Gereon Niedner

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...

Charge transfer and structured vibrational distributions in H++CH4 low‐energy collisions

Inelastic and charge transfer collisions of protons with methane molecules have been investigated in a perpendicular‐plane crossed beam experiment via the detection of the scattered protons and H atoms, respectively. Time‐of‐flight analysis of the protons and H atoms at scattering angles 0°≤θ≤10° and collision energies 10≤E≤30 eV provided information on internal energy distributions of the CH4 and CH+4 products. Excitation of the n(ν1 ,ν3) +m (ν2 ,ν4) type vibrations, with n,m=0, 1, 2,⋅⋅⋅was found to be the most probable assignment of the observed structured energy distributions of CH4 (1 A1 ) at θ≤4°. At θ>4°, the energy transfer increases steeply up to the dissociation limit while the vibrational structure was no longer resolved. In the case of charge transfer, the observed narrow internal energy distributions corresponding to a most probable average internal energy of CH+4 of about 0.95 eV was centered at the recombination energy of the proton indicative of quasiresonant charge transfer. In addition, f...

Vibrationally resolved inelastic and charge transfer scattering of H+ by H2O

Inelastic and charge transfer scattering of protons by water molecules at collision energies of 27.0 and 46.0 eV have been investigated in a high‐resolution crossed beam experiment up to the rainbow scattering angles. Excitation of the stretching (symmetric or asymmetric) and bending mode vibrations within the electronic ground state, X 1A1, of H2O was observed in the proton energy‐loss spectra. In the case of charge transfer, formation of H2O+ in the X 2B1 and A 2A1 electronic states was identified in the corresponding H‐atom spectra; the vibrational states within the X and A bands were for the most part resolved and, at small angles (θ≤2°), they were found to be nearly the same as in photoionization (symmetric stretch and bending mode excitation within the X state and pure bending mode excitation within the A state). The vibronic transition probabilities deviate, however, considerably from the corresponding Franck–Condon factors in favor of the enhancement of the quasiresonant states. For both the...

A comparison between theoretical and experimental state‐to‐state charge transfer cross sections for H++H2 at 20 eV: Evidence for quantum effects

A three−dimensional close coupling quantum mechanics calculation is performed for hydrogen ion−molecule collisions. The results are compared to those obtained from the trajectory surface hopping method. (AIP)

Selective vibrational excitation and mode conservation in H++CO2/N2O inelastic and charge transfer collisions

Total angular distributions and vibrationally resolved time‐of‐flight spectra have been measured for H++CO2/N2O at collision energies of 9.8 and 30 eV and scattering angles up to θ=15°. Results are available for the scattered protons as well as for H atoms from charge transfer collisions into the electronic ground states of CO+2/N2O+. For both systems, the H+ and H product channels exhibit practically identical total angular distributions with marked rainbow structures in the CO2 case. The time‐of‐flight distributions, on the other hand, reveal strongly selective excitation of the ν3 fundamental modes and their overtones for both target molecules and both product channels. In addition, at each scattering angle, the ν3 transition probability distributions for CO2 and N2O are remarkably similar to those for CO+2 and N2O+, respectively. The dominance of the ν3 mode excitation in the neutral molecules is in accord with what is expected from the combination of dipole‐ and valence‐type interaction mechanisms on...

Potential energy curves for the (ArH)+ and (NeH)+ systems from the interplay of theory and experiments

The ground state potential energy curves for protons interacting with Ar and Ne atoms are determined by the analysis of new, highly accurate measurements of the elastic differential cross sections at a laboratory collision energy of 14.8 eV. Accompanying theoretical results from SCF-CI calculations are used as starting points to generate analytic potentials that are able to fit all available experimental cross sections for both systems. The final results provide the full shape of the potential curves and give the best existing fit to the measured cross sections for elastic scattering at several energies from 2eV to 30eV.

Vibronic energy distribution of H2O+ produced in charge transfer scattering of D+ by H2O

A crossed beam study of the charge exchange reaction between deuterons and water molecules is reported. Individual vibronic states of the triatomic molecular ion produced in collisions are resolved.(AIP) e

H++Xe low-energy collisions: Opposite-phase oscillations of the elastic and charge transfer differential cross sections

Elastic as well as charge transfer collisions of H++Xe have been investigated in a crossed beam experiment atECM≈30 and 50 eV. Opposite-phase oscillations have been observed in the elastic differential cross section with respect to the charge transfer differential cross section for the formation of Xe+(2P1/2). Taking advantage of the asymptotic quasi-degeneracy of the channels in question, this behavior has been qualitatively interpreted in terms of a simplified two-curve crossing model. The conditions of the validity of the model are discussed and its relation to the potential symmetry scattering in homonuclear systems is pointed out.