We present a novel procedure for vibrationally resolved sum-frequency generation (SFG) in which a broad-bandwidth IR pulse is mixed with a narrow-bandwidth visible pulse. The resultant SFG spectrum is dispersed with a spectrograph and detected in parallel with a scientific-grade CCD detector, permitting rapid and high signal-to-noise ratio data acquisition over a 400-cm(-1) spectral region without scanning the IR frequency. Application to the study of a self-assembled monolayer of octadecanethiol is discussed.

Vibrationally resonant sum frequency generation (VR-SFG) has been used to study the absolute molecular orientational distribution of the pendant phenyl groups at the free surface of polystyrene (PS) thin films on oxidized Si substrates. Characterization of the dependence of the VR-SFG on film thickness allows unique identification of the origin of the signal, e.g., free surface, bulk, or buried interface. For films <400 nm thick, the dominant source of vibrationally resonant signal is the PS/air interface, while the dominant source of the nonresonant background is the Si/SiO2 interface. VR-SFG of a self-assembled phenylsiloxane layer is used to calibrate the relative phase between the vibrationally resonant phenyl ring hyperpolarizability and the Si/SiO2 interface nonresonant hyperpolarizability. It is found that the phenyl groups are ordered at the PS/air interface and are oriented away from the polymer film. Quantitative analyses of the orientational distribution for both the PS free interface and the phenylsiloxane monolayer are reported. The phenyl groups at the PS free surface are tilted away from the surface normal in an angular range near 57°.

No abstract

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 &lt;1 ps and monochromator resolution &lt;1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD&lt;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&gt;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.

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm-I (VI) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that VI is anharmonic ally coupled to the frustrated translation near 32 cm-I (V4), so that transient shifts in VI indicate population changes in V4' The VI response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a =2 ps time constant to an intermediate value, which then decays on a =200 ps time scale. The data agree well with a model whereby V4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.

Optically driven surface reactions are attracting an increasing level of attention in the physical chemistry community. Not only have there been recent advances in establishing the viability of laser driven surface reactions, but there has also been an increased awareness of the need to understand the underlying reaction mechanisms. The necessity of accounting for energy-transfer processes that occur on the femtosecond time scale is now apparent. In this review the experimental and theoretical basis of our current understanding is surveyed, and prospective areas of advancement are considered.

No abstract

No abstract