RELAXATION OF INTERNALLY EXCITED HIGH-MASS IONS SIMULATED UNDER TYPICAL QUADRUPOLE ION-TRAP STORAGE-CONDITIONS

Collisional relaxation of internally excited high-mass (>1 kDa) ions h as been simulated under typical quadrupole ion trap storage conditions . Two models have been employed that are expected to bracket the range of cooling rates that prevail for such ions present in similar to 1 m Torr of room temperature helium. A diffuse scattering model that assum es that the helium target atom thermally equilibrates with the high-ma ss ion upon collision is expected to yield a maximum cooling rate. A r andom walk algorithm using the exponential model for inefficient colli ders and a relatively small average energy down-step size provides an estimate for the lowest cooling rates that might be expected. The two models give cooling rates that differ by about a factor of three and f all within the range of 200-2000 s(-1) for the ions and energies consi dered in the simulations. Unimolecular dissociation rates have also be en determined for the same model ions. Random wall simulations employi ng collisional cooling and dissociation clearly show how a rapid input of internal energy, as with the absorption of an ultraviolet photon, can either result in dissociation of a large fraction of the ions or c an lead to an insignificant degree of dissociation, depending largely on the unimolecular dissociation rate of the ion. (Int J Mass Spectrom 177 (1998) 163-174) (C) 1998 Elsevier Science B.V.