Infrared photophysics in an ion tap

A Monte Carlo model has been developed which provides a very detailed picture of conditions during multiphoton infrared fragmentation experiments, as performed in an ion trap. Typically, two types of ion traps are used, an ion cyclotron resonance (ICR) instrument, and a quadrupole ion trap. Experiments fall into three separate categories: Low background gas pressure combined with either high or low intensity laser radiation, and moderate background pressures with low intensity laser radiation. Each set of experimental conditions brings to the simulation a dependency on a particular set of variables, and these can be refined to give a self-consistent picture of the complete photofragmentation process. At the low gas pressures ( ~ 10-7?mbar) found in ICR traps, the simulation of experiments run at low laser intensities shows that radiative decay has an important influence on photofragment yield. In the same type of trap, but at high laser intensities, pulse shape and stimulated emission become important. Finally, at pressures found in a typical quadrupole ion trap ( ~ 10-4?mbar), collisions with the helium background have a significant effect on the outcome of infrared excitation; however, the time scale of an experiment is such that radiative decay can also influence the results. The model has been applied to the infrared photofragmentation of the protonated diethyl ether dimer, [(C2H5)2O]2H+, where it successfully accounts for experimental results recorded under each of the three conditions identified above. Under circumstances where photofragmentation is in competition with either radiative or collisional relaxation, the calculations show that fragmentation requires the absorption of up to 20 photons (assumed to come from a CO2 laser, h? ˜ 0.11?eV), as opposed to the 12 photons necessary to match the critical energy of reaction.