Knowledge Bank

The method of chemical timing, that is to say, the restriction of fluorescence lifetimes by addition of an electronic quencher such as $O2$, has been used to monitor the time evolution of vibrational structure in $S1-S0$ fluorescence. In both p-difluorobenzene (pDFB) and p-fluorotoluene (pFT), structure from the pumped $S1$ vibrational level is observed in fluorescence at short times (10's of psec) after excitation. That structure disappears at longer observation times as intramolecular vibrational redistribution (IVR) destroys the initial vibrational character. Kinetic analysis reveals that the IVR lifetime is near 10 psec for three levels $(\in vib\approx 1200-2000cm-1)$ in pFT IVR dynamics are characterized for 11 levels $(\in vib\approx 1600-3300cm-1)$ in pDFB. There is a trend from long IVR lifetimes (500 psec) to short lifetimes (20 psec) as $\in vib$ increases. Within this trend, however, there is marked sensitivity of IVR lifetimes to the identity of the initially pumped zero-order level. The addition of a low frequency mode $(\nu 30\prime \approx 120cm-1)$ to the pumped level identity is particuarly effective in accelerating the IVR rate.