On the temperature of large biomolecules in ion-storage rings

A method to determine the temperature of molecular ions in an ion-storage ring is presented. Molecular ions were repeatedly irradiated by laser pulses over several hundred milliseconds, and the rate of fragmentation was used to determine the temperature of the photoexcited ions. The initial temperature of the ions before photoabsorption was in turn found from the microcanonical caloric curve for the molecule of interest. The temperature evolution of the protonated GFP chromophore in the ELISA storage ring was found for different starting conditions by this method. We find that the initial temperature of the ions when entering the ring depends on the ion-trap temperature and the amount of buffer gas used in the trap. In particular, collisional heating during acceleration after the ion trap can be significant. Protonated GFP chromophores, produced under different conditions, were used to determine temperature effects on the gas-phase absorption spectra.