Deexcitation probability

In this chapter, we shall describe the basic theories of molecular energy transfer in nonreactive collisions, up to their present state of development. We shall then discuss the various experimental techniques of measuring collisional excitation or deexcitation probabilities. Finally, we will list some experimental results in both diatomic and polyatomic systems. 

The Co nucleus decays with a half-life of 5.27 years by /5 emission to the levels in Ni. These levels then deexcite to the ground state of Ni by the emission of one or more y-rays. The spins and parities of these levels are known from a variety of measurements and require that the two strong y-rays of 1173 and 1332 keV both have E2 character, although the 1173 y could contain some admixture of M3. However, from the theoretical lifetime shown ia Table 7, the E2 contribution is expected to have a much shorter half-life and therefore also to dominate ia this decay. Although the emission probabilities of the strong 1173- and 1332-keV y-rays are so nearly equal that the difference cannot be determined by a direct measurement, from measurements of other parameters of the decay it can be determined that the 1332 is the stronger. Specifically, measurements of the continuous electron spectmm from the j3 -decay have shown that there is a branch of 0.12% to the 1332-keV level. When this, the weak y-rays, the internal conversion, and the internal-pair formation are all taken iato account, the relative emission probabilities of the two strong y-rays can be determined very accurately, as shown ia Table 8. 

The rate of transfer for a homogeneous system of donors and acceptors has been shown to be linear with acceptor concentration in dilute systems.(43,44) This can be understood simply by presuming that the donor has a sphere of influence, the radius of which is equal to the Forster range R0.If an acceptor molecule lies inside this sphere, the excitation is transferred otherwise the donor deexcites by fluorescence. The probability that an acceptor will lie within the sphere of influence of an excited donor is directly proportional to the acceptor concentration, and so the transfer is linear with acceptor concentration in dilute systems. 

The probability per unit time for a single ion embedded in a crystal excited to state K to deexcite by spontaneous emission of electric-dipole radiation to a lower state M has been given by Axe (28) as... 

Vibrational and Rotational Excitation in Gaseous Collisions probability of excitation or deexcitation is determined from... 

The case 4+1 — 80, representing, for example, He + HBr, was shown by Kelley to illustrate the difficulty with which vibrational energy is transferred when the Br end is struck (m = 0.0008) as opposed to the values obtained when the H atom is struck (m = 3.8). It was also demonstrated that, considering all values of 0O for this case, the predominant deexcitation mechanism is intramolecular V-R transfer. Since this V-R process depends upon particle masses and collision energy in a manner different from the V-T process, a colinear collision model will not always lead to a proper description of vibrational deexcitation. This conclusion probably applies as well to more complicated systems, such as noble gas collisions with CH4. 

A close-packed monolayer of long, planar molecules must necessarily contain at least "islands" of local order in which molecules are packed plane-to-plane. One notices that crystals of these dyes tend to pack in sheets, within which the molecules have their long axes parallel to one another and parallel to the plane of the sheet. The molecular short axes are nearly perpendicular to the plane of the sheet, and the intermolecular contacts within the sheet tend to occur at the graphitic distance, except for molecules which are bent or twisted out of planarity. Twisted molecules ("overcrowded" in Brooker s classification) generally make poor sensitizers, probably because they have a fast route of deexcitation directly from the excited singlet state to the ground state opened by the existence of the twisted, stressed structure. 

Consider a closed system characterized by a constant temperature T. The system is prepared in such a way that molecules in energy levels are distributed in departure from their equilibrium distribution. Transitions of molecules among energy levels take place by collisional excitation or deexcitation. The redistribution of molecular population is described by the rate equation or the Pauli master equation. The values for the microscopic transition probability kfj for transition from ith level toyth level are, in principle, calculable from quantum theory of collisions. Let the set of numbers vr be vibrational quantum numbers of the reactant molecule and vp be those of the product molecule. The collisional transitions or intermolecular relaxation processes will be described by ... 

When an electron in an atom or molecule is excited into an unoccupied bound state, it leaves an electron vacancy, or hole, behind. The electron-hole pair thus created exhibits a Coulomb attraction that is modified by the screening of all the other electrons in the system. The same phenomenon occurs when an electron is excited—by light (or electron beam) of appropriate energy—into a bound state above the Fermi level in an semiconductor. The electron-hole pair created in this circumstance is called an exciton, and its attractive Coulomb interaction is screened by the static dielectric constant of the solid. There is a finite probability that the exciton may migrate from atom to atom (or molecule to molecule) through the solid before deexcitation, the destruction of the electron-hole pair by recombination, occurs. Exciton... 

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