Yongkai Deng

Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.

Double Ionization of Nitrogen from Multiple Orbitals

In intense femtosecond laser fields, molecules could be tunnel ionized from multiple valence orbitals, which produces molecular ions in different electronic states. In this article, we have used a reaction microscope to study double ionization of nitrogen by intense femtosecond laser pulses. It is found that some doubly charged molecular ions N(2)(2+) are metastable while others dissociate through charge symmetric dissociation (N(2)(2+) --> N(+) + N(+)) or charge asymmetric dissociation (N(2)(2+) --> N(2+) + N). The kinetic energy releases and angular distributions of atomic ions are measured for the dissociation channels. With the aid of the CASSCF and MRCI calculations, the electronic states are identified and the contributions of the valence orbitals are discussed for these dissociated molecular dications.

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.

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.

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.

Optimizing single-nanoparticle two-photon microscopy by in situ adaptive control of femtosecond pulses

Single-nanoparticle two-photon microscopy shows great application potential in super-resolution cell imaging. Here, we report in situ adaptive optimization of single-nanoparticle two-photon luminescence signals by phase and polarization modulations of broadband laser pulses. For polarization-independent quantum dots, phase-only optimization was carried out to compensate the phase dispersion at the focus of the objective. Enhancement of the two-photon excitation fluorescence intensity under dispersion-compensated femtosecond pulses was achieved. For polarization-dependent single gold nanorod, in situ polarization optimization resulted in further enhancement of two-photon photoluminescence intensity than phase-only optimization. The application of in situ adaptive control of femtosecond pulse provides a way for object-oriented optimization of single-nanoparticle two-photon microscopy for its future applications.