Intensity-resolved measurement of above-threshold ionization of Ar-H$_2$O

Above-treshold ionization (ATI) by femtosecond laser pulses centered at 515 nm is studied for a gas mixture containing the Van-der-Waals complex Ar-H$_2$O. By detecting photoions and -electrons in coincidence, the ATI spectra for various atomic and molecular targets, i.e. Ar, Ar$_2$, \HHO, and Ar-\HHO, are clearly discerned. Using an intensity-scanning technique, we observe the red-shift of the ATI spectra, proportional to the laser intensity. The intensity-dependent shift of the ATI peak positions observed for Ar-H$_2$O and H$_2$O match but significantly differ from the ones measured for Ar and Ar$_2$. We find reasonable agreement of our results with simulations taking into account the ground state polarizability and the intensity-volume effect. Based on the ATI peak positions, the vertical ionization potential of Ar-H$_2$O is determined as $(12.4 \pm 0.1)$ eV. Measurements of coulomb explosion of the Ar-H$_2$O dication are consistent with the predicted values of the equilibrium bond length in neutral Ar-H$_2$O. Finally, we observe an enhancement of photoelectron rescattering in Ar-H$_2$O, as compared to H$_2$O, suggesting that the relatively large Ar atom acts as a scattering center, which influences the ionization dynamics.