PRESSURE BROADENING PARAMETERS IN VIBRATION-ROTATION SPECTRA BY ANALYSIS OF LINESHAPES OF LASER-STARK TRANSITIONS

$^{1}$M. R\""{o}mheld, private communicationAuthor Institution:At the lowest pressures (1-10 mtorr) the lineshapes of laser Stark transitions in molecules with predominantly first-order Stark effects are Gaussian functions of the applied electric field. The widths of such transitions depend on the Doppler width for the sample and the derivative of the frequency of the transition with respect to the Stark field. As the pressure is increased the transitions are pressure broadened and the lineshapes become Voigt profiles. We have used a minicomputer to control the electric field in a laser Stark spectrometer and to record the output of the spectrometer as a function of field. For a variety of transitions in $CH_{3} F$ and $CH_{3}CN$ at low pressure we have shown that the lineshape is well represented by a Gaussian function with the expected width. The relative intensities of transitions at different fields for a single laser line closely match the predicted values. The $J^{\prime}=2, K^{\prime}=1, M^{\prime}=1 \leftarrow J^{\prime\prime}=1, K^{\prime\prime}=1, M^{\prime\prime}=1$ transition in $CH_{3}CN$ is resonant with the p(20) line of the $N_{2} O$ laser at low enough Stark $field^{1}$ that the transition can be recorded at pressures up to 1 torr. Analysis of the Voigt profiles obtained at several pressures leads to a preliminary value of the pressure-broadened half-width at half height of 65 MHz/torr

INFRARED-MICROWAVE TWO-PHOTON SPECTROSCOPY OF THE ν2\nu_{2} BAND OF NH3NH_{3} AND THE ν3\nu_{3} BAND OF CH3F.∗CH_{3}F.^{*}

$^{*}$This work was supported by the National Science Foundation. $^{1}$ S. M. Freund and T. Oka, Phys. Rev. A13, 2178 (1976). $^{2}$ H. Jones, Appl. Phys. 15, 261-264 (1978). $^{3}$ R. L. Poynter and R. K. Kakar, Astrophysical J. Supp. Ser. 29, 87 (1975). $^{4}$ S. Urban, V. Spirko, D. Papousek, R. S. McDowell, N. G. Nereson, S .P. Belov, L. I. Gershstein, A. V. Maslovsky, A. F. Krupnov, J. Curtis and K. Narahari Rao, J. Mol. Spectrosc. 79, 455-495 (1980).Author Institution:A computer-controlled infrared-microwave two-photon spectrometer with sample cells outside the laser cavity has been used to record two-photon transitions in the $\nu_{2}$ bands of $^{14}NH_{3}$ and $^{15}H_{3}$ and in the $\nu_{3}$ band of $CH_{3}F$. By fitting the lineshapes to an approximate lineshape expression, it has been found possible to determine the line centers to a precision of a few tenths of a MHz (\sim 0.00001 \mbox{cm}^{-1}). The absolute accuracy of the frequencies is approximately 10 times poorer as a result of fluctuations in the frequency of the laser stabilized to the maximum of the laser gain curve. Because of the broad-band characteristics of the sample cells and phase-locked microwave sources, the spectra are close to theoretical lineshapes. The $^{14}NH_{3}$ data have been fit separately and together with previous two-photon $data^{1,2}$ to a simple rotation vibration Hamiltonian. Inversion frequencies in the lower state obtained by microwave $spectroscopy^{3}$ and in the upper state obtained by millimeter-wave $spectroscopy^{4}$ were assumed. The results of a number of fitting procedures will be reported. The two-photon frequencies obtained for $CH_{3}F$ and the parameters obtained by fitting to a simple Hamiltonian will also be presented