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We have investigated laser‐assisted reactions in Hg/HgBr2 gas mixtures by measuring fluorescence from the product, HgBr radical, following simultaneous IR (1064 nm) and UV (253.7 nm) laser excitation. The product emission has one‐photon dependence on both IR and UV laser power, and the results indicate that the excited HgBr product channel is accessed through single photon absorptions by the {Hg–HgBr2} complex. This constitutes the first experimental evidence for an electronically excited state of the {Hg–HgBr2} collision complex, having an energy level >4.9 eV but

An LCAO–MO energy level diagram and bond length estimates are used to describe a planar ethylenic molecule model for the N2H4+ defect at an NH4+ site in ammonium halides. The unpaired spin orbital is found to be an antibonding combination of 2p orbitals on each nitrogen perpendicular to the molecular plane. The model is then further specified and used to analyze the low‐temperature EPR data for NH4Cl in detail, from which hyperfine, bond length, and bond angle parameters are extracted and compared with LCAO–MO calculations. Thereby, more information about the defect and confirmation of the model are obtained. Satellite intensity and linewidth angular variations in the high‐temperature spectra are explained, and the relation of high‐ to low‐temperature spectra is discussed.