Working in the Fourier-transform ion cyclotron resonance ion trap, the competitive photodissociation of n-butylbenzene ions at 440 nm has been developed as a thermometric probe to monitor the instantaneous internal energy of the ion population. Using this probe, the heating of initially thermal ions by CO2 laser irradiation was followed as a function of time at several laser intensities. The IR multiphoton dissociation of the population was followed simultaneously. The ions were observed to heat up progressively during the induction period, during which no dissociation was observed. Dissociation commenced as the ions leveled off at a steady-state internal energy approximately 0.8 eV, and dissociation then proceeded approximately as a first-order decay at constant average internal energy. After heating of the population, the CO2 laser was turned off and IR-radiative cooling of the ions was observed with a cooling rate constant of 0.97 s-1. The kinetics of heating, cooling, and dissociation followed the qualitative expectations from previous work, but quantitative modeling required some modification of the previously used simple harmonic photophysical picture. It seemed most likely that this ion possesses a modest inverse pumping bottleneck with up pumping slowing by a factor of 3 at high internal energies (above 0.8 eV). Enhanced radiative relaxation at internal energies above 1.0 eV can also bring the kinetic modeling into quantitative agreement with experiment.
