Rapid scan spectrometer

Snively, C.M., Katzenberger, S., Oskarsdottir, G. et al. (1999) Fourier-transform infrared imaging using a rapid-scan spectrometer. Opt. Lett., 24, 1841. 

A modern variation on the rapid scan spectrometer, which is under development, uses a laser-generated plasma as a high intensity broad-band IR source (65). This method has been used to probe the vc—o absorption of W(CO)6. Another technique TRISP (time-resolved IR spectral photography), which involves up-conversion of IR radiation to the visible, has also been used to probe transients (66). This method has the enormous advantage that efficient phototubes and photodiodes can be used as detectors. However, it is a technically challenging procedure with limitations on the frequency range which depend on the optical material used as an up-converter. 

Both Porter s original flash photolysis apparatus and Pimentel s rapid scan spectrometer recorded the whole spectral region in a time which was short compared to the decay of the transient species. Kinetic information was obtained by repeatedly firing the photolytic flash lamp and making each spectroscopic measurement at a different time delay after each flash. The decay rate could then be extracted from this series of delayed spectra. Such a process clearly has limitations, particularly for IR measurements, where the decay must be slow compared to the scan rate of the spectrum. 

In order to observe a short-lived species it may be necessary to employ a rapid-scanning spectrometer, such as a diode-array instrument (Sms for a 240nm-800nm spectrum). In addition, the absorbances of electrogenerated species can be very small and signal-averaging or phase-sensitive detection may be necessary to achieve the required signal-to-noise ratio (cf. EMIRS and FTIR). 

For visual observation of the cell interior through the sapphire windows a lamp mounted behind one end is used. A mirror and stereo microscope at the other end facilitate the observation. The microscope is equipped with a normal camera or a video camera. Normally the phenomena within the cell are continuously observed and controlled with video camera and colour monitor. A video recorder serves for documentation, for inspection of short time processes and for the production of standing flame pictures for size and shape determination. Instead of the microscope a Jarrell-Ash diode array rapid scan spectrometer can be attached to the cell to obtain flame spectra in the visible and UV-regions. 

Studies by Crawford Rotenberg (Ref 4) who used a rapid-scan spectrometer in conjunction with a strand burning apparatus to examine NG-NC low temperature decomposition and flames. During the decomposition of commercial double-base propellants, the gas products were NO, N20, C02 CO. When these propellants were burned under 100-150 psi nitrogen pressure at a linear velocity of 100 cm/sec, C02 CO absorption bands appeared even at 2 cm away from the burning surface. Nitric oxide was barely detectable and N20 was completely absent... 

If we have a single-beam spectrometer, we may separately record spectra Bm(x), Um(x), and Z>M(x) and apply Eq. (45) later in the computer. With special rapid-scanning spectrometers this approach may be practical, but... 

The transparency of the electrode also enables spectra to be recorded of electrogenerated species as well as of any species produced as a result of a homogeneous chemical reaction. Such spectra have been recorded with rapid scanning spectrometers that are capable of recording as many as 100 or more spectra per second in the UV-visible range [22]. Spectra can be useful for structural identification of intermediate components in the reaction sequence and for... 

Transient absorption spectroscopy, wherein one measures the electronic absorption spectrum of a molecule in an excited state, is still in its infancy, but the growing availability of ultra-high-speed, rapid-scan spectrometers augurs well for this area of spectroscopy. Thus one may, in the future, routinely probe excited state absorption spectra as well as ground state absorption spectra. The former can be expected to be as valuable in obtaining information about the excited state as is the latter for the ground state. 

All that is needed to make such a detector into a MULT1-WAVELENGTH/RAPID SCANNING spectrometer for liquid chromatographic purposes is to disperse the light exiting from an LC cell onto the surface of the photodiode array by means of a monochromater. 

Spectra of intermediates can also be obtained using rapid scanning spectrometers after a potential step (Strojek et al., 1969). 

J.-J. Meyer, P. Ixvoir, and R. Dubest, Upgrading a rapid-scanning spectrometer with microcompu-terized data acquisition and treatment to measure spectrokinetic parameters of photochromic compounds, Analyst 120, 447-452 (1995) and references therein. 

The rapid-scanning spectrometer can be realized with a moving radiation... 

With slow kinetics of the involved processes or with interfaces where electrode potential modulation might be detrimental because of crystallographic changes in the metal surface, other spectroscopic techniques have to be used. The whole spectrum of interest can be scanned or registered within a few milliseconds with a rapid scan spectrometer or a multichannel (diode array) spectrometer. Repeated acquisition provides the required signal-to-noise ratio. After a potential step, the acquisition is repeated and spectral calculation yields AR/R. This single potential step procedure allows investigation of systems where repeated potential modulation has failed. 

Other detectors that are useful in the near- and mid-infrared regions are bolometers and pyroelectric detectors. Both these detectors have very large bandwidths and can operate at room temperature however, they have long response times compared to the photodetectors and they have low D s. Pyroelectric detectors are useful in the far-infrared region with rapid-scanning spectrometers whereas Golay cell detectors are often used with slow scanning far-infrared interferometers. These cells are modulated at or below 20 Hz. 

A more elegant, but more expensive, way of obtaining spectra is to use a rapid scanning spectrometer (RSS) [4, 5]. This enables a 1000 point, 450 nm... 

Kemp, Moore, and Quick " have used a rapid-scanning spectrometer to characterize the transient intermediates in the dissociation in water of nickel(II) poly amine complexes. While they were unable to give a complete analysis for [Ni(trien)(OH2)2] because of the complexity of the spectra involved and the fact that the consecutive stages of dechelation are not significantly separated along the time axis, they could confirm a previous suggestion that the dissociation of [Ni(dien)(OH2)3] occurred in two stages ... 

Figure 19.2. Three-dimensional plot of the spectra obtained from the basic hydrolysis of methyl monochloroacetate and NaOH (100 mM each) using a rapid-scanning spectrometer. The time difference between subsequent spectra was 65 s. The first five spectra were measured during the flow-on period. (Reproduced from [1], by permission of the Society for Applied Spectroscopy copyright 2001.)...

Mechanistic information is often available from spectroelectrochemical measurements. To illustrate the acquisition of semiquantitative information using a rapid scan spectrometer (RSS), the reduction of methyl viologen (the 1,1 -dimethyl-4,4 -bipyridilium dication) under semi-infinite linear diffusion conditions is presented. Methyl viologen (MV +) undergoes two consecutive one-electron reductions to the radical cation (MV " ) and neutral species (MV ) in an EE mechanism. In acetonitrile at an OTE coated with a... 

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