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.

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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

Multiple digital data pages (480 kbits per page) were holographically recorded and retrieved with low bit-error rates in thick (~250- and ~500-mum) photopolymer media. The photopolymer systems were fabricated with the optical quality and low level of scatter required for digital data storage. We believe that these results represent the first demonstration of holographic storage of high-capacity digital data pages in photopolymer media with the thickness that will be required for such storage densities.