Photon-Assisted Collisional Energy Transfer

The energy transfer by inelastic collisions between excited atoms or molecules A and ground-state atoms B 

If such a reaction (13.37) proceeds inside the intense radiation field of a laser, a photon may be absorbed or emitted during the collision which may help to satisfy energy conseration for small AEj j even if AEjj is large. Instead of (13.37) the reaction is now 

For a suitable choice of the photon energy h(jJ the cross section for a nonre-sonant reaction (13.37) can be increased by many orders of magnitude through the help of the photon which makes the process near resonant. Such photon-assisted collisions will be discussed in this section. 

If we start with A+B the inverse process of stimulating photon emission will result in a transition from the upper into the lower potential, which means an energy transfer from B to A for the separated atoms. 

For the experimental realization of such photon-assisted collisional energy transfer one needs two lasers the pump laser LI excites the atoms A into the excited state A and the transfer laser L2 induces the transition between the two potential curves V(A B) - V(AB ). 

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Fig. 8.34 Photon-assisted collisional energy transfer (a) molecular model (b) dressed-atom model...

The first experimental demonstration of photon-assisted collisional energy transfer was reported by Harris and coworkers [1092], who studied the process... 

Fig. 8.35 Term diagram for photon-assisted collisional energy transfer from Sr (ip< )to Ca(4/72i5) [1092]...

Fig. 8.36 Experimental setup for studies of photon-assisted collisional energy transfer [1097]...

In addition, new tandem mass spectrometry technologies were also among the important innovations. Apart from traditional collision-induced dissociation (CID) [89-91], a variety of activation methods (used to add energy to mass-selected ions) based on inelastic collisions and photon absorption have been widely utilized. They include IR multiphoton excitation [92,93], UV laser excitation [94—97], surface-induced dissociation (SID) [98-100], black body radiation (101, 102], thermal dissociation [103], and others. As the fragmentation of peptide/protein ions is a central topic in proteomics, there is strong interest in such novel ion dissociation methods as electron capture dissociation (ECD) [104, 105] and electron transfer dissociation [22]. These new methods can provide structural information that complements that obtained by traditional collisional activation. Also, very recently, ambient ion dissociation methods such as atmospheric pressure thermal dissociation [106] and low temperature plasma assisted ion dissociation [107] have been reported. 

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