Laser mode selection

In the case of a common lower level, the second absorption transition would show this narrowing effect when probed with a tunable monochromatic laser line. This example can be realized if atoms or molecules in a magnetic field are pumped by a laser, oscillating simultaneously on two cavity modes 324). if the Zeeman splitting of the probe equals the mode spacing of the laser, both transitions are pumped simultaneously and each laser mode selectively eats... 

Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

Quack M 1991 Mode selective vibrational redistribution and unimolecular reactions during and after IR-laser excitation Mode Selective Chemistry ed J Jortner, R D Levine and B Pullman (Dordrecht Kluwer) pp 47-65... 

B2.5.5.4 LASER ISOTOPE SEPARATION AND MODE-SELECTIVE REACTIONS... 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

Bond- or Mode-selective Laser Chemistry. Suppose we wish to break a specific bond in a molecule or cause a molecule to rearrange in a specific way, and the desired transformation is not the one which will occur if the molecule is simply heated (i.e., it is not the weakest coordinate in the molecule). Can we, by selectively exciting with a laser the bond or motion in question, cause the desired transformation to occur in greater than thermal yield ... 

The last method has been pushed to an impressive sensitivity by putting the probe inside the cavity of a cw dye laser oscillating on several modes close above threshold. The sensitivity of such a broad-band dye laser to selective intracavity absorption on a single mode is proportional to the number of oscillating modes due to... 

Energy Redistribution in Isolated Molecules and the Question of Mode-Selective Laser Chemistry Revisited, N. Bloembergen and A. H. Zewail, J. Phys. Chem. 88, 5459 (1984). 

My answer would be, using an analogy between molecules and human beings, that it is neither nice nor possibly easy to use brute force on the molecules. However, often the molecules may be in a state where they do not really know what they want to do. Then we might use some very mild means to seduce them to do what we would wish them to do. As an early example for such mild seduction I might quote the theoretical scheme for potentially mode selective infrared laser chemistry of ozone [1, 2], which predates some of the more widely publicized subsequent schemes using excited electronic states. 

Third, ultrashort laser pulses can provide for selective electronic or vibrational excitation of molecules, which will probably allow one to effect a high selective excitation of certain vibrational degrees of freedom, that is, to effect mode-selective photochemical reactions. 

Mode-selective chemistry The use of laser beams to control the outcome of a chemical reaction by exciting specific energy states of the reactants. 

Ordinary Raman scattering is an inefficient process, and in general it is less sensitive than IR absorption. However, in certain lignin samples, conjugation, resonance, or pre-resonance Raman effects can arise, and for particular vibrational modes a higher level of sensitivity can be achieved (Long 1977, Schmid and Brosa 1971). In lignins, the latter effect can be induced by proper laser frequency selection. 

Polyatomic molecules are characterized by complex internal motions leading to several possible rearrangements. With the advent of ultrafast lasers, much hope arose for achieving mode selectivity in chemical reactions based on the interaction between the laser pulses and the dynamics of the molecule. In the quest to steer complex systems, an especially attractive control scheme is the adaptive optimal... 

Quantum dynamical calculations on the IRMPE/D of 614, 615 O3. Quantum interference effects and discussion of the possibility of mode-selective excitation and reaction Confirmation that OCS does not undergo IRMPD at 616 high laser fluences (ca. 250 J cm ). Laser-induced dielectric breakdown in OCS, OCS-He, and OCS-Ar does lead to dissociation, giving CO + S Ab initio study of SO2 IRMPE using the most proba- 617 ble path approximation to select the most important paths within the semiclassical Floquet matrix. Conclude that collisionless MPD of SO2 will not occur at laser field strengths <20 GW cm ... 

The conventional narrowband CARS process probes one particular vibrational mode selectively. Conversely, so-called broadband CARS measurements, using ultrashort pulsed laser sources, can probe multiple RS-active vibrational modes simultaneously [19, 29-31]. In the case of two-beam broadband CARS method, one of the two beams has a narrow bandwidth and the other a broad bandwidth. Therefore, the technical issue is how to generate these beams from a single laser source. Typically, subpicosecond pulses from a conventional solid-state femtosecond laser... 

For the coherent control of reactions at surfaces, the manipulation of adsorbate motion is essential. Cs/Pt(lll) is a suitable system which provides us with a good opportunity to test whether or not we can excite preferentially one of the two modes whose frequencies are very close to each other by using tailored laser pulses. We have demonstrated the mode-selective excitation of coherent surface phonon modes on Cs/Pt(lll) by synthesized femtosecond pulse trains. 

Laser-induced formaldehyde dissociation has been intensively studied experimentally and theoretically in the last decade.51-54 Detailed calculations of the potential energy surface have been reported and various studies have been undertaken regarding the possibility of mode selective decay, The low density of vibrational states at the typical energies studied make H2CO an interesting candidate for possible non-RRKM effects, a point to which we return later. 

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