TIME-RESOLVED CARS OF THE VIBRON LINE-SHAPE OF CRYSTALLINE NITROGEN AT LOW-TEMPERATURE AND HIGH-PRESSURE

Temperature-and pressure-dependent vibrational dephasing results for t he alpha, gamma, and epsilon phases of crystalline nitrogen are obtain ed by time-resolved CARS measurements at high pressure (0-36 kbar) and low temperature (5-35 K). The very small homogeneous contribution to the vibrational line width in crystalline nitrogen at low temperature leads to vibronic dephasing that is dominated by weak inhomogeneous ef fects. Specifically, crystalline nitrogen cannot be cooled to low temp erature without experiencing polymorphic solid-state phase transitions , For example, the beta-N-2 --> alpha-N-2 phase transition at ambient pressure, or the beta-N-2 --> gamma-N-2 transition between 4 and 21 kb ar, may generate defects leading to an inhomogeneous distribution of v ibronic site energies. In each case, crystalline beta-N-2 is formed st arting from the supercritical fluid, by either cooling or compression. The negligible homogeneous contribution to the vibron line width for nitrogen at low temperature (<10 K) allows one to probe details of the extremely small inhomogeneous contributions to the vibron line shape. At higher temperature the vibronic dephasing rate increases and is an alyzed in terms of a dominant quartic dephasing process involving ther mally populated optical phonons.