Yunquan Liu

Nonadiabatic tunneling ionization of atoms in elliptically polarized laser fields

We theoretically investigate the nonadiabatic effects in strong field tunneling ionization of atoms in elliptically polarized laser fields by solving the 3D time-dependent Schrodinger equation (TDSE). Comparing our TDSE results with those of two semi-classical methods, i.e., the quantum-trajectory Monte Carlo simulation (QTMC) and the Coulomb-corrected strong field approximation (CCSFA), we confirm the existence of the nonadiabatic effects with its fingerprint in the nonzero initial lateral velocity at the tunneling exit in the laser polarization plane. Our study shows that these nonadiabatic initial lateral momentum effects become significant in high ellipticity or circularly polarized laser field. These results indicate that the calibration of the experimental laser intensity in this situation should be performed nonadiabatically, which may strongly affect the results of the real tunneling time delay measurements.

Communication: Determining the structure of the N2Ar van der Waals complex with laser-based channel-selected Coulomb explosion

We experimentally reconstructed the structure of the N2Ar van der Waals complex with the technique of laser-based channel-selected Coulomb explosion imaging. The internuclear distance between the N2 center of mass and the Ar atom, i.e., the length of the van der Waals bond, was determined to be 3.88 Å from the two-body explosion channels. The angle between the van der Waals bond and the N2 principal axis was determined to be 90° from the three-body explosion channels. The reconstructed structure was contrasted with our high level ab initio calculations. The agreement demonstrated the potential application of laser-based Coulomb explosion in imaging transient molecular structure, particularly for floppy van der Waals complexes, whose structures remain difficult to be determined by conventional spectroscopic methods.

Towards non-sequential double ionization of Ne and Ar using a femtosecond laser oscillator

We report on first proof-of-principles results on non-sequential double ionization of argon and neon achieved by using a newly developed long-cavity Ti:sapphire femtosecond oscillator with a pulse duration of 45 fs and a repetition of 6.2 MHz combined with a dedicated reaction microscope. Under optimized experimental conditions, peak intensities larger than 2.310(14) W/cm(2) have been achieved. Ion momentum distributions were recorded for both rare gases and show significantly different features for single as well as for double ionization. For single ionization of neon a spike of zero-momentum electrons is found when decreasing the laser intensity towards the lowest ionization rate we can measure which is attributed to a non-resonant ionization channel. As to double ionization, the longitudinal momentum distribution for Ne(2+) displays a clear double-hump structure whereas this feature is found to be smoothened out with a maximum at zero momentum for Ar(2+).

Revealing backward rescattering photoelectron interference of molecules in strong infrared laser fields

Photoelectrons ionized from atoms and molecules in a strong laser field are either emitted directly or rescattered by the nucleus, both of which can serve as efficiently useful tools for molecular orbital imaging. We measure the photoelectron angular distributions of molecules (N2, O2 and CO2) ionized by infrared laser pulses (1320 nm, 0.2 ~ 1 × 1014 W/cm2) from multiphoton to tunneling regime and observe an enhancement of interference stripes in the tunneling regime. Using a semiclassical rescattering model with implementing the interference effect, we show that the enhancement arises from the sub-laser-cycle holographic interference of the contributions of the back-rescattering and the non-rescattering electron trajectory. It is shown that the low-energy backscattering photoelectron interference patterns have encoded the structural information of the molecular initial orbitals and attosecond time-resolved dynamics of photoelectron, opening new paths in high-resolution imaging of sub-Ångström and sub-femtosecond structural dynamics in molecules.

Revealing the Sub-Barrier Phase using a Spatiotemporal Interferometer with Orthogonal Two-Color Laser Fields of Comparable Intensity

We measure photoelectron momentum distributions of Ar atoms in orthogonally polarized two-color laser fields with comparable intensities. The synthesized laser field is used to manipulate the oscillating tunneling barrier and the subsequent motion of electrons onto two spatial dimensions. The subcycle structures associated with the temporal double-slit interference are spatially separated and enhanced. We use such a spatiotemporal interferometer to reveal sub-barrier phase of strong-field tunneling ionization. This study shows that the tunneling process transfers the initial phase onto momentum distribution. Our work has the implication that the sub-barrier phase plays an indispensable role in photoelectron interference processes.

Scaling Laws of the Two-Electron Sum-Energy Spectrum in Strong-Field Double Ionization

The sum-energy spectrum of two correlated electrons emitted in nonsequential strong-field double ionization (SFDI) of Ar was studied for intensities of 0.3 to 2×10^{14} W/cm^{2}. We find the mean sum energy, the maximum of the distributions as well as the high-energy tail of the scaled (to the ponderomotive energy) spectra increase with decreasing intensity below the recollision threshold (BRT). At higher intensities the spectra collapse into a single distribution. This behavior can be well explained within a semiclassical model providing clear evidence of the importance of multiple recollisions in the BRT regime. Here, ultrafast thermalization between both electrons is found occurring within three optical cycles only and leaving its clear footprint in the sum-energy spectra.

Three-body fragmentation of CO2 driven by intense laser pulses

Dissociative ionization dynamics were studied experimentally for CO2 driven by intense laser pulses. Three-dimensional momentum vectors of correlated atomic ions were obtained for each three-body fragmentation event using triple ion coincidence measurement. Newton diagram demonstrated that three-body fragmentation of CO2 (n+) (n = 3-6) can occur through Coulomb explosion process and sequential fragmentation process depending on the fragmentation channels. The experimental data from these two processes were disentangled by using correlation diagram of correlated ions. Based on the accurate Coulomb explosion data, we reconstructed the bond angle distributions of CO2 (n+) at the moment of fragmentation, which are close to that of neutral CO2 before laser irradiation.

Coincidence imaging of photoelectrons and photo-ions of molecules in strong laser fields

In recent years, reaction microscopes have become a powerful technique to image ultrafast dynamics in atoms and molecules. Here, we present the specific details of our reaction microscope that was designed to study molecular dynamics driven by intense femtosecond laser pulses. A supersonic molecular beam and laser focusing system was specially designed to confine molecules to the peak intensity of the laser focus. Reaction channels can be precisely identified and three-dimensional momentum vectors can be accurately acquired for correlated reaction products with high resolution. The molecular structure information can be extracted based on the experimentally measured momentum vectors of the photoelectrons and the photo-ions that are generated in the laser–molecule interaction.

Fully differential measurement on above-threshold ionization of CO and CO 2 molecules in strong laser fields

We present a fully differential measurement on above-threshold ionization of diatomic (CO) and linear triatomic (CO2) molecules in strong femtosecond laser fields (25fs, 795nm) at a laser intensity of ∼8×1013W/cm2. The photoelectron longitudinal momentum distribution of CO in the laser polarization direction shows a broader distribution near zero momentum in the laser polarization plane with a shoulder around 0.38a.u., whereas it shows a clear maximum for CO2. The relative photoelectron yields near zero longitudinal momentum are less dependent on the laser intensity. The difference may result from the molecular structure. Even though molecular targets have different binding energy and outer orbitals, the near-threshold photoelectrons have similar angular distribution, and the dominant angular momentum of the near-threshold photoelectron is L=4 (ng Rydberg state) for both targets.

Multiple ionization of oxygen studied by coincident measurement

We experimentally study double and triple ionization of oxygen using a reaction microscope. The kinetic energy releases (KERs) and angular distributions are obtained through coincidentally measuring the ionic fragments of doubly or triply charged parent ions. The pathway O(2+)2 ? O (+) O(+)?proceeds through some excited electronic states. The KERs exhibit definite structures independent of the laser intensity and the pulse duration. However, the angular distribution of coincident O(+) reflects the symmetry of the highest occupied molecular orbital (HOMO) only for few-cycle laser pulses at low intensity. The pathways O(2+) 2 --> O(2+) +O and O(3+) 2 --> O(2+) + O(+) occur through some repulsive states. The KERs show a single broad peak and decrease with increasing the pulse duration. The decrease of KER comes from the stretch of the internuclear distance in intense laser fields.