Photon assisted collisions

A more general term for laser-assisted collision is photon-assisted collision (PAC), since it is not always necessary that lasers provide the light. Both photon-assisted collisions and collision-induced absorption may be... 

Both photon-assisted collisions and collision-induced absorption deal with transitions which occur because a dipole moment is induced in a collisional pair. The induction proceeds, for example, via the polarization of B in the electric multipole field of A. A variety of photon-assisted collisions exist for example, the above mentioned LICET or pair absorption process, or the induction of a transition which is forbidden in the isolated atom [427], All of these photon-assisted collision processes are characterized by long-range transition dipoles which vary with separation, R, as R n with n — 3 or 4, depending on the symmetry of the states involved. Collision-induced spectra, on the other hand, frequently arise from quadrupole (n = 4), octopole (n = 5) and hexadecapole (n = 6) induction, as we have seen. At near range, a modification of the inverse power law due to electron exchange is often quite noticeable. The importance of such overlap terms has been demonstrated for the forbidden oxygen —> lD emission induced by collision with rare gases [206] and... 

Experimental data of photon-assisted collisions are much sparser than those of collision-induced absorption. To some extent, this fact may be due to the greater difficulty in working with excited states [176]. There was an early hope that collision-induced emission or LICET processes could result in new laser sources. When these methods did not work well for various reasons, some of the interest in these phenomena waned. 

One other area of interest in photon-assisted collisions concerns intense fields [9, 427], which are not well understood [26]. 

Fig. 15.5 Observed Na 18p ion signal after the population of the 18s level vs the static field with a 15.4 GHz microwave field. Trace (a) corresponds to no microwave power input to the cavity and shows the set of four zero-photon collisional resonances. Traces (b), (c), (d), and (e) correspond, respectively, to 13.5, 50, 105, and 165 V/cm microwave field amplitudes inside the cavity and show additional sets of four collisional resonances corresponding to one, two, and three-photon radiatively assisted collisions. The peaks labelled 0,1,2, and 3 correspond to the lowest field member of the set of four resonances corresponding to zero-, one-, two-, and three-photon assisted collisions, (0,0)°, (0,0), ...

To obtain the cross section for the m-photon assisted collision in the presence of a microwave field mw we integrate the transition probability over impact parameter. Explicitly... 

A. M. F. Lau. On Laser-induced Inelastic Collisions. In N. K. Rahman and C. Guidotti (ed.), Photon-Assisted Collisions and Related Topics, Harwood Academic, New York, 1982, pp. 55-92. 

As shown by Fig. 8 we can see collisions in which stimulated emission of up to three microwave photons occurs using very low microwave powers. It is interesting to note that laser assisted collisions typically require optical intensities of MW/cm2 to be visible at all [Falcone 1977], In Fig. 8 it appears that the one photon assisted collision disappears only to reappear at higher microwave fields. This unexpected feature becomes more apparent if we plot the resonance signal vs the microwave field, as shown in Fig. 9. Also shown by the lines in Fig. 9 are the results of a model describing collisions in which from zero to three photons are emitted. 

Consequently the cross section for the j photon assisted collision is... 

For a suitable choice of the photon energy hco, the cross section for a nonresonant reaction (8.41) 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. 

Fig. 15.2 Stark energy level diagram of the Na mt = 0 states, relevant to the multiphoton-assisted collisions. The vertical lines indicate the colhsional transfer and are drawn at the fields where they occur. The thick arrows correspond to the emitted photons (from ref. 8).

To describe the shifts and intensities of the m-photon assisted collisional resonances with the microwave field Pillet et al. developed a picture based on dressed molecular states,3 and we follow that development here. As in the previous chapter, we break the Hamiltonian into an unperturbed Hamiltonian H(h and a perturbation V. The difference from our previous treatment of resonant collisions is that now H0 describes the isolated, noninteracting, atoms in both static and microwave fields. Each of the two atoms is described by a dressed atomic state, and we construct the dressed molecular state as a direct product of the two atomic states. The dipole-dipole interaction Vis still given by Eq. (14.12), and using it we can calculate the transition probabilities and cross sections for the radiatively assisted collisions. 

The new and interesting field of light-assisted collisions (often called optical collisions), where absorption of laser photons by a collision pair results in an effective excitation of one of the eollision partners, is briefly treated in the last section of this chapter. For further studies of the subject covered in this chapter, the reader is referred to books [958-960], reviews [961-967], and conference proceedings [968-971]. 

The new and interesting field of "light-assisted collisions", where absorption of laser photons by a collision pair results in effective excitation of one of the collision partners, is briefly treated in the last section of this chapter. 

In Fig. 15.1(b) we show the level system of a collision radiatively assisted by the absorption of one photon of frequency m. In this case the interaction leading to the production of the p and d states is given by... 

Figure 9 Relative cross sections for collisions assisted by zero photons (o and solid line), one photon ( and dashed line), two photons (A and long-short dashed line), three photons ( , dot-dashed line). The lines are calculated from Eq. (14).

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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