Excitation transitions

Here we mention as an example that in the coordination-chemistry field optical MMCT transitions between weakly coupled species are usually evaluated using the Hush theory [10,11]. The energy of the MMCT transition is given by = AE + x- Here AE is the difference between the potentials of both redox couples involved in the CT process. The reorganizational energy x is the sum of inner-sphere and outer-sphere contributions. The former depends on structural changes after the MMCT excitation transition, the latter depends on solvent polarity and the distance between the redox centres. However, similar approaches are also known in the solid state field since long [12]. 

If the semiconductor is doped, the excitation transition may be a MMCT transition. An example is Cr in Ti02 or SrTiOj where irradiation promotes an electron from the Cr ion to the conduction band which is essentially titanium 3d. This type of transition was discussed in the first part of this section. 

For transitiog+metal complexes an intense eel as it was observed for Ru(bipy) seems to be rather an exception. It is certainly difficult to draw definite mechanistic conclusions based on small eel efficiencies because eel may originate from side reactions in these cases. However, our results do show that electron transfer reactions with large driving forces can generate electronically excited transition metal complexes as a rather general phenomenon. 

Northrup, F. J., and Sears, T. J. (1992), Stimulated Emission Pumping Applications to Highly Vibrationally Excited Transition Molecules, Ann. Rev. Phys. Chem. 43, 127. 

Electron configuration of Bp" is (6s) (6p) yielding a Pip ground state and a crystal field split Pap excited state (Hamstra et al. 1994). Because the emission is a 6p inter-configurational transition Pap- Pip. which is confirmed by the yellow excitation band presence, it is formally parity forbidden. Since the uneven crystal-field terms mix with the (65) (75) Si/2 and the Pap and Pip states, the parity selection rule becomes partly lifted. The excitation transition -Pl/2- S 1/2 is the allowed one and it demands photons with higher energy. 

Fig. 5.49. Configurational coordinate diagram with emission and excitation transitions in Bi ...

The importance of the JT effect in the spectroscopy of s2 ions can be easily observed from the splitting of the 1S0->3f,1, 1P1 absorption (excitation) transitions [6]. Figure 7 gives as an example the spectra of Sb3 + (5s2) in Cs2NaScCl6 at 4.2 and 300 K. The SbClg" octahedron is cubic. The 1S0-3P1 transition at about 30000 cm -1 splits into two components, the 1S0-1/>1 transition at about 40000 cm-1 into three components [24]. This, together with their temperature dependence, is proof of the dynamic JT effect in the 3P state. 

There are two different temperature regimes of diffusive behavior they are analogous to those described by Holstein [1959] for polaron motion. At the lowest temperatures, coherent motion takes place in which the lattice oscillations are not excited transitions in which the phonon occupation numbers are not changed are dominant. The Frank-Condon factor is described by (2.51), and for the resonant case one has in the Debye model ... 

Applying this new exciting transition metal dtc-based catenane high yielding synthetic procedure to the construction of novel redox-controlled molecular machines and switches is the subject of ongoing research within the group. 

A molecule can only absorb infrared radiation if the vibration changes the dipole moment. Homonuclear diatomic molecules (such as N2) have no dipole moment no matter how much the atoms are separated, so they have no infrared spectra, just as they had no microwave spectra. They still have rotational and vibrational energy levels it is just that absorption of one infrared or microwave photon will not excite transitions between those levels. Heteronuclear diatomics (such as CO or HC1) absorb infrared radiation. All polyatomic molecules (three or more atoms) also absorb infrared radiation, because there are always some vibrations which create a dipole moment. For example, the bending modes of carbon dioxide make the molecule nonlinear and create a dipole moment, hence CO2 can absorb infrared radiation. 

In this review article, we discuss the fundamental basis of the bimolecular electron-transfer reactions of electronically excited transition metal complexes and then collect and examine the data so far obtained in this field. Although a wide range of systems are discussed, we focus primarily on quantitative studies, the majority of which involves Werner-type complexes in fluid solution. 

Figure 5-7. Dispersed emission spectra of aniline(N2)i clusters following excitation to several vibrational states of St. Relative energy is the shift, in wavenumbers, from the excited transition. The top spectrum (TJ excitation) shows an inset trace for an expanded scale about the 0° intense feature 10b, 0 J, and JJ emission can be observed. Note that the relaxed cluster emission from 0 J (following IVR) is broad as expected (compare with 6aJ + 55 cm -1 and 6aJ excitation).

For molecules, the energy density required to saturate the excited transition can be as much as three orders of magnitude higher than for atoms. This may be seen from Equation 23 in terms of a saturation spectral energy density with a result similar to but more complicated than that of Equation 20. 

The method described here for detecting NO in flames is based on the use of a frequency-doubled tunable dye laser to excite transitions in the (0,0) Y-band of NO in the range of 2250 to 2270 A. Fluorescence is observed at wavelengths associated with the bands involving the (0,0), (0,1), (0,2), and higher ground-state vibrational transitions of the Y-band system. 

With reference to absorption spectroscopy, we deal here with photon absorption by electrons distributed within specific orbitals in a population of molecules. Upon absorption, one electron reaches an upper vacant orbital of higher energy. Thus, light absorption would induce the molecule excitation. Transition from ground to excited state is accompanied by a redistribution of an electronic cloud within the molecular orbitals. This condition is implicit for transitions to occur. According to the Franck-Condon principle, electronic transitions are so fast that they occur without any change in nuclei position, that is, nuclei have no time to move during electronic transition. For this reason, electronic transitions are always drawn as vertical lines. 

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