Triplet excited states probed by cavity ringdown spectroscopy

Despite their photochemical importance, molecular triplet states have received relatively little attention in spectroscopic studies. This is primarily due to the challenge of detecting the spin-forbidden Tn ˆS0 transitions. To meet the challenge, we have implemented cavity ringdown (CRD) spectroscopy, a high-sensitivity absorption technique, in our studies of photochemically relevant triplet species. In this article we outline the sensitivity requirements for observing singlet–triplet transitions and show how CRD detection achieves the needed sensitivity. We also specify the construction and operating details we used to set up the CRD spectroscopy system in our laboratory. Finally, we review investigations from our laboratory and others that have exploited the sensitivity of CRD detection to obtain triplet data that was not previously available. These studies include measurement of vibronically resolved Tn ˆS0 spectra of some cyclic enones and conjugated hydrocarbons. q 2004 Elsevier B.V. All rights reserved.