JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Pulsed high-pressure lamp for the vacuum ultraviolet

Zohar Ophir, Uzi Even, Baruch Raz, and Joshua JortnerDepartment of Chemistry, Tel-Aviv University, Tel-Aviv, Israel

(Received 5 March 1974)

A high-pressure continuous light source has been developed for the near-vacuum-ultraviolet region, downto 1100 A. This source is based on a pulsed discharge in rare gases and is characterized by high intensity,small dimensions, and low consumption.

Index Headings: Sources; Spectra; Ultraviolet; Argon; Krypton.

In this paper, we report the construction of a continuumsource in the vacuum-ultraviolet spectral region. Thissource was designed to be used for photoemissionstudies of solid insulators, such as solid rare gases, inwhich the high-energy gap requires application of highphoton energies (above 6 eV). In such studies, a highflux of photons emitted by the source into the mono-chromator is also required. Two other importantrequirements are time stability of the source and lowlevel of electrical noise. Time stability is importantbecause of the use of the long integration times of theorder of 10 s. These lead to a slow wavelength (orenergy) scanning times, of the order of 30 min perspectrum; this, in turn, determines the time-stabilityrequirement. In order to tackle the interference causedby the emitted electrical noise, there are two possi-bilities: (i) increasing the fraction of the total powerthat produced the flux of photons at the desired energy,(ii) minimizing the geometric dimensions of the source(including its power supply) in order that their elec-trical shielding be practical. The two approaches are,of course, complementary.

We have not found the specified combination in anyconventional sources for the vacuum ultraviolet.' TheTanaka-type pulsed-discharge rare-gas continualamps",2 emit about 107 photons/s/A when used withan approximately 0.5-kW power supply. Such a powersupply produces an appreciable amount of electricalnoise. Because of geometric considerations, it is verydifficult to suppress this noise by electrical shielding.High-pressure dc arc lamps3 are brighter, because of thehigher rate of formation of excited homonuclearmolecules, than the Tanaka lamps. The time stabilityof these lamps is, however, impaired by the nature ofa dc arc. The fact that the arc is anchored to definitepoints on the electrodes causes intensive local heatingand contamination of the plasma. The power supply ofthe dc arc lamps, which uses about 1 kW, is a source ofconsiderable electric noise, which is generated in therectifying stages. Owing to its dimensions, the powersupply is difficult to shield.

The new light source that we constructed combinesthe advantages of the medium-pressure Tanaka-typedischarge lamp and the high-pressure dc arc. It is ahigh-pressure pulsed-discharge lamp; the pulse ratevaries between 10-300 Hz, and the operating pressure

is 1-6 atm. This lamp is much-less sensitive to thepresence of impurities than is the Tanaka-type lamp,and is characterized by an appreciably higher intensity.It is also adequate for use with a boxcar integrator andother signal-averaging electronic detector systems. Itssmall size, and the low power consumption (- 10 W)of its power supply, makes its shielding very simple.

LAMP DESIGN

The electrode geometry shown in Fig. 1 was em-ployed. The discharge in the rare gas is initiated byfield-emitted electrons. These are obtained by applyinga voltage to the junction of a tungsten wire and aferroelectric material, barium titanate, which has adielectric constant greater than 4000. The titanate isgrounded and connected to the cathode. Applying avoltage pulse to the junction causes a sudden increaseof the field at the junction and results in emission ofelectrons from the cathode, starting an avalanchedischarge in the rare gas. The discharge is stabilized andgoverned by a series of tungsten electrodes with delayed

I

28

7 3

4

5FIG. 1. Electrode configuration of the lamp, 1-anode, 2,3,4,7,8-

auxiliary electrodes, 5-cathode, 6-trigger electrode.

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SEPTEMBER 1974VOLUME 64, NUMBER 9

OPHIR et al.

Lamp 8

FIG. 2. Schematic electrical layout of the lampand its power supply.

triggering. The electrode system of an EG&G modelFX-6A Xenon lamp, originally designed for the produc-tion of short pulses in the near ultraviolet, was used inour setup. As a driver, we used an EG&G FY litepac.A block diagram of the electric power supply is shownin Fig. 2. The small (2-3 mm) spacing between theguiding electrodes insures that the arc produced in thisway is very stable. By this arrangement, we managedto overcome the difficulties involved in the initiation ofthe spark at high pressures and to produce a spatiallystable arc.

In order to operate the lamp in a pulsed mode, weused a SCR-triggered gate operated by a square-wavevoltage from a pulse generator. The mean powerdissipated in the lamp is of the order of only 1 W.

The lamp container is shown in Fig. 3. It was madeof a 80X80X80-mm stainless-steel cube, connected tothe monochromator with the aid of a flange, intowhich a LiF window is fixed. The cube is pumped by a100-1/min rotary pump equipped with an activated-alumina oil-vapor trap, and a 10-1/s triode ion pump.The cylinder of the rare gas (Matheson Research grade)and a pressure gauge for measuring 0-10 atm are con-nected directly to the cube. The vacuum chamber waspumped down to less than 107 torr before filling thelamp with 1-6 atm of the pure rare gas.

RESULTS

In Figs. 4 and 5, we present the emission spectra ofthe high-pressure discharge lamp, operating with 2 atm

0-tO AtmPressureG on9,

Raft Ga,Cy inde,

EtectrcFeedhrough

Pump

FRONT VIEW

Ion Pump

SlDE VIEW

FIG. 3. Lamp chamber and auxiliary systems.

0 6x

C-)

2.3

2

1100 1200 1300 1400 1500

FIG. 4. Emission continuum of an argon lamp. The pressurein the lamp was 2.0 atm, and the pulse rate 100 Hz.

of Ar and Kr. A pulse-repetition rate of 100 Hz waschosen in order to obtain good statistical averagingwithin reasonable integration periods. This makes thelamp very suitable for photoelectric detection, whichwas with an EMI 9524B photomultiplier and sodiumsalicylate converter. Owing to the very great intensityof the lamp, we applied an accelerating voltage of only600 V to the photomultiplier, in order to avoid satura-tion. The intensity of our new source was comparedwith that of a conventional Tanaka-type discharge

10

at-0x

I-z

L

C-,

E

1700

FIG. 5. Emission continuum of a krypton lamp. The pressurein the lamp was 2.0 atm, the pulse rate 100 Hz.

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PULSED HIGH-PRESSURE LAMP FOR vuv

lamp 4 operated at 100 mA. An intensity ratio of 5 X 102was found. This ratio is uncertain within one order ofmagnitude because of uncertainty of the acceleratingvoltages applied on the photomultiplier in the twomeasurements. The lamp and the detector were attachedto the entrance and exit slits (slit width of 0.1 mm,spectral resolution 1.3 A) of a McPherson 218 Czerny-Turner monochromator, respectively. This mono-chromator was equipped with a 2400-lines/mm flatgrating, blazed at 1500 A.

CONCLUSIONS

The high-pressure, rare-gas discharge lamp, emittingcontinua in the vacuum ultraviolet (i) emits a flux of

photons 2-3 orders of magnitude greater than we ob-tain4 by use of the Tanaka-type low-pressure rare-gasdischarge lamps, (ii) dissipates a much lower averagepower (its power supply is therefore far smaller thanthat of the conventional lamps), and (iii) its pulse-repetition rate can be regulated within a wide range,using a simple pulse generator.

REFERENCES

'J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy(Wiley, New York, 1967).

2 (a) Y. Tanaka, J. Opt. Soc. Am. 45, 710 (1955); (b) R. E.Huffman, J. C. Larrabee, and Y. Tanaka, Appl. Opt. 4, 1581(1965).

'P. M. Johnson, J. Opt. Soc. Am. 60, 1669 (1970).4B. Raz, J. Magen, and J. Jortner, Vacuum 19, 571 (1969).

Technical Council

ROBERT V. POLE (Chairman), IBM Corporation,Monterey and Cottle Roads, San Jose, California95193

ANTHONY J. DEMARIA (Vice Chairman), United Air-craft Research Laboratories, East Hartford, Con-necticut 06108

The chairmen of the Technical Groups comprise(ex officio) the Technical Council.

Aeronautics and Space Optics-LLOYD G. MUNDIE,Rand Corporation, 1700 Main Street, SantaMonica, California 90406

Atmospheric Optics-GILBERT N. PLASS, Departmentof Physics, Texas A & M University, CollegeStation, Texas 77843

Color-S. LEON GUTH, Department of Psychology,Indiana University, Bloomington, Indiana 47401

Information Processing, Holography, & Coherence-ADAM KOZMA, Environmental Research Institute ofMichigan, P.O. Box 618, Ann Arbor, Michigan48107

Lasers and Electro-Optics-William B. Bridges,Hughes Research Laboratories, 3011 Malibu Can-yon Road, Malibu, California 90265

Lens Design-THOMAS I. HARRIS, Optical ResearchAssociates, 1774 North Sierra Madre Villa Avenue,Pasadena, California 91107

Optical Fabrication and Testing-Frank Cooke, 66Summer Street, North Brookfield, Massachusetts01535

Optical Materials-IRVING H. MALITSON, A-251Physics Building, National Bureau of Standards,Washington, D. C. 20234

Radiometry and Photometry-BRUCE W. STEINER,B-213 Metrology Building, National Bureau ofStandards, Washington, D. C. 20234

Raman-JAMES E. GRIFFITHS, Bell Laboratories,Murray Hill, New Jersey 07974

Spectroscopy-JACK SUGAR, A-167 Physics Building,National Bureau of Standards, Washington, D. C.20234

Thin Films & Interferometry-KENNETH M. BAIRD,Applied Physics Division, National Research Coun-cil, Ottawa KMA OSI, Canada

Vision-GERALD WESTHEIMER, Physiology-AnatomyDept., 2575 LSB, University of California, Berkeley,California 94720

1177September 1974