Infrared spectroscopy of the protonated nitrogen dimer: The complexity of shared proton vibrations

The proton-bridged dimers of nitrogen, e. g., N-2-H+-N-2 and N-2-D+-N-2, are produced in a pulsed-discharge supersonic nozzle source, mass selected in a reflectron time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy using the method of messenger atom tagging with argon. Both complexes are studied from 700-4000 cm(-1). These spectra reproduce the high frequency vibrations seen previously but discover many new vibrational bands, particularly those in the region of the shared proton modes. Because of the linear structure of the core ions, simple vibrational spectra are expected containing only the antisymmetric N-N stretch and two lower frequency modes corresponding to proton stretching and bending motions. However, many additional bands are detected corresponding to various combination bands in this system activated by anharmonic couplings of the proton motions. The anharmonic coupling is stronger for the H+ system than it is for the D+ system. Using anharmonic proton vibrations computed previously and combinations of computed harmonic frequencies, reasonable assignments can be made for the spectra of both isotopomers. However, advanced anharmonic computational treatments are needed for this system to confirm these assignments. (C) 2009 American Institute of Physics. [doi:10.1063/1.3224155]