F. Sheldon Wettack and C. H. Bibart
Hope College Holland, Michigan 49423
Vibronic Energy Transfer A
physical chemistry experiment
c u r r e n t research in physical chemistry includes extensive work in the general area of photochemistry and electronic energy t,ransfer. Techniques have been developed recently to determine direct,ly the lifetimes of excited electronic states down to t,he nanosecond region. This capabilit,y has allowed the photochemist to determine bimolecular rate const,ants for various intermolecular processes involving excited states by ~t~udying the effect of these processes on the st,ate whose lifet,ime is known The determination of these absolnte bimolecular rate constants is providing significant new knowledge about the nature of electronic energy t,ransfer. I n order to familiarize our chemistry majors with we have develwork in this area of physical chemi~t~ry, oped a new experiment for the third year laboratory course. This experiment involves a study of the qucnching of the fluorescence of henzene by various added molecules in the gas phase, and serves t o introduce the student to such concepts as excit,ed st,ate energy levels and energy transfer rates and to the techniques involved in measuring fluorescence in the gas phase. The experiment has been so well received by the students that we feel the following inf~rmat~ion concerning it may be of interest t o others. Benzene Photochemirtry-A
/
'B 'B
1B
-- +
--
B aB X
hv,
k2
(2)
k,
(3)
h
(4)
%-X ks (5) Step (1) represents the formation of 'B by absorption of a photon; R, is the rate of absorption which, strictly speaking, is kl[B01 [hvl. Step (2) is t,he.fluorescence step and occurs with arate const,ant.k2. Step (3) represents the formation of the triplet state (spins unpaired) of benzene and is called intersystem crossing. The formation (from the singlet state) of any isomers of henzene or the formation of t,he ground electronic &ate is indicated by st,ep (4) while step (5) represents similar reactions of the triplet state. With 254 nm excit,at,ion roughly 20y0 of the 'B molecules undergo reaction (2) while -70% undergo r e a d o n (3) and the remainder reaction (4) ( 1 , Z ) . The yield of isomers is very small when excitation is at 22.5 nm(5) and, hence, the X in reactions (4) and (5) is, in the main, BQ. The fluorescent efficiency of benzene, Q,, is defined by
Brief Description
When benzene absorbs light in the region of 265-230 nm, it is excited from the ground electronic state, which we will symbolize as BQ, to the first excited singlet state,'B,. Absorption at short,er wavelengt,hs (
