2001and only (M2 -H)+ and MzH+ appear in the mass spectrometer for both the Ni / 3 and laser ionization source even a t the lowest concentration levels studied. Below 150 "C the laser ionization signal becomes very weak due to lack of hot-band population. Although direct ionization of the neutral dimer is possible, at low concentration, formation of M+ followed by MH+ + M -MzH+ is believed to be the mechanism of dimer formation.
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