B3Π(0+) states of IF, ICl and IBr. Part 2.—Observation and analysis of the excitation spectra of IF and ICl

Abstract

Narrow band tunable radiation from a nitrogen pumped dye laser has been used to excite fluorescence in the B³Π(0⁺)–X¹∑⁺ systems of IF (430 to 500 nm) and of ICI (568 to 650 nm). The kinetically labile IF molecules were produced in a fast flow system by the rapid elementary reaction F + ICl → IF + Cl. No laser excitation or absorption spectrum of IF has been described previously. Onset of pre-dissociation has been observed at J′= 12, v′= 10 of IF B³Π(0⁺), and a new value for the dissociation energy of IF, D″₀(IF)⩽(22 700 ± 15) cm^–1, is reported. Criteria for the commencement of pre-dissociation in IF spectra are discussed. The mechanism of formation of the B and A states of IF in F₂+ I₂ systems is considered to arise from initial production of IIF, in the elementary step, I₂+ F₂→ IIF + F. Laser fluorescence of ICI from the excited B³Π(0⁺) state (v′= 0, 1, 2) has been observed. The 0–2 band of the B–X system of I³⁵CI has been analysed, providing the first spectro-scopic constants for the level v′= 0 of the B state. A complete evaluation of constants for this shallow, weakly-bound state, has thus been possible. The onset of predissociation in ICI B³Π(0⁺) near J′= 70 of v′= 2 is discussed in the light of linewidth measurements of other workers.