Propensities toward C2H(Ã 2Π) in acetylene photodissociation

Abstract

When expansion-cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH-stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(Ã 2Π). This differs markedly from one-photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X̃ 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(Ã 2Π) exclusively.

Fast-Flow Study of the CH + CH Reaction Products

“…The atomic hydrogen velocity distribution obtained in the photodissociation of C 2 H 2 at Lyman-R wavelength by Lai et al 6b and Zhang et al 15 showed an important contribution of small velocities associated with a production of C 2 H mainly in the excited A state. The comparison between the CH + CH reaction and the photodissociation of C 2 H 2 at the Lyman-R wavelength (121 nm) is difficult due to the lack of theoretical information about the CH + CH reaction particularly on the multiplicity of C 2 H 2 (either in singlet or triplet state) produced in the addition step (the excitation of C 2 H 2 at Lyman-R promotes C 2 H 2 in the 3R′′( 1 Π u )2 0 1 and the 2 CH + 2 CH reaction could produce 1 C 2 H 2 or 3 C 2 H 2 ).…”

Atmospheric Pressure Acetylene Detection by UV Photo-Fragmentation and Induced C2 Emission

“…This possibility is supported by vibrationally mediated photodissociation studies, 13 which have shown that vibrationally excited C 2 H 2 (X) preferentially yields C 2 H (Ã), whereas direct single-photon excitation of the ground state C 2 H 2 yields products dominated by C 2 H (X), even though the total excitation energies differ by only 1220 cm À1 . Moreover, with sufficient vibrational excitation (v…”

Photodissociation of CCH: Classical trajectory calculations involving seven electronic states

“…2 Figure 1 of this work suggests that, the product ratio, X to à , is Ϸ4:1, and vibrational state distributions are as shown in Table I. The photolysis of C 2 H 2 with 193 nm laser seems to result with no excitation in CH stretch, and only a modest excitation in CC mode, i.e., up to 3 ϭ1.…”

“…18 -20 C 2 H radical is found to be formed in electronic states, X 2 ⌺ g ϩ1 (0 2 0) with 2 ϭ0 -7, X 2 ⌺ g ϩ1 (0 2 1) with 2 ϭ0 -5, and A 2 ⌸(0 2 0) with 2 ϭ0 -1, from photodissociation of acetylene at 193 nm. 2,21 Time-of-flight ͑TOF͒ and laser induced fluorescence studies of C 2 H, where it is produced from a precursor molecule, are carried out to analyze photofragments. 1,3,22,23 Several electronic states of C 2 are identified as a product of the photolysis, such as, X 1 ⌺ g ϩ , a 3 ⌸ u , b 3 ⌺ g Ϫ , A 1 ⌸ u , B 1 ⌬ g , and BЈ 1 ⌺ g ϩ .…”

“…The ethynyl radical, C 2 H, has long been a subject to experimental [1][2][3][4][5][6] and theoretical investigations, [6][7][8][9][10][11][12][13][14][15] which can be attributed to its important role in combustion, 16 its prominent presence in astrophysics as a major source of C 2 in comets and interstellar media, 17 and for understanding the photochemistry and spectroscopy of this molecule. The strong vibronic interaction between the ground state X 2 ⌺ ϩ ( 4 5 1 ) and the first excited state A 2 ⌸( 3 5 2 ) of C 2 H, that are separated only by 3600 cm Ϫ1 , restrains the definite spectroscopic characterization of C 2 H radical.…”