Interpretation of the pressure-induced Raman frequency shift of the ν1 stretching bands of CH4 and N2 within CH4–CO2, N2–CO2 and CH4–N2 binary mixtures

The pressure-induced frequency shift of the CH4 and N2 bands is interpreted by quantitatively attributing to the attractive and repulsive solvation mean-force variation using the Lennard–Jones 6-12 potential and the perturbed hard-sphere fluid model.

Statistics of pulse enrgy fluctuations in a solid-state Raman laser

In this paper, we present the results of the study of the statistics of pulse energy fluctuations in a Raman laser under optical pump by the multimode nanosecond pulses. A system of coupled differential equations for slowly varying envelopes of the pump field and first three Stokes lines was integrated numerically with taking into account spatial inhomogeneity of the pump beam, spontaneous noise, and optical feedback. Data of the numerical simulation revealed a sharp increase in the fluctuation amplitude in the nonlinear regime of Raman frequency conversion when the optical length of the Raman cavity was matched with the cavity length of the multimode pump laser. At a mean 1st Stokes conversion efficiency of 3.5–3.8 %, the calculations showed an increase in the coefficient of variation (CV) of a random value from 9 % to 118 %. In the linear regime of Raman frequency conversion, when the conversion efficiency was 0.2–0.03 %, a further increase in the CV value up to 270–500 % was predicted. It is also numerically shown that the fluctuation statistics under the conditions of the cavity length matching is essentially non-Gaussian and described by the L-type probability density distributions (PDDs) with long tails and maxima located near zero. The numerical data were quantitatively confirmed by an experiment for a Raman laser on a barium nitrate crystal operated near the Raman threshold, when the 1st Stokes conversion efficiency did not exceed 0.3 %. A Raman cavity was formed by two flat mirrors providing a double-pass pump configuration. The Raman laser was excited by the linearly polarized frequency-doubled radiation of a Q-switched Nd:YAG laser generating multimode pulses with a duration of 7–8 ns. A Raman laser operating regime characterized by the hyperexponential PDDs with CVs reaching 480 %, which is 2–2.5 times higher than those observed earlier for the single-pass conditions of stimulated Raman scattering, was realized.

Calculation of the thermodynamic functions from the Raman frequency shifts close to the 𝜀–δloc–δ transitions and Pippard relations in nitrogen

Thermodynamic functions of the thermal expansion [Formula: see text], isothermal compressibility [Formula: see text] and the difference in the heat capacity [Formula: see text] are calculated as a function of temperature ([Formula: see text] GPa) close to the transitions of [Formula: see text][Formula: see text]–[Formula: see text][Formula: see text] and [Formula: see text][Formula: see text]–[Formula: see text][Formula: see text] in the solid nitrogen. This calculation is performed by using the observed Raman frequency shifts of vibrons [Formula: see text] and [Formula: see text]. Also, by using the observed [Formula: see text]–[Formula: see text] data, those thermodynamic functions are predicted at various pressures for the fluid–solid transition in nitrogen. For both calculations, observed data are used from the literature. From the temperature and pressure dependences of the thermodynamic functions studied, the Pippard relations are examined close to the [Formula: see text][Formula: see text]–[Formula: see text][Formula: see text][Formula: see text]–[Formula: see text][Formula: see text] transitions and also fluid–solid transition in nitrogen.We find that the thermodynamic functions can be predicted from the Raman frequency shifts and that the Pippard relations can be established for both the [Formula: see text][Formula: see text]–[Formula: see text][Formula: see text][Formula: see text]–[Formula: see text][Formula: see text] and fluid–solid transitions in nitrogen. This method of predicting the thermodynamic functions can also be applied to some other molecular solids.

Raman Spectra of Quartz and Pb4+-Doped SiO2 Crystals at Different Temperature and Pressure

In order to seek for new Raman crystals, the manuscript presented the discussion on the Raman frequency of α-quartz, β-quartz, as well as Pb4+-doped SiO2 crystals at different temperature and pressure. The results demonstrated that as the pressure increased, the Raman lines shifted towards higher frequency for α-SiO2 crystal, while for β-SiO2 crystals, the main lines moved to lower frequency. Several new peaks were generated from the Pb4+-doped α-SiO2 crystals. The Pb4+-doped β-SiO2 crystals presented strong and ample Raman beams in the low-frequency range. The airfoil-shaped spectra appeared and broadened at high temperature and pressure. It could be inferred that the Pb4+-doped SiO2 crystals were excellent Raman crystal candidates.