Abstract
We report studies of ultrafast proton transfer (PT) reaction of PhOH-NH3 cation complex by using femtosecond pump-probe photoionization-photofragmentation spectroscopy (fs-PIPF). Neutral PhOH-NH3 complexes prepared in a free jet are photoionized by femtosecond [1+1] resonance-enhanced multiphoton ionization via the S1 state, and the subsequent dynamics occurring in the cations is probed by delayed pulses that result in ion fragmentation. The spectrum of the relative [PhOH-NH3]+ ion depletion yields measured at ~6 ps with λ_probe = 392 ~ 410 nm was found to be similar to the photofragmentation spectrum of [PhOH-NH3]+ reported by Mikami et al. and it resembles the X ̃→C ̃ band of the phenoxy radical. Thus, it suggests that the observed temporal evolutions of the photofragmentation spectra are consistent with an intracomplex PT reaction. Besides, we also compare fs-PIPF [PhOH-NH3]+ ion transients at a series of pump and probe wavelengths with [PhOH-H2O]+ and [PhOH-Ar]+ transients. The very different temporal evolutions of [PhOH-H2O]+ and [PhOH-Ar]+ transients from those of [PhOH-NH3]+ further confirm the above assignments. The experiments revealed that PT in [PhOH-NH3]+ cation proceeds in two distinct steps: an initial wave-packet motion (∼70 fs) followed by a slower relaxation (∼1 ps) that stabilizes the system into the equilibrium PT structure. Our results support the mechanism of intracomplex PT in [PhOH-NH3]+ cation complex and suggest that the proton affinity of the base is an important factor for the proton-transfer dynamics.