Oscillator strength of electronic transition

It is very important for semi-empirical calculations to have experimental data measured accurately and identified correctly. Otherwise, it is impossible to achieve good correspondence between calculated and measured quantities. To overcome these dangers one has to analyse the structure of the eigenfunctions, energy levels and oscillator strengths of electronic transitions along the isoelectronic sequences. As a rule, all these quantities vary fairly smoothly and, therefore, the occurrence of any inconsistency in their behaviour usually indicates that there are uncertainities or errors in the data utilized. 

Certain semi-empirical methods are aimed at choosing the effective potential, allowing one to determine as accurately as possible values of oscillator strengths of electronic transitions others, more complex, are also directed at achieving the best correspondence between calculated energy spectra and experimental ones. 

Oscillator strengths,/, of electronic transitions in the dipole length and velocity formulations are ... 

The CIS approximation is used in a majority of semiempirical SCF methods to calculate energies and oscillator strengths of electronic transitions (CNDO/S, INDO/S, CINDO-E/S, NDDO/MC, NDDO-G). 

Transition energy (ev). Oscillator strength. Note that INDO/S usually overestimates oscillator strengths of electronic transitions by a factor of 2-3." Estimated using = 24,000 mol L cm and Ai/2 = 6,500 cm ... 

Figure 3 Experimental (solid line) and calculated (INDO/S/ dash-dot line TDDFT, dash line) spectrum of Ni(C0)4. The calculated spectra were simulated using Gaussian bands with Ai/2 = 6,500 The oscillator strengths of electronic transitions from INDO/S calculations were reduced by a factor of 2.

On the other hand, the luminescence quantum efficiency can decrease. Apart from a decrease of the oscillator strengths themselves, many other mechanisms can cause such reduced quantum efficiencies, as, for example, enhanced electron-phonon coupling, generation of new paths for deexcitation or energy transfer processes. In fact, these processes will not affect the oscillator strengths of the transition but simply influence the occupation of the excited level. Therefore, the oscillator strength of a given transition may still increase under pressure, however, this increase is completely covered up by a fast depletion of the excited level. 

More quantitatively, the value (0.66) that we obtain for the intensity ratio (0— 1 )/(0—0) of the b lines disagrees with the measured value (0.52). [The calculations by Philpott et al. Provide, on the contrary, too weak a value (0.37)]. In addition to the lack of accuracy inherent in methods of measuring oscillator strengths of strong transitions, it appears probable that inclusion of upper electronic states in (2.87) would, by the induction of a complementary collective coupling, improve the accord with the measured values. 

Theories about the origin and nature of the universe have as test data the composition of stars and even of our own sun. The abundance of, say, boron in the atmosphere of the sun can be estimated from measurements of the BH electronic spectrum in the sun s emission coupled with known values of the oscillator strength of the transition. If the spectrum cannot be produced in the laboratory then only calculated values of the transition probability can be used. 

The Thomas Reiche Kuhn f-sum rule shows that the summation of the oscillator strengths of all transitions in a chromophore is equal to the number of electrons. In this way, we could envisage an electron count in the same way NMR and electron paramagnetic resonance (EPR)... 

Table 5 Oscillator strengths of electronic dipole transitions between the five lowest electronic states of the fluorinated benzene cations. An empty entry (—) means that the quantity vanishes by symmetry...

As shown in Table 8, it is not possible to account for the transitions involving the tc electrons of the simplest organic molecules by non-empirical calculations based on an independent particle model. Both the singlet-triplet separation and the oscillator strength of the transition are overestimated. 

It would be tempting to venture that both (2) and (3) are contributing to be observed fine structure while this is possible, there is no evidence for this in the high resolution spectrum of Pearson and Innes [75] the observed C-C stretching and CH3 deformation quanta can well belong to the same electronic transition. This latter point needs further clarification. On the other hand the total oscillator strength of these transitions is about 0.28 in very satisfactory agreement with the observed... 

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