Rate state-specific ionization

At 2000 K and 1 atm, Hollander s state-specific rate constant becomes k. = 1.46 x 1010 exp(-AE/kT) s-1, where AE is the energy required for ionization. For each n-manifold state the fraction ionized by collisions is determined, as well as the fraction transferred to nearby n-manifold states in steps of An = 1. Then the fractions ionized from these nearby n-manifold states are calculated. In this way a total overall ionization rate is evaluated for each photo-excited d state. The total ionization rate always exceeds the state-specific rate, since some of the Na atoms transferred by collisions to the nearby n-manifold states are subsequently ionized. Table I summarizes the values used for the state-specific cross sections and the derived overall ionization and quenching rate constants for each n-manifold state. The required optical transition, ionization, and quenching rates can now be incorporated in the rate equation model. Figure 2 compares the results of the model calculation with the experimental values. 

FIGURE 16.11 Specific and general acid-base catalysis of simple reactions in solution may be distinguished by determining the dependence of observed reaction rate constants (/sobs) pH and buffer concentration, (a) In specific acid-base catalysis, or OH concentration affects the reaction rate, is pH-dependent, but buffers (which accept or donate H /OH ) have no effect, (b) In general acid-base catalysis, in which an ionizable buffer may donate or accept a proton in the transition state, is dependent on buffer concentration. 

Effect of pH on the ionization of the active site The concentra tion of H+ affects reaction velocity in several ways. First, the cat alytic process usually requires that the enzyme and substrate have specific chemical groups in either an ionized or unionized state in order to interact. For example, catalytic activity may require that an amino group of the enzyme be in the protonated form (-NH3+). At alkaline pH, this group is deprotonated, and the rate of the reaction, therefore, declines. 

Since Vd(r) is only nonzero near r = 0 the matrix element of Eq. (6.51) reflects the amplitude of the wavefunction of the continuum wave at r 0. Specifically, the squared matrix element is proportional to C, the density of states defined earlier and plotted in Fig. 6.18. From the plots of Fig. 6.18 it is apparent that the ionization rate into a continuum substantially above threshold is energy independent. However, as shown in Fig. 6.18, there is often a peak in the density of continuum states just at the threshold for ionization, substantially increasing the ionization rate for a degenerate blue state of larger This phenomenon has been observed experimentally by Littman et al.32 who observed a local increase in the ionization rate of the Na (12,6,3,2) Stark state where it crosses the 14,0,11,2 state, at a field of 15.6 kV/cm, as shown by Fig. 6.19. In this field the energy of the... 

Since there have been no previous studies of spin-polarized electron induced reaction asymmetries in adsorbed chiral molecules, the exact manner by which the enhancement occurs is unclear. If the orbital occupied during DEA is sufficiently diffuse so as to sample the regions of the molecule responsible for the chiral structure [92] then enantiomeric specific dissociation will result. On the other hand, it has been theorized that two enantiomers will be ionized at different rates by longitudinally spin-polarized electrons [126]. If there are sufficient numbers of higher energy spin-polarized secondary electrons and the final state reached following ionization is dissociative, then this could lead to chiral enhancement. 

All mechanisms exhibit first-order kinetics in substrate. Only transition states with considerable carbanion character are considered in this table. "Specific base catalysis predicted if extent of substrate ionization reduced from almost complete. Effect on rate assuming no change in mechanism is caused, as steric factors upon substitution at C-a and C-P have not been considered. The rate predictions are geared to substituent effects such as these giving rise to Hammett reaction constants on P- and a-aryl substitution. Depends on whether ion pair assists in removal of leaving group. 

As discussed in Section 8.2, superexcited states, AB, can decay by both autoionization and dissociation (more specifically, by predissociation). Decay by spontaneous fluorescence can be neglected for superexcited states because, generally, the predissociation or autoionization rates (l/rnr 1012 to 1014s-1) are much faster than the fluorescence rate (l/rr < 108s-1). Only two examples of detected spontaneous fluorescence from superexcited states have been reported (for H2, Glass-Maujean, et ai, 1987, for Li2, Chu and Wu, 1988). The H2 D1 e-symmetry component is predissociated by an L-uncoupling interaction with the B 1B+ state (see Section 7.9 and Fig. 7.27). Since a 4E+ state has no /-symmetry levels, the /-components of the D1 A-doublets cannot interact with the B E+ state and are not predissociated. The v = 8 level of the D1 state, which lies just above the H/ X2E+ v+ = 0 ionization threshold, could in principle be autoionized (both e and / components) by the X2E+ v+ = 0 en continuum. However, the Av = 1 propensity rule for vibrational autoionization implies that the v = 8 level will be only weakly autoionized. Consequently, the nonradiative decay rate, 1 /rnr, is slow only for the /-symmetry component of the D1 v = 8 state. Thus, in the LIF spectrum of the D1] —... 

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