Vibronic excitation

Weak coupling (U AE, Aw U As) The interaction energy is much lower than the absorption bandwidth but larger than the width of an isolated vibronic level. The electronic excitation in this case is more localized than under strong coupling. Nevertheless, the vibronic excitation is still to be considered as delocalized so that the system can be described in terms of stationary vibronic exciton states. 

Intermediates for the CO Complexes. Absorption of a 531 nm photon in the Q-band would produce a vibronically excited singlet state... 

S . S = nth singlet energy state and vibronically excited state Tn, T = nth triplet energy state and vibronically excited state IC = internal conversion, ISC = intersystem crossing F fluorescence emission, P = phosphoresence emission. 

The rate determining step in intersystem crossing is the transfer from the thermally relaxed singlet state to the vibronically excited triplet state S/ >7 (j > k). This is followed by vibrational relaxation. The spin-orbital interaction modifies the transition rates. A prohibition factor of 10 — 10 is introduced and the values of kiSc lie between 101 and 107 s-1. The reverse transfer from the relaxed triplet to vibronically excited singlet is also possible. 

E. Photoisomerization of benzene. Photolysis of liquid benzene by excitation to its (7t, jr ) states brings about interesting photochemical transform ations from vibronically excited singlet as well as triplet states (Figure 7.8). 

The effect of vibrational excitation is examined in Fig. 4. Shown here is the difference diffraction pattern of the molecule in the Si electronic state with excitation to vibration 16a8, vs. the vibrationless level 0° of the S, electronic state. The gray-scale indicates the difference in the total diffraction signals of the two vibronically excited states, divided by the diffraction signal of the molecule in the ground electronic and vibrational state. Important to note is that this difference pattern has a strong feature at a = 0°, i.e. along the direction of the laser... 

In the nanosecond biphotonic photolysis the vibronically excited level reached by absorption of the first photon relaxes by IC to the first excited state (Figure 3), which is stronger acid than the ground state by up to six orders in magnitude [11]. Quantum chemical calculations showed that the O - H bond becomes a bit longer and the C - OH bond becomes shorter and more rigid. The lifetimes of the first excited singlet state of the sterically hindered phenols... 

IT excitation of heteronuclear mixed-valence cyanometallates leads to the formation of a vibronically excited valence-isomeric species ( see Figure 6 ) consisting of octacyanomolybdate (V) and the corresponding reduced form of the metal center M. Due to the kinetic lability of[Mo(CN) ] - (32-34) fast cyanide aquation can be expected which conpetes with the back electron... 

These decay measurements on the state excited can be repeated by a TRSEP technique (Hineman et al. 1994) to verify the IVR cluster kinetics. This has been done for the aniline(N2)1 P- vibronic excitation. The experiment involves excitation of the P- state, followed by stimulated emission with a time delayed pulse to deplete the P" population. The total emission from the excited Sj cluster and bare molecule as a function of time delay between the excitation and dump... 

A new ion source has been developed for rapid, non-contact analysis of materials at ambient pressure and at ground potential [8,9], The new source, termed direct analysis in real time (DART), is based on the atmospheric pressure interactions of long-lived electronic excited-state atoms or vibronic excited-state molecules with the sample and atmospheric gases. Figure 5 shows a schematic diagram of the DART ion source. 

Infrared spectra of vibronic excitations in metals are not observed because of the strong shielding effect of free carriers. Raman spectra of conventional two-atomic or multi-atomic metals, such as Zn (Fraas and Porto, 1970) and V3Si (Klein and Dierker, 1984), have been reported, but failed to gain importance. 

Fig. 11. Reaction coordinate diagram for ECL system involving rubrene (A) and 9,10-diphenylan-thracene (B). Potential energy curves are presented in the zero-order approximation, without removing the degeneracy at the crossing points of the potential energy curves. Broken lines represent the vibronically excited triplet state.

Equation (49) describes the rate of formation of the rubrene geminate triplet-triplet pair in a similar way. Strongly exergonic formation of the excited triplets and ground states lies in the inverted Marcus region and therefore Eq. (24) must be applied. In both cases, vibronic excitation of the reaction products can take place, leading to an increase in the electron transfer rate. The electron transfer rate constitutes a superposition of a solvent-dynamics-controlled contribution and a... 

The results obtained clearly demonstrate that the Marcus model for ECL processes may be used for qualitative as well as for quantitative descriptions of this kind of electron transfer reactions. The more sophisticated approach, taking into account the vibronic excitation in the reaction products (important in the inverted Marcus region), solvent molecular dynamics (important in the case of large values of the electronic coupling elements), as well as the changes in the electron transfer distance, should be used. The results indicate that the Marcus theory may also be used for predicting the ECL efficiency, provided that some conditions are fulfilled. Especially, during the ECL process, only the annihilation of ions should occur, without any competitive reactions. The necessary rate constants can be evaluated from pertinent electrochemical and spectroscopic data. 

Fig. 2b is an idealized illustration of a single, uncomplicated Cotton effect. In reality, the occurrence of a complete curve in the electronic spectrum is rare. Complete dispersions are more likely to be observed in the vibrational spectral range because of the increased spectral resolution. However, even there, dispersions are too often complicated by extensive band overlap. The same is true for electronic spectra where hidden absorption bands coupled vibronic excitations and interferences from bands associated with other chiral chromophores contribute to producing anomalous ORD curves that are so complex they have little utility in quantitative analytical applications (Fig. 3). 

Electron loss spectroscopy (ELS, HREELS) electrons/same electrons 0.5-2 electronic and vibronic excitation... 

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