The thermal instability of a finitely conducting hydromagnetic composite and compressible medium is studied to include the frictional effects with neutrals. The effect of compressibility is found to be stabilizing. In contrast to the nonoscillatory modes for (C_p/g)β > 1 in the absence of a magnetic field;C_v, β andg being specific heat at constant pressure, uniform adverse temperature gradient and acceleration due to gravity respectively, the presence of magnetic field introduces oscillatory modes in the system. The overstable case is also discussed. The magnetic field is found to have a stabilizing effect on the system for (C_p/g)β > 1.

The effects of compressibility, finite Larmor radius (FLR) and Hall currents are considered on the thermal instability of a plasma in the presence of a uniform horizontal magnetic field. For stationary convection, the compressibility has a stabilizing effect whereas FLR and Hall currents have stabilizing as well as destabilizing effects. For (C_pβ/g)<1, the system is stable. The magnetic field, FLR and Hall currents introduce oscillatory modes in the system for (C_pβ/g)>1.

Two-photon optogalvanic transitions in Ar glow discharge with Nd: YAG laser pumped dye laser excitation in the frequency range 13520–16520 cm⁻¹ has been studied using linear and circular polarization. The intensities of two-photon optogalvanic transitions are very sensitive to changes in the incident laser power which is not the case with one-photon transitions. Intensity ratio for circular and linear polarized light for two photon transitions 6s′[1/2]°1←4s[3/2]°2, 6s′[1/2]°0←4s[3/2]°2, and 5d[1/2]°0←4s[3/2]°2, 5d[1/2]°1←4s[3/2]°2 are quite different from the other two-photon transitions. This has been explained as due to near one photon resonance of 4p′[3/2]1 level for the first pair and 4p′[1/2]1 for the second pair of transitions. The ratio of optogalvanic intensity for circular to linear polarized light has been theoretically estimated and compared with the observed results.