Laser mechanism

The impressive progress of photophysics in recent decades has been possible because of the laser, which provides short and intense light pulses. The basic laser mechanism is sununarized in Section 12.2.4. Later, the basic rules for absorption and emission of light (or electromagnetic [EM] radiation in general) are derived from time-dependent quantum mechanics. 

Imaging plates are exposed similar to radiographic films. They are read out by a LASER-scanner to a digital image without any developing process. After optical erasing of the virtual picture the same IP can be used cyclic up to more than 1000 times. The life time is limited by the mechanical stability of the IP s. An IP consists of a flexible polymer carrier which is coated with the sensitive layer. This layer is covered with a thin transparent protective foil. 

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

Laser-based profilometry systems have also been adapted for unique applications in nuclear power generating plants. Applications where quantitative information with regard to surface condition for mechanisms such as surface pitting and flow-assisted corrosion are candidates for this NDT method. 

Cohen-Tannoud]i C N and Phillips W D 1990 New mechanisms for laser cooling Phys. Today 42 33-40... 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Contradictory evidence regarding the reaction to fonn 8 and 9 from 7 led the researchers to use TREPR to investigate the photochemistry of DMPA. Figure B1.16.15A shows the TREPR spectrum ofthis system at 0.7 ps after the laser flash. Radicals 6, 7 and 8 are all present. At 2.54 ps, only 7 can be seen, as shown in figure B1.16.15B. All radicals in this system exliibit an emissive triplet mechanism. After completing a laser flash intensity sPidy, the researchers concluded that production of 8 from 7 occurs upon absorption of a second photon and not tiiemially as some had previously believed. 

FT-EPR spectra of the ZnTPPS/DQ system in a solution of cetyltriinethylaininonium chloride (CTAC), a cationic surfactant, are shown in figme BE 16.21. As in the TX100 solution, both donor and acceptor are associated with the micelles in the CTAC solution. The spectra of DQ at delays after the laser flash of less than 5 ps clearly show polarization from the SCRP mechanism. While SCRPs were too short-lived to be observed in TXlOO solution, they clearly have a long lifetime in this case. Van Willigen and co-workers... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

The molecular beam and laser teclmiques described in this section, especially in combination with theoretical treatments using accurate PESs and a quantum mechanical description of the collisional event, have revealed considerable detail about the dynamics of chemical reactions. Several aspects of reactive scattering are currently drawing special attention. The measurement of vector correlations, for example as described in section B2.3.3.5. continue to be of particular interest, especially the interplay between the product angular distribution and rotational polarization. 

This book presents a detailed exposition of angular momentum theory in quantum mechanics, with numerous applications and problems in chemical physics. Of particular relevance to the present section is an elegant and clear discussion of molecular wavefiinctions and the detennination of populations and moments of the rotational state distributions from polarized laser fluorescence excitation experiments. 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

It has been possible to determine transition structures computationally for many years, although not always easy. Experimentally, it has only recently become possible to examine reaction mechanisms directly using femtosecond pulsed laser spectroscopy. It will be some time before these techniques can be applied to all the compounds that are accessible computationally. Furthermore, these experimental techniques yield vibrational information rather than an actual geometry for the transition structure. 

A diode, or semiconductor, laser operates in the near-infrared and into the visible region of the spectmm. Like the mby and Nd YAG lasers it is a solid state laser but the mechanism involved is quite different. 

Such a situation suggests the possibility of creating a population inversion and laser action between two such states, since any molecules in the repulsive ground state have an extremely short lifetime, typically a few picoseconds. A laser operating by this mechanism is a... 

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