Laser resolution

Because MALDI is a desorption technique, it is particularly suited for the analysis of surfaces such as biological tissues [50]. In this application, the matrix is applied on the complete surface of the tissue. The laser resolution is about 100 pm and complete analyte distribution images (low molecular weight compounds, peptides, proteins) can be recorded [51, 52]. 

An alternate approach used by the Miller group is to pulse amplify a CW dye laser. This approach degrades the laser resolution from about 1 MHz to about 200 MHz but results in a high-power pulsed laser beam with excellent mode quality. This high peak power and high spectral resolution results in excellent spectra with an improved S/N ratio. The only serious drawback of this scheme is the increased complexity and cost of the laser systems. 

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. 

P ) The system BAS 2000 (bio-imaging analyser) from Fuji Film. The system included flexible IP s, in a 20 X 40 cm and 20 X 25 cm format (type BAS ni) and an IP without proteetive layer (BAS-TR2040). With the system s laser scanner (flat bed) one can read these IP s with a resolution of 100 pm or 200 pm and a digital dynamie of 8 or 10 bits. The system is not on the market anymore. 

PES of neutral molecules to give positive ions is a much older field [ ]. The infomiation is valuable to chemists because it tells one about unoccupied orbitals m the neutral that may become occupied in chemical reactions. Since UV light is needed to ionize neutrals, UV lamps and syncln-otron radiation have been used as well as UV laser light. With suitable electron-energy resolution, vibrational states of the positive ions can be... 

Quack M 1993 Molecular quantum dynamics from high resolution spectroscopy and laser chemistry J. Mol. Struct. 292 171-95... 

Quack M 1992 Time dependent intramolecular quantum dynamics from high resolution spectroscopy and laser chemistry Time Dependent Quantum Molecular Dynamics Experiment and Theory. Proc. NATO ARW 019/92 (NATO ASI Ser. Vol 299) ed J Broeckhove and L Lathouwers (New York Plenum) pp 293-310... 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

For the highest resolution and sensitivity, laser-based spectrometers must be used. These have tire advantage that the resolution depends on the linewidth of the laser, rather than the monocln-omator. Furthennore, at any given moment, all of the power is at the frequency of mterest, rather than being spread out over the whole IR... 

Nesbitt D J 1994 Fligh-resolution, direct infrared-laser absorption-spectroscopy in slit supersonic ]ets—intermolecular forces and unimolecular vibrational dynamics in clusters Ann. Rev. Phys. Chem. 45 367-99... 

Nonnal spontaneous Raman scahering suffers from lack of frequency precision and thus good spectral subtractions are not possible. Another limitation to this technique is that high resolution experiments are often difficult to perfomi [39]. These shortcomings have been circumvented by the development of Fourier transfomi (FT) Raman spectroscopy [40]. FT Raman spectroscopy employs a long wavelength laser to achieve viable interferometry. 

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

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