Internal conversion process

AIterna.tives to y-Ray Emission. y-Ray emission results ia the deexcitation of an excited nuclear state to a lower state ia the same nucHde, ie, no change ia Z or. There are two other processes by which this transition can take place without the emission of a y-ray of this energy. These are internal conversion and internal pair formation. The internal-conversion process iavolves the transfer of the energy to an atomic electron. 

Internal Conversion—Process in which a gamma ray knocks an electron out of the same atom from which the gamma ray was emitted. The ratio of the number of internal conversion electrons to the number of gamma quanta emitted in the de-excitation of the nucleus is called the "conversion ratio."... 

Internal-conversion electrons, 21 309 Internal-conversion process, 21 300, 306-309... 

Model II C B X Internal-Conversion Process in the Benzene Cation... 

Parameters of Model II, Which Represents a Three-State Eive-Mode Model of the Ultrafast C — B — X Internal-Conversion Process in the Benzene Cation [179, 180] ... 

Figure 1. Quantum-mechanical (thick lines) and mean-field-trajectory (thin lines) calculations obtained for Model 1 describing the S2 — Si internal-conversion process in pyrazine. Shown are the time-dependent population probabilities Pf t) and Pf (t) of the initially prepared adiabatic and diabatic electronic state, respectively, as well as the mean momenta pi (t) and P2 t) of the two totally symmetric modes Vi and V( of the model.

Let us next turn to Model II, representing the C —> B —> X internal-conversion process in the benzene cation. Figure 2 demonstrates that this (compared to the electronic two-state model, Model I) more complicated process is difficult to describe with a MFT ansatz. Although the method is seen to catch the initial fast C —> B decay quite accurately and can also qualitatively reproduce the oscillations of the diabatic populations of the C- and B-state, it essentially fails to reproduce the subsequent internal conversion to the electronic X-state. Jn particular, the MFT method predicts a too-slow population transfer from the C- and B-state to the electronic ground state. 

Figure 46. Diabatic population (a) and modulus of the autocorrelation function (b) of the initially prepared state for a four-mode model describing the S2 Si internal-conversion process in pyrazine. The full line is the quantum result, the dashed-dotted line is the result of the semiclassical spin-coherent state propagator, and the dashed line depicts the result of Suzuki s propagator. The semiclassical data have been normalized. Panel (c) shows the norm of the semiclassical wave functions.

Photodyncimics of metalloporphyrins have been extensively investigated on account of its importance in the understanding of photosynthesis and other processes of biological importance ( ). Particular atten-sion has been paid to the reason why the excited metalloporphyrins possess unique characteristics from the viewpoint of redox (2-4), energy transfer ( ), and other photodynamical processes (6,7). In comparison with the considerable knowledge accumulated on the photochemical properties of the lowest excited states, little has been known on the S2 - Sq fluorescence and Si Sq internal conversion processes which can also be regarded as unusual characters of metalloporphyrins. 

The ko. A, and Fa parameters obtained for a few alkanes are collected in Table 3. kg is around 10 sec A 10 to 10 sec and Fa 10 to 20 kJ mol h In principle, the decay of excited states may involve Si- Sx-type internal conversion transitions [IC, where Sx is some singlet state that gives the product(s) of chemical decomposition] and Si T -type intersystem crossing processes (ISC). The temperature-independent decay was attributed, on the basis of the size of the rate parameter (ko 10 sec ), to Si T -type intersystem crossing. At the same time the temperature-activated decay with a frequency factor of 10 to 10 sec was attributed to an internal conversion process that takes place by overcoming a barrier of Fa 10-20 kJ mol and leads finally to some... 

If there exists an effective pathway for the internal conversion process... 

In a low-concentration sample, with both levels stable, they applied bursts of excitation to the 5D3 level while the emissions from the 5Z>4 were monitored. As expected, a rise and decay of fluorescence was observed. This again indicates that a slow internal-conversion process is occurring from the 5Z)3 to the 5D4 states. The rise time of the fluorescence turned out to be 189 /xsec whereas the decay time was 566 /xsec. 

The rates of internal conversion from the 5Z)3 to the 5D4 states were also measured. The backup oxide in this case was yttrium. This information was obtained by determining the rise time of the 5Z)4-state green fluorescence as a function of time, when the 5Z>3 state was excited. The rise time of the 5Z)4 state is, of course, the decay time of the 5Z>3 state. It was assumed that the decay of the 5Z)3 was predominantly due to an efficient internal conversion process to the 5D4. Measurements of the decay time of the 5Z)3 state directly were not possible, since the emission from this state is very weak if not, indeed, absent. The result of this study is shown in Fig. 23, where it can be seen that the internal-conversion time remains constant at about 17 fxsec up to a terbium oxide concentration of 1 mole per cent. At higher concentrations, the internal conversion time falls rapidly, until at 10 mole per cent terbium oxide the value is about 1.7 /xsec. This is down by a factor of 10 over samples containing 1 mole per cent or less of terbium oxide. 

The chemistry of the excited states of molecules induced by light absorption in the visible and ultraviolet range is the normal realm of photochemistry. Because of the great rapidity of internal conversion processes in which highly excited electronic states are converted to lower electronic states with the energy difference distributed among the various vibrational modes as dictated by the Franck-Condon principle, the photo-... 

The highly excited states of molecules produced by high-energy radiation that arc chemically important are mainly the ionic states because of the rapidity of internal conversion processes. Primary excitation is relatively unimportant while secondary excitation is quite common. In the condensed phases energy dissipation is very rapid because of colli-sional deactivation, the cage effect, and excitation energy transfer processes all of which act to negate the chemical effects of secondary excitation,... 

Vibronic mixing of levels, leading to an overall finite value of a transition probability, even when /9el is zero because of symmetry, may occur if nonsymmetric vibrational levels of the product are used in the internal conversion process. The effect is well known in the case of optical transitions and may be the best interpretation of the Woodward-Hoffman rules. 

An internal conversion process recently discovered in our laboratory may shed light on the subject. Murovla found that quadricyclene, 6, is a powerful quencher of the excited singlet states of naphthalene and other aromatic hydrocarbons. In the course of the quenching reaction, the quencher was extensively converted to bicyclo [2.2.1] hepta-2,5-diene, 7. The following mechanism was suggested. 

The rapid internal conversion process (VII-164) in comparison with fluorescence in ketene may be treated as a case of a so-called statistical limit... 

All excited molecules will not undergo conversion to the metastable colored form, so that will generally be less than unity. Competing deactivating processes for the excited molecules include fluorescence, phosphorescence, permanent chemical reaction and internal conversion processes in which the excitation energy ultimately appears as thermal energy in the system. 

The aforementioned E — D nonadiabatic interactions also lead to efficient, i.e., femtosecond internal conversion processes which we now briefly address. The presence of such processes is indicated indirectly by experimental studies, where Bz+ has been prepared initially in the E state, but fragmentation been found to occur via lower (even the X) electronic states [45,46]. To describe this non-radiative decay... 

---