Vol. 56

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 56, NUMBER 6

Detection of Self-Reversed Spectrum Lines by a Time-ResolvingSpectroscopic Technique

A. BARD6cz AND U. M. VANYEx

University for Technical Sciences, Hungarian A cadewny of Sciences, Budapest, Hungary

(Received 30 July 1965)

Time-resolution techniques have been used to observe self-reversal of iron lines as a function of time.The spectrum of iron was excited with an electronically regulated ac arc source controlled to a high precisionin time at currents up to 200 A. Time resolution was achieved with the aid of a disk, having an Archimedesspiral slot, rotating in front of the silt of a 2-m Ebert spectrograph. In the present spectral region, two typesof self-reversed iron lines could be easily distinguished for transitions terminating on levels above the groundstate. For lines terminating on the a5 !F levels, the width of the self-absorbed core is greatest at the end of thedischarge. For lines terminating on levels above a5F', the self-reversed core either disappears or becomesvery narrow at the end of the discharge.

INDEX HEADINGS: Spectroscopy; Sources; Detection.

INTRODUCTION

THE detection of spectrum lines exhibiting self-absorption and self-reversal is of paramount im-

portance in nearly all spectroscopic work. For instance,estimation of absolute or relative intensities of spectrumlines without consideration of this effect leads toerroneous results. When calibrating a photographic

emulsion with a series of spectra having known relativeintensities,'- 3 we must use lines which do not exhibitself-absorption or self-reversal under strongly varyingexperimental conditions. In spectrochemical analyses,

I H. M. Crosswhite, Spectrochim. Acta 4, 122 (1950-52).2 G. H. Dieke and H. M. Crosswhite, J. Opt. Soc. Am. 33, 425

(1943).3 E. S. Hodge, Soc. Appl. Spectry. Bull. 5, 4 (1951).

JUNE 1966

756

June1966 TIME-RESOLVED, SELF-REVERSED SPECTRAL LINES

lines showing this effect should be avoided in the deter-mination of compositions. 4

Detection of lines exhibiting self-reversal is importantin the classification of spectra. Lines exhibiting self-absorption arise from transitions to the ground state orlow-lying energy levels of the atom; therefore, linesending on these levels can be sorted out. This possi-bility has special importance for spectra of elementshaving very many lines.

In light sources having high vapor densities self-absorption and self-reversal is more pronounced.Examples of such light sources are the electrodelessdischarges with high metal-vapor pressures describedby Worden et al.A and the arc discharge lamps withvertical electrodes discussed by Ahrens.6 Since thedegree of self-reversal increases exponentially with theproduct of line strength and the number of absorbingatoms through which the radiation passes,' sources forthe detection of lines exhibiting self-reversal should bepreferred which have a high-intensity core surroundedwith a high-density vapor mantle. For this reason, high-current discharges have been used for this purpose. Areview of several experimental methods suitable for thedetection of self-absorbed lines can be found in the articleby Sugar8 in which he describes an arc source in whicha square current pulse of short duration and highamperage is used.

FIG. 1. Experimental arrangement for the production of time-resolved spectra. Electric circuit: Electronically controlled ac-arcsource. F=analytical gap. Ml, M2, M3=rimaging lenses.N = rotating disk for time resolution. O =magnetic pick-up.B-M4-M5-G-MG-P= 2-m Ebert spectrograph.

4 J. K. Hurwitz and J. Convey, J. Opt. Soc. Am. 42, 24 (1952).6 E. F. Worden, R. G. Gutmacher, and J. G. Conway, Appl.

Opt. 2, 707 (1963).6 L. H. Ahrens and S. R. Taylor, Spectrocliemical Analysis

(Addison Wesley Publ. Co., Inc., Reading, Mass., 1961), seconded.

7 R. D. Cowan and G. H. Dieke, Rev. Mod. Phys. 20, 418(1948).

8 J. Sugar, J. Res. Natl. Bur. Std. (U. S.) 66A, 321 (1962).

FIG. 2. Rotating disk N with aperture V for time resolution.B =slit of the spectrograph.

Steinhaus et al.,9 using a technique described byCowan et al.,7 have investigated the self-absorption ofspectrum lines as a function of time in high-voltagespark discharges. Preliminary time-resolution investi-gations of similar low vapor-density high-voltage sparkspectra showed that it is possible to use this techniquefor the classification of spectral lines.'"'"

It is the aim of this paper to describe a time-resolutiontechnique for investigating a high vapor density sourceand present preliminary results obtained for the ironspectrum.

EXPERIMENTAL ARRANGEMENT

The apparatus, consisting of a 2-m Ebert spectro-graph, time-resolving optics, and an ac arc source, isschematically represented in Fig. 1. Light from thearc gap F is conducted through the imaging systemonto the slit B of the spectrograph. P is the photo-graphic plate. To achieve time resolution, the rotatingdisk N is placed in front of the slit of the spectrograph.This disk has an aperture in the form of an Archimedesspiral, Fig. 2, the radial elevation of which is 12 mmand the width is 0.2 mm. The ac arc burns 0.005 sec,i.e., during one half-cycle of the ac from the supplyvoltage which corresponds to 900 rotation of the disk N.However, to make sure that the light originating fromthe afterglow of the discharge reaches the spectrograph,the angle included by the Archimedes spiral was chosento include a 120° portion of the cycle.

The ac arcs were produced by use of an electronically

I D. W. Steinhaus, H. M. Crosswhite, and G. H. Dieke, J. Opt.Soc. Am. 43, 257 (1953).

lo A. Bard6cz, Appl. Spectry. 11, 167 (1959).u A. Bard6cz, Rev. Univ. Mines 15, 344 (1959).

757

A. BARD6CZ AND U. M. VANYEK

controlled ac interrupted-arc source operating with highprecision in time as shown in Fig. 1.12,13 The electroniccontrol of the source has two purposes, first to assurea precise synchronization between the rotational phaseof the disk N and the onset of the discharge, and secondto assure operation with a high current. With conven-tional ac interrupted-arc sources it is not possible tooperate with 200 A, because generally the suppliescannot furnish this current and because at conventionalarc-repetition rates the electrodes in the test gap Fwould melt. With the aid of the electronic control of thesource and with low arc-repetition rates, it is possibleto operate with conventional power supplies and with-out excessive heating of the electrodes. The repetitionrate can be adjusted to any desired value since the ro-tating aluminum disk in Fig. 1 contains a soft iron insertwhich passes in front of the magnetic coil 0 thus pro-ducing a signal which is fed to the pulse generator JG.The total impedance of the arc circuit, drawn in heavylines, is only about 1.5 Q and the circuit is able to handlepulses of about 200 A. A more detailed description ofthis apparatus will appear elsewhere.

OBSERVATIONS IN THE TIME-RESOLVEDHIGH-CURRENT ARC SPECTRUM

Figure 3 displays three ac-arc spectra of iron. Thefirst and second spectra are of the conventional type,the third is time resolved. All three spectra were ob-tained in the second order of the spectrograph with adispersion of 1.7 A/mm. Ignition of the arc occurred atthe peak of the ac supply voltage. The operating peakwas 300 V. The arc-repetition rate for the first spectrumwas 50/sec; for the second and third spectra, the ratewas 5/sec. The first spectrum was taken with a peakcurrent of 5 A, the second and third with a peak currentof 200 A, with other experimental conditions being thesame. Twenty discharges were required to produce the

3580 3590 3500 3510

Is

FIG. 3. ac-arc spectra of iron. Top spectrum-5A; Middle spec-trum-200 A; bottom spectrum-same as middle spectrum buttime resolved.

i2 A. Bard6cz, Spectrochim. Acta 7, 238 (1955).

i3 A. Bard6cz, Spectrochim.,Acta 8, 152 (1956).

-10

T

T

I I I I 1100

FIG. 4. Microphotometer traces of the spectrograms in Fig. 3.First: a trace of the middle spectrogram in Fig. 3. Subsequentfour: traces taken from the time-resolved spectrograms in Fig. 3at time intervals of 0.001 sec. T=transmittance.

758 Vol. 56

0

i

I L I

TIME-RESOLVED, SELF-REVERSED SPECTRAL LINES

second spectrum and 480 to produce the third spectrum.At the top of the time-resolved spectrum in Fig. 3,some short lines superimposed from the ignition sparkindicate the zero-time position of the spectrum.

From the time scale in Fig. 3 it can be seen that theduration of the discharge was 0.005 sec, i.e., the durationof the fourth period of the ac voltage wave.

No self-absorption can be detected in the first, or low-current, spectrum, but a series of self-reversed linesappear in the high-current spectrum. Juxtaposition ofthe low- and high-current arc spectra makes it easy todistinguish between self-reversed lines and doublets inthe high-current spectrum.

In the time-resolved high-current spectrum, numer-ous additional lines which exhibited no self-reversal inthe conventional high-current arc are found to be self-reversed. Similarly, weak self-reversed lines in the con-ventional spectrum show prominent self-reversal in thetime-resolved spectrum.

To obtain more detailed information concerning self-reversal as a function of time, microphotometer traces(Fig. 4) were made from both conventional and time-resolved spectrograms. The traces were obtained forthe portion of the spectrum involving the Fe lines at3605.46, 3606.68, 3608.86, and 3610.16 A. The firsttrace, obtained from the conventional high-current arcspectrogram, shows clear self-reversal only for the Feline at 3608.86 A. The following four traces in Fig. 4are of spectrograms taken at time intervals of 0.001sec; the first was made 0.001 sec after the onset of thedischarge.

Two general kinds of self-reversed lines occur in thetime-resolved spectrum in Fig. 3. For one type of lines,the self-reversal begins at the onset of the discharge orsometime later; the most important characteristic isthat the width of the self-reversed core increases intime and at the end of the discharge the width of theself-reversed core is greatest. This type of line showsthe additional characteristic that the self-reversed coreat one part of the line is much broader than at the other.For the other type of lines the self-absorbed coreappears, in general later than for the previous type. Thecore becomes broader with the passing of time, attainsa maximum width, after which the core becomesnarrower. At the end of the discharge the self-absorbedcore is very narrow or has disappeared completely.These latter lines usually have very narrow self-reversed cores.

TABLE I. Wavelengths and excitation potentials for Fe i lines.

Lower excitationWavelength potentials Lower term

A eV designations

3608.86 1.01 oaFl3586.99 0.99 a5F23585.71 0.91 a5F43585.32 0.96 oaF33581.19 0.86 oa5F53610.16 2.81 276°3606.68 2.69 aIG53605.46 2.73 a3G43603.21 2.69 &aG53594.64 2.85 z7F4°3586.11 3.24 b3H63584.66 2.69 &aG5

For a better understanding of the behavior of thespectra in Fig. 4 it is necessary to consider the termsinvolved in these transitions. The lower levels of thesetransitions are summarized in Table I. The term valuesand term designations have been taken from Russellet al.14

In the spectral range chosen for the present investi-gation, no lines terminate on the ground level, a5D, ofFe i. A series of lines terminates on the next higher level,a1F, all of which show the first type of self-xeversal. Inthe time-resolved high-current spectrum in Fig. 3 sevenadditional lines show self-reversal, but these are identi-cal in appearance with the second type of self-reversal(see Table I).

By consideration of the lower excitation levels of thelines investigated we can make an unambiguous dis-tinction between the lines ending on the level a1F andthose ending on levels listed in the second group inTable I. The excitation potentials of the two groups oflines listed in Table I differ by less than 2 eV.

We are extending this study on self-reversed lines tothe whole arc spectrum of iron and hope to be able topublish a complete list of self-reversed spectrum linestogether with their self-reversal classification in the nearfuture.

ACKNOWLEDGMENTS

We are indebted to T. V6rds for the discussion of thepaper, to M. E. Aususztinovics, I. Ninausz, andG. Vladir for the experimental work and measurements.

14 H. N. Russell and C. E. Moore, Trans. Am. Phys. Soc. 34,113 (1944).

June 1966