Photoionization of gas-phase versus ion-beam-desorbed dopamine with femtosecond laser pulses.

We have investigated the photoionization of gas-phase and ion-beam desorbed dopamine using femtosecond laser pulses at wavelengths of 800, 400, 267, and 200 nm. Photoionization of gas-phase dopamine is found to produce the molecular ion, and three fragment ions at all four wavelengths, with the branching ratios strongly wavelength dependent. Photoionization at 400 and 267 nm yields the highest molecular ion signal, while that at 800 and 200 nm produces very little molecular ion signal. An excited-state lifetime of approximately 10 ps following 267-nm excitation has been measured for dopamine using time-resolved pump-probe techniques. The short-lived excited state suggests that internal conversion, intersystem crossing, and/or dissociation is a concern when ionizing at this wavelength using longer laser pulses. Photoionization of ion-beam-desorbed dopamine exhibits a large degree of fragmentation at all four wavelengths, though 267-nm photoionization produces the highest yield of dopamine fragment ions. Power dependence studies show a high degree of internal excitation. A direct comparison of ion yields obtained for photoionization of ion-beam-desorbed dopamine at 267 nm to that for SIMS shows a 20-fold increase in signal.