Photodissociation dynamics of C4H2 at 164.41 nm: Competitive dissociation pathways

Abstract

Photodissociation dynamics of C 4 H 2 at 164.41 nm through the Rydberg state R( 1 u ) have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show that two different predissociation pathways exist to form the ground C 4 H (X 2 + ) and electronically excited C 4 H (A 2 ) products: the former has statistical and isotropic translational energy distribution through internal conversion (IC) to the ground state, while the latter has non-statistical and anisotropic translational energy distribution through IC to the excited repulsive state. The vibrational progressions of C 4 H (A 2 ) products have also been observed and assigned.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…31 The TKER spectra of the H + C4H products formed when exciting C4H2 at shorter wavelengths (in the range 127.5    164.4 nm) chosen to match with resolved Rydberg features in the parent absorption spectrum show peaks sitting on a continuous background. 32,33 Such structure reflects the formation of excited C4H(A 2 ) state radicals, with specific vibrational excitation in the CCC bend and CC stretch modes. As with C2H2, these products arise via non-radiative transfer to the corresponding 1 * continuum, while the underlying (isotropic) signalthat peaks at low TKERis logically attributed to unimolecular decay of highly internally excited C4H2(S0) molecules.…”

Formation of C9H2 and C10H2 from Reactions C3H + C6H2 and C4H + C6H2

“…In contrast, radicals C m H ( m ≥ 1) have yet been detected in Titan’s atmosphere due to their high reactivity and the dense atmosphere with a surface pressure of 1.5 bar. Nonetheless, it is believed that C 2 H and C 4 H are producible from C 2 H 2 and C 4 H 2 by solar UV-light photolysis, − which might drive formation of larger polyynes on Titan. The mole fractions of C 6 H 2 and C 8 H 2 were derived to be 8.0 × 10 –7 and 2.0 × 10 –7 , respectively, by ion chemistry fitting to the ion mass spectrum measured in Titan’s atmosphere .…”

“…C m H ( m = 1–8) were observed in the line of sight toward Taurus Molecular Cloud (TMC-1) − and the circumstellar envelope of carbon star IRC+10216 . Besides, C 2 H, C 3 H, C 4 H, and C 5 H were observed also toward the circumstellar envelope of CRL 618. , C m H are producible from C m H 2 by photolysis − in a photon-rich environment or from chemical reactions like C + C m –1 H 2 (and C 2 + C m –2 H 2 ) → C m H + H. , Bimolecular reaction rates are slow in a typical molecular cloud with a density of 10 3 –10 5 cm –3 , which enables the observation of highly reactive C m H radicals survived in a molecular cloud. In contrast, it is tough to detect C m H radicals in a typical flame that has a gas density as high as 10 18 –10 19 cm –3 .…”

Quantum state-to-state vacuum ultraviolet photodissociation dynamics of small molecules

“…This angular anisotropy and high rotational excitation of the dissociation products are believed to be caused by the CI between the S 1 and S 0 PESs of CH 4 . In the case of HNCO [93] and C 4 H 2 [98,99], multiple dissociation pathways with different nonadiabatic interactions among various PESs were observed by comparing the different TKER spectra obtained at parallel and perpendicular excitation schemes. Similar complicated non-adiabatic interactions have also been observed in the case of VUV photodissociation of H 2 O, which has already been well studied [58].…”