Fe(CO)(5) was used as a probe of the efficiency of cation and anion trapping in cryogenic matrices under electron-bombardment, matrix-isolation spectroscopy (EBMIS) conditions. Ar gas containing Fe(CO)(5) (1: 1000-3000; Fe:Ar) was subjected to electron bombardment with 150-300 eV electrons above a 16 K spectroscopic window. FTIR spectroscopic analysis of the matrix-isolated products demonstrates the presence of the known neutral and anionic subcarbonyl species, Fe(CO)(n) (n = 2, 3, and 4) and Fe(CO)(m)(-) (m = 3 and 4). Yields for both neutrals and anions diminish significantly with decreasing carbonyl ligand number. Annealing studies in Ar indicate facile conversion of Fe(CO)(4) and Fe(CO)(4)(-) to higher nuclearity iron polycarbonyls, including Fe-2(CO)(9). Irradiation with visible light of samples of electron bombarded Fe(CO)(5) in Ar was found to stimulate loss of Fe(CO)(4) to form Fe(CO)(5) by photoinduced CO recombination. Further irradiation with UV-visible light resulted in additional loss of Fe(CO)(5) and Fe(CO)(4)(-), and loss of features attributed to CO perturbed by Fe(CO)(4)(-), with concomitant production of Fe(CO)(4). The mechanism for formation of these products and the efficiency of ion trapping was further investigated through electron bombardment of Fe(CO)(5) in Ar containing small amounts of CH4 or N-2, and in pure Kr, Xe, CH4, and N-2. Essentially the same distribution of neutral and anionic products was observed in Kr and Xe, but with somewhat diminishing yields. The presence of small amounts of CH4 (4%) or N-2 (2%) in Ar resulted in decreased yields of all products (<50%), with preferential loss of features associated with Fe(CO)(4) compared with Fe(CO)(4)(-). Experiments in pure methane gave very small product yields (<10%), and in pure N-2, no products were observed. Anion product yields are consistent with gas-phase studies of the dissociative capture of low-energy electrons by iron pentacarbonyl, and therefore, the matrix-isolated products appear to be qualitatively representative of the gas-phase product distribution for such a process. The absence of cationic species is in contrast to the efficient production of all cationic subcarbonyls in gas-phase EI experiments. This difference is believed to be due to the low efficiency with which Ar, Kr, or Xe trap cationic species, as well as the high concentration of scattered secondary electrons in the EBMIS environment, resulting in the isolation of the neutral counterparts of cationic species.