Two- and three-photon double ionization of helium by soft x-ray free-electron laser pulses

Abstract

The multiphoton double ionization of He was investigated using a soft x-ray free-electron laser (SX-FEL), in the perturbative intensity regime (<1013 W cm−2). The photon energy was tuned to the transition between the 1s and 2p levels of He+ ions. At this resonance energy of 40.8 eV, both sequential three-photon double ionization (S3PDI) and two-photon double ionization (2PDI) were measured by electron spectroscopy and by ion time-of-flight mass spectroscopy. We distinguished S3PDI from 2PDI by electron spectroscopy. Based on these experimental data, SX-FEL pulse-energy dependences of electron and ion yields for both S3PDI and 2PDI were discussed. The angular distribution of 2p photoelectrons emitted from He+(2p) ions in the S3PDI process was examined as an example of the simplest photoemission dynamics.

Interference effects on intense XUV pulse induced resonant transitions between core ionized states of helium

“…The ionization rates used in this work are estimated from the magnitudes of photoionization cross sections (σ) taken from literature. The magnitudes of the relevant cross sections are σ a 1 g = 3.1 × 10 −18 cm 2 [36], σ 1 = 0.7 × 10 −52 cm 4 s [36] and σ a 2 = 1.2 × 10 −19 cm 2 [37] respectively. It is important to note that the magnitude of the ionization cross section σ (2) a 2 g for the He-He + (2p) ionization channel reported by different literatures is significantly different from each other.…”

“…This approach ensures that the important physics is not lost due to inaccurate choice of cross sections. The transition dipole matrix element is, μ a 2 a 1 = 0.37 [36]. The central photon energy of the applied laser is 40.8 eV.…”

XUV-Initiated Dissociation Dynamics of Molecular Oxygen (O2)

“…In this way, the evolution of the nuclear wave packet on the unperturbed potential energy curves can be probed by the second XUV pulse. This can be done by either recording photoelectron 11 or fragment kinetic energy release 12 , 13 spectra of the target but also via the detection of the molecule’s absorption spectrum. 14 , 15 …”

“…In this way, the evolution of the nuclear wave packet on the unperturbed potential energy curves can be probed by the second XUV pulse. This can be done by either recording photoelectron 11 or fragment kinetic energy release 12,13 spectra of the target but also via the detection of the molecule's absorption spectrum. 14,15 Photoelectron/photofragment ion spectroscopy gives detailed information on all produced fragments and electrons including their kinetic energies over a broad bandwidth.…”