TDDFT study of multiphoton excitation and nuclear dynamics in HHG |
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| Xi Chu | |
| The University of Montana, Missoula, USA | |
| Minima in HHG spectra may be used to probe dynamics with subfemtosecond resolution. The 39 eV minimum for N$_2$ has attracted special attention because it appears not to be influenced by laser parameters or alignment angles. We reproduce this minimum and found that excitation to a linear combination of the $c'_4,^1Sigma^+_u$ and $c_3,{^1Pi}_u$ Rydberg states gives rise to an orientation invariant minimum analogous to the `Cooper minimum" in argon. A radial node passes through the atomic centers,therefore only when the molecular axis is parallel to the polarization direction of the field, the Cooper-like minimum coincides with the minimum predicted by a modified two-center interference model that considers the de-excitation of the ion and symmetry of the Rydberg orbital.
Including nuclear dynamics is a challenge for TDDFT methods. As an initial step, using the measured isotope effect of H$_2$ we benchmark a method in which the nuclear and electronic degrees of freedom are separated and both treated quantum mechanically. Our calculation reproduce the measured D$_2$ to H$_2$ HHG ratios up to the 25th harmonic when the 35th is the classical cutoff. Beyond the 25th harmonic, however, our results become extremely sensitive to the laser intensity. Analysis reveals an $R$-dependent phase factor as the cause of the isotope effect in our calculation. We isolated this phase factor and proposed a strong field approximation (SFA)-phase model, which reproduces all the experimental data. |