Neon in ultrashort and intense x-rays from free electron lasers

Abstract

We theoretically examine neon atoms in ultrashort and intense x rays from free electron lasers and compare our results with data from experiments conducted at the Linac Coherent Light Source (LCLS). For this purpose, we treat in detail the electronic structure in all possible nonrelativistic cationic configurations using a relativistic multiconfiguration approach. The interaction with the x rays is described in rate-equation approximation. To understand the mechanisms of the interaction, a path analysis is devised which allows us to investigate what sequences of photoionization and decay processes lead to a specific configuration and with what probability. Thereby, we uncover a connection to the mathematics of graph theory and formal languages. In detail, we study the ion yields and find that plain rate equations do not provide a satisfactory description. We need to extend the rate equations for neon to incorporate double Auger decay of a K-shell vacancy and photoionization shake off for neutral neon. Shake off is included for valence and core ionization; the former has hitherto been overlooked but has important consequences for the ion yields from an x-ray energy below the core ionization threshold. Furthermore, we predict the photon yields from xuv and x-ray fluorescence; these allow one insights into the configurations populated by the interaction with the x rays. Finally, we discover that inaccuracies in those Auger decay widths employed in previous studies have only a minor influence on ion and photon yields.Keywords: ultrashort and intense x rays, neon atom, multiconfiguration Dirac-Hartre-Fock, rate equations, free-electron laser, ion yield, photon yield I. INTRODUCTIONAtoms are the basic constituents of aggregates of matter realized in molecules, clusters, and solids. In this way, the study of the interaction of intense and ultrafast x rays with atoms is of fundamental importance for all research involving matter in such light. Intense x rays offer manifold novel perspectives for science such as diffraction experiments with single molecules [1] and x-ray quantum optics [2].Experimentally, research with intense and ultrafast x rays has only recently become a reality by the novel x-ray free electron lasers (FELs) of which there are presently four producing soft to hard x rays: the Linac Coherent Light Source (LCLS) [3,4] in Menlo Park, California, USA, the SPring-8 Angstrom Compact free electron LAser (SACLA) [5] in Sayo-cho, Sayo-gun, Hyogo, Japan, the SwissFEL [6] in Villigen, Switzerland, and the European X-Ray Free-Electron Laser (XFEL) [7] in Hamburg, Germany.The novel FEL facilities which produce x rays with unprecedented characteristics inspire one to investigate processes, known from the strong-field interaction of optical lasers with atoms, but with x rays that go be- * World Wide Web: www.christianbuth.name, electronic mail; christian.buth@web.de yond the well-established one-x-ray-photon science only possible at synchrotron light sources [8]. With x rays the sequential absorption of multiple pho...

LCLS in—photon out: fluorescence measurement of neon using soft x-rays

“…We theoretically examine the ionization of neon atoms with x-rays from the LCLS and calculate the fluorescence photon yields. The details of the theoretical framework have been discussed elsewhere [12]. The electronic structure of neon in all possible nonrelativistic cationic configurations is treated using a relativistic multiconfiguration approach based on Grasp2K [17].…”

“…Photoionization cross sections for a hydrogen-like ion are found with FAC [19]. We are interested in configuration-averaged quantities, so we performed a probabilistic average over the fine-structure-resolved results [12]. To gain insights into the time-dependent absorption of x-rays and the resulting decay processes, we use the rate-equation approximation.…”

“…In principle, we need to solve the rate equations for a very long time interval for radiative transitions where no competing Auger decay is available. However, as all probability trapped in such a configuration decays radiatively, we can calculate the photon emission probability up to a chosen time when all other decays have taken place and then solve a decay equation [12]. From the probability for x-ray emission, we calculate the photon yield which is the probability for photon emission on a specific transition normalized to the sum of the probabilities for photon emission on all x-ray transitions [12].…”

“…Three distinct peaks result for neon following photoionization shake-off and Auger decay. Using the theory [12], we provide the identification of the peaks resulting thereof. Our measurements confirm two of the peaks resulting from 1s 1 2s 2 2p 6 → 1s 2 2s 2 2p 5 , and a combination of 1s 1 2s 2 2p 5 → 1s 2 2s 2 2p 4 and 1s 1 2s 1 2p 6 → 1s 2 2s 1 2p 5 transitions, identified in earlier experiments measuring K emission spectrum of neon [13].…”

Dissociating diatomic molecules in ultrafast and intense light

“…The first step selects the starting state vector and is unmodified. The second step uses the probability (49) to decide whether to quantum jump or not. The third step uses (48) to temporally propagate.…”