IONIZATION MODELING OF MATRIX MOLECULES IN ULTRAVIOLET MATRIX-ASSISTED LASER DESORPTION IONIZATION/

A scheme of matrix ionization in UV matrix-assisted laser desorption/i onization is presented. Excitation to an upper lying electronic single t state by the absorption of two photons and subsequent thermionic emi ssion by vibronic coupling is proposed as the primary route for the fo rmation of unprotonated matrix ions within the desorbed gas-phase plum e. Under conditions where matrix/matrix collisions are sufficiently fr equent to establish a thermal distribution of energy amongst the matri x vibrational states, thermionic emission is assumed to proceed as a u nimolecular reaction that draws upon the vibrational energy reservoir of individual molecules. The dominant free-electron loss mechanism is assumed to be electron/neutral attachment, thus providing an equal num ber of positive and negative matrix ions whilst maintaining the overal l ionization level. Predictions based on excited-state thermionic emis sion and a photoionization model shows that the former provides a cons iderably better agreement with experimentally determined ion yield vs. fluence characteristics:of matrices. For example, for a 3 ns (full wi dth at half maximum) Gaussian N-2 laser (337 nm) incident fluence (F) of 15 mJ cm(-2), the predicted fractional ion yield is similar to 9.8 x 10(-4) scaling as F-. (C) 1997 by John Wiley & Sons, Ltd.