Changes of Vibrational Lifetimes with Minor Structural Modification of Small Polyatomic Molecules

With the help of ultrashort laser pulses it has been possible to measure directly the population Lifetime of welldefined vrbrational modes of polyatomrc molecules in the liquid state. For a number of molecules the populatron decay was found to be of the order of picoseconds [l-l]. Special attention was pard to CH-stretching modes whrch are of considerable unportance for the radiatronless decay of electroruc excitations. The investigation of small molecules IS facditated by the small number of normal modes [S]. Frequently we know quite accurately the uutially prepared states and have strong indications as to the fmal states. Recently, It became apparent that mixing by Fermi resonance represents a maJor coupling mechanism between the imtial and the final states, I.e. intramolecular anharmonic coupling between neighboring states of like symmetry is of major importance for the hfetime of the vrbrational states [2]. In tb paper we present new expenmental endence of the importance of Ferrm resonance for the hfetimes of CH-stretchmg modes. Small changes m the molecular structure can give rise to large changes of Fermi resonance rruxing whch m turn has a profound effect on the populatron lifetune. As examples we show data on CH1=CCII and trans CHCkCHCI where only two atoms exchange therr positions while the basic structure and the number of atoms are preserved. In addition, we compare the hfetrmes of the CH modes of benzene C,H, with the partially deutcraled form 1,3,5C,H-,D,.