Collisional effects in organometallic multiphoton dissociation/ionization experiments: The modeling of townsend discharges

We have measured the effects of added buffer gas on the multiphoton ionization (MPI) signal of various arene chromium tricarbonyls and Cr(CO)₆. Using basic gaseous electronics theory and experimental results on ion multiplication behavior in pure buffer gases, we have found the simplest method for quantitative analysis of collisional effects in MPI systems. We have applied this discharge model to the multiphoton ionization signal of chromium compounds as a function of helium, argon, and xenon pressures at a number of wavelengths and ion collection plate voltages. Where simple ion multiplication behavior holds, the measured ion signal is directly proportional to the initial MPI yield. This is of crucial importance for MPI studies where product branching ratios are being determined. When multiplication theory does not model the ion signal pressure dependence, other processes which may influence ion production in the system must be considered.