Delocalization of electronic energy in the lowest triplet states of molecules

Abstract

A variety of molecules are examined for the existence of metastable (τ≳1 msec) electronic states under isolated molecule conditions. A beam is excited by electron impact and detected after a 1 msec drift by Auger electron emission from a cesium surface. Metastable states were observed for benzene, toluene, m‐xylene, acteylene, cyanogen, SO2, N2, and CO. Molecules observed to not have metastable electronically excited states were formaldehyde, acetone, cyclohexanone, 2‐cyclopenteneone, 2‐cyclohexeneone, bromobenzene, benzylbromide, propyne, hydrogen cyanide, acetonitrile, carbon dioxide,carbon disulfide, carbonyl sulfide, ammonia, ethylene, 1,3‐butadiene, c i s‐2‐butene, t r a n s‐2‐butene, cyclopentene, cyclohexene, cyclopropane, and allene. A correlation is observed between nonmetastability of the lowest triplet state and a large change in geometry between the ground electronic state and the lowest triplet state. Mixing of the lowest triplet state of a molecule with high vibrational levels of the ground electronic state is discussed in light of the experimental findings. The generalized Franck–Condon factors of the presently accepted theory of radiationless transitons are found to be the crucial factors which determine the metastability or nonmetastability of the lowest triplet state of most molecules. Cross sections for production of high vibrational levels of the ground electronic state (E vib≳20 000 cm−1) by electron bombardment excitation of a triplet state which is strongly coupled to these levels of the ground state may be as large as a few tenths of a square angstrom.