Zero-kinetic-energy photoelectron spectroscopy and excited-state intramolecular proton transfer in a ‘double’ benzoxazole

Resonant two-color photoionization spectroscopy has been used for the first time to study a compound which exhibits an enol → keto reaction after electronic excitation. Detecting only photoelectrons with zero kinetic energy (ZEKE), we observe two energy thresholds for ionization from selected vibronic levels of the jet-cooled molecule. The thresholds are assigned to the excited enol and keto forms. The implications for the ultrafast time-resolved spectroscopy of the reaction are discussed.