Unimolecular dissociation of cyclopentadiene and indene

Abstract

The dissociation of hydrogen atoms from the methylene group of cyclopentadiene (CP) and indene (ID) excited with a 193 nm photon has been studied by hydrogen atom laser induced fluorescence. The rate of dissociation of IND was 7.4 × 10⁶ s⁻¹ but that of CP was too fast to measure. The ratio of H atoms to D atoms generated from 5‐deuteriocyclopentadiene (5‐dCP) was 3.91 ± 0.46. Rice–Ramsberger–Kassel–Marcus theory was used to calculate the rates of dissociation of CP and 5‐dCP. The quantum yield for dissociating H atoms from CP was 0.85 ± 0.07. The ejected H atoms have a Maxwell velocity distribution with temperatures which are equal to the vibrational temperatures, 3690 and 2479 K for CP and IND, respectively. The most important result of the work is this confirmation of an earlier finding on a different set of molecules that the translational temperature of the fragments after the dissociation is equal to the vibrational temperature before the dissociation. This is explained by the assumption that the motion of the fast, light hydrogen atom is partly decoupled from that of the heavier, slower atoms.