Energetics of fragmentation of CH5, H3O, and NH4 from neutralized ion-beam experiments

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Abstract
Fragmentation energies for radicals of the type RH2 (RH=CH4, NH3, and H2O) produced by electron capture interactions of 5 keV RH2 + ion with Na or K atoms are reported. The experimental technique involves measurement of spatial beam profiles resulting from dissociation of neutral radicals following their formation in a near resonant electron transfer process. Cross sections for RH2 +–Na capture reactions are typically 1x10−14 cm2. Fragmentation energies from measurements with Na target atoms are −2.65±0.14, −0.22±0.03, and −1.12±0.07 eV for CH5, NH4, and H3O, respectively. From our results with Na and K targets and published values for proton affinities, the vertical electron affinities of CH5 + and H3O+ are calculated to be 5.3±0.2 eV and 5.1±0.3 eV, respectively. Beam profiles for ND4 show this species to be metastable with a lifetime of about 1 μs. From this we estimate a potential barrier to dissociation in NH4(ND4) between 0.36 and 0.48 eV, indicating this species should be stable at low temperatures. Comparison of these experimental results with theoretical calculations indicates areas of disagreement.