Fig. 2.

Scheme describing the onset of spectral diffusion by site exchange in a temperature-controlled cluster using selective photodissociation. The populations in the five spectroscopically distinct sites, color-coded to match those identified in Fig. 1 A and G, are indicated by the fractional filling of the circles. The panels correspond to three temperature regimes: (A) a low-temperature, static ensemble in which the red isotopomer is removed at t = 0, and subsequent interrogation of the ensemble after a delay time Δt reveals persistent removal of the red species from the ion packet; (B) an intermediate temperature where only two sites, red and brown, undergo interconversion on the timescale of the experiment. In this case, initial depletion of the red species, and hence its signature vibrational band, partially recovers after time Δt while the population of the brown isotopomer decreases to restore equilibrium with red; and (C) a sufficiently high temperature that all sites interconvert after initial depletion of red such that, after delay Δt, the red population is largely restored while those in the other four sites exhibit minor depletions upon returning to steady state.