Identification of isomeric hydrocarbons by Rydberg photoelectron spectroscopy

Many saturated and unsaturated organic hydrocarbons can assume multiple isomeric forms. At high temperatures, the identification of such isomers is difficult with conventional spectroscopic techniques, posing a challenge to the exploration of important processes such as the combustion of hydrocarbons. A recently developed technology using resonance enhanced multi-photon ionization via Rydberg states shows promise as an analytical technique, because the resultant photoelectron spectra provide well-resolved peaks that are sensitive to the molecular structure even at high temperatures. We tested this new method on isomeric hydrocarbon systems with the chemical formulas C5H8, C6H8, C7H8, and C11H10. Three-photon ionization with femtosecond pulses near 400 nm shows that in all systems the observation of distinct Rydberg features can serve to identify the isomers. In the C5H8 system, the photoelectron spectra of the isomers show Rydberg peaks from different quantum states, making the spectral identification of those isomers especially facile. Thus it appears that photoionization from Rydberg states could be developed into an analytical tool that is capable of distinguishing isomeric hydrocarbons even under adverse conditions such as flames and other combustion phenomena.