SUPPRESSION OF VIBRON STATE FORMATION IN ARX(N2)1-X MIXED-CRYSTALS

The dephasing of the k congruent-to 0 states of the vibrons in Ar(x)(N2)1-x crystals was studied by picosecond time-resolved coherent anti-Stokes Raman scattering as a function of temperature (0 < T < 40 K) and Ar concentration (0 < x < 0.15). Homogeneous (exponential) and inhomogeneous (Gaussian) contributions to the observed dephasing signal were separated. In the alpha phase, an increase of the inhomogeneous linewidth was obtained starting from small concentrations of Ar atoms. For larger Ar concentrations, a decrease of the exponential dephasing time T2 and a temperature dependence of the increasingly faster Gaussian decay is observed. Substitution of N2 molecules by Ar atoms increases the librational amplitude. Through anharmonic terms in the crystal Hamiltonian the vibron state formation is reduced and as a result, the T2 time decreases. This suppression of vibron state formation, which counteracts the reduction of the inhomogeneous broadening, is responsible for the increasing and temperature dependent inhomogeneous linewidth. The A(g) - T(g) factor group splitting increases almost linearly with Ar concentration. Surprisingly, no inhomogeneous line broadening could be resolved for beta-Ar(x)(N2)1-x crystals and the observed dephasing time T2 remains long compared to the alpha phase, even for high Ar concentrations.