Gas-phase infrared multiple photon dissociation spectroscopy of mass-selected molecular ions

In this review, we present an overview of our recent work on the infrared spectroscopy of mass-selected gas-phase molecular ions. The ions are stored and isolated in a quadrupole ion trap where they are investigated with infrared multiple photon dissociation spectroscopy using a free electron laser (FEL). The wide and continuous tunability of FELIX, the FEL for Infrared eXperiments at the FOM institute for Plasma Physics in The Netherlands, allows us to explore the interesting mid-infrared range between 500 and 2000 cm−1, sometimes referred to as the fingerprint region. The experiments have focused on polycyclic aromatic hydrocarbons because of their hypothesized occurrence in the interstellar medium, both in neutral and cationic forms. Furthermore, several related species of fundamental chemical interest have been investigated, such as molecular fragment ions and protonated species. The spectra are compared to theoretical spectra calculated at the density functional theory level. The effects of multiple photon excitation and dissociation dynamics on the appearance of the spectrum are extensively discussed. A mathematical model to account for the effects of anharmonicity is described. It explains for instance, qualitatively, how the observed spectral response can vary (slightly) with the fragment channel, which allows one to “direct” dissociation reactions in a non-coherent fashion by the choice of the infrared excitation wavelength.