A Fast High-Intensity-Pulse Light Source for Flash Photolysis Ernst G. Niemann and Max Klenert A new pulse light source has been developed for flash photolytic investigations. It consists of a z-pinch operated with a 33-kV, 4.85-kJ condenser bank, and produces light pulses of 0.2 usec duration with a maximum continuum radiation density exceeding 100 W A-' cm-' sr-'. Details of construction and measurement techniques are given, lysis are discussed. Introduction The detection and identification of short-lived radicals and intermediates by the flash photolysis technique often involve the use of intense and fast uv sources.'- 4 Some of the properties required for these light sources are (1) pulse duration below 1 Musec, (2) very high light emission, especially in the uv region, (3) linear extended light emitter to enable optimal coupling to an irradiation cuvette. It is un- likely that these requirements can be achieved by further development of the capillary spark flashlamps which are used in flash photolysis at the present time. With this type of lamp, increase in light intensity by increase of discharge energy also leads to an extension of light pulse duration. ' 4 It can be expected however that a gas plasma, heated by shock waves to very high temperatures, will achieve the conditions given above. For this reason a fast z- pinch assembly has been constructed and tested with regard to its suitability as an irradiation light source for flash photolysis. Arrangements of this kind are used in plasma physics to obtain extremely high temperatures. 5 A condenser bank is discharged through a cylindrical vessel, the top and bottom of which consist of flat electrodes. The vessel is filled with low pressure gas. The ions produced at the beginning of the discharge are contracted by Lorentz forces toward the axis of the cylinder forming a high velocity shock wave. At maximum contraction a very high temperature is generated and part of the kinetic energy is emitted in the form of electromagnetic radia- tion. Then the gas expands again and cools down rapidly. The effect of different gases and gas pressures was investigated in an attempt to achieve a discharge The authors are with Institut fur Strahlenbiologie, Technische Hochschule, 2 Herrenhduserstrasse, 3 Hannover, Germany. Received 10 April 1967. and the possibilities of application of this light source in flash photo- most suited to the special requirements of a flash photo lysis light source. Experimental Figure 1 gives an over-all view of the assembly. A bank of eight capacitors, (Hydra, 1.11 F; 33 kY), maximum stored energy 4.85 kJ, is arranged con- centrically around the discharge vessel and the vacuum pumps. In order to reduce the total inductance, the condensers are individually connected to the dis- charge tube by short copper bands lying closetogether and separated by 1 mm of polyethylene foil. Each capacitor has its own triggered spark gap. This concentric arrangement was chosen to provide a toroidal current distribution in the supply conductors and in the vessel. This again causes cylindrical shock waves and high plasma temperatures. The discharge vessel [Fig. 2(a)-(c)] is a cylinder of hard glass (Duran 50) with an inner diameter of 20 cm and a height of 20 cm. A copper electrode is cemented to each end and a side tube with a quartz window in the middle of the cylinder wall allows optical investigations of the discharge. In a later stage of this work a quartz tube containing the substance to be irradiated was built into the discharge vessel parallel to the cylinder axis. The analyzing light beam is passed through this tube in the vertical direction. The eight spark gaps 6 (Fig. 3) in the main discharge circuit are fired from a preignition circuit (30 kY) which again is fired by a ninth triggered spark gap. The trigger pulse for this prespark gap is generated by a thyratron circuit with an air transformer. The jitter of the total trigger system is well below 0.5 sec. The discharge current was measured by means of a Rogowski coil 7 with an integration network and a Tektronix 551 dual beam oscilloscope. The measure- ment of the time behavior of the light source was carried out using the streak technique with a rotating mirror camera 8 of our own construction. This camera has a maximum streak velocity of 3 mm/,4sec on the February 1968 / Vol. 7, No. 2 / APPLIED OPTICS 295
Transcript
A Fast High-Intensity-Pulse Light Source for Flash Photolysis
Ernst G. Niemann and Max Klenert
A new pulse light source has been developed for flash photolytic investigations. It consists of a z-pinchoperated with a 33-kV, 4.85-kJ condenser bank, and produces light pulses of 0.2 usec duration with amaximum continuum radiation density exceeding 100 W A-' cm-' sr-'. Details of construction andmeasurement techniques are given,lysis are discussed.
Introduction
The detection and identification of short-livedradicals and intermediates by the flash photolysistechnique often involve the use of intense and fastuv sources.'- 4 Some of the properties required forthese light sources are (1) pulse duration below 1Musec, (2) very high light emission, especially in the uvregion, (3) linear extended light emitter to enableoptimal coupling to an irradiation cuvette. It is un-likely that these requirements can be achieved byfurther development of the capillary spark flashlampswhich are used in flash photolysis at the present time.With this type of lamp, increase in light intensity byincrease of discharge energy also leads to an extensionof light pulse duration. ' 4
It can be expected however that a gas plasma, heatedby shock waves to very high temperatures, will achievethe conditions given above. For this reason a fast z-pinch assembly has been constructed and tested withregard to its suitability as an irradiation light sourcefor flash photolysis. Arrangements of this kind areused in plasma physics to obtain extremely hightemperatures. 5 A condenser bank is dischargedthrough a cylindrical vessel, the top and bottom ofwhich consist of flat electrodes. The vessel is filledwith low pressure gas. The ions produced at thebeginning of the discharge are contracted by Lorentzforces toward the axis of the cylinder forming a highvelocity shock wave. At maximum contraction a veryhigh temperature is generated and part of the kineticenergy is emitted in the form of electromagnetic radia-tion. Then the gas expands again and cools downrapidly. The effect of different gases and gas pressureswas investigated in an attempt to achieve a discharge
The authors are with Institut fur Strahlenbiologie, TechnischeHochschule, 2 Herrenhduserstrasse, 3 Hannover, Germany.
Received 10 April 1967.
and the possibilities of application of this light source in flash photo-
most suited to the special requirements of a flash photolysis light source.
Experimental
Figure 1 gives an over-all view of the assembly.A bank of eight capacitors, (Hydra, 1.11 F; 33 kY),maximum stored energy 4.85 kJ, is arranged con-centrically around the discharge vessel and the vacuumpumps. In order to reduce the total inductance,the condensers are individually connected to the dis-charge tube by short copper bands lying close togetherand separated by 1 mm of polyethylene foil. Eachcapacitor has its own triggered spark gap. Thisconcentric arrangement was chosen to provide a toroidalcurrent distribution in the supply conductors and inthe vessel. This again causes cylindrical shock wavesand high plasma temperatures.
The discharge vessel [Fig. 2(a)-(c)] is a cylinder ofhard glass (Duran 50) with an inner diameter of 20cm and a height of 20 cm. A copper electrode iscemented to each end and a side tube with a quartzwindow in the middle of the cylinder wall allows opticalinvestigations of the discharge. In a later stage of thiswork a quartz tube containing the substance to beirradiated was built into the discharge vessel parallelto the cylinder axis. The analyzing light beam ispassed through this tube in the vertical direction.
The eight spark gaps6 (Fig. 3) in the main dischargecircuit are fired from a preignition circuit (30 kY) whichagain is fired by a ninth triggered spark gap. Thetrigger pulse for this prespark gap is generated by athyratron circuit with an air transformer. The jitterof the total trigger system is well below 0.5 sec.
The discharge current was measured by means of aRogowski coil7 with an integration network and aTektronix 551 dual beam oscilloscope. The measure-ment of the time behavior of the light source wascarried out using the streak technique with a rotatingmirror camera8 of our own construction. This camerahas a maximum streak velocity of 3 mm/,4sec on the
film and a time resolution of 50 nsec. The same device,slightly modified, was used to take streak spectra of thedischarge by smearing an image of the hot plasma overthe entrance slit of a 3.4-m Ebert spectrograph (Hilgerand Watts). The spectral distribution of radiationdensity of the discharge was determined by comparisonwith a standard carbon arc9 l 0 and an Osram uvnormal." " 2 In these measurements the Hilger spectro-graph was used as a monochromator with variousphotomultipliers as light detectors [EMI 9529B for uv;FS 9 A (Fernseh GmbH) for visible] together with theTektronix 551 oscilloscope.
ResultsBy varying the gas (air, Ar, He, Kr, Xe) and the
pressure (0.001-100 torr) it was found that optimumdischarge characteristics occurred at a gas pressure of0.03 torr and with argon as the filling gas. Increasingpressure caused a reduction of contraction velocity,longer light pulses, and lower brightness. Pressuresbelow 0.03 torr Ar decreased the brightness of thelight source as well. Figure 4 gives a comparisonbetween streak photographs of the discharge at varyingpressures in argon. The effective exposure time onPolapan 200 film on these photographs is 50 nsec, theoriginal time scale 3 mm/Asec. From these figures,the contraction velocity of the plasma sheath wascalculated to be 3 X 10' cm/sec for 0.03 torr. The dia-meter of the light emitting plasma column at the mo-ment of maximum contraction was below 7 mm.
Figure 5 shows an oscillogram of the radiation densityat 4000 A with a time scale of 0.2 usec per large division.The half-width of the light pulse is below 0.2 usec.Neither a tail nor a secondary contraction are foundunder these conditions.
In streak spectra of the pinch, a very intense con-tinuum could be seen during maximum contractionover the total wavelength range under observation
(3000-6000 X). Some weak lines were also present andhad a lifetime of some jisec, but their total intensity wasnegligible compared with the continuum brightness.
The spectral radiation densities S* in W A-' cm-2sr-' in the continuum of the pinch and some otherlight sources are plotted in Fig. 6. They have beenmeasured and calculated by comparison of the dif-ferent light sources with the known radiation densitiesof the standard carbon arc9 and the uv normal," usingthe same beam path and detector for all of them.The argon pinch has a maximum radiation density ofmore than 102 W A-' cm-2 sr-' and the curve shows asteady increase toward the short wavelength region.The equivalent blackbody temperature,. calculatedfrom the Planck function in the 5000-A region is60,0000 K. At lower wavelengths the pinch plasmais partially transparent for its own radiation andtherefore cannot be regarded as a blackbody. Theradiation densities of the pinch discharge under opti-mum conditions of the other gases investigated werenear or below the air values. These gases howeverare either too expensive for routine use or have toomany spectral lines superimposed upon the continuum.For instance the Heii line at 4686 A is very strongand broad and dominates the pinch spectrum in He.
Integration of the spectral radiation density of theargon pinch over the wavelength range from 2000 A to6000 A, the pinch surface, solid angle, and discharge timegives a value of E 40 Wsec for the total energyemitted in this part of the spectrum. Thus theenergy efficiency of the discharge in this wavelengthregion is about 0.9%.
From measurements with a Rogowski coil,7 the max-imum current was calculated to be JO = 3.3 X 105 A,which is in relatively good agreement with the value ofJ = 4.3 X 105 A obtained from the initial electricaldata.
The current curve proved to be a damped oscillation(Fig. 7). From this curve the total inductance andthe resistance of the discharge circuit may be calculated.
They are L 50 nH, R 20 mg. These dataand the contraction velocity of the plasma sheathresult in an electron temperature being in the order of5 X 10' K corresponding to an average thermalenergy of 50 eV. The degree of ionization must there-fore be very high.
Discussion
In contrast to the requirements of plasma physics,the objective of the present work was to develop a z-pinch device as a pulse light source of high intensityand short pulse duration. The results obtained showthat the technical effort involved has been worthwhilefor the purposes of pulse photolysis. The pulse lengthhas been reduced by more than two orders of magnitude(for equal light output) compared with the usualcapillary spark in xenon. Thus it is now possible toobtain a time resolution in flash photolysis similarto that used in pulse radiolysis'3-"5 and to comparethe results of both methods.
Fig. 2. (a) Photograph of the discharge tube withcovered spark gaps, Rogowski coil, and built-inirradiation tube. (b) Horizontal cross sectionthrough the discharge tube assembly. (c) Ver-tical cross section through the discharge tubeassembly.
Owing to the good spectral continuum of the z-pinch, this device could (in principle) be used as ananalysis light source for flash spectroscopy as well.The linear extension of the light emitting plasmahowever is not very suitable for the illumination ofspectroscopic systems. For this purpose a Thetapinch assembly'6 using the end-on light emission ofthe plasma would be more useful. A further advantageof this source would be a greater optical thickness of the
radiating plasma resulting in a better approach toblackbody radiation.
One problem in the application of the z-pinch ap-paratus as an irradiation light source in flash photo-lysis is the optimum location of the substance to beirradiated. The usual arrangement of experimentalmaterial and light source in the line foci of an ellipticmirror causes sme difficulties because of the presenceof spark gaps and condensers and the need of a large
Fig. 4. Streak photographs of z-pinch in argon,tion; 33 kV, 4.85 kJ.
Fig. 5. Oscillogram of radiation density at 4000 I Time scale,0.2 pusec per large division.
quartz container for the discharge tube. The locationof the irradiation cuvette inside the discharge tube, asshown in Fig. 2, avoids these difficulties and has theadditional advantage that, because of the good vacuumbetween plasma and cuvette during maximum con-traction, vacuum uv irradiations become possible.The disturbance of symmetry caused by the built-inquartz tube turned out to be of minor importance forthe discharge characteristics. Even filling the cuvettewith conducting liquids or with metals did not causemore than a small dislocation of the contraction line,and did not essentially influence radiation density orpulse length. Disadvantages of this arrangement are(a) that shock waves directly impinge upon the ir-radiation tube and may cause trouble in analysis,
and (b) that a selective filtering for distinct wavelengthranges is possible only by using cuvettes of differentmaterials.
These difficulties might be overcome by using adouble-walled irradiation tube, the interspace of whichcould either be evacuated or filled with filter fluids.
At present, investigations are being carried out inour laboratory concerning the question whether thedischarge chairacteristics can be further improved byvarying the dimensions of the discharge vessel.
The first results of flash photolytic investigationsobtained with this equipment will be published else-where.
The authors want to thank Professor Glubrecht andProfessor Bartels for numerous helpful discussions andencouragement, the Federal Ministry of ScientificResearch for financial support, and U. Lesch andPrampain dit Boulan for interested and active technicalassistance.
fRaii1 1to&
tO .
o -
JO'
Argon Pinch0,03mm Hg
Air Pinch0,06mm Hg
Xenon Flashlamp
Standard Carbon Arc
2000 4000 6000
Fig. 6. Spectral radiation densities of some light sources.Xenon flashlamp: Philips, type 103723; 65-J tungsten filament
lamp; Osram, type Wi: 35, 5V, 16 A.
Fig. 7. Oscillogram of discharge current (integrated Rogowskicoil signal). Time scale, 2 isec per large division.
1. R. G. W. Norrish and G. Porter, Discussions Faraday Soc.17, 40 (1954).
2. L. I. Grossweiner, in Advances in Radiation Biology, L.Augenstein, R. Mason, and M. Zelle, Eds. (Academic PressInc., New York, 1966), Vol. 2.
3. A. D. McLaren and D. Shugar, Photochemistry of Proteinsand Nucleic Acids (Pergamon Press, London, 1964).
4. G. Porter, Proc. Roy. Soc. A 200, 284 (1950).
5. S. Glasstone and R. H. Lovberg, Controlled ThermonuclearReactions (D. Van Nostrand Co., Inc., Princeton, 1960),Sec. 3.25.
6. H. Zwicker und M. Kaufmann, Z. Phys. 180, 255 (1964).
S. L. Leonard, in Plasma Diagnostic Techniques, R. H.Huddlestone and S. L. Leonard, Eds. (Academic Press, Inc.,New York, 1965), p. 8 ff.
8. S. L. Leonard, in Ref. 7, p. 30ff.9. J. P. Mehltretter, Dissertation, Heidelberg (1962).
10. J. Euler, Ann. Physik 11, 203 (1953).11. H. Krefft, F. Rossler, und A. Rittenauer, Z. Tech. Phys.
18, 20 (1937).12. F. Rossler, Ann. Physik 34, 1 (1939).13. J. W. Boag, Phys. Med. Biol. 10, 457 (1965).14. J. P. Keene, J. Sci. Instr. 41, 493 (1964).15. L. M. Dorfman and M. S. Matheson, Prog. Reaction Kine-
tics 3, 237 (1965).16. S. Glasstone and R. H. Lovberg, Controlled Thermonuclear
Reactions (D. Van Nostrand Co., Inc., Princeton, 1960), Sec.11.1-11.8.
OSA Detroit-photos D. L. MacAdam
Alan Monkewicz Notre Dame University.
L. W. Nichols Naval Weapons Center, China Lake.
M. B. Wells Radiation ResearchAssociates at the AtmosphericOptics Technical Group Session.
Volume 57, No. 12 JOURNAL OF THE OPTICAL SOCIETY OF AMERICA December 1967
Analysis of the He ii 4686-A (n = 4 to n = 3) Line Complex Excited in an Atomic-Beam Light Source .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... . . Harold P. Larson and Robert W. Stanley 1439
Excitation of the 4F States of Helium .. . . .... . . . . .. . .. . . . . . John D. Jobe and Robert M. St. John 1449
Spectroscopic Measurement of the Nuclear Spin and Magnetic Moment of 3 Ar ........... . . . . . . . . . ..XV . . . . . . . . . . . . . . . . . . . . . . . .W. Traub, F. L. Roesler, M. M. Robertson, and V. W. Cohen 1452
Spectrum of Doubly Ionized Lanthanum (La I) .Halis Odabasi 1459
Rotational Spectrum of Hydrogen Fluoride: Frequencies and Linewidths . . . . . . Arthur A. Mason and Alvin H. Nielsen 1464
Experimental Values of the Atomic Absorption Cross Section of Potassium Between 580 A and 1000 A . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . Robert D. Hudson and Virginia L. Carter 1471
Far-Infrared Spectrum of Cadmium Telluride . . . . . . . . . . . . 0. M. Stafsudd, F. A. Haak, and K. Radisavljevi6 1475
Multiple Scattering and the Method of Rytov ........................... G. R. Heidbreder 1477
Nonlinearity in Optical Imaging Systems. . . . . . . . . . . . . . . . . Richard J. Becherer and George B. Parrent, Jr. 1479
Transfer Function for Cascaded Optical Systems . . . . . . . .. . . . . . John B. DeVelis and George B. Parrent, Jr. 1486
Integral-Transform Formulation of Diffraction Theory . . . . . . . . . . . . . . . . . . . . . . . George C. Sherman 1490
Diffraction Images of Annular and Disk-like Objects under Partially Coherent Illumination . . . . . . . . . . . S. C. Som 1499
Illustration of the Use of Characteristic Functions in Lens Design . . . . . . . . . . . . . . . . . . .Donald R. Wilder 1510
Visibility of Light Sources Against a Background of Uniform Luminance . . . . . . . . ... . . . . Melvin H. Horman 1516
Evaluation of Hologram Aberrations by Ray Tracing . . . . . . . . . . . . . . . I. A. Abramowitz and J. M. Ballantyne 1522
Coherence Requirements in Holography . . . . . . . . . . . . . . . . . . . . M. Bertolotti, F. Gori, and G. Guattari 1526
Translation of Light Frequency by Moving Grating ...... . . . . . . . . . . . . . . Takeo mi Suzuki and Ryuichi Hioki 1551Optical Constants of Black Polyethylene ....... . . . . . . . . . . . . . . . Robert J. Bell and Geoffrey M. Goldman 1552Generation of Matrix Elements of Angular-Momentum Operators ....... . .. . . . . .. . . . . . . . R. P. Hudson 1552
Book Reviews:Interferometry. By W. H. Steel . .... ... . .................... Reviewed by C. Frank Mooney 1553Telescopes. How to Make Them and Use Them. Edited by Thornton Page and Lou Williams Page .... . .
