Photoexcitation of rare-gas neon and argon clusters doped with H2O

We have studied the fluorescence of electronically excited OH*, H-* and H2O+* dissociation fragments after VUV excitation (hp greater than or equal to 11.6eV) of rare-gas clusters (Rg = Ne, Ar) doped with H2O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH*(A(2)Sigma(+) --> X(2)Pi) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH*(A) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H2O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons.