Stray radiation, effect

Figure 11.24. Experimental arrangement used by Ernst and Kindt [44] in their pump/probe microwave/optical double resonance study of a rotational transition (18.2 GHz) in the ground state of CaCl. The photomultiplier tubes which monitor fluorescence are situated on the axis perpendicular to both the laser beam and the molecular beam. The C region, where the molecular beam is exposed to microwave radiation, is magnetically shielded to minimise stray Zeeman effects. The microwave power was amplitude modulated at 160 Hz and the modulated fluorescence detected by photomultiplier B. 

Figure 1. The effect of near field stray radiation on the spectral line profile (546-nm Hg emission line). The detector was a microchannel-plate image intensified diode array (Reticon model RL-5I2SF, Princeton Instruments, Inc. model IRY-512). Key a, full line profile b, upper half (FWHM)ofline andc, lower portion of line, 0-10% relative emission...

Analogously to spectrophotometry, stray radiation can alter the measured turbidance (Eq. 4.6), the limitation becoming more severe at lower transmitted power. The presence of suspended matter in the processed sample is inherent to turbidimetry and leads to an amplification of stray light due to scattering effects. This effect has not been systematically investigated in flow analysis. 

Photomultiplier tubes have also been developed with response limited to the ultraviolet region (160 to 320 nm), the so-called solar-blind photomultipliers. They are helpful in reducing stray light effects from visible radiation and are useful as UV detectors in nondispersive systems. 

The scattered radiation, however, is often made up of wavelengths to which the instrument is highly sensitive. Thus, the effects of stray radiation can be greatly enhanced. Indeed, in some instances the output signal produced by the stray radiation may exceed that produced by the monochromator output beam. In such cases, the component of the measured transmittance due to the stray radiation may be as large as or exceed the true transmittance. 

The richness of spectral features enhances the probability of overlapping absorption bands. In addition, with older dispersive instruments, the narrowness of bands and the effects of stray radiation make absorbance measurements critically dependent on the slit width and the wavelength setting. Finally, the narrow-path-length cells required for many analyses are inconvenient to use and may lead to significant analytical uncertainties. For these reasons, the analytical errors associated with a quantitative IR analysis often cannot be reduced to the level associated with ultraviolet and visible methods, even with considerable care and effort. 

In some instances encounter between accumulated masses of matter and antimatter across the interface releases enormous quantities of energy by mutual annihilation. As a result of such activity intergalactic space is saturated with stray radiation, plasma and electromagnetic fields. The visible effects of such activity are difficult to interpret and often highly confusing because of multiple images created by complicated involution of space-time. 

The above characteristics make these systems very promising for radiation effect studies and in particular for metrological applications such as the measurement of the Rydberg constant directly in frequency units. One can indeed expect very narrow resonances between circular states, with spectral lines only quadratically sensitive to stray electric fields and frequencies depending only slightly upon the atomic ion core properties and being easily related to the hydrogen frequencies via the determination of very small quantum defects corrections. 

McCall, S. H. C. P., Pierre, J. E., Mabee, A., Tennyson, R., Morison, D., Kleiman, J., Iskanderova, Z., and Gudimenko, J. (1994). The Atomic Oxygen Exposure Effects Module of the Database for the Properties of Black, White, Reflective and Transmissive Spectrally Selective Surfaces, SPIE Proceedings, Vol. 2260. Presented at the SPIE Conference, Stray Radiation in Optical Systems II, July 24-29, San Diego. 

The effects of scattered radiation will show up as false intensity data, usually a decrease in the peak intensity, and the accuracy of the results will obviously suffer. As stated in Chapter 2 (Section 2.2B), the intensity of the scattered radiation can be measured and a correction factor applied to the data. If this is done, the data should be redetermined and the factor recalculated periodically. It is better to eliminate or reduce an error than to correct for it a short-wavelength filter placed in the sample beam will cut down on the stray radiation. 

Imperfections in monochromators result in the presence of a small proportion of unwanted wavelengths in the incident radiation. Such stray light results in a deviation from a Beer-Lambert relationship (Figure 2.14) and the effect is that absorbance measurements are lower than they should be. 

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