Molecular ions of n-butylbenzene trapped in the ion cyclotron resonance ion trap were dissociated by cw CO2 -laser irradiation at pressures near 1×10−8 Torr where collisions play a negligible role in the kinetics. The dissociation as a function of time after laser onset showed a well-defined induction time τind, followed by exponential decay of a parent ion with first-order rate constant kdiss. Data were collected over a range of laser intensities giving kdiss values from 0.3 to 10.5 s−1. Computer simulation of the dissociation curves was carried out using a random-walk method to solve a master equation for the kinetics, and assuming infrared radiative relaxation as the mechanism of ion deactivation. The simulations yielded curves also showing well-defined induction times and dissociation rate constants. It was necessary to consider both the induction times and the dissociation rates to constrain the fit of experiment and computer simulation. The resulting fit was good. The thermal picture of low-pressure infrared multiphoton dissociation kinetics developed in previous work was applied to the data, and thermal kinetics were found to give an excellent description of the observed kinetics. The observed activation energies were successfully interpreted in terms of the modified Tolman’s theorem.

No abstract

No abstract

Ion trajectory calculations have confirmed that space charge interactions can be a source for mass discrimination seen in Fourier transform-ion cyclotron resonance (FT-ICR) spectra. As compared with the previously recognized mechanism of z-axis excitation, ion-ion repulsion is a mechanism which specifically affects relative peak heights of ions close in mass, and is most severe for low excitation radiofrequency (rf) amplitudes. In this mechanism, Coulomb repulsion significantly perturbs the motion of the ion clouds during excitation and alters the final cyclotron orbital radii. Under these conditions peak heights do not accurately reflect the true ion abundances in the FT-ICR spectrometer. Mass discrimination can be minimized by using low numbers of ions, low ion densities, and a short, high amplitude rf excitation waveform. Experimental observation of the relative peak heights of the m/z 91, 92, and 134 ions in n-butylbenzene gives quantitative confirmation of the results of the trajectory calculations. Chirp, SWIFT, and impulse excitation were modeled: impulse excitation was found to be most effective in minimizing the effects of space charge interactions.

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 ∼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.