Multiphoton ionization of xenon in the vuv regime

Robin Santra and Chris H. Greene

Phys. Rev. A 70, 053401 – Published 3 November 2004

Abstract

In a recent experiment at the vuv free-electron laser facility at DESY in Hamburg, the generation of multiply charged ions in a gas of atomic xenon was observed. This paper develops a theoretical description of the multiphoton ionization of xenon and its ions. The numerical results lend support to the view that the experimental observation may be interpreted in terms of the nonlinear absorption of several vuv photons. The method rests on the Hartree-Fock-Slater independent-particle model. The multiphoton physics is treated within a Floquet scheme. The continuum problem of the photoelectron is solved using a complex absorbing potential. Rate equations for the ionic populations are integrated to take into account the temporal structure of the individual vuv laser pulses. The effect of the spatial profile of the free-electron laser beam on the distribution of xenon charge states is included. An Auger-type many-electron mechanism may play a role in the vuv multiphoton ionization of xenon ions.

Received 2 July 2004

DOI:https://doi.org/10.1103/PhysRevA.70.053401

Authors & Affiliations

Robin Santra and Chris H. Greene

Department of Physics and JILA, University of Colorado, Boulder, Colorado 80309-0440, USA

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Vol. 70, Iss. 5 — November 2004

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Figure 1
Experimental [27] ion detection signal is plotted versus the ionic time of flight. Free-electron laser pulses with a peak intensity of about $10^{13} W ∕ {cm}^{2}$ , a duration of approximately $100 fs$ , and a photon energy of $12.7 eV$ produce, in an atomic xenon beam, ionic species of various charge states [26]. This time-of-flight mass spectrum [27] is an average over 100 consecutive FEL pulses.Reuse & Permissions
Figure 2
The dots in this figure correspond to a specific eigenvalue of the family of complex symmetric matrices $H (η_{n})$ [Eq. (6)], where $η_{n} = δ (κ^{n} - 1) ∕ (κ - 1)$ [ $n = 0, \dots, 99$ ; $δ = 3 \times 10^{- 9}$ ; $κ = 1.1$ ]. In the absence of photons and absorbing potential, this eigenvalue equals the $5 p$ , $m = 0$ level of atomic xenon (which defines the origin in the figure). Due to the interaction with $12.7 − eV$ photons, this level is shifted as well as broadened. The point of stabilization of the $η$ trajectory implies a dynamic Stark shift of $7.31 \times 10^{- 6} a.u.$ and an ionization rate of $3.05 \times 10^{- 5} a.u.$ , at a radiation intensity of $1 \times 10^{11} W ∕ {cm}^{2}$ .Reuse & Permissions
Figure 3
The power in the DESY free-electron laser pulse is shown as a function of time. The vuv laser pulses supplied by the free-electron laser source at DESY, Hamburg, are ultrashort, with an average width of about $50 fs$ , and intense, with a pulse energy of order $10 μ J$ [23]. The pulses shown in this figure [95] have been calculated using an FEL simulation program [93, 94].Reuse & Permissions
Figure 4
The average number $⟨ N_{q} ⟩$ of ${Xe}^{q +}$ ions produced per VUV-FEL laser pulse, as calculated on the basis of the multiphoton ionization cross sections in Eqs. (17, 18, 19, 20, 21), the laser pulse properties [Eqs. (22, 23) and Fig. 3], the rate equations (24), and the integral over the interaction volume in Eq. (25). Note the logarithmic scale along the ordinate of the inset.Reuse & Permissions
Figure 5
Schematic depiction of energy levels associated with the excitation of xenon ions from the $5 s$ subshell. For instance, ${Xe}^{+}$ can be photoionized via the absorption of two vuv photons by a $5 p$ electron. A second ionization path, alluded to in this figure, involves a photon absorption that virtually excites a $5 s$ electron to the $5 p$ shell. Simultaneously, a $5 p$ electron is excited to an autoionizing state associated with a Rydberg series converging to the $5 s$ $5 p^{5}$ threshold of ${Xe}^{2 +}$ . Electron correlation then induces a transition from $5 p$ to $5 s$ accompanied by the emission of an electron.Reuse & Permissions
Figure 6
The same as in Fig. 4, the only difference being the fact that the multiphoton ionization cross sections underlying the calculation were taken from Eqs. (27, 28, 29, 30, 31), not Eqs. (17, 18, 19, 20, 21). Here $⟨ N_{q} ⟩$ is the average number of ${Xe}^{q +}$ ions produced per VUV-FEL radiation pulse.Reuse & Permissions

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