Infrared fluorescence relaxation of photoexcited gas-phase ions by chopped-laser two-photon dissociation

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Abstract
Two-photon photodissociation in the ion cyclotron resonance ion trap, using repetitively pulsed laser excitation at low pressure, was exploited to measure the noncollisional energy dissipation rate for a number of gas-phase ions. Since the photoexcited ions initially contain 2–5 eV of excess vibrational energy, the observations give cooling rates for the ions at very high vibrational temperatures. Calculated as first-order rate constants for internal energy dissipation, the values lie in the range of 3–16 s−1, with the more polar molecules generally showing faster relaxation. Under these collisionless conditions, it is presumed that the cooling mechanism is infrared fluorescence. The observed rates of cooling are compared with the rates that would be observed if the infrared radiative characteristics of the ions were the same as those of the corresponding neutrals. The ions all show faster radiative relaxation than the neutrals, often being faster by about a factor of 2. This is consistent with a general increase in vibrational transition dipole moment due to charge movement involving the free positive charge on the ion.