Relaxation processes of molecules on a solution surface studied by the two-color pump-probe ionization technique

A propanol (PrOH) solution of aniline (AN) was introduced into a vacuum cha mber as a continuous liquid flow (liquid beam) and was irradiated with pump and probe laser beams having wavelengths of 266 and 500-600 nm, respective ly. Ions produced by multiphoton absorption and ejected from the liquid bea m surface were analyzed by a time-of-flight mass spectrometer, The mass spe ctra show that AN(+)(PrOH)(n) and H+(PrOH)(n) are produced. The AN(+)(PrOH) (n) and H+(PrOH)(n) intensities were measured as a function of the delay ti me between the pump and the probe lasers, The lifetime for relaxation of an excited aniline molecule on the liquid surface was obtained to be 7.8 ns f rom the dependence of the AN(+)(PrOH)(n) intensity on the delay time. In co mparison with the lifetimes of an isolated aniline molecule in the gas phas e and an aniline molecule in bulk solution, it is concluded that the anilin e molecule does not strongly interact with ifs surrounding propanol molecul es on the surface. Variation of the H+(PrOH)(n) intensity with the delay ti me indicates that H+ produced by dissociative ionization of an aniline mole cule is ejected with several propanol molecules as H+(PrOH)(n).