The Basic Principles of Lasers

The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Similar to the way in which transistor systems are available to generate and amplify electrical signals, with the advent of lasers we have at onr disposal devices that are able to generate and amplify coherent light. 

Both the active medium and the resonator determine the light frequencies generated. 

The quantum mechanical perturbation theory that allows us to calculate the probability of absorption and stimulated emission shows (see Section 5.3) that both 

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The basic principle of laser-induced chemical reactions is schematically illustrated in Fig. 10.7. The reaction is initiated by one- or multiphoton excitation of one or more of the reactants. The excitation can be performed before the reactants collide (Fig. 10.7a) or during the collision (Fig. 10.7b and Sect. 8.6). 

For nonspecialists, however, or for people who are just starting in this field, it is often difficult to find from the many articles scattered over many journals a coherent representation of the basic principles of laser spectroscopy. This textbook intends to close this gap between the advanced research papers and the representation of fundamental principles and experimental techniques. It is addressed to physicists and chemists who want to study laser spectroscopy in more detail. Students who have some knowledge of atomic and molecular physics, electrodynamics, and optics should be able to follow the presentation. 

Semiconductor lasers have undergone a considerable metamorphosis during the past 30 years. They have grown and developed into a whole range of sophisticated opto-electronic devices. It is beyond the scope of this section to give a detailed description of the different semiconductor lasers, but we shall summarize the basic principles of this type of laser. 

A schematic of the basic principles of a matrix-assisted laser desorption/ion source is shown in Figure 2.35. By the interaction of a focused laser beam with short pulses and a suitable matrix, the energy of the photons is transferred to the matrix molecules. In MALDI mostly pulsed UV (e.g., nitrogen, X = 337 nm, pulse duration 3-10 ns), but also IR lasers (e.g., Er YAG, X = 2.94 (xm or C02, X = 10.6(xm with a higher pulse duration of up to 600 ns) are used. The MALDI mass spectra obtained during soft ionization by UV and IR lasers are identical. The energy density... 

When the fine structure frequencies fall below 100 MHz they can also be measured by quantum beat spectroscopy. The basic principle of quantum beat spectroscopy is straightforward. Using a polarized pulsed laser, a coherent superposition of the two fine structure states is excited in a time short compared to the inverse of the fine structure interval. After excitation, the wavefunctions of the two fine structure levels evolve at different rates due to their different energies. For example if the nd3/2 and nd5/2 mf = 3/2 states are coherently excited from the 3p3/2 state at time t = 0, the nd wavefunction at a later time t can be written as40... 

The basic principle of LII is the rapid heating of nanoparticles often up to their sublimation temperature within a few nanoseconds by means of a short intense laser pulse and the subsequent detection and evaluation of the enhanced thermal radiation. First, the particles are heated up by absorbing the laser radiation, which results in an increased internal energy. Considering carbonaceous particles, their maximum particle... 

Laser vaporization reactor. At LTT-Erlangen, first investigations with other nanomaterials have been carried out in cooperation with the group of Staupendahl at the University of Jena in a laser vaporization reactor (LVR) (Staupendahl, 2003). The basic principle of this reactor is dispersing raw material by a fountain into the CO2 laser beams, in which particles are vaporized and nanoparticles are formed by the subsequent condensation. As the LII measurement volume was located slightly above the vaporization zone, it was not possible to prevent coarse structures of material to occur inside the measurement volume (Figure 20). 

Whereas in LIMS only one laser with defined wavelength (e.g., Nd YAG - 1064 nm) is used for direct vaporization and ionization of solid samples in laser plasma, in resonance ionization mass spectrometry (RIMS) " one or more lasers are tuned precisely to the wavelength required for the excited states and ionization of evaporated atoms in order to get a highly selective ionization of the analyte. The basic principles of resonant ionization were first described by Hurst and coworkers at Oak Ridge National Laboratory as well as by Letokhov et in Russia. The technology... 

The basic principle of the method requires that the molecules to be analysed can be ionised in a gaseous phase after desorption from the matrix on which it was previously adsorbed. The energy that provokes desorption and ionisation of the species to be analysed is provided by a laser pulse working in the ultraviolet domain. Each ionised molecule coming from the sample and desorbed from the matrix has an acceleration that is inversely proportional to its own mass and proportional to its charge. This means that the smallest ions reach the detector first and the heaviest ions reach the detector last, as shown in Figure 2.3. 

Fig.16 shows a picture and a overall scheme of the experimental setup for holographic recording using the LC-R 2500 LCoS modulator. The basic principle of the operation is the same as described in third part of the chapter. With the help of the aligned mirror 2, radiation of the laser 1 is delivered onto the optical filter 4. The X./2 plate 3, placed in front of the optical filter, makes it possible to vary the angle of the laser beam polarization relative to the modulator 6 optical axis. 

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