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

Abstract

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, fragmentation of CH+4 formed in charge transfer collisions of H+ with CH4 was investigated in an independent experiment using mass spectrometric analysis to identify the individual fragment species. The relative intensities of the parent and fragment ions (i.e., of CH+4, CH+3, and CH+2) were found to be in good agreement with the known values of the appearance potentials of the fragment ions and the distribution of the CH+4 internal energy as obtained from the differential cross sections. A mechanism is proposed to explain the experimental results based on vibronic symmetry correlation theory. This mechanism deals with vibronic interactions in the compound quasimolecule CH+5 and explains the origin of the unexpected excitation of infrared inactive modes [e.g., ν2 (E)] of the tetrahedral methane. The effects of Jahn-Teller distortions of the CH+4 charge transfer product are also discussed.

Dynamics for the dynamic Frank Harris: Exploring H+ + CF4 at Elab = 20 and 30 eV

“…However, unlike H ϩ ϩ CH 4 , H ϩ ϩ CF 4 poses a significant challenge for time-dependent, direct-dynamics simulations because of its elevated number of electrons (42 electrons in total). For that reason, this END investigation will only explore some reactive and scattering aspects of the H ϩ ϩ CF 4 experiments at E Lab ϭ 20 and 30 eV [8]. The results of this preliminary investigation are very promising and give an impetus for more exhaustive studies of the present systems and of the remaining experiments [10] in the future.…”

“…However, because of the computational demands by the 42-electron CF 4 target, only one representative orientation will be considered in this preliminary study. Thanks to the almost spherical symmetry of CF 4 [8], the initial orientations of this target should not strongly affect the scattering results [8]. The selected orientation has the carbon atom C at the center of the coordinates ͑0.0, 0.0, 0.0͒ and the four fluorine atoms F(1), F(2), F(3), and F(4) at the points ͑ϩa,ϩa,ϩa͒, ͑Ϫa,Ϫa,ϩa͒, ͑Ϫa,ϩa,Ϫa͒, ͑ϩa,Ϫa,Ϫa͒, respectively, where a ϭ 1.443356 a.u.…”

“…This investigation is the natural continuation of the previous END study of H ϩ ϩ CH 4 at E Lab ϭ 30 eV [15] because it permits comparing the reactivity of two related tetrahedral molecules with different chemical properties. The H ϩ ϩ CF 4 system has been studied experimentally by the Toennies group in two separate investigations [8,10]. One investigation [8], at the collision energies E Lab ϭ 20 and 30 eV, concentrated on the scattering properties of H ϩ ϩ CF 4 (e.g., differential cross sections, DCS) in comparison with those of H ϩ ϩ CH 4 .…”

“…The H ϩ ϩ CF 4 system has been studied experimentally by the Toennies group in two separate investigations [8,10]. One investigation [8], at the collision energies E Lab ϭ 20 and 30 eV, concentrated on the scattering properties of H ϩ ϩ CF 4 (e.g., differential cross sections, DCS) in comparison with those of H ϩ ϩ CH 4 . The other investigation [10], at lower collision energies (E Lab ϭ 14.6 eV and others), concentrated on the CF 4 vibrational excitations during collision.…”

“…In those studies, time-of-flight analyses of the scattered projectiles (protons in nonchargetransfer reactions and hydrogen atoms in chargetransfer ones) revealed information about the targets' excitations during the collisions. Molecules investigated in that way included: H 2 [1,3], N 2 [1,2], O 2 [4], CO [1,2], NO [2], HF [1,5], HCl [1], H 2 O [6], CO 2 [7], N 2 O [7], CH 4 [8], C 2 H 2 [9], CF 4 [10], and SF 6 [10] inter alia. Although those experiments rendered detailed descriptions of the scrutinized reactions, several microscopic aspects in those processes were not accessible by experimental means.…”