A threshold-photoelectron fluorescence–photon coincidence study of radiationless transitions in the B2Π state of BrCN+

Abstract

The fluorescence decay characteristics and intramolecular dynamics of selected vibronic energy levels of BrCN⁺ have been investigated by the T-PEFCO technique in which coincidences are counted between threshold photoelectrons and the fluorescence photons emitted by photoions produced by synchrotron radiation excitation in the 13.5–15 eV excitation energy range. The ν= 0 level of the òΣ⁺ state fluoresces with a monoexponential decay constant τ= 2750 ± 100 ns. Selected vibrational levels of the B²Π state over the internal energy range E_ν′= 0–5000 cm^–1 exhibit a biexponential fluorescence decay. The results are interpreted in terms of the small-molecule resonance limit case of radiationless transitions. Values of the interelectronic state coupling matrix elements are derived and their variation with E_ν′ is discussed. The threshold photoelectron spectrum (TPES) of BrCN was measured. Analysis of the 13.5–15 eV region of the TPES leads to revision of previous He I photoelectron spectroscopic assignments in this energy region.