1 Metastable levels Molecular dissociation

Figure 3 Scheme of a two laser pump-pump experiment for the production of internal energy-selected molecular ions in a reflectron time-of-flight mass spectrometer (from ref. /15/). Laser 1 produces state-selected molecular ions and 200 ns later laser 2 excites these ions to a well defined internal energy level above dissociation threshold. The dissociation rate constants of the energy-selected ions are measured by the technique of detection and energy analysis of metastable ions. 

Desorption can proceed via several mechanisms. For solids with a negative electron alSnity such as Ar [49,149-151] and N2 [153], the extended electron cloud around a metastable center will interact repulsively with the surrounding medium and metastables formed at the film-vacuum interface will be expelled into vacuum (the so-called cavity expulsion mechanism [161]). Also permitted in solids with positive electron affinities (e.g., CO) is the transfer of energy intramolecular vibration to the molecule-surface bond with the resulting desorption of a molecule in lower vibrational level [153,155,158-160]. Desorption of metastables via the excitation of dissociative molecular (or excimer) electronic states is also possible [49,149-151,154,156,157]. A concise review of the topic can be found in Ref. 162. 

FIGURE 7 Energy-level diagram for the COIL illustrating the resonant energy transfer from the metastable energy reservoir 02(a A) to the upper laser level of atomic iodine, l( Pi/2)- The energy levels for several key states Involved in the dissociation of molecular iodine are also provided. 

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