RESUMO
High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H(2)O and D(2)O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by solving the time-dependent Schrödinger equation to simulate strong field ionization and high harmonic generation from the water isotopes. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.
RESUMO
The ionization probability of N2, O2, and CO2 in intense laser fields is studied theoretically as a function of the alignment angle by solving the time-dependent Schrödinger equation numerically assuming only the single-active-electron approximation. The results are compared to recent experimental data [D. Pavicic, Phys. Rev. Lett. 98, 243001 (2007)] and good agreement is found for N2 and O2. For CO2 a possible explanation is provided for the failure of simplified single-active-electron models to reproduce the experimentally observed narrow ionization distribution. It is based on a field-induced coherent core-trapping effect.