Electronic to vibrational energy

The increased cross sections for these three states are attributed to resonant electronic to vibrational energy transfer. Table 11.1 identifies the three atomic transitions and the resonant molecular transitions in CH4 and CD4. For example the rapid depopulation of the Na 7s state by CD4 is attributed to the Na 7s — 5d transition. To verify this assignment the cross section for the 7s — 5d transfer was measured for both CH4and CD4 by observing the 5d-3p fluorescence as well as the 7s-3p fluorescence. The 7s — 5d cross sections are 215 A2 for CD4 and 15 A2 for CH4. As shown by Fig. 11.16, the 7s CD4 cross sections is —240 A2 above the smooth dotted curve in good agreement with the 7s — 5d cross section. Similar confirmations were carried out for the other two resonant collisional transfers. 

The CO laser resonance absorption technique is a useful tool for studying the dynamics of chemical reactions that involve the initial production of vibrationally excited CO molecules. We have recently applied this technique to study various atomic and free radical reactions related to combustion and electronic-to-vibrational energy transfer processes U—6). In this brief account, we discuss mainly the dynamics of 0(3P) + 1-alkynes and associated free radical reactions. 

Figure 4.14 The variation with temperature of the cross sections for 6 /2 +-62P3/2 mixing in cesium. A, Cs-CH4 0. Cs-CH3D , Cs-CH2D2 V, Cs-CHD3 0, Cs-CD4 , Cs-CF4. The points are experimental and the solid curves represent theoretical calculations [170]. The theory does not apply to the case of CF4 where only resonant electronic-to-vibrational energy transfer is likely. No specific functional relation is implied by the dashed curve.

Fto. 3. Cascade of electronic to vibrational energy. (For simplicity, the electrons in the radical cations are drawn in paired orbitals—see also Fig. 5.)... 

The efficiency of electronic-to-vibrational energy transfer in (10) is in some doubt, the only measurement reported to date suggesting that ca. 40% of the available 45 kcal/mole is converted into vibration (Slanger and Black, 1974). However, in the related deactivation process. 

Merkel PB, Kearns DR. Radiationless decay of singlet molecular oxygen in solution. An experimental and theoretical study of electronic-to-vibrational energy transfer. J Am Chem Soc 1972 94 7244-7253. 

Fig. 6. Nascent vibrational population distribution resulting from the electronic to vibrational energy transfer process I + I2(.X )...

C-H bonds in the solvent, as anticipated on the basis of Eqs. (20) and (21), was far better than with optical density parameters (Eq. (18)), and that such a situation was in accord with an electronic to vibrational energy transfer process of the exchange type. Thus, coupling between the highest frequency mode of the solvent and a vibronic transition of 02(1Ag) can be expressed... 

The conclusion that most amine quenching occurs at the pre-equilibrium limit and therefore rate constants will depend, in part, on the equilibrium constant for exciplex formation, which in turn depends on the amine electron-donating ability, raises the possibility of an alternative mechanism. Here formation of the exciplex would simply facilitate the electronic to vibrational energy transfer discussed for solvent quenching where kA (cf. Eq. (21)) replaces fcisc in Eq. (34). This would be much more effective than solvent quenching where interaction simply involves encounter complexation. That such a mechanism does not operate is demonstrated in Figure 6 which shows a plot [82] of the first-order constant for decay of 02( Ag) luminescence in benzene as a function of DABCO and DABCO-2HI2 [83], There is clearly no isotope effect and the mechanism of Eq. (31) appears very firmly established. A similar conclusion has been drawn from recent work [84] which shows that, as expected, hydrazines also quench 02(1Ag) via the same mechanism. The hydrazine 4 is a particularly efficient quencher with kq values in benzene and acetonitrile about twice those of strychnine. 

A detailed theory of the quenching of singlet oxygen by solvents in terms of transfer of electronic-to-vibrational energy has been given by Kearns (9 17) It has been determined that the diffusion path of singlet oxygen ) ia thin... 

Electronic to Vibrational Energy Transfer from Excited Halogen Atoms... 

Three areas of our studies covering photodissociation (by the absorption of single and multiple photons), electronic-to-vibrational energy transfer and combustion related reactions are discussed. Because of space limitation, we present primarily results obtained in this Laboratory and related work. 

The efficient quenching of the atomic and molecular fluorescence by collisions has been observed in early studies of the luminescence of gaseous compounds (for a review of early work see Ref. 2). In a large number of cases these processes have been explained by the electronic-to-vibrational energy transfer, charge transfer or excited-complex (excimer or exciplex) formation. There remains, however, an important class of collisional processes corresponding to the essentially intramolecular relaxation induced (or assisted) by collisions with chemically inert partners. In such... 

ELECTRONIC TO VIBRATIONAL ENERGY TRANSFER FROM EXCITED HALOGEN ATOMS... 

Grimley and Houston have indirectly monitored electronic to vibrational energy transfer between Br and three isotopic forms of hydrogen by observing subsequent vibrational to vibrational energy transfer between hydrogen and CO. The energy flow pathways for the Hj and HD systems are shown in Fig. 8. From the decay of Br fluorescence at 2.71 /am, they found the total deactivation rates of Br by H2, HD, and D2 to be 8.7X 10 , 2.1 X 10, and 2.2X 10 s torr respectively. These rate constants are substantially lower than those of a previous determination. ... 

Analysis of the relative fluorescence amplitudes by Grimley and Houston revealed that the quenching of I by HjO and HDO is, in fact, due entirely to an electronic to vibrational energy-transfer mechanism that produces two quanta of stretching vibration in the water molecule. A lower excitation limit of 1 stretching quanta for DjO was estimated. 

Electronic to vibrational energy transfer from I has been directly observed to populate the vibrational modes of NH3, C2H2, and HCN. Although complete studies of these systems have not been performed, it... 

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