Bend Motion Research Articles

Hydrogen vibrational density of states of adsorbed water on rare-earth modified zirconia

Rare-earth (RE) doped ZrO 2 prepared by a method of coprecipitation from aqueous solution shows many properties, such as good thermal stability and large surface area ( ∼ 80m 2/g), that are suitable for use as catalyst supports and sorbents for a variety of molecules. We have measured the vibrational densities of states of surface hydroxyl groups as well as physisorbed water molecules in La 0.1Zr 0.9O 1.95 and Nd 0.1Zr 0.9O 1.95 by inelastic neutron scattering. The spectrum of dry RE-ZrO 2 exhibits a peak at about 455 meV characteristic of the O H stretch vibrations of surface hydroxyl groups. At submonolayer coverage of H 2O this peak broadens and shifts to a slightly lower energy. At higher coverage three bands, corresponding to the O H stretch (∼ 430 meV), H O H bend (∼ 200 meV) and librational motion ( ∼ 80 meV), were observed. The decreasing energy and larger width of the O H stretch band with increasing H 2O coverage indicate the influence of hydrogen bonding on the motion of water molecules.

The oxidation of methanol on Cu(100)

Different aspects of the catalytic oxidation of methanol on Cu(100) have been investigated. CH3O is a surface intermediate formed in the reaction and infrared spectroscopy has been used to study its vibrational modes. For the C–O stretch mode there is a large coverage dependent shift of the vibration frequency caused by the dipole–dipole interaction between the molecules. In order to study the reaction mechanisms the vibration frequency was used as a measure of the CH3O coverage, which then indicated the probability for a methanol molecule to react with a surface oxygen atom under various conditions. LEED was used for structural studies and showed that the CH3O could form both a c(2×2) and a hexagonal structure. A model for the oxidation of methanol on Cu(100) is proposed, stressing the structural aspects. The model implies that the reaction can only occur if there exist empty nearest neighbor adsorption sites adjacent to the oxygen atom and that the reaction can be prevented by the presence of an excess of oxygen atoms or highly mobile CH3O molecules on the surface. The dehydrogenation of CH3O to H2CO was shown to proceed via the adsorption of a hydrogen atom on the surface during a thermally excited bend motion. Finally, it is reported that the infrared absorption peak width of the C–H stretch modes of CH3O is caused by inhomogeneous broadening and is not a measure of the vibrational damping, as previously assumed.

Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments

In this and the following paper, we present a full account of our earlier report [Syage et al., Chem. Phys. Lett. 88, 268 (1982)] on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jet-cooled excitation and dispersed fluorescence spectra of t-stilbene-h12 and -d12 are reported and assigned for the Bu ← Ag electronic excitation. The 000 wavelengths for h12 and d12 (in excitation) are 3101.4 and 3092.5 Å, respectively. Previously unidentified low frequency modes (as low as 20 cm−1 in S0 for -h12) have been observed and tentatively assigned as out-of-plane modes of au symmetry in C2h. This indicates that t-stilbene has a propeller-like geometry involving phenyl rotation (i.e., C2 symmetry). A Franck–Condon analysis of the low frequency modes and particularly the ag, ν25 in-plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from the Ce–Ce bond to Ce–φ bond. The geometry change of the in-plane Ce–Ce–φ between S1 and S0 was determined from the Franck–Condon and a normal mode analysis to be 1.3°±0.3°. The rms amplitude of this bend motion for the symmetric ν25 bend mode (for one quanta in S0) is ‖〈Δφ〉2‖1/2=1.0±0.2°. Most ag modes involving benzene-type vibrations (other than C–H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly with S1 vibrational energy. The spectra were completely diffuse above 1200 cm−1 which is consistent with the barrier for isomerization being at about 1100–1200 cm−1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cm−1. In the accompanying paper, we present results on the dynamics of isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic).

Semiclassical determination of adiabatic barriers on a three-dimensional potential energy surface

A recently proposed method, based on periodic orbits, for finding vibrationally adiabatic barriers and wells in collinear collisions is generalized to the full three-dimensional case. The main idea is a consistent use of the adiabatic approximation—one first solves for the fast vibrational motion to obtain an effective Hamiltonian for the slower bend motion which in turn is solved to obtain an effective Hamiltonian for the overall rotation. The method is applied to the hydrogen exchange reaction. We find the bend-vibration adiabatic barrier levels for the H2(v=1) state. The zero point motion in the bend degree of freedom is found to be substantial (0.1 eV) and is a source for nonnegligible discrepancies between approximate theories such as the infinite order sudden and quasiclassical trajectory approach and exact quantal scattering computations. Having found the barrier levels we are able to evaluate the collision cross section. Our analysis points out that differences between experimental cross sections and theoretical predictions may be due to inaccuracy in the potential energy surfaces. The available surfaces probably overestimate the adiabatic barrier height.