Laser Energy Transfer

There is a complex interplay of kinetic processes in these energy transfer lasers. The conditions under which they operate are extreme in terms of the concentrations of transient species and the local gas temperatures. Many of the key reactions are difficult to study in isolation as they involve interactions between pairs of radicals or other transient species (e.g. collisions between electronically and vibrationally excited molecules). The impetus to obtain mechanistic and kinetic data for chemical laser systems (potential as well as demonstrated) has stimulated the development of new experimental techniques and produced a substantial body of kinetic data. Beyond the practical value of this work, the kinetic data that have been obtained are of fundamental scientific interest. They provide insights regarding the underlying principles that govern energy transfer processes and rare examples of reactions involving electronically and/or vibrationally excited species. 

Kobayashi Y., Kurokawa Y., final Y., Muto S. A transparent alumina film doped with laser dye and its emission properties. J. Non-Cryst. Solids 1988 105 198-200 Kobayashi Y., Sasaki H., Muto S., Yamazaki S., Kurokawa Y. Preparation of a transparent alumina film doped with fluorescence dye and its energy transfer laser emission. Thin Solids Films 1991 ... 

Most infrared molecular lasers, such as the CO2 laser, the HCl laser, and many other chemical lasers, are based on colli sional energy transfer between vibrational-rotational levels of the laser molecule and other atoms or molecules. Such lasers may therefore be called energy-transfer lasers [12.17]. Also in the visible region many molecular cw laser systems, where the laser transition terminates on rotation-vibration levels of the electronic ground state, rely on efficient collisional energy transfer between these levels and neighboring vibration-rotation levels in order to achieve a sufficiently fast... 

Resonant processes of some importance include resonant electronic to electronic energy transfer (E-E), such as the pumping process of the iodine atom laser... 

Another near resonant process is important in the hydrogen fluoride laser, equation (A3.13.37), where vibrational to vibrational energy transfer is of interest ... 

On the experimental side, small molecule energy transfer experiments may use molecular beam teclmiques [65. 66 and 67] (see also chapter C3.3 for laser studies). 

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. 

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). 

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-... 

Flynn G W and Weston R E Jr 1993 Diode laser studies of oollisional energy transfer J. Phys. Chem. 97 8116-27... 

Moore C B 1969 Laser studies of vibrational energy transfer Accounts Chem. Res. 2 103-9... 

With tlie development of femtosecond laser teclmology it has become possible to observe in resonance energy transfer some apparent manifestations of tire coupling between nuclear and electronic motions. For example in photosyntlietic preparations such as light-harvesting antennae and reaction centres [32, 46, 47 and 49] such observations are believed to result eitlier from oscillations between tire coupled excitonic levels of dimers (generally multimers), or tire nuclear motions of tire cliromophores. This is a subject tliat is still very much open to debate, and for extensive discussion we refer tire reader for example to [46, 47, 50, 51 and 55]. A simplified view of tire subject can nonetlieless be obtained from tire following semiclassical picture. 

Seilmeier A and Kaiser W 1988 Ultrashort intramoleoular and intermoleoular vibrational energy transfer of polyatomio moleoules in liquids Ultrashort Laser Pulses and Applications (Topics in Applied Physics 60) ed W Kaiser (Berlin Springer) pp 279-315... 

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

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]. 

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