This work is aimed to study of the ν1 band of the methane Raman spectrum in the pressure range of 1–80 atm. The wavenumber calibration was performed using the rotational–vibrational structure of the ν3 methane band. It was established, that pressure shift coefficient is about –0.02 cm–1 /atm, pressure broadening coefficient is about 0.005 cm– 1 /atm. According to the obtained experimental data, in the region of 2914–2916 cm–1 , with an increase in pressure, either the van der Waals methane complexes begin to make a noticeable contribution to the intensity of the ν1 band, or collision–induced Raman scattering increases in this range.
In the range of 2500–3500 cm –1 of methane Raman spectrum there are two bands (2ν4 and 2ν2) which are under Fermi resonance with an intense ν1 band. It was found that when the pressure changes from 1 to 55 atm, the peak intensity of the 2ν4 band increases by ~ 9% and peak intensity of the 2ν2 band increases by ~ 6.5% relative to the ν1 intensity. An analysis of the integrated intensities showed that in this pressure range the ratio 2ν4/ν1 increases by ~ 7%, the ratio 2ν2/ν1 (taking into account the additional contribution of the lines of the ν3 band) increases by ~ 0.7%.
The methane absorption spectra in the range between 6600 and 9200 cm-1 have been recorded at 296 K and 109 K by the Fourier spectrometer IFS – 125M at pressures from 11 to 300 mbar, and spectral resolution 0.03 cm-1. Low temperature cell with a length of 220 cm was used, which provided the threshold sensitivity to absorption of the order of 10-7 cm-1. Empirical values of the lower state energy level were derived from the intensity ratios of the lines measured at 200 K and 296 K. Line assignment of the the 3ν3 band of the 12CH4 molecule was performed. The parameters of Voigt, SDV, HTP contours (the line center, intensity) were determined at 296K.
We report results on comparative study of SHG in powder of promising nonlinear γ-Ga2S3 crystal. Digallium trisulfide powders with particle size from 20 μm to 500 μm were tested in comparison with powders of well-known LBO, BBO, KABO, KDP, and LN crystals under the pumping by 7 ns 1064 Nd:YAG laser. Laser-induced damage threshold of different powder fractions were determined. The γ-Ga2S3 shown high damage threshold and large SHG intensity: 56 times to that in LBO powder, 15 in BBO, 50 in KABO, 67 in KDP, and 3 in LN (for particle size: 20–50 μm), that renders it amongst the most promising crystal for frequency conversion of high-intense nanosecond radiation of near-IR lasers by optical rectification technique.