FEMTOSECOND STUDIES OF THE PHASE-TRANSITION IN TI2O3

We report femtosecond time-resolved pump-probe DECP experiments using a colliding pulse mode-locked laser performed on Ti2O3 as the sample l attice temperature T-L is raised from 300 K through; the ''soft transi tion'' at 450 K to a temperature of 570 K. We have observed DECP spect ra through the transition, with oscillations in reflectivity of a few percent associated with the low frequency A(1g) mode. A thermodynamic relation is found between the low frequency A(1g) equilibrium displace ment and the number of excited electrons removed from the valence band . When applied to T-L-dependent equilibrium coordinate data for Ti2O3 obtained in x-ray experiments, the theory allows a determination of th e band overlap vs lattice temperature. The band overlap at T-L = 621 K is found to be similar to 0.06 eV. The A(1g) mode frequency nu(ph), t he electronic relaxation rate (1/tau(el)), and the phonon relaxation r ate (1/tau(ph)) have all been followed through the transition. nu(ph) decreases, and shows a partial recovery in agreement with other Raman studies. The behavior of (1/tau(el)) can be understood as due to an in crease in available states for interband electron-phonon scattering as the band crossing takes place. Applying a deformation potential model to the data for (1/tau(el)) before band crossing, with the low freque ncy A(1g) mode as the dominant scattering mechanism, a value of /D/ co ngruent to 2.0 eV is obtained for the valence band deformation potenti al associated with this mode. (1/tau(ph)) does not show a clearcut cor relation with bandcrossing due to greater scatter in the data. The tem perature dependence is partially explained by the two-phonon decay of the coherent phonon excited in DECP, and may also have a component due to interaction with hot electrons as well as a dephasing contribution .