Some vacuum aspects of vacuum ultraviolet spectroscopy

Some vacuum aspects Of vacuum ultraviolet spectroscopy B Raz, J Magen and J Jortner,

Departmentof Chemistry, TeI-AvivUniversity, TeI-Aviv,Israel

Experimental problems in the near vacuum ultraviolet (1900 A-1100 A) spectra/region will be reviewed. Vacuum requirements in this region will be considered with an empha~sis on low temperature spectroscopic measurements. Vacuum aspects will also be considered in discussing continuous light sources for this spectra/region. A cell for absorption spectroscopy in simple liquids and solids will be described, and some applications for spectroscopy of dense, disordered systems will be presented. Spectroscopic studies in the vacuum ultraviolet region provide valuable information on extravalence excitations in atoms and molecules, on ionization autoionization and predissociation processes in atomic and molecular systems and on the electronic structure of insulators such as the rare gas solids. The spectral region of vacuum ultraviolet lies between 1900A and 100~, the lower wavelength limit being arbitrary. A further sub-division is generally made: (a) The "conventional" vacuum ultraviolet region. 1900~1100A is the spectral region in which single crystal windows may be employed. In this range it is possible to work in closed cells or with solids deposited on a cooled window. Co) The "windowless" vacuum ultraviolet region. In the region of shorter wavelengths, no conventional window is transparent. Recently, windows of a very thin metal layer are being used in this region, but these have very little mechanical strength. V a c u u m aspects

tion damage, these products tend to adhere to optical elements in the spectrograph and gradually deteriorate their performance. An oil vapour trap such as molecular sieve roughing line traps, is highly recommended to eliminate these undesirable effects. CO) In some types of measurements, the spectrograph is filled with examined gas. In such a case, desorption of interfering gases should be maintained at a very low level. This is generally attained by pumping to a far lower pressure than the working pressure.

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Table 1 and Figures 1-3 summarize the necessary data for the evaluation of the maximum permissible pressure at room temperature to make possible spectroscopic measurements in the vacuum ultraviolet region. 0,5

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It is evident from these data that oxygen is responsible for most of the absorption in room temperature measurements. The pressures with which one could contend oneself are no lower than 10-t torr, in a l-meter optical path length setup. The above conclusion is somewhat oversimplified for two reasons: (a) There is an accumulation of degradation products of oil vapours, which are originated from the pump and suffer radia-

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:Figure 2. U V absorption spectrum of gaseous water. (Jarrel Ash-Technical Report).

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Figure 4. Absorption spectrum of deposit outgassed onto Li window at 77°K from vacuum system at 10-6 torr. Vacuum requirements do change drastically when low temperature vacuum ultraviolet experiments have to be performed. Vapour pressures of many interfering gases and primarily water are extremely low at 4°K and even 77°K. Cooling the sample results in its becoming a cold trap for condensible gases, a fact that has a considerable bearing upon maximum permissible pressure level in such experiments. To demonstrate this point, Figure 4 shows the absorption spectrum of an impurity deposited on a window cooled to 77°K when the spectrograph was operated at the pressure of 10 -6 mm HR. This deposit is due to ice resulting from desorption of water vapour. Using a Veeco 4 inch diffusion pumping unit we had to maintain pressures well below 1 × 10 -6 tort in order to obtain clear windows at 77°K. Normally, the working pressure did not exceed 2 × 10 -~ torr over the liquid nitrogen cold trap. In Figure 5 a photograph of the vacuum system is presented. Special stress has been placed on obtaining lowest pressure around the low temperature absorption cell. Light sources Continuous stable light sources are rather rare in the vacuum ultraviolet region. A very thorough discussion of light sources is presented in Reference 2. Synchrotron radiation provides a very bright and continuous 572

Figure 5. Pumping system of monochromator and cryostat. light source 1, however, it cannot be considered a standard light source for obvious reasons. The "sliding spark" source a,4, laser induced black body radiation from a plasma 5 and noble gas discharge lamps e are more suited for conventional work. Each of these lamps is based on spark operation and the yields of the sparks vary with respect to the light intensity, as well as to the spectral distribution. As a result, integration of some sort is necessary, and the most common mode of integration is that of photography. Due to the non linear dependence of the response of a photographic plate to the light intensity, this method is somewhat unreliable when line shapes and peak energies of very broad absorption bands are investigated. In the alternative method of photoelectric detection, integration times are much lower (generally up to 10 see). Therefore, high repetition rate of the discharge is of great importance, and the light source to stand up to the requirement is, at present, that of the Tanaka discharge lamps in rare gases1. Repetition rate in such a source is in the order of magnitude of 1 kc. Figures 6 and 7 represent our version of a rare gas discharge lamp which is used in our laboratory. The water-cooled inner glass tube extends just over the forward electrode so as to reduce the effect of sputtering of electrode material on the LiF window. The gas used in our lamp is Matheson research grade (total impurities less than 1 ppm). The electrodes were constructed of aluminium 99.999 per cent pure. Figures 8 and 9 represent typical spectra of Argon and Krypton lamps operated by us. It was found that vacuum level and specifically the degree of degassing has a marked effect on the sputtering rate as well as the "cleanliness" of the resulting spectrum. Baking the lamp at 4500C at 5 x 10 -e torr for at least 48 hr eliminates all the emission lines which could be detected 140~

Figure 6. Rare gas discharge lamp present work version.

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B Raz, d Magen and J Jortner: Some vacuum aspects of vacuum ultraviolet spectroscopy spectroscopic c.¢lls, especially at low temperatures. Fragility of window dictates a scaling method which equilibrates mechanical stresses. High coefficient pf thermal expansion (37 x 10 -e) makes dynamic thermal expansion compensation imperative. In the compensation method we have chosen, use is made of a disc-like spring. The choice of spring force constant is somewhat delicate. An optimum should be sought between the obvious necessity of having a spring with sufficiently high force constant to push the window against the sealing material (an indium ring in our case) and the painful revelation that the use of too strong a spring makes homogeneous dissipation of mechanical stress practically impossible, resulting in the breaking of the window. Figure 10 shows a schematic representation of the cell and its

Figure 7. Rare gas discharge lamp, photographed with upper part of Faraday cage removed.

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Figure 8. Argon lamp emission continuum. in previously published spectra'. The only emission lines which we were unable to remove are the a-Lyman line of hydrogen in the case of the Ar lamp and the resonance line of Xe impurity (about 5 ppm) in the Kx lamp.

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Windows and spectroscopic cells Lithium fluoride crystals of the thickness of 1 mm are transparent up to 1050k and thus constitute a proper window material for the "conventional" vacuum ultraviolet region. These windows have many thermal and mechanical drawbacks which have to be compensated before using them in

Figure 10. Expanded view of absorption cell.

573

B Raz J Magen and d dortner: Some vacuum aspects of vacuum ultraviolet spectroscopy ~ I

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scaling components. The most important features are: (a) Disc spring compensation. (b) Design eliminating any unequilibrated mechanical stress. Figure 11 shows the result of a spectroscopic study of Xe doped 1 cm thick solid krypton crystal. The solid was prepared by cooling the doped liquid rare gas in a closed cell under the pressure of 5 arm. The physical motivations for this study seem to fall out of the scope of this article. The spectroscopic technique, however, unites all of the above-mentioned elements. The light source used was a line-free Krypton continuum. Vacuum conditions inside and out of the spectroscopic cell were such that solidification could be achieved at a very slow rate without any interference from foreign condensable impurities. The above result is in very good agreement with previous data obtained from very thin crystalline solids (thickness about 2000A) 7.

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References x K Codling and R P Madden, J Appl Phys, 36, 1965, 380. z j A R Samson, Techniques of Vacuum Ultraviolet Spectroscopy, John Wiley (1967). a G Ballofet, Am Phys, 5, 1960, 1256. 4 H Damany, J Y Roncin and N Damany-Atoin, Appl Optics, 5, 1966, 297. s A W Ehler and G L Weissler, Appl Phys Letts, 8, 1966, 89. e y Tanaka, J Opt Soc Am, 45, 1955, 710. 7 R E Huffman, J C Larrabee and Y Tanaka, Appl Opt, 4, 1965, 1581. s G Baldini, Phys Rev, 137, 1965, A508.