INTERNAL ENERGY OF NEUTRAL MOLECULES EJECTED BY MATRIX-ASSISTED LASER-DESORPTION

Neutral molecules ejected by matrix-assisted laser desorption were detected by photoionization with coherent vacuum-ultraviolet radiation. The photoionization mass spectra of laser-desorbed neutrals were compared to temperature- and wavelength-dependent photoionization mass spectra of vapor-phase molecules. Mass spectra were obtained for primary and secondary alkylamines desorbed from various matrices. Ionic fragmentation in the photoionization mass spectra showed that alkylamine neutrals ejected by matrix-assisted laser desorption contained between 0.6 and 0.9 eV of internal energy even with desorption irradiances near threshold. These internal energies corresponded to ''temperatures'' between 440 and 520 K. The temperatures were comparable to the sublimation temperatures of the matrices used. The internal energy increased with increasing desorption laser irradiance but was independent of the sample probe temperature. Varying the time delay between the desorption and photoionization pulses showed that high-kinetic energy neutrals also had the highest internal energies while low-kinetic energy neutrals also had the lowest internal energies.