Electron rescattering in above-threshold photodetachment of negative ions

It is all about phases: ultrafast holographic photoelectron imaging

Photoelectron holography constitutes a powerful tool for the ultrafast imaging of matter, as it combines high electron currents with subfemtosecond resolution, and gives information about transition amplitudes and phase shifts. Similarly to light holography, it uses the phase difference between the probe and the reference waves associated with qualitatively different ionization events for the reconstruction of the target and for ascertaining any changes that may occur. These are major advantages over other attosecond imaging techniques, which require elaborate interferometric schemes in order to extract phase differences. For that reason, ultrafast photoelectron holography has experienced a huge growth in activity, which has led to a vast, but fragmented landscape. The present review is an organizational effort towards unifying this landscape. This includes a historic account in which a connection with laser-induced electron diffraction is established, a summary of the main holographic structures encountered and their underlying physical mechanisms, a broad discussion of the theoretical methods employed, and of the key challenges and future possibilities. We delve deeper in our own work, and place a strong emphasis on quantum interference, and on the residual Coulomb potential.

Double Photodetachment of F-·H2O: Experimental and Theoretical Studies of [F·H2O]. J Phys Chem Lett 2018;9:6808-6813. [PMID: 30433784 DOI: 10.1021/acs.jpclett.8b02562]

Double photodetachment of the cluster F^-·H₂O in a strong laser field is explored in a combined experimental-theoretical study. Products are observed experimentally by coincidence photofragment imaging following double ionization by intense laser pulses. Theoretically, equation of motion coupled cluster calculations (EOM-CC), suitable for modeling strong correlation effects in the electronic wave function, shed light on the Franck-Condon region, and ab initio molecular dynamics simulations also performed using EOM-CC methods reveal the fragmentation dynamics in time on the lowest-lying singlet and triplet states of [F·H₂O]⁺. The simulations show the formation of H₂O⁺ + F, which is the predominant experimentally observed product channel. Suggestions are proposed for the formation mechanisms of the minor products, for example, the very interesting H₂F⁺, which involves significant geometrical rearrangement. Analysis of the results suggests interesting future directions for the exploration of photodetachment of anionic clusters in an intense laser field.

Photoelectron momentum distributions of F- ions by a few-cycle laser pulse

The two-dimensional photoelectron momentum distributions (PMD) of F^- ions induced by a linearly polarized few-cycle laser pulse are analyzed with the saddle-point (SP) method. The validity of the SP method is confirmed by comparing the PMD with those obtained from direct numerical integration of the transition probability amplitude in the context of strong-field approximation (SFA). We analyze the intra- and inter-cycle interference patterns in the two-dimensional PMD and show that the two-dimensional PMD can be effectively monitored by changing the carrier-envelope phase of few-cycle laser pulse. In addition, by separately calculating the two-dimensional PMD formed in the different detachment steps, we find that the rich oscillatory patterns in the two-dimensional PMD can be mainly attributed to the interference effects of electronic wave packets in the classical propagation step after the ionization, and part of intra-cycle interference fringes' shape is affected by the sub-barrier phase.

Intense-Field Multiple-Detachment of F2¯: Competition with Photodissociation

The competition of intense-field multiple-detachment with efficient photodissociation of F₂¯ is studied as a function of laser peak intensity. The main product channels are disentangled and characterized by 3D coincidence fragment imaging. The presented kinetic energy release spectra, angular distributions, as well as two-color pump-probe measurements allow identification of competing sequential and nonsequential mechanisms. Dissociative detachment, producing two neutral atoms (F + F), is found to be dominated by a sequential mechanism of photodissociation (F¯ + F), followed by detachment of the atomic anion fragment. In contrast, dissociative ionization (F + F⁺) shows competing contributions of both a sequential two-step mechanism as well as a nonsequential double-detachment of the molecular anion, which are distinguished by the kinetic energy released in the dissociation. Triple-detachment is found to be nonsequential in nature and results in Coulomb explosion (F⁺ + F⁺). Furthermore, the measured kinetic energy release for dissociation on the ²Σ_g⁺ state provides a direct measurement of the F₂¯ dissociation energy, D₀ = 1.26 ± 0.03 eV.

Femtosecond photodetachment of silver anions

The two- and three-photon detachment of negative silver ions in a femtosecond infrared laser field is studied using photoelectron velocity map imaging methods. Photoelectron angular distributions (PADs) are obtained for these detachment channels; these PADs change dramatically when the laser wavelength and intensity are changed. Theoretical predictions, which are based on the adiabatic Keldysh-Faisal-Reiss saddle point method, are in good agreement with our experiment. The dependence of the PAD on the laser wavelengths and intensities is due to the interference between the different partial wave functions. The relative contributions of the different partial waves to the detachment amplitude are altered by changing the laser parameters and, as a result, the shape of the PAD. Close to the detachment threshold, the two-photon detachment process also follows the Wigner threshold law. Near the detachment threshold, the large differences between the calculated results and our experimental results indicate that the ponderomotive energy shifts caused by the femtosecond laser fields must be taken into account in the theoretical model. The three-photon detachment of Ag(-) is also observed and compared with theoretical calculations.

Scaling of the low-energy structure in above-threshold ionization in the tunneling regime: theory and experiment

A calculation of the second-order (rescattering) term in the S-matrix expansion of above-threshold ionization is presented for the case when the binding potential is the unscreened Coulomb potential. Technical problems related to the divergence of the Coulomb scattering amplitude are avoided in the theory by considering the depletion of the atomic ground state due to the applied laser field, which is well defined and does not require the introduction of a screening constant. We focus on the low-energy structure, which was observed in recent experiments with a midinfrared wavelength laser field. Both the spectra and, in particular, the observed scaling versus the Keldysh parameter and the ponderomotive energy are reproduced. The theory provides evidence that the origin of the structure lies in the long-range Coulomb interaction.

Interference carpets in above-threshold ionization: from the Coulomb-free to the Coulomb-dominated regime

The velocity map recorded in above-threshold ionization of xenon at 800 nm exhibits a distinct carpetlike pattern of maxima and minima for emission in the direction approximately perpendicular to the laser polarization. The pattern is well reproduced by a numerical solution of the time-dependent Schrödinger equation. In terms of the simple-man model and the strong-field approximation, it is explained by the constructive and destructive interference of the contribution of the long and the short orbit. Strictly perpendicular emission is caused by ionization at the two peaks of the laser field per cycle, which results in a 2ħω separation of the above-threshold ionization rings.

Low-energy structures in strong field ionization revealed by quantum orbits

Experiments on atoms in intense laser pulses and the corresponding exact ab initio solutions of the time-dependent Schrödinger equation (TDSE) yield photoelectron spectra with low-energy features that are not reproduced by the otherwise successful work horse of strong field laser physics: the "strong field approximation" (SFA). In the semiclassical limit, the SFA possesses an appealing interpretation in terms of interfering quantum trajectories. It is shown that a conceptually simple extension towards the inclusion of Coulomb effects yields very good agreement with exact TDSE results. Moreover, the Coulomb quantum orbits allow for a physically intuitive interpretation and detailed analysis of all low-energy features in the semiclassical regime, in particular, the recently discovered "low-energy structure" [C. I. Blaga, Nature Phys. 5, 335 (2009) and W. Quan, Phys. Rev. Lett. 103, 093001 (2009).