Laser resonant interaction

Next we model the laser-driven electron dynamics quantum mechanically to reveal analogies and differences to the simple classical model. In view of the SPODS mechanism, which is based on resonant interactions, we consider only two quantum states at hrst, the ground state and the resonantly excited state. Eor this purpose, we briehy recapitulate the relevant equations for a two-state system driven on... 

The limit cycle is an attractor. A slightly different kind occurs in the theory of the laser Consider the electric field in the laser cavity interacting with the atoms, and select a single mode near resonance, having a complex amplitude E. One then derives from a macroscopic description laced with approximations the evolution equation... 

Before it is possible to interpret on a rigorous basis the behavior of the carbonyl stretching frequencies of a series of isostructural and isoelectronic complexes complete vibrational analyses are necessary. However, it is only within the last few years that far-infrared 137) and laser Raman 84) spectrometers have become available generally. Hence, in the general absence of the data they have provided, earlier complete analyses were limited to the spectra of simple metal carbonyls (for which such information was available). Even for these complexes, the number of force constants exceeds the number of observable frequencies, and model force fields had to be used. Since Urey-Bradley type force fields proved to be unsuitable for carbonyl complexes 86,105, 106), Jones 80-82) developed a resonance interaction valence force field which reduced the number of force constants by interrelating several on the basis of orbital overlap. This approach is not readily adaptable to less symmetrical substituted carbonyl complexes. Alternative models had, therefore, to be investigated. 

If the exciting laser field interacts with the CNT resonantly, the magnitude of the hyperpolarizabilty tensor is also affected by the lifetimes of the involved electronic levels. Lifetime of electronic states in a CNT is mostly determined by electron-electron scattering events. As far as electrons in a CNT are treated as a Fermi liquid (despite there are experimental evidence of Luttinger liquid behaviour [27]) the lifetime of an electronic state with an energy E above the Fermi level Ef, is given by... 

Now we have a driving force E(t) = Eq exp(—iu t) so that the molecule a oscillates with this laser field frequency u>l and due to the Fermi resonance interaction across the interface this leads to oscillations of molecules b with frequency 2ujl. As a result, we obtain an algebraic system of equations for the amplitudes A, B, Bu... ... 

Eq.(l) shows velocity distibution of gas in a gas laser resonator. Two propagating wave components in the opposite directions to each other of a standing electromagnetic wave interact with atoms with a velocity, v, in eq,(2) due to Doppler effect. In a gas laser resonating with a single frequency, v, downward induced transition increases for atoms with the velocities of eq.(3). This is called Lamb dip. 

The core components of soUd-state lasers are laser materials that allow for the inversion of population and amplification of radiation through stimulated emission. The properties of the laser materials determine the ways to design pumping system and laser resonator of a soUd-state laser. Because the characteristics of laser active centers are determined by the physical processes related to the laser materials, while there are various possible interactions between the active centers and the electromagnetic radiations, the interrelationship among the composition, stmcture, properties, and functionality of laser materials is very complicated, leading the research in this field to be unlimited. 

The excited ions of the pumped laser materials in a laser resonator can be de-excited by various radiative (either laser or luminescence) and nonradiative (electron-phonon interaction or energy transfer) processes. Also, the amount of ions participating in laser emission is dependent on the laser emission efficiency ( /i). The laser emission is produced by the excited ions inside the laser mode volume and pumped above the threshold. The excited ions inside the pumped volume but outside the laser mode volume and those that form the inversion of population at the laser threshold can be de-excited by luminescence and nonradiative processes. 

Fig. 3.5 Experimental arrangement for intracavity Raman spectroscopy with an argon laser CM, multiple reflection four-mirror system for efficient collection of scattered light LM, laser-resonator mirror DP, Dove prism, which turns the image of the horizontal interaction plane by 90° in order to match it to the vertical entrance slit S of the spectrograph FPE, Fabry-Perot etalon to enforce single-mode operation of the argon laser LP, Littrow prism for line selection [315]...

It is also possible to observe the Ramsey resonances at z = 2L without the third laser beam. If two standing waves at z = 0 and z = L resonantly interact with the molecules, we have a situation similar to that for photon echoes. The molecules that are coherently excited during the transit time t through the first field suffer a phase jump at r = T in the second zone, because of their nonlinear interaction with the second laser beam, which reverses the time development of the phases of the oscillating dipoles. At r = 2T the dipoles are again in phase and emit coherent radiation with increased intensity for co = a>ik (photon echo) [1258,1263]. 

In atomic laser spectroscopy, the laser radiation, which is tuned to a strong dipole transition of the atoms under investigation, penetrates the volume of species evaporated from the sample. The presence of analyte atoms can be measmed by means of the specific interaction between atoms and laser photons, such as by absorption techniques (laser atomic absorption spectrometry, LAAS), by fluorescence detection (laser-induced fluorescence spectroscopy, LIFS), or by means of ionization products (electrons or ions) of the selectively excited analyte atoms after an appropriate ionization process (Figures lA and IB). Ionization can be achieved in different ways (1) by interaction with an additional photon of the exciting laser or of a second laser (resonance ionization spectroscopy, RIS, or resonance ionization mass spectrometry, RIMS, respectively, if combined with a mass detection system) (2) by an electric field applied to the atomization volume (field-ionization laser spectroscopy, FILS) or (3) by collisional ionization by surrounding atoms (laser-enhanced ionization spectroscopy, LEIS). 

If a particle resonantly absorbs a photon from the laser beam, the particle is left in an excited energy state. Such a state is unstable and will decay spontaneously, emitting a photon again. As has been discussed earlier, the excited state of finite lifetime emits its photon on return to a lower energy level in random directions. It is this fact that allows one to measure an absorption signal directly, as outlined in Chapter 6. Conveniently, the fluorescence is observed at 90° to a collimated laser beam. In principle, a very small focal volume Vc may be defined in the imaging set-up, resulting in spatial resolution of the laser-particle interaction volume note that spatial resolution cannot normally be realized in an experiment, which measures the absorption directly. 

Altogether, we thus see a striking similarity, up to details, between the nuclear and the cluster responses for this clean Coulomb excitation mechanism with the prominent feature that the resonance modes turn out to represent robust, harmonic oscillations. It remains to be seen to what extent this behavior persists in other experimental situations. In the following we focus on heavy-ion collisions and laser-cluster interactions as complementary and widely studied excitation mechanisms. 

If, for instance, the laser beam is focused into the interaction region with absorbing molecules, the beam waist of the laser resonator has to be transformed into a beam waist located in this region. The beam parameters in the waists are purely imaginary, that is. 

Contents Introduction. Elements of the Theory of Resonant Interaction of a Laser Field and Gas.—Narrow Saturation Resonances on... 

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