Vacuum UV spectra of photolyzed C2H2 in solid Ar at 8 K

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Abstract
The results are reported from a study of absorption spectra in the range 1200 to 3000 Å of the products from the photolysis of acetylene trapped in Ar matrices at ∼8 K. The principal features of the observed spectra are two‐band systems appearing at 1660 and 1950 Å. The band system at 1660 Å is identified as the 3R‐X Rydberg transition of the C4H2 diacetylene molecule. The vibrational structure exhibits vibrations in the stretching mode ν′2∼2020 cm−1 and overtones of the C–H bending mode 2ν′6∼550 cm−1 in the (σ2u π4u π3g 3pσ) 1Πu Rydberg state. These frequencies are shifted to 1930 and 400 cm−1, respectively, on deuteration of the parent molecule. It is proposed that the most likely assignment for the band system at 1946 Å is the C2H ethynyl radical and that the principal progression in ∼2175 cm−1 and shorter progressions in ∼630 cm−1 correspond to the ν′3 C≡C stretching mode, and even multiples 2ν′2′ of the deformation mode. These vibrational assignments are supported by the effects of deuterating the C2H2 parent: ν′3 remains essentially unchanged at ∼2180 cm−1 while 2ν′2 shifts to ∼520 cm−1. On the basis of the extended progression in the C≡C stretching mode, the increase in the ν′3 mode from 1848 cm−1 in the (4σ2 1π4 5σ)X 2Σ+ ground state to 2175 cm−1 in the excited state, and the expected relative strength of the 2Σ+ ← X 2Σ+ transition, it is argued from simple molecular orbital considerations that the upper state of the 1946 Å transition is (4σ1π4 5σ2)2Σ+, with C2H retaining its linear geometry.

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