Formation of CH3CFCl+ from photoionization of CH3CFCl2: An application of threshold photoelectron photoion coincidence (TPEPICO) technique

The dissociation of energy-selected CH3CFCl2+ to form CH3CFCl+ was studied with a molecular-beam/threshold-photoelectron-photoion-coincidence system u sing synchrotron radiation for ionization. Maximal releases of kinetic ener gy in the channel CH3CFCl+ + Cl at five photon energies were determined fro m half-widths of CH3CFCl+ time-of-flight peaks in coincidence mass spectra. A thermochemical threshold for this channel was determined to be 11.10 +/- 0.09 eV after taking kinetic energy releases into account. With this thres hold and heats of formation of Cl and CH3CFCl2, we derived Delta (f)H(0)deg rees (CH3CFCl+) = 149.0 +/- 2.1 kcal mol(-1). Ab initio calculations were a lso performed to predict heats of formation of CH3CFCl2, CH3CFCl, and their corresponding cations. Our experimental results agree satisfactorily with calculations. On the basis of experimental and theoretical results, we cons truct an energy diagram for dissociative photoionization of CH3CFCl2 to for m CH3CFCl+ + Cl.