Energy deposition process electronic excitation

In MALDI, the sample is deposited on a target and co-ciystallized with a solid matrix [14-15]. The target is transferred to vacuum and bombarded by photon pulses from a laser, in most cases a nitrogen laser (337 nm) nowadays. The ionization results from efficient electronic excitation of the matrix and subsequent transfer of the energy to the dissolved analyte molecules, which are desorbed and analysed as protonated or cationized molecules [7]. The ionization process is not fully understood. Extremely high molecular-mass compounds, e.g., in excess of 200 kDa, can be analysed using the MALDI, if performed on a time-of-flight mass spectrometer (Ch. 2.4.3). 

Various chemical steps generally take place between the end of purely physical processes of energy deposition in polymers and the resulting chemistry. When the energy imparted to a molecular electron is lower than its lowest ionization potential, the resulting excitation (Scheme 1) proceeds according to optical selection rules. 

Non-homogeneity of energy deposition in radial direction is of importance when elementary processes are concerned or fluence is low. Upon beam incident to material, energy deposition would cause track structure. Substructure is presumed (physical) core at which an electronic ionization and excitation can... 

---