Ch. Schlier

Genaue Potentialbestimmung aus Streumessungen: Alkali-Edelgas-Systeme

Recent improvements in precision and the increase in number of molecular beam scattering experiments demand a reanalysis of the experimental data. The commonly used potential functions show a serious lack of flexibility, and fitting procedures are not unambigious.A new potential ansatz (which is a continuous modification of the (12-6)-potential) has been used to fit all available data on the alkali-rare gas systems Li-Kr, Na-Ar, Na-Kr, Na-Xe, K-Ar, and K-Kr. Aχ2-minimum method is used as the procedure to fit the data and to estimate the errors of the parameters obtained (see Table 1).It turns out, that the shape of the potential is similar for all systems evaluated here but probably different from that derived for rare gas-rare gas systems.

Complex formation in proton—D2 collisions

Abstract Classical trajectory calculations are used to compute the formation cross section (suitably defined) for strongly interacting collision complexes formed in H + + D 2 collisions in the kinetic energy range from 0.1 to 4 eV. This cross section corresponds to the usual Langevin cross section only if the kinetic energy is less than 0.2 eV, and provided that little initial excitation is present, while for higher kinetic energies it drops exponentially. It is in much better agreement with absolute integral cross sections observed experimentally than the latter. Further study shows that it is the contribution from large orbital anglular momenta, which the Langevin cross section overestimates. Orbiting complexes (of H + around D 2 ) play a negligible role, and are very short-lived. The lifetime of strongly coupled complexes is estimated to be 450 E −1.3 fs, where E is the total energy in eV. The use of trajectory data to improve Lights phase space theory is discussed.

Observation of vibrationally resolved charge transfer in H++H2 at ECM=20 eV

The doubly differential cross sections for both the scattered protons and H atoms have been measured at ELAB=30 eV (ECM=20 eV) from θLAB=0° to 12° (θCM=0° to 18°) for the reactions H++H2(v=0)→H++H2(vf) and →H+H+2(vf) . The energy resolution is sufficient to resolve final vibrational states in both channels. The comparison of both the angular and energy loss distributions for the two product channels provides the first clear experimental evidence of a two‐step charge transfer mechanism: Vibrational excitation on the lower H++H2 surface is followed by charge transfer in the outgoing collision for only those H2 molecules which are excited vibrationally high enough (vf≥4) to overcome the endoergic barrier (ΔE=1.83 eV). The final vibrational distributions of H+2 appear to be very similar to those of H2 for vf≥4 indicating that for the angular range observed the charge transfer probability is the same for all vibrational states with vf≥4. The comparison with classical trajectory surface hopping (TSH) calculatio...

High-order symplectic integration: an assessment

We report tests of some new symplectic integration routines of sixth and eighth order applied to the integration of classical trajectories for a triatomic model molecule. This system has mixed regular and chaotic phase space. Especially for long-lived trajectories, which are trapped in the stochastic layers of the phase space, the eighth-order integrators are very powerful. Among a great number of integrating routines tested by the authors they are the most efficient ones, i.e. they need the smallest computational expense at a prescribed accuracy level.

Proton-Hydrogen collisions:

Abstract The trajectory surface hopping (TSH) method is used to compute product cross sections for the reactions H+ + D2, D+ + D2 and D+ + H2. They are compared with (mostly) integral cross sections obtained with a guided beam apparatus. The agreement is excellent in view of the absolute character of this comparison. Some systematic deviations of the computation can be traced to deficiencies of the potential (DIM), and of the TSH method.

Lifetimes of triatomic collision complexes

Abstract Lifetimes of long-lived collision complexes of H + + H 2 and some isotopic variants have been determined from classical trajectory calculations. A lifetime range of 0.1–50 ps could be covered, while a typical vibrational period of the complex is 10 fs. The results in agree with the expectation from RRKM theory: independence of τ of the origin of the energy of the complex (translation, vibration, or rotation), and a power law dependence of τ on E /( E + D ) with exponent 2. A closer look shows a dependence of τ on total angular momentum J , which is also the main reason for non-exponential decay curves of samples for which J is not specified, and for much of the apparent dependence of τ on mass distributions. Our data show that an induction time is needed before the complex obtains its (statistical) properties.

Bound states embedded in the continuum of H+3

Recently, Kennedy and Carrington found new quasibound states of H+3, which lie up to 1 eV above the dissociation limit with lifetimes as long as 1 μs. In an effort to understand the structure of these states, we investigate classically bound states embedded in the dissociative continuum of this molecule. In the first part, we assume J=0, and specialize to one of the two symmetries, C∞V or C2V. Poincare surfaces of section are used to demonstrate the existence of a small region of bound phase space in these 2D problems, but stability analysis of the periodic orbits show that most of them are unstable in 3D. We conclude that J=0 or, more generally, low J states cannot explain the experiments. In the second part we treat the case J>0. A total angular momentum centrifugal barrier provides a classically rigorous boundary, which separates the phase space into two parts: a dissociative and a bound region. Wells and double wells exist. Trajectories in these wells show quasiperiodic or chaotic character, depending...

Ion–molecule reactions of N+ with CO: Integral reactive cross sections in the collision energy range 0.2–13 eV

The guided beam technique, which allows the precise measurement of integral reactive cross sections of ions, has been used to measure the products of the collision of N+ with CO in the energy range 0.2–20 eV (lab). All five possible ionic reaction products CO+, NO+, C+, CN+, and O+ have been observed. The cross sections of the exothermic CO+ and NO+ channels show the usual decrease with E at low E (below 3 eV lab, i.e., 1 eV c.m.), then increase again to a flat maximum (near 12 eV lab, i.e., 5 eV c.m.). The CN+ and O+ channels show the expected thresholds, then a maximum (near 8 eV lab, i.e., 6 eV c.m.), whereas the C+ channel seems to have an activation energy and is still increasing at 13 eV c.m. There is no indication, in the form of kinks, of excited product channels except for dissociation. In addition, time‐of‐flight measurements have been done, giving some information on the forward component of the product velocity. The results are discussed in the light of simple theories, and of correlation diag...

All the adiabatic bound states of NO2

We calculated all 2967 even and odd bound states of the adiabatic ground state of NO2, using a modification of the ab initio potential energy surface of Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. The calculation was performed by harmonic inversion of the Chebyshev correlation function generated by a DVR Hamiltonian in Radau coordinates. The relative error for the computed eigenenergies (measured from the potential minimum), is 10−4 or better, corresponding to an absolute error of less than about 2.5 cm−1. Near the dissociation threshold the average density of states is about 0.2/cm−1 for each symmetry. Statistical analysis of the states shows some interesting structure of the rigidity parameter Δ3 as a function of energy.

ELECTRONIC EXCITATION OF K ATOMS IN COLLISIONS WITH DIATOMIC MOLECULES: THRESHOLDS AND ENERGY DEPENDENCE FROM 1-5 eV

Abstract A velocity selected atomic potassium beam produced by sputtering has been utilized to study collisional excitation of K atoms by diatomic molecules. K*(42P) was detected by its radiative decay. For N2. NO and O2 the excitation thresholds coincide with the endoergicity of the 42P a 42P transition, 1.6 eV (c.m.s.) whereas for CO the threshold occurs at 2.1 eV. The cross sections rise about linearly with velocity above threshold and are of the order of 2A2 at 4 eV.