Laser-induced fluorescence and microwave-optical double resonance spectra of the ICl(A←X,19←0) vibronic band: Measurement of the Cl atom hyperfine structure

Fluorescence excitation spectra of the A, ³Π₁ ← X, ¹Σ⁺ electronic transition of I³⁵Cl have been obtained by using a single-frequency dye laser and a collimated molecular beam. The 5-MHz resolution of the spectra is sufficient to resolve the Cl atom hyperfine structure of the A, ³Π₁, state for the first time. The values obtained for the I atom hyperfine constants also are improved. Assignments of the highly structured spectra were confirmed by microwave-optical double resonance using previously assigned microwave transitions in the X, ¹Σ⁺, ground electronic state. Unlike the hyperfine constants of the X and B, ³Π₀ states, those of the A state differ markedly from the predictions of simple models based on molecular orbital descriptions of the electronic structure. This data will provide a very sensitive test of quantum calculations for the excited electronic state.