DEPHASING TIMES OF THE VIBRONS IN ALPHA-N-2 AND IN ALPHA-(N-15(2))X (N-14(2))1-X MIXED-CRYSTALS

Picosecond time-resolved coherent anti-Stokes Raman spectroscopy measurements were performed on the vibrons in single crystals of -N2 and on mixed crystals of -(15N2)x(14N2)1-x with 0x0.086. The observed dephasing of the vibrons in pure -N2 crystals is faster than exponential, and this is well described by adding a Gaussian factor. The Gaussian decay component is assigned to inhomogeneous broadening and could be observed as a result of the high time resolution of our setup. This indicates that the approximations often employed in the case of weak disorder are not applicable in the short-time region of the decay. The exponential dephasing is ascribed to scattering from anharmonic librons and low-frequency phonons. The Ag-Tg factor group splitting and its temperature dependence were determined through analysis of the beating pattern on the spectra, with an accuracy up to 0.007 cm-1. In the isotopically mixed crystals, it is shown that the N215 molecules do not act as trapping center for the N214 vibrons. The homogenous dephasing time for the N215 molecules is surprisingly short compared to the N214 decay. © 1990 The American Physical Society.