Infrared laser spectroscopy of mass-selected carbocations.

Small carbocations are of longstanding interest in mass spectrometry, organic chemistry and astrophysics, but there are few measurements of their spectroscopy in the gas phase. Of existing infrared measurements, few are available across the full range of IR frequencies. In new work described here, a pulsed-discharge supersonic-nozzle ion source produces higher densities of carbocations at low temperatures (20-100 K). Mass-selected photodissociation spectroscopy and the method of rare gas "tagging", together with new broadly tunable infrared OPO lasers, produce IR spectra for a variety of small carbocations including C(2)H(3)(+), C(2)H(5)(+), C(3)H(3)(+), C(3)H(5)(+), the tert-butyl cation, protonated benzene, and protonated naphthalene. Spectra in the frequency range 600-4500 cm(-1) provide new IR data for these ions and evidence for the presence of coexisting isomeric structures (e.g., C(3)H(3)(+) is present as both cyclopropenyl and propargyl). Protonated naphthalene has bands at 6.2, 7.7, and 8.6 μm matching prominent features in the interstellar unassigned infrared (UIR) emission spectra.