Relativistic effects in nonrelativistic ionization

Abstract

Phys. Rev. Lett. 106, 193002 (2011)] in atomic ionization by circularly polarized light are readily explained by the relativistic strong-field approximation (RSFA). The physical picture that emerges is determined by linear and angular momentum properties of the photons required for ionization, and it is largely independent of the atom being ionized. Radiation pressure follows from linear momenta carried by photons, and the angular momenta of the photons require the photoelectron to be in a circular orbit around the remnant ion. Two special features of the Smeenk et al. experiments are highlighted by the analysis. One is that this is the first verification of true relativistic effects in ionization. The other is that the circular trajectory picture from the RSFA analysis directly contradicts the physical picture that emerges from the length gauge. This is an exceptionally striking example of gauge-dependent differences in physical interpretations. Nevertheless, the distribution of momentum found by the Smeenk et al. experiments is accurately predicted by the RSFA. Results herein provide a universal theory to which the analysis of Titi and Drake [A. S. Titi and G. W. F. Drake, Phys. Rev. A 85, 041404(R) (2012)] represents an approximation. The physical properties of ionization by circularly polarized light also cast doubt on some predictions of rescattering theory.

Physical restrictions on the choice of electromagnetic gauge and their practical consequences

“…Each photon carries a momentum of ω/c, with all photon momenta aligned in the direction of propagation of the laser field, so the field-induced momentum in the propagation direction is just U p /c, and this is independent of the atom being ionized when the field is strong. This is precisely what the laboratory measurements reveal [26,28]. Transverse-field concepts produce insightful and quantitatively accurate results as shown in the span of three sentences given above, as contrasted with three journal articles [11,26,27].…”

Magnetic fields alter strong-field ionization

“…[5][6][7] Here, by utilizing a novel experimental approach of two counter-propagating laser pulses, we present a detailed study on the effects of the photon momentum in strong-field ionization. The high precision and selfreferencing of the method allows to unambiguously demonstrate the action of the light's magnetic field on the electron while it is under the tunnel barrier, confirming theoretical predictions [1][2][3]8 , disproving others 5,9,10 . Our results deepen the understanding of, for example, molecular imaging 11,12 and time-resolved photoelectron holography 13 .…”

Supercontinuum in ionization by relativistically intense and short laser pulses: Ionization without interference and its time analysis

“…(26) is not shifted by the vector (q 0 − p 0 )n, independently of the gauge choice. This term, among others, is responsible for the so-called radiation pressure [61] which has recently been discussed in the literature (see, e.g., [47,[62][63][64][65][66][67]). …”