HAL Id: jpa-00217461
https://hal.archives-ouvertes.fr/jpa-00217461
Submitted on 1 Jan 1978
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
VACUUM ULTRAVIOLET RADIOMETRY
James A. R. Samson
To cite this version:
JOURNAL DE PHYSIQUE Colloque C4, suppldment au no 7, Tome 39, Juillet 1978, page C4-227
VACUUM ULTRAVIOLET RADIOMETRY
JAMES A. R. SAMSON
Behlen Laboratory of Physics, University of Nebraska, Lincoln, Nebraska 68588, U.S.A.
RCsumC. - Les diffkrentes mkthodes utilisees pour la determination de l'emissivitk spectrale des
sources ou l'efficacitk des detecteurs dans l'ultraviolet lointain ont CtB exposees au cours de la table ronde sur la radiometrie. Le dkveloppement des travaux dans ce domaine a abouti
a
la mise au point des sources et de dktecteurs Ctalons. Cette revue en dCcrit les principales caracteristiques ainsi que les progrks qui restent a effectuer pour couvrir l'ensemble du domaine spectral.Abstract. - Various methods have been used to determine the spectral radiance of a source or
the response of a detector in the vacuum ultiaviolet spectral region. With the increased activity in the area of vacuum UV radiometry absolute and transfer standards have been developed to the point where certain sources and detectors are emerging as the prime standards. The field is reviewed with respect to these standards indicating areas where gaps in our technology still exist.
1. Introduction. - Before 1960 no distinction was made between radiometry in the vacuum ultraviolet and the visible region of the spectrum. Radiometric measurements utilized an absolute standard black- body source of visible radiant energy from which secondary standard lamps and detectors could be calibrated. There was no absolute standard detector. Thermocouples were calibrated in the visible region of the spectrum against the standard blackbody source or calibrated tungsten strip lamps and use was made of their flat response as a function of photon energy for calibration measurements at other photon energies. However, Tomboulian and Hartman [l] as early as 1956 had studied synchrotron radiation and proposed that it could be used as an absolute standard source. Nevertheless, radiometry in the vacuum UV was confined to calibrated thermocouples [2, 31. In the 1960's vacuum UV radiometry and its unique pro- blems became a specialized field of study. Standard detectors were developed [4-61, synchrotron radiation was exploited [7-91, the wall-stabilized argon arc was introduced to produce blackbody limited emission lines in the vacuum UV [10], and the method of line ratios or branching ratios was shown to be prac- tical [l 1-16]. Much of this activity was stimulated by the need for reliable short wavelength intensity measu- rements in plasma physics, solar physics, and the general area of vacuum UV spectroscopy (especially in photoionization studies). Work has continued on into the 1970's in all of the above classes of standards including the study of suitable transfer standards. Definite progress has been made with the development of new absolute and transfer standard sources and
with the extension to shorter wavelengths of the absolute and transfer standard detectors. It was appropriate, therefore, that a panel, consisting of the major international laboratories interested in absolute photon flux measurements, should be convened during the fifth international meeting on vacuum UV radiation physics to review the progress and current status of vacuum UV radiometry. The present paper will attempt to review the field and to summarize the panel discussion.
2. Standard sources. - There are now three abso- lute standard sources of calculable spectral radiance in addition to the conventional blackbody radiator. These are : synchrotron radiation, the continuum emitted from a high power hydrogen arc [17-191, and the blackbody-limited lines emitted by trace constituents in an argon arc [10].
Ott et al. [l91 have used a high power wall-stabilized arc with hydrogen at atmospheric pressure to develop an absolute standard source. When the arc current is about 100 A the temperature on the axis of the discharge approaches 20,000 K. At this temperature the continuum emission coefficient reaches a broad maximum and the spectral radiance can be calculated. There is no temperature measurement necessary only an observation of the point where maximum emission has been reached. The source can be used between 140 and 360 nm with calibration uncertainties less than
+
5%
and down to 130 nm with+
9%
uncertainties. To shorter wavelength the Starkbroadened optically thick Lyman line series of atomic hydrogen dominates the spectrum down to 94 nm. The uncertainties in line-16
C4-228 JAMES A. R. SAMSON
broadening theory increase the calibration errors in this spectral region.
The blackbody line radiator extends the absolute calibrations down to 95.3 nm. This technique, deve- loped by Boldt [10], uses an argon arc of known temperature and in local thermodynamical equi- librium. Small quantities of N,, CO,, Kr, or H, are introduced into the arc such that the temperature is barely changed. The radiant energy of the resonance emission lines from the atoms are found to be black- body limited and, therefore, their spectral radiance can be calculated.
The third absolute standard source utilizes the radiation emitted from a synchrotron or a storage ring. Synchrotron radiation was analyzed theoreti- cally by Schwinger [20] and first studied experimentally by Tomboulian and Hartman [l]. The theoretical characteristics of synchrotron radiation have been experimentally verified [7, 9, 211 and the relative spectral distribution has been verified in the spectral range 220-550 nm to within about
+
2%
[9] by use of the conventional radiation standard sources (eg. calibrated tungsten strip lamp). Determination of the absolute spectral radiance of a synchrotron requires knowledge of the number of circulating electrons. This can be difficult to obtain with a high degree of accuracy. Lemke and Labs [g] made a direct electrical measurement of the number of circulating electrons in the 6 GeV DESY machine and estimated their accuracy to be f 15%.
This problem is less severe in storage rings and presumably will be measured to higher degrees of accuracy in the future than is pre- sently possible. The outstanding advantage of the synchrotron is that it is the only absolute source that covers the spectrum from the infrared to the X-ray region. However, in practice the synchrotron has only been used as a standard of relative spectral distri- bution. The calibration results have been put on an absolute basis by calibrating the flux from the synchro- tron in the visible region against a tungsten strip lamp or a photodiode plus filter that has been previously calibrated against a blackbody radiator. This optical measurement establishes the number of circulating electrons.Having established sources of absolute spectral radiance in the vacuum UV it is necessary to find suitable transfer standards similar to the tungsten strip lamp. Two continuum sources have emerged : the deuterium lamp, suitable between 165 and 370 nm [22-261 and the argon mini-arc [27], which has a range from 110 to 330 nm. The deuterium lamp has been used to make the important comparison between the three absolute standard sources. Pitz [22] and Einfeld et al. [25] calibrated different deuterium lamps against the DESY synchrotron. These lamps were subse- quently calibrated against the NBS hydrogen arc by Bridges et al. [28]. The agreement was within
+
3%
for wavelengths above 170 nm. Stuck and Wende [23] compared the synchrotron standard with plasma
blackbody lines using the deuterium lamp as a transfer standard. Agreement was found to within
+
20%.
Recently, Key and Preston [29] have repeated this technique using improved methods to determine the plasma temperature. Deuterium lamps were again used to compare synchrotron radiation (Daresbury) with the plasma blackbody lines. Agreement was found to be typically within 1.5%.
The work described above establishes confidence in the absolute standards and the usefulness of deuterium lamps as transfer standards. However, because of the short wavelength limit of the deuterium lamp (z 165 nm) other transfer standards are required for shorter wavelengths. The argon mini-arc extends the wavelength range down to 110 nm and is more intense than the deuterium lamp where their spectral outputs overlap. Figure 1 shows a comparison of the spectral radiances of several standard sources. Of prime importance now is the development of suitable transfer standards for wave- lengths less than 100 nm.'
?--+-l2 ~ O O K ' BLACKBODY B W B O D Y LIMITED LINES FROM l 2 5 0 0 K ARC10'
IIN
ARGONFIG. 1. - Comparison of spectral radiances for far UV sources [27, 181.
Recent reviews on the use of synchrotron radiation as a radiation standard have been given by Ederer et al. [30] and Rusbuldt and Thimm [31].
3. Branching ratio method. - The branching ratio
VACUUM ULTRAVIOLET RADIOMETRY C4-229
where the A's are transition probabilities and the A's are the wavelengths of the two lines in the vacuum UV (VUV) and in the visible region. This technique is of practical interest in plasma physics where the plasma itself becomes the standard source emitting line pairs either from impurities within the plasma or from additive trace gases [l 1-16]. A major draw-back of this technique has been the scarcity of line pairs with accurately known transition probabilities. This situa- tion has improved recently with the large list of published transition probabilities for the lithium isoelectronic sequence [32] and in the availability of transfer standard sources for wavelengths as short as 11 5 nm. This means that the longer wavelength of the line pair,
A,,,
can be chosen in the range down to about 115 nm. Thus, a large number of new line pairs with known transition probabilities are now available.The branching ratio method can be extended even further by utilizing double ratios [33]. For example, transitions from the 5d 'D level of C IV to the 3p 2P0 and 4p 'P0 levels produce the line pairs 77 nm and 240.5 nm, respectively, while the NV transitions from the 4s =S level to the 2p 2P0 and 3p 'P0 levels produce, respectively, the line pairs 19 nm and 77.8 nm. From the measured intensity of the C IV 240.5 nm line the intensity of the 77 nm line is obtained from the known ratio of the transition probabilities, see equation (3.1). Because of the proximity of the N V line at 77.8 nm to the C IV line at 77 nm the intensity of the two lines can be compared using any type of detector. From this measurement of the N V 77.8 nm line the branching ratio method gives the intensity of the 19 nm line. A
summary of the positions of suitable lines for the branching ratio method is given in figure 2.
4. Standard detectors. - The absolute standard detectors are the rare gas ionization chamber [5, 341 and the Geiger or proportional counter [5, 6, 35-37]. The principle of the ion chamber is that it produces
one ion-pair for each photon absorbed within the chamber at all wavelengths from the first ionization potential of the gas until the threshold for double ionization, that is, its photoionization efficiency is unity. The wavelength range of the ion chamber is thus from the Xe threshold at 102.2 nm to the double ionization threshold in He at 15.7 nm. However, it can be extended indefinitely to shorter wavelengths [34] provided the efficiency for multiple ionization is known. These have now been measured for some of the rare gases down to wavelengths of about 5 nm [38- 411. The assumption that the ionization efficiency of the rare gases is unity at wavelengths,longer than the double ionization threshold is based on the fact that the only absorption process that can occur is ionization and scattering. Photon scattering is extre- mely small at these wavelengths and ionization is assumed to be the only measurable process. This hypothesis has been tested by comparing the efficiency of one gas against another ; they were found to be identical within the experimental error of f 5
%
[4]. Further, the ionization efficiency of Xe was measured against a calibrated thermopile [42] and found to be unity within the experimental error. Finally, Canfield et al. [43] at the National Bureau of Standards madea careful comparison between a calibrated thermopile and the rare gas double ionization chamber at the wavelengths 58.4 and 73.6 nm. They found that the two methods agreed within a probable error of f 3
%.
Thus it seems reasonable to take the efficiency as known and equal to unity. For wavelengths where the ejected photoelectrons have sufficient energy to cause secondary ionization care must be taken to account for these extra charges. Samson and Haddad [34] have shown that this can be done by taking data as a function of gas pressure and extrapolating the apparent radiant energy to zero pressure. Recently, Masuoka and Oshio [44] have proposed using a four- stage ionization chamber with an increasing distance between the repellor and collector electrodes for eachTOTAL: Li-ltke ions:
I
A I X I t O A r X ZI
Be-like ions. H, He I, He 11:om
C 7 Yon
C I E!I
1
1
JJ
'l kW\\111
P
y
f l ~ ~ ~ ' l
\\,([\'l
Nem
N V Nem NIP: OYcm
----
double ratiosC4-230 JAMES A. R. SAMSON
stage thereby varying the amounts of ionization under each electrode caused by the secondary ionization. The apparent radiant energy obtained from each stage is plotted against the average electron path- length. The curves are then extrapolated to a threshold length for which the photoelectrons just start to produce secondary ionization. At this point the true radiant energy can be read off the curve. The advan- tage of this method is that the data are all obtained simultaneously at one gas pressure. Regardless how the rare gas ion chamber is used its short wavelength limit depends on the ability to measure the ion current and on a knowledge of the efficiency for multiple ionization. If a single ion chamber is used the absorp- tion cross section of the gas must also be known.
The Geiger counter has been used as an absolute standard for wavelengths generally less than 30 nm but particularly in the soft X-ray region less than 10 nm. The important principle is that the counter should register one count when one photon is absorbed within it. Two problems exist : when a photoelectron is released will it cause an avalanche with a subsequent count ? Is the photoionization efficiency of the gas at least 100
%
? When a molecular gas is used in the counter the ionization efficiency may not be unity. However, in the soft X-ray region the efficiency is usually unity. But these problems must be carefully checked. For example, NO .has a high affinity for electrons and NO- states can be produced thereby inhibiting an avalanche. Manson [36] has used pure methane gas and operated the counter in the pro- portional mode between 16 and 30.4 nm. The photon flux measured with both the counter and the double ion chamber agreed within the error of the experiment (+ 10%>.
4.1 TRANSFER STANDARD DETECTORS. - Thermo- couples and thermopiles have long been the conven- tional transfer standards. In fact they are the primary transfer standard for wavelengths greater than 102.2 nm, because no absolute standard detectors exist at wavelengths longer than the Xe ionization potential. Thus, thermocoupleswith their flat response can be calibrated by the standard blackbody radiator or by a tungsten strip lamp. Marette [45] has built a calibration laboratory for work in the spectral range 135 to 350 nm based on a blackbody source and 'thermocouples. He has also reviewed the status of vacuum
uv
radiometry ~. [46]. The use, of thermo-couples for absolute radiometry in the-far UV has been discussed by Johnston and ~ a d d , e n [47]. For wavelengths shorter than 100 nm thermocouples are rather insensitive for the weaker sources of radiant energy encountered and are generally difficult to use in a windowless atmosphere. More convenient transfer standards have been developed for wavelengths longer than 110 nm, although these are usually cali- brated against a calibrated thermocouple. These standards are simple vacuum photodiodes with catho-
des of Cs2Te or Cs3Sb enclosed with MgF, or LiF windows [48].
For shorter wavelengths windowless photodiodes must be used, thus the photocathode should be stable on exposure to air. Cathodes made from pure metals have been used for some time as transfer standards, either calibrated against thermocouples [49, 501 or against the double ion chamber [51, 521. These have proved to be extremely useful and stable detectors. The National Bureau of Standards have developed a rather stable aluminum photocathode with an A1203 surface layer approximately 15 nm thick that makes excellent photodiodes [53, 541. These photo- diodes have been calibrated against the rare gas double ion chamber at wavelengths as short as 5 nm [55].
Often it is necessary to have more sensitive transfer standards. A pure metal photocathode typically has
an efficiency that varies between 2 and 20
%
for wavelengths below 100 nm, which rivals the sensitivity of many photomultiplier's photocathode. However, the simple photodiode has no multiplying structure. Thus, there has been interest in using open structure multipliers as transfer standards, especially channel- tron electron multipliers (CEM) and microchannel plate detectors [56, 571. However, a major problem has been the linearity of response across the face of the CEM's. This non-linearity does not occur with the microchannel plates. Other problems are encountered ; such as reflection of the photons down each micro- channel, which causes avalanches away from the input. This effect eliminates the normal operating plateau in the curve of voltage vs count rate. Further, the high resistance of each microchannel (z 5 X 10i3 Cl)limits the dynamic range of the channel array plate. Efforts to date are aimed at producing curved micro- channels (to minimize ion feedback) with high- conductivity. With a solution to these problems the microchannel array plate appears to be a promising transfer standard detector.
Another interesting device, which not only acts as a detector but also measures the spectral irradiance of a source, is the photoelectron energy analyzer [58-611. The analyzer acts as a monochromator by transferring the spectral distribution of the radiant energy source,
via photoionization of an atomic gas, into an electron energy distribution through the Einstein equation,
h v = I + E,, (4.1)
VACUUM ULTRAVIOLET RADIOMETRY
then by use of the known photoionization cross sections of the analyzer gas, the analyzer is capable
of providing the absolute spectral irradiance of a 30 38 nm source.
5
ELECTRON ENERGY :eV)
FIG. 3. - Electron energy spectrum of an undispersed source of Ar I1 radiation. The analyzer used argon gas a s the source of photoelectrons. Thus. the spectrum consists of doublets. The solid vertical lines refer t o the 'P,,, electrons whereas the dashed vertical lines refer to the 'P,,, electrons. The dashed base line indicates the extent of the scattered electrons. Wavelength identification is in
Angstrom units of length [58].
Figure 3 shows the Ar I1 spectrum emitted by a duoplasmatron light source as obtained with a cylin- drical mirror electron energy analyzer. Argon gas was used in the analyzer. The spectrum, therefore, contains electron energy doublets for each wavelength. The 2P,12,,1, doublet separation for argon is 178 mV which is equivalent to a wavelength separation of about 0.5 nm at 60 nm. The analyzer resolution was 43 mV which is equivalent to a wavelength resolution of 0.125 nm a t 60 nm and 0.03 nm at 30 nm. The spectral distribution of the duoplasmatron and the relative irradiance of the lines is shown by the vertical lines in figure 3. The solid lines represent the 2P312 electron energy group while the dashed vertical represent the 2P112 group. The dashed lines are one- half the height of the solid lines reflecting the fact that the ratio of the : is about two [62]. The wavelength identification is based on the more intense electron energy group. The dashed base line is the true base line and indicates the effect of scattered electrons on the spectrum, reminiscent of the effect of scattered radiation in a conventional mono- chromator.
The results of Sato et al. [60] are shown in figure 4. They observed a d.c. high-density helium plasma with a retarding grid type electron energy analyzer. Neon
35 25 20 15
PHOTOELECTRON ENERGY leV1
FIG. 4. - Integrated photoelectron counts for He I1 Lyman series emitted by a stationary hlgh-density helium plasma [60].
was used as the analyzer gas. The Lyman series of
He II is clearly observed. The intensity ratig of the
first four members of the series was found to be 120 : 9 : 3.5 : 1. From these results they were able to esti- mate the temperature of the helium plasma.
5. Summary. - Absolute standard sources now
exist from the infrared to the hard X-ray region. Transfer standard sources have been tested and used down to 110 nm. No source has yet been put forward as a transfer standard for wavelengths less than 110 nm.
Absolute standard detectors are now available from 102.2 nm into the X-ray region. However, none exist for wavelengths longer than 102.2 nm. Transfer standard detectors exist from the infrared into the X-ray region. However, sensitivity is lacking in the detectors (windowless photodiodes) used for weak sources of extreme UV and X-radiation. Considering the present activity in vacuum UV radiometry we can expect the few remaining gaps to be closed and improvements made on the existing sources and detectors within the next few years.
Acknowledgment. - I should like to express my
appreciation to the members of the panel on VUV Radiometry for the various preprints of their latest work, which- aided in the preparation of this review. The panel members were : Drs. B. Vodar, P. Fieffe- Prevost, B. Wende, P. J. Key, W. L. Wiese, T. Sasaki, J. G. Timothy, E. Pitz and R. P. Madden. It is also with pleasure that we acknowledge the National Aero- nautics and ,Space Administration, under Grant # NGR28-004-021, for support of our program on vacuum UV radiation and in the preparation of this review.
References
C4-232 JAMES A. R. SAMSON
[4] SAMSON. J. A. R., J. Opt. SOC. Am. 54 (1964) 6.
[5] LUKIRSKII, A. P.. RUMSH. M. A. and SMIRNOV. L. A.. Opt.
Spektrosk. 9 (1969) 505; Eng. Trans.. Opt. Spectrosc. 9
(1960) 262
161 EDERER. D. L and TOMBOULIAN. D. H.. Appl. O p t 3 (1964) 1073.
[7] CODLING. K. and MADDEN. R. P,. J. Appl. Phys. 36 (1965) 380. [S] BATHOW, G., FREYTAG, E. and HAENSEL, R., J. Appl. Phys. 37
(1966) 3449.
[9] LEMKE. D. and LABS. D., Appl. Opt. 6 (1967) 1043.
[l01 BOLDT, G.. Proc Fifih Int. Conference on Ionization Pheno-
mena in Gases (North Holland, Munich). 1961. p 925 and
J. Quant. Spectrosc. Radiat. Transfer 5 (1965) 91. [l l ] GRIFFIN, W. G . and MCWHIRTER. R. W. P,. in Conference on
Optical Instruments and Techniques, ed. K. J. Habell
(Chapman and Hall, London), 1962, p. 14.
[l21 ZAIDEL, A. N.. MALYSHEV. G . M. and SHREIDER. E. Ya..
Zh. Tekh. Fiz. 31 (1961) 129.
[l31 GLADUSHCHAK. V. I. and SHREIDER. E. Ya.. Opr. Spektrosk. 14 (1963) 815 ; English Trans., Opt. Spectrosc 17 (1964) 75.
[l41 AARTS, J . F. M. and DE HEER, l-. J.. J . Opt. Soc. An?. 58 (1968)
1666.
[l51 MCCONKEY, J. W., J. Opt. SOC. Am. 59 (1969) 110. [l61 MUMMA, M. J.. J. Opt. SOC. Am. 62 (1972) 1459.
[l71 OTT. W. R.. FIEFFE-PREVOST. P. and WIESE, W. L., AppI Opt. 12 (1973) 1618.
[l81 OTT. W. R. and WIESE. W. L.. Opt. Eng. 12 (1973) 86 1191 OTT. W. R., BEHRINGER. K. and GIERES, G., AppL O p f 14
(1975) 2121.
[20] SCHWINGER, J.. Phys. Rev. 70 (1946) 798 and 75 (1949) 1912. [21] HAENSEN, R. and KUNZ. C.. Z. Angew. Phys. 23 (1967) 276. [22] PITZ. E., Appl. O p t 8 (1969) 255
[23] STUCK. D and WENDE. B.. 1. Opt SOC Am. 62 (1972) 96
1241 EINFELD, D. and STUCK. D.. Opt. Commun. 19 (1976) 297. [25] EINFELD. D.. STUCK. D. and WENDE. B., Int. J. Sci. M e r r o l o ~ ~ . .
1261 KEY. P. J and WARD. T H., Metrologitr I4 (1978) 17 [27] BRIDGES. J. M. and OTT. W. R., Appl. Opt 16 (1977) 367. [28] BRIDGES. J. M.. OTT. W. R.. PITZ, E.. SCHULZ. A.. EINFELD. D.
and STU< K. D.. At)/)/. O ~ J I . 16 ( 1977) 1788
(291 KFY. P. J. and P R E % T O ~ , R C . AppI. Opt. 16 (1977) 1477 (301 EDERER. D. L.. SALOMAN. E B , EBNER, S. C and M A D I X : ~ .
R. P.. J Res. Not. Bur Stand. 79A (1975) 761
1311 RUSBULDT, D and THIMM. K.. Nucl. Instrunl Method.\ 116
(1974) 125.
1321 MARTIN. G. A. and WIESE. W. L . J. Plry.c. Chem. Rrf Dtrttr 5
(1976) 537.
1331 WIESE. W . L
.
Private communlcatlon.1341 SAMSON. J . A R. and HADDAD, G. N.. J Opt. So(. Am 64 (1 974) 47.
1351 TOMBOULIAN. D. H.. Jpn J. Appl. Phys. 4 (1965) 542, Sup- plement I.
1361 MANSON. J. E.. Bull. Am. Phys. Soc. 22 (1977) 115 and Astro-
phys. J. 153 (1968) L191.
[37] CARUSO, A. J. and NEUPERT, W. M,, Appl. Opt. 4 (1965) 247. [38] SAMSON. J. A. R. and HADDAD, G . N.. Phys. Rev. Lett. 33
(1974) 875.
1391 SCHMIDT, V., SANDER. N.. KUNTZEMULLER. H., DHEZ. P,. WUILLEUMIER. F. and KALLNE. E.. Phys. Rev. A 13 (1976) 1748.
1401 CARLSON, T. A., Phys. Rev. 156 (1967) 142.
[41] WIGHT. G R. dnd VAN DER WIEL. M. J., J . Phys. B : Atomic and Molecular Physics 9 (1976) 1319.
[42] MATSUNAGA, F. J., JACKSON, R. S. and WATANABE. K.. J.
Quant. Spectrosc. Radiat. Transfer 5 (1965) 329.
1431 CANFIELD. L. R., JOHNSTON. R. G.. CODLING. K. and MADDEN. R P., Appl. Opt. 6 (1967) 1886.
[44] MASUOKA. T. and OSHIO. T., Jpn J. Appl. Phys. 15 (1976) 65 and proceedings of thls conference.
1451 MARETTE. G.. Appl. Opt. 15 (1976) 440. [46] MARETTE. G.. E S A Sci. Tech. Rev. 2 (1976) 55.
1471 JOHNSTON. R. G . and MADDEN. R. P., Appl. Opt. 4 (1965) 1574. [48] FISHER, G . B.. SPICER. W. E., MCKERNAN, P. C., PERESKOK.
V F. and WANNER, S. J., Appl. Opt. 12 (1973) 799. [49] WALKER. W. C.. WAINFAN. N. and WEISSLER, G. L., J. Appl.
Pl!,r. 26 (1955) 1366
1501 HINTEIIEGGER. H. E. and WATANABE. K.. J. Opt. Soc. Am. 43 (1953) 604.
1511 SAMSON. J. A. R. and CAIRNS. R. B.. Rev. Scr. Instrum. 36 (1965) 19.
1521 C A I R N S . R. B. and SAMSON. J. A. R.. J. Opt Soc. Am. 56 (1966)
1 568.
[53] CANFIELD, L. R.. JOHNSTON. R. G and MADDEN, R. P,, Appl. Opt. 12 (1973) 1611.
1541 S A I O M A N . E. B. and EDERER, D. L., Appl. Opt. 14 (1975) 1029. [55] MAI)I)I:N. R P.. private communication.
1561 TIMOTHY. J . G . and LAPSON. L. B.. Appl. Opt. 13 (1974) 1417 (571 T l r l o l ~ u , J. G. and Byetb. R. L.. Rev. Scr. ltr.ctrutn. 48 (1977)
292
(581 S A M S O ~ . J. A. R.. P r ~ c . Intern. Symp. fi)r Si*nehrotrori Rtrtltcr- !roil L!srr.\. eds. Marr. G. V. and Munro. 1. M . (Daresbury Nuclear Physlcs Laboratory. Daresbury. England) 1973.
p 127.
[60] SAT(). K.. SUGAWARA. H.. ODA. T.. HOSOKAWA. M,, ISHII. K and SASAKI. K.. Phys. Lett. 58A (1976) 310. See also proceedings of t h ~ s conference.
1611 KRAUSE. M. 0 . WUILLEUMIER, F. and NESTOR. C. W. Jr.,
Phvs. Rev 6 (1972) 871 and Chem. Phvs Le!!. 10 (1971) 65.
1621 SAMSON, J. A. R.. GARDNER. J. L. and STARACE. A. F.. Phys.
