Gaussian-Type Orbital Calculations for High Harmonic Generation in Vibrating Molecules: Benchmarks for H2.

The response of the hydrogen molecular ion, H2+, to few-cycle laser pulses of different intensities is simulated. To treat the coupled electron-nuclear motion, we use adiabatic potentials computed with Gaussian-type basis sets together with a heuristic ionization model for the electron and a grid representation for the nuclei. Using this mixed-basis approach, the time-dependent Schrödinger equation is solved, either within the Born-Oppenheimer approximation or with nonadiabatic couplings included. The dipole response spectra are compared to all-grid-based solutions for the three-body problem, which we take as a reference to benchmark the Gaussian-type basis set approaches. Also, calculations employing the fixed-nuclei approximation are performed, to quantify effects due to nuclear motion. For low intensities and small ionization probabilities, we get excellent agreement of the dynamics using Gaussian-type basis sets with the all-grid solutions. Our investigations suggest that high harmonic generation (HHG) and high-frequency response, in general, can be reliably modeled using Gaussian-type basis sets for the electrons for not too high harmonics. Further, nuclear motion destroys electronic coherences in the response spectra even on the time scale of about 30 fs and affects HHG intensities, which reflect the electron dynamics occurring on the attosecond time scale. For the present system, non-Born-Oppenheimer effects are small. The Gaussian-based, nonadiabatically coupled, time-dependent multisurface approach to treat quantum electron-nuclear motion beyond the non-Born-Oppenheimer approximation can be easily extended to approximate wavefunction methods, such as time-dependent configuration interaction singles (TD-CIS), for systems where no benchmarks are available.

State-selective vibrational excitation and dissociation of H2+ by strong infrared laser pulses: below-resonant versus resonant laser fields and electron-field following.

The quantum dynamics of vibrational excitation and dissociation of H(2)(+) by strong and temporally shaped infrared (IR) laser pulses has been studied on the femtosecond (fs) time scale by numerical solution of the time-dependent Schrödinger equation with explicit treatment of nuclear and electron motion beyond the Born-Oppenheimer approximation. Using sin(2)-shaped laser pulses of 120 fs duration with a peak intensity of I(0) > 10(14) W/cm(2), it has been found that below-resonant vibrational excitation with a laser carrier frequency of ω < ω(10)/2 (where ω(10) is the frequency of the |v = 0> → |v = 1> vibrational transition) is much more efficient than a quasi-resonant vibrational excitation at ω ≈ ω(10). In particular, at the below-resonant laser carrier frequency ω = 0.3641 × 10(-2) au (799.17 cm(-1)), dissociation probabilities of H(2)(+) (15.3% at the end of the 120 fs laser pulse and 21% at t = 240 fs) are more than 3 orders of magnitude higher than those obtained for the quasi-resonant laser frequency ω = 1.013 × 10(-2) au (2223.72 cm(-1)). Probabilities of state-selective population transfer to vibrational states |v = 1>, |v = 2>, and |v = 3> from the vibrational ground state |v = 0> of about 85% have been calculated in the optimal below-resonant cases. The underlying mechanism of the efficient below-resonant vibrational excitation is the electron-field following and simultaneous transfer of energy to the nuclear coordinate.

Long-range energy transfer and ionization in extended quantum systems driven by ultrashort spatially shaped laser pulses.

The processes of ionization and energy transfer in a quantum system composed of two distant H atoms with an initial internuclear separation of 100 atomic units (5.29 nm) have been studied by the numerical solution of the time-dependent Schrödinger equation beyond the Born-Oppenheimer approximation. Thereby it has been assumed that only one of the two H atoms was excited by temporally and spatially shaped laser pulses at various laser carrier frequencies. The quantum dynamics of the extended H-H system, which was taken to be initially either in an unentangled or an entangled ground state, has been explored within a linear three-dimensional model, including the two z coordinates of the electrons and the internuclear distance R. An efficient energy transfer from the laser-excited H atom (atom A) to the other H atom (atom B) and the ionization of the latter have been found. It has been shown that the physical mechanisms of the energy transfer as well as of the ionization of atom B are the Coulomb attraction of the laser driven electron of atom A by the proton of atom B and a short-range Coulomb repulsion of the two electrons when their wave functions strongly overlap in the domain of atom B.