Dispersive monochromator setting

Simple, low-dispersion monochromators or even interference filters are used for most flame emission applications since few atomic line spectral interferences are expected as a result of the limited population of the higher-lying excited states. For high-temperature sources such as ICPs, higher-dispersion spectrometers are typically used. Instruments set up to do simultaneous multielemental analysis can use direct readers with PMT detection. However, most modern detections systems for this type of source for simultaneous multielemental analysis employ a high-dispersion eschelle grating spectrometer and an array detector such as a CCD or CID. 

One further point to note is that a grating monochromator disperses light in a series of orders. Thus a monochromator setting at 500 mn will also pass light whose wavelength is 250 nm since the second order of dispersion of li t at X will occur at 2Xj. Placing an appropriate cut-off filter after a monochromator, i.e. a P3nrex filter in this example, can efficiently remove this second-order dispersion. 

A monochromator set at wavelength X nm, will also transmit wavelengths of XJ 2, X/3, XIA... run, produced by second order, third order, fomth order, etc. dispersion. Thus, in the absence of any filtering, an excitation monochromator set at say 500 nm will also let through unwanted 250 nm radiation from the source. Similarly, scanning an emission monochromator to obtain the emission spectrum for a sample excited using say 225 mn radiation will show bands at 450 and 675 nm due to the second and third orders of the 225 nm excitation wavelength. If these artifacts become troublesome then judicious use of filters can usually remove them. 

Modern conventional laser-Raman systems for recording spontaneous Raman spectra comprise essentially an appropriate source, some dispersing and collection optics and a sensitive detector. The two most common approaches are either single-channel detection using a photomultiplier in combination with a multi (double or triple) monochromator set-up, or a multi channel option with an array detector coupled to a spectrograph [12, 20, 21]. The recent commercial development of FT-Raman systems, either as standalone spectrometers or as FTIR adjuncts, has added to the armoury of techniques, and without doubt offers a cost-effective, rapid, readily applicable technique for a wide range of polymer studies [22-24]. 

The previous discussions of the signal are nicely illustrated by an extremely simple model analysis using real fields and signals for two Lorenzian resonances at frequencies a and b. The sample is irradiated with two very short pulses whose spectra are flat. The real generated field from the sample is the real part of Eq. (21) or Eq. (33) with T set equal to zero for convenience since is in any case a multiplicative factor. In time-domain interferometry, this is measured directly along the indicated time axes as described above. In spectral interferometry the real generated field along with a real local oscillator field, delayed by time d, is dispersed (i.e., Fourier-transformed) by a monochromator, then squared by the detection to yield a spectrum on the array detector at each value of t ... 

Fig. 4.1. Schematic of an experimental set-up for absorption measurements at low temperature incorporating a Perkin-Elmer Model 99G monochromator. Si, S2 and S3 are IR sources selectable with plane mirrors Mi and M2. FM focusing spherical mirrors. Ei and E2 entrance and exit slits. CM off-axis paraboloid collimating mirror. G plane reflection grating. Beam 1 from Si is converted by CM into a parallel beam dispersed by G. One wavelength is diffracted in a direction where it can be intercepted by first mirror M as beam 2 and focused on the internal chopper Ch. Modulated beam 2 is redirected toward G as beam 3 and re-dispersed a second time as beam 4. Beam 4 intercepted by IM is focused on E2 and re-focused on the sample by FM. The divergent monochromatic beam is finally focused on thermocouple D by ellipsoidal mirror EFM. Fi, F2 and Pol are locations for transmission filters and a polarizer. Beam 1 can be blocked by shutter Sh (after [37]). With permission from the Institute of Physics...

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