B. W. BULLOCK AND S. SILVERMAN Vol. 43

3. Radiation for CO2 at 2.8,u lags behind the peakpressure by about 0.015 sec which is equivalent toseveral radiative lifetimes.

4. There is evidence of radiative nonequilibriumshortly after the combustion wave reaches the wall forboth CO2 and CO as shown in Fig. 8.

5. The flare-up of radiation during cooling is mostpronounced for regions of high optical density. Thecorresponding changes in brightness temperature are

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

as much as 200-300° K, and an approximate peri-odicity (10-15 cps) is noted in the brightness flare-up.

Further experiments are under way on the generalproblem of nonequilibrium phenomena in the CO-oxygen reaction, and these will be reported elsewhere.

The authors wish to express their appreciation toMr. B. H. Nall for his help in experimental mattersand to Miss Doris Rubenfeld, who is responsible forthe drawings.

VOLUME 43, NUMBER 5 MAY, 1953

Self-Reversal of the Ha Hydrogen Line in the High Pressure Spark Discharge*HEINZ FISCHER

A iy Force Cambridge Research Center, Camibridge, Massachlusetts(Received December 1,1952)

With increasing gas pressure and spark energy, the Balmer lines in the hydrogen spectrum (with theexception of H,) become so wide and overlapping that they form a continuous spectrum, as is well known.Only Ha remains relatively sharp and separated even with the highest energies obtained so far.

By increasing the energy density in the spark channel substantially beyond the present limits, Ha alsowidens considerably and finally shows reversal in its center. The spectral range of the reversal widens withincreasing energy and gas pressure. The phenomenon is discussed.

I. INTRODUCTION

THE Balmer lines of the hydrogen spectrum inself-reversal are produced in a nonuniforml2

atmosphere of H-atoms excited to the n= 2 level(-10.2 e-volts). In the case of thermal equilibriumbetween exciting electrons and H-atoms, this requiresextremely high gas temperatures (T>80 0000 K) orextremely large optical path T (concentration timesdistance) and gas pressure p. Such conditions have beenfound so far only in stellar atmospheres.

Self-reversal of Balmer lines, however, can also beobtained theoretically with any low gas temperature,provided that a nonuniform atmosphere with sufficientoptical depth, rk (kx= absorption coefficient), of excitedH-atoms can be produced by any onthermal mechanismof excitation. Indeed, reversal of Balmer lines hasalready been observed by several authors.'-5 Objectionsto the use of the term self-reversal in these cases maybe raised, however, since two separate systems wereused. A "strongly" excited gas discharge (tube 1),serving as a light source, produced broadened Balmerlines in emission. This light was passed through a"weakly" excited discharge (tube 2) containingsufficient excited H-atoms (end-on observation) in order

* Presented at the Conference for Gaseous Electronics, Prince-ton, New Jersey, September 4, 1952.

1 R. C. Cowan and G. H. Dieke, Revs. Modern Phys. 20, 418(1948).

2 H. Bartels, Z. Physik 125, 597 (1947); 126, 108 (1949).3 V. Schumann, Astronomy and Astrophysics 12, 159 (1893).4 M. Kimura and G. Nakamura, Japan J. Phys. 2, 53 (1923).6 R. W. Wood, Phil. Mag. 2, 876 (1926).

to produce absorption in the center of the lines emittedfrom tube 1; but it was also reported that the reversal'could still be observed when tube 2 was not excited atall.

The present paper deals with self-reversal of theBalmer line H,,,, produced from the channel of a highpressure spark discharge (C= 0.65 F, L =0.16 1uH,tungsten-electrodes, gap= 0.07 cm). The radiation wasobserved from a 90 degree angle of the gap. The effectwas obtained by increasing the energy density in thespark channel considerably beyond those reported sofar. This was done mainly by decreasing the inductanceL of the spark circuit and by increasing the gas pressure.The method of obtaining large energy densities in thespark channel and its limitations will be discussed in alater article. Previously only extremely broadenedBalmer lines in emission were repogted. 6 -

II. EXPERIMENTAL RESULTS

The discharge is a damped oscillation, f-0.5 mega-cycle. Figure 1 (a) gives the time function of the sparkcurrent in arbitrary units as measured with the Tek-tronics-Scope, 513D, one unit of the time basis corre-sponding to one microsecond. Reproducible time func-tions of the radiation so far could be taken only withrelatively small spark energies. Here the intensity de-

' See also W. Finkelnburg, Continuous Spectra (Julius Springer,Blerlin, 1938).

7 J. D. Craggs and I. M. Meek, Proc. Roy. Soc. (London) A186,241 (1947).

8 G. Glaser, Optik 7, 33 (1950).9 W. Finkelnburg, Z. Physik. 70, 375 (1931).

394

SELF-REVERSAL OF Ha HYDROGEN LINE

(a)

(b)

FIG. 1. Time functions of current and radiation. (The ripples on the trace seem to be characteristic for this specific coaxial capacitorarrangement.) (a) Spark current (17 at, 17 kv, 1 flash). (b) Spark radiation (17 at, 17 kv, 1 flash; photomultiplier lp22, Wratten filter53).

creases to less than approximately 10 percent of itsmaximum value within approximately 6 microseconds,see also Fig. 1(b). It is expected that with increasingspark energy the intensity drops faster from its maxi-mum because of an increased amount of free-free radia-tion.6

Figure 2 shows the hydrogen spectrum as emittedfrom the spark channel and photographed on Kodak103a-F(3) through a Gaertner quartz spectrometer. Itrepresents the radiation integrated over the total dura-tion of the breakdown, not only the radiation in the timeof the radiation maximum. Fortunately, however, theafterglow in hydrogen is relatively small,7 as also can beseen in Fig. 1 (b), so that the integrated spectrum showsthe same characteristics as that at maximum tempera-

ture. The slit is parallel to the spark channel, distance1.8 meters. No focusing system was used.

Figure 2 demonstrated the increase of the line widthof Ha as a function of the gas pressure p and the ca-pacitor energy K; No. 1 is the mercury spectrum forwavelength comparison; Nos. 2 and 11 show Ha withlow energy. Self-reversal of Ha is seen in Nos. 7 to 10.The reversal widens with increasing p and K, as isdemonstrated. The intensity of Ha increases consider-ably from 1 to 7 atmospheres; from here on the maxi-mum intensity in the wings only increases slowly with p.

Intensity and shape of Ha do not change when filteredthrough a polarization filter, with positions 0, 60, 90,and 120 degrees to the spectrometer slit.

The technical data of the hydrogen spectra in Fig. 2

395May 1953

HEINZ FISCHER

6

7

8

9

10

1

are given in Table I; K is the total capacitor energy, ofwhich only a small and unknown fraction goes intothe spark~channel; 7<0.01 has been found by Glaser"(see also Table II). The relative energy input, specifi-cally that during the first half-oscillation of the current(see Fig. 1(a)), may be somewhat larger under ourconditions because of the decreased electron mobilityin the channel (increased resistance) because of highergas pressure p and larger ionization degree x.

The values Kp may be used to compare roughly theenergy densities E in the spark channel, since thechannel cross section is expected to go approximatelywith 1/p; but E also increases with decreasing timeconstant of the circuit, and therefore the valuesKp/-r in Table II may also be used for comparison.

Table II shows comparison with conditions obtainedby different authors. Largest energy densities in hydro-gen so far were obtained by Finkelnburg9 ; his K and Kpvalues are comparable to ours, but his Kp/r values arestill smaller by a factor 10, more or less. (This value isonly a guess since L is not given in the publication.)For this reason Finkelnburg may not have obtainedself-reversal of the Ha. It may be mentioned that ourpresent apparatus allows Kp/r values of 1.OX105 andlarger.

FIG. 2. H with increasing gaspressure and energy of the sparkdischarge.

The values of Craggs and Meek7 are supposedlyvalues of the true energy put into the channel, i.e., theequivalent K values are considerably larger. We wouldexpect the equivalent K values not to be larger than1-10, which is confirmed by the published spectra whichshow sharper lines Hp and H1 than were observed in ourspectrum No. 2 with the technical data given in Table I.

III. CONCLUSIONS

The energy density in the spark channel was raisedconsiderably beyond that reported previously (Tables

TABLE I. Technical data of hydrogen spectra in Fig. 2.

p u n K Kp p dNo. atmos kv flashes watt sec watt sec Xatmos atmos Xcm

2 1 5 20 8.1 8.1 0.073 7 14 1 63.0 440.0 0.494 14 15 1 73.0 1020.0 0.985 21 18 1 103.0 2170.0 1.416 28 21 1 142.0 4000.0 1.957 42 24 1 197.0 8250.0 2.98 56 28 1 255.0 14300.0 3.99 70 24a 1 197.0 13700.0 4.9

10 95 30 1 294.0 28000.0 6.611 1 4 20 5.2 5.2 0.07

'The discharge was not triggered, which explains the unsteadiness in theincrease of breakdown voltage with p.

396

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Vol. 43

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SELF-REVERSAL OF Ha HYDROGEN LINE

Table II. Data of some spark discharges in hydrogen reported so far.

C L d P U K KP Kp/T Pd TAuthor F pH p see cm at kv Watt sec Deg. Kel.

W. Finkelnburg, Z. 0.72 >10 m10 0.1 30 30 320 9500 950 3.0Physik 70, 375 (1931)J. Craggs and I. Square pulse 4.0 1.0 1.0 * (7.2X10< 7.2X10<) 1.0 -12000Meek, Proc. Roy. 120 ampSoc. (London) A186,241 (1947)G. Glaser, Optik 7, 0.1 0.1 0.62 1.0 10.3 10 5 50 80 10.333 (1950) 0.1 0.5 1.4 1.0 10.3 10 5 50 36 10.3

Self-Reversal 0.65 0.16 2.0 0.07 42 24 197 8250 4125 2.9Ha observed 2.0 0.07 95 30 294 28000 14000 6.6

I and II). Reversal of the Balmer line Ha is obtained byincreased energy density in the channel as demonstratedin Fig. 2, and Table I.

The effect is true self-reversal; Stark effect as causedby the electric field of the electrodes can be ruled out,

as well as Zeeman effect as caused by the magnetic fieldof the spark current.t

The approaching opacity of Ha is demonstrated by itsintensity, which increases more or less directly withincreasing energy at relatively small p and K (Fig. 2),but becomes almost constant shortly before self-reversal is observed. The reversal itself widens with pand K as may be expected.

The hydrogen atmosphere across the spark channelmust be nonuniform because of the observed self-reversal. It means that the ion-density N+ in thechannel, as well as the gas temperature T, does de-

crease from the axis to the outside, since the broadening-of the Ha is assumed to be primarily determined by

intermolecular Stark-effect as caused by N+. Structurewithin the channel at a short time spark discharge hasnot been observed so far.

t The magnetic field though extremely strong is not uniformenough to account for the sharp contour of the line-reversal.

The reversing layer may be expected close to theboundary layer of the channel, which under such con-ditions is assumed" to expand mainly by diffusion ofshort-wave ultraviolet radiation.

This geometry shows some approach to that of stellaratmospheres, but the question of whether or not there islocal thermal equilibrium in the channel in the sense ofBoltzmann distribution will not be discussed here. Itmust be pointed out, however, that the relaxation timeswill be much shorter under the prevailing conditionsthan have been found elsewhere7' 1 -12 (-one micro-second), because of the much higher gas pressure p andlarger ionization degree x which will be close to 1.

The other Blamer lines (besides Ha) show no reversalunder the present conditions, as may be expected sincethey are so wide and overlapping that they cannot bedistinguished; its intensity increases strongly with pand K. Also, the Balmer continuum in absorption can-not be observed.

A more thorough analysis of the broadening andreversal of Ha will be tried at a later date with a time-resolved spectrum.

'1 W. Weizel and R. Rompe, Ann. Physik 1, 285 (1947).11 Margenau, McMillan, Dearnley, Peassall, and Montgomery,

Phys. Rev. 70, 349 (1946)..12 L. Blitzer and W. Cady, J. Opt. Soc. Am. 41, 440 (1951).

397May 1953