# Laser Field Interactions with Atoms and Molecules

## Abstract

This dissertation explores multiple areas of laser matter interactions. It studies four main topics: multi-photon (few photon) double ionization of He, double photoionization of He-like systems from level resolved states, single and double photoionization of the diatomic molecule--Li$_{2}$, and non-equilibrium modeling of the Fe XVII 3C/3D line intensity ratio for an intense x-ray free electron laser. A time-dependent close-coupling (TDCC) method is used to calculate the five-photon double ionization of He. It is found that the generalized cross section used in the past for two-photon double ionization of He cannot be extended to five-photon double ionization of He. Therefore only five-photon double ionization probabilities that depend on specific radiation field pulses can be calculated. A TDCC method is then used to calculate the multiphoton double ionization of He using femtosecond laser pulses with linear and circular polarization. Total double ionization probabilities are calculated for 2, 3, 4, and 5 photon absorption in the photon energy range from 10 to 60 eV. Single and triple differential probabilities are calculated for 2, 3, 4, and 5 photon absorption at energies where the total ionization probability is near a maximum. For circular polarization the total and differential probabilities are consistently smaller compared to linear polarization as the number of photons absorbed is increased while keeping the radiation field intensity constant. For linear polarization, the total and differential probabilities vary substantially as a function of photons absorbed due to the presence of more absorption pathways. A semi-relativistic TDCC method is developed. The TDCC($l_1j_1l_2j_2J$) for He only includes the spin-orbit interaction, whereas the TDCC($l_1j_1l_2j_2J$) includes the spin-orbit, mass-velocity, and Darwin interactions. Double photoionization cross sections of He from the $1s^{2}$, $1s2s$, and $1s2p$ configurations and Ne$^{8+}$ from the $1s^{2}$ configuration are carried out using both TDCC($l_1l_2L$) and TDCC($l_1j_1l_2j_2J$). TDCC methods are used to study the single and double photoionization of Li$_{2}$. Formulations for both one-active and two-active electron methods make use of Hartree with local exchange potentials for the core electrons. Both the single and double photoionization cross sections for $Li_{2}$ are found to be larger for linear polarization than for circular polarization, in sharp contrast to that found before for $H_{2}$. In particular the double photoionization cross sections for $Li_{2}$ are found to be approximately five times larger than for $H_{2}$ and thus more easily observed by future experiments. A review is presented for two methods used to model recent LCLS experimental results for the 3C/3D line intensity ratio of Fe XVII \cite{Bernitt2012}, the time-dependent collisional-radiative method and the density-matrix approach. These are described and applied to a two-level atomic system excited by an X-ray free electron laser. A range of pulse parameters is explored and the effects on the predicted Fe XVII 3C and 3D line intensity ratio are calculated. We reaffirm the conclusions from Oreshkina et al.~\cite{Oreshkina2014,Oreshkina2015}: the non-linear effects in the density matrix are important and the reduction in the Fe XVII 3C/3D line intensity ratio is sensitive to the laser pulse parameters, namely pulse duration, pulse intensity, and laser bandwidth. It is also shown that for both models the lowering of the 3C/3D line intensity ratio below the expected time-independent oscillator strength ratio has a significant contribution due to the emission from the plasma after the laser pulse has left the plasma volume. Laser intensities above $\sim1\times10^{12}$~W/cm$^{2}$ are required for a reduction in the 3C/3D line intensity ratio below the expected time independent oscillator strength ratio.