Franck desorption

Figure 1 Schematic energy diagram for the DIET process due to the MGR model illustrating the relaxation and desorption processes. Electronic excitation due to laser irradiation occurs via the Franck-Condon transition. After a residue time t at the intermediate excited state, relaxation occurs with an excess energy ZA surpassing the surface barrier for desorption. The value of depends strongly on t, and no desorption occurs when t is shorter than the critical residence time tc. The Absicissa is the adsorbate-substrate distance.

The impulse model is applied to the interpretation of experimental results of the rotational and translational energy distributions and is effective for obtaining the properties of the intermediate excited state [28, 68, 69], where the impulse model has widely been used in the desorption process [63-65]. The one-dimensional MGR model shown in Fig. 1 is assumed for discussion, but this assumption does not lose the essence of the phenomena. The adsorbate-substrate system is excited electronically by laser irradiation via the Franck-Condon process. The energy Ek shown in Fig. 1 is the excess energy surpassing the dissociation barrier after breaking the metal-adsorbate bond and delivered to the translational, rotational and vibrational energies of the desorbed free molecule. 

The second application is to the direct measurement of adsorption-desorption processes using the Auger peak height of the particular element as a monitor. The principal limitation is the possible influence of the electron beam on the adsorbate, which can result in beam-induced desorption, adsorption or dissociation. The basis of electron-stimulated desorption (ESD) was established some time ago independently be Menzel and Gomer [38] and Redhead [39]. Electron impact causes Franck—Condon transitions of bound electrons in the adsorbed species into excited states. There is, therefore, a probability of dissociation with subsequent desorption of the particular species involved. As an example of these effects on semiconductor surfaces, Joyce and Neave [40] have reported results on silicon, while Ranke and Jacobi [41] have discussed the electron-stimulated oxidation of GaAs. 

Franck, J., Arafah, K., Barnes, A., Wisztorski, M., Salzet, M. and Fournier, I. (2009) Improving tissue preparation for matrix-assisted laser desorption ionization mass spectrometry imaging. Part 1 Using microspotting. Anal. Chem. 81, 8193-8202. 

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