Femtosecond infrared spectroscopy and vibrational dynamics in condensed phases and on surfaces

Vibrational dynamics in condensed phases and on surfaces were studied using femtosecond infrared laser spectroscopy. Infrared pump-probe and infrared polarization spectroscopy were used to measure the vibration relaxation times (T$\\sb1$) and rotational reorientation times (T$\\sb{\\rm R}$) of the pseudohalide ions N$\\sb3\\sp{-}$, NCS$\\sp{-}$ and NCO$\\sp{-}$ in various solvents. The importance of the Columbic force in these systems was demonstrated by rapid vibrational relaxation time (a few ps) and slow reorientation. The nature of the force served to simultaneously retain the ion-solvent structure and provide friction on the ions as revealed by the reciprocal correlation between the T$\\sb1$ and T$\\sb{\\rm R}$. The relative interaction strengths with the solvents were determined by comparison of T$\\sb1$ and T$\\sb{\\rm R}$ in corresponding solvents. The role of hydrogen bonding was seen by relatively weaker coupling of N$\\sb3\\sp{-}$ in the aprotic solvent HMPA, and the involvement of specific internal modes of water was suggested by the H$\\sb2$O/D$\\sb2$O isotope effect of N$\\sb3\\sp{-}$. Transient response of internal stretch vibration of CO molecule adsorbed on Cu(111) surface was measured following an impulsive substrate excitation by a 300 fs optical laser pulse. The observed rapid spectral shift and broadening in the C-O transition were attributed to the anharmonic coupling to the surface frustrated translation mode ($<$60 cm$\\sp{-1}$) modeled as a Brownian oscillator, so that a transient population change of the Brownian oscillator (induced by exchanging energy with the substrate hot Fermi-Dirac electrons and phonons) resulted in a coherence change of the C-O stretch. Coupling of the Brownian oscillator to both electrons and phonons was found important, and associated coupling rates on the order of a few ps for both. Surface transient sum frequency generation (SFG) was used to measure femtosecond infrared coherent transients for the CO stretch vibration on Cu(111). A 2 ps dephasing time (T$\\sb2$) was measured from the free induction decay, and the relative phase and amplitude of the resonant and nonresonant second-order susceptibility of CO/Cu(111) were determined by SFG spectra at different coverages.