E. E. DABY,J. S. HITT,AND G. J. MAINS
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Properties of the e-Pinch Flash Lampla by Eric E. Daby, Joe S. Hitt, and Gilbert J. Mainslb Departments of Chemistry and Electrical Engineering, University of Detroit, Detroit, Michigan (Received June 86, 1970)
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The &pinch plasma device has been investigated as a light source for flash photochemical applications. The electrodeless flash lamp emits a many line spectrum superimposed upon an underlying continuum. The intensity and duration of the optical emission was studied as a function of circuit inductance, capacitance, drive coil geometry, gas composition, gas pressure, and lamp geometry. Plash intensities greater than 1022 quanta cm-2 sec-‘have been attained at flash lifetimes less than 10 Msec. The &pinch flash lamp is shown to be a reliable and inexpensive high-intensity light source suitable for photochemical studies from the vacuum ultraviolet through the visible regions of the spectrum. The vacuum ultravioletflash photolysis of acetaldehyde and the kinetic spectroscopic study of naphthalene are briefly described applications of the &pinch flash lamp.
Introduction The fast magnetic compression of plasmas has been the subject of intensive research for the past two decades.2 These “pinch devices” are usually classified according to the motion of the plasma current as lOtrsli\.--.$ 0 0
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Figure 4. Signal response of 929 photodiode to typical &pinch discharge.
0.0 Unfortunately, neither the light emission nor the discharge current is readily amenable to such a simple mathematical analysis because the effective resistance of the plasma changes during the discharge.'O Nonetheless, from a practical viewpoint a knowledge of the qualitative effects of these parameters is necessary for the intelligent design of a &pinch lamp system for photochemical studies. For this reason we report the effects of varying the capacitance, discharge voltage, and circuit inductance and direct the interested reader t o Silberg's excellent paperlo for insight into the mathematical analysis. The lamp parameters, to be discussed later, were fixed for the following sequence of experiments. The lamp geometry was identical with that shown in Figure 1. The lamp diameter was 24 mm; the annulus diameter was 10 mm; the lamp was constructed of fused quartz and filled with a gas mixture [lo0 ppm Xe in Ar] to a pressure of 10 Torr. Capacitance. Three capacitors were employed to provide a range of 0.33 to 3.0 .uF by wiring in series and in parallel. The drive coil was a single turn of copper wire and the discharge voltage maintained a t 35 kV. The results are given in Table I. Instead of the ex-
Table I : Plasma Emission Dependence on Capacitance Capacitance,
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pected linear dependence of energy dissipated per flash upon capacitance ( E = C V 2 / 2 ) ,a much more complicated situation was encountered. Because of the The Journal of Physical Chem&Ty, Vol. 74,No. $4, 1070
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Figure 5. Light output as a function of stored energy: C = 1.0 pF; one-turn drive coil; 6-~seclifetime.
low inductance in the electrical circuit any rewiring of the capacitors also produced a significant change in the circuit inductance which, in turn, affected the life time of the discharge and the efficiency with which energy was dissipated in the plasma. It is readily seen from Table I that additional wiring for the 0.33 p F (series hook-up) and 3.0 pF (parallel hook-up) both caused significant increases in the discharge lifetime. I n fact, the simplest circuit (one capacitor, 1.0 pF) was the most efficient in our experiments. Therefore, the only efficient way to increase energy storage via increasing capacitance is to utilize larger single capacitors. The use of a large bank of capacitors, common in the usual flash lamp systems, is simply not practical for the &pinch lamp. Discharge Voltage. Provided the inductance and the capacitance of the circuit were fixed, the light output was a linear function of the stored energy, C V 2 / 2 . This is illustrated by the results, presented in Figures 5 and 6, of experiments in which the discharge voltage was varied from 20 to about 40 kV. r n Figure 5, the light output as measured by chemical actinometry is presented as a function of the stored energy of a single 1-pF capacitor. Each point represents the average of three actinometric determinations. The reproducibility of the &pinch lamp emission is demonstrated by the results presented in Figure 6 . I n t,his experiment the phototube signal was used to monitor the light output. Each point represents a single measurement of the emission from the lamp driven by a capacitor bank of three parallel 1-pF capacitors. The flash lifetime was essentially constant over this voltage range at 6
PROPERTIES OF THE &PINCH
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ENERGY (kjoules) Figure 6. Light output as a function of stored energy: C = 3.0 p F ; one-turn drive coil; 22-psec lifetime.
and 22 psec for the 1 and 3-pF capacitors, respectively. Above the threshold energy for plasma generation, light output was linearly dependent on the energy stored in the capacitor. It should be noted here that the threshold energy was markedly dependent on the geometry of the spark gap switch. Spark gap designs which lowered the switch inductance decreased the threshold energy. Coil Inductance and Geometry. The problems arising from the low impedance requirements of the &pinch circuitry became most evident when attempts were made to examine the effect of the drive coil in the emission process. Coil sizes ranged from a single turn, 1-in. diameter coil to a seven-turn, 2-in. diameter coil. Qualitatively, as the coil inductance was increased by either increasing the coil diameter or the number of turns, the emissive output was increased. Furthermore, the lifetime of the discharge was not measurably changed during this variation in coil geometry suggesting that the drive coil inductance was a small part of the circuit inductance. These qualitative observations are in agreement with Silberg's study'O which employed a 1.125-in. diameter lamp filled with 5 Torr of Ar. It is also pertinent to note that Silberg found that 59% of the stored energy (measured calorimetrically) could be dissipated in such a lamp using an eight-turn drive coil! Such efficiencies are indeed comparable to those obtained using conventional flash lamps. Lamp Parameters. The parameters considered in this study were the nature of the lamp gas, the pressure of the gas, and the lamp geometry. For these experiments the most efficient circuit parameters were employed, i e . , one capacitor (1.0 pF), discharge voltage fixed at 28 kV, and a single-turn, 1-in. diameter drive coil. The most critical lamp parameter was found to be gas pressure. Figur'e 7 shows the dependence of
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L A M P FILLING P R E S S U R E (torr) Figure 7. Light output as a function of lamp-filling pressure (see text for details).
light emission on filling pressure for the gas mixture, 100 ppm of Xe in Ar. Below 1 and above 20 Torr the plasma could not be formed without altering the circuit parameters. The light flux at the maximum corresponded to 3 X loz2quanta cm-l sec-' in the annulus. Argon, krypton, and xenon were also investigated in this study and exhibited pressure dependencies similar to that of Figure 7 . The pure gases exhibited maxima in the same pressure range but lower maximum light flux than the mixture. That is, the observed order of light flux emission was as follows: 100 ppm of Xe in Ar > pure Xe > pure Kr > pure Ar. However, there was less than a factor of ten variation between the gases and discussion of the reasons for these observations is not necessary here. The final parameter, lamp geometry, had very little effect on the light output except for the interdependence of lamp diameter and drive coil diameter noted earlier. It should be noted, however, that very short lamps tended to confine the plasma and reduce the light output. While we do not report the spectral distributions of the lamps employed in this research, it is pertinent to note that they are similar to those reported by Feldmana9 Lines in the emission from a krypton discharge have been assigned to Kr(1V) transitions and to Si (arising from quartz walls). Some of the transitions have not been assigned and may be the result of dielectronic excitation in the plasma. These characteristic lines are broadened and superimposed upon continuum to which a black-body temperature in excess of 10,OOO"Ii has been assigned. Thus, the light emission from a dense &pinch lamp may be thought of in terms of a continuum of almost uniform intensity throughout the visible-uv region with characteristic lines from the filling gas and wall material superimposed. The Journal of Physical Chemistry, Val. 74, No. $4, 1970
E. E. DABY,J. S. HITT, AND G. J. MAINS
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Figure 8. Kinetic bpinch photolysis appnrntiis: A, xenon flnqh lamp: k, lamp housing; B, bpineh lamp; b, reaction v s d ; c, discharge mil; C, spectrograph; d, slit; e, photographic plate holder.
Applicalioiis lo Kinetic Speclroscop?l. One of the earliest applications of conventional fliisli lamps was in the area of kinetic spectroscopy. The techniques, involving conventional flashlamps, have becnadequntely reviewed by Xorrish” and 1’0rter.’~ To determine the suitability of the 8-pinch flash lamp for such studies the apparatus, diagrammed in Figure S, was eonstructcd. The annular design @-pinch lamp is shown i n section B of Figure 8. The reaction vcssel, designated as “b,” is a fused quartz cylinder, 25 mm diameter, 300 mm in length, closed a t either end with optical quality fused quartz windows. The 8-pinch lamp, surrounding the reaction vessel, is 55 mm in diameter and 200 mm in length and was filled with 10 Torr of Xe. The drive coil was seven turns of 0.2Sin. copper tubing which loosely fitted the @-pinchlamp. Thr other circuit parameters for the 0-pinch lamp were as follows: capacitance, 1.0 pF; discharge voltage, 25 kV. The lifetime of the &pinch flash was 18 psec. The spectrographic flash lamp, designated as section A of Figure 8, was a special design, U-shaped, xenon flash lamp constructed by E G G . Co. A 0.5-pF capacitor, charged to 10 kV, was discharged through the spectrographic flash within 25 psec. Conventional circuitry’0 was used to delay the spectrographic flash for 0-1000 psec after the &pinch flash. The absorption spectrum of the sample in the reaction vessel v a s recorded on Kodak spectrographic plates, Type 103-F, using the Hilger medium quartz spectrograph, Model E-498, designated as section C on Figure 8. For the experiments reported here the reaction vessel was filled with a 0.031 M solution of naphthalene in ethanol and thoroughly outgassed using conventional high vacuum techniques. Spectra, recorded a t various time intervals following the @-pinchflash, are presented in Figure 9. T w o transient absorptions were observed. The shorter lived transient near 3900 A and the longer lived absorption near 4100 A have been observed previously by Forter’O and have been assigned t o To T, and To- TItransitions of the triplet state of naphthalene. The relative ease with which the &pinch flash lamp was adapted to kinetic spectroscopic experiments suggests that further refinements (Le., increasing the light output of the &pinch and decreasing the lifetime of the two flashes) will be forthcoming in the near future.
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The Journd of P h h d Chnnir(ry. Vol. 74. No. t 4 . 1870
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Figure 9. Plasma pinch kinetic flash spectmsmpy of 0.031 A I naphthalene in ethanol. Spectrographic flash without the plasma pinch i3n.h (1 j, fired 0.00 m c after plasma pinch flash (Zj,fired 21, ,see after flash (3), fired 50 m c after flash (4), fired 7; pwc after flash (nj, fired 90 ,set after flash (6).
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NUMBER OF F L A S H E S Figure 10. Product yields from acetaldehyde as a function of the number of bpinch flashes (see text for details). (17) 11. G. W. Norrish, “Nobel Symposium 5 Fast Reaotions and Primary Processes in Chemical Kinetics.” Stig Cl-n. Ed.. 1867,
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(IS) G. Porter. ref 17. p 141. (18) G .Porter. “Technique 01 Orgnnic Chemistry." A. W e h b e r m . Ed.. Interscience. N e w York. N. Y..1963. 1, 1055. (20) M . Windsor and G. Porter, Dkussionr For&t! &.. 17, 178 (1854).
PROPERTIES OF THE &PINCH FLASH LAMP Although it must be admitted that the &pinch kinetic spectroscopy system will never become competitive with the recently developed &-spoiled laser systemsZ1,Z2 for kinetic spectroscopy because of the nanosecond resolution times of the latter, it must also be pointed out that the &pinch kinetic spectroscopy system is currently considerably less expensive, requires less technical skill to bu'ild and maintain, and is not limited to a single wavelength as is the laser system. $-Pinch Vacuum Uv Flash Photolysis. I n view of the high-temperature black-body continuum which underlies the emission from the dense &pinch flash it is not surprising that considerable radiation must be emitted a t wavelengths shorter than 2000 A as limited by fused quartz. This is simply demonstrated by using a epinch lamp of the end-on design, illustrated in Figure 3, with polished LiF windows which transmit to 1150 A. Carbon dioxide, which absorbs light below 1650 A at the pressures employed here and decomposes into carbon monoxide, was employed as an actinometer. Based on the yield of carbon monoxide per flash, we estimate a light emission of,2 X 10l6quanta ern+ flash-l in the region 1650-1150 A. I n view of the fact that the light emission in the region 2000-5770 A was only 2.3 X 10'' quanta cm-2 flash-' (see Table I) it must be concluded that the $-pinch is indeed a very intense flash lamp in the vacuum ultraviolet. To demonstrate the utility of the &pinch flash lamp in the vacuum uv region some preliminary results from the flash photolysis of acetaldehydeza are presented in Figure 10. The initial pressure of acetaldehyde was 5 Torr a t 25" and 0.5% was decomposed per flash. The products were analyzed using a C.E.C. 21-103C (modified) mass spectrometer. The linearity of the product yields suggests that secondary reactions can be ignored up to 5% conversion in the vacuum uv photolysis, simplifying the interpretation of the data. A more complete report, utilizing mixtures of acetaldehyde and
4209 perdeuterioacetaldehyde, will be publishedz3 and the mechanistic implications of Figure 10 will be discussed in more detail then. These data are presented here solely to illustrate the versatility of the &pinch flash lamp.
Conclusions The &pinch flash lamp is a convenient and inexpensive flash lamp system for simple flash photolysis experiments or kinetic spectroscopy experiments. Because of the electrodeless nature of the &pinch lamp, construction and maintenance are simple and inexpensive. Although the light flux is of the same order of magnitude as conventional flash lamps, the microsecond lifetime region is more easily accessible. Also, because of the higher black-body temperature of &pinch lamps they are efficient sources in the vacuum ultraviolet. However, the low-inductance circuitry of the e-pinch devices is not readily "scaled up" as has been done in the case of conventional flash lamp systems. Also, because a microsecond or so is usually required to build up the plasma and pinch it, the &pinch flash will not be extended into the nanosecond lifetime region. I n spite of these limitations the authors believe that the &pinch flash lamp is a useful addition to the light sources available for photochemical studies.
Acknowledgments. The U. S. Air Force Office of Scientific Research has patiently supported this new research effort at the University of Detroit and must be thanked by the authors. Mr. Randolph Mateer constructed the kinetic spectroscopy system and Mr. Cesar Castillo obtained the data recorded in Figure 9; their contributions are gratefully acknowledged. (21) J. R. Novak and M. Windsor, J. Chern. Phys., 47, 3075 (1967). (22) M . R. Topp and G. Porter, Proc. Roy. SOC., Ser. A, 315, 163 (1970). (23) E. E. Daby and G. J. Mains, to be published.
T h e Journal of Physical Chemistry, Vot. 74, No. 34, 1970
