Power-law decay of collisionally excited amino acids and quenching by radiative cooling

Abstract

We have investigated the time dependence of the fragmentation of protonated amino acids stored at 22 keV in the electrostatic ring ELISA. The ions were produced in an electrospray source and after bunching in a quadrupole trap they were excited by collisions in a Ne gas. As in earlier studies of metal clusters and fullerenes produced in “hot” ion sources we find that the dissociation of metastable molecules follows approximately a 1/t decay law until a time $\tau$ after which the yield falls off much more rapidly. We interpret this reduction as a result of radiative cooling with a characteristic cooling time, $\tau_c \simeq G\tau$ , where G is the magnitude of the exponent in an Arrhenius expression for the rate of the dominant fragmentation process. The values of $\tau$ obtained from fits to the data are in the range 9-17 ms corresponding to cooling times of a few hundred milliseconds, in good accord with the expected rate of cooling by emission from IR-active vibrations. The power-law behaviour for $t < \tau$ varies somewhat between the different amino acids, with powers between -0.9 and -1.1. We argue that this may be due to a competition between fragmentation channels with different Arrhenius parameters.