Energy transfer studied with lasers

Although modem laser teelmiques ean in prineiple aehieve mueh narrower energy distributions, optieal exeitation is frequently not a viable method for the preparation of exeited reaetive speeies. Therefore ehemieal aetivation—often eombined with (laser-) flash photolysis—still plays an important role in gas-phase kmeties, in partieular of unstable speeies sueh as radieals [ ]. Chemieal aetivation also plays an important role in energy-transfer studies (see chapter A3.13). 

The problem of quenching alkali resonance radiation in E-VR energy-transfer collisions with simple molecules is important as a model case for basic processes in photochemistry and serves its own right for a variety of practical applications, such as in laser physics. It has been studied for many years in the past, but only recent progress has led to information of the final internal energy of the molecule. In particular, crossed-beam experiments with laser-excited atoms allow a detailed measurement of energy-transfer spectra. There can be no doubt that the curve-crossing... 

Measuring Doppler widths of rotational lines by laser-probe techniques gives velocity distributions in just the same way as measuring Doppler widths of atomic lines by conventional means. In this method a laser beam with a very narrow band width is tuned over the spectral line to determine the profile of the Doppler broadened line. The line shape can be interpreted to give the average velocity of the product. As yet, this method has been applied only to rotational energy transfer studies however, with the availability of mode-locked lasers providing narrow band widths, this procedure may become more widely used. 

Small-scale laboratory-sized chemical lasers provide convenient sources for studies of the distribution of vibrational energy in chemical reaction products and for measurements of vibrational and rotational energy transfer processes with the laser-induced fluorescence and laser double-resonance techniques. " ... 

Superieure de Chimie, Mulhouse, Fr. J. Photopolym. Sci. Technol. (2000), 13 (2), 237-242 (Eng.). The interactions of excited states of a coumarin or a ketocoumarin photosensitizer with a bisimi-dazole deriv., mercaptobenzoxazole photoinitiators, and titanocene were studied by laser absorption spectroscopy and by photocond. The redox potential of the compds. was measured and used in calcn. of the free enthalpy change for a possible electron transfer reaction. The coumarin forms radicals through an electron transfer reaction, while the ketocoumarin undergoes an energy transfer reaction with bisimidazole and a hydrogen abstraction reaction with a benzoxazole deriv. Thus, coumarin or ketocoumarin/free radical initiator combinations are suitable as initiators of radical polymn. reactions, esp. those applicable to laser imaging. [1707-68-2]. 

A recent study of the vibrational-to-vibrational (V-V) energy transfer between highly-excited oxygen molecules and ozone combines laser-flash photolysis and chemical activation with detection by time-resolved LIF [ ]. Partial laser-flash photolysis at 532 mn of pure ozone in the Chappuis band produces translationally-... 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

The efficiency of solid-state lasers (that is the ratio of laser output power to pump power) can often be increased by energy transfer from other excited ions with which the crystal was doped additionally 0. In this way energy transfer processes can be studied... 

In another picosecond laser study of Forster energy transfer, Sato et al. [165] have studied systems of rhodamine 6G (R6G) and/or 3,3 -diethyl-thiacarbocyanine iodide (DODCI). Satisfactory agreement between the experimentally observed decay of R6G fluorescence and that based on the Forster kinetics [eqn. (85) with a calculated R0 — 5.9 nm and r = 0] was noted. However, from eqn. (85), Uiim 10-9 m2 s-1, so that Forster... 

Figure 5.4, one can easily understand why the interfacial electron transfer should take place in the 10-100 fsec range because this ET process should be faster than the photo-luminescence of the dye molecules and energy transfer between the molecules. Recently Zimmermann et al. [58] have employed the 20 fsec laser pulses to study the ET dynamics in the DTB-Pe/TiC>2 system and for comparison, they have also studied the excited-state dynamics of free perylene in toluene solution. Limited by the 20 fsec pulse-duration, from the uncertainty principle, they can only observe the vibrational coherences (i.e., vibrational wave packets) of low-frequency modes (see Figure 5.5). Six significant modes, 275, 360, 420, 460, 500 and 625 cm-1, have been resolved from the Fourier transform spectra of ultrashort pulse measurements. The Fourier transform spectrum has also been compared with the Raman spectrum. A good agreement can be seen (Figure 5.5). For detail of the analysis of the quantum beat, refer to Figures 5.5-5.7 of Zimmermann et al. s paper [58], These modes should play an important role not only in ET dynamics or excited-state dynamics, but also in absorption spectra. Therefore, the steady state absorption spectra of DTB-Pe, both in...

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