M. Noll

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...

Vibrational state resolved measurements of differential cross sections for H++O2 charge transfer collisions

Vibrational state‐to‐state charge transfer cross sections have been obtained for H++O2 collisions at Ecm =23.0 eV in a differential scattering experiment by measuring the product H atom energy distribution in the range 0°≤θ≤11°. The results show a strong dependence of the distribution of vibrational transition probabilities P(O2, v=0→O+2, v″) on the scattering angle. At very low angles (θ≲2°) we find a strongly enhanced contribution of the more resonant states v″=3–6 compared to a Franck–Condon distribution, which is peaked at v″=1 and, on the other hand, at moderate angles of about θ=4° comes very close to the measured spectra. At even larger angles increasing deviations from the Franck–Condon distribution towards larger excitation of higher vibrational states are observed. An explanation of these and the related results for the nonexchange collisions leading to neutral O2 vibrational excitation is given in terms of the underlying potential energy surfaces.

Time‐of‐flight spectroscopy of high‐overtone mode‐selective vibrational excitation of CF4 and SF6 in collisions with H+(D+) at energies between 10 and 28 eV

Time‐of‐flight spectra for H+(D+)–CF4 and SF6 collisions have been measured with an improved resolution and at higher collision energies (10≤Elab ≤28 eV) compared to earlier work. In the low energy region (≤13 eV) new distinct peaks are resolved for both molecules corresponding to small contributions from the second infrared active ν4 mode in addition to the dominant ν3 mode observed previously. Reexamination of experimental vibrational transition probabilities reveals an almost perfect agreement with a Poisson distribution for both modes up to the n=6 overtone transition of ν3. A simple straight line theory is used to calculate the energy transfer in small angle scattering from the long‐range potential in good agreement with a full classical trajectory calculation. With this theory dipole moment derivatives can be determined directly from the observed energy transfers and are found to agree well with previous infrared measurements. At larger collision energies (≥16 eV) an additional low intensity vibrati...

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...

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...

Predominance of CF‐stretch vibrational excitation in state resolved inelastic scattering of protons from 11 small fluorohydrocarbon molecules

Time‐of‐flight spectra of H+ scattered from all ten different fluorinated hydrocarbon molecules of type CHmFn and C2HmFn (m+n=4) as well as C2F6 have been measured for small scattering angles (θlab≤15°) at 9.8 eV collision energy. For the pure hydrocarbons the spectra show a complicated structure suggesting unresolved excitation of several modes and a significant effect of charge transfer collisions. In contrast all the spectra for fluorine containing molecules exhibit more or less sharp maxima indicating high overtone excitation of predominantly one vibrational mode with ωv≂1200 cm−1 corresponding to a CF‐stretch vibration. At some scattering angles the overtone transition probabilities deviate somewhat from a Poisson distribution suggesting small unresolved contributions from additional vibrational modes. Over‐ all, however, there is good agreement for the low symmetry molecules, some of which have sizable dipole moments, with the predictions of the simple forced oscillator model previously tested only ...

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.

Observation of very high overtone vibrational levels (n ⩽ 14) IN CF4 by proton inelastic scattering

Abstract The inelastic scattering of protons from Cl: 4 has been further investigated using a high-resolution time-of-flight tecnique. At a collision energy of 18.5 eV and angles less than the rainbow ( O R ≈ 20°) two structured energy-loss distributions have been observed the second of which with greater average energy transfer (≈ 1.5 eV) is attributed to repulsive collisions. Both distributions are resolved and reveal a progression of ν 3 -overtone levels extending up to the n = 11 overtone state. With significant but not uniquely, assignable structure at even higher energy transfers ( n ⩽ 14).

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