Application of a complex Gaussian approach to study electron and photon impact ionization of molecules

We present a theoretical investigation of single 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. Since they are 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. The approach has been applied [2] to study molecular photoionization 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. Here we present also an application to the ionization of such molecules but by electron impact. Extension of our approach to a multicentric initial state description is under current investigations [3].