Spectroscopic instruments for vacuum ultraviolet radiation stud

Weissler, G. L.

English

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Department of Physics 
University of Southern California 
Los Angeles 7, California 
Semi-Annual Status Reports 
No. 1, 2, and 3 
f o r  the 
18 month period ending 28 February, 1963 
8 
on 
VACUUM ULTRAVIOUT RADIATION 
AND SOLID STATE PHYSICS 
sponsored by the 
National Aeronautics 'ana 
I .*j 
Grant No. NsG-178-61 
submitted by 
G. L. Weissler 
Chief Investigator 
March 15, 1963 
. . 
a 
"Vacuum Ultraviolet Radiation and 
Solid State Physics" 
Department of Physics 
University of Southern California 
Los Angeles 7, California 
Since this report covers an 18-month period, up to 
and including February 28, 1963, the material presented 
here may be regarded as intermediary between the normal 
type of brief status report and a technical report. 
This NASA grant concerns itself primarily with 
vacuum ultraviolet radiation and its interactions with 
solid materials, and the various activities initiated 
under the auspices of this grant will be described under 
separate headings. 
1. Vacuum W Photon Interactions with Surfaces Kept in an 
Ultra-High Vacuum 
An ultra-high vacuum system, as shown in Fig. 1, has 
been designed, built, and put into preliminary operation. 
After the first bake-out a vacuum of 4 x 
been obtained. Several modifications have been made in the 
vacuum manifold, which is now being readied for final 
assembly. Aside from the commercially purchased pumps 
the bakable vacuum system consists entirely of stainless 
steel. Various sections are shown which are made vacuum 
mm Hg has 
kight by special copper gasket seals. Two bakable liquid 
nitrogen traps with oil-creep barriers and radiation 
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shields have been incorporated in this design, keeping in 
all essentials to the most advanced ideas from the 
Livermore Radiation Laboratory and from the Princeton 
Stellarator project. 
At the present time, the uppermost experiment 
chamber is provided with four sapphire windows which 
transmit radiation from the infra-red down to 1500 
Angstrom units. Initial experiments are planned on 
barium, which will be evaporated under ultra-high 
vacuum conditions, of the order of 10 mm Hg. The 
reflectivity of this surface will then be studied as 
a function of wavelength down to 1500 Angstrom units and 
for 2 angles of incidence. A literature search has 
indicated that pronounced changes in the reflectivity are 
to be expected in the short wavelength region. These 
reflectivity data will clarify the variation of the 
optical constants, namely the index of refraction, n, 
and the extinction coefficient, k, which will be cal- 
culated with computer h e l p  from the Xramers-Kr8nig 
dispersion relation. 
new results will be obtained. These results should be 
characteristic of barium only and not of contaminations, 
such as oxides, carbonates, etc., which are unavoidable 
under conditions of ordinary high vacua, of the order 
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It is anticipated that interesting 
of 10-5 to m Hg. 
Fig. 2 shows a plan view of the ultra-high vacuum 
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system containing a sample of barium. 
is shown on the left of the figure and provides beyond 
its exit slit monochromatic radiation from the near infra- 
red down to 1500 Angstrom units. The two angles at which 
reflectivity measurements will be undertaken are shown in 
this figure. 
The monochromtor 
All of this equipment shown in Figs. 1 and 2 has 
been designed and built by the USC Physics Shop. The 
ultra-high vacuum system is ready for final operation 
an3 the monochromator associated with it ( a s  shown in 
Fig. 2) is nearly finished and should be assembled in 
approximately one month from the time of this writing. 
Future extension of this work will be to study the 
optical constants of a variety of materials, all under 
conditions of ultra-high vacua, and eventually to de- 
crease the wavelength range down to approximately 500 
Angstrom units. 
2, Plasmon Radiation Excited in Thin Films by 50 KV Electrons 
When electrons of high energy pass through thin 
films, theory predicts that certain charactersitic energy 
losses will be suffered by these electrons which are 
equal to the plasma frequency. This plasma frequency lies 
in the range of between 5 and 50 electron volts, depending 
on film material, etc, If an electron stream excites 
such a plasma to its characteristic oscillations, it is 
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also predicted that the plasma will re-radiate photons in 
energy equal to the equivalent plasma frequency. For 
most metals this plasmon re-radiation lies in the wave- 
length range of the vacuumultraviolet. In order to 
observe this type of radiation, the apparatus described 
in Fig. 3 has been designed, built, and is nearing completion. 
Electrons from a filament at ground potential (lower left 
of Fig. 3 )  are accelerated towards a thin film deposited 
on a target holder, which is maintained at +50 KV. The 
impact point of the electrons on the target is equivalent 
to the position of a primary slit of a vacuum spectrograph 
(upper portion of Fig. 3 ) .  
excited by the electron beam impinging on the thin target 
film, will travel towards the grating of the vacuum spectro- 
graph. From the grating i t  will be dispersed and registered 
Plasmon radiation, which is 
on a photographic film as indicated in Fig. 3 .  This photo- 
graphic film will cover a wavelength range from 4000 Angstrom 
units down to approximately 400 Angstrom units, a region 
which should contain the plasmon wavelengths of a wide 
variety of materials. It is hoped that this experiment 
will be ready for first trial runs within one or two months. 
3 .  Light Source ami Detector Developmnt of Vacuum W Radiation 
A versatile new vacuummonochromator has been built 
which can be operated in two modes: 
shown in Fig. 4, and as a grazing incidence spectrograph, 
shown in Fig. 5. 
as a Seya monochromator, 
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The Seya mounting covers a wavelength region from 
12,000 to 500 Angstrom units, with a reciprocal linear 
dispersion of 17 Angstroms per m, In contrast, the 
grazing incidence mounting covers a much shorter and 
narrower wavelength region, from 500 to 150 Angstrom 
units, with a much higher reciprocal linear dispersion, of 
the order of 3 Angstroms per mm. This instrument will be 
used in both modes to test new ideas with respect to 
high intensity light sources in this general spectral 
region and to develop new types of detectors, capable 
of counting absolute numbers of photons leaving the 
exit slit. 
This work is now actively under way, and it is 
hoped to produce either a technical report or a contributed 
paper to a scientific meeting or both during 1963. 
Beyond source and detector development work, this 
instrument will also be used to look for vacuum ultra- 
violet polarization effects. This will be accomplished 
in two independent ways: 
a) by looking for the vectorial effect in the emission 
of photo-electrons, and 
b) by looking for Zeemann components when the light 
source is placed in a strong magnetic field (a 12- 
inch electromagnet capable of 30,000 gaussis indicated 
in Fig. 5 ) .  
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4. Photo-Electric Yields, Transmissivities, and Reflectivities 
,at Grazing Incidence 
Dr. B. Vodar of the Centre National de la Recherche 
Scientifique in Bellevue (near Paris) has designed a 
grazing incidence monochromator, which covers the wave- 
length region from 1000 to 100 Angstrom units. 
characteristic feature of this instrument is that in 
the scanning process the exit slit remains fixed, while 
both the grating as well as the primary slit (to which 
the light source is attached) move along straight lines. 
For our purposes this appeared to be an exceptionally 
useful instrument. For this reason a purchase order was 
placed with Dr. Vodar and the instrument was received 
in the fall of 1962. 
system is fitted to it and a detector, to be placed behind 
the exit slit, is being constructed. A grating, lightly 
ruled on glass, has been specially made for the instrument 
and will be installed during the next few weeks. 
anticipated that this grazing incidence monochromator 
will be focused and operational by early summer, 1963. 
The 
At the present time a vacuum 
It is 
The principle purpose of this instrument is to study 
reflectivities of surfaces in the extreme ultraviolet, 
where very little information is available at this time, 
Specifically, it is hoped t o  determine grating efficiencies 
in this wavelength region and to check by direct measure- 
m n t  to what extent calculated blaze angles really provide 
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enhancement of intensities at the predicted short wavelengths. 
A program of routinely checking such grating efficiencies 
is now being set-up. 
In addition, it is planned to study the transmission 
properties of thin films, of the order of 500 to 1500 
Angstrom units thick, of such materials as aluminum, 
indium, tellurium, zinc, and others. Earlier researches 
at this laboratory have shown pronounced light transmission 
in the region below 1000 Angstrom units. It is essential 
that these transmission properties be determined at wave- 
lengths shorter than 500. 
Finally, this instrument is also designed for the 
determination of photo-electric yields from various 
surfaces. A considerable gap exists between our data 
taken at longer wavelengths and those from Russian 
investigators obtained in the soft X-ray region. 
is hoped that these experiments will bridge this gap. 
It 
In order to accomplish these various researches, a 
single, multi-purpose target chamber has been designed 
and is shown, together with the Vodar monochromator, 
in a side view in Fig. 6 and in a plan view in Fig. 7. 
Most of the vacuum system shown in Fig. 6 has been 
built. However, the target chamber itself is still in 
the design stages and will not be ready for the machine 
shop until later in the spring of 1963. 
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During the 18-month period of these three status 
reports, a considerable variety of  research and develop- 
ment projects have been initiated as indicated by the 
material presented above, It is felt that a number of 
interesting research results will be forthcoming during 
the course of this year. These will be distributed 
either in the usual form of technical reports or in 
the form of reprints of published notes and articles. 
25 copies respectfully submitted, - 
G. L. Weissler 
Professor of Physics 
Chief Investigator 
NASA Grant No. NsG-178-61 
March 15, 1963 


Vacuum ultraviolet radiation and solid state physics semiannual status reports no. 1, 2, and 3, period ending 28 feb. 1963

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