The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≊2106 cm−1 ) of top-site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO-covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution <1 ps and monochromator resolution <1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD<0 show features characteristic of a perturbed free induction decay, which are expected based on multiple-level density-matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the time scale for relaxation do not depend (within experimental error) on coverage for 0.1≤ΘCO≤0.5 ML or on temperature for 150≤Ts≤300 K. A model for coupled anharmonic oscillators qualitatively explains the tD>0 spectra in terms of a population-dependent decrease in frequency of the one-phonon band, as opposed to a transition involving a true CO(v=2) two-phonon bound state. The rapid relaxation time and its insensitivity to Ts and ΘCO are consistent with electron–hole pair generation as the dominant decay mechanism.

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Picosecond infrared pump–probe experiments determined the vibrational population lifetimes (T1) of the hydroxyl fundamental stretching mode OH(v=1) in 12 alcohols (R3COH) and 8 silanols (R3SiOH) in dilute room temperature CCl4 solutions. T1 for the silanols is in the range 185<T1<292 ps, while T1 for the alcohols is much less (T1<80 ps). The deuterium-exchanged analogs (COD and SiOD) exhibit population relaxation times similar to protonated hydroxyls. An analysis of the vibrational energy levels corresponding to modes involving the four bonds nearest the hydroxyl groups of these molecules is used to qualitatively explain the trends of the observed T1 lifetimes for these systems. Solution T1 lifetimes are also compared to those previously measured for OH(v=1) on the surface of silica and in other condensed-phase, room temperature systems.

Picosecond infrared transmission spectroscopy was used to directly measure the vibrational energy relaxation time T1 of hydroxyl groups chemisorbed on the surface of colloidal silica (SiO2). T1 was obtained for OH(νstretch=1) in the strongly bound ‘‘isolated sites’’ of fumed silica particles in vacuum and dispersed in several liquids at T=293 K. At the SiO2/vacuum interface, T1=204±20 ps. When the SiO2 particles are surrounded by solvents, the relaxation time of the surface OH(v=1) groups decreases: for the liquids CCl4, CF2Br2, CH2Cl2, and C6H6, T1(ps)=159±16, 140±30, 102±20, and 87±30, respectively. T1 does not depend on the size of the SiO2 particles for the range 70 Å≤ diameter ≤150 Å, or on the surface OH coverage up to an average density of 4 OH/100 Å2. Significant amounts of physisorbed water (5 H2O/100 Å2) decreased T1 for the isolated OH(v=1) to T1=56±10 ps. For comparison to the surface hydroxyls, the vibrational deactivation time for OH(v=1) groups in the bulk of fused silica (OH/SiO2≊130 ppm by weight) was determined to be T1=109±11 ps. These observations are discussed in terms of the possible mechanisms of vibrational energy flow in these systems. The observed T1 values demonstrate that the spectral linewidths (e.g., IR and Raman) observed for these surface vibrations are too large (by factors of 200–2000) to be caused solely by T1 uncertainty broadening. The slow transfer of vibrational energy between surface and lattice vibrations may have important implications for surface chemistry.

A picosecond time-resolved infrared technique has been used to measure the transient response of vibrationally excited CO adsorbed on the surface of a Pt(lll) single crystal. Transient-bleaching signal decay is interpreted as giving T\ for damping of vibrationally excited CO. Transient absorption is discussed in terms of transitions from the excited adsorbate band to overtone levels.

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Vibrational overtone photodissociation is used to examine the spectroscopy and vibrational predissociation lifetimes of HN3 in its ground electronic state. Direct overtone pumping of the N–H stretching levels 5νNH and 6νNH prepares molecules in selected states (v,J,K) near 15 100 and 17 700 cm−1 of vibrational energy; spin-forbidden NH(X 3 Σ−) dissociation fragments are detected by laser-induced fluorescence. Photodissociation spectra of beam-cooled HN3 display mixing of individual rotational levels of the nνNH vibrations with several background states, with derived coupling matrix elements in the range 0.01–0.1 cm−1. Vibrational predissociation lifetimes of mixed components of 5νNH are state specific, with variations of a factor of 2 for only 0.1 cm−1 energy differences. Average lifetimes for low J, K are 210 ns for 5νNH and 0.95 ns for 6νNH. The ratio of decay rates for the two overtone levels, k(6νNH)/k(5νNH)=220, is much greater than predicted by statistical theory, which gives a ratio of 4.