Ionization Modelling of Matrix Molecules in Ultraviolet Matrix-assisted Laser Desorption/Ionization

✍ Scribed by D. A. Allwood; P. E. Dyer; R. W. Dreyfus

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 498 KB
• Volume: 11
• Category: Article
• ISSN: 0951-4198
• DOI: 10.1002/(sici)1097-0231(199703)11:5<499::aid-rcm880>3.0.co;2-2

✦ Synopsis

A scheme of matrix ionization in UV matrix-assisted laser desorptiodionization 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 matridmatrix 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 electrodneutral 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 NZ laser (337 nm) incident fluence (F) of 15 mJ cm-2, the predicted fractional ion yield is -9.8 x lo4 scaling as p5.