Showing papers by "Sydney Leach published in 1983"

Intramolecular relaxation of excited states of C6F6

Abstract: Fluorescence quantum yields, φF(v′), and lifetimes, τ(v′), of selected vibrational levels of electronic excited states of C6F6+ have been measured by counting coincidences between threshold photoelectrons and ion-fluorescence photons (T-PEFCO technique). The derived non-radiative rates knr(v′) for the B2A2u state correspond to intramolecular electronic non-radiative transitions (ENRT) via coupling to high vibrational levels of the X2E1g ground state. Radiative rates kr and their variation with internal energy reflect electronic properties whereas the variations of knr also reveal vibrational dynamics of the ion. The knr(v′) values exhibit (i) a monotonic quasi-exponential increase for each of the two vibrational progressions 1n and 1n21 with increasing vibrational energy Ev and (ii) mode-selective behaviour, knr(v′) being enhanced when mode 2 is excited. Model calculations of the vibrational part of knr(v′) were carried out on a non-communicating, harmonic-oscillator basis; the results reproduce qualitatively the experimental findings. Good quantitative agreement is found between experimental and calculated values of knr(v′) for low values of Ev. Progressive deviations which occur as Ev increases are interpreted as indicating the gradual onset of vibrational redistribution in the B state. This constitutes a new method for studying vibrational non-radiative transitions (VNRT). With increasing Ev, the VNRT processes are considered to be analogous to the sparse-level (I; Ev= 0–2500 cm–1), intermediate (II; Ev= 2500–4500 cm–1) and statistical-limit (III; Ev > 4500 cm–1) cases, respectively, of ENRT. Emission in coincidence with excitation of the C2B2u state is discussed and shown to involve coupling between C and mixed B,X vibronic levels.