Review Paper

Vacuum Ultraviolet Spectroscopy

Giulio Milazzo and Gaetano Cecchetti Istituto Superiore di SanitY, Rome, Italy (Received 25 November 1968)

A review is presented on vuv spectroscopy, discussing the optics necessary, instrumentation used iu absorption and emission vuv, and measurement of wavelength and intensities. Finally, several applications of the technique of interest to the chemist are presented. INDEX I-[EADING : Vacuum ultraviolet spectroscopy ; Applications ; l~eview.

INTRODUCTION

In 1893, Schumann built the first spectrograph suitable for investigt~ting the region beyond about 2100 A. The region was named the Schumann region, after its discoverer, and is also called the vacuum ultraviolet (vuv). I t is located between about 2100 A and the limit of soft x rays ( ~ 1 0 A). The delay in investigating this region was due to the atmospheric oxygen which absorbs nearly all radiat ion beyond 2100 A. I t is necessary therefore to carry out the in- vestigation in vacuum or, in some cases, in one of the few gases t ransparent in the region.

Although many impor tant chemical problems may be studied by vuv, the method had been neglected by chemists. The principal reason was due to the lack of availability of commercial instrumentat ion. Anyone interested in the method had to construct his own instrument, and this required him to some extent, to master optics and mechanics.

I. OPTICS

Figures 1 and 2 show one of the first instruments constructed in 1931 by Carlo and Schmit t -Ot t . 1 By comparision of this ins t rument with following ones, technical difficulties prevailing at tha t t ime for in- vestigating this spectral region, may be realized. The spectrograph was a classical-type instrument, based on fluorite optics (prism and lenses), and could only be used down to 1200 A (limit of fluorite trans-

parency). Later developments in optics, and in par- ticular, in the .construction of diffraction gratings (concave and plane), dramatically changed vuv in- s trumentat ion. Today, a choice between several com- mercial spectrographs and scanning spectrometers (monochromators) is available. In contrast to the small size of the first apparatus, Fig. 3 shows the giant 10.7-m Jarrel l-Ash spectrograph using a concave grating in an Eagle mount (this type of mount is described and illustrated in Fig. 6).

Diffraction gratings have largely replaced the prisms in modern-day instrumentat ion. The plane grating re- quires concave mirrors for focusing, while the concave grating is self-focusing and does not require ancillary optics. The simplest mount is the normal incidence mount, utilizing a concave grating and is illustrated irl Fig. 4. Figure 5 shows a normal incidence spectrograph (McPherson model 240). A derivation of this mount is the out-of-plane Eagle s mount (Figs. 3 and 6).. This mount is very useful because of its compactness. The diffracted spectrum in the Eagle mount is focused al- most to the same point on the Rowland circle as the entrance slit. Interferences are avoided by placing the slit somewhat below the plane perpendicular to the grooves of the grating and containing the center of the concave grating. The spectrum is then focused sym- metrically over this plane.

A more sophisticated mount using a concave grating with parallel light is the Wadsworth mount, 3 which requires one concave mirror (see Fig. 7). This mount

FIG. 1. Carlo and Schmitt-Ott spectrograph, general view.

Volume 23, Number 3, 1969

FIG. 2. Carlo and Schmitt-Ott spectrograph: optics.

APPLIED SPECTROSCOPY 197

FiG. 3. A 10.7-m vacmtm spectrograph, Eagle moun t (courtesy Ja r re l -Ash Co.).

FIO. 4. Normal incidence moun t : G = concave grating, 0 = center of the l~owland circle, N = normal to the grating, P = plate holder, S = ent rance slit.

is not generally used in vuv, and represents, in a sense, the transit ion from concave grating mounts to the plane grating mounts. The la t ter use parallel light beams, and thus require two concave reflecting surfaces for focusing (concave mirrors). The E b e r t - Fastie 4 (Fig. 8) and the Czerny-Turne r 5 mounts (Fig. 9) are examples of plane grating mounts. Figure 10 shows a Cze rny-Turne r mount. Both of these mounts have a focal plane, and can be used, with ap- propriate accessories, as a scanning monoehromator , a spectrograph, or as a direct-reading ins t rument (see Fig. 11). A special mount part icularly suited for use as a monochromator is the Seya-Namioka, 6 which utilizes the unique properties of a fixed 70 ° angle of diffraction (see Figs. 12 and 13).

For very short wavelengths, the use of the normal (or nearly normal) incidence radiation is no longer

FIG. 5. Normal incidence spec t rograph (courtesy Mc Pherson Company) .

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FIG. 6. Eagle m o u n t : G = c o n c a v e grating, O = c e n t e r of the Rowland circle, N = n o r m a l to the grating, R = R o w l a n d circle, P =plate~holder, S = e n t r a n c e slit.

\ P

Fio. 7. Wadswor th moun t : G = c o n c a v e grating, N = n o r m a l to the concave grating, P = p l a t e holder, M = concave mirror, S = en- t rance slit.

advantageous because of a large decrease in reflective power. In this case the grazing incidence principle has been found useful. ~ This is i l lustrated in Fig. 14. The limit of use for normal or nearly normal incidence is ~300 A, whereas with grazing incidence one can work to ~ 1 0 A.

II. MEASUREMENTS OF WAVELENGTH AND INTENSITIES

As in other regions, it is necessary to measure the wavelength and the intensi ty of the lines and bands in the vuv spectra. With monochromators, the wave- length focused on the exit slit is given by the instru- mental and grating's constants and by the grating's position. I t is usually read directly on a scale fixed on the instrument. With a spectrograph, a wavelength atlas and tables are needed. They are available today, s,9 Figure 15 shows a table of the spectrum of aluminum obtained using hollow-cathode discharge excitation. Similar tables for C, Cu, Hg, Fe, Ge, Si, and H2 are also available.

Intensities may be measured by photographic pho tomet ry or photometrically. The usual photo- graphic emulsion used in spectroscopy is useless

198 Volume 23, Number 3, 1969

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FiG. 8. Ebert-Fastie mount: S~=entrance slit, M=coneave mirror, G =plane grating, N = normal to the grating, S~ ~ exit slit.

Fro. 9. Czerny-Turner mount: S~=entrance slit, M~ and M2=concave mirrors, G=plane grating, N=normal to the grating, S.~ = exit slit.

because the gelatine absorbs practically all the radia- tions at ~<2200 A. Special emulsions are necessary, like those first prepared by Schumann, and which con- sisted of AgBr grains at the surface of a very thin layer of gelatine film. Plates and films coated with this kind of emulsion are now commercially available from American, ~° European, u and Russian 12 manufacturers. The vuv emulsions are of poorer quality than the common spectral emulsions used in the near uv or visible region. Further, they show poor constancy of the sensitivity on different points of the surface. On the other hand, this is somewhat compensated because of their higher contrast factor (for the Kodak SWR film, ~> 2). In explorative work, emulsions may be used after sensitization with sodium salicylate or another suitable fluorescent material. Using sodium salicylate this is easily done by dipping the photo- graphic plate or film in a methanol solution of the salt (about 2%), and drying the adhering solution~for a

FIG. 10. Czerny-Turner spectrometer (courtesy Jarrel-Ash Company).

few seconds in the dark room before use. In this case no desensitizing bath is needed prior to development. Emulsions coated with fluorescent lacquer are also commercially available. TM With monochromators, the most common way for measuring intensities involves a good commercial photomultiplier externally coated with a thin film ( ~ 1 mg/cm 2) of sodium salicylate. For very short wavelengths, a windowless photo- multiplier may be employed, which uses special sensi- tive materials. These often have the added advantage of being insensitive to the longer wavelength. No order sorter is thus required for eliminating diffracted radi- ations of longer wavelengths, when a higher order for the desired short wavelength radiation is used. A variety of other detectors and devices for measuring relative and absolute intensities has been described, 14 but few have been extensively used.

The necessary equipment for an efficient vacuum is well known today and need not be described in this paper.

I I I . APPLICATIONS

Two areas of investigation in vuv exist : absorption and emission. Additional equipment is needed for absorption work; i.e., a light source which produces an extended continuous spectrum. The ideal light source producing a continuous spectrum over the whole vuv is nonexistent. The only continuous emitting light sources are those producing the Lyman continuum, 15 the rare gas discharge lamps (including hydrogen), and that described by Damany et al . ~6 The Lyman continuum is produced by a highly con-

FIG. 11. Accessories for Czerny-Turner spec- trometer-spectrograph (courtesy McPherson Company).

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APPLIED SPECTROSCOPY 199

$2 Fro. 12. Seya-Namioka mount: Sl=entrance slit, G = concave grating, O = cen- ter of the Rowland circle, N,N'=normal to the grating (two positions of the grating), R=Rowland circle, S2=exit slit.

densed flash discharge ( ~ 0 . 5 - 1 uF) through a small diameter tube containing a gas at low pressure. The most impor tan t condition for obtaining the Lyman cont inuum is an instantaneous current density of 30 000-50 000 A/cm'-'. The lamp produces a continuous spectrum from the visible to 900 A, regardless of the chemical nature of the gas. Thes imul taneous require- ment of high tensions (Editor 's Note : The author prefers to use tension ra ther than voltage) and high capacitances are disadvantages. A practical discharge tube for producing this type of cont inuum was de- signed by G a r t o n Y The rare gas or hydrogen continua are emit ted after excitation by a continuous electric discharge, usually ac at moderately high tension, through the gas under moderately low pressure. A typical design of such a source is i l lustrated in Fig. 16. This lamp operated with hydrogen is at 2 mm Hg pressure and about 1000 V, produces a good cont inuum from the visible to 1700 A, and is able to support a current intensi ty as high as 1 A. A lamp of similar construction filled with one of the rare gases (He, Ne, Ar, or Kr) at appreciate pressure (up to 100 mm of Hg) and with appropriate electrical feeding emits

FIG. 13. Seya-Namioka monochromator (courtesy McPherson Company).

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FIG. 14. Grazing incidence mount: S = entrance slit, G = grating, P =plate holder.

spectra having each a continuous region that may reach 750/~, each continuum overlapping one another, depending on the rare gas used. These sources were developed by Huffman and Tanaka et al. 19 Another very powerful source was recently described by Damany et al., 16 and is based on a discharge (initiated by a sliding spark discharge), which uses a metal of high atomic number like uranium as the anodic material. A very strong continuum is emitted in the region near the anodic electrode, and the source is well suited for investigating the absorption spectra of gases or solids. Figure 17 illustrates the apparatus used for gases and Fig. 18 shows the apparatus used for solids, mono- or polycrystalline materials deposited on thin t ransparent plates. 2° Both devices use LiF windows, which are t ransparent to 1100 A. At shorter wavelengths, windowless equipment is necessary. The investigation of liquids in vuv is difficult for the following reasons: (1) difficulties in obtaining suffi- ciently thin layers; and (2) difficulties in the con- struction of cells using t ransparent material. Figure 19 illustrates a simple device 21 which operates at atmo- spheric pressure using a gas tha t is t ransparent , and high-quality silica cells ( t ransparent to ~ 1600 A).

The results obtained in vuv absorption spectros- copy are similar to those obtained in the visible or near uv. Invest igat ion of substances not absorbing in the near uv is possible, and observation of more than one electronic transit ion can be made, allowing one to make a bet ter comparison of experiment with theory22 In particular, the determinat ion of the position of the lines of the Rydberg series for various molecules and therefore of the convergence limit giving the ionization potential is possible by vuv more precisely than by other experimental techniques (e.g., electron impact).

In emission, the first application of vuv was the investigation of the line spectra of elements to de- termine the high energy states of the atoms and ions. The most promising applications to the chemist are in the area of spectrochemical analyses. A number of elements (mainly nonmetals, like P, C, S, As, and halogens) cannot be analyzed by the usual techniques because of their lack of sensitive lines in the visible or near uv, and yet the analysis of these elements is very important , part icularly in metallurgy. This prob- lem has been solved in the past with varying degrees of success in three ways : (1) the classical technique using

200 Volume 23, Number 3, 1969

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FiG. 15. Atomic spectra from 2250-1100 ~ (A1) : Table of the aluminum spectrum in the vuv from 2250-1100 £; excitation by hollow- cathode discharge. (Instituto Superiore di SanitS., Roma. Specola Vaticana: Laboratorio Astrofisico.)

a v a c u u m spec t romete r wi th a convent iona l spark opera ted in an a rgon a tmosphere . A typ ica l ins t ru- m e n t of this t y p e is the Quan tovac , m a n u f a c t u r e d by A R L , and consist ing of a d i rec t - reading spec t romete r wi th a n u m b e r of fixed exit slits, each equipped wi th an appropr ia te photomul t ip l ier . This t y p e of appa- ra tus gave C, P, and S analysis in the range of 0 . 0 0 5 % - 5 % C, 0 . 0 0 5 % - 1 % P, 0 . 0 0 5 % - 0 . 2 % S. i typ ica l indica t ion of the reliabil i ty of this t echnique is given by Belder, 23 who ment ions a mean square error of 0 .0012% for a C con ten t of 0 .32%, 0 .0007% for a P con ten t of 0 .021%, and 0 .0006% for a S con ten t of 0 .032%. Similar results have been ob ta ined by o ther

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FIG. 17. Experimental setup for investigating absorption spectra of vapors and gases: (1) connection with the substance reservoir for introduction of the sample; (2) connection with the pumping system; (3) thermostated reservoir at higher temperature; (4) connection with the vapor pressure-measuring device ; (5) thermo- stated reservoir at lower temperature; (6) connection with pumps ; (7) connection with the spectrograph; (8) connecting piece between spectrograph and absorption tube; (9) absorption tube of definite length; (10) LiF window; (11) connection with the continuous spectral source; (12) connection with atmosphere.

APPLIED SPECTROSCOPY 201

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Fro. 18. Experimental setup for investigating absorption spectra of thin solid films: (1) connection with a pressure-measuring device; (2) reservoir for liquid nitrogen; (3) connection with the pumping system; (4) connection with the atmosphere; (5) connection with the spectrograph; (6) and (9) LiF windows; (7) glass piece for the solid sample; (8) refrigerated metal block supporting the solid film (eventually deposited on another transparent window); (10) connection with the continuous spectral source.

invest igators ; (2) by the method of Vodar et al2 4 Vodar employed a sliding spark to avoid the l imita- tions imposed by the operat ion of the spark in a gase- ous a tmosphere and the difficulties of using a high v a c u u m spark (disruptive firing, nonreproducibil i ty, and high tensions). He found tha t if a semiconductor or an insulating mater ia l is used to hold the electrodes

(instead of separat ing t hem by high vacuum) a less disrupt ive spark is fired, requiring lower tension and having improved reproducibil i ty. The light source described by D a m a n y et al. 16 for the product ion of

• the L y m a n cont inuum can be employed for analyt ical purposes using the light emi t ted somewhat far f rom the t ip of the electrodes. A windowless ins t rument is necessary in this case. The spectra of highly ionized a toms are emi t ted in these sources. I n some cases, under appropr ia te exper imental conditions, the spect ra of a toms possessing only one electron in their outer shell, and having t rue s-p t ransi t ions are emit ted. The authors claim tha t these lines are the mos t sensitive lines found for each element and therefore are suitable for spectrochemical analysis. A sophisti- cated, direct-reading ins t rument was described by Vodar and his associates for quan t i t a t ive analysis giving working curves t ha t are quite linear. T h e sca t ter of results for repeated analyses is of the order of several percent. The order of magni tude of precision and accuracy of spectrochemical and chemical analysis is about the same; (3) the method of Milazzo et al. ~5

uses a hollow cathode as the light source. This special type of gas discharge was discovered by Paschen a half century ago, bu t was neglected by chemists until recently. I t is not the aim of this review to discuss the theory of hollow-cathode discharge. However , several of its characterist ics m a y be ment ioned : (1) The spec- t r um emit ted by this l ight source is simpler than those emit ted by conventional arc or spark ; (2) the electric

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Fzo. 19. Experimental setup for investigating liquids and solutions in a transparent atmosphere : (1) cover; (2) rotating axis; (3) standard- taper conical guide; (4) exit of the transparent gas; (5) operating crown with stops for rotating the cells; (6) stopping index; (7) inspecting windows; (8) pentagonal supporting plates; (9) cells; (10) cylindrical metallic container; (11) rings for fastening the windows; (12) LiF window; (13) standard-taper glass connection; (14) glass-metal sealing material; (15) entrance of transparent gas.

202 Volume 23, Number 3, 1969

FIG. 20. Hollow-cathode source: (1) standard conical fitting, male, shaft bearing part; (2) standard conical fitting, female part; (3) carrier-gas outlet; (4) cooling-water outlet; (5) cathodic block; (6) hollow cathode; (7) and (8) channels for carrier gas; (9) sealing O-ring; (10) connection ring, cathodic side; (11) insulating glass rings; (12) insulating quartz disk; (13) connection ring, anodic side; (14) anodic block; (15) carrier-gas inlet; (16) standard conical connection with the spectrograph; (17) LiF lens (if desired); (18) vacuum-tight closing LiF window; (19) fastening ring for (18); (20) protecting LiF window; (21) cylindrical fastening for (20); (22) cooling-water inlet; (23) extracting hole for (21).

field within the hollow cathode, (i.e., the space where the material is excited) is negligible, and the tempera- tures produced are much lower than in the arc or spark method. As a consequence, Stark and Doppler effects are negligibly small, and the lines are sharp, and back- ground free ; (3) the escape of materials being analyzed can be, to some extent, prevented and so atoms can be excited many times, thus improving the sensit ivity; (4) the operation at low pressure with rare gas makes this lamp part icularly suited for obtaining atomic spectra in the vuv. Figures 20 and 21 show the lamp designed and used by one of the present authors? 5

The electrical input is made by a d c stabilized generator giving a peak electrical tension of about 1000 V for firing. I t works between 100-300 V and can be operated up to 600 mA. An example of the results obtained with this lamp are those of iodine impuri ty in a 1 mg KC1 matrix. 25 Exci tat ion was made in an aluminum hollow cathode and the intense lines at 1830.4 and 1782.4 A were used for analysis. The minimum detectable quant i ty of iodine under the

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given experimental conditions was found to be 0.01 ~g, and a quant i ta t ive analysis was found possible be- tween 10 -6 to 3 0 -4 g iodine.

1. G. Carlo and H. D. Schmidt-Ott, Z. Physik 69, 719 (1931). 2. A. Eagle, Astrophys. J. 31, 120 (1910); Proc. Phys. Soc. 23,

233 (1911). 3. F. L. O. Wadsworth, Astrophys. J. 3, 54 (1896). 4. H. Ebert, Wied. Ann. 38, 489 (1889); W. G. Fastie, J. Opt.

Soc. Am. 42, 641, 647 (1952). 5. M. Czerny and A. F. Turner, Z. Physik 61, 792 (1930). 6. M. Seya, Sci. Light (Tokyo) 2, 8 (1952); T. Namioka, Sci.

Light (Tokyo) 3, 15 (1954) ; J. Opt. Soc. Am. 49, 951 (1959). 7. See for example, J. A. R. Samson, Techniques of Vacuum

Ultraviolet Spectroscopy (J. Wiley & Sons, Inc., New York, 1967), p. 48.

8. J. Junkes, E. Salpeter, and G. Milazzo, Atomic Spectra in the Vacuum Ultraviolet from 2250 to 1100 ~ (Specola Vaticana, Citta del Vaticano, 1965).

9. R. L. Kelly, Vacuum Ultraviolet Emission Lines, University of California Radiation Laboratory, Rept. UCRL-5612, (1959); A. N. Zaldel, V. K. Prokofev, and S. M. Rafskii, Tables of Spectrum Lines (VEB Verlag Technik, Berlin, 1961).

10. Eastman Kodak Co., Rochester, New York, SWR emulsion. 11. Socidt~ Kodak Path~ Paris, France, Emulsions type C. 12. V. M. Uvarova, N. K. Sukhodrev, P. M. Shpol'sky, and A. B.

Kovanova, Izv. Akad. Nauk. S.S.S.R. Ser. Fiz. 26, 367 (1962); T. A. Kalinkina, A. B. Kovanova, A. A. Pankova, N. K. Sukhodrev, and V. M. Uvarova, Zh. Nauchn. Prikl. Fotogr. Kinematogr. 9, 286 (1964); T. A. Kalinkina, A. N. Oshurkova, A. A. Pankova, V. M. Uvarova, G. I. Christova, and M. P. Shpol'sky, Zh. Prikl. Spektr. 2, 475 (1965); A. N. Ryabtzov and A. N. Sukhodrev, Twelfth Colloquium Spectrosc. Intern. (Hilger and Watts Ltd. London, 1965), p. 601.

13. Eastman Kodak Co., Rochester, New York, Emulsion 103-UV type.

14. See Ref. 7, chapt, on Detectors, p. 224 and the following. 15. T. Lyman, Astrophys. J. 60, 1 (1924). 16. H. Damany, J. Y. Roucin, and N. Damany Astoin, Appl.

Opt. 5, 297 (1966). 17. W. R. S. Garton, J. Sci. Instr. 30, 119 (1953); 36, 11 (1959);

W. R. S. Garton, I. W. Celnik, H. Hessberg, and J. E. G. Wheaten, Proc. Fourth Intern. Conf. Ioniz. Phenomena Gases (North-Holland Publ. Company, Amsterdam, 1960), p. 518.

18. G. Milazzo, Mikrochim. Acta, 1955, 547 (1955). 19. Y. Tanaka, R. E. Huffman et al., Sci. Papers Inst. Phys.

Chem. Res. (Tokyo) 39, 465 (1942); J. Opt. Soc. Am. 48, 304 (1958); Optics 2, 617 (1963); J. Opt. Soc. Am. 55, 101 (1965); Appl. Opt. 4, 1145, 1581 (1965); J. Opt. Soc. Am. 51,693 (1961); 52, 851 (1962); 45, 710 (1955).

20. G. Milazzo and P. de Gasperis, J. Chem. Phys. 65, 1171 (1968).

21. G. Milazzo, "work in progress." 22. The most comprehensive books on electronic molecular

spectra are, G. Herzberg, Molecular Spectra and Molecular Structure, (D. Van Nostrand Company Inc., Princeton, 1950 and 1966), 2nd ed. Vols. I and III. For diatomic molecules a considerable amount of work has been published since the appearance of the Herzberg books. The first quoted is in our opinion still one of the best suited ones for introductory study of molecular spectroscopy.

23. M. de Belder, Eight Colloquium Spectrosc. Intern. Luzern. (Verlag H. R. Sauerlander and C. Aarau, Schweiz, 1959), pp. 244, 248.

24. J. Romand and R. Berneron, Thirteenth Colloquium Spectrosc. Intern. Ottawa (Hilger and Watts Ltd., London, 1967), p. 434.

25. G. Milazzo, Appl. Spectry. 21, 185 (1967); G. Milazzo and N. Sopranzi, Appl. Spectry. 21, 172, 256 (1967).

APPLIED SPECTROSCOPY 203