Vibrational effects on valence electron momentum distributions of CH2F2

Abstract

We report an electron momentum spectroscopy study of vibrational effects on the electron momentum distributions for the outer valence orbitals of difluoromethane (CH2F2). The symmetric noncoplanar (e,2e) experiment has been performed at an incident electron energy of 1.2 keV. Furthermore, a theoretical calculation of the electron momentum distributions of the CH2F2 molecule has been carried out with vibrational effects being involved. It is shown from comparisons between experiment and theory that it is essential to take into account influences of the CH2 asymmetric stretching and CH2 rocking vibrational modes for a proper understanding of the electron momentum distribution of the 2b1 orbital having the CH-bonding character. The results of CH2F2and additional theoretical calculations for (CH3)2O and H2CO molecules strongly suggest that vibrational effects on electron momentum distributions tend to be appreciable for non-total symmetry molecular orbitals delocalized over some equivalent CH-bond sites.

Influence of molecular vibrations on the valence electron momentum distributions of adamantane

“…Comparisons between experiment and theory has shown that taking into account vibrational effects lead to appreciable enhancement of the low p component of the 7t 2 , {2t 1 +3e} and {5t 2 +5a 1 } momentum profiles and considerably improve the agreement with experiment. Further analysis based on the HAQM approach has revealed the significant role of vibrational modes associated with the CH bonds, in line with previous results on molecules with several CH bonds 30,[34][35][36]38 in their contribution to the change of momentum profiles.…”

“…From such an analysis, we have found that vibrational effects for the 7t 2 , {2t 1 +3e}, and {5t 2 +5a 1 } orbitals cannot be ascribed to only a few specific normal modes; the contributions are distributed over a number of modes due to the large degrees of vibrations freedom of the adamantane molecule. This is in contrast with findings made recently for smaller molecules, such as C 2 H 4 30 and CH 2 F 2 , 34 where only a few normal modes play dominant roles. To facilitate the interpretation of results, the classification of vibrational modes proposed by Jensen 50 has then been adopted, where each of modes is assigned to one of nine types of motions: CC stretch, CH stretch, CCC bend, CCC wag, CH wag, CH 2 scissors, CH 2 twist, CH 2 wag, and CH 2 rock motions.…”

“…Since the theory behind the HAQM approach has been fully described elsewhere, 30,34,35 only a short account is given here. Under the assumption of the PWIA the EMS cross section can be written as…”

“…where f ee is the electron-electron collision factor and Μ(p) is the structure factor corresponding to a momentum profile. 23 In the frame of the Born-Oppenheimer approximation, the electron momentum profile for a transition from the initial neutral electronic state i to the final ion state f can be expressed as follows: 30,34,35 ( )…”

“…[25][26][27][28] Because of its unique ability to image individual electron orbitals in momentum space, EMS can provide a powerful means to investigate how molecular orbital patterns are affected by the vibrational motions of molecules. Indeed, recent EMS studies on various polyatomic molecules [29][30][31][32][33][34][35][36][37][38] have shown that electron momentum profiles may considerably be affected by molecular vibrations, and that EMS experiments in conjunction with recently developed theoretical approaches are capable of disentangling the interplay between the electronic and nuclear dynamics. 30,31,[34][35][36][37][38] It is therefore of interest to apply the EMS technique to the study of the nuclear dynamics effects for diamondoids.…”