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Surface CH stretching features on diamond nanocrystals of different origins and sizes have been characterized by Fourier transform infrared spectroscopy and density functional calculations. Independent of the origins of the samples, a single sharp transition, corresponding to the CH stretch on C(111)-1×1, is observed at 2834 cm[sup -1] for particles of sizes (d) larger than 100 nm. The feature, however, gradually diminishes as the particle size decreases and disappears nearly completely at d=5 nm. We interpreted the observation in terms of a domain size effect, namely, the domains of the {111}-type facets on the 5 nm diamonds are too small to yield the characteristic CH stretching feature [Chen et al., J. Chem. Phys. 116, 1211 (2002)]. The interpretation is supported by the present density functional calculations performed at the B3LYP/6-31G[sup *] level for three H-truncated diamond model clusters, C[sub 22]H[sub 28], C[sub 38]H[sub 42], and C[sub 58]H[sub 58]. Both “isolated” and “coupled” frequencies of the C–H stretching features on the {111}-type facets of these clusters are calculated and the results agree satisfactorily with the values obtained from the mixed isotope measurements conducted on diamond C(111)-1×1 single crystal surfaces [Cheng et al., J. Chem. Phys. 106, 7411 (1997)]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Infrared spectra of CH stretches have been investigated on the surfaces of synthetic diamond crystallites (5, 100, and 700 nm in diameter) to provide insights into the mid-infrared emission bands of stardusts in interstellar media. While the spectrum of 5-nm diamonds fails to display the distinct 2835 cm-1 (or 3.53 μm) band, a remarkably good match of the absorption bands of larger diamond grains with the emission observed for the stars Elias 1 and HD 97048 is obtained. Suggested by this study, the nanodiamonds that can display such characteristic emission bands at 3.53 μm in these two interstellar systems have a size significantly larger than 5 nm. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a phenomenological model to account for the observations of the hydrogen interchange tunneling at N=v1+v2=1–3 of the free (v1) and the bound (v2) HF stretches of (HF)2. Good agreement is obtained between calculations and observations at the levels (v1,v2) of v1-v2=±1 for both N=1 and 3, suggesting that the observed hydrogen tunneling splittings in these levels are direct rather than the results of many extraneous perturbations. The model also predicts well the ratios of the rates of vibrational predissociation at these states, in addition to the splittings. We attribute the unusually large vibrational dependence of the splittings upon valence bond excitation to the result of interbond coupling between the two HF local oscillators. Significant differences in the vibrational energy of the level (1,2) as well as the tunneling splitting at (2,0) between observation and prediction are, however, noted. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the clustering of water on protonated molecular ions by vibrational predissociation spectroscopy. Resemblance of the hydroxide stretch spectra in the protonated ion-water clusters; Supersonic expansion of the ions to establish complexation with water.

Investigates the infrared absorption intensity and isotope-dependent frequency shifts of hydrocarbon stretching on diamond single-crystal surfaces by Fourier transform infrared spectroscopy (IRS). Use of the single-pass direct absorption and in situ surface oxidation methods; Systematic measuring of how band intensity depends on hydrogen etching time.

Studies the dephasing and energy relaxation contributions to the line width in infrared and sum-frequency generation spectra of adsorbates derived from the generalized master equation approach. Expressions for the line shift; Anharmonic interaction between the adsorbate and substrate; Phonon processes that contribute to energy relaxation.

The temperature dependence of infrared absorption spectra of CH and CD on diamond nanocrystal surfaces has been investigated. Phase relaxation was closely examined by analyzing frequency shifts and line broadening in the spectra. Based on the model of Persson and Ryberg [Phys. Rev. B 40, 10 273 (1989)], coupling phonons responsible for the pure dephasing process were found to resonate at ω0≊1200 cm-1 for the CH stretch. By including both the phase and energy relaxation in the linewidth analysis and assuming that energy relaxes via three-phonon emission, we estimate a pure dephasing time of T*2≊340 ps at room temperature. This value is one order of magnitude larger than the energy relaxation time, T1≊19 ps, measured by Chin et al. [Europhys. Lett. 30, 399 (1995)] on a C(111) single crystal surface. We interpret the anomalous observation to be the result of the high frequency of the coupling phonons. For the CD stretches, however, severe line broadening due to exceedingly rapid energy relaxation disallows accurate determination of T*2. Since CH and CD stretches have similar thermal and inhomogeneous broadening characteristics, the lifetime of the latter is estimated to be T1≊220 fs. This T1 value is one of the shortest vibrational energy relaxation times for any adsorbate on a dielectric surface. Systematic comparison of these anomalous results with the measurements of SiH stretches on Si(100) and Si(111) is given. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The stretching motions of CH on diamond C(111) single- and nanocrystal surfaces have been investigated using Fourier transform infrared spectroscopy. The observations at 800 K indicate a band center of 2835.7 cm-1 and a heterogeneous broadening of 3 cm-1 for the monohydride CH stretch on ideally hydrogen-terminated C(111)-1×1 surfaces at 0 K. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Cluster isomers of H[sup +](CH[sub 3]OH)[sub 4]H[sub 2]O have been identified by vibrational predissociation spectroscopy in combination with mass-selected detection of photofragments. Ab initio calculations indicate that the cluster ion can exist in either CH[sub 3]OH[sub 2][sup +](CH[sub 3]OH)[sub 3]H[sub 2]O or H[sub 3]O[sup +](CH[sub 3]OH)[sub 4] isomeric forms. They can dissociate via a methanol loss or water loss channel, depending on the structure of the isomers. While water loss is the dominant channel of the dissociation, the methanol loss channel is readily accessible by the H[sub 3]O[sup +]-centered cluster isomer. We demonstrate in this study that mass-selected detection of photofragments produced by vibrational excitation is an effective way of identifying cluster isomers in the gas phase. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Behaviors of an excess proton in solute-containing water clusters were investigated using infrared spectroscopy and ab initio calculations. This investigation characterized the structures of protonated methanol-water clusters, H[sup +](CH[sub 3]OH)(H[sub 2]O)[sub n] with n=2-6, according to their nonhydrogen-bonded and hydrogen-bonded OH stretches in the frequency range of 2700-3900 cm-1. Ab initio calculations indicated that the excess proton in these clusters can be either localized at a site closer to methanol, forming a methyloxonium ion core (CH[sub 3]OH[sub 2][sup +]), or at a site closer to water, forming a hydronium ion core (H[sub 3]O[sup +]). Infrared spectroscopic measurements verified the calculations and provided compelling evidence for the coexistence of two distinct structural isomers, CH[sub 3]OH[sub 2][sup +](H[sub 2]O)[sub 3] and H[sub 3]O[sup +](CH[sub 3]OH)(H[sub 2]O)[sub 2], in a supersonic expansion. The spectral signatures of them (either CH[sub 3]OH[sub 2][sup +] or H[sub 3]O[sup +] centered) are the free-OH stretching absorption band at 3706 cm-1 of a single-acceptor-single-donor H[sub 2]O, and the band at 3673 cm-1 of a single-acceptor CH[sub 3]OH. At n=4-6, the clusters adopt structures similar to their pure water analogs with five-membered rings starting to form at n=5. The position of the excess proton in them varies sensitively with the number of solvent water molecules as well as the geometry of the clusters. To further elucidate the behaviors of the excess proton in these clusters, we analyze in detail the potential energy surface along the proton transfer coordinate for two specific isomers of n=2 and 4: MW2II and MW4I. It is found that the proton can be nearly equally shared by methanol and the water dimer subunit in the form of CH[sub 3]OH-H[sup +]-(H[sub 2]O)[sub 2], as substantiated by hydrogen bond cooperativity and zero-point vibrational effects. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]