Direct observation of desorption kinetics with perylene at ultrafine aerosol particle surfaces

A new method for the investigation of the desorption dynamics from ultrafine particle surfaces is presented. It uses photoelectric charging of the substrate particles by a pulsed light source. The yield of such charging near photoelectric threshold depends on the state of the surface with submonolayer sensitivity. If the substrate particles are suspended in a gas at ambient conditions, a time resolution of a few milliseconds is achieved. As an example, we measured the thermal desorption of perylene (C20H12) from sodium chloride (NaCl) and carbon particles. The first-order rate constant of perylene desorption from NaC1 particles was determined for temperatures between 66.3 and 86.0 degrees C. The time required for the reduction of the perylene concentration to l/e of the initial value was found to be t(d) = 396 ms for T = 66.3 degrees C and t(d) = 36 ms for T = 86.0 degrees C. The desorption activation energy was determined to E-d = 122.9(3.0) kJ/mol. This value is in excellent agreement with the result obtained by an equilibrium method and taken from the literature.(1) Thermal desorption of perylene from the surface of carbon particles was studied for temperatures between 67.5 and 104.5 degrees C. The obtained time constants are larger than for NaCl particles. (At T = 83.0 degrees C the time constant is t(d) = 94 ms compared to t(d) = 54 ms for perylene desorption from NaCl particles.) Thermal desorption from carbon particles cannot be explained in terms of a single desorption activation energy. The experimental data are interpreted as desorption from a distribution of adsorption sites with different desorption activation energies.