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Overpopulated state

Now consider the kinetics of the flow of spins between spin states. For each pair of spin states, we calculate the difference in population and compare it to the equilibrium difference. If these two are not the same, there will be a flow of spins from the overpopulated state to the underpopulated state at a rate that is proportional to the rate constant for that transition and to the amount by which the transition is out of equilibrium. The rate constants, or relaxation rates, for each transition are determined by... [Pg.410]

Figure A1.8 Nuclear spin states and spectrum for product 5. At the top the four states are again shown in an energy-level diagram. Heavy lines are the states with enhanced populations. A downward-pointing arrow indicates a net transfer of molecules from an overpopulated higher spin state to a less populated lower one, and corresponds to net emission. The spectrum shows the multiplet effect of type El A. From S. H. Pine, J. Chem. Educ., 49, 664 (1972). Reproduced by permission of the Division of Chemical Education. Figure A1.8 Nuclear spin states and spectrum for product 5. At the top the four states are again shown in an energy-level diagram. Heavy lines are the states with enhanced populations. A downward-pointing arrow indicates a net transfer of molecules from an overpopulated higher spin state to a less populated lower one, and corresponds to net emission. The spectrum shows the multiplet effect of type El A. From S. H. Pine, J. Chem. Educ., 49, 664 (1972). Reproduced by permission of the Division of Chemical Education.
A predissociation, which may or may not be related to the one just discussed, is observed in hot flames147 and in cool atomic flames148. For rotationless states the predissociating curve appears to cross the bound 2E+ state very near v = 2. The corresponding inverse predissociation has been proposed149,150 as an explanation for the observed overpopulation of the first and second vibrational levels of OH(2E+) in flames where there is a considerable excess population (over thermodynamic equilibrium) of O and H atoms. This process may produce a population inversion in nozzle expansion of a dissociated gas 15 x. [Pg.148]

Fig. 22 shows the results of photometry of plates similar to that illustrated in Fig. 21. The relative intensities of suitable transitions were determined from the asymptotic limit at long time delays when the system attains equilibrium. (These resemble, but are not identical to, the relative/ values because of the usual instrumental effects which depend on line width.) The time variation of the relative concentrations is shown in Fig. 23 the upper four levels attain Boltzmann equilibrium amongst themselves after 100 /isec, to form a coupled (by collision) system overpopulated with respect to the 5DA state. The equilibration of the upper four levels causes the initial rise (Fig. 22) in the population of Fe(a5D3). Thus relaxation amongst the sub-levels is formally similar to vibrational relaxation in most polyatomic molecules, in which excitation to the first vibrational level is the rate determining step. In both cases, this result is due to the translational overlap term, for example, in the simple form of equation (14) of Section 3. Fig. 22 shows the results of photometry of plates similar to that illustrated in Fig. 21. The relative intensities of suitable transitions were determined from the asymptotic limit at long time delays when the system attains equilibrium. (These resemble, but are not identical to, the relative/ values because of the usual instrumental effects which depend on line width.) The time variation of the relative concentrations is shown in Fig. 23 the upper four levels attain Boltzmann equilibrium amongst themselves after 100 /isec, to form a coupled (by collision) system overpopulated with respect to the 5DA state. The equilibration of the upper four levels causes the initial rise (Fig. 22) in the population of Fe(a5D3). Thus relaxation amongst the sub-levels is formally similar to vibrational relaxation in most polyatomic molecules, in which excitation to the first vibrational level is the rate determining step. In both cases, this result is due to the translational overlap term, for example, in the simple form of equation (14) of Section 3.
According to the radical pair theory [54], the CIDNP effect results from the competition between a geminate spin dependent radical (ion) pair reaction and the separation of the radicals (radical ions) by diffusion [55]. The free radicals (radical ions) may react with diamagnetic species (e.g. solvent) and accordingly, the escape products have certain nuclear spin states. overpopulated. An enhanced absorption or an emission line in the NMR spectrum is observed. [Pg.233]

For OH, experimental evidence indicates the intermediate case as illustrated in Figure 4, which shows the deviation of each rotational sublevel from its normalized Boltzmann value. Not surprisingly, the laser coupled state is overpopulated compared to other states. [Pg.69]

Rotational Population Distributions. As expected, there is an overpopulation in the A state level and an underpopulation in the X state level connected by the laser, compared to a thermal distribution (see Fig. 1). Higher rotational levels (N 6) in the A-state are described by a Boltzmann distribution with T 940°K, well less than the gas temperature and reflecting the energy dependence of the or. The high-N levels of the X-state are described by a Boltzmann distribution with very high T (3200°K) but this may be an artifact of the model, due principally to the assumed... [Pg.139]

Consider first the single-quantum transition between the afi and pp states (Ha(2) transition, Fig. 10.1). This is an Ha transition with relaxation rate W. The equilibrium difference in population for this transition is Pap — Ppp = 28. If this equality does not hold, then the overpopulation of the pp state is given by Ppp — Pap + 28, and the rate of spins dropping down from the pp state to the ap state is Wf(Ppp — Pap + 28). If this were the only transition available (i.e., if there were no double-quantum or zero-quantum pathways), we could write down the rate of change of population as... [Pg.411]

Earlier experiments showed a vibrational overpopulation of the first excited vibrational state in desorption that was higher than the vibrational ground-state population by a factor of nine [55]. This result was later questioned on the basis of the quantum calculations which only found an overpopulation by a factor of 2.5 [54]. When the experiments were repeated, the theoretical predictions were confirmed [50], as Fig. 5 demonstrates. This indicates that in the field of surface science theory has reached a level of reliability that makes predictions possible and allows a fruitful and close collaboration with experiment. [Pg.10]

In the natural state, populations of wildlife tend to increase and decrease in cycles. Humans, however, have changed so much of the natural environment for our own use that the natural cycles for wildlife are no longer a constant. Thus, animals such as deer often die slowly in agony from hunger, cold, or disease frequently caused by overpopulation in a restricted environment. Hunters harvest animals under the control of wildlife scientists. Tranquil-... [Pg.63]

At the end of their article, Golde and Thrush comment that population inversions must occur in some afterglows, but that it is difficult to obtain a sufficient (absolute) overpopulation of the excited state to sustain laser action. The so-called bound-free molecular systems, which are characterized by a repulsive ground state and a bound excited state, have attracted a great deal of attention as potential lasers [403-410]. Examples of such systems are (noble gas + noble gas) or (noble gas + alkali metal atom) ... [Pg.88]

Programmed cell death allows the removal of overpopulated cells, pathogen-infected cells, malformed cells, or cells that have sustained genetic damage (1). Several disease states are hnked either to a lack of apoptotic response or to unwanted cell death... [Pg.1580]

Not only is the electron-spin state conserved in all these reactions but also the nuclear spin state. Hence, the nuclear spin polarizations present in the pairs are transferred into the diamagnetic products, where they can be detected. Those nuclear spin configurations that slowed down intersystem crossing are over-populated in the product(s) of the exit channel on the same side as the entry channel of the scheme those that accelerated it are overpopulated in the product(s) of the exit channel on the other side. [Pg.87]

Figure 8 (a) Diagram of the energy levels in the AX spin system of an hydrogenated product showing the overpopulation of the aP and Pa states due to para-Hj and (b) schematic representation of the resulting H NMR spectrum. [Pg.369]

The exponentially parabolic Treanor distribution function, which provides a significant overpopulation of the highly vibrationally excited states, was illustrated in Fig. 3-3. To analyze the quite complicated W flux (3-122), it can be divided into linear and non-linear components ... [Pg.116]


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