- Email: [email protected]
High-resolution infrared absorption spectroscopy of the CF3l ν2 band
JOURNAL OF MOLECULAR SPECTROSCOPY 118, 3 10-3 12 ( 1986)
NOTES High-Resolution infrared Absorption Spectroscopy of the CF31 v2 Band One of the current interests in infrared photochemistry involves the study of IR multiphoton excitation of polyatomic molecules under supercooled conditions. Lowering of the rotational and vibrational temperature of molecules by supersonic free expansion largely simplifies analysis of the experimental results by confining the initial states of excitation to a small number of low-lying rotational levels. For such studies, however, the rotational distribution of the supersonic free jet must first be measured in order to define the initial population distribution. Once accurate molecular constants are known this distribution can be measured by observing the relative intensities of infrared absorption lines in a supersonic free jet source (I, 2). In the present work, molecular constants for the Y*band of CFJ have been determined to assist measurements of the rotational distribution relevant to infrared photochemical studies of this molecule (3). The vibrational spectra of CF,I have been studied by infrared (d-9), Raman (IO), and infrared-microwave double resonance (II, 12) spectroscopy. However, the molecular constants reported in these studies are not accurate enough to reproduce the observed spectrum for temperature measurements. We have measured the v2band, a CF bending parallel band at 743 cm-i, with a tunable diode laser at Doppler limited resolution. This band was chosen because it had a simple band structure and was free from perturbations. A I-m-long Pyrex cell was cooled by solid carbon dioxide to suppress hot bands. A spectrum taken at room temperature was found to be almost useless because many hot bands overlapped the fundamental. Figure 1 shows a part of the observed spectrum where the K structure of P(49) is clearly seen. The line positions were determined by using IR reference lines of CO2 v2(13, 14) and C2H2 v5(15) bands. Interference fringes of an air-spaced etalon were used as wavenumber markers. The v2 spectrum was recorded for P(5 I)-P(36), P(29)-R( I l), and R(28)-R(36) in the 738- to 747cm-i range. The molecular constants were determined by a least-squares fit of the observed wavenumbers to an
I 738.22
I
I
I
.24
826
-28
I 738.30 cm-l
FIG. I. The K structure observed in P(49) of CFJ. The linewidth increases as K increases due to quadrupole hyperhne splittings. The observed linewidth for K = 24 is approximately 55 MHz (FWHM) while the width calculated from ~-MHZ Doppler linewidth is 22 MHz (FWHM). The K = 9 line is overlapped by one of the hot band peaks. Note the intensity alternation due to spin weight which is twice as large for the k = 3n (n: positive integer) states. 0022-2852186 $3.00 Copyright0 1986 by Academic Pxs, Inc. All a-i&s of reproductionin any form reserved.
310
311 TABLE I Molecuhu Constants for the u2Rand of CFrI (cm-i) v2
state A’
0.190
B’ DJ’
0.050 9.0
771 (5) x
8 (2) 10 -9
DJX’
2.6
(4)
10 -8
v2 ground
854
743.369
x 12
6
(71a
(12)
state
A”
0.191
B” DJ*’
0.050 8.8
ob (4)
811’ x
10 -9
DJKn
2.7
(4)
x
10 -8
a One standard deviation in units of the last digit. b Fixed at the vahte calculated from molecular geometry [Ref. (IO)]. ’ Fixed at the value in Ref. (12). ordinary Ha~toni~ for a parallel band of a symmetric top. The ground state constant B” was ftxed at the value derived From microwave spectroscopy (12, IQ, and A” at the vahre calculated from the molecuhu geometry (10.16). The centrifirgal distortion constant I& was neglected since 0; - DE was smaller than 1 SD. The root mean square of the residual was 0.0005 cm-‘. The constants determined from 346 lines are listed in Table I. The centrifugal distortion constants for the ground vibrational state are in good agreement with the values horn microwave spectroscopy (12). The small difference in D, or DIK between the upper and lower states suggests that the v2band is almost free from perturbations. The list of observed transitions is in the Appendix (Table II).’ The iodine nucleus of CFsI induces large electric quadrupole hypefine splittings (17). The coupling constant has been determined to be -2 I50 MHz (16). The influence of hypertine structure is clearly visible in Fig. I asan increase of linewidth with increase of K. If the coupling constant is assumed to be equal for the lower and upper states, the splitting of the strongest hyperhne components, AF = AJ, for the P(49) K = 24 line is calculated to be 14 MHz while the Doppler width is 8 MHz(HWHM) at 200 K. The observed linewidth can not be compared directly with the calculated vahte since the observed lines are broadened by frequency modulation of the laser diode, carried out to record the interference fringes of the etalon, and also by fi-equency ktuations arising from mechanical vibration of the coId head. However, the obse~ed finewidth is 2.5 times as large as the calculated value although the observed K dependence of the linewidth agrees qualitatively with the calculated one which shows a monotonic increase of hyperfine structure splittings as K increases. Thus far, it is dikth to explain the linewidth quantitatively. In the present analysis, the quadrupole hyperhne structure was ignored since the splitting simply broadened the hnewidth for most of the observed lines. Estimates of the rotational temperature for CFsI in various supersonic free jet expansions, using the derived constants and observed q absorption contours, are in progress. ACKNOWLEDGMENT We are grateful to Dr. H. Jones for useful comments on the ground state rotational constants of CFJ.
’ The Appendix has been placed on deposit in the editorial office of this Journal. A limited number of copies are also availabIe directly from one of the authors (M. Takami).
312
NOTES REFERENCES
1. Y. MIZUGAI,H. KUZE, H. JONES,AND M. TAKAMI,Appt. Phys. B32,43-47 (1983). 2. M. TAKAMIANDH. KUZE, J. Chem. Phys. 80,5994-5998 (1984). 3. See, for example, AA. S. SUDBO,P. A. SCHULTZ,E. R. GRANT, Y. R. SHEN,AND Y. T. LEE,J. Chem. Phys. 70,912-929 (1979); M. QUACKAND G. SEYFANG,J. Chem. Phys. 76,955-965 (1982). 4. W. F. EDGELLAND C. E. MAY, J. Chem. Phys. 20, 1822-1823 (1952). 5. E. K. PLYLERAND N. ACQUISTA,J. Res. NBS 48,92-97 (1952). 6. P. R. MCGEE, F. F. CLEVELAND, A. G. MEISTER,C. E. DECKER,AND S. I. MILLER,J. Chem. Phys. 21,242-246 (1953). 7. W. F. EDGELLANDC. E. MAY, J. Chem. Phys. 22, 1808-1813 (1954). 8. L. C. HOSKINSAND C. J. LEE,J. Chem. Phys. 59,4932-4936 (1973). 9. W. Fuss, Spectrochim. Acta 38A, 829-840 (1982). 10. R. J. H. CLARKANDO. H. ELLESTAD, Mol. Phys. 30, 1899-1911 (1975). 11. H. JONESAND F. KOHLER,J. Mol. Spectrosc. 58, 125-141 (1975). 12. F. KOHLER,H. JONES,AND H. D. RUDOLPH,J. Mol. Spectrosc. 80, 56-70 (1980). 13. R. PASO,J. KAUPPINEN, AND R. ANTTILA,J. Mol. Spectrosc. 79, 236-253 (1980). 14. K. JOLMA,J. KAUPPINEN, AND V.-M. HORNEMAN,J. Mol. Spectrosc. 101,300-305(1983). 15. J. HIETANEN ANDJ. KAUPPINEN, Mol. Phys. 42,41 l-423 (1981). 16. J. SHERIDAN AND W. GORDY, J. Chem. Phys. 20,591-595 (1952). 17. C. H. TOWNESAND A. L. SHAWLOW,“Microwave Spectroscopy,”McGraw-Hill, New York, 1955. YOSHIYASUMATSUMOTO MICHIOTAKAMI Institute of Physical and Chemical Research Wako, Saitama 351, Japan PETERA. HACKETT Chemistry Division NRC of Canada Ottawa OlA R16, Canada
* IPCR VisitingScientistin 1984.
NOTES High-Resolution infrared Absorption Spectroscopy of the CF31 v2 Band One of the current interests in infrared photochemistry involves the study of IR multiphoton excitation of polyatomic molecules under supercooled conditions. Lowering of the rotational and vibrational temperature of molecules by supersonic free expansion largely simplifies analysis of the experimental results by confining the initial states of excitation to a small number of low-lying rotational levels. For such studies, however, the rotational distribution of the supersonic free jet must first be measured in order to define the initial population distribution. Once accurate molecular constants are known this distribution can be measured by observing the relative intensities of infrared absorption lines in a supersonic free jet source (I, 2). In the present work, molecular constants for the Y*band of CFJ have been determined to assist measurements of the rotational distribution relevant to infrared photochemical studies of this molecule (3). The vibrational spectra of CF,I have been studied by infrared (d-9), Raman (IO), and infrared-microwave double resonance (II, 12) spectroscopy. However, the molecular constants reported in these studies are not accurate enough to reproduce the observed spectrum for temperature measurements. We have measured the v2band, a CF bending parallel band at 743 cm-i, with a tunable diode laser at Doppler limited resolution. This band was chosen because it had a simple band structure and was free from perturbations. A I-m-long Pyrex cell was cooled by solid carbon dioxide to suppress hot bands. A spectrum taken at room temperature was found to be almost useless because many hot bands overlapped the fundamental. Figure 1 shows a part of the observed spectrum where the K structure of P(49) is clearly seen. The line positions were determined by using IR reference lines of CO2 v2(13, 14) and C2H2 v5(15) bands. Interference fringes of an air-spaced etalon were used as wavenumber markers. The v2 spectrum was recorded for P(5 I)-P(36), P(29)-R( I l), and R(28)-R(36) in the 738- to 747cm-i range. The molecular constants were determined by a least-squares fit of the observed wavenumbers to an
I 738.22
I
I
I
.24
826
-28
I 738.30 cm-l
FIG. I. The K structure observed in P(49) of CFJ. The linewidth increases as K increases due to quadrupole hyperhne splittings. The observed linewidth for K = 24 is approximately 55 MHz (FWHM) while the width calculated from ~-MHZ Doppler linewidth is 22 MHz (FWHM). The K = 9 line is overlapped by one of the hot band peaks. Note the intensity alternation due to spin weight which is twice as large for the k = 3n (n: positive integer) states. 0022-2852186 $3.00 Copyright0 1986 by Academic Pxs, Inc. All a-i&s of reproductionin any form reserved.
310
311 TABLE I Molecuhu Constants for the u2Rand of CFrI (cm-i) v2
state A’
0.190
B’ DJ’
0.050 9.0
771 (5) x
8 (2) 10 -9
DJX’
2.6
(4)
10 -8
v2 ground
854
743.369
x 12
6
(71a
(12)
state
A”
0.191
B” DJ*’
0.050 8.8
ob (4)
811’ x
10 -9
DJKn
2.7
(4)
x
10 -8
a One standard deviation in units of the last digit. b Fixed at the vahte calculated from molecular geometry [Ref. (IO)]. ’ Fixed at the value in Ref. (12). ordinary Ha~toni~ for a parallel band of a symmetric top. The ground state constant B” was ftxed at the value derived From microwave spectroscopy (12, IQ, and A” at the vahre calculated from the molecuhu geometry (10.16). The centrifirgal distortion constant I& was neglected since 0; - DE was smaller than 1 SD. The root mean square of the residual was 0.0005 cm-‘. The constants determined from 346 lines are listed in Table I. The centrifugal distortion constants for the ground vibrational state are in good agreement with the values horn microwave spectroscopy (12). The small difference in D, or DIK between the upper and lower states suggests that the v2band is almost free from perturbations. The list of observed transitions is in the Appendix (Table II).’ The iodine nucleus of CFsI induces large electric quadrupole hypefine splittings (17). The coupling constant has been determined to be -2 I50 MHz (16). The influence of hypertine structure is clearly visible in Fig. I asan increase of linewidth with increase of K. If the coupling constant is assumed to be equal for the lower and upper states, the splitting of the strongest hyperhne components, AF = AJ, for the P(49) K = 24 line is calculated to be 14 MHz while the Doppler width is 8 MHz(HWHM) at 200 K. The observed linewidth can not be compared directly with the calculated vahte since the observed lines are broadened by frequency modulation of the laser diode, carried out to record the interference fringes of the etalon, and also by fi-equency ktuations arising from mechanical vibration of the coId head. However, the obse~ed finewidth is 2.5 times as large as the calculated value although the observed K dependence of the linewidth agrees qualitatively with the calculated one which shows a monotonic increase of hyperfine structure splittings as K increases. Thus far, it is dikth to explain the linewidth quantitatively. In the present analysis, the quadrupole hyperhne structure was ignored since the splitting simply broadened the hnewidth for most of the observed lines. Estimates of the rotational temperature for CFsI in various supersonic free jet expansions, using the derived constants and observed q absorption contours, are in progress. ACKNOWLEDGMENT We are grateful to Dr. H. Jones for useful comments on the ground state rotational constants of CFJ.
’ The Appendix has been placed on deposit in the editorial office of this Journal. A limited number of copies are also availabIe directly from one of the authors (M. Takami).
312
NOTES REFERENCES
1. Y. MIZUGAI,H. KUZE, H. JONES,AND M. TAKAMI,Appt. Phys. B32,43-47 (1983). 2. M. TAKAMIANDH. KUZE, J. Chem. Phys. 80,5994-5998 (1984). 3. See, for example, AA. S. SUDBO,P. A. SCHULTZ,E. R. GRANT, Y. R. SHEN,AND Y. T. LEE,J. Chem. Phys. 70,912-929 (1979); M. QUACKAND G. SEYFANG,J. Chem. Phys. 76,955-965 (1982). 4. W. F. EDGELLAND C. E. MAY, J. Chem. Phys. 20, 1822-1823 (1952). 5. E. K. PLYLERAND N. ACQUISTA,J. Res. NBS 48,92-97 (1952). 6. P. R. MCGEE, F. F. CLEVELAND, A. G. MEISTER,C. E. DECKER,AND S. I. MILLER,J. Chem. Phys. 21,242-246 (1953). 7. W. F. EDGELLANDC. E. MAY, J. Chem. Phys. 22, 1808-1813 (1954). 8. L. C. HOSKINSAND C. J. LEE,J. Chem. Phys. 59,4932-4936 (1973). 9. W. Fuss, Spectrochim. Acta 38A, 829-840 (1982). 10. R. J. H. CLARKANDO. H. ELLESTAD, Mol. Phys. 30, 1899-1911 (1975). 11. H. JONESAND F. KOHLER,J. Mol. Spectrosc. 58, 125-141 (1975). 12. F. KOHLER,H. JONES,AND H. D. RUDOLPH,J. Mol. Spectrosc. 80, 56-70 (1980). 13. R. PASO,J. KAUPPINEN, AND R. ANTTILA,J. Mol. Spectrosc. 79, 236-253 (1980). 14. K. JOLMA,J. KAUPPINEN, AND V.-M. HORNEMAN,J. Mol. Spectrosc. 101,300-305(1983). 15. J. HIETANEN ANDJ. KAUPPINEN, Mol. Phys. 42,41 l-423 (1981). 16. J. SHERIDAN AND W. GORDY, J. Chem. Phys. 20,591-595 (1952). 17. C. H. TOWNESAND A. L. SHAWLOW,“Microwave Spectroscopy,”McGraw-Hill, New York, 1955. YOSHIYASUMATSUMOTO MICHIOTAKAMI Institute of Physical and Chemical Research Wako, Saitama 351, Japan PETERA. HACKETT Chemistry Division NRC of Canada Ottawa OlA R16, Canada
* IPCR VisitingScientistin 1984.