Absorption didymium

Marignac, Lecoq de Boisbaudran, Cleve, and Bohuslav Brauner all believed didymium to be a mixture of elements, but none of them were able to make the difficult separation (49). In 1882 Professor Brauner of the University of Prague examined some of his didymia fractions with the spectroscope and found a group of absorption bands in the blue region (A=449-443) and another in the yellow (A.=590-568) (53, 66)." These two groups of bands are now known to belong to two earths, praseodymia and neodymia, respectively, which Baron Auer von Welsbach obtained in 1885 by splitting didymia (3, 30, 32, 58). 

With broader-bandpass instruments, a didymium filter may be used to verify wavelength settings. This filter should show a minimum percent transmittance at 530 nm against an air blank (Figure 3-11). Because didymium has several absorption peaks, the setting should be verified grossly by... 

As in the case of dispersive Raman spectrometers (cf Section 4.4.1), it is necessary to calibrate the wavelength scale of dispersive UV/VIS spectrometers. The most accurate standards for checking the UV/VIS wavelengths are lasers of various types. The inexpensive helium-neon laser can be used to check at 632.8 nm. For spectrometers with a deuterium source, spectral lines at 486.6 and 656.1 nm can be used for calibration. A common method for wavelength calibration is the use of optical filters. A filter of didymium glass has many sharp absorption peaks, which can be used as a second wavelength standard (precision within 0.5 nm). 

In 1878 Delafontaine made an important observation. He isolated didymium both from cerite and from the mineral samarskite . Absorption spectra obtained during examination of the two didymium samples were different. To Delafontaine this was an indication that didymium was not a homogenous element This interested Bois-baudran in France. Unlike Delafontaine he used emission and not absorption spectroscopy. He found Hnes showing the presence of a previously unknown element In 1879 he announced the discovery with the information that its name was samarium after the mineral. 

Fig. 14.5 A variety of typical filters, a An interference bandpass filter showing the typical mirrored surface of these types of filter b two square silvered glass neutral density filters and a round neutral density filter of a colloidal dispersion in glass c a series of round, coloured absorption cut-off filters, and d a pale blue glass heat filter e a Wratten filter of dyed gelatin, which is flexible and easily cut to shape f a polariser g a didymium absorption wavelength standard. The 5p UK coin, included top left to give some idea of scale, has a diameter of 18 mm...

Slit width effect is treated by Gibson in Mellon s book which also shows visible spectra of a didymium glass taken at nominal slit widths equivalent to 4 and 8 mu of spectrum, pointing out both differences in recorded transmittance and shifts of wavelengths of maximum absorption. 

Although it was easy to see that the depth with which one could probe into the absorption bands above was limited by the thickness of the stylus employed, it is less obvious that the wavelength at which a band occurs is also slit-width dependent. Repeating Gibson s work by running a didymium glass at two widely different slit widths is a convincing experiment. 

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