Resolving multi-orbital effects on high harmonic generation from aligned N2 molecules in linearly and elliptically polarized intense laser fields

We perform an experimental study of the multi-orbital effect on the high-order harmonic generation(HHG) from aligned N_2 molecules in both linearly and elliptically polarized intense laser fields.Measured by a home-built extreme ultraviolet(XUV) flat grating spectrometer with the pump-probe method, the angular distributions of different orders of HHG are obtained, which show distinctive behaviors for harmonics in the plateau and the cut-off regions.The ellipticity dependence of HHG is investigated by aligning the molecular axis parallel or perpendicular to the laser polarization.Our results indicate that both the highest occupied molecular orbital(HOMO) as well as the lower one(HOMO-1) contribute to the HHG of N2 molecules, in either linearly or elliptically polarized intense laser field.The study paves the way for understanding the ultrafast electron dynamics of molecules exposed to an intense laser field.

High-order Harmonic Generation of Aligned Acetylene in Elliptically Polarized Strong Laser Fields

We perform an experimental study on high-order harmonic generation （HHG） of aligned acetylene molecules induced by a 35-fs 800-nm strong laser field, by using a home-built HHG spectrometer. It is observed that the molecular HHG probability declines with in- creasing the laser ellipticity, which is in consistence with the deduction fl＇om the well-known tunneling-plus-rescattering scenario. By introducing a weak felntosecond laser pulse to non- adiabatically align the molecules, we investigated the molecular orbital efl）ct on the HHG in both linearly and elliptically polarized driving laser fields. The results show that the harmonic intensity is maximum for the molecular axis aligned perpendicularly to the laser electric field. It indicates that both the highest occupied molecular orbitals （HOMO） and HOMO-1 contribute to the strong-field HHG of acetylene molecules. Our study should pave the way for understanding the interaction of molecules with ultrafast strong laser fields.

Accurate all-electron calculation on the vibrational and rotational spectra of ground states for O2 and its ions

The potential energy curves, spectroscopic constants, and low-lying vibration–rotation levels of ground-state O2 and its cation O2+ and anion O2- were calculated with the explicitly correlated multireference configuration interaction method.The zeroth-order reference wavefunction was treated with the complete active space multiconfigurational self-consistent field method, in which the active space was carefully selected, and an additional molecular orbital πu was added into the full valence active space.The electron correlation of the 1s core in the oxygen atom was considered in the computations.The Davidson correction on molecular energy was considered to account for higher electron excitation.The relativistic effects, including the scalar relativistic effect and spin–orbit coupling, were considered in the computation of potential energy curves.These physical effects on the spectroscopic constants were examined.The low-lying levels of vibration–rotation spectra of O2 and its ions were determined based on the computed potential energy curves.Comparisons with available experiments were made and excellent agreement was obtained for the vibrational and rotational parameters.The spectroscopic constants and vibration–rotation spectrum of O2-, which is sparse in experiments, were provided.Our study will shed some light on further theoretical and experimental studies on these simple but important molecular systems.

Quantum interference of multi-orbital effects in high-harmonic spectra from aligned carbon dioxide and nitrous oxide

We investigate experimentally multi-orbital effects in high-order harmonic generation(HHG) from aligned CO2 and N2O molecules by intense femtosecond laser fields with linear and elliptical polarizations.For either of the aligned molecules, a minimum in the harmonic spectrum is observed, the position of which shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser.This indicates that the minimum originates from the dynamic interference of different channels, of which the tunneling ionization and recombination are contributed via different molecular orbitals.The results show that both the highest occupied molecular orbital(HOMO) and low-lying HOMO-2 in CO2(or HOMO-1 in N2O) contribute to the molecular HHG in both linearly and elliptically polarized strong laser fields.Our study would pave a way for understanding multi-electron dynamics from polyatomic molecules irradiated by strong laser fields.

Minimum structure of high-harmonic spectra from aligned O_(2) and N_(2) molecules

We experimentally investigated the high-order harmonic generation(HHG)from aligned O_(2) and N_(2) molecules in a linearly polarized laser field,and presented the dependence of the harmonic spectrum on the driving laser intensity and molecular alignment angle.The minimum position of HHG of O_(2) varies with changing the laser intensity,which is caused by multi-orbital interference.However,the location of the observed minimum structure in N_(2) harmonic spectrum remained unchanged upon changing the laser intensity.The mechanism of the spectral minimum for N_(2) case is regarded as a Cooperlike minimum in HHG associated with the molecular electronic structure.This work indicates that harmonic spectroscopy can effectively uncover information about molecular structure and electron dynamics.

Enhanced ionization of vibrational hot carbon disulfide molecules in strong femtosecond laser fields

By using a heated molecular beam in combination with a time-of-flight mass spectrometer, we experimentally study the ionization of vibrational-hot carbon disulfide（CS2） molecules irradiated by a linearly polarized 800-nm 50-fs strong laser field. The ion yields are measured in a laser intensity range of 7.0 × 10^（12） W/cm^2–1.5 × 10^（14） W/cm^2 at different molecular temperatures of up to 1400 K. Enhanced ionization yield is observed for vibrationally excited CS2 molecules.The results show that the enhancement decreases as the laser intensity increases, and exhibits non-monotonical dependence on the molecular temperature. According to the calculated potential energy curves of the neutral and ionic electronic states of CS2, as well as the theoretical models of molecular strong-field ionization available in the literature, we discuss the mechanism of the enhanced ionization of vibrational-hot molecules. It is indicated that the enhanced ionization could be attributed to both the reduced ionization potential with vibrational excitation and the Frank–Condon factors between the neutral and ionic electronic states. Our study paves the way to understanding the effect of nuclear motion on the strongfield ionization of molecules, which would give a further insight into theoretical and experimental investigations on the interaction of polyatomic molecules with strong laser fields.