Multiple reflection cell

It is interesting to note that a similar specttum of the 0-0 band of the a-X system, leading to the same value of the absorption intensity, has been obtained using a Fourier transform spectrometer (see Section 3.3.3.2) but with an absorption path, using a multiple reflection cell, of 129 m and half the pressure of gas. 

See reference (7) for optical arrangement and reference (13) for multiple reflection cell details. 

Fig. 6. (a) The IR spectrum of gallane vapor (i) trapped in a solid nitrogen matrix at ca. 20 K and (ii) at a pressure of ca. 5 mm Hg and temperature near 270 K (contained in a cell fitted with Csl windows and having a path length of 10 cm), (b) Part of the IR spectrum of gallane vapor at a pressure of ca. 0.05 mm Hg contained in a multiple-reflection cell set to a path length of 6.5 m and maintained at ca. 290 K (reproduced with permission from Ref. 56 copyright 1991, American Chemical Society). 

Fig. 9. Portions of the IR spectrum of gallaborane vapor at a pressure of 0.15 mm Hg and maintained at ca. 290 K, showing the two v(B—Ht) fundamentals the vapor is contained in a multiple-reflection cell set to a path length of 3.9 m (37, 65).

Thermostated sample cells for all physical states are commercially available. To obtain vapor phase spectra of materials with low vapor pressure, heatable gas cells with a standard length or multiple reflection cells are needed. 

The size of the beam should match that of the cell, particularly when quantitative information is to be gained from absorbance measurements. It can be modified to some extent by means of scatter plates interposed between the exit window and the cell (if the energy of the electron beam allows this to be done) or by means of focusing magnets. In general, the beam is kept small and as uniform as possible and the optical path length is increased by use of multiple reflection cells (see below). For conductivity experiments (see below), the size of the beam should match the volume between the electrodes. 

Hi) Most of the bands in the infrared spectrum of gallane vapor carry unmistakable signs of rotational structure no better example is provided than that of the two absorptions near 1980 cm" the appearance of which, as measured at moderately high resolution with the aid of a multiple reflection cell, is illustrated in Fig. 5. There are, it appears, two types of band. One type, exemplified by the band at 1976 cm S displays all the features characteristic of a parallel band of a highly prolate symmetric top molecule (see Section III.B.l). The other type has the attributes of a perpendicular band of such a molecule (as witness the band at 1993 cm in Fig. 5). 

With the exception of the studies of Pickering and Eckstrom (11) on evaporated metal films in a multiple reflection cell, most of the work... 

A new flash photolysis instrument, especially designed to generate hydrated electrons and study their reactions, is described in detail. With a new three-dimensional multiple reflection cell and its capacity to produce up to 10 7M e aq in a single 40-fisec. light pulse, this instrument provides adequate sensitivity for determining e aq rate constants and for use in analytical chemistry. With this instrument less than 10 9M e aq may be detected. 

These NDIR instruments are mainly used for gas samples although multiple reflection cells are available in a variety of materials that are applicable for the analysis of liquids. Filters are chosen according to the analyte to be determined and the other components of the matrix. Carbon dioxide, carbon monoxide, ethylene, nitrogen oxides and sulfur dioxide are typical analytes. 

Infrared spectrophotometry of ozone based on the principal absorption band near 9.5 pm is relatively free from interference by the bands of other atmospheric constituents. However, a long optical path is necessary for detection of atmospheric ozone. A White cell (multiple reflection cell, 10-1000 m path) combined with a Fourier-transform infrared (FTIR) spectrometer (spectral resolution of lcm or better) with a HgCdTe detector is often used in multi-component air-monitoring and smog chamber experiments. 

In Munich, we have developed our spectrometer around a Jarrell-Ash double monochromator equipped with holographic gratings, using photomultiplier detection [48-51]. An extracavity multiple-reflection cell [49] as well as intracavity excitation with transfer of the Raman-scattered light with optical fibres [50,51] were applied for signal enhancement. The multiple reflection cell has been transferred to Florence and installed at a Jobin-Yvon UlOOO spectrometer equipped with a charge-coupled device (CCD) camera [52]. 

Measurements on gases require much longer path-lengths than those for condensed phases 10 cm is usual for compounds at relatively high concentrations. For trace concentrations multiple reflection cells provide pathlengths of several metres in compact designs. Detection limits are well below 1 ppm for many compounds. 

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