Direct observation of the ionization threshold of triplet methylene by photoionization mass spectrometry
- Chemistry Division, Argonne National Laboratory, Argonne, Illinois60439-4831 (United States)
The photoionization spectrum of the ionization threshold region of methylene has been recorded for the first time. The CH{sub 2} radical was produced {ital in situ} by successive hydrogen abstractions from methane precursor. The observed steplike onset corresponds to the vibrationless transition CH{sub 2}{sup +}{tilde X}{sup 2}A{sub 1}{l_arrow}CH{sub 2}{tilde X}{sup 3}B{sub 1} and leads to the adiabatic ionization energy of CH{sub 2} of 10.393{plus_minus}0.011eV. This value is slightly higher than the nominal midrise of the threshold step structure, which is depressed by rotational autoionization effects. In a separate set of experiments, the threshold region of the CH{sub 2}{sup +} fragment from CH{sub 3} was recorded at room temperature. The fragment appearance energy was accurately determined by fitting to be 15.120{plus_minus}0.006 eV at 0 K. The combination of these two measurements provides the best current experimental value for the bond dissociation energy of the methyl radical, D{sub 0}(H{endash}CH{sub 2})=4.727{plus_minus}0.012 eV=109.0{plus_minus}0.3 kcal/mol (corresponding to 110.4{plus_minus}0.3 kcal/mol at 298 K), and yields {Delta}H{sub f0}{sup {circle_minus}}(CH{sub 2},{tilde X}{sup 3}B{sub 1})=93.2{plus_minus}0.3 kcal/mol (93.3{plus_minus}0.3 kcal/mol at 298 K) and {Delta}H{sub f0}{sup {circle_minus}}(CH{sub 2},{tilde a}{sup 1}A{sub 2})=102.2{plus_minus}0.3 kcal/mol (102.3{plus_minus}0.3 kcal/mol at 298 K). The latter makes the reaction CH{sub 2}({tilde a}{sup 1}A{sub 2})+H{sub 2}O{r_arrow}CH{sub 3}+OH essentially thermoneutral, {Delta}H{sub r0}{sup {circle_minus}}=0.0{plus_minus}0.3 kcal/mol. {copyright} {ital 1998 American Institute of Physics.}
- OSTI ID:
- 627739
- Journal Information:
- Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 16 Vol. 108; ISSN JCPSA6; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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