Instrumentation continuous wave systems

The first successful application of the continuous wave (CW) He-Ne gas laser as a Raman excitation source by Kogelnik and Porto (14) was reported in 1963. Since that time, significant improvements in instrumentation have been continually achieved which have circumvented a great number of problems encountered with mercury lamp sources. The renaissance of Raman spectroscopy has also been due to improvements in the design of monochromators and photoelectric recording systems. 

So far, we have shown where the signal comes from, but how do we measure it There are two main technologies continuous wave (CW) and pulsed Fourier transform (FT). CW is the technology used in older systems and is becoming hard to find these days. (We only include it for the sake of historical context and because it is perhaps the easier technology to explain). FT systems offer many advantages over CW and they are used for all high field instruments. 

Fourier transform spectroscopy technology is widely used in infrared spectroscopy. A spectrum that formerly required 15 min to obtain on a continuous wave instrument can be obtained in a few seconds on an FT-IR. This greatly increases research and analytical productivity. In addition to increased productivity, the FT-IR instrument can use a concept called Fleggetts Advantage where the entire spectrum is determined in the same time it takes a continuous wave (CW) device to measure a small fraction of the spectrum. Therefore many spectra can be obtained in the same time as one CW spectrum. If these spectra are summed, the signal-to-noise ratio, S/N can be greatly increased. Finally, because of the inherent computer-based nature of the FT-IR system, databases of infrared spectra are easily searched for matching or similar compounds. 

Although first demonstrated by Chantry et al. in 1964, FT-Raman spectroscopy did not attract significant attention until the development of commercially available instrumentation with excitation in the near-infrared region (1064 nm) from a continuous wave Nd/YAG (neodymium-yttrium-aluminum-gamet) laser. The FT-Raman spectrometer is a frequency-division multiplexing system in which all (scattered) wave-... 

ISA s (Spex s) Fluorolog Tau-3 lifetime system and Spectronic Instruments SLM-AMINCO 48000 DSCF spectrofluorimeter both use xenon flash lamp excitation (t5 ically 150-450 W) and have modulation frequencies of up to 310 MHz. (Most systems can also be operated as steady-state fluorimeters). SLM also manufactures a multi-harmonic s3rstem based on a pulse- modulated continuous wave light source. 

Other groups have built tunable far-inffared spectrometers which do not involve high-frequency backward-wave oscillators. Verhoeve, Zwart, Versluis, Drabbels, ter Meulen, Meerts, Dymanus and McLay [61] have described a system in which fixed frequency far-inffared radiation is mixed with tunable microwave radiation in Schottky barrier diodes. This instrument has been operated up to 2.7 THz, and used to study OD and N2H+. A similar system, combined with a continuous supersonic jet, has been described by Cohen, Busarow, Laughlin, Blake, Havenith, Lee and Saykally [62], This instrument was used to study rare gas/water clusters. 

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