Negative ion photoelectron spectroscopy of the negative cluster ion H−(NH3)1

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Abstract
The negative ion photoelectron spectrum of the negative cluster ion H− (NH3)1 has been recorded with 2.540 eV photons. This negative cluster ion was prepared in a supersonic nozzle‐ion source involving the injection of electrons into an expanding jet. While the spectrum is dominated by a broadened peak centered at 1.430±0.019 eV, there is also a small feature centered at 0.997±0.031 eV. Our interpretation of this spectrum is that the main peak contains the origin of the photodetachment transition, and that the smaller one is due primarily to the excitation of a stretching mode in the ammonia solvent during photodetachment. An upper limit to the dissociation energy of H−(NH3)1 into H− and NH3 is found to be 0.36 eV. This result is in good agreement with calculations by Kalcher, Squires, and Schleyer. The separation between the main and the small peaks is 3490±130 cm−1, and the symmetric stretching frequency of free NH3 is 3506 cm−1. The small peak also shifts appropriately upon deuteration in support of this assignment.

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