The intramolecular vibrational energy redistribution threshold in S 1 deuterated p-difluorobenzene

Abstract

The intramolecular vibrational energy redistribution (IVR) in S 1 deuterated p-difluorobenzene (pDFB–d 4 or –d 4) has been studied to determine the IVR threshold. For this, the S 1←S 0 fluorescence excitation (FE) spectrum of jet-cooled d 4 was investigated in the 2000–3250 cm −1 vibronic energy range of the S 1 electronic state, and single vibronic level fluorescence (SVLF) spectra have been acquired by exciting selected levels lying between 750 and 2850 cm −1 in vibrational energy in the S 1 excited state. Congestion of the dispersed fluorescence in this molecule first appears as the vibrational level energy climbs above 2000 cm −1. By comparing the SVLF spectra of pDFB–d 4 with those of p-difluorobenzene (pDFB or –h 4), it is obvious that IVR threshold in –d 4 is localized with a few hundreds cm −1 lower than that in pDFB. This decrease is entirely due to the increase in vibrational state density due to deuteration.