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 Ã2Σ+ state fluoresces with a monoexponential decay constant τ= 2750 ± 100 ns. Selected vibrational levels of the B2Π state over the internal energy range Eν′= 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 Eν′ 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.