High-order Harmonic Generation Of H2 Research Articles

Coulomb-induced emission time shifts in high-order harmonic generation from H2+

Accurate emission times of high-order harmonic generation (HHG) are vital for high-precision ultrafast detection in attosecond science, but a quantitative analysis of Coulomb effects on this time is absent in the molecular HHG. Here, we investigate the Coulomb-induced emission-time shift in HHG of H2+ with two different internuclear distances R, where the times obtained via the Gabor transform of numerical data from solving the time-dependent Schrödinger equation are used as simulation experiment results. Based on the molecular strong-field approximation, we develop a trajectory-resolved classical model that takes into account the molecular two-center structure. By selecting appropriate electron trajectories and including Coulomb interactions, the classical trajectory method can reproduce Gabor emission times well. This consistence reveals that Coulomb tails cause an emission-time shift of ∼35 as at the R = 2.0 a.u. case and of ∼40-60 as at the R = 2.6 a.u. case under the present laser parameters when compared to the Coulomb-free quantum-orbit model. Our results are of significance to probe the attosecond dynamics via two-center interference.

Orientation dependences of high-order harmonic generation in H2 and H2+ molecules

We propose a modified version for the strong field theory of high-order harmonic generation (HHG) in molecules and we apply it to analyze the orientation-dependences of HHG from the hydrogen molecule and it molecular ion H2+ when exposed to linearly polarized laser light. Our model predicts the presence of clear interference minima in the spectra of the high-order harmonics. We further exploit the orientation-dependence of the interference minima’s position and the harmonics polarization properties through proposed model. We found that the position of the interference minima are associated with the polarization properties of harmonics.

The inhomogeneous field effect on quantum path control of H2+ driven by PG scheme

The inhomogeneous field effect on high-order harmonic generation of H2+ driven by the polarization gate scheme is theoretically investigated. The harmonic emission process is explored by the electron probability density evolution. The research shows that it is the influence on the electron movement imposed by inhomogeneous field that causes the quantum paths to be manipulated. In view of these simulations, only one short quantum path is significantly enhanced but others are suppressed by selecting optimized inhomogeneous factor ε and laser pulse duration. Meanwhile, a ultrabroad supercontinuum up to 300 orders harmonic plateau and an isolated 48-as pulse are generated.

Time-resolved interferences in the high-order harmonic generations from H2+ and HeH2+

The molecular high-order harmonic generations (HHGs) from H2+ and HeH2+ are theoretically investigated by solving the time-dependent Schrödinger equation based on the Born-Oppenheimer approximation. The contributions of different recombination pathways in the high-order harmonic generation of H2+ and HeH2+ are discussed via the wavelet time-frequency analysis. By the analysis of the temporal structures of the different recombination pathways, the spectral minima in the molecular HHG are attributed to the destructive interferences of the recombination pathways from the two localized states. The result provides a new insight into clarifying the spectral minima in the molecular HHG.

A redshift mechanism of high-order harmonics: Change of ionization energy

We theoretically study the high-order harmonic generation of H2+ and its isotopes beyond the Born-Oppenheimer dynamics. It is surprising that the spectral redshift can still be observed in high harmonic spectra of H2+ driven by a sinusoidal laser pulse in which the trailing (leading) edge of the laser pulse is nonexistent. The results confirm that this spectral redshift originates from the reduction in ionization energy between recombination time and ionization time, which is obviously different from the nonadiabatic spectral redshift induced by the falling edge of the laser pulse. Additionally, the improved instantaneous frequency of harmonics by considering the changeable ionization energy can deeply verify our results. Therefore, this new mechanism must be taken into account when one uses the nonadiabatic spectral redshift to retrieve the nuclear motion.

Resonance enhanced high-order harmonic generation in H2+ by two sequential laser pulses.

We investigate high-order harmonic generation in H2+ by using two sequential laser pulses, which consist of a 800-nm pump pulse and a time-delayed 1600-nm probe pulse. Based on the solution of the time-dependent Schrödinger equation, we demonstrate that the harmonic cutoff in our two-pulse scheme is significantly extended compared to that in the 1600-nm probe pulse alone. Meanwhile, the harmonic efficiency is enhanced by 2-3 orders of magnitude due to charge-resonance-enhanced ionization steered by the 800-nm pump pulse. By using a probe pulse with longer wavelength, our scheme can be used for efficient high harmonic generation in the water window region. In addition, the influence of the intensity of the pump pulse and the relative time delay of the two laser pulses on the harmonic generation are also investigated.

The spatial distribution in high-order harmonic generation of H2+ with different time delays of the two-color laser fields

We theoretically investigate the high-order harmonic generation (HHG) and the spatial distribution in HHG of the H2+ molecule by introducing a time-delayed two-color laser field which consists of the mid-infrared and near-infrared laser field. For the different time delays of the two-color laser fields, the pulse shapes are changed which result in the variation of the electron recombined with the nuclei along the positive- or negative-z direction. When the time delay is 0fs (1.34fs), a smooth harmonic plateau from the electron recombined with the nuclei along the negative (positive)-z direction can be achieved. An isolated attosecond pulse with a duration of about 99 as is generated when the time delay is 1.34fs. We perform the classical analysis which is consistent with the numerical results from the one-dimensional non-Born-Oppenheimer time dependent Schrödinger equation (TDSE). We also investigate emission time of harmonics in terms of a time-frequency analysis to further understand the underlying physical mechanism.