Homogeneous Bottleneck Model of Matrix-assisted Ultraviolet Laser Desorption of Large Molecules

The factors affecting the yield of high-mass molecules by matrix-assisted ultraviolet laser volatilization are examined in a simple model. The key material factors appear to be a low heat of sublimation, subcritical concentration of the guest molecules and a high irradiance input in a short time compared to the sublimation induction period. The model is homogeneous in that the energy density is taken to be uniform within the ‘hot region’ of the matrix. The two competing effects are the rates of energy transfer from the matrix to the guest molecules and the desorption by sublimation. It is the bottleneck for energy transfer to the embedded guest molecules that makes their energy content lag behind that of the matrix. This is particularly the case for an initially cold sample. When a sufficiently high rate of sublimation can be achieved (e.g., using a high-power laser), the guest molecules (or adduct ions) will desorb internally cold and will thus not fragment. Numerical simulations of the sublimation kinetics using realistic laser and material parameters support the conclusions and delineate the ranges of the critical factors.