Lasers radiation

In modern spectroscopy, it is necessary to have access to good sources of light, ultraviolet (UV), and infrared (IR) radiation. The laser is useful since it anits coherent EM radiation at a very high intensity and, if necessary, in a narrow frequency range. 

The idea of amplification of EM radiation by stimulated emission was first realized for microwaves. The MASER (microwave amplification by stimulated emission of radiation) was developed in the Soviet Union in the early part of the 1950s, particularly by N. Basov and A. Prokhorov. At the same time, it was developed by C. H. Townes, A. L. Schawlow, and others in the United States. Originally, the laser was called light-maser, but the name was changed to LASER (light amplification by stimulated emission of radiation). 

The first laser, the ruby laser, was constructed by T. Maiman in the Hughes Laboratories in California. The same year, it was considerably improved in the Bell Laboratories. It consists of a corundum (AI2O3) crystal with quite sparse substitutions of AP+ with Cr +. The corundum device is shaped in the form of a long cylinder. 

One wants to achieve population inversion, meaning that level 2 has a higher occupation than level 1. When the electrons start to deexcite and reoccupy the ground state, the intensity starts to increase in the frequency range of the emission. The deexcitation is now a stimulated emission and its intensity increases manifold. The laser is fired by itself and a light pulse is emitted. The emitted light penetrates the semitransparent mirror. 

FIGURE 12.3 Three-level ruby laser. The levels shown are ligand field levels. Level 1 is the A ground state level 2 is the Ti and Tj excited states, and level 3 is the excited state. 

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We need to point out that, if the wavelengths of laser radiation are less than the size of typical structures on the optical element, the Fresnel model gives a satisfactory approximation for the diffraction of the wave on a flat optical element If we have to work with super-high resolution e-beam generators when the size of a typical structure on the element is less than the wavelengths, in principle, we need to use the Maxwell equations. Now, the calculation of direct problems of diffraction, using the Maxwell equations, are used only in cases when the element has special symmetry (for example circular symmetry). As a rule, the purpose of this calculation in this case is to define the boundary of the Fresnel model approximation. In common cases, the calculation of the diffraction using the Maxwell equation is an extremely complicated problem, even if we use a super computer. 

In the first approximation, the flat optical element may be described as an element which transforms laser radiation as it passes through the element, as is shown by the following formula ... 

For the application, the problems of focusing laser radiation into the curve L are very interesting In this case we have to find function (p ... 

The fundamental confirmation of the theory of focusing laser radiation may be formulated in the following way [3,4] ... 

Let the problem of focusing laser radiation into the smooth curve L have a smooth solution function (p, rf)e.C (G). Then the inverse image of each point M ff) EiL is a certain segment F (ff) S G. ... 

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

There are several requirements for this to be a suitable deteetion method for a given moleeule. Obviously, tire moleeule must have a transition to a bound, exeited eleetronie state whose wavelength ean be reaehed with tunable laser radiation, and the band system must have been previously speetroseopioally assigned. If the moleeules are fonned with eonsiderable vibrational exeitation, the available speetroseopie data may not extend up to these vibrational levels. Transitions in the visible ean be aeeessed direetly by the output of a tunable dye laser, while transitions in the ultraviolet ean be reaehed by Ifequeney-doubled radiation. The... 

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. 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Apart from the obvious property of defining pulses within short time intervals, the pulsed laser radiation used in reaetion kineties studies ean have additional partieular properties (i) high mtensity, (ii) high monoehromatieity, and (iii) eoherenee. Depending on the type of laser, these properties may be more or less pronouneed. For instanee, the pulsed CO2 lasers used in IR laser ehemistry easily reaeh intensities between... 

Strategies for aehieving intra- and intennoleeular seleetivity are the subjeet of a very aetive field of eurrent researeh with many open questions. Under the label eoherent eontrol it ineludes approaehes that exploit the eoherenee properties of laser radiation to eontrol ehemieal reaetions. Figure B2.5.18 suimnarizes the different sehemes of intra- and intennoleeular seleetivity. 

In FT-Raman spectroscopy the radiation emerging from the sample contains not only the Raman scattering but also the extremely intense laser radiation used to produce it. If this were allowed to contribute to the interferogram, before Fourier transformation, the corresponding cosine wave would overwhelm those due to the Raman scattering. To avoid this, a sharp cut-off (interference) filter is inserted after the sample cell to remove 1064 nm (and lower wavelength) radiation. 

An FT-Raman spectrometer is often simply an FTIR spectrometer adapted to accommodate the laser source, filters to remove the laser radiation and a variety of infrared detectors. 

Laser radiation is emitted entirely by the process of stimulated emission, unlike the more conventional sources of radiation discussed in Chapter 3, which emit through a spontaneous process. 

Laser radiation has four very remarkable properties ... 

The cavity of a laser may resonate in various ways during the process of generation of radiation. The cavity, which we can regard as a rectangular box with a square cross-section, has modes of oscillation, referred to as cavity modes, which are of two types, transverse and axial (or longitudinal). These are, respectively, normal to and along the direction of propagation of the laser radiation. 

The excimer laser radiation is pulsed with a typical maximum rate of about 200 FIz. Peak power of up to 5 MW is high compared with that of a nitrogen laser. 

The state may decay by radiative (r) or non-radiative (nr) processes, labelled 5 and 7, respectively, in Figure 9.18. Process 5 is the fluorescence, which forms the laser radiation and the figure shows it terminating in a vibrationally excited level of Sq. The fact that it does so is vital to the dye being usable as an active medium and is a consequence of the Franck-Condon principle (see Section 7.2.5.3). 

Laser radiation is very much more intense, and the line width much smaller, than that from, for example, a mercury arc, which was commonly used as a Raman source before 1960. As a result, weaker Raman scattering can now be observed and higher resolution is obtainable. 

The reason why the spacings are equal, and not the 1-0, 2-1, 3-2,... anharmonic intervals, is explained in Figure 9.21. The laser radiation of wavenumber Vg takes benzene molecules into the virtual state Fj from which they may drop down to the v = level. The resulting Stokes scattering is, as mentioned above, extremely intense in the forward direction with about 50 per cent of the incident radiation scattered at a wavenumber of Vg — Vj. This radiation is sufficiently intense to take other molecules into the virtual state V2, resulting in intense scattering at Vg — 2vj, and so on. 

Figure 9.22 illustrates how a CARS experiment might be carried out. In order to vary (vj — V2) in Equation (9.18) one laser wavenumber, Vj, is fixed and V2 is varied. Here, Vj is frequency-doubled Nd YAG laser radiation at 532 nm, and the V2 radiation is that of a dye laser which is pumped by the same Nd YAG laser. The two laser beams are focused with a lens L into the sample cell C making a small angle 2a with each other. The collimated CARS radiation emerges at an angle 3 a to the optic axis, is spatially filtered from Vj and V2... 

In the discussion in Section 9.1.6 of harmonic generation of laser radiation we have seen how the high photon density produced by focusing a laser beam into certain crystalline materials may result in doubling, tripling, etc., of the laser frequency. Similarly, if a laser beam of wavenumber Vl is focused into a cell containing a material which is known to absorb at a wavenumber 2vl in an ordinary one-photon process the laser radiation may be absorbed in a two-photon process provided it is allowed by the relevant selection rules. 

The phenomenon of multiphoton dissociation finds a possible application in the separation of isotopes. For this purpose it is not only the high power of the laser that is important but the fact that it is highly monochromatic. This latter property makes it possible, in favourable circumstances, for the laser radiation to be absorbed selectively by a single isotopic molecular species. This species is then selectively dissociated resulting in isotopic enrichment both in the dissociation products and in the undissociated material. 

Measurements of ozone concentration in the ozone layer in the stratosphere are made in the less intense Huggins band to avoid complete absorption of the laser radiation. Again, the two or three wavelength DIAL method is used to make allowance for background aerosol scattering. A suitable laser for these measurements is the XeCl pulsed excimer laser (see Section 9.2.8) with a wavelength of 308 nm, close to the peak absorption of the Huggins... 

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