The Journal of Physical Chemistry, Vol. 82,No. 27, 7978 2277
4-Methylumbelliferone Fluorescence
TABLE I: Measured I n t e n s i t y Ratios of Fluorescence at -490 nm for A N F / o - D i c h l o r o b e n z e n e a t t h e Pulse M a x i m u m and after Franck-Condon Relaxation temp,
K
331 319 295 259
intensity ratio 2.6 i. 0 . 3 5.0 f 0 . 3 6.7 i. 0 . 3 22 f. 4
will be proportional to the lifetime. On the other hand, the recorded intensity of the long-lived tail will be constant. Thus, the ratio of the two intensities will allow an estimate of the relative decay times of the Franck-Condon states a t different temperatures. (See Table I.) Since the relaxation time of the fluorescence profiles is not distinct from the pulse (Le., the full widths a t halfmaximum intensity are virtually identical at -20 ps), we may assume an upper limit for the relaxation time of < l o ps a t 259 K. Thus, the computed decay time a t 295 K would be
