A Laser Vision tunable optical parametric oscillator/optical parametric amplifier (OPO/OPA) (mid-IR output 3200-3800 cm -1 , ~2 mJ/pulse, 3 cm -1 spectral line width) generated IR light. The IR pulse preceded the excitation/ionization pulses by ~200 ns. We used two variations of IR double resonant spectroscopy. In mode I, the IR laser scanned across IR wavelengths, while the ionization pulse was held constant at a resonant R2PI transition for a given tautomer. When the IR pulse was resonant with a vibrational mode, it modified the ground state vibrational population, taking the excitation pulse out of resonance and causing a decrease in signal. This produced the IR spectrum for the specific tautomer that was being ionized. In mode II, the IR pulse was held constant at a resonant vibrational transition, while the excitation pulse scanned the R2PI spectrum. In this method, a tautomer was labeled by a vibrational stretch and signal decreased wherever the vibrational transition was shared. Digital chopping created control pulses by firing the IR pulse (pump pulse) 200 ns after the excitation/ionization pulses (probe pulses), generating ions that have not interacted with the IR light.