DIRECT OBSERVATION OF THE IONIZATION THRESHOLD OF TRIPLET METHYLENE BY PHOTOIONIZATION MASS-SPECTROMETRY

The photoionization spectrum of the ionization threshold region of met hylene has been recorded for the first time. The CH? radical was produ ced in situ by successive hydrogen abstractions from methane precursor . The observed steplike onset corresponds to the vibrationless transit ion CH2+(X) over tilde(2)A(1) <-- CH2 (X) over tilde(3)B(1) and leads to the adiabatic ionization energy of CH2 of 10.393 +/- 0.011 eV. This value is slightly higher than the nominal midrise of the threshold st ep structure, which is depressed by rotational autoionization effects. In a separate set of experiments, the threshold region of the CH2+ fr agment from CH3 was recorded at room temperature. The fragment appeara nce energy was accurately determined by fitting to be 15.120 +/- 0.006 eV at 0 K. The combination of these two measurements provides the bes t current experimental value for the bond dissociation energy of the m ethyl radical, D-0(H-CH2) = 4.727 +/- 0.012 eV = 109.0 +/- 0.3 kcal/mo l (corresponding to 110.4 +/- 0.3 kcal/mol at 298 K), and yields Delta H-f0(circle minus)(CH2, (X) over tilde B-3(1)) = 93.2 +/- 0.3 kcal/mo l (93.3 +/- 0.3 kcal/mol at 298 K) and Delta H-f0(circle minus)(CH2, < (alpha)over tilde> (1)A(2)) = 102.2 +/- 0.3 kcal/mol (102.3 +/- 0.3 kc al/mol at 298 K). The latter makes the reaction CH2 (<(alpha)over tild e> (1)A(2)) + H2O --> CH3 + OH essentially thermoneurtral, Delta H-r0( circle minus) = 0.0 +/- 0.3 kcal/mol. (C) 1998 American Institute of P hysics.