Electron Dissociative Attachment on HCl Molecules

Abstract

The dissociative attachment (DA) of slow electrons $E \lesssim 1eV$ on HCl molecules:

$$ {e^{ - }} + HCl \to C{l^{ - }} + H $$

has been shown experimentally by Allan and Wong ¹ to be rapidly varying with the internal energy of the target molecule (vibrational or rotational energy). This process is associated with a (Cl^–-H) state of ²Σ⁺ symmetry at large distance, and the attached electron has predominantly a s wave symmetry. As a consequence, the HC1^{– 2}Σ⁺ state, that is bound at large distances, cannot be correlated with a standard resonant state at small R. The usual picture of the DA process is no more applicable, one rather has to consider that the incoming electron induces some vibrational excitation in the target molecule, and when the nuclei are separated enough the electron gets captured into a bound state. We present a theoretical study of DA, that is able to describe such a process. It complements a previous study on the reverse reaction in associative detachment ³, and also extends a previous study of dissociative attachment in the ZRP approximation which solved the collision problem by a first order perturbation 7, The eleetron-static molecule interaction is described in an effective range theory (ERT), that is an extension of the zero range potential approximation 2,3,4: when the electron is far from the molecule (r>r_c), it feels the polarisation potential of the molecule; and the short range forces are taken into account via a boundary condition ar r=r_c on the logarithmic derivative of the electron wavefunction. In this formalism, the electronic and vibrational motions are only coupled via the boundary condition. The HCl long range dipolar field has been neglected⁴.

laboratoire associé au CNRS n_o 281