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, a nd 200 nm, Photoionization of gas-phase dopamine is found to produce the mo lecular ion, and three fragment ions at all four wavelengths, with the bran ching ratios strongly wavelength dependent. Photoionization at 400 and 267 nm yields the highest molecular ion signal, while that at 800 and 200 mn pr oduces very little molecular ion signal. An excited-state lifetime of simil ar to 10 ps following 267-mm excitation has been measured for dopamine usin g time-resolved pump-probe techniques. The short-lived excited state sugges ts that internal conversion, intersystem crossing, and/or dissociation is a concern when ionizing at this wavelength using longer laser pulses. Photoi onization of ion-beam-desorbed dopamine exhibits a large degree of fragment ation at all four wavelengths, though 267-mm photoionization produces the h ighest yield of dopamine fragment ions. Power dependence studies show a hig h degree of internal excitation. A direct comparison of ion yields obtained for photoionization of ion-beam-desorbed dopamine at 267 mn to that for SI MS shows a 20-fold increase in signal.