Intramolecular relaxation of excited states of C6F6+

Abstract

Fluorescence quantum yields, φ_F(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 k_nr(v′) for the B²A_2u state correspond to intramolecular electronic non-radiative transitions (ENRT) via coupling to high vibrational levels of the X²E_1g ground state. Radiative rates k_r and their variation with internal energy reflect electronic properties whereas the variations of k_nr also reveal vibrational dynamics of the ion. The k_nr(v′) values exhibit (i) a monotonic quasi-exponential increase for each of the two vibrational progressions 1ⁿ and 1ⁿ2¹ with increasing vibrational energy E_v and (ii) mode-selective behaviour, k_nr(v′) being enhanced when mode 2 is excited. Model calculations of the vibrational part of k_nr(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 k_nr(v′) for low values of E_v. Progressive deviations which occur as E_v 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 E_v, the VNRT processes are considered to be analogous to the sparse-level (I; E_v= 0–2500 cm^–1), intermediate (II; E_v= 2500–4500 cm^–1) and statistical-limit (III; E_v > 4500 cm^–1) cases, respectively, of ENRT. Emission in coincidence with excitation of the C²B_2u state is discussed and shown to involve coupling between C and mixed B,X vibronic levels.