Observation of CH A 2 Delta -->X 2 Pi r and 2 Sigma --->X 2 Pi r emissions in gas-phase collisions of fast O(3P) atoms with acetylene.

Abstract

Optical emissions in single-collision, beam-beam reactions of fast (3--22-eV translational energy) O${(}^{3}$P) atoms with ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$ have been measured in the wavelength range 300--850 nm. Two features were observed, one with a peak wavelength at 431 nm, corresponding to the CH A $^{2}\mathrm{X}$ $^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{r}}$ transition, and a second weaker emission in the range 380--400 nm corresponding to the B $^{2}\mathrm{\ensuremath{\Sigma}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}X $^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{r}}$ transition. Both the A\ensuremath{\rightarrow}X and B\ensuremath{\rightarrow}X emissions were fit to a synthetic spectrum of CH(A) at a vibrational temperature ${\mathit{T}}_{\mathit{v}}$ of 10 000 K (0.86 eV) and a rotational temperature ${\mathit{T}}_{\mathit{r}}$ of approximately 5000 K (0.43 eV); and CH(B) to ${\mathit{T}}_{\mathit{v}}$=2500 K (0.22 eV) and ${\mathit{T}}_{\mathit{r}}$=1000 K (0.09 eV). The energy threshold for the A\ensuremath{\rightarrow}X emission was measured to be 7.3\ifmmode\pm\else\textpm\fi{}0.4 eV (lab) or 4.5\ifmmode\pm\else\textpm\fi{}0.2 eV (c.m.). This agrees with the energy threshold of 7.36 eV (lab) for the reaction O${(}^{3}$P)+${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$\ensuremath{\rightarrow}CH(A)+HCO.