Sensing collisional

If collisional systems involving one or more molecules are considered, the internal degrees of freedom of the molecule(s) (e.g., rotation, vibration) have to be taken into account. This often leads to cumbersome notations and other complications. Furthermore, we now have to deal with anisotropic intermolecular interactions which again calls for a significant modification of the formal theory. In that sense, this Chapter differs from the previous one but otherwise the reader will find here much the same material, techniques, etc., as discussed in Chapter 5. 

Let us first discuss a system which is traditional for optical pumping in the Kastler sense [106, 224, 226], namely an optically oriented alkali atom A (see Fig. 1.1) in a noble gas X buffer surrounding. It is important to take into account the fact that in alkali atoms, owing to hyperfine interaction, nuclear spins are also oriented. However, in a mixture of alkali vapor with a noble gas alkali dimers A2 which are in the 1SJ electronic ground state are always present. There exist two basic collisional mechanisms which lead to orientation transfer from the optically oriented (spin-polarized) atom A to the dimer A2 (a) creation and destruction of molecules in triple collisions A + A + X <—> A2 + X (6) exchange atom-dimer reaction... 

Collisional dissociation (process (9) of Table I) differs essentially from the other three particle processes in the sense that the diabatic transitions, responsible for ionization, take place inside the molecule XY. Therefore this process is more related with two-particle collisions than with three-particle... 

By definition irreversible electronic relaxation processes cannot occur in isolated small and too-many level small (intermediate) case molecules because of the insufficient density of final levels. For long times the molecule senses the presence of a finite number of possible final levels instead of the effective continuum that is required to drive irreversible electron relaxation. When collisional processes are appended, it is clear that the continuous density of states of the colliding pair can provide the necessary driving force for irreversible relaxation. The observed magnitudes of electronic relaxation rates as well as dependencies on the initial state, perturbing molecules, temperature, and so on, are the aspects of the processes that are of central interest. 

The analysis of cometary observations suggests the existence of very fluffy dust aggregates. Differences are observed in the light-scattering properties, e.g. stracture of the comae, polarization phase curves maxima and minima, polarization wavelength dependence. They could be a clue to the temporal evolution of the physical properties of the dust particles, with collisional processes as well as evaporation of icy mantles and organic compoimds. Table 1 presents some polarization properties of dust particles in comets, asteroids, in the interplanetary dust cloud, and on Mars, as retrieved by remote sensing. 

One common approach to fluorescence sensing is to rely on fluorophores which are collisionally quenched by the analyte. There are many known collisional quenchers (andytes) which alter the fluorescence intensity and decay time. These include O2 (27-31), chloride (32-33), chlorinated hydrocarbons (34), iodide (35), bromate (36), xenon (37), acrylamide (38), succinimide (39), sulfur dioxide (40), and halothane (41), to name a few. The quenching process obeys the Stem-Volmer equation ... 

Any phenomenon which results in a change of fluorescence intensity wavelength, anisotropy, or lifetime can be used for sensing. The simplest mechanism to understand is collisional quenching (Figure 19.6, middle). In this case, one identifies a fluorophoie which is quenched by the analyte. Collisional quenching results in a decrease in the intensity or lifetime of the fluorophore, either of which can be used to determine the analyte concentration. 

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