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The ho~ogeneous. (single-cluster) and in~omogeneous contributions to the low temperature electronic absorptIon spectrum of 35-50 A diameter edSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure but their electr.onic states are strongly quantum confined. The inhomogeneous broadening ~f these featu~es ~n~es because t~e spectrum depends upon cluster size and shape, and the samples contam SImIlar, but not Identical, clusters. The homogeneous spectrum, which consists of a peak 14? cm -I (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 -I, or seven times the homogeneous width, in the most monodisperse sample.

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 of this mode has previously been estimated by density-functional calculations [T. T. Rantala and A. Rosen, Phys. Rev. B 34, 837 (1986)], and has recently been calculated by extrapolation of ab initio Hartree–Fock electronic structure calculations for CO on copper clusters [M. Head-Gordon and J. Tully, preceding paper, J. Chem. Phys. 96, 3939 (1992)]. The calculated lifetimes in both cases are in the 1–3 ps range, in good agreement with the experimentally measured value.

No abstract

A quantitative model is presented to describe the formation of volume holograms in a polymeric medium containing photopolymerizable acrylate monomers that undergo spatially modulated gelation as a result of exposure to a visible ''write'' beam. The model refines the simple diffusion model of Zhao and Mouroulis ͓J. Mod. Opt. 41, 1929 ͑1994͔͒, by including cure dependence of both the photoreaction kinetics and the monomer diffusivity. These dependences are determined by experimental measurements, using near infrared spectroscopy to quantify the degree of cure and the time dependence of the hologram formation to infer the cure-dependent diffusivity. The cure-dependent diffusion coefficient can be fit by an expression from a free-volume theory, and the cure-dependent reaction rate coefficient is found to be proportional to the diffusivity, showing the reaction rate to be diffusion limited. With the model parameters determined experimentally, predictions are then made of the first, second, and third harmonics of the grating profile, and these are found to be in good agreement with the measured values. The results show the validity of the model and its usefulness in predicting the optimal exposure conditions and performance of a given holographic material.

Picosecond transient absorption measurements from 1000-295 nm are used to monitor the recombination dynamics of iodine after photodissociation in a variety of inert solvents. The high time resolution and signal-to-noise ratio of these measurements permits the development of a detailed model of this reaction, which should resolve disagreements over the time scales of geminate recombination and vibrational relaxation and over the role of excited electronic state trapping. Most of the atoms which undergo geminate recombination do so in < 15 ps, in agreement with the predictions of existing molecular dynamics simulations. The subsequent vibrational and electronic energy relaxation of the recombined molecule is relatively slow and accounts for most of the transient absorption dynamics. The relaxing X-state vibrational population distribution is extracted with an approximate method using calculated spectra of the excited vibrational levels and is compared to recent models. Vibrational relaxation times vary from -15 ps near the middle of the ground state well to -150 ps for complete relaxation to v = O.The vibrational relaxation rates do not provide support for the predicted role of resonant vibration-to-vibration energy transfer to chlorinated methane solvents, but some evidence for this mechanism is found in alkane solvents. B-state predissociation times of 10--15 ps and A '-state lifetimes of 65-2700 ps are found depending on the solvent. Current theory is not able to satisfactorily explain the large variation of the A '-state lifetime in various solvents. 788

The lifetime of the first excited level of the symmetric C-H stretching mode of CH3S chemisorbed on a Ag(lll) surface was measured by picosecond vibrational spectroscopy. A biexponential decay (--3and 63-ps lifetimes at 300 K) was observed, with a substantial temperature dependence of the slow component. Both decay processes are assigned to intramolecular vibrational relaxation. The decay rates are 2 orders of magnitude too fast to be explained by electron-hole-pair damping by the metal substrate.PACS numbers: 82.20. Rp, 34.30.+h, 78.47.+p Despite the fundamental importance of vibrational energy transfer to chemical processes at metal surfaces, there have been no time-domain measurements of vibrational lifetimes for adsorbates on single-crystal surfaces. ] This Letter reports such measurements for the first excited level of the symmetric C-H stretching mode of methyl thiolate, CH3S, chemisorbed on a Ag(l 11) surface. Previous indirect measurements based on linewidth have assigned the vibrational relaxation of this mode in similar adsorbates [CH 3 O/Cu(100) and CH 3 C/Pt(l 11)] to electron-hole-pair excitation in the metal. 2 The present results show that the vibrational linewidth is not determined in CH3S by vibrational energy relaxation. In addition, the relaxation itself is assigned to intramolecular energy transfer. The previous assignments must be reconsidered. The results demonstrate the importance of intramolecular energy transfer for small polyatomic molecules even where electron-hole-pair damping has previously been assigned dominance.To measure vibrational relaxation times, a resonant infrared pulse populates the first excited state of the symmetric C-H stretching mode at 2918 cm -1 . The subsequent population relaxation is monitored by transient sum-frequency-generation (SFG) spectroscopy. 3 In this method, an SFG signal is generated by mixing a visible probe pulse with a vibrationally resonant infrared probe pulse at the surface. The vibrationally resonant SFG signal is proportional to the square of the population difference between the lower and upper levels of the transition probed. 3 Its square root is therefore a measure of the population difference on the vibrational transition. If So is the SFG signal with no pump and S(r^) is the signal at delay r

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)-(1̄1̄2)] surface is rotated 37°±4° from the surface normal in good agreement with the 31° predicted by ab initio cluster techniques, but significantly larger than the 12.5° predicted by pseudopotential slab calculations. In contrast to both theoretical predictions, the normal modes of this step dihydride display little concerted motion indicating that subsurface relaxation near the step edge reduces steric interactions much further than predicted. The observed anisotropic etch rates, evidenced by the production of atomically straight steps, are explained in terms of the measured distortion at the step edge.