Collisions disorienting

Chief Phillip A. Napier of the GVW Fire and Rescue Department, who also runs Napier s Hardware Store in downtown Graniteville, South Carolina remembers, We received a call that the train had possibly hit a building in Graniteville. Upon our response, I told the men to report to the station, and I would try to locate the incident site and find out what was involved. And evidently, by listening to the tapes at a later date, it is evident I was already beginning to be disoriented. I couldn t see. I called for an ambulance, and I couldn t say what I wanted it for. When I pulled up to the man on the railroad track, and rolled the window down, he told me they had had a head on collision with the train, they had a chemical leak, and he couldn t breathe. Then he fell to the ground, and later died. 

If the molecule, from the moment of absorption up to the moment of emission, is not subjected to external influence, e.g. to disorienting collisions or external fields, then the multipole moments bPQ entering into... 

If the excited molecules undergo disorienting collisions before emitting light, then the bpQ of various rank K relax at different rates T/c. The anisotropy of polarization % permits us to find the ratio between the population relaxation rate To and that of the alignment relaxation r2 ... 

Studies of the depolarization process in elastic collisions for excited states of a number of diatomic molecules, such as H2(S1E+) [311], Li2(A1E+) [147, 332], NaR II) [24, 291], CdH(A2II1/2) [130], Na2(S1nu) [237], BaO(A1E+) [350], Se2 [205] and Te2 [154] have shown that, unlike the case of atoms, the efficiency of purely disorienting collisions is mostly very low in comparison with quenching collisions. The results of the works in which the differences 02 — < i — o have been... 

In the classical model the inefficiency of purely disorienting elastic collisions can easily be understood if one takes into account the fact that such disorientation is connected with a turn of the angular momentum J by a tangible angle (collisional randomization of 3(0,rotational level, i.e. the collision becomes an inelastic one. 

Table 2.3. Cross-sections of purely disorienting collisions...

It is assumed here that the molecule in its ground (initial) state a either crosses the light beam without collisions, or passes, as the result of collisions with rate 7, into another state with different rovibronic numbers. Elastic collisions, including disorienting collisions, are neglected in this case if they do not remove molecules from state a. The corresponding fluorescence intensity has to be calculated from (3.39), where pa is replaced by p according to (3.40). 

The interaction between a dipole and an induced dipole is independent of the disorienting effect of thermal motion, whereas the dipole-dipole interaction between two permanent dipoles does vary with the relative orientation of the two dipoles. This is because the induced dipole follows the direction of the permanent dipole even as the molecules change their orientations as a consequence of molecular collisions. 

On the basis of the use of the orientation of atomic and nuclear spins, Kastler formulated a generalization of the Franck-Condon principle In a rapid process involving the electronic configuration (a spectral transition, disorienting collision, or exchange collision), the position and orientation of the atomic nuclei remain unchanged (Kastler 1966). 

Compared to the newer modulation techniques, polarization spectroscopy has the advantage of greater simplicity. And its reliance on atomic orientation or alignement can provide valuable information on angular momenta and disorienting collisions. However, the technique is limited to spectral regions where good polarizers are available, and considerable care is necessary to reach shot-noise limited sensitivity. 

Since the observed relaxation times in uncoated cells are of this order of magnitude, we can conclude that at every collision with the walls of the cell the spin of the alkali atom is completely disoriented. In fact the atom is usually absorbed on to the surface and a finite time elapses before it is re-evaporated into the cell. During this time the atom is subjected to the large local magnetic and electric fields produced by the ions and molecules forming the cell walls, and consequently the orientation of its magnetic moment with respect to the external magnetic field is completely destroyed. 

---