Spectral line shape profile of rovibrational transitions of CO embedded in p-H2 crystals studied by high resolution IR diode laser spectroscopy

Line profiles of rovibrational transitions of CO embedded in p-H-2 crystals were studied by high resolution midinfrared diode laser spectroscopy. The line profile analysis for the R(0)(parallel to), R(0)(perpendicular to), P(1)(parallel to), and P(1)(perpendicular to) transitions shows that spectral line shapes are well reproduced by a convolution of Gaussian and Lorentzian functions. The temperature dependence of the Lorentzian Gamma(L)(T) and Gaussian widths Gamma(G)(T) shows that there is a nonzero linewidth contribution to each at the T=0 K limit. The main part of the Lorentzian width Gamma(L)(T=0) shows anisotropy in the hcp structure and is explained by spontaneous decay of the rotational excited state energy to phonon modes. A smaller part of Gamma(L)(T=0) is attributed to inhomogeneous broadening due to the point defects of other CO molecules in the crystal. On the other hand, the Gaussian width Gamma(G)(T=0) is explained by inhomogeneous broadening due to dislocations. In the T>0 region, Gamma(L)(T) shows strong temperature dependence but Gamma(G)(T) does not. The center frequencies of the R(0)(perpendicular to) and P(1)(parallel to) transitions show blueshifts and those of the R(0)(parallel to) and P(1)(perpendicular to) transitions show redshifts with increasing temperature. This phenomenon is explained by a decrease in the anisotropy in the crystal field, which is caused by the averaging of thermal lattice fluctuations. Furthermore, the contribution of vibration and rotation to the linewidth is discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3429252]