Vacuum ultraviolet photoelectron spectroscopy of transient species

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Abstract
The HeI photoelectron spectrum of the NH2(X 2 B 1) radical produced from the rapid reaction has been recorded. Three bands were observed corresponding to ionization of NH2(X 2 B 1) to the X 3 B 1, 1 A 1 and 1 B 1 states of NH2 +. Measurement of the first two adiabatic ionization potentials of NH2 gave a value for the separation of the ground vibrational levels in NH2 +(X 3 B 1) and NH2 +(1 A 1) of 0·99 ±0·02 eV. Ionization energies of NH2 have been estimated using ab initio calculations which include the effects of electron correlation. These calculations indicate that two other bands arising from ionization of NH2, corresponding to formation of the 3 A 2 and 1 A 2 stages, should be observed in the HeI region but no clear evidence of these was obtained experimentally because of overlapping band problems. Spectra of the deuteriated radical were obtained and are consistent with these conclusions. Calculated and observed separations of the two lowest states of NH2 + are compared with those for the analogous states of the isoelectronic methylene radical. The values obtained strongly support a CH2 3 B 1-1 A 1 separation in the range 0·27–0·43 eV. The position of the 1 B 1←1 A 1 NH2 + optically allowed transition is also estimated. Evidence is also presented to indicate that the first ionization potential of NH, corresponding to the process NH+(X 2Π) ←NH(X 3Σ-), has been observed at 13·49 ± 0·01 eV.

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