Mapping Molecular Excited States via Relative Ion Yield Photodissociation Spectroscopy

Laser photodissociation spectroscopy of gaseous ions is a widely applicable technique used to investigate the electronic excited states and photochemistry of charged systems ranging from inorganic nanoclusters through to biomolecules. Spectra obtained by this technique provide important benchmarks for theoretical studies of dissociative excited states and give insight into light-induced fragmentation pathways. In this concept article, we will highlight the advantages of obtaining and analysing photodissociation action spectra in terms of photon-energy-dependent relative ion yield plots. This new approach to analysing ion photodissociation spectra has been demonstrated by our laboratory over recent years and is based on the photochemical concept of quantum yields. Importantly, relative ion yield analysis allows experimental vertical excitation energies of charged systems to be straightforwardly identified for comparison to theory. The application of relative ion yield analysis also provides greater insight into excited-state decay dynamics than is evident from inspection of individual photofragment action spectra. To provide a suitable introduction to the relative ion yield concept for non-expert readers, we begin the article by reviewing the broader topic of ionic photofragment action spectroscopy. Results from several UV filter molecules are then presented to illustrate the advantages of relative ion yield analysis.