Bibliothek

Notizen: The results of a molecular beam surface scattering experiment with preferentially orientated NO on Ag[111] are given. In adsorption molecules oriented with the O end towards the surface have a larger trapping probability.

Notizen: The energies and eigenfunctions of hyperfine levels belonging to the J=||K||=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos cursive-theta), where cursive-theta is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos cursive-theta, can be characterized by two moments only: 〈P1〉 the orientation and 〈P2〉 the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions with Ca atoms, down to a field strength of a few volts per centimeter.At lower field strengths the orientation falls off more rapidly than predicted by the theory. This behavior is tentatively attributed to the variation in the direction of the electric field, and hence in the axis of quantization, across the reaction volume, at low field strengths. This may, in addition, cause a breakdown of the adiabatic approximation. In the case of methyliodide, quantitative agreement between the theoretical and the experimental asymmetry parameters of photodissociation is obtained if the latter ones are provisionally corrected for systematic errors. Contrary to earlier preliminary conclusions, the results show that within experimental error the adiabatic approximation is applicable. Up until now, no experimental data has been available for methylbromide. The calculations clearly demonstrate the inadequacy of restricting theoretical calculations to limiting cases, i.e., weak field and strong field, particularly because the Stark curves reveal the presence and the precise position of avoided crossings. Dependent on the field strength used for state selection and focusing, these crossings may, in principle, have a marked effect on the orientation obtained, given that the final orientation field at the reaction site remains the same. In addition, knowledge of the detailed Stark curves suggests a novel and feasible way to reverse the orientation of the molecules without interference with the detection system, which may be particularly useful if ionic products are formed.

Notizen: A classical molecular dynamics study is applied to simulate the scattering of NO from Pt(111) in the energy range of 0.3–1 eV. The solid consists of a large number of crystal atoms that interact via an anharmonic nearest-neighbor potential. The NO–Pt(111) interaction potential is constructed as a pairwise additive potential with a well depth of 1 eV for the N end of the molecule towards the surface and purely repulsive for the O end. The in-plane scattering results obtained with this model potential are compared with recent experiments for NO–Pt(111). The angular intensity distributions, the final translational energy, as well as the rotational energy distributions with the corresponding alignment are in qualitative agreement with those experimental results. A detailed examination of the collision dynamics shows that multiple collisions with the surface results predominantly in superspecular scattering. The rotational angular momentum of the scattered molecules exhibits a preference for cartwheeling alignment and the rotational energy distributions for specular and normal exit angles can be described with a Boltzmann distribution, whereas for grazing exit angles they are distinctly non-Boltzmann. The latter structure results from a cutoff in the rotational excitation by the attraction of the well. The high rotational excitation clearly originates from molecules that initially are oriented with the O end towards the surface, whereas for the low rotational excitation this orientation preference disappears. © 1996 American Institute of Physics.

Notizen: Rotational excitation of NO scattered from Pt(111) has been measured for incoming energies from 0.3 to 1.6 eV. For an initial energy of 0.3 eV a clear rotational rainbow, which we assign to O-end collisions, is visible at superspecular exit angles. This is very surprising, since NO–Pt(111) is a chemisorption system with a binding energy of 1 eV. Sharp, pronounced rainbows are visible in the range of incoming kinetic energies where the initial sticking coefficient is high. For an initial energy of 0.3 eV the initial sticking coefficient is 0.9, and at this energy the clearest rotational rainbow is observed. In contrast, at an initial energy of 1.6 eV the sticking coefficient is 0.5 and no rotational rainbow is observed. At subspecular exit angles the distributions are indistinguishable from a Boltzmann distribution at all the energies investigated, and show a clear energy dependence. Boltzmann-type distributions at high incident energy indicate a thorough redistribution of the available energy, although they cannot be explained in terms of a simple statistical model. The scattering results are interpreted as indicating a competition between direct scattering from the repulsive wall and indirect scattering via the deep potential well. Most of the direct scattering can be assigned to O-end collisions with the surface, where the binding energy is expected to be much smaller. Indirect scattering becomes more important at higher energies as more molecules are able to escape the well promptly. © 1995 American Institute of Physics.

Notizen: The complexity of the absorption spectrum of NO2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of 2A1 symmetry and the first electronically excited state of 2B2 symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm−1 above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm−1, so that the fine- and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K−=0 stack. In the present paper we extend the analysis to transitions of the K−=1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO2 molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO2 molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants. © 2000 American Institute of Physics.