We recorded the electronic absorption spectra of these clusters using a mass spectrometrybased technique that alleviates the need for purification of the as-synthesized reaction mixturethe mass spectrometer is effectively the separation device. The combination of exact selectivity, minimal environmental effects, and low temperature of this experiment yields highly reproducible, well resolved spectra of unambiguously-purified nanoclusters. All experiments were conducted on a home-built ion trap time-of-flight (TOF) mass spectrometer. The experimental process has been explained in detail previously. Briefly, the clusters are introduced into the instrument using electrospray ionization (ESI) of solutions of ∼ 1 mg/mL in methanol. Upon storage in a cryogenically-cooled octopole ion trap containing helium, the ions are collisionally cooled to near the trap temperature (3.6-3.8 K), at which point tens of He atoms (referred to as "tags") adsorb onto the clusters. A pulsed optical parametric oscillator, spanning the region 409-719 nm, is intersects the tagged clusters in the cryogenic trap, 10 µs prior to their extraction. Absorption of a photon resonant with the cluster's electronic transitions results in desorption of the all tags, yielding an untagged peak in the mass spectrum. The integrated intensities of the tagged and untagged clusters are recorded as a function of wavelength, and a linear absorption spectrum is generated from the ratio of the latter to the total cluster signal after correcting for the laser power spectrum.