Excited Ions in Intense Femtosecond Laser Pulses: Laser-Induced Recombination

Abstract

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied. DOI: 10.1103/PhysRevLett.99.173002 PACS numbers: 32.80.Rm, 32.80.Wr Intense ultrashort laser pulses are an important tool for probing the nonlinear physics of atoms, molecules and clusters in the inherent strong fields. Such studies have been used to elucidate fundamental quantum processes in the regime where electric (E) field strengths are of a similar order of magnitude to that binding electrons and where pulse time scales approaching a femtosecond constrain the quantum dynamics. In addition some of the resulting processes are being harnessed, for example, high-harmonic generation to produce attosecond x-ray pulses [1] in order to further probe atomic systems [2] and for practical nanotechnological applications [3,4].Arguably the single most important aspect of intense field studies is the ''rescattering'' of an ionized electron from its ionic core in a linearly polarized pulse. Proposed in a seminal paper by Corkum [5], the process relies on the ejected electron periodically revisiting the core as it is driven to and from it by the oscillating E-field of the laser. Acceleration in the external field, coupled with Coulombic acceleration and focussing may lead to recollision with the ion core. The narrow momentum distribution of the wave packet results in a significant probability of electron-ion interaction. This opens up the fascinating prospect of studying electron-ion scattering phenomena, notoriously difficult to investigate in a conventional beam-beam approach [6], with the added bonus that the electron ''pulse'' is on a femtosecond time scale [7].The possible electron collision mechanisms, i.e., elastic scattering, inelastic scattering, and recombination, have all been observed by various means. Elastic scattering leads to momentum transfer, such that the scattered electron is unlikely to further recollide with the core and will be observable at the high energy extreme of the detected electron spectrum, contributing to the above-threshold ionization process [8]. Inelastic scattering contributes to the nonsequential ionization process, either directly by electron impact ionization [9] or indirectly by excitation followed by tunnelling ionization in the field [10]. Recombination, which may be regarded as the inverse of above-threshold ionization, has been observed by detection of the stabilizing photon, the source of high-harmonic generation [11]. However, the underlying effect of laserinduced recombination has not been independently experimentally studied in the femtosecond pulse regime, and indeed recombination has never been detect...

Measurements of intense ultrafast laser-driven D3+fragmentation dynamics

“…The highest peak focal intensity for both pulse durations is 10 16 W/cm 2 and lower intensities are sampled by crossing the ion beam with the laser away from the center of the laser focus, that is, shifting the laser focus along its propagation direction while keeping the ion beam position fixed (e.g., Refs. [32][33][34]), a variant of the intensity selective scanning (ISS) method [35,36].…”

Frustrated tunneling ionization during laser-induced D2fragmentation: Detection of excited metastable D*atoms

“…This has been used to resolve attosecond timescale motion of molecular dynamics [11][12][13][14]. Electron recombination with the parent ion has been observed by detecting the high-harmonic photons emitted in this process (see, e.g., [15]) or through detection of the particles following the recombination event [16]. Electron recombination is the foundation of the modern era of optical attosecond science [17][18][19].…”

Frustrated tunnelling ionization during strong-field fragmentation of D3+

“…From the experimental evidence, we conclude that the origin of the feature is due to frustrated tunnelling ionization, the first observation of this mechanism in a polyatomic system. Furthermore, we unravel evidence of frustrated tunnelling ionization in dissociation, both two-body breakup to D + D + 2 and D + + D 2 , and three-body breakup to D + + D + D. Gesellschaft processes involving either elastic scattering [6][7][8], inelastic scattering [9, 10], or electron-ion recombination [11]. These phenomena have led to the birth of new areas of research such as high-harmonic generation and attosecond science [12][13][14][15], laser-driven electron diffraction imaging [5][6][7][8], molecular orbital tomography [16,17] and electron wavepacket probing of molecular dynamics [18-21]-naming only a few.Related to the electron recollision process, recently Nubbemeyer et al [22] reported a new phenomenon dubbed frustrated tunnelling ionization (FTI).…”