Method for calculating multiphoton above-threshold processes in atoms: Two-photon above-threshold ionization

The two-photon above-threshold ionization of atoms is calculated using numerical algorithms of the Padé approximation in the model-potential method with the Coulomb asymptotics. The total and differential cross sections of the above-threshold ionization of helium and alkali metal atoms by elliptically polarized radiation are presented. The dependence of the angular distribution of photoelectrons on the sign of the ellipticity of radiation (the elliptic dichroism phenomenon) is analyzed in the above-threshold frequency range.     

Attosecond electron wave packet dynamics in strong laser fields

We use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (approximately 20 eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes.     

Experimental evidence of above-threshold photoemission in solids

Nonlinear photoemission from a silver single crystal is investigated by femtosecond laser pulses in a perturbative regime. A clear observation of above-threshold photoemission in solids is reported for the first time. The ratio between the three-photon above-threshold and the two-photon Fermi edges is found to be 10(-4). This value constitutes the only available benchmark for theories aimed at understanding the mechanism responsible for above-threshold photoemission in solids.     

阈上电离过程中光电子角分布的精细结构

应用非微扰散射理论并考虑自发辐射 ,研究了阈上电离过程中光电子的角分布。由于自发辐射的影响 ,电子的角分布具有精细结构 ,成功地解释了Nandor等人的实验观测。     

Conclusive evidence of an attosecond pulse train observed with the mode-resolved autocorrelation technique

We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.     

Role of XUV Photons in Atomic High-Order Above-Threshold Ionization Processes in IR+XUV Two-Color Laser Fields

We investigate the above-threshold ionization of an atom in a combined infrared(IR) and extreme ultraviolet(XUV) two-color laser field and focus on the role of XUV field in the high-order above-threshold ionization(HATI)process. It is demonstrated that, in stark contrast to previous studies, the XUV laser may play a significant role in atomic HATI process, and in particular, the XUV laser can accelerate the ionized electron in a quantized way during the collision between the electron and its parent ion. This process cannot be explained by the classical three-step model. Our results indicate that the previously well-established concept that HATI is an elastic recollision process is broken down.     

Threshold-related enhancement of the high-energy plateau in above-threshold detachment

We present nonperturbative theoretical results showing a resonant-like enhancement of above-threshold detachment spectra in the region of the high-energy plateau as the laser intensity sweeps across channel thresholds. This enhancement has a pure quantum origin stemming from well-known threshold phenomena in multichannel problems whose features are clearly demonstrated in our numerical results. Similar well-known anomalies at neutral atom thresholds are expected to explain experimentally observed resonant-like enhancements of above-threshold ionization spectra.     

Energy- and angle-resolved photoelectron spectra of above-threshold ionization and detachment

We analyze strong-laser-field induced above-threshold ionization and above-threshold detachment processes within the strong-field approximation. Energy and angular distribution of photoelectrons are presented for two cases: (1) high-order above-threshold ionization of electrons from neutral atoms by a linearly polarized laser field and (2) above-threshold detachment of electrons from negative ions by a bicircular laser field. In particular, for the case (1) we compare the results obtained using a long pulse with the flat-top spatial intensity distribution in the laser focus with the results obtained using the Gaussian distribution. We also analyze the intensity-dependent resonant-like enhancements. We show that the position and shape of these enhancements strongly depend on the atomic ground state parity. The bicircular field of the case (2) consists of two counter-rotating circularly polarized fields having the frequencies rω and sω with integer r and s. The properties of the photoelectron spectra generated by such bicircular field are illustrated using an ω–4ω example. The spectra obtained are explained in terms of the interference of partial detachment amplitudes which correspond to different complex solutions of the saddle-point equations.     

Relativistic nonperturbative above-threshold phenomena in strong laser fields

Relativistic features of various nonperturbative above-threshold phenomena in strong laser fields are discussed and compared. This includes above-threshold ionization of multiply charged ions as well as pair production in an ultrastrong laser wave, superimposed with either a nuclear Coulomb field or another counterpropagating laser wave. For the probability of above-threshold pair production, a new scaling relation is given. Particular attention is paid to similarities among these processes, regarding the energy and angular spectra of the particles as well as the total reaction rates.     

Excited-state relaxation of Ag8 clusters embedded in helium droplets

Neutral silver clusters Ag(N) are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for Ag8, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of (6.89+/-0.09) eV for the vertical ionization potential of Ag8. The influence of the helium matrix on photoemission is discussed.     

Frequency-resolved optical gating of isolated attosecond pulses in the extreme ultraviolet

The pulse shape and phase of isolated attosecond extreme ultraviolet (XUV) pulses with a duration of 860 asec have been determined simultaneously by using frequency-resolved optical gating based on two-photon above-threshold ionization with 28-eV photons in He. From the detailed characterization, we succeeded in shaping isolated XUV pulses on an attosecond time scale by precise dispersion control with Ar gas density or by changing the driving pulse width. These results offer a novel way to excite and observe an electron motion in atoms and molecules.     

Photoelectron angular distributions from two-photon ionization of atoms near ionization threshold

Photoelectron angular distributions(PADs) from two-photon ionization of atoms in linearly polarized strong laser fields are obtained in accordance with the nonperturbative quantum scattering theory.We also study the influence of laser wavelength on PADs.For two-photon ionization very close to the ionization threshold,most of the ionized electrons are vertically ejected to the laser polarization.PADs from twophoton ionization of atoms are determined by the second order generalized phased Bessel function at which the ponderomotive parameter plays a key role.In terms of dependence of PADs on laser wavelength,corresponding variations for the ponderomotive parameter are demonstrated.     

Production of doubly charged helium ions by two-photon absorption of an intense sub-10-fs soft x-ray pulse at 42 eV photon energy

We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.     

Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ～ 21 eV, ～ 5 TW/cm2), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e? and Ar2(+) + 2hν→Ar(+) + e?. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).     

真实脉冲激光作用下的原子超阈值离化

在文献[18]的基础上进一步讨论真实脉冲激光作用下的原子超阈值离化,在考虑了初始离化时刻的影响后,结果清楚地表明,随激光脉宽减小而产生峰红移,以及同时出现的离化电子谱峰的增宽现象.     

The spectrum of above-threshold electrons produced upon atom ionization by a short light pulse

The Landau-Dykhne approximation for the spectrum of above-threshold electrons produced upon ionization of atoms by a monochromatic laser field is used to calculate the spectrum in the case of ionization by a short laser pulse. The spectra obtained in this way are compared with those calculated by the adiabatic theory. The comparison shows that the Landau-Dykhne approximation with the properly chosen exponential factor describes well the averaged spectrum.     

Above-threshold ionization of xenon by intense laser field

Above-threshold ionization of xenon atom is treated by the KFR theory withconsideration of screening effects in the initial state.Comparison is made with experiment us-ing circularly polarized laser, screening effects on the above-threshold ionization is also dis-cussed.Better agreement with experiment is found than the previous results using hydrogenicwavefunction.     

Recoil-ion momentum distributions for two-photon double ionization of He and Ne by 44 eV free-electron laser radiation

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (variant Planck's over omega=44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of approximately 1 x 10(14) W/cm2 exploring the dynamics of the two fundamental two-photon-two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.     

Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (～24.1 eV, ～5 TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr?dinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.     

Nonsequential two-photon double ionization of atoms: identifying the mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process can be described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.