particular K a value, the lines are split by asymmetry, and the intensity alternation arising from the spin statistical weight consideration is clearly seen in the observed spectrum (Fig. 1A, B and 2A, B). This feature was quite useful in confirming the assignments. Subsequent high J transitions were assigned by the “bootstrap” method. This way, for the ground state, a total of 424 new transitions with J # 60 and K a # 21 and for the v 4 5 1 state, a total of 225 new transitions with J # 48 and K a # 9 have been assigned. A list of the newly measured frequencies is given in Table 1 (ground state)
and 2 (v 4 5 1 state). Asymmetry splitting was observed up to K a # 12 in the ground state. To obtain the molecular parameters from the observed frequencies, a weighted least-squares program based on the Hamiltonian of Watson (16) using I r representation in A as well as in S reduction was used. All the data of Table 1 and microwave data of Hirota and Sahara (13) were used for the ground state. The standard deviation obtained from independent fits of the various data sets (s i) were used as weights (1/s i2) in the combined weighted least-squares analysis. The standard deviation of the combined fit was found to be close to unity indicating the
Copyright © 1999 by Academic Press
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DEO AND KAWAGUCHI
TABLE 2 Observed Millimeter-Wave and Submillimeter-Wave Transitions (in MHz) for the v 4 5 1 State of CD 2F 2
compatibility of the weights of the various data sets. A similar weighted least-squares analysis was carried out for the v 4 5 1 state using the data of Table 2 and Ref. (13). The overall standard
deviation of the fit in both the cases, A and S reduction, was 25 kHz for the ground as well as for the v 4 5 1 state, which is well within the experimental accuracy.
Copyright © 1999 by Academic Press
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2
217
TABLE 3 Molecular Constants (in MHz) of the Ground State of CD 2F 2
The final constants obtained from the fit in the A and S reductions for the ground and v 4 5 1 states are listed in Tables 3 and 4, respectively. For comparison, the parameters obtained from the independent fit of Ref. (13) are also included in the tables. The rotational constants obtained in the present study agree well with those reported from previous microwave studies (13), and at the same time, the precision of these constants has significantly improved. All the parameters are well determined. In conclusion, more than 400 rotational transitions up to J # 60
and K a # 21 in the ground state and more than 200 rotational transitions up to J # 48 and K a # 9 in the v 4 5 1 vibrational state have been observed in the mm/submm-wave spectrum. The present experimental data extending to high J and K a transitions made it possible to determine the sextic centrifugal distortion constants as well as to improve the precision of the rotational constants and quartic centrifugal distortion constants for the ground state and v 4 5 1 state of CD 2F 2. The availability of the present parameters for the ground state should facilitate reliable
Copyright © 1999 by Academic Press
218
DEO AND KAWAGUCHI
TABLE 4 Molecular Constants (in MHz) of the v 5 1 State of CD 2F 2
analysis of the high-resolution infrared data of the excited vibrational state of interest, which in turn will be useful in assigning laser emission lines.
authors are also grateful to Dr. R. D’Cunha and Dr. T. K. Balasubramanian for a critical reading of the manuscript.
ACKNOWLEDGMENTS
1. J. C. Peterson, D. Igner, and G. Duxbury, J. Mol. Spectrosc. 100, 396 – 402 (1983). 2. J. C. Deroche and E. K. Benichou, Opt. Commun. 54, 23–26 (1985). 3. J. C. Deroche, E. K. Benichou, G. Guelachvili, and J. Demaison, Int. J. Infrared Millim Waves 7, 1653–1675 (1986).
M. N. Deo gratefully acknowledges financial support from the Japan Society for the Promotion of Science (JSPS), P96330, Japan during his stay at NRO, Nobeyama as JSPS fellow. He is thankful to Dr. A. P. Roy for his interest. The
REFERENCES
Copyright © 1999 by Academic Press
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2 4. B. W. Davis and A. Vass, Int. J. Infrared Millim Waves 9, 279 –293 (1988). 5. S. C. Zebetto, E. C. C. Vasconcellos, L. R. Zink, and K. M. Evenson, Int. J. Infrared Millim Waves 18, 2301–2306 (1997). 6. E. C. C. Vasconcellos, T. R. Peterson, and K. M. Evenson, Int. J. Infrared Millim Waves 2, 705–711 (1981). 7. E. Hirota, J. Mol. Spectrosc. 69, 409 – 420 (1978). 8. L. Martinache, J. C. Deroche, D. Boucher, and J. Demaison, J. Mol. Spectrosc. 119, 225–228 (1986). 9. M. Carlotti, G. D. Nivellini, F. Tullini, and B. Carli, J. Mol. Spectrosc. 132, 158 –165 (1988).
219
10. M. N. Deo, R. D’Cunha, and V. A. Job, J. Mol. Spectrosc. 161, 403– 415 (1993). 11. Janina Matuszeski and Mark D. Marshall, Spectrochim. Acta A 50, 1069 – 1077 (1996). 12. E. Hirota, T. Tanaka, A. Sakakibara, Y. Ohashi, and Y. Morino, J. Mol. Spectrosc. 34, 222 (1970). 13. E. Hirota and M. Sahara, J. Mol. Spectrosc. 56, 21–38 (1975). 14. I. Suzuki and T. Shimanouchi, J. Mol. Spectrosc. 46, 130 –145 (1973). 15. E. Kagi and K. Kawaguchi, submitted for publication. 16. J. K. G. Watson, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 6, pp. 1– 89, Elsevier, Amsterdam, 1977.
Copyright © 1999 by Academic Press
Millimeter- and Submillimeter-Wave Spectrum of Methylene Fluoride-d 2 M. N. Deo and K. Kawaguchi Spectroscopy Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India; and Nobeyama Radio Observatory, Nobeyama, Minamimaki, Minamisaku, Nagano-ken 384-1305, Japan E-mail: [email protected] Received January 5, 1999; in revised form March 26, 1999
The millimeter- and submillimeter-wave spectrum of methylene fluoride-d 2 (CD 2F 2) has been observed in the region between 200 and 415 GHz. The spectrum was recorded using a frequency-modulated millimeter- and submillimeter-wave spectrometer. More than 400 rotational transitions up to J # 60 and K a # 21 in the ground state and more than 200 rotational transitions up to J # 48 and K a # 9 in the v 4 5 1 vibrational state have been assigned. The lines assigned are simultaneously least-squares fitted along with the microwave data in the Watson’s A- and S-reduced Hamiltonian. Improved values have been obtained for the rotational constants and quartic centrifugal distortion constants, including for the first time, sextic distortion constants of the ground state as well as the v 4 5 1 state. © 1999 Academic Press
in that four (n 3, n 5, n 7, n 9) of the nine modes have their fundamental frequencies in this narrow region (14). As a result, the infrared spectrum is extremely crowded and complexity is further enhanced by strong Coriolis interactions between the various modes. As CD 2F 2 is a relatively light molecule (A 5 34.7 GHz, B 5 10.2 GHz, and C 5 8.8 GHz), the greater part of its pure rotational spectrum lies above 200 GHz. Therefore, in order to obtain the higher order centrifugal distortion constants by including high J and high K a transitions, it is necessary to measure the mm/submm-wave spectra. We have undertaken a systematic study of the mm/submm spectra of the ground and v 4 5 1 state, and the high-resolution FTIR spectra in the 9 –10 mm region (n 3, n 5, n 7, n 9, and 2n 4). Although the main infrared absorption of the pump laser is expected to be associated with transitions in the n 3, n 7, and n 9 bands, some absorption due to hot band transitions is to be expected from the low-lying n 4 vibrational state which has about 8% population at room temperature. Therefore, as a first step, in the present paper we report the results of the mm/submm-wave measurements and analysis of the rotational spectra in the ground state and v 4 5 1 state.
I. INTRODUCTION
CH 2F 2 and its isotopic species, in particular CD 2F 2, are among the most efficient laser medium for the generation of strong far-IR laser emission optically pumped by CO 2 lasers (1– 6). The practical importance of these molecules is because of their large permanent dipole moment (;2 D), dense energy levels introduced by asymmetry (k 5 ;– 0.9), and very strong absorption in the 9 –10 mm region of CO 2 lasers. Numerous submillimeter-wave emissions from CH 2F 2 and CD 2F 2 pumped by CO 2 lasers have been reported (1– 6). However, to assign the energy levels involved in the pumping and emission process without ambiguity, it is necessary to know the accurate ground and upper state rotational energies. The ground state of CH 2F 2 has been extensively studied by microwave (7), mm/submm-wave (8), and far-IR (9) techniques, and accurate molecular parameters of the ground state have been reported. The 9 –10 mm region (n 3, n 9, n 7 bands) of CH 2F 2 has also been studied to some extent at high resolution (3, 10, 11). Deroche et al. (3) have reported the assignment of many laser lines. The situation is much less satisfactory for CD 2F 2, for which only microwave spectra (12, 13) in 8 – 68 GHz region and low-resolution infrared data (14) have been reported. The microwave spectrum of CD 2F 2 was first reported by Hirota et al. (12). Hirota and Sahara (13) reinvestigated the microwave spectra in detail up to J 5 19 and determined the rotational constants and quartic centrifugal distortion constants of the ground states and limited number of excited vibrational states. CD 2F 2 has an unusual spectral feature in the 9 –10 mm region 212 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
II. EXPERIMENTAL DETAILS
The mm/submm-wave spectra of CD 2F 2 were measured at Nobeyama Radio Observatory, Japan with a source modulated mm/submm-wave spectrometer (15), equipped with phaselocked Gunn oscillators combined with Schottky diode multipliers as tunable radiation sources. The radiation from the
213
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2
ment by symmetry lies along the b axis, the pure rotational spectrum has b-type transitions that follow the selection rules DK a 5 61, 63, . . . ;
DK c 5 61, 63, . . . ; and
DJ 5 0, 61.
Assignments of the observed transitions were initiated on the basis of the line positions predicted from the reported ground state parameters of Hirota and Sahara (13). The centrifugal distortion constants in Ref. (13) which are in an alternate but equivalent form were first converted to the more commonly reported form in A as well as in S reduction (16). For the two D nuclei in CD 2F 2, each has a nuclear spin of 1 while each F has a nuclear spin of 21. These can combine to give a nuclear statistical weight of 21 for K a, K c 5 (even, odd) or (odd, even) levels and a weight of 15 for
FIG. 1. Typical spectrum observed for the ground state of CD 2F 2: A: Low J transitions with asymmetry splitting, B: High J and high K a transitions with asymmetry splitting, C: High J transition.
multiplier was detected by a liquid-helium-cooled InSb bolometer through a 1.2 m free-space absorption cell (10 cm in diameter). The measurements were performed at room temperature and pressures below 8 mTorr. The spectrum was measured in the 208 – 411 GHz frequency range. The transitions are rather strong and thus more that 600 lines with good signalto-noise ratios have been observed. Each line frequency was determined by averaging one or two pairs of upward and downward frequency sweep measurements. Figures 1(A–C) and 2(A–C) show examples of the observed spectral lines. III. ANALYSIS, RESULTS, AND DISCUSSION
CD 2F 2 is a near-prolate asymmetric rotor (k 5 20.89) with C 2v symmetry point group. Since the permanent dipole mo-
FIG. 2. Typical spectrum observed for the v 4 5 1 state of CD 2F 2: A: Low J transitions with asymmetry splitting, B: Medium J and K a transitions with asymmetry splitting, C: High J transition.
Copyright © 1999 by Academic Press
214
DEO AND KAWAGUCHI
TABLE 1 Observed Millimeter-Wave and Submillimeter-Wave Transitions (in MHz) for the Ground State of CD 2F 2
Copyright © 1999 by Academic Press
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2
215
TABLE 1—Continued
K a, K c 5 (even, even) or (odd, odd) levels. For a particular K a value, the lines are split by asymmetry, and the intensity alternation arising from the spin statistical weight consideration is clearly seen in the observed spectrum (Fig. 1A, B and 2A, B). This feature was quite useful in confirming the assignments. Subsequent high J transitions were assigned by the “bootstrap” method. This way, for the ground state, a total of 424 new transitions with J # 60 and K a # 21 and for the v 4 5 1 state, a total of 225 new transitions with J # 48 and K a # 9 have been assigned. A list of the newly measured frequencies is given in Table 1 (ground state)
and 2 (v 4 5 1 state). Asymmetry splitting was observed up to K a # 12 in the ground state. To obtain the molecular parameters from the observed frequencies, a weighted least-squares program based on the Hamiltonian of Watson (16) using I r representation in A as well as in S reduction was used. All the data of Table 1 and microwave data of Hirota and Sahara (13) were used for the ground state. The standard deviation obtained from independent fits of the various data sets (s i) were used as weights (1/s i2) in the combined weighted least-squares analysis. The standard deviation of the combined fit was found to be close to unity indicating the
Copyright © 1999 by Academic Press
216
DEO AND KAWAGUCHI
TABLE 2 Observed Millimeter-Wave and Submillimeter-Wave Transitions (in MHz) for the v 4 5 1 State of CD 2F 2
compatibility of the weights of the various data sets. A similar weighted least-squares analysis was carried out for the v 4 5 1 state using the data of Table 2 and Ref. (13). The overall standard
deviation of the fit in both the cases, A and S reduction, was 25 kHz for the ground as well as for the v 4 5 1 state, which is well within the experimental accuracy.
Copyright © 1999 by Academic Press
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2
217
TABLE 3 Molecular Constants (in MHz) of the Ground State of CD 2F 2
The final constants obtained from the fit in the A and S reductions for the ground and v 4 5 1 states are listed in Tables 3 and 4, respectively. For comparison, the parameters obtained from the independent fit of Ref. (13) are also included in the tables. The rotational constants obtained in the present study agree well with those reported from previous microwave studies (13), and at the same time, the precision of these constants has significantly improved. All the parameters are well determined. In conclusion, more than 400 rotational transitions up to J # 60
and K a # 21 in the ground state and more than 200 rotational transitions up to J # 48 and K a # 9 in the v 4 5 1 vibrational state have been observed in the mm/submm-wave spectrum. The present experimental data extending to high J and K a transitions made it possible to determine the sextic centrifugal distortion constants as well as to improve the precision of the rotational constants and quartic centrifugal distortion constants for the ground state and v 4 5 1 state of CD 2F 2. The availability of the present parameters for the ground state should facilitate reliable
Copyright © 1999 by Academic Press
218
DEO AND KAWAGUCHI
TABLE 4 Molecular Constants (in MHz) of the v 5 1 State of CD 2F 2
analysis of the high-resolution infrared data of the excited vibrational state of interest, which in turn will be useful in assigning laser emission lines.
authors are also grateful to Dr. R. D’Cunha and Dr. T. K. Balasubramanian for a critical reading of the manuscript.
ACKNOWLEDGMENTS
1. J. C. Peterson, D. Igner, and G. Duxbury, J. Mol. Spectrosc. 100, 396 – 402 (1983). 2. J. C. Deroche and E. K. Benichou, Opt. Commun. 54, 23–26 (1985). 3. J. C. Deroche, E. K. Benichou, G. Guelachvili, and J. Demaison, Int. J. Infrared Millim Waves 7, 1653–1675 (1986).
M. N. Deo gratefully acknowledges financial support from the Japan Society for the Promotion of Science (JSPS), P96330, Japan during his stay at NRO, Nobeyama as JSPS fellow. He is thankful to Dr. A. P. Roy for his interest. The
REFERENCES
Copyright © 1999 by Academic Press
MM/SUBMM-WAVE SPECTRUM OF CD 2F 2 4. B. W. Davis and A. Vass, Int. J. Infrared Millim Waves 9, 279 –293 (1988). 5. S. C. Zebetto, E. C. C. Vasconcellos, L. R. Zink, and K. M. Evenson, Int. J. Infrared Millim Waves 18, 2301–2306 (1997). 6. E. C. C. Vasconcellos, T. R. Peterson, and K. M. Evenson, Int. J. Infrared Millim Waves 2, 705–711 (1981). 7. E. Hirota, J. Mol. Spectrosc. 69, 409 – 420 (1978). 8. L. Martinache, J. C. Deroche, D. Boucher, and J. Demaison, J. Mol. Spectrosc. 119, 225–228 (1986). 9. M. Carlotti, G. D. Nivellini, F. Tullini, and B. Carli, J. Mol. Spectrosc. 132, 158 –165 (1988).
219
10. M. N. Deo, R. D’Cunha, and V. A. Job, J. Mol. Spectrosc. 161, 403– 415 (1993). 11. Janina Matuszeski and Mark D. Marshall, Spectrochim. Acta A 50, 1069 – 1077 (1996). 12. E. Hirota, T. Tanaka, A. Sakakibara, Y. Ohashi, and Y. Morino, J. Mol. Spectrosc. 34, 222 (1970). 13. E. Hirota and M. Sahara, J. Mol. Spectrosc. 56, 21–38 (1975). 14. I. Suzuki and T. Shimanouchi, J. Mol. Spectrosc. 46, 130 –145 (1973). 15. E. Kagi and K. Kawaguchi, submitted for publication. 16. J. K. G. Watson, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 6, pp. 1– 89, Elsevier, Amsterdam, 1977.
Copyright © 1999 by Academic Press