Transient frequency modulation spectroscopy and 266 nm photodissociation of bromoform

A high-resolution absorption spectrometer, based on the combination of frequency-modulation (FM) and diode laser techniques, has been constructed to investigate the reaction intermediates in the 266 nm laser photolysis of bromoform (CHBr₃). The absorption spectrum of HCBr corresponding to the ùA″ ← X̃¹A′ electronic transition was successfully obtained. We also found that the nascent HCBr is electronically excited. These data indicate that in the reaction pathway we observed, the first step of the 266 nm photolysis is a two-photon process and HCBr is one of the primary intermediates. The electronically excited HCBr absorbs one or two more photons to produce the electronically excited CH radical.