Broadband spectral sources

The spectra in Figure 2.3c and d both have Lorentzian profiles and yield sinusoidal interferograms with an exponentially decaying envelope. The narrower the width of the spectral band, the greater is the width of the envelope of the interferogram. For a monochromatic source, the envelope of the interferogram will have an infinitely large width (i.e., it will be a pure cosine wave). Conversely, for broadband spectral sources, the decay is very rapid. 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

Most optical spectral measurements, where the measurement of multiple wavelengths is required, will feature some type of polychromatic or broadband light source. There are a few exceptions here, such as tunable laser sources and source arrays. In such instances, the source is effectively "monochromatic at a given point in time. These sources are covered separately under monochromatic sources. 

Most optical spectral measurements, where the measurement of multiple wavelengths is required, will feature some type of polychromatic or broadband light source. There are a... 

A mid-infrared absorption instrument generally consists of a Fourier transform design with the same basic components as noted above for the Fourier transform near-infrared spectrometers (broadband light source, Michelson interferometer, and detector optimized for the mid-infrared spectral region.)... 

The TERS strategy is not suitable for IR absorption spectroscopy (IRAS). This is because a broadband light source is used in IRAS and it is difficult to create a constant enhancement over the entire spectral range. Another problem stems from the much longer wavelength of IR compared to visible light. In other words, the far-field excitation area will be much larger than in the case of TERS, and the SNR will deteriorate. In order to overcome these problems, s-SNOM is commonly used for near-field IRAS [32]. 

The alternative to tunable sources is the absorption spectrometer, composed schematically from a broadband incoherent source, a monochromator and a detector. Additional equipment is also needed, as additional sources for band gap excitation. Each part of the equipment is specific to the spectral range... 

An important consideration in spectroscopic measurements concerns the bandwidth of the laser sources. In order to resolve the vibrational resonances in a conventional approach, one needs, in the conventional scheme, a tunable source that has a narrow bandwidth compared to the resonance being studied. For t5q)ical resolutions, this requirement implies, by uncertainty principle, that IR pulses of picosecond or longer duration must be used longer. On the other hand, ultrafast pulsed IR sources with broad bandwidths are quite attractive from the experimental standpoint. In order to make use of these sources, two t5q)es of new experimental techniques have been introduced. One technique involves mixing the broadband IR source ( 300 cm ) with a narrowband visible input ( 5 cm ). By spectrally resolving the SF output, we may then obtain resolution of the IR spectrum limited only by the linewidth of the visible source [M, M]- This result follows from the fact that SF vis satisfied for the SFG process. The second new approach involves the... 

Various broadband lamp sources have been employed to optically pump Nd tungsten, mercury, xenon, and krypton. The last source provides an especially good spectral match to the near-infrared absorption bands of Nd in YAG. To reduce lattice heating resulting from the multiphonon emission cascade to F3/2, semiconductor diodes and laser sources at 0.8 /im have been used for pumping Nd lasers. Sun-pumped Nd and chromium-sensitized Nd lasers have been... 

Broadband spectral levels fall also with increasing borehole depth, but narrow spectral lines due to cultural sources and or waveguide trapping may show less depth dependence. This may result in very different noise decay and signal-to-noise ratio behavior with depth (see section Installations in Subsurface Mines, Tunnels, and Boreholes ). 

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. 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

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