Knowledge Bank

Acetylene cooled in He supersonic expansion was photodissociated by excitation in the 201-216 nm region of the $A¯1Au\leftarrow X¯1\Sigma g+$ 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 $HCCH+h\nu \to C2H+H$ process. Measurement of the maximum H atom velocity produced by photodissociation of acetylene through the $A¯1Au\leftarrow X¯1\Sigma g+V5K01$. $V07K01$. and $101V04K01$ vibronic transitions gave a value for $D00(HCC-H)$ of $131\pm 1$ keal/mol. Other channels producing hydrogen atoms (including $HC2hv\to C2+H$ and $HCCHhv\to HCCH+C2H++H$) 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 $-500J/m2$ 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.