Intramolecular vibrational relaxation and forbidden transitions in the SEP spectrum of acetylene

Abstract

à 1Au→X̃ 1Σg+ SEP spectra of acetylene near EVIB=7000 cm−1 show that Darling–Dennison resonance between the cis- and trans-bending vibrations is the first step in a series of anharmonic resonances which can transfer nearly all the vibrational energy out of the Franck–Condon bright states at higher energy. In addition to allowed ‖ΔK‖≡‖K′−ℓ″‖=1 rotational transitions, nominally forbidden ‖ΔK‖=0,2,3 rotational transitions have also been observed due to axis-switching and rotational-ℓ-resonance. Although the range of detectable fluorescence dips is only about 30, the range of detectable SEP intensities in these spectra is probably about 500.