Dissociative multi-ionization of N2O molecules in strong femtosecond laser field.

Multi-ionization and subsequent Coulomb explosion (CE) of the N2O molecule irradiated by a linearly polarized 800 nm laser field is investigated by a reaction microscope, where a number of CE channels of N2Oq+ with q ≤ 5 for two-body fragmentation and q ≤ 8 for three-body fragmentation were observed. For two-body CE, by analyzing the internuclear separations extracted from kinetic energy releases (KERs), dissociation branching fractions, and laser intensity dependence, interestingly, we found that fragmentation N2O5+ → N3+ + NO2+ is produced directly from dissociating N2O3+ via non-sequential stairstep ionization, whereas most of the others result from the sequential stairstep ionization. For three-body CE, 25 fragmentation channels of N2Oq+ (q = 3-8) are distinguished in the present charge-encoded multi-photoion coincidence plot, and the concerted fragmentation mechanism is nominated in a typical Dalitz plot. With the help of the numerical computation with the measured KERs and momentum correlation angles, the geometric structures of molecular ions prior to fragmentation are reconstructed, which display the bending motion and simultaneous two-bond stretching before the CE. Increasing of the bond length for high charged N2Oq+ indicates the dominating stairstep ionization in the three-body fragmentation.

Third-order harmonic generation in a bi-chromatic elliptical laser field.

The low-order harmonic generation induced by a strong laser field produces a bright, ultrashort, supercontinuum radiation ranging from the terahertz to ultraviolet band. By controlling the phase-delay and ellipticity of the bi-chromatic laser fields, the third harmonic generation is experimentally and theoretically investigated for elucidating the mechanism of the low-order harmonics. The third harmonic generation is found to be strongly suppressed in the counter-rotating bi-chromatic laser field due to the selection rule for harmonic emissions. The continuum-continuum transition in the strong field approximation is extended to explain the third harmonic generation as a function of the phase delay and ellipticity of the bi-chromatic laser fields. Compared with the semi-classical photocurrent model, the continuum-continuum transition on the basis of quantum-mechanical treatment achieves better agreement with the experimental observations. Our work indicates that the overlapping in continuum states via different quantum paths of a single electron plays a role in low-order harmonics generation under elliptical bi-chromatic laser fields.

Experimental evidence for terahertz emission of continuum electrons in the dual-color laser field.

Terahertz (THz) wave generation (TWG) in a dual-color laser is investigated with joint measurements between THz and third-harmonic generation, where the relative phase delay of dual-color fields is determined in situ in sub-wavelength accuracy, allowing for the clarification of the TWG mechanism in a direct comparison with various theoretical predictions. The delay- and polarization-dependent experiment validates that the continuum-continuum transition within the escaped electron wavepacket in the single atom gives birth to THz emission, while the bound energetic level does not contribute to TWG. TWG from atoms and molecules would provide an all-optical, vacuum-free, and ultrafast tool to record the spatiotemporal evolution of tunneling electron wavepackets.

Fragmentation dynamics of nitrogen trifluoride induced by electron collision.

The fragmentation dynamics of nitrogen trifluoride (NF3) in collisions with a 500 eV electron is studied by using a momentum imaging spectrometer. The kinetic energy releases of two-body, three-body, and four-body fragmentation channels of NF3 q+ (q = 2, 3) are investigated. The fragmentation dynamics of three-body, as well as four-body, dissociation channels is analyzed by the Dalitz plot and the Newton diagram. It is found that for all of the dissociation channels, the fragment including N atom (ion) always shares significant momenta, regardless of whether it is charged. For F atom, however, it is always emitted with negligible momenta.

Fragmentation dynamics of CS2 in collisions with 1.0 keV electrons.

The dissociation dynamics of CS2 molecules in collisions with 1.0 keV electrons is studied. We observe a series of two- and three-body fragmentation channels which are identified from the correlation map between fragment ions. For all of the channels, the kinetic energy release (KER) distributions are obtained. The Dalitz plot and Newton diagram are adopted to analyze the fragmentation dynamics of the three-body dissociation channels. For C S 2 3 + and C S 2 4 + , both the concerted and sequential fragmentation mechanisms are observed where the concerted mechanism dominates. For C S 2 5 + , only the concerted mechanism is observed. Two types of Coulomb explosion models considering the molecular vibration are adopted to simulate the experimental KER distributions of the three-body channels. While obvious deviations are observed considering each ion during the whole dissociation process with an integer charge, good agreement can be achieved within deviation less than 5% if the charge state of the ions are adopted from ab initio calculations.

Fragmentation dynamics of carbonyl sulfide in collision with 500 eV electron.

The fragmentation dynamics of OCSq+ (q = 2, 3, 4) induced by electron collision at an impact energy of 500 eV is studied. By using the momentum imaging technique, the three dimensional momentum vectors of all the fragments are obtained, which enables us to analyse both the kinetic energy release and the momentum correlations for a certain fragmentation channel. Up to fifteen dissociation channels are analyzed including six, five, and four channels for two-body, and incomplete and complete three-body Coulomb fragmentations. For three-body dissociation, the fragmentation mechanisms are investigated with the help of Dalitz plot and Newton diagram. It is found that the sequential fragmentation involves in OCS2+→O+C++S+ with S+ emitted first and in OCS3+→O++C++S+ with O-C and C-S bonds breaking first. The remaining channels, however, always dissociate through a concerted mechanism. The relative intensities of the channels are also presented in this work.

Note: A high-efficiency multi-coincidence method designed for molecular fragmentation studies.

A high-efficiency multi-coincidence method is developed based on the hardware electronic multiple coincidence units. The multi-hit signals originating from one single detector can be selected and measured in coincidence. The performance of the method is tested by the electron impact three-body fragmentation of CO2(3+). Compared to the conventional method, the relative and absolute coincidence efficiencies of the triple-coincidence measurement are improved by about 200 and 3 times, respectively.