Electron and photon impact ionization of molecules using a complex Gaussian representation of continuum states

This theoretical work deals with ionization of small molecules by photon or electron impact. The focus is on the description of the electron ejected into the continuum.  

Real Gaussian-type orbitals (rGTOs) are widely used in molecular bound state calculations. Nodeless and exponentially decreasing to zero at large distances, they are not suited to represent oscillating and non-decreasing continuum wavefunctions. Alternatively, complex Gaussian-type orbitals (cGTOs) --i.e. Gaussians with a complex exponent-- intrinsically oscillate and should be more adapted for this task. In a recent study [1], we have developed an efficient non-linear optimization method to provide sets of cGTOs able to reproduce accurately --within a large radial box--  continuum-type functions.These sets were used with success to evaluate analytically all matrix elements involved in the benchmark case of atomic hydrogen ionization, under photon or electron impact.

In the present work, we pursue this approach by employing cGTOs to study molecular ionization within a one-active-electron model and a single-center approach.Using Slater-type expansions for the initial molecular target, all the necessary transition integrals are expressed in closed form. Results for water, ammonia and methane compare very well with other theoretical calculations and with available experimental data. Work is ongoing to extend our proposal to a multicentric initial state description.

[1] Ammar A et al 2020 J. Comput. Chem., 41, 2365