Non-intrusive detection of combustion intermediates by photoionization via Rydberg states and microwave backscattering

A non-intrusive, in-situ detection technique for combustion intermediates in flame environments yields highly resolved spectra that are largely insensitive to molecular vibrations and thus temperature. The technique is based on laser photoionization of target compounds via Rydberg states, followed by detection of the laser-induced plasma with microwave radiation. The feasibility of this approach is tested on methyl radicals that are detected in methane, propane, and hexane fuel-rich flames. The methyl radicals are prepared in the 3s Rydberg state using photoexcitation at 216.4 nm. Using tunable VIS/IR radiation, the molecules are promoted from 3s to the 3pxy and 3pz Rydberg states. Photoionization out of the 3p states with 355 nm photons results in a small-volume plasma that is detected by microwave backscattering. The previously unobserved 3pxy state is found to have a binding energy of 2.57 eV.