Photochemistry of Cyanoacetylene at 193.3 nm

Abstract

Cyanoacetylene (CA) is an important minor constituent in the Titan atmosphere and is present in the interstellar medium. The absorption cross section of CA has been measured in the region from 190 to 255 nm with a resolution of 1 nm. The photochemistry of CA at 193.3 nm has been studied using a quadrupole mass spectrometer and a Fourier transform infrared spectrometer for product analysis. From the photolysis of HC 3 N−D 2 and HC 3 N−CD 4 mixtures and a plateau value of 0.3 for the quantum yield (QY) of DC 3 N (C 3 N + D 2 → DC 3 N + D), it is concluded that the main dissociation process is HC 3 N + h ν → H + C 3 N with a QY of 0.30 ± 0.05 and a minor process is HC 3 N + h ν → C 2 H + CN with a QY equal to or less than 0.02. The remaining process is the formation of metastable CA (a triplet or carbene). The photolysis of CA induces a noticeable pressure decrease and a concomitant formation of a mist. The QY of CA disappearance is 4.5 ± 0.5, which is much higher than that of diacetylene (QY = 2.0 ± 0.5) and of acetylene (QY = 2.3). The rapid mist formation in CA may explain a haze observed in the Titan atmosphere. A detailed mist formation process is not known. The C 3 N radical disappears partially by C 3 N + HC 3 N → C 6 N 2 + H and 2C 3 N → C 6 N 2 . To explain the formation of minor products, HCN, C 2 H 2, HC 5 N, and C 4 N 2, two processes involving an unspecified CA metastable state or states may be proposed: HC 3 N* + HC 3 N → HC 5 N + HCN and C 4 N 2 + C 2 H 2 .