Two-color resonance enhanced two-photon ionization andmass analyzed threshold ionization spectroscopy of p-chlorobenzonitrile

The vibrational features of p-chlorobenzonitrile in its first electronically excited state S1 and cationicground state D0 have been investigated by two-color resonance enhanced two-photon ionization and massanalyzed threshold ionization spectroscopy. The excitation energy of S1 <- S0 and the ionization energy of 35Cland 37Cl isotopomers of p-chlorobenzonitrile are determined to be 35818 +/- 2, and 76846 +/- 5 cm-1, respectively.These two isotopomers have similar vibrational features. Most of the active vibrations in the S1 and D0 statesare related to the motions of the in-plane ring deformation. The stable structures and vibrational frequencies ofp-chlorobenzonitrile are also calculated by the B3LYP/aug-cc-pVDZ method for the S0 and D0 states, and TD-B3LYP/aug-cc-pVDZ method for the S1 state. The changes in the molecular geometry are discussed in the S1 <- S0 photoexcitation process and the D0 <- S1 photoionization process. The comparisons between the transitionenergy of p-chlorophenol, p-chloroaniline, p-chloroanisole, and p-chlorobenzonitrile with those of phenol, anisole,aniline, and benzonitrile provide an insight into the substitution effect of Cl atom.