Photodissociation of Acetylene in the 201-216 nm Region: Determination of D0(HCC-II).

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Ohio State University

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Acetylene cooled in He supersonic expansion was photodissociated by excitation in the 201-216 nm region of the $\bar{A}^{1}A_{u} \leftarrow \bar{X}^{1}\Sigma^{+}_{g}$ transition. Subsequent ionization of the H atom fragments by 2+1 (243 nm) REMPI, and mass-selected ion imaging allowed analysis of the velocity distribution of H-atoms from the $HCCH + h\nu \rightarrow C_{2}H + H$ process. Measurement of the maximum H atom velocity produced by photodissociation of acetylene through the $\bar{A}^{1}A_{u} \leftarrow \bar{X}^{1}\Sigma^{+}_{g} V_{5}K_{0}^{1}$. $V^{7}_{0}K_{0}^{1}$. and $1^{1}_{0}V^{4}_{0}K^{1}_{0}$ vibronic transitions gave a value for $D^{0}_{0}(HCC-H)$ of $131 \pm 1$ keal/mol. Other channels producing hydrogen atoms (including $HC_{2} \stackrel{\rightarrow}{hv} C_{2} + H$ and $HCCH \stackrel{\rightarrow}{hv} HCCH^{+} C_{2}H^{+}+ H$) were detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obseure measurement of the maximum velocity of H atoms produced by single-photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to $-500 J/m^{2}$ gave a background multiphoron signal of -5\% of the peak single-photon signal. Because this multiphoton background limits the detectability of fast H atoms from single-photon dissociation of acetylene, the dissociation energy reported here is an upper limit.


Author Institution: Combustion Research Facility, Sandia National Laboratories; Combustion Research Facility, Stanford University; Combustion Research Facility, Sandia National Laboratories