Collisional orientation

In studies where cholesteric phase order appears to influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dlmethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered glasses, the cis-syn dimer is formed almost exclusively in liquid-crystalline media. Furthermore, selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO). Since all of the... 

The total cross section (Ttot( o) will be sensitive to the intermolecular potential, which operates at the different collisional orientations. From the order of magnitude of experimentally determined dissociation rate constants, one can conclude that fftot( o) is of the order of the gas kinetic cross section. However, pronounced differences in collision efficiencies, e.g. of water or some atoms as collision partners, may be ascribed to long range forces which increase intermolecular forces obtained from vibrational relaxation studies can probably also be used for dissociation. However, the influence of these forces on vibrational relaxation times and on dissociation rates is completely different, owing to the difference between complex collisions on the one hand and simple transitions between levels separated by large energy intervals on the other. 

Then the ith collision occurs and the orientation changes by g,. The form of the corresponding collisional operator T(g,) is to be determined below. Finally... 

A rotation of the H2 molecule through 180° creates an identical electric field. In other words, for every full rotation of a H2 molecule, the dipole induced in the collisional partner X oscillates twice through the full cycle. Quadrupole induced lines occur, therefore, at twice the (classical) rotation frequencies, or with selection rules J — J + 2, like rotational Raman lines of linear molecules. Orientational transitions (J — J AM 0) occur at zero frequency and make up the translational line. Besides multipole induction of the lowest-order multipole moments consistent with... 

We note that even the unmixed molecular gases absorb infrared radiation by collisional induction, in contrast to the unmixed monatomic gases which do not. This is so because pairs of like atoms possess an inversion symmetry that is inconsistent with a dipole moment pairs of like molecules, on the other hand, may be found in various orientations and rotovibrational excitations that are consistent with a dipole moment of the complex. We will, therefore, consider in this Chapter mostly the unmixed molecular gases (without neglecting mixtures unduly). 

One of the more interesting aspects of the collision process is the calculated dependence on the orientation of v relative to E. In particular, do the lineshapes for collisions with v E and v 1E differ as dramatically as shown in Fig. 14.6 To answer this question unambiguously required two improvements upon the initial measurements. First, the Na atoms must be in a well defined beam, otherwise v is not well defined. This requirement is easily met by enclosing the interaction region with a liquid N2 cooled box, which ensures that the only Na atoms in the interaction region are those in the beam. Second, the field homogeneity must be adequate, 1 part in 104, to resolve the intrinsic lineshape of the collisional resonances. A pair of Cu field plates 1.592(2) cm apart with 1 mm diameter holes in the top plate to allow the ions to be extracted is adequate to meet this requirement. 

So far we have discussed various studies on the relaxation of the population bPo of excited state rovibrational levels in elastic and inelastic collisions. To this end the intensity of fluorescence was measured in one or the other way. If an analysis of the state of polarization of the radiation is performed, one may obtain information on the behavior of alignment and orientation of the molecular angular momenta in elastic and inelastic collisions. If we register, under collisional conditions, the polarization properties of a directly laser-induced rovibrational level of the molecule, then, according to (2.35) and (2.39), it is possible to determine the rates... 

The simplest and most straightforward idea for producing collisionally polarized molecules in a thermal cell consists of using collisions with the participation of particles which are polarized in the laboratory frame. It seems that the earliest one was the method based on collisions of atoms which have been optically pumped (optically oriented in their ground state) by the Kastler method see Section 1.1, Fig. 1.1. If the gas constitutes a mixture of a molecular and an atomic component, the conditions being specially created in such a way as to produce such optical orientation of the atoms, we must expect, from considerations of spin conservation in molecular reactions, that polarization of the molecular component must also emerge. 

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... 

Manabe, T., Yabuzaki, T. and Ogawa, T. (1981). Observation of collisional transfer from alignment to orientation of atoms excited by a single-mode laser, Phys. Rev. Lett., 46, 637-640. 

Okunevich, A.I. (1981). Excited-state collisional relaxation by optical orientation of atoms with arbitrary electronic angular momentum,... 

Free atoms are spherically symmetrical, which implies conservation of their angular momenta. Quantum-mechanically this means that both Lz and L2 are constants of the motion when V = V(r). The special direction, denoted Z, only becomes meaningful in an orienting field. During a chemical reaction such as the formation of a homonuclear diatomic molecule, which occurs on collisional activation, a local held is induced along the axis of approach. Polarization also happens in reactions between radicals, in which case it is directed along the principal symmetry axes of the activated reactants. When two radicals interact they do so by anti-parallel line-up of their symmetry axes, which ensures that any residual angular momentum is optimally quenched. The proposed sequence of events is conveniently demonstrated by consideration of the interactions between simple hydrocarbon molecules. 

Since the value of A depends on the value chosen for the cross-section, and the values are base on taking the values for non-reactive collision cross section. Hence, orientation requirement can be taken into account by replacing the collision cross section a2 in equation (2.50) by the reactive cross section o 2. A more conventional procedure takes the view that the cross section for reaction can be expressed in terms of the collisional cross section and stearic factor P such that a 2. 

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