Photoelectron spectra in strong-field ionization by a high frequency field

We analyze atomic photoelectron momentum distributions induced by bichromatic and monochromatic laser fields within the strong field approximation (SFA), separable Coulomb-Volkov approximation (SCVA), and ab initio treatment. We focus on the high frequency regime -- the smallest frequency used is larger than the ionization potential of the atom. We observe a remarkable agreement between the ab initio and velocity gauge SFA results while the velocity gauge SCVA fails to agree. Reasons of such a failure are discussed.Comment: Completely rewritten paper. Ionization by a two-color field is adde

Physics of correlated double ionization of atoms in intense laser fields : Quasistatic tunneling limit

We revisit the recollision picture of correlated multiphoton double ionization of atoms in strong laser fields and develop consistent semiclassical model in the tunneling limit. We illustrate the model by applying it to helium and obtain quantitative agreement with recent experiments [B. Walker, E. Mevel, Baorui Yang, P. Breger, J. P. Chambaret, A. Antonetti, L. F. DiMauro, and P. Agostini, Phys. Rev. A 48, R894 (1993); B. Walker, B. Sheehy, L. F. DiMauro, P. Agostini, K. J. Schafer, and K. C. Kulander, Phys. Rev. Lett. 73, 1227 (1994)]. Developing the model, we address several problems of general interest, such as the reduction of intense field-assisted electron-ion collision to the field-free one and the total-cross-sections that include all inelastic channels. We describe a set of important physical effects responsible for the surprisingly high yield of doubly charged ions of noble gas atoms. All effects originate from the key role of the Coulomb potential and its interplay with the laser field. In addition to the Coulomb focusing of the oscillating trajectories onto the parent ion, other effects include transient trapping of electrons after tunneling in the vicinity of the parent ion, the creation of high-velocity electrons at all phases of the laser field, and the dominant role of collisional excitation of the parent ion followed by laser-assisted ionization.Peer reviewed: YesNRC publication: Ye

The origins and spread of domestic horses from the Western Eurasian steppes

This is the final version. Available on open access from Nature Research via the DOI in this recordData availability: All collapsed and paired-end sequence data for samples sequenced in this study are available in compressed fastq format through the European Nucleotide Archive under accession number PRJEB44430, together with rescaled and trimmed bam sequence alignments against both the nuclear and mitochondrial horse reference genomes. Previously published ancient data used in this study are available under accession numbers PRJEB7537, PRJEB10098, PRJEB10854, PRJEB22390 and PRJEB31613, and detailed in Supplementary Table 1. The genomes of ten modern horses, publicly available, were also accessed as indicated in their corresponding original publications57,61,85-87.NOTE: see the published version available via the DOI in this record for the full list of authorsDomestication of horses fundamentally transformed long-range mobility and warfare. However, modern domesticated breeds do not descend from the earliest domestic horse lineage associated with archaeological evidence of bridling, milking and corralling at Botai, Central Asia around 3500 BC. Other longstanding candidate regions for horse domestication, such as Iberia and Anatolia, have also recently been challenged. Thus, the genetic, geographic and temporal origins of modern domestic horses have remained unknown. Here we pinpoint the Western Eurasian steppes, especially the lower Volga-Don region, as the homeland of modern domestic horses. Furthermore, we map the population changes accompanying domestication from 273 ancient horse genomes. This reveals that modern domestic horses ultimately replaced almost all other local populations as they expanded rapidly across Eurasia from about 2000 BC, synchronously with equestrian material culture, including Sintashta spoke-wheeled chariots. We find that equestrianism involved strong selection for critical locomotor and behavioural adaptations at the GSDMC and ZFPM1 genes. Our results reject the commonly held association between horseback riding and the massive expansion of Yamnaya steppe pastoralists into Europe around 3000 BC driving the spread of Indo-European languages. This contrasts with the scenario in Asia where Indo-Iranian languages, chariots and horses spread together, following the early second millennium BC Sintashta culture

Nonadiabatic tunnel ionization : looking inside a laser cycle

We obtain a simple closed-form analytical expression for ionization rate as a function of instantaneous laser phase ?(t), for arbitrary values of the Keldysh parameter ?, within the usual strong-field approximation. Our analysis allows us to explicitly distinguish multiphoton and tunneling contributions to the total ionization probability. The range of intermediate ??1, which is typical for most current intense field experiments, is the regime of nonadiabatic tunneling. In this regime, the instantaneous laser phase dependence differs dramatically from both quasistatic tunneling and multiphoton limits. For cycle-averaged rates, our results reproduce standard Keldysh-like expressions.Peer reviewed: YesNRC publication: Ye

Correlated multiphoton double ionization of helium: The role of non-adiabatic tunneling and singlet recollision

We study the role of singlet coupling during double ionization of Helium atoms in intense laser fields, within the framework of the recollision model. The singlet cross sections for inelastic e+He+ scattering, integrated over all excitation and ionization channels, are used. Nonadiabatic corrections during tunneling of the active electron are also included.Peer reviewed: YesNRC publication: Ye

Adaptation of the modified adiabatic approximation to strong-field ionization

Savichev\u2019s modified adiabatic approximation [Sov. Phys. JETP 73, 803 (1991)] is used to obtain a general form of a quantum-mechanical amplitude of ionization by a low-frequency laser field. The method possesses only one requirement that the frequency of the laser field must be low. Connections of the obtained result with the quasiclassical approximation and other previous investigations are discussed.Peer reviewed: YesNRC publication: Ye

Attosecond photoionization of a coherent superposition of bound and dissociative molecular states: effect of nuclear motion

We study numerically the possibility for monitoring electron motion in a dissociating molecule using an attosecond XUV probe pulse which photoionizes a coherent superposition of two nuclear wave packets. We present the photoelectron spectra and forward\u2013backward asymmetries in these spectra obtained from a numerical solution of the time-dependent Schr\uf6dinger equation for one electron and two protons both moving in 1D, along the laser polarization vector. In our (non-Born\u2013Oppenheimer) approach the 1D dissociative ionization of the model H\u207a\u2082 with softcore potential is described exactly. We find that in general the nuclear motion in a fast moving light molecule does not wash out the oscillations as function of the time delay between XUV probe and the pump pulses as expected from the model with fixed nuclei.Peer reviewed: YesNRC publication: Ye

Ultrafast multiphoton forest fires and fractals in clusters and dielectrics

We describe the interaction of ultrashort infrared laser pulses with clusters and dielectrics. Rapid ionization occurs on a sub-laser wavelength scale below the conventional breakdown threshold. It starts with the formation of nano-droplets of plasma which grow like forest fires, without any need for heating of the electrons promoted to the conduction band. The dimensionality of the damaged area can be fractal and changes during the laser pulse. This mechanism is operative in both rare gas clusters and dielectrics interacting with ultrashort, moderately intense laser pulses which include only several periods of the driving field, so that the traditional avalanche mechanisms have no time to develop.Peer reviewed: YesNRC publication: Ye