Probing photoproducts ejected from liquid surfaces

A method is presented for studying the photochemistry on clean liquid surfaces in a high vacuum. After irradiation of C12H25I and C18H37I with a laser pulse at 275 nm, the desorbed photoproducts are characterized by time-of-flight spectroscopy and quadrupole mass analysis. It was found that the photoexcitation pulse is followed by a transient enhancement of liquid evaporation, the ejection of partially thermalized photoproduct I and thermalized recombination product I2. At laser fluences > 10 mJ/cm2 explosive desorption is observed. Calculations of the flight time distributions including one-dimensional diffusion in the liquid show that the desorbed photoroduct I2 originates from depths up to several micrometers below the surface.