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Ionization Modelling of Matrix Molecules in Ultraviolet Matrix‐assisted Laser Desorption/Ionization
Author(s) -
Allwood D. A.,
Dyer P. E.,
Dreyfus R. W.
Publication year - 1997
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/(sici)1097-0231(199703)11:5<499::aid-rcm880>3.0.co;2-2
Subject(s) - chemistry , photoionization , ionization , atomic physics , thermionic emission , electron ionization , soft laser desorption , excited state , ion , thermal ionization , ionization energy , electron , desorption , matrix assisted laser desorption/ionization , physics , quantum mechanics , organic chemistry , adsorption
A scheme of matrix ionization in UV matrix‐assisted laser desorption/ionization is presented. Excitation to an upper lying electronic singlet state by the absorption of two photons and subsequent thermionic emission by vibronic coupling is proposed as the primary route for the formation of unprotonated matrix ions within the desorbed gas‐phase plume. Under conditions where matrix/matrix collisions are sufficiently frequent to establish a thermal distribution of energy amongst the matrix vibrational states, thermionic emission is assumed to proceed as a unimolecular 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 number of positive and negative matrix ions whilst maintaining the overall ionization level. Predictions based on excited‐state thermionic emission and a photoionization model shows that the former provides a considerably better agreement with experimentally determined ion yield vs. fluence characteristics of matrices. For example, for a 3 ns (full width at half maximum) Gaussian N 2 laser (337 nm) incident fluence ( F ) of 15 mJ cm −2 , the predicted fractional ion yield is ∼9.8 × 10 −4 scaling as F 8.5 . © 1997 John Wiley & Sons, Ltd.