Our most common method of ionization is through Resonant Two-Photon Ionization (R2PI). The electronic energy level diagram for this process is shown on the right. As indicated in the name R2PI, two photons are used to ionize the molecule. The first photon is resonant with an excited electronic state, S_n. The second photon (either from the same laser in one-color R2PI or from a different laser in two-color R2PI) excites the molecule to the ionization continuum where the molecule loses an electron, producing an positively charged ion. The ions are detected by time-of-flight mass spectrometry in one of our two time-of-flight chambers.

Since we only detect ions when the molecules absorb UV light that is resonant with an electronic transition, we tune our dye laser through the region of interest and collect the signal from the detector that corresponds to the mass of our molecule. This method of detection allows us to obtain the mass-resolved ultraviolet excitation spectrum, which is important if a molecule is prone to decomposition or the sample contains impurities (such as starting materials).

Laser induced fluorescence (LIF) (energy level diagram shown on the right) is a more sensitive method than R2PI, but lacks mass resolution. As in R2PI, LIF begins when a molecule absorbs a photon and is promoted to an excited electronic state. After a short time (usually a few nanoseconds), the molecule will fluoresce, or lose energy by emitting a photon. We monitor the total fluorescence as a function of exciting wavelength to generate an ultraviolet excitation spectrum. All species in the expansion that absorb at these wavelengths and subsequently fluoresce will contribute to this LIF spectrum.

We can also obtain emission spectra by using a monochromator to disperse the fluorescence. A schematic for this experiment is shown on the right.