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 Ar, Ar${}_2$, \HHO, and Ar-\HHO 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-H${}_2$O and H${}_2$O match but significantly differ from the ones measured for Ar and Ar${}_2$. This indicates that the photoelectron is emitted from the \HHO site of the complex and the vertical ionization potential of Ar-H${}_2$O is determined as $(12.4\pm 0.1)$,eV. For resacttered electrons, however, an enhancement of high-order ATI is observed for 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.