Theory of x-ray absorption by laser-aligned symmetric-top molecules

TL;DR

This paper develops a theoretical framework for understanding x-ray absorption in laser-aligned symmetric-top molecules, enabling new possibilities for quantum control of x-ray radiation.

Contribution

It introduces a novel theory that separates laser interaction from x-ray absorption, allowing for numerical simulation of aligned molecules' x-ray absorption dynamics.

Findings

The theory successfully models adiabatic alignment of bromine molecules.

Ab initio methods determine the polarizabilities needed for the model.

Application to Br2 demonstrates the theory's capability to analyze x-ray resonance absorption.

Abstract

We devise a theory of x-ray absorption by symmetric-top molecules which are aligned by an intense optical laser. Initially, the density matrix of the system is composed of the electronic ground state of the molecules and a thermal ensemble of rigid-rotor eigenstates. We formulate equations of motion of the two-color (laser plus x rays) rotational-electronic problem. The interaction with the laser is assumed to be nonresonant; it is described by an electric dipole polarizability tensor. X-ray absorption is approximated as a one-photon process. It is shown that the equations can be separated such that the interaction with the laser can be treated independently of the x rays. The laser-only density matrix is propagated numerically. After each time step, the x-ray absorption is calculated. We apply our theory to study adiabatic alignment of bromine molecules (Br2). The required dynamic polarizabilities are determined using the ab initio linear response methods coupled-cluster singles (CCS), second-order approximate coupled-cluster singles and doubles (CC2), and coupled-cluster singles and doubles (CCSD). For the description of x-ray absorption on the sigma_g 1s --> sigma_u 4p resonance, a parameter-free two-level model is used for the electronic structure of the molecules. Our theory opens up novel perspectives for the quantum control of x-ray radiation.