Calibration of the absorption spectra at 14–15 µm obtained with a tunable diode laser
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Calibration of the absorption spectra at 14-15 μm obtained with a tunable diode laser John S. Knoll, Gregory L. Tettemer, Walter G. Planet, K. Narahari Rao, Da-Wun Chen, and L. A. Pugh The first three authors named are with NOAA, National Environmental Satellite Service, Washington, D.C. 20233; the other authors are with Ohio State University, Physics Department, Columbus, Ohio 43210. Received 18 August 1976. In recent years the technological advances made with tun- able diode lasers have enabled the observation of the rota- tional structure of vibration-rotation bands with very high resolving power. Half-value widths of about 0.002 cm -1 have been clearly measured on resolved absorption lines. For in- stance, the Q branch of the ν 2 band of 12 C 16 O 2 at 15 μm (667 cm -1 ) has been completely resolved 1,2 (see also Fig. 1). Likewise the structure of the 2v 2 -v 2 band Q branch of the same molecule observed at a slightly longer wavelength (618 cm -1 ) was recorded, and molecular constants were evaluated from the data. 3 Again, the observation of the fine structure split- tings of the ν 3 band of the SF 6 molecule at about 10.5 μm (948 cm -1 ) was possible 4 because of the superior spectral resolution capability of the technique. It appears, therefore, that one should be able to make studies in the entire stretch from 10 μm to 15 μm by employing tunable diode laser spectrometers. Although this technique has been demonstrated to work in the near ir also, mention is made here of this longer wavelength spectral region because it is an important region for investi- gations of the low frequency vibration-rotation fundamentals of polyatomic molecules; and the information obtainable from their study is useful for structural determinations. Obviously, Table I. Wavenumbers (expressed in cm -1 -vac) of the v, Band of 12 C 16 O 2 α α Molecular constants (in cm" 1 ) used for calculating the above line positions are: Fig. 1. (a) The rotational structure of the Q branches of the carbon dioxide bands at 15 μm along with the Fabry-Perot etalon fringes. Pathlength: 20 cm; pressure: 0.231 Torr; temperature: 298 K; etalon: Ge, 2.54 cm (1.0 in.) thick. (b) Spectrum of CO 2 and HCN mixture at 15 μm. Pathlength: 10 cm; estimated partial pressures: CO 2 = 0.6 Torr, HCN = 0.3 Torr; temperature: 298 K. Half-value width = 0.002 cm -1 . The full width at half power is indicated as 0.0015 cm -1 . Doppler width is calculated to be about 0.00125 it is necessary to consider the different possibilities for wavenumber calibrations of the spectra obtained by this technique. One of the procedures that is being used is to in- sert a germanium flat into a collimated portion of the optics to impress a set of Fabry-Perot fringes of known spacing on the ir spectrum. This provides an accurate wavenumber scale the absolute position of which is established with reference to absorption lines of known wavenumbers. The ν 2 funda- mentals of both the CO 2 and HCN molecules in the 14-15-μm region can be observed easily by using relatively short path- lengths and low pressures. They are conveniently located to serve as absorption standards. The present experiment has been designed to examine the current status of the wave- number data available for these bands by employing a tunable diode laser spectrometer located at the Satellite Experiment Laboratory, NOAA, Suitland, Maryland. (This instrument was developed by Laser Analytics, Inc., 38 Hartwell Avenue, Lexington, Massachusetts.) In this work, a spectral interval of about 1 cm -1 in the region of the Q branch of the ν 2 band of 12 C 16 O 2 at 667 cm -1 was scanned. The P(15) line of the ν 2 band of HCN also occurs in this wavenumber interval. In Fig. 1 the top portion (marked A) shows the Q branch lines of CO 2 , and the bottom portion (marked B) shows a portion of the scan made with CO 2 and HCN kept in the same December 1976 / Vol. 15, No. 12 / APPLIED OPTICS 2973
Transcript of Calibration of the absorption spectra at 14–15 µm obtained with a tunable diode laser
Calibration of the absorption spectra at 14-15 μm obtained with a tunable diode laser John S. Knoll, Gregory L. Tettemer, Walter G. Planet, K. Narahari Rao, Da-Wun Chen, and L. A. Pugh
The first three authors named are with NOAA, National Environmental Satellite Service, Washington, D.C. 20233; the other authors are with Ohio State University, Physics Department, Columbus, Ohio 43210. Received 18 August 1976. In recent years the technological advances made with tun
able diode lasers have enabled the observation of the rotational structure of vibration-rotation bands with very high resolving power. Half-value widths of about 0.002 c m - 1 have been clearly measured on resolved absorption lines. For instance, the Q branch of the ν2 band of 12C16O2 at 15 μm (667 cm - 1) has been completely resolved1,2 (see also Fig. 1). Likewise the structure of the 2v2-v2 band Q branch of the same molecule observed at a slightly longer wavelength (618 cm - 1) was recorded, and molecular constants were evaluated from the data.3 Again, the observation of the fine structure splittings of the ν3 band of the SF6 molecule at about 10.5 μm (948 cm - 1) was possible4 because of the superior spectral resolution capability of the technique. It appears, therefore, that one should be able to make studies in the entire stretch from 10 μm to 15 μm by employing tunable diode laser spectrometers. Although this technique has been demonstrated to work in the near ir also, mention is made here of this longer wavelength spectral region because it is an important region for investigations of the low frequency vibration-rotation fundamentals of polyatomic molecules; and the information obtainable from their study is useful for structural determinations. Obviously,
Table I. Wavenumbers (expressed in cm-1-vac) of the v, Band of 12C16O2
α
α Molecular constants (in cm"1) used for calculating the above line positions are:
Fig. 1. (a) The rotational structure of the Q branches of the carbon dioxide bands at 15 μm along with the Fabry-Perot etalon fringes. Pathlength: 20 cm; pressure: 0.231 Torr; temperature: 298 K; etalon: Ge, 2.54 cm (1.0 in.) thick. (b) Spectrum of CO2 and HCN mixture at 15 μm. Pathlength: 10 cm; estimated partial pressures: CO2 = 0.6 Torr, HCN = 0.3 Torr; temperature: 298 K. Half-value width = 0.002 cm-1. The full width at half power is indicated as 0.0015 cm-1. Doppler width is calculated to be about 0.00125
it is necessary to consider the different possibilities for wavenumber calibrations of the spectra obtained by this technique. One of the procedures that is being used is to insert a germanium flat into a collimated portion of the optics to impress a set of Fabry-Perot fringes of known spacing on the ir spectrum. This provides an accurate wavenumber scale the absolute position of which is established with reference to absorption lines of known wavenumbers. The ν2 fundamentals of both the CO2 and HCN molecules in the 14-15-μm region can be observed easily by using relatively short path-lengths and low pressures. They are conveniently located to serve as absorption standards. The present experiment has been designed to examine the current status of the wave-number data available for these bands by employing a tunable diode laser spectrometer located at the Satellite Experiment Laboratory, NOAA, Suitland, Maryland. (This instrument was developed by Laser Analytics, Inc., 38 Hartwell Avenue, Lexington, Massachusetts.) In this work, a spectral interval of about 1 c m - 1 in the region of the Q branch of the ν2 band of 12C16O2 at 667 c m - 1 was scanned. The P(15) line of the ν2 band of HCN also occurs in this wavenumber interval.
In Fig. 1 the top portion (marked A) shows the Q branch lines of CO2, and the bottom portion (marked B) shows a portion of the scan made with CO2 and HCN kept in the same
December 1976 / Vol. 15, No. 12 / APPLIED OPTICS 2973
absorption cell, which is about 10 cm in length. It was estimated that this mixture consists of 0.6 Torr of CO2 and 0.3 Torr of HCN. Since the HCN line is well placed with respect to the CO2 lines, a measurement of the HCN line was made with respect to the CO2 lines.
Tables I and II give the wavenumbers (expressed in cm - 1 ) of the v2 band lines of 12C16O2 and HCN. They are calculated values obtained by making use of the available molecular
Table I I . Wavenumbers (expressed in cm-1-vac) of the ν2 Band of H12C14Nα
α Molecular constants (in cm-1) used for calculating the above line positions are:
2974 APPLIED OPTICS / Vol. 15, No. 12 / December 1976
constants. For 12C16O2, the vibrational constants were taken from the set5 derived by Chedin and Cihla, who made extensive theoretical calculations for determining an accurate potential energy curve for this molecule by including all the observed energy levels. (A. Chedin and Z. Cihla have been associated with CNRS, Orsay, France; Ref. 5 is a review article which quotes their latest constants.) The rotational constants for CO2 for these evaluations were obtained from the most recent experimental laboratory work.6 A similar analysis of the available constants7,8 for the HCN molecule resulted in the wavenumbers given in Table II. Five repetitive scans were obtained of the spectra illustrated in the bottom portion of Fig. 1. The average wavenumber of the P(15) line of the ν2 of HCN calculated by using the data for 12C16O2 given in Table I was 667.6323 cm - 1 , as compared with 667.6315 c m - 1 appearing in Table II. Although the difference between these two values is only 0.0008 cm - 1 , which is within the uncertainty of the available HCN or CO2 data, it would be useful to be more definitive because the spectral resolution available definitely can allow us to make measurements to an accuracy of at least ±0.0002 cm - 1 . This is, in fact, the accuracy with which the HCN line could be located, as noted from the reproducibility within the five repetitive scans made. New measurements of the ν2 band lines of HCN and CO2 would be desirable to improve the wavenumbers given in Tables I and II to reap the full benefit of the available spectral resolution, which is quoted9 to be at least 0.0001 c m - 1 for tunable diode laser spectrometers.
References 1. W. G. Planet, J. R. Aronson, and J. F. Butler, J. Mol. Spectrosc. 54,
331 (1975). 2. J. R. Aronson, P. C. von Thüna, and J. F. Butler, Appl. Opt. 14,
1120 (1975). 3. J. P. Aldridge, R. F. Holland, H. Flicker, K. W. Nill, and T. C.
Harman, J. Mol. Spectrosc. 54, 328 (1975). 4. J. P. Aldridge, H. Filip, H. Flicker, R. F. Holland, R. S. McDowell,
and N. G. Nereson, J. Mol. Spectrosc. 58, 165 (1975). 5. K. Narahari Rao, High Resolution Infrared Spectroscopy: As
pects of Modern Research in Physical Chemistry Series Two, D. A. Ramsay, Ed. (Butterworths, London, 1976), Vol. 3.
6. T. K. McCubbin, Jr., J. Pliva, R. Pulfrey, W. Telfair, and T. Todd, J. Mol. Spectrosc. 49, 136 (1974); For a summary of earlier work, see review by K. Narahari Rao in "Exploration of the Planetary System" A. Woszczyk and C. Iwaniszewska, Eds. (D. Reidel Publishing Co., Holland, 1974) ρρ 41-56.
7. P. K. L. Yin and K. Narahari Rao, J. Mol. Spectrosc. 42, 385 (1972).
8. A. G. Maki, J. Mol. Spectrosc. 58, 308 (1975). 9. E. D. Hinkley and C. Freed, Phys. Rev. Lett. 23, 277 (1969).
