Intensity-resolved measurement of above-threshold ionization of Ar-H2O

Above-threshold ionization (ATI) by femtosecond laser pulses centered at 515 nm is studied for a gas mixture containing the van der Waals complex Ar-H2O. By detecting photoions and photoelectrons in coincidence, the ATI spectra for Ar, Ar-2, H2O, and Ar-H2O are discerned and measured simultaneously. Using an intensity-scanning technique, we observe the red shift of the ATI spectra as a function of the laser intensity. The intensity-dependent shift of the ATI peak positions observed for Ar-H2O and H2O match but significantly differ from the ones measured for Ar and Ar2. This indicates that the photoelectron is emitted from the H2O site of the complex and the vertical ionization potential of Ar-H2O is determined as (12.4 +/- 0.1)eV. For rescattered electrons, however, an enhancement of high-order ATI is observed for Ar-H2O, as compared to H2O, suggesting that the relatively large Ar atom acts as a scattering center, which influences the ionization dynamics.