One-colour (~220 nm) resonance-enhanced (S1–S0) multi-photon dissociation of acetylene: probe of the C2A1Πu–X1Σ+g band by frequency-modulation spectroscopy
In a recent paper, we demonstrated that one-colour (~220 nm), resonance-enhanced (S1–S0), photodissociation of acetylene generates strong $$C_2$$ Swan band ($$d^3Π_g–a^3Π_u$$) and $$C_2$$ Deslandres-d'Azambuja band ($$C^1Π_g–A^1Π_u$$) fluorescence, and long-lived (>3 µs) fluorescence from an electronically-excited $$C_2$$H* species. It was not known whether the $$C_2A^1Π_u$$ and $$X^1Σ^+_g$$ states are also directly populated in this process. In this paper, multiple vibration-rotation transitions between the $$C_2A$$-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 S1–S0 transitions that populate individual rotational levels of the S1$$_{trans}$$-conformer $3^2,3^3,$ and $3^4$ vibrational states. By comparing the phase of the FM signals from the $$C_2A–X$$ transitions with that from the Rb D1-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 S1 level is excited by the second photon to an acetylene dissociation precursor state, which undergoes sequential C-H bond-breaking to produce the $$C_2X$$ state. The dissociation precursor is assigned as the $$1^1B_g(trans-bent)/1^1B_1(cis-bent)$$ valence state, which correlates to a doubly-excited configuration, $$(1π_u)^2(1π_g)^2$$, at linear geometry. Based on the rotational distributions of the $$C_2X$$-state fragments, we believe that at least one of the transition states involved in the photolysis via S1$3^4$ has a larger CC-H bend-angle for the departing H-atom than that involved in the S1$3^2$ and $3^3$ photolysis.
- Research Organization:
- Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; USDOE National Nuclear Security Administration (NNSA); US Air Force Office of Scientific Research (AFOSR); USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- FG02-87ER13671; AC52-07NA27344; FA9550-16-1-0117; FG0287ER13671
- OSTI ID:
- 1831789
- Alternate ID(s):
- OSTI ID: 1634293; OSTI ID: 1733362
- Report Number(s):
- LLNL-JRNL-796903; FA9550-16-1-0117; DE-AC52-07NA27344; TRN: US2216630
- Journal Information:
- Molecular Physics, Vol. 118, Issue 15; ISSN 0026-8976
- Publisher:
- Taylor & FrancisCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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