Ultrafast absorption of intense x rays by nitrogen molecules

TL;DR

This paper presents a comprehensive theoretical model for the ultrafast absorption of intense x-ray pulses by nitrogen molecules, incorporating atomic ionization, decay processes, and molecular fragmentation to match experimental ion yields.

Contribution

It introduces a detailed rate-equation framework combining atomic and molecular physics to explain nitrogen's response to intense x-ray irradiation.

Findings

Ion yields and charge states match experimental data.

Multiple ionization and fragmentation pathways are elucidated.

Role of core-hole states and metastable states in molecular dynamics.

Abstract

We devise a theoretical description for the response of nitrogen molecules (N2) to ultrashort and intense x rays from the free electron laser (FEL) Linac Coherent Light Source (LCLS). We set out from a rate-equation description for the x-ray absorption by a nitrogen atom. The equations are formulated using all one-x-ray-photon absorption cross sections and the Auger and radiative decay widths of multiply-ionized nitrogen atoms. Cross sections are obtained with a one-electron theory and decay widths are determined from ab initio computations using the Dirac-Hartree-Slater (DHS) method. We also calculate all binding and transition energies of nitrogen atoms in all charge states with the DHS method as the difference of two self-consistent field calculations (Delta SCF method). To describe the interaction with N2, a detailed investigation of intense x-ray-induced ionization and molecular fragmentation are carried out. As a figure of merit, we calculate ion yields and the average charge state measured in recent experiments at the LCLS. We use a series of phenomenological models of increasing sophistication to unravel the mechanisms of the interaction of x rays with N2: a single atom, a symmetric-sharing model, and a fragmentation-matrix model are developed. The role of the formation and decay of single and double core holes, the metastable states of N_2^2+, and molecular fragmentation are explained.