Very low-energy electron scattering from benzene: experiment and theory

Abstract

Low-energy collision studies have shown that the scattering cross section for electrons by benzene rises sharply at electron impact energies below ∼ 200 meV. This was attributed in earlier work to electron attachment and formation of benzene anions with lifetimes τ 1 µs. A detailed study of electron attachment to benzene is reported here that makes use of Rydberg atom techniques. The data provide no evidence for the formation of long-lived benzene anions, although a small collisional ionization signal is observed which is consistent with the formation of short-lived excited anions (τ 3 ps). An alternative explanation for the rapid rise in cross section at low energy is presented, based on ab initio scattering calculations. These calculations indicate that virtual state scattering takes place at low energies. Possible implications of this are briefly discussed.

Giant Resonances in Cold Electron Scattering byCS2

“…2 and is seen to be less than 0.5 at higher energies and to rise towards the limiting value of 0.5 at the lowest energies shown. As explained in [7,8], this suggests that A 0 is indeed negative. Virtual state scattering also requires that the negative ion of the target should be unbound to autodetachment in the equilibrium geometry of the neutral and that the target molecule should be able to form, through structural rearrangement, a negative ion that is stable to nonadiabatic electron detachment.…”

“…While no rigorous analysis is possible because of the obscuring effects of the giant resonances in the scattering spectrum, the general rise in cross section at low energy is likely to be due to virtual state scattering. As discussed in detail in [7,8], the behavior with energy of the ratio R B = T is diagnostic of the sign of the s-wave scattering length, A 0 , which must be negative for the virtual state process. The value of R as a function of electron energy is shown in Fig.…”

“…The scattering and attachment of low energy electrons by atomic and molecular targets exemplify fundamental phenomena of quantum scattering [1][2][3][4][5], such as Feshbach resonances [6], virtual state scattering [7,8], or strong suppression of rotationally inelastic scattering [1,9,10]. The scattering of cold electrons is a subject closely related to cold atom collisions, since de Broglie wavelengths are comparable and interactions tend to be governed by asymptotic long-range potentials [11].…”

Dynamics of formation of anthracene anions in molecular clouds and protoplanetary atmospheres

“…The presence of virtual state formation during electron scattering from anthracene has been suggested in experiments [22,25] that measured backward scattering processes down to meV of collision energy. It therefore seems interesting to try to verify this suggestion by following the scattering features using quantum scattering calculations as already performed successfully for benzene [25]. The angular distributions, calculated over a broad range of energies and down to 1 × 10 −6 eV, are reported by the data of figure 1.…”

“…In a series of earlier studies on ultralow energy scattering experiments [22,25], it was shown that both the total cross sections, σ tot , and the backward scattering cross sections, σ back , exhibit a marked increase in size at vanishing collision energies, thereby confirming the presence of a temporary anion as a virtual state. The data for the anthracene are provided in the two panels of figure 3, where we compare them with the present calculations.…”