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Register a new userReading molecular messages from the intersections of high-order harmonic spectra at different orientations
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We investigate the orientation dependence of high-order harmonic generation (HHG) from H$_2^+$ with different internuclear distances irradiated by intense laser fields both numerically and analytically. The calculated molecular HHG spectra are found to be sensitive to molecular axis orientation relative to incident laser field polarization and internuclear separation. In particular, the spectra calculated for different orientation angles demonstrate a kind of intersection, which is identified as arising due to intramolecular two-center interference in the HHG. The striking intersection phenomenon can be used to probe the molecular instantaneous structure.
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We illustrate an iterative method for retrieving the internuclear separations of N$_2$, O$_2$ and CO$_2$ molecules using the high-order harmonics generated from these molecules by intense infrared laser pulses. We show that accurate results can be retrieved with a small set of harmonics and with one or few alignment angles of the molecules. For linear molecules the internuclear separations can also be retrieved from harmonics generated using isotropically distributed molecules. By extracting the transition dipole moment from the high-order harmonic spectra, we further demonstrated that it is preferable to retrieve the interatomic separation iteratively by fitting the extracted dipole moment. Our results show that time-resolved chemical imaging of molecules using infrared laser pulses with femtosecond temporal resolutions is possible.
We investigate high-order harmonic generation in inhomogeneous media for reduced dimensionality models. We perform a phase-space analysis, in which we identify specific features caused by the field inhomogeneity. We compute high-order harmonic spectra using the numerical solution of the time-dependent Schrodinger equation, and provide an interpretation in terms of classical electron trajectories. We show that the dynamics of the system can be described by the interplay of high-frequency and slow-frequency oscillations, which are given by Mathieus equations. The latter oscillations lead to an increase in the cutoff energy, and, for small values of the inhomogeneity parameter, take place over many driving-field cycles. In this case, the two processes can be decoupled and the oscillations can be described analytically.
Based on high-order harmonic generation (HHG) spectra obtained from solving the time-dependent Schrodinger equation for atoms, we established quantitatively that the HHG yield can be expressed as the product of a returning electron wave packet and the photo-recombination cross sections, and the shape of the returning wave packet is shown to be largely independent of the species. By comparing the HHG spectra generated from different targets under identical laser pulses, accurate structural information, including the phase of the recombination amplitude, can be retrieved. This result opens up the possibility of studying the target structure of complex systems, including their time evolution, from the HHG spectra generated by short laser pulses.
We formulate the concept of dominant interaction Hamiltonians to obtain an integrable approximation to the dynamics of an electron exposed to a strong laser field and an atomic potential leading to high harmonic generation. The concept relies on local information in phase space to switch between the interactions. This information is provided by classical integrable trajectories from which we construct a semiclassical wave function. The high harmonic spectrum obtained is in excellent agreement with the accurate quantum spectrum. The separation in the atomic potential and laser coupling interactions should facilitate the calculation of high harmonic spectra in complex systems.
We investigate the orientation dependence of molecular high-order harmonic generation (HHG) both numerically and analytically. We show that the molecular recollision electronic wave packets (REWPs) in the HHG are closely related to the ionization potential as well as the particular orbital from which it ionized. As a result, the spectral amplitude of the molecular REWP can be significantly different from its reference atom (i.e., with the same ionization potential as the molecule under study) in some energy regions due to the interference between the atomic cores of the molecules. This finding is important for molecular orbital tomography using HHG[Nature textbf{432}, 867(2004)].
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