High-Order-Harmonic Generation using Gas-Phase H₂O Molecules

We investigate high-order-harmonic generation of isotropically distributed gas-phase H₂O molecules exposed to an intense laser field. The induced dipole of each individual molecule by the laser field is first calculated using the recently developed quantitative rescattering theory. In a thin medium, harmonic spectra generated coherently from all the molecules are then calculated by solving Maxwell\u27s equation of propagation. By using accurate transition dipoles of H₂O, we show that the harmonics in the lower plateau region are quite different from models that employ the simpler strong-field approximation. We also examine the magnitude and phase of the harmonics and their dependence on laser focusing conditions