Photodissociation of HNCO: Three competing pathways

Th Droz-Georget, M. Zyrianov, A. Sanov, H. Reisler

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29 Scopus citations


The unimolecular decomposition of expansion-cooled isocyanic acid (HNCO) via channels (1) 3NH + CO, (2) H+NCO, and (3) 1NH + CO [where 3NH and 1NH denote NH(X3-) and NH(a1δ), respectively] has been investigated following photoexcitation to the S1(1A″) state in two energy regimes: (i) in the region of the 1NH + CO threshold (41700-45500cm-1; 240-220nm), and (ii) ∼ 3200cm-1 above D0(1NH + CO), at around 46000 cm-1 (217.6nm). Several complementary experiments are presented: NCO, 3NH and 1NH photofragment yield spectra and relative 1NH/3NH branching ratios are obtained by laser induced fluorescence (LIF); photofragment ion imaging is used to record CO angular recoil distributions, and 1NH rotational distributions correlated with specific CO(v,J) levels. HNCO excited to S, undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (1C) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the 3NH yield spectrum is observed above the opening of channel (3), and is interpreted as the imprint of short-time dynamics characteristic of the ISC step. State selectivity in the photodissociation is revealed by comparing the photofragment yield spectra of the three channels. In region (ii), product state distributions for channel (3) exhibit clear dynamical signatures, as expected for dissociation on S1 At low excess energies channel (2) derives from dissociation on S0, but the respective roles of S0 and S1 at higher energies are not well established yet. The results are discussed in terms of vibronic levels of mixed electronic character coupled directly or via radiationless decay to the various continua. The competition between the different processes depends sensitively on photolysis energy and excitation conditions.

Original languageEnglish (US)
Pages (from-to)469-477
Number of pages9
JournalBerichte der Bunsengesellschaft/Physical Chemistry Chemical Physics
Issue number3
StatePublished - 1997


  • Energy transfer
  • Molecular beams
  • Photochemistry
  • Radiationless transitions

ASJC Scopus subject areas

  • Chemical Engineering(all)


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