Strong-Field Double Ionization of Ar below the Recollision Threshold

Abstract

Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at 4-7 10 13 W=cm 2 was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed. DOI: 10.1103/PhysRevLett.101.053001 PACS numbers: 32.80.Rm, 31.90.+s, 32.80.Fb, 32.80.Wr Within the past three decades extensive studies on the interaction of intense laser fields with atoms and molecules have resulted in a profound understanding of various strong-field phenomena. Prominent examples are abovethreshold ionization (ATI) [1] or high-order harmonic generation [2], both essentially treatable within the single active electron (SAE) approximation. Correlated fewelectron processes, on the other hand, most important, for example, in nonsequential double (multiple) ionization (NSDI) (for a recent review see, e.g., [3]) have, until the present day, resisted any comprehensive modeling.Recently, however, a breakthrough was achieved at high intensities ( PW=cm 2 ) [4]. The application of many-particle imaging techniques [reaction microscopes and cold target recoil ion momentum spectroscopy (COLTRIMS)] [5] has allowed recording (multi)differential data on strong-field few-electron reactions and sophisticated calculations (see, e.g., [6] and references therein) have advanced their theoretical interpretation. As a result, a commonly accepted though simple picture has emerged, characterizing NSDI. Here an electron first tunnels into the field, is then accelerated, and finally thrown back onto its parent ion by the oscillating laser field. During ''recollision'' n-fold ionization might occur either in a direct e; ne -like encounter or indirectly via recollision-induced excitation of the ion plus subsequent field ionization (RESI) [7]. Signatures of the former are that both electrons are exclusively emitted into the same hemisphere along the polarization direction leading to ''double-hump'' shaped parallel momentum distributions of the ions (compensating the electron momenta). For RESI instead, according to the present understanding, the electrons can be emitted either parallel or back to back, thus filling the valley in between the double hump for the ions. Beyond the well-accepted simple scenario, however, major questions about the correlated electron emission are still far from being understood and are extensively investigated [6,8] because recollision is at the very heart of attoscience, molecular tomography, or imaging [9].At low intensities one intriguing though still widely unexplored question did arise early on within the above picture [10]. What happens when the energy of the recolliding electron of up to 3:17U P is not sufficient t...

Attosecond tracing of correlated electron-emission in non-sequential double ionization

“…The ion and electron optics are symmetric in reality, and a few electrodes have been removed for visualization of the laser path. 14 W cm − 1 (narrow and wide black spectra respectively). The gray area in between the two black curves represents the error bars of ± 20% in the determination of the peak intensity.…”

“…Recording the momentum spectra of doubly 10,11 and multiply 7 charged ions provided clear evidence that the emission of the second electron is triggered by the laser-driven recollision of the first electron with its parent ion. Even more insight into the process was gained in kinematically complete studies where the correlated two-electron momentum distributions were measured 11 and investigated for different laser parameters [12][13][14][15] . Measurements of the dynamics of NSDI in near-single cycle laser pulses have been restricted to recoil-ion spectra, which did not allow for the direct retrieval of the correlated electron-emission dynamics 16,17 .…”

Exploring few-photon, few-electron reactions at FLASH: from ion yield and momentum measurements to time-resolved and kinematically complete experiments

“…Still, the subtle details of the underlying dynamics are not completely settled and, thus, subject of continuing experimental as well as theoretical effort (see e.g. [19][20][21][22] is the ponderomotive potential, I the light intensity and ω it's frequency). In the first decade of the 21 st century the rapid development of laser-(high-harmonic generation, HHG) [23,24] and accelerator-based (free-electron lasers, FEL) [25][26][27][28] light sources enabled generating intense, ultrashort VUV, EUV, and even X-ray pulses.…”

Double-Electron Ionization Driven by Inhomogeneous Fields

“…This observable has allowed to disentangle the common sequential and non-sequential double RESI, rescattering impact ionization and laser-field assisted rescattering ionization mechanisms [12,13,37]. Fig.…”