Figure 3.

(a) Photofragmentation decay of mass-isolated protonated isoindigo as a function of the number of IR FEL pulses at fixed frequencies diagnostic for the trans-isomer (1745 cm^–1, ●) and for the cis-isomer (1686 cm^–1, ★, ☆: replicate data). Data points indicated with ▷ were taken with the laser set at 763 cm^–1, where both isomers absorb leading to complete depletion of the precursor ion population. (b) Results of similar experiments for protonated indigo, performed at frequencies resonant with trans (1725 cm^–1, ●, ○: replicate), and with both cis + trans (1604 cm^–1, ★, ☆ replicate, and 745 cm^–1, ▷). Each data point is obtained from 10 averaged mass spectra. Solid curves are single-exponential decay fits to a limited range of the experimental data points, except for trans-isoindigo where all data points are included.