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Tunable diode laser measurements of intensities and Lorentz broadening coefficients of lines in the ν2 band of 12CH4
J. Quant. Spectros¢. gadiat. Trans[er Vol. 29, No. I, pp. 45--47, 1983
0022-4073/83/010045-03503.00/0 Pergamon Press Ltd.
Printed in Great Bd"rain.
TUNABLE DIODE LASER MEASUREMENTS OF INTENSITIES AND LORENTZ BROADENING COEFFICIENTS OF LINES IN THE ~'2 BAND OF 12CH4 V. MALATHYDEVI,B.
FRIDOVICH,D. G. S. SNYDER,and G. D. JONES
National Oceanic and Atmospheric Administration, National Earth Satellite Service, Washington, 20233, U.S.A.
and PALASH P. DAS Tachisto Inc., 13 Highland Circle, Needham, MA 02194, U.S.A.
(Received 24 February 1982) Al~tract--Absolute line strengths and self-, nitrogen-, and air-broadened half-widths, for five lines in the R-branch of the ~,~fundamental of methane have been measured at room temperature using a tunable diode laser spectrometer. INTRODUCTION
An extensive literature of the ~'3,2z,3, 3;,3, and ~4 bands of methane exists; H~ however, studies of the ~2 band are sparse./2-15 We may attribute this situation to the fact that the ;'2 band is a forbidden band, which becomes active because of Coriolis interactions between the ~'3 and ~4 bands and hence is weak. The v2 band also falls in the region of the strong 6.3 tLm H20 band. In order to study this band experimentally, both high resolution and high sensitivity techniques are needed. An atlas of methane in the region from 1200 to 1800 cm -1, obtained by using a Nicolet Fourier Transform Spectrometer at 0.06 cm -~ resolution, was compiled by Blatherwick et al. 12 Robiette, ~4 using experimental data from various sources, has analyzed the ~'2 band and reported line assignments, molecular constants, and relative strengths for the ;'2 band of t2ca4. In the present work, we report the first experimental measurements on absolute line intensities and on self-, nitrogen-, and air-broadened half-widths for five lines in the 1,2 R-branch of J2CH4, using a diode laser spectrometer. EXPERIMENTAL DETAILS
The diode laser spectrometer used in this work has been described elsewhere/6 A diode laser operating from 1570 to 1590 cm -1 was used. For line strength and foreign gas broadening measurements, a double pass cell with a total length of 9 m was used. For self-broadening measurements, shorter cells were used. Pressure measurements were made using an MKS Baratron gauge with heads in the 0--1, 0--10, and 0-100 torr ranges. Ultra-high purity methane, nitrogen, and air purchased from the Matheson Company were used without further purification. LINE STRENGTHS
The five lines measured were located spectrally in the operating range of the diode laser used in our previous s t u d i e s ) 6'17 The wavenumbers and assignments for these lines were taken from the work of Margolis) 5 Sample pressures ranged from 0.4 to 1 torr. Three or more pressures were used for each absorption line, and several repetitive scans were made at each pressure. Line strengths were determined from equivalent widths and were measured on strip chart records in the standard way by linear interpolation in the tables of Jansson and Korb. 18 The results are presented in Table 1. Column headings are as follows; ;' is the wavenumber (cm -I) of the transitions involved; J is the total angular momentum; C refers to the irreducible representations AI, A2, E, Ft or F2 of the symmetry group To; N characterizes the different states that have the same values of J and C; single and double primes refer to upper and lower states, respectively. In order to compare our experimentally determined line strengths with the calculated relative peak intensities of Robiette ~4 (listed in Table 1), we normalized both sets of values to the first line listed. The two sets of ratios are listed in Table 1. The agreement is good. 45
V. M. DEv[ et al.
46
Table 1. Line strengths for some ~,_,R-branch lines of '2CH~at 296 K. J'
C'
N'
J"
C"
N"
(cm-))
~ ( c m -2
Relative a t m -1 )
peak
Normalized
i n t e n s i t y i;
Present
Intensities
Work
Robiette
1586.9466
5
FI
3
4
F2
I
0.00180+0.00003
0.03544
1.00
1587.8341
5
FI
2
4
F2
1
0.00339+0.00006
0.06812
1.88
1.92
|588.0310
5
E
I
4
E
I
0.00251*0.00003
0.05111
1,39
1.44
1588.3281
5
F2
2
4
F1
~
0.00216+0.00004
0.04479
1.20
1.26
1589.7348
5
A2
1
4
AI
I
0.00444+0.00010
0.0891(]
2.47
2.51
Uncertainties 11 F o r
details
quoted
are
see Ref.
14.
one
standard
1.00
deviation.
LINE WIDTHS AND SHAPES The half-widths of these five lines were measured at room temperature with various pressures of CH4 or CH4-N2 and CH4-air mixtures to derive the Lorentz broadening coefficients bcu4-cH,, 0 0 0 bCH4-N2 and bcH4-air. Self-broadening measurements were made using a pyrex cell 30.59 cm long with pressures from 30 to 50 tore For the nitrogen- and air-broadened measurements, the partial pressures of methane ranged from 0.5 to 1.2 torr while the broadener pressures ranged from 25 to 50 torr. We assumed that the absorption lines had a Voigt profile and measured the half-width by. The technique for extracting the broadening parameters from by has been described in detail elsewhereJ 6": The measurements were repeated for three or more pressures of CH4, CH4-N2 or CH4-air for each line. At least six repetitions were made at each pressure. The results are presented in Table 2. Since the Lorentz broadening coefficients in the present study belong to one multiplet, no comparison could be made with other multiplets. However, even within a multiplet, strong dependencies exist among the various symmetry species. Similar symmetry-dependent half-widths were reported earlier by Tejwani et al) 9-zt for the ,~ and 2 v4 bands of methane.
Table 2. Self-, nitrogen- and air-broadeningcoefficients for some ,2 band lines of '2CH4 at 296 K, b" CH4-CH4
b° C H 4 -N~
(cm-latm -I )
(cm -] a r m -I )
( :m-latm -] )
1586.9466 "
0.085+0.001
0.068+0.002
0.068+0.001
1587.8341
0.069+0.002
0.054*0.000
0.054+0.001
1588.0310
0.065+0.001
0.056+0.001
0.053+0.001
(cm -I )
4
-Air
1588.3281
0.067+0.003
0.058+0.003
0.057+0.001
1589.7348
0.072+0.004
0.069+0.002
0.068+0.002
}. Assignments
arc
civeh
%[Incertainti~,~s q u o t e d
1. 2. 3. 4. 5. 6. 7.
CH
]n T a b l e ,ire o n e
]1.
:6tatldard
deviation.
REFERENCES P. Varanasi, JQSRT 11, 1711 (1971) and the cited references. J. S. Margolis, JQSRT 13, 1097 (1973). L. Darnton and J. S. Margolis, JQSRT 13, 969 (1973). P. Varanasi, JQSRT 15, 281 (1975). M. Dang-Nhu, A. S. Pine, and A. G. Rohiette, J. Molec. Spectrosc. 77, 57 (1979). K. Fox, G. W. Halsey, S. J. Daunt, W. E. Blass, and D. E. Jennings, J. Chem. Phys. {Letters) 72, 4657 (1980). S. Gherissi, A. Henry, M. Loete, and A, ValentimJ. Molec. Spectrosc. 86, 344 (1981~
Tunable diode laser measurements
47
8. P. Varanasi and G. D. T. Tejwani, JQSRT 12, 849 (1972). 9. D. E. Jennings, Appl. Opt. 17, 2695 (1980). 10. G. Restelli and F. Capellani, J. Mol. Spectrosc. 78, 161 (1979). ll. P. Varanasi and F. K. Ko, JQSRT2$, 307 (1981). 12. R. D. Blatherwick, A. Goldman, B. L. Lutz, P. M. Silvaggio,and R. W. Boese, Appl. Opt. 18, 3798 (1979). 13. B. L. Lutz, Paper ME3 presented at the 36th symposium on Molecular Spectroscopy, The Ohio State University, Columbus, Ohio (June 1981). 14. A. G. Robiette, J. Mol. Spectrosc. g6, 143 (1981) and the references therein. 15. J. S. Margolis (private communication). 16. V. Malathy Dcvi, B. Fridovich, G. D. Jones, D. G. S. Snyder, P. P. Das, J.-M. Flaud, C. Camy-Peyret, and K. Narahari Rao, J. Molec. Spectrosc. 93, 179 (1982). 17. V. Malathy Devi, B. Fridovich, G. D. Jones, D. G. S. Snyder, and A. Neuendortfer, Appl. Opt. 21, 1537 (1982). 18. P. A. Jansson and C. Laurence Korb, JQSRT 8, 1399 (1968). 19. G. D. T. Tejwani and P. Varanasi, J. Chem. Phys. 55, 1075 (1971). 20. G. D. T. Tejwani and K. Fox, J. Chem. Phys. 60, 2021 (1974). 21. G. D. T. Tejwani, P. Varanasi, and K. Fox, JQSRT 15, 243 (1975).
0022-4073/83/010045-03503.00/0 Pergamon Press Ltd.
Printed in Great Bd"rain.
TUNABLE DIODE LASER MEASUREMENTS OF INTENSITIES AND LORENTZ BROADENING COEFFICIENTS OF LINES IN THE ~'2 BAND OF 12CH4 V. MALATHYDEVI,B.
FRIDOVICH,D. G. S. SNYDER,and G. D. JONES
National Oceanic and Atmospheric Administration, National Earth Satellite Service, Washington, 20233, U.S.A.
and PALASH P. DAS Tachisto Inc., 13 Highland Circle, Needham, MA 02194, U.S.A.
(Received 24 February 1982) Al~tract--Absolute line strengths and self-, nitrogen-, and air-broadened half-widths, for five lines in the R-branch of the ~,~fundamental of methane have been measured at room temperature using a tunable diode laser spectrometer. INTRODUCTION
An extensive literature of the ~'3,2z,3, 3;,3, and ~4 bands of methane exists; H~ however, studies of the ~2 band are sparse./2-15 We may attribute this situation to the fact that the ;'2 band is a forbidden band, which becomes active because of Coriolis interactions between the ~'3 and ~4 bands and hence is weak. The v2 band also falls in the region of the strong 6.3 tLm H20 band. In order to study this band experimentally, both high resolution and high sensitivity techniques are needed. An atlas of methane in the region from 1200 to 1800 cm -1, obtained by using a Nicolet Fourier Transform Spectrometer at 0.06 cm -~ resolution, was compiled by Blatherwick et al. 12 Robiette, ~4 using experimental data from various sources, has analyzed the ~'2 band and reported line assignments, molecular constants, and relative strengths for the ;'2 band of t2ca4. In the present work, we report the first experimental measurements on absolute line intensities and on self-, nitrogen-, and air-broadened half-widths for five lines in the 1,2 R-branch of J2CH4, using a diode laser spectrometer. EXPERIMENTAL DETAILS
The diode laser spectrometer used in this work has been described elsewhere/6 A diode laser operating from 1570 to 1590 cm -1 was used. For line strength and foreign gas broadening measurements, a double pass cell with a total length of 9 m was used. For self-broadening measurements, shorter cells were used. Pressure measurements were made using an MKS Baratron gauge with heads in the 0--1, 0--10, and 0-100 torr ranges. Ultra-high purity methane, nitrogen, and air purchased from the Matheson Company were used without further purification. LINE STRENGTHS
The five lines measured were located spectrally in the operating range of the diode laser used in our previous s t u d i e s ) 6'17 The wavenumbers and assignments for these lines were taken from the work of Margolis) 5 Sample pressures ranged from 0.4 to 1 torr. Three or more pressures were used for each absorption line, and several repetitive scans were made at each pressure. Line strengths were determined from equivalent widths and were measured on strip chart records in the standard way by linear interpolation in the tables of Jansson and Korb. 18 The results are presented in Table 1. Column headings are as follows; ;' is the wavenumber (cm -I) of the transitions involved; J is the total angular momentum; C refers to the irreducible representations AI, A2, E, Ft or F2 of the symmetry group To; N characterizes the different states that have the same values of J and C; single and double primes refer to upper and lower states, respectively. In order to compare our experimentally determined line strengths with the calculated relative peak intensities of Robiette ~4 (listed in Table 1), we normalized both sets of values to the first line listed. The two sets of ratios are listed in Table 1. The agreement is good. 45
V. M. DEv[ et al.
46
Table 1. Line strengths for some ~,_,R-branch lines of '2CH~at 296 K. J'
C'
N'
J"
C"
N"
(cm-))
~ ( c m -2
Relative a t m -1 )
peak
Normalized
i n t e n s i t y i;
Present
Intensities
Work
Robiette
1586.9466
5
FI
3
4
F2
I
0.00180+0.00003
0.03544
1.00
1587.8341
5
FI
2
4
F2
1
0.00339+0.00006
0.06812
1.88
1.92
|588.0310
5
E
I
4
E
I
0.00251*0.00003
0.05111
1,39
1.44
1588.3281
5
F2
2
4
F1
~
0.00216+0.00004
0.04479
1.20
1.26
1589.7348
5
A2
1
4
AI
I
0.00444+0.00010
0.0891(]
2.47
2.51
Uncertainties 11 F o r
details
quoted
are
see Ref.
14.
one
standard
1.00
deviation.
LINE WIDTHS AND SHAPES The half-widths of these five lines were measured at room temperature with various pressures of CH4 or CH4-N2 and CH4-air mixtures to derive the Lorentz broadening coefficients bcu4-cH,, 0 0 0 bCH4-N2 and bcH4-air. Self-broadening measurements were made using a pyrex cell 30.59 cm long with pressures from 30 to 50 tore For the nitrogen- and air-broadened measurements, the partial pressures of methane ranged from 0.5 to 1.2 torr while the broadener pressures ranged from 25 to 50 torr. We assumed that the absorption lines had a Voigt profile and measured the half-width by. The technique for extracting the broadening parameters from by has been described in detail elsewhereJ 6": The measurements were repeated for three or more pressures of CH4, CH4-N2 or CH4-air for each line. At least six repetitions were made at each pressure. The results are presented in Table 2. Since the Lorentz broadening coefficients in the present study belong to one multiplet, no comparison could be made with other multiplets. However, even within a multiplet, strong dependencies exist among the various symmetry species. Similar symmetry-dependent half-widths were reported earlier by Tejwani et al) 9-zt for the ,~ and 2 v4 bands of methane.
Table 2. Self-, nitrogen- and air-broadeningcoefficients for some ,2 band lines of '2CH4 at 296 K, b" CH4-CH4
b° C H 4 -N~
(cm-latm -I )
(cm -] a r m -I )
( :m-latm -] )
1586.9466 "
0.085+0.001
0.068+0.002
0.068+0.001
1587.8341
0.069+0.002
0.054*0.000
0.054+0.001
1588.0310
0.065+0.001
0.056+0.001
0.053+0.001
(cm -I )
4
-Air
1588.3281
0.067+0.003
0.058+0.003
0.057+0.001
1589.7348
0.072+0.004
0.069+0.002
0.068+0.002
}. Assignments
arc
civeh
%[Incertainti~,~s q u o t e d
1. 2. 3. 4. 5. 6. 7.
CH
]n T a b l e ,ire o n e
]1.
:6tatldard
deviation.
REFERENCES P. Varanasi, JQSRT 11, 1711 (1971) and the cited references. J. S. Margolis, JQSRT 13, 1097 (1973). L. Darnton and J. S. Margolis, JQSRT 13, 969 (1973). P. Varanasi, JQSRT 15, 281 (1975). M. Dang-Nhu, A. S. Pine, and A. G. Rohiette, J. Molec. Spectrosc. 77, 57 (1979). K. Fox, G. W. Halsey, S. J. Daunt, W. E. Blass, and D. E. Jennings, J. Chem. Phys. {Letters) 72, 4657 (1980). S. Gherissi, A. Henry, M. Loete, and A, ValentimJ. Molec. Spectrosc. 86, 344 (1981~
Tunable diode laser measurements
47
8. P. Varanasi and G. D. T. Tejwani, JQSRT 12, 849 (1972). 9. D. E. Jennings, Appl. Opt. 17, 2695 (1980). 10. G. Restelli and F. Capellani, J. Mol. Spectrosc. 78, 161 (1979). ll. P. Varanasi and F. K. Ko, JQSRT2$, 307 (1981). 12. R. D. Blatherwick, A. Goldman, B. L. Lutz, P. M. Silvaggio,and R. W. Boese, Appl. Opt. 18, 3798 (1979). 13. B. L. Lutz, Paper ME3 presented at the 36th symposium on Molecular Spectroscopy, The Ohio State University, Columbus, Ohio (June 1981). 14. A. G. Robiette, J. Mol. Spectrosc. g6, 143 (1981) and the references therein. 15. J. S. Margolis (private communication). 16. V. Malathy Dcvi, B. Fridovich, G. D. Jones, D. G. S. Snyder, P. P. Das, J.-M. Flaud, C. Camy-Peyret, and K. Narahari Rao, J. Molec. Spectrosc. 93, 179 (1982). 17. V. Malathy Devi, B. Fridovich, G. D. Jones, D. G. S. Snyder, and A. Neuendortfer, Appl. Opt. 21, 1537 (1982). 18. P. A. Jansson and C. Laurence Korb, JQSRT 8, 1399 (1968). 19. G. D. T. Tejwani and P. Varanasi, J. Chem. Phys. 55, 1075 (1971). 20. G. D. T. Tejwani and K. Fox, J. Chem. Phys. 60, 2021 (1974). 21. G. D. T. Tejwani, P. Varanasi, and K. Fox, JQSRT 15, 243 (1975).