Short-wavelength cut-off

Figure 8.10 The intensity of the continuum radiation from an X-ray tube and the short wavelength cut-off vary as the applied voltage varies. This plot is of a tungsten X-ray tube.

Absorptions in the UV/VIS are assodated with electronic transitions from the ground level to an excited state. The strongest transitions are a —> a, they are observed below 200 run (vacuum UV). Typical examples are C—C and C—H bonds. Because aU molecules indude a electrons, a a transitions constitute the short-wavelength cut-off of the routine UV/VIS range. Saturated compound with pairs of free electrons exhibit n —> a transitions in a wavelength range from 150 to 250 run, their absorption coefBdents do not exceed 1000 1 cm moU. ... 

Samples are prepared as described in the preceding subchapters, mostly by dilution with a suitable solvent. The appHcation range of solvents is given by their short wavelength cut-off (Tab. 6.5). 

These intramolecular transitions result in the characteristic intense bands of unsaturated organic compounds, observable in the usual solvent range of A > 200 nm. They also produce the short wavelength cut-off of solvents with a tt electron system. For example the intense band centred around 198 nm in DMF and DMA is considered to consist of 71 7T and tt —> tt transitions. ... 

Including diffusion of the activator in addition to the convective term, apparently brings about the short-wavelength cut-off in the dispersion relations and the appearance of a critical velocity (below which no instability is possible), similar to the case considered above (Sections 2.2.1 and 2.2.3). For this reason it may be difficult to use the dispersion relations to distinguish the two differential flow instabilities. 

Solution filters. They are both short-wavelength cut-off or wide band-pass filters (a and c in Fig. 4.2), mainly constituted by aqueous solutions of inorganic compounds. Several of them, which cover the wavelength interval between 200 and 650 nm, have been described in detail in [2] and references therein. It is however to be noted that some of these filters may undergo thermal or photochemical reactions, so that a periodic control of their absorbance is recommended. 

The absorption coefficient of silicon is shown in Figure 11. Considering this curve it can be seen that the short and long wavelength cut off points of the cells spectral response are determined by two factors. 

The near UV spectral range is divided in three bands the short wave band or UV-C emission runs from 180 to 280 nm with a peak at 254 nm. The medium wave band or UV-B runs from 280 to 320 nm with a peak at 312 nm, while the UV-A long wave band runs from 320 to 380 nm with a peak at 365 nm. The long wave emission is commonly called "Black light" or "Wood s light". Only the UV-A portion of the lamp spectrum is useful for UV reflectivity measurements, since wavelengths under 400 nm are cut off by the glass optics of the objective. However, the full power of the lamp is used for UV fluorescence measurements. 

Irradiating at rather short wavelengths (approx. 300 nm) is essential for realizing the unique regioselectivity. When short wavelengths (below 325 nm) were cut off, the weakest C-H bonds were carbonylated methylene in alkanes and benzylic methyl in toluene (Scheme 7). 

Figure 5. Different methods used for efficient generation of short-wavelength radiation down to the LiF cut-off (From Ref. [41]).

The calculated spectrum (see chapter 4) is modified at the long wavelength end by absorption due to beam line windows or any airpaths in the Laue camera. Also, the short wavelengths are cut off if a reflecting mirror is used (and this may be the case for focussing or to aid more precise definition of spot multiplicity). 

---