I. M. Grigoriev

Study of the ν1 band shape of the H2O⋯HF, H2O⋯DF, and H2O⋯HCl complexes in the gas phase

The band shape of the ν1 hydrogen fluoride stretch in H2O⋯HF and H2O⋯DF complexes was studied in the gas phase. The spectra of H2O/HF mixtures at 293 K in cells 20 and 1200 cm long were recorded in the range 4200–3000 cm−1 at a resolution of 0.2−0.02 cm−1. The spectra of the 1 : 1 complex in the region of the ν1(HF) absorption band were obtained by subtracting the calculated spectra of free H2O and HF molecules from the experimental spectra. The asymmetric ν1 band of H2O⋯HF has a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. The ν1 band shape was reconstructed nonempirically as a superposition of rovibrational bands of the ν1(HF) fundamental transition and hot transitions from excited states of low-frequency modes. The reconstruction was based on an ab initio calculation of the potential energy and dipole moment surfaces and subsequent variational multidimensional anharmonic calculations of the vibrational energy levels, the frequencies and intensities of the transitions considered, and the rotational constants. The calculated spectrum reproduces the structure of the experimental spectrum, in particular, the relative intensities of the peaks. However, the assignment of spectral features differs from that generally accepted. The central, most intense, peak is associated with the transition from the ground state, while the lowest-frequency peak with the P branch head of transition from the v6(B2) = 1 state. This leads to a value of 3633.8 cm−1 for the ν1(HF) stretch frequency of H2O⋯HF, which is higher than the commonly adopted value of 3608 cm−1. Similar calculations of H2O⋯DF predict a value of 2689 cm−1 for the ν1(DF) stretch and a less structured band shape. On formation of a 1 : 1 complex with water the frequency is shifted by −331.8 cm−1 and −229.4 cm−1 and the intensity is increased by a factor of 3.87 and 3.51 for HF and DF, respectively. Similar calculations of H2O⋯HCl predicted a value of 2726.5 cm−1 for the ν1 fundamental, a lower frequency for the hot transition from the v6(B2) = 1 excited state, and a ν1(HCl) band shape in agreement with the results of recent low-temperature experiments.

Estimation of line parameters under line mixing effects: the ν3 band of CH4 in helium

Spectra of CH4-He mixtures were measured in the P and R branch regions of nu3 vibration-rotation band at lower pressures of 0.26-1.0 bar and for the whole band at higher pressures up to 90 bar. The line broadening coefficients were found from lower pressure data for the lines of the P branch. These coefficients were calculated in the framework of the Robert and Bonamy semi-classical approach. In general, they agree with the experimental ones, but a small J-dependent deviation was observed. Line mixing effects were observed at all pressures. At higher pressures they were interpreted in terms of the adjusted branch-coupling model. At lower pressures line mixing effects were found to be especially pronounced in the region of the relatively weak lines forming the clusters from R(16) to R(19).

Line parameters and shapes of high clusters: R branch of the ν3 band of CH4 in He mixtures

The IR absorption spectra of CH4 in pure gas and in mixture with helium were studied in the region of nu3 band at higher J line clusters R(17)-R(22). The frequencies and intensities of rotation-vibration lines were estimated from the experimental spectra at Doppler shape conditions. The line frequencies and intensities were calculated and used for the attribution of overlapped lines in clusters. The calculated line intensities are close to the experimental values. The calculated frequency structure of the higher J manifolds are somewhat wider than the observed one. The shapes of helium-broadened line clusters were compared with those calculated accounting for line mixing. The relaxation matrix W, which is necessary in shape calculations, was constructed using semiclassical collision rate constants. The calculated shapes are in satisfactory accordance with the measured ones.

Line-mixing effects in the ν3 parallel absorption band of CH3F perturbed by rare gases

Abstract We have measured the absorption profiles of CH3F mixed with argon and helium at room temperature in the interval 940–1110 cm−1 of the ν3 band. The spectra measured at pressures of 0.25–3 bar were used for evaluation of the line-broadening parameters. At higher pressures up to 20 bar significant deviations from the Lorentz bandshape were detected. At the maximum of the Q branch the absorption is super-Lorentzian, the differences reaching up to 60%. In the lower frequency wing of the P branch the observed absorption is up to five times lower than the Lorentzian one. The discrepancies observed are sensitive to the perturbing molecule, being more pronounced in the case of helium. They are explained by line-mixing effects and interpreted by means of a simple model with a single adjustable parameter.

LINE SHAPES IN THE ROTATIONAL SPECTRA OF HF IN Ar GAS: NEW EXPERIMENTAL DATA AND CALCULATIONS OF LINE INTERFERENCE

Abstract The absorption spectra of HFAr mixtures have been registered in the spectral range of 30–410 cm −1 . The line asymmetry has been detected. The widths, shifts and asymmetry parameters for lines R(0)–R(9) have been obtained. The data are compared with the recently calculated lineshape parameters by Green and Hutson. The 20% discrepancy of the experimental and calculated width cross-sections is observed. The agreement of the experimental and calculated shift cross-sections is satisfactory for the lines at lower frequency but is rather poor for the lines at higher frequency. The observed line asymmetry is explained by the line mixing effect in the framework of semiclassical approach.