A. Levy

Noble-gas broadening of vibration-rotation lines belonging to diatomic molecules—I. Experimental results for HCl lineshifts and widths

Abstract Pressure effects in the vibration-rotation spectra of gaseous hydrogen chloride broadened by argon and xenon have been investigated under high perturber pressures with the use of high-resolving power spectrometers, particularly a SISAM. The experimental determination of the instrument function is discussed in detail. Observed shifts and widths are compared with previous results, and their variation with vibrational transitions is determined.

Air-broadened linewidths of nitrous oxide: An improved calculation

Abstract The semiclassical theory developed by Robert and Bonamy was used to obtain the linewidths of N 2 O broadened by itself, by N 2 and by O 2 . The main features of the formalism are as follows: (a) The anisotropic potential is expressed by using, besides the quadrupole-quadrupole contribution, an atom-atom interaction model (without any adjustable parameter) which takes both long- and short-range forces into account. (b) The geometry of the collision is described through the so-called “equivalent” straight path, more appropriate than the usual one. (c) The matrix elements of the relaxation operator are computed by means of the linked-cluster theorem, so that the treatment remains nonperturbative and no resort to cutoff precedures is needed. In addition to being more realistic the present formalism has the advantage of making the computation tractable for complex molecular systems such as linear-linear ones. Careful comparison was made with the available experimental results. For self-broadened N 2 O very satisfactory agreement is obtained both at 300 and 204 K. This is also the case for nitrogen broadening at room temperature. Regarding oxygen-broadened linewidths, very few experimental data exist. Anyway, the present results reveal substantial improvement as compared to the usual calculations based upon Anderson-Tsao-Curnutte model. From these results a predictive tabulation was obtained for the values of air-broadened N 2 O linewidths at 300 and 204 K.

Line mixing effects in the Q branch of the 1000←0110 transition of CO2

Abstract Line mixing effects have been investigated in the Q branch of CO 2 near 720 cm −1 , by means of a tunable diode laser. Spectra were obtained at room temperature, in the 100–1000 torr pressure range. In the high-frequency wing of the Q branch, below the Q (2) line, the observed absorption is nearly 50% less than that calculated on the assumption of purely additive Lorentz lines, whereas the Q branch itself is nearly insensitive to line coupling in the pressure range investigated. From the line broadening coefficients alone, a simple scaling law allows an estimate of the state-to-state rotationally inelastic transfer rates. Calculation of line mixing coefficients using these rates yields corrected absorption coefficients, in very good agreement with the observed ones, all over the analyzed spectral range. The results appear to be of importance for modeling outgoing radiances in temperature sounding channels near 14 μm.

A parametric deconvolution method: Application to two bands of N2O in the 1.9-μm region

Abstract A simple deconvolution method, intended for the analysis of moderately dense spectra is presented and its practical setting up discussed. The method has been first tested on synthetic lines that simulated the distortion of a Lorentz profile by a Gaussian distribution. Conditions and limits of applicability of the procedure have been investigated. The procedure has been used to study, at pressure less than 1 atm, the 40°0-00°0 and 32°0-00°0 transitions of N 2 O near 1.9 μm. Line strengths and self-broadened linewidths have been accurately measured and the data were used to obtain the vibrational band intensities and the pure vibrational transition moment for both transitions. The results for the transition moments are respectively 1.212 × 10 −3 and 1.201 × 10 −3 D for the 40°0-00°0 and 32°0-00°0 transitions.

Line strengths and self-broadened linewidths of N2O in the 2-μm region: 2400-0000 and 0112-0000 transitions

The strenghths and self-broadened linewidths of the parallel 2400-0000 and perpendicular 0112-0000 bands of N2O have been measured with a precision better than 3%, using a deconvolution procedure. For both transitions, the coefficient of the vibration-rotation interaction polynomial, the values of the rotationless dipolar transition moment, and the band intensity have been calculated from the line strengths. For the total intensity the values found are S00002400 = (1.325 ± 0.021) × 10−2 cm−2·atm−1 and S00000112 = (1.209 ± 0.018) × 10−2 cm−2·atm−1.

Method for the simultaneous determination of line strengths and collisional widths from high-resolution Fourier transform spectra.

The considerable increase in high-resolution Fourier transform spectra has led to a practical method of deriving individual line parameters from Fourier data. This method concerns in the main those spectra exhibiting moderately dense structure, so that spectral analysis can be carried out in pressure conditions where collisional broadening dominates: (1) For any line under study, the strength and width values can be determined simultaneously from the same spectrum. (2) Instrumental distortions can be reduced by pressurizing the gas sufficiently. In the present case the relevant corrections do not exceed 3%, in a few cases 5%. (3) Thus, from the analytical expression of the apodized apparatus function, it is possible by means of a convolution calculation to draw up a series of tabulations. These give the true linewidth and the dimensionless quantity K(sigma(0))l, respectively, as functions of the width and maximum transmission of the observed contour. The calculation has been extended to the case of doublets of the same intensities, widths, and known spacing. The method has been tested by investigating a few self-broadened lines in the 12 degrees 0-00 degrees 0 transition of N(2)O and some nitrogen-broadened lines in the fundamental band of NO recorded at the laboratory by G. Guelachvili.

Noble-gas broadening of vibration-rotation lines belonging to diatomic molecules—III. Theoretical calculations of CO linesshifts and widths

Abstract The Anderson-Tsao-Curnutte theory is generalized to account for the vibrational dependence of line shifts and widths in the case of noble-gas-broadened HCl lines. Buckinghams intermolecular potential is used and good agreement with experimental values is obtained. Comparison is made with Herman and Tippings semi-empirical potential. The conditions under which the theory should be applied are pointed out, especially cutoff procedures, trajectory shapes, and velocity averaging.

Linewidths and strengths in the Q branch of the 1000 ← 0110 transition of CO2 near 14 μm

Abstract The 10 0 0 ← 01 1 0 transition of CO 2 has been investigated in order to determine accurate values of the line intensities and half-widths in the Q branch. A double-beam TDL mounting was used. Slow scans of the spectra were performed. All measurements were carried out at room temperature for pressures ranging from 5 to 20 Torr in order to avoid overlapping of lines. Eighteen lines were analyzed from Q (6) to Q (48). The transition moment is found to be equal to 0.119 D. The obtained linewidths exhibit a systematic departure of 10–15 × 10 −3 cm −1 atm −1 as compared to the averaged values currently observed so far in a great number of CO 2 parallel bands.

Argon and nitrogen broadening in the pure rotational spectrum of HCl

Abstract The Ar- and N 2 -broadened linewidths have been measured for the first four pure rotational transitions of HCl. The spectra were taken by means of a Fourier transform spectrometer working in the range 10–100 cm −1 with an apodized resolution of 0.05 cm −1 . The results obtained generally agree with those of other authors. Comparison is also made with the most recent results of various theoretical calculations.

Collisional broadening and line intensities in the pure rotational spectrum of PH3

Abstract The H 2 - and He-broadened widths have been measured at room temperature for seven pure rotational lines of phosphine in the spectral range 10–80 cm −1 . With the available resolution of the instrument (≈0.060 cm −1 ) the K structure of lines was not resolved. The retrieval of linewidths was carried out, on the assumption of identical linewidths for the J + 1 K components within a given multiplet J + 1 ← J . The results obtained are in good agreement with the previous determination reported by H. M. Pickett, R. L. Poynter, and E. A. Cohen for the 1 ← 0 line ( J. Quant. Spectrosc. Radiat. Transfer 26 , 197–198 (1981)). The linestrengths were also determined for five of the lines under study. The measured values compare well with the calculated ones, derived by using the recommended value of the dipole moment for PH 3 , μ = 0.574 D.