Excited state relaxation processes of MALDI-matrices studied by luminescence spectroscopy

The process of matrix-assisted laser desorption/ionization (MALDI) has been investigated by studying the luminescence of the matrix molecules after UV-laser irradiation. A grating spectrograph equipped with an optical multichannel analyzer (OMA) has been used for the measurements. The broad shape of the spectrum and the fact that the molecules form hydrogen bonds with neighbouring molecules in the solid phase support the hypothesis that the molecules form excimers after laser irradiation. The increase of the decay time and the intensity with decreasing temperature suggests that backbone vibrations play an important role and that photon emission occurs simultaneously with the emission of a phonon. The activation energy for this process has been estimated to be 0.023 eV. At laser fluences above 2 mJ/cm2 the luminescence is quenched, presumably by S1-S1 interactions leading to the excitation of higher excited states Sn or to electron emission. Both S1-S1-annihilation and electron emission can influence the MALDI-process.