Absorption spectrum of isolated tris(2,2′-bipyridine)ruthenium(II) dications in vacuo

Abstract

Here we report the gas-phase action spectrum of tris(2,2′-bypyridine)ruthenium(II) dications, Ru(bipy) 3 2+, recorded over a broad wavelength region from 210 nm to 650 nm. Ions formed by electrospray ionization were thermalized in collisions with helium buffer gas at room temperature in a 22-pole ion trap. A bunch of ions was then accelerated to keV-kinetic energies and injected into an electrostatic ion storage ring in which they circulated with a revolution time of 69 microseconds. After several milliseconds to allow for the decay of metastable ions, the ions were photoexcited and their decay monitored in time. Dissociation was found to occur on the microsecond time scale. Based on the time spectra, the initial number of photoexcited ions was estimated at each wavelength (relative and not absolute numbers). This approach circumvents the problem of kinetic shifts often encountered when dissociation is sampled within a finite time interval in the case of finite length time-of-flight instruments. The obtained absorption spectrum is found to be similar to the solution phase spectrum (acetonitrile solvent) but with a blueshift of the lowest energy metal-to-ligand charge transfer transition. This finding indicates a localized electronic transition in solution involving only one of the bipyridine ligands. There are small differences in the relative intensities of the bands between the gas-phase and solution-phase spectra. Finally, our gas-phase spectrum serves as a reference to benchmark theoretical calculations of excited states.