The infrared spectra of the NH3-d(n)(+) cations trapped in solid neon

When a dilute mixture of NH3 in neon is subjected to Penning ionization and /or photoionization by neon atoms in their first excited states, between 16 .6 eV and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes three relatively prominent product absorptions which agree well with the positions determin ed in earlier gas-phase studies for the three infrared-active fundamentals of (NH3+)-N-14. The corresponding fundamentals of (NH3+)-N-15, (ND3+)-N-14, and (ND3+)-N-15 have been observed for the first time, as have been many o f the fundamentals of the partially deuterium-substituted cations, in exper iments on isotopically enriched NH3 samples. When the effects of anharmonic ity are considered (including the occurrence of "negative anharmonicity" fo r the out-of-plane deformation fundamental), the positions of these absorpt ions agree satisfactorily with those predicted in an earlier study from the fit of an ab initio potential surface to the experimental data for (NH3+)- N-14, as well as with those predicted in the present study from a least-squ ares force constant adjustment to the frequencies observed for the various isotopomers.