Vibrational-electronic energy transfer

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

On the other hand, the similarity between metals is a characteristic of the electron-mediated vibrational energy transfer models. Here one must only calculate the population of excited electron-hole pairs at the energy of vibrational excitation, a quantity that is quite similar for many metals.b... 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

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. 

Electronic-vibration and electronic-translation energy transfer with AE 1 eV... 

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. 

A state-of-the-art description of broadband ultrafast infrared pulse generation and multichannel CCD and IR focal plane detection methods has been given in this chapter. A few poignant examples of how these techniques can be used to extract molecular vibrational energy transfer rates, photochemical reaction and electron transfer mechanisms, and to control vibrational excitation in complex systems were also described. The author hopes that more advanced measurements of chemical, material, and biochemical systems will be made with higher time and spectral resolution using multichannel infrared detectors as they become available to the scientific research community. 

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.

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. 

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