Globars

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. 

H2 and D2 lamp tungsten lamp Xe arc lamp Nernst glower globar... 

Mid- and near-infrared Nernst filament globar NaCl or KBr Grating interferometer Golay cell thermocouple bolometer pyroelectric photoconductive semiconductor... 

The PIA-investigations were carried out under dynamic vacuum (p< 10 5 mbar) and at 77 K with films cast from toluene solution onto KBr substrates. For the dispersive method [29, 30] the globar, the KBr-prism premonochromator, and the grating monochromator of a Perkin Elmer 125 lR-spcctrometer were used in the spectral range of 0.25 to 1.24 eV. The pump beam was chopped mechanically... 

The main sources of infrared radiation used in spectrophotometers are (1) a nichrome wire wound on a ceramic support, (2) the Nernst glower, which is a filament containing zirconium, thorium and cerium oxides held together by a binder, (3) the Globar, a bonded silicon carbide rod. These are heated electrically to temperatures within the range 1200- 2000 °C when they will glow and produce the infrared radiation approximating to that of a black body. 

Globars" (silicon carbide rods) are the next most frequently used furnace heating elements. They will operate continuously at 1450 °C. and intermittently at 1500 C. A newer t3q>e of element. Mo wire coated with silicide, i.e.- MoSi2(to protect the Mo wire against oxidation), heis become... 

The pump source in the dispersive experiment is typically a nanosecond laser the probe source can be broadband IR light from a globar or tunable IR light from a CO laser or a semiconductor diode laser. Although CO and diode lasers can produce... 

One solution to this dilemma has been advanced by Hamaguchi and co-workers who have made use of a MoSi2 IR source newly developed by JASCO that provides approximately twice the emissive intensity of conventional globar sources. This probe source was incorporated into a dispersive TRIR spectrometer that allows access to the entire mid-IR spectrum with high sensitivity (A A < 10 ) and sufficient time (50 ns) and frequency (4-16 cm ) resolution to probe a wide range of transient intermediates in solution. 

Mulheim globar + monochromator flash lamp InSb/MCT I.Spsec... 

Now that the parameters of typical prototype systems have been considered, it seems worthwhile to consider further variations that could be used to enhance the power and/or versatility of the basic apparatus. An obvious improvement would be to use a completely tunable IR probe source. As mentioned above, a globar source is completely tunable but with a lower than desired number of photons per unit bandwidth (60,73). There are a number of other means of producing completely tunable IR probe radiation with enough intensity to pull the detector out of the blackbody-noise regime. 

Radiation in the infrared region of the spectrum is obtained from heated ceramic devices such as the Nemst glower or Globar. The Globar is made of silicon carbide and is heated to approximately 800-1500°C to emit black-body radiation in the infrared region of the spectrum. Coils of nichrome wire also emit infrared radiation when electrically heated. 

Infrared radiation in the range 4 000 to 200 cm-1 is commonly obtained from a silicon carbide globar, heated to near 1 500 K by an electric current. The globar emits radiation with approximately the same spectrum as a blackbody at 1 000 K (Box 20-1). 

Fourier transform infrared (FTIR) spectroscopy has been extensively developed over the past decade and provides a number of advantages. The main part of FTIR spectrophotometer is the Michelson interferometer. Radiation containing all IR wavelengths (e.g., 4000-400 cm 1) is emitted by source of infrared radiation (Globar) and is split into two beams. One beam is of fixed length, and the other is of variable length (movable mirror). 

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