J.-C. Lin

Steric Control of the Reaction of CH Stretch–Excited CHD3 with Chlorine Atom

Spectroscopy elucidates the complex interplay between orientational and vibrational effects in a simple chemical reaction. Exciting the CH-stretching mode of CHD3 (where D is deuterium) is known to promote the C-H bond’s reactivity toward chlorine (Cl) atom. Conventional wisdom ascribes the vibrational-rate enhancement to a widening of the cone of acceptance (i.e., the collective Cl approach trajectories that lead to reaction). A previous study of this reaction indicated an intriguing alignment effect by infrared laser–excited reagents, which on intuitive grounds is not fully compatible with the above interpretation. We report here an in-depth experimental study of reagent alignment effects in this reaction. Pronounced impacts are evident not only in total reactivity but also in product state and angular distributions. By contrasting the data with previously reported stereodynamics in reactions of unpolarized, excited CHD3 with fluorine (F) and O(3P), we elucidate the decisive role of long-range anisotropic interactions in steric control of this chemical reaction.

Vibrational Enhancement Factor of the Cl + CHD3(v1 = 1) Reaction: Rotational-Probe Effects.

The vibrational enhancement factor in the Cl + CHD3(v1 = 1) reaction is revisited over the collisional energy range of 2-5.9 kcal mol(-1). Contrary to the previous results obtained by probing the low-|N, K⟩ states of CD3(v = 0) products, CH stretching excitation becomes more efficacious than the same amount of translational energy in promoting the HCl(v) + CD3(v = 0) product pairs when all-|N, K⟩ states are probed. Whereas the new vibrational enhancement factors, which are three to four times larger than the previous report, agree reasonably well with a recent reduced-dimensionality quantum dynamics calculation, a cautious note is made on the different initial |J,K⟩ rotational selections of the CHD3 reactants in the present theory-experiment comparison.

Hydrogen chemisorption and thermal desorption on the diamond C(111) surface

Temperature programmed desorption (TPD) and low energy electron diffraction (LEED) were utilized to study the interaction of atomic hydrogen with single crystal diamond C(111) surface. From isotherm and isostere analysis of TPD spectra acquired at various sample heating rates ranging from 0.6 K/s to 30 K/s, the kinetic parameters were extracted. It is found that molecular hydrogen desorption from the C(111) surface exhibits the first-order kinetics. This result is confirmed by no apparent shift in peak temperatures of TPD spectra for hydrogen coverage above 0.2 ML. At lower coverage regime, the isothermal desorption experiment also indicates the first-order desorption kinetics. A nearly coverage-independent activation energy of (3.7±0.1) eV and a prefactor of (9.5±4.0)×1013 s−1 are obtained except at relatively low coverages (below ∼0.2 ML). In addition, the half-order LEED spots intensity decreases linearly with increase of the hydrogen coverage and drops to zero at ∼0.5 ML. These results are interpreted...

The vibrational dephasing and relaxation of CH and CD stretches on diamond surfaces: An anomaly

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 a...

The absolute absorption strength and vibrational coupling of CH stretching on diamond C(111)

This research investigates the infrared absorption intensity and isotope-dependent frequency shifts of CH stretching on diamond C(111) single-crystal surfaces by Fourier transform infrared spectroscopy (IRS). By employing single-pass direct absorption and in situ surface oxidation methods, a single sharp feature at νm=2832.2±0.9 cm−1 with a FWHM of Γ≈6 cm−1 is observed at 800 K. Systematic measuring of how band intensity depends on hydrogen etching time indicates that a well hydrogen-terminated C(111)-1×1 can be prepared only after prolonged exposure of the surface to H, generated by hot W filaments, at 1100 K. A study of the band intensity at saturation, and assuming an electronic polarizability of αe=0.65 A3 for the CH bond as that in CH4, yields an integrated cross section σz=5.5×10−18 cm for the CH stretching motion along the internuclear axis. Additional measurements of band position as a function of mixed isotope concentrations afford a stretching frequency of νi=2816.2±0.9 cm−1 for a single CH iso...

Effect of temperature on the infrared and sum-frequency generation spectra of adsorbates

The dephasing and energy relaxation contributions to the line width in infrared (IR) and sum-frequency generation (SFG) spectra of adsorbates are derived from the generalized master equation approach. Expression for the line shift is also obtained. The anharmonic interaction between the adsorbate and the substrate is expanded in a polynomial in terms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to two-phonon processes, while two-phonon, three-phonon or four-phonon processes can contribute to energy relaxation, depending on the relative values of the adsorbate vibrational and the phonon frequencies. The temperature-dependence data of the IR absorption for C(111):H is found to be consistent with the theory, and the large line width for C(111):D can be accounted for by the efficient two-phonon energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H.

Characterization of CH stretches on diamond C(111) single‐ and nanocrystal surfaces by infrared absorption spectroscopy

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.

Interaction of atomic hydrogen with a Ge(111) surface: low-energy electron diffraction and surface Raman studies

Abstract We report the preparation and characterization of a sufficiently ordered Ge(111)-1×1:H surface by prolonged hydrogenation of Ge(111)-c(2×8) at elevated temperatures. For both annealed and sputtered/annealed c(2×8) surfaces, a (1×1) pattern with distinct primary-order spots was observed by low-energy electron diffraction (LEED) after extensive hydrogenation treatment. We demonstrated that a surface Raman spectroscopic method based on polarization effects can be used successfully to characterize such a prepared Ge(111)-1×1:H surface, which is flat enough to yield a single prominent peak of the monohydride GeH stretch. The possible mechanism for surface smoothing by atomic hydrogen is also discussed. The smoothness of this surface makes various spectroscopic characterization methods feasible.

Structural instability of the diamond C(111) surface induced by hydrogen chemisorption

The low energy electron diffraction technique was used to study the hydrogen chemisorption induced structural instability on the diamond C(111) surface. From the quantitative analysis of diffraction spots intensity on the as-dosed, partially desorbed, and annealed hydrogenated C(111) surfaces, the correlation between the (1×1)↔(2×1) phase transformation, hydrogen coverage, and surface temperature is shown. Thermal treatment with partial hydrogen desorption on the fully hydrogenated C(111) surface induces a (1×1)–(2×1) reconstruction with the observable half-order spots intensity (I1/2) emerging only after heating the substrate to 1270 K. Conversely, thermal annealing of the partially hydrogenated C(111) surface without desorbing H causes the size shrinking of the (2×1) domains as well as the relaxation of the hydrogenated domains. The temperature effect of I1/2 summarized from both thermal studies reveals that the (2×1) domain instability originated from the relaxation of the hydrogenated domains at eleva...