Quasiclassical modelling of helium double photoionization

Abstract

We applied the method known as fermion molecular dynamics (FMD) to the description of a helium atom interacting with a short pulse of intense, long-wavelength laser radiation, for both linear and circular polarization. We describe the results of these calculations, insofar as they bear on the question of the mechanisms leading to double electron ejection. In the case of linear polarization, boomeranging trajectories leading to double ionization were observed at all laser intensities above threshold. The probability of occurrence was low, and almost independent of laser intensity. However, very near to threshold, boomeranging trajectories leading to double ionization were found to occur with a probability comparable to that for all other independent electron (sequential) processes. This produced a shoulder in the curve of double ionization probability versus laser intensity. The size of this shoulder was found to depend on laser wavelength and pulse length. No such trajectories were found for circular polarization.

Double ionization of helium with classical ensemble simulations

“…1 that the double ionization probability increases with intensity, the "knee" signature is clearly predicted in classical calculations at the wave- length of 532 nm, 780 nm and 1024 nm, whereas at laser wavelength of 248 nm the NSDI is not obvious. This tendency is similar to the experimental and semi-classical results [16,17]. At 1 × 10 14 W/cm 2 , the double ionization probability is the largest at the wavelength of 1024 nm.…”

“…), (17) where [I] represents the laser pulse. We defined these three processes as survival, single ionization and double ionization.…”

Many-electron strong-field physics

“…In the standard SPA, the action (19) in the matrix element (18) is expanded to second order about the solutions to the saddle-point Equations (21), whereupon the integrations can be carried out and yield…”

“…The other is entirely rooted in classical physics and studies very large ensembles of classical two (or more) electron trajectories to find those that lead to double (or multiple) ionisation [17,18]. Intermediate are fermion molecular dynamics [19] and classical-trajectory calculations that maintain the quantum element of tunnelling [20].…”

Few-body dynamics in ultrashort laser pulses

“…[7,8]): An electron, excited due to the interaction with the intense laser field in the continuum, is accelerated by the field, gains energy and can be driven back to the parent ion when the field changes its sign. Upon recollision it exchanges energy with the second electron via electron correlation interaction and either both electrons instantaneously leave the atom together or only one is detached leaving the second in an excited state, which is subsequently ionised by the field [9,10,11]. We may note that the rescattering mechanism can be seen as a strong-field extension of the two-step-one mechanism (TS1), known for single photon double ionization at low photon energies [12].…”