EVIDENCE OF ROTATIONAL AUTOIONIZATION IN THE THRESHOLD REGION OF THE PHOTOIONIZATION SPECTRUM OF CH3

The photoionization spectrum of tile threshold region of CH3 equilibra ted at room temperature has been recorded and compared to the zero ele ctron kinetic energy (ZEKE) spectrum of Blush et al. [J. Chem, Phys, 9 8, 3557 (1993)]. The ionization onset region is at similar to 70 cm(-1 ) higher energy than previous high-temperature photoionization work [C hupka and Lifshitz, J. Chem, Phys. 48, 1109 (1967)], but still similar to 34 cm(-1) lower than that implied by invoking only direct ionizati on. The residual discrepancy can be accounted for by including fully a llowed quadrupole-induced and partially allowed dipole-induced rotatio nal autoionization, thus making the observed onset completely congruou s with the ZEKE ionization potential. In addition, the fragment appear ance potential of CH3+ from CH4 was redetermined by accurate fitting a s AP(0)(CH3+/CH4) = 14.322+/-0.003 eV. With the very precise ZEKE ioni zation potential, this yields the best current value for the bond diss ociation energy in methane, D-0(H-CH3)=4.484+/-0.003 eV =103.40+/-0.07 kcal/mol (104.96+/-0.07 kcal/mol at 298 K). (C) 1997 American Institu te of Physics. [S0021-9606(97)03247-9].