Excition diffusion

By optical excitation with argon and krypton laser lines, continuous laser oscillation on A -> X and B -> X transitions of Li Na2 and K molecules can be achieved dimer lasers show such interesting features as multiline emission, extremely low threshold pump intensities and forward-backward amplification asymmetry. Basic principles, operating conditions and applications of these lasers will be discussed. The dimer lasers operate between bound electronic states, resulting in the emission of discrete lines. To achieve tunable laser oscillation, continuous emission bands from bound-free transitions have to be considered. Some possibilities for alkali dimers are outlined and recent spectroscopic investigations on UV excited diffuse bands are reported. 

In a broad family of cases the excited vibration loses its energy to the lattice in two steps. In the first step, which is frequently rate determining, it excites localised vibrations, in particular a rotational type of motion of the guest molecule or a translational motion of the same in the lattice cage 19K Secondly the localised excitation diffuses away to the farther reaches of the lattice. 

As mentioned in Chapter 3, the type of excitable behavior discussed there may be considered as arising from a quasi-bistable dynamics in which one of the involved states, the excited one, is not really stable but lasts only for a finite time. Thus excitable diffusive systems have some similarities with bistable ones, but present an additional level of complexity. 

Fig. 5. Excitation diffusion cross section for He(P5) + He (2 / ) as a function of incident energy. The meaning of C = 23.75 and C = 0 is the same as before. The isotope effect of He + He is negligibly small in this case.

INCIDENT ENERGY IN ELECTRON VOLTS 8. Excitation diffusion cross section for He(l 5) +... 

Exchange interactions Phonon assisted energy transfer Excitation diffusion Sensitization of luminescence... 

Fig. 1. Schematic diagram of the laser-excited diffusion cloud apparatus.

There are several possible mechanisms through which this might occur - namely, the Induced dipole interaction, the exchange mechanism, radiative transfer, and excitation diffusion. 

V = 5x 10 cm/s and t = 10 s, we obtain d = 5 x 10 cm = 0,5 ym, which shows that the fluorescence from excited molecules with lifetimes t < 10 s is.essentially emitted from the interaction zone where the molecules are excited. Diffusion of the excited molecules out of this zone becomes noticeable only for t > 10" s. In these cases either the diffusion has to be reduced by increasing the pressure or the field of observation has to be enlarged. 

Phillips, D. (ed.), Polymer Photophysics , Chapman and Hall, New York, 1985. Studies on luminescence spectroscopy to show molecular motion, order and excitation diffusion and energy transfer are presented. 

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

The statistics of the detected photon bursts from a dilute sample of cliromophores can be used to count, and to some degree characterize, individual molecules passing tlirough the illumination and detection volume. This can be achieved either by flowing the sample rapidly through a narrow fluid stream that intersects the focused excitation beam or by allowing individual cliromophores to diffuse into and out of the beam. If the sample is sufficiently dilute that... 

Reilly P D and Skinner J L 1993 Spectral diffusion of single molecule fluorescence a probe of low-frequency localized excitations in disordered crystals Phys. Rev. Lett. 71 4257-60... 

Excitable media are some of tire most commonly observed reaction-diffusion systems in nature. An excitable system possesses a stable fixed point which responds to perturbations in a characteristic way small perturbations return quickly to tire fixed point, while larger perturbations tliat exceed a certain tlireshold value make a long excursion in concentration phase space before tire system returns to tire stable state. In many physical systems tliis behaviour is captured by tire dynamics of two concentration fields, a fast activator variable u witli cubic nullcline and a slow inhibitor variable u witli linear nullcline [31]. The FitzHugh-Nagumo equation [34], derived as a simple model for nerve impulse propagation but which can also apply to a chemical reaction scheme [35], is one of tire best known equations witli such activator-inlribitor kinetics ... 

If the electrodes are moved closer together, the positive column begins to shorten as it moves through the Faraday dark space because the ions and electrons within it have a shorter distance through which to diffuse. Near the cathode, however, the electric-field gradient becomes steeper and electrons from the cathode are accelerated more quickly. Thus atom excitation through collision with electrons occurs nearer and nearer to the cathode, and the cathode glow moves down toward the electrode. 

Whereas the emission spectrum of the hydrogen atom shows only one series, the Balmer series (see Figure 1.1), in the visible region the alkali metals show at least three. The spectra can be excited in a discharge lamp containing a sample of the appropriate metal. One series was called the principal series because it could also be observed in absorption through a column of the vapour. The other two were called sharp and diffuse because of their general appearance. A part of a fourth series, called the fundamental series, can sometimes be observed. 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

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