A. L. Harris

Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum

The homogeneous (single‐cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Vibrational energy transfer of CO/Cu(100): Nonadiabatic vibration/electron coupling

Vibrational energy relaxation of the internal C–O stretching mode of carbon monoxide in the c(2×2) overlayer on the Cu(100) surface at 120 K is measured by picosecond pump–probe spectroscopy. A resonant 1.5 ps infrared pulse at ν=2085 cm−1 pumps the C–O stretching mode. The energy relaxation is monitored by sum frequency generation from a delayed pair of 1.5 ps infrared and visible pulses. A single component decay, with a decay time of 2.0 ±0.5 ps, is reported. Uncertainties in the actual excited state lifetime are discussed, and the actual lifetime is estimated to be 2.0 ±1.0 ps. This lifetime is close to the lower limit of 1.2 ps set by the observed vibrational linewidth of 4.5 cm−1. The energy relaxation process is interpreted to occur by nonadiabatic energy transfer to the electrons (electron‐hole pair excitations) of the copper substrate, and the measurement supports previous assertions that the nonadiabatic energy transfer rate for this system is very rapid. The nonadiabatic energy transfer lifetime...

Vibrational energy transfer on hydrogen-terminated vicinal Si(111) surfaces : interadsorbate energy flow

We report measurements of excited‐state lifetimes for Si–H stretching vibrational modes of steps and terraces on chemically prepared, hydrogen‐terminated vicinal Si(111) surfaces using picosecond pump–probe surface spectroscopy. The steps present on these vicinal surfaces are shown to play an important role in the vibrational energy relaxation pathways. Three types of vicinal Si(111) surfaces are studied, all having monohydride‐terminated terraces but differing in step termination or in step density. Two surfaces are cut along the 〈112〉 direction, 9° and 5° from the (111) plane, respectively. Both of these surfaces have dihydride‐terminated steps. A third surface is cut 9° from the (111) plane along the 〈112〉 direction and has monohydride‐terminated steps. Two normal modes of the dihydride‐terminated steps show vibrational energy relaxation times of ∼100 ps [≤80 ps and 130(20) ps, uncertainty in parentheses], while the monohydride‐terminated steps relax 10 times more slowly with an 1100(120) ps lifetim...

VIBRATIONAL ENERGY RELAXATION OF A POLYATOMIC ADSORBATE ON A METAL SURFACE: METHYL THIOLATE (CH3S) ON AG(111)

The lifetime of the first excited level of the symmetric C‐H stretching mode of methyl thiolate (CH3S) bonded to Ag(1 1 1) is measured by populating the level with a picosecond infrared pulse and probing the population by transient sum frequency generation spectroscopy. The population transient shows a biexponential decay across the experimental temperature range from 110 to 380 K. The fast decay component has a lifetime of 2.5–3 ps at all temperatures. The slow relaxation component lifetime varies from 55 ps at 380 K to 90 ps at 110 K. Neither relaxation component shows decay rates that are compatible with direct energy transfer to phonons or electron‐hole pairs of the metal substrate. Both relaxation components are instead assigned to intramolecular energy transfer to excited vibrational levels of other vibrational modes of the molecule.

Vibrational energy relaxation in a molecular monolayer at a metal surface

Picosecond time‐resolved measurements of vibrational energy relaxation from ν=1 to ν=0 for C–H stretching modes of the terminal methyl group in 9 Cd stearate Langmuir–Blodgett monolayer on an evaporated silver film were made. The experiments used infrared–visible sum spectroscopy to dynamically probe vibrational level populations. To the authors’ knowledge, this is the first direct measurement of vibrational energy relaxation for molecular adsorbates at a bulk metal surface. Multicomponent decay processes with lifetimes of 3 ps to >1 ns indicate complex intramolecular vibrational energy transfer processes in these ordered monolayer films, which may be different than for similar molecules in liquids.

Measuring the structure of etched silicon surfaces with Raman spectroscopy

We have measured the unenhanced, nonresonant surface Raman spectra of one monolayer of hydrogen bound to flat and stepped Si(111) surfaces prepared using a novel, aqueous fluorine etch. The orientation and normal mode composition of adsorbate vibrations are obtained from polarized, angle‐resolved Raman spectra using a 3‐layer dielectric model. This approach requires the experimental determination of both the anisotropy in the dynamic polarizability of the adsorbate bond and the effective dielectric constant in the vicinity of the adsorbate. The measured Si–H bond anisotropy is 0.263±0.028 in good agreement with gas phase measurements. The adsorbate dielectric constant is measured to be 3.78±0.20; this response is clearly nonlocal and predominantly due to polarization of the underlying silicon lattice. Using this technique, we find that the step dihydride on a Si[6(111)‐(112)] surface is rotated 37°±4° from the surface normal in good agreement with the 31° predicted by ab initio cluster techniques, but s...

Surface vibrational energy relaxation by sum frequency generation : five-wave mixing and coherent transients

Vibrational energy relaxation of molecular adsorbates at solid surfaces has been measured in several recent experiments by pumping vibrational transitions with resonant picosecond infrared laser pulses and probing the excited state vibrational populations with transient infrared–visible sum frequency generation (SFG). In this paper, we develop a description of the infrared‐pump, SFG‐probe experiment as a five‐wave mixing process in the perturbation limit. Five‐wave mixing describes the effect of finite vibrational coherent response times (T2) on the experimental time resolution and includes coherent interactions between the pump and probe infrared fields which contribute to transient coherent artifacts in the experimental signal. Neither of these effects is included in the previous steady state descriptions of the SFG vibrational probe. The five‐wave mixing expression is developed for a three‐vibrational‐level model of the adsorbate molecule and is integrated numerically for Gaussian pulse shapes to illus...

Vibrational energy transfer among adsorbate modes: Picosecond dynamics on stepped H/Si(111)

Direct measurements of interadsorbate vibrational energy flow among Si–H stretching modes on hydrogen‐terminated, stepped vicinal H/Si(111) surfaces are made. A two‐color picosecond infrared method is used in which one vibrational mode is pumped by a resonant infrared pulse and other vibrational modes are probed by vibrationally resonant sum frequency generation to observe energy transfer. The surfaces are prepared by chemical etching in HF solutions and have monohydride‐terminated (111)‐(1×1) terraces, and average terrace widths of approximately five atoms. Two types of surfaces, differing in having either monohydride‐ or dihydride‐terminated steps, are examined. The results on both surfaces confirm that interadsorbate energy transfer competes efficiently with energy relaxation to the substrate. On the dihydride‐stepped surface, the energy flow is analyzed to give a relatively complete kinetic model of the energy equilibration pathways. The model confirms that the fast relaxing dihydride‐terminated steps (60–120 ps lifetime) drain a large fraction (∼2/3) of the terrace Si–H mode energy (the terrace mode intrinsic lifetime is fit to be ∼1.4 ns). The model is consistent with terrace–step energy transfer by dipole–dipole coupling between Si–H oscillators. On the monohydride‐stepped surface, the experimental results suggest even stronger terrace–step coupling, but the monohydride step lifetime is long (≳500 ps) and does not drain the terrace mode energy. The coupling of the monohydride steps to the terraces by dipole interactions is in fact calculated to be strong enough so that the step and terrace modes mix, and detailed kinetic analysis of the monohydride‐stepped surface is therefore ambiguous because of strong spectral interactions of the modes.

INFRARED SPECTROSCOPY OF H-TERMINATED SILICON SURFACES

In this review, the present level of infrared spectroscopy at surfaces is described by using hydrogen-terminated silicon surfaces as model systems. The electronic structure of the adsorbate, H, and the large mass difference between H and Si simplify the interpretation of the data and make it possible for the theories to give reliable quantitative information. In particular, ab initio cluster calculations provide an accurate structural description and precise vibrational frequencies for various surface configurations, and are used as the basis of a priori simulations of the line shape of H on silicon. A special emphasis is given to the recent discovery of chemical etching to prepare H-terminated silicon surfaces because it has greatly helped in understanding structural and dynamical properties of H-terminated silicon surfaces. In particular, both the energy and phase relaxation of the Si-H stretching vibration on the flat, ideally hydrogen terminated Si(111) surface have been measured directly and evidence for vibrational energy diffusion has been obtained on vicinal, H-terminated Si(111) surfaces. The data and current theoretical understanding of the chemically prepared Si(111) surfaces are presented and discussed.

PICOSECOND STUDIES OF THE TEMPERATURE DEPENDENCE OF HOMOGENEOUS AND INHOMOGENEOUS VIBRATIONAL LINEWIDTH BROADENING IN LIQUID ACETONITRILE

The temperature dependence of homogeneous and inhomogeneous vibrational linewidth broadening is reported for the symmetric CH3‐stretching vibration in acetonitrile over its entire liquid range at P=1 atm. A selective excite‐and‐probe vibrational dephasing experiment based on transient stimulated Raman scattering in high laser depletion is used to measure the homogeneous dephasing times T2. The separation of homogeneous and inhomogeneous broadening processes is accomplished using the combined results of isotropic spontaneous Raman studies and selective picosecond vibrational dephasing experiments. As a function of temperature, the relative contributions of homogeneous and inhomogeneous broadening are shown to change significantly in opposing directions. Agreement between experiment and theory supports previous suggestions that homogeneous broadening is caused by rapidly varying processes which affect the vibration via short range repulsive forces. The results also suggest that inhomogeneous broadening is c...