Laser studies of energy- and charge-transfer dynamics

Abstract:

This thesis presents the results of three experiments which use lasers to investigate energy-transfer and charge-transfer dynamics. The dynamical processes studied include nanosecond vibrational energy transfer in molecules, subpicosecond electron relaxation in semiconductors, and subpicosecond initiation of surface bimolecular reactions on a metal crystal. In experiments using time-resolved coherent Raman spectroscopy to probe infrared multiphoton excited molecules, we study CO 2-laser excited SO2 and SF6. In SO2 we observe direct n1-mode excitation and distinguish between this process and excitation of the nearly resonant n2-mode overtone. In SF6, we directly observe n3-mode excitation followed by collisional energy redistribution to a heat bath of non-pumped modes. Quantitative modeling of the SF6 spectra yields excited vibrational population distributions and resolves some long-standing inconsistencies between different previously published reports. In an experiment using time-resolved photoelectron spectroscopy, we observe the subpicosecond evolution of an optically-excited nonequilibrium electron distribution in silicon. We observe an electron thermalization time of less than 120 fs, electron equilibration with the lattice in 1 ps, and an energy-dependent electron cooling rate consistent with published calculations of the electron-phonon scattering rate. The results indicate the formation, in 1 ps, of a surface space-charge electron layer with an electron density two orders of magnitude greater than the bulk electron density. In an experiment using 100-fs laser pulses to induce desorption of O2 and reaction of O2+CO to form CO2 on a Pt(111) surface, we present desorption and reaction data obtained over an absorbed fluence range of 1- 20 mJ/ cm2 at wavelengths of 800, 400, and 266 nm. We observe a highly nonlinear desorption and reaction yield fluence dependence; the data are fit by a power law model in which the yield is proportional to fluence to the power p = 5.9 and 3.8 for the 800 nm and 400 nm data respectively. The ratio of O2 to CO2 desorption is found to be 14:1, 12:1 and 3:1 at 800, 400, and 266 nm respectively. At 800 nm, the desorption and reaction are independent of laser pulsewidth in the range 100 fs to 3.6 ps. Finally, this thesis describes the design, development and operation of new equipment used for the surface reaction experiment: a state-of-the- art amplified femtosecond Ti:sapphire laser, and an ultrahigh-vacuum surface- science chamber.