Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

Abstract

The dominant mechanism of low-energy positron annihilation in polyatomic molecules is through positron capture in vibrational Feshbach resonances (VFR). In this paper we investigate theoretically the effect of anharmonic terms in the vibrational Hamiltonian on the positron annihilation rates. Such interactions enable positron capture in VFRs associated with multiquantum vibrational excitations, leading to enhanced annihilation. Mode coupling can also lead to faster depopulation of VFRs, thereby reducing their contribution to the annihlation rates. To analyze this complex picture, we use coupled-cluster methods to calculate the anharmonic vibrational spectra and dipole transition amplitudes for chloroform, chloroform-d1, 1,1-dichloroethylene, and methanol, and use these data to compute positron resonant annihilation rates for these molecules. Theoretical predictions are compared with the annihilation rates measured as a function of incident positron energy. The results demonstrate the importance of mode coupling in both enhancement and suppression of the VFR. There is also experimental evidence for the direct excitation of multimode VFR. Their contribution is analyzed using a statistical approach, with an outlook towards more accurate treatment of this phenomenon.

Reduction of OH vibrational frequencies in amino acids by positron attachment

“…Conversely, Sanchez et al have proposed that annihilation rate enhancement can be described through the vibrational couplings arising from the positron‐induced target response although the corresponding couplings are estimated around the neutral minima. The results of the present study suggest that the multimode vibrational quantum dynamics calculation on a positronic potential energy surface (as well as a positron–electron contact density surface) may shed light on understanding of the detailed mechanisms of annihilation rate enhancement phenomena …”

Effect of chlorination on positron binding to hydrocarbons: Experiment and theory

“…In this work we explore positron binding to chlorinated hydrocarbon molecules. Measured binding energies have previously been reported for a range of singly and multiply chlorinated methanes and ethylenes [26,31,37,38], and nhexyl chloride [24]. Here we summarize these data and report newly measured binding energies for methylene chloride, cis-1,2-dichloroethylene, trichloroethylene, and singly and doubly chlorinated propanes.…”

Theoretical calculation of positron annihilation spectrum using positron‐electron correlation‐polarization potential

“…Their importance has been extensively studied in a recent theoretical study. [ 66 ] The multimode excitation may also increase the magnitude of the CH‐stretch peak at E ~ 0.23 eV. In addition to this disagreement, it should be mentioned that the peak intensity in the calculated annihilation spectrum is much smaller than that of the experimental spectrum by a factor of about 10.…”

“…However, it should be emphasized that the present calculations do not include the contribution of vibrational inelastic channels, which can significantly affect the calculated width. [ 66 ] Thus, its contribution to the annihilation spectrum would be an open question.…”

“…This can be improved by the introduction of positron enhancement factors. [21,23] In addition, the contribution of multimode excitations [66] should be taken into account to calculate more reliable positron annihilation spectra. F I G U R E 5 Comparison of the annihilation spectrum of benzene between experiment and theory.…”