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 rotationally 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 benzene¹ (using two-photon excitation), and jet-cooled spectra of tetrazine,² pyrazine³ 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 rotationally resolved spectra one needs stable, ultranarrow bandwidth lasers together with a scheme to reduce Doppler broadening to less than several megahertz…