Doppler-free time-resolved polarization spectroscopy of large molecules: Measurement of excited state rotational constants

Measurement of the rotational spectra and constants of molecules can be a powerful probe of excited state geometries and intramolecular dynamics. The conventional approach for obtaining rationally resolved spectra is to use high-resolution (frequency domain, time-integrated) laser excitation. For medium-sized molecules, recent advances in these high-resolution techniques have made it possible to obtain Doppler-free spectra of benzene(1) (using two-photon excitation), and jet-cooled spectra of tetrazines,(2) pyrazine,(3) and others.(4) These results on medium-sized molecules have provided valuable information on geometries, (1,2) and on the dynamics of intramolecular singlet-triplet coupling(3,5) and the "channel 3" decay in benzene.(1) For large molecules, to obtain rationally resolved spectra one needs stable, ultra-narrow bandwidth lasers together with a scheme to reduce Doppler broadening to less than several megahertz.