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One-colour (∼220 nm) resonance-enhanced (S1 − S0) multi-photon dissociation of acetylene: probe of the C2 A1 Πu − X1 Σ+ g band by frequency-modulation spectroscopy

par Jiang, Jun;Du, Zhenhui;Liévin, Jacques ;Field, Robert R.W.
Référence Molecular Physics, 118, 15
Publication Publié, 2020-03-01

Article révisé par les pairs

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Résumé :

In a recent paper, we demonstrated that one-colour (∼220 nm), resonance-enhanced (S (Formula presented.) S (Formula presented.)), photodissociation of acetylene generates strong (Formula presented.) Swan band ((Formula presented.)) and (Formula presented.) Deslandres-d'Azambuja band ((Formula presented.)) fluorescence, and long-lived (>3 µs) fluorescence from an electronically-excited (Formula presented.) H (Formula presented.) species. It was not known whether the (Formula presented.) and (Formula presented.) states are also directly populated in this process. In this work, multiple vibration-rotation transitions between the (Formula presented.) -state v = 2 and the X-state v = 0 level are examined by time-resolved frequency-modulation (FM) spectroscopy. The photolysis laser wavelength is tuned into resonance at the one-photon level with S (Formula presented.) S (Formula presented.) transitions that populate individual rotational levels of the S (Formula presented.) -conformer (Formula presented.), (Formula presented.), and (Formula presented.) vibrational states. By comparing the phase of the FM signals from the (Formula presented.) transitions with that from the Rb D (Formula presented.) -line absorption transition, we determine that, for all of the probed A−X transitions, the X-state level is more populated than the A-state level. We propose that the acetylene S (Formula presented.) level is excited by the second photon to an acetylene dissociation precursor state, which undergoes sequential C-H bond-breaking to produce the (Formula presented.) state. The dissociation precursor is assigned as the (Formula presented.) valence state, which correlates to a doubly-excited configuration, (Formula presented.), at linear geometry. Based on the rotational distributions of the (Formula presented.) -state fragments, we believe that at least one of the transition states involved in the photolysis via S (Formula presented.) has a larger CC-H bend-angle for the departing H-atom than that involved in the S (Formula presented.) and (Formula presented.) photolysis.