Excited state hydrogen transfer in fluorophenol·ammonia clusters studied by two-color REMPI spectroscopy

Abstract

Two-color (1 + 1′) REMPI mass spectra of o-, m- and p-fluorophenol·ammonia (1 ∶ n) clusters were measured with a long delay time between excitation and ionization lasers. The appearance of NH₄(NH₃)_n−1⁺ with 100 ns delay after exciting the S₁ state is a strong indication of generation of long-lived species via S₁. In analogy with the phenol·ammonia clusters, we conclude that an excited state hydrogen transfer reaction occurs in o-, m- and p-fluorophenol·ammonia clusters. The S₁-S₀ transition of o-, m- and p-fluorophenol·ammonia (1 : 1) clusters were measured by the (1 + 1′) REMPI spectra, while larger (1 ∶ n) cluster (n = 2–4) were observed by monitoring the long-lived NH₄(NH₃)_n−1 clusters action spectra. The vibronic structures of m- and p-fluorophenol·ammonia clusters are assigned based on vibrational calculations in S₀. The o-fluorophenol·ammonia (1 : 1) cluster shows an anharmonic progression that is analyzed by a one-dimensional internal rotational motion of the ammonia molecule. The interaction between the ammonia molecule and the fluorine atom, and its change upon electronic excitation are suggested. The broad action spectra observed for the o-fluorophenol·ammonia (1 : n) cluster (n ≧ 2) suggest the excited state hydrogen transfer is faster than in m- and p-fluorophenol·ammonia clusters. The different reaction rates between o-, m- and p-fluorophenol·ammonia clusters are found from comparison between the REMPI and action spectra.