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Photo-Ionization of Caesium by Line Absorption

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Photo-Ionization of Caesium by Line Absorption . RP186 PHOTO-IONIZATION OF CESIUM BY LINE ABSORPTION By F. L. Mohler and C. Boeckner ABSTRACT Method.—The ionization of caesium by line absorption has been investigated by means of the space-charge method. Light resolved by a monochromator is used and the relative sensitivity in the region of line absorption, as compared to the region of continuous absorption beyond the limit, is measured. Ionization by a continuous spectrum.—The effect is directly proportional to the light intensity. Relative sensitivity is independent of electrical conditions and temperature of the vapor. The reduction of the effect by an absorbing column of caesium vapor in the light path indicates that the effective absorption line width is about 0.007 A at 0.003 mm pressure. This width increases with pres- sure. Relative sensitivity rises rapidly with vapor pressure between 0.0002 and 0.004 mm and drops slowdy above 0.01 mm. Ionization by monochromatic light. —A helium line at 3888 A is coincident with one of the caesium lines and measurements of the absorption of this line by caesium and of the photo-ionization produced give the probablity of ionization of an excited atom in the 4 Pi state as a function of the vapor pressure. Discussion.— Measurements of relative sensitivity to continuous spectrum excitation are corrected for absorption and slit width to give the probability of ionization of excited atoms in states 4 to 8 P and higher states near 9 P, 12 P, 16 P, and 25 P. This probability is shown to be proportional to the chance that an excited atom collides with another caesium atom during the life of the excited state. The probability of ionization at a collision varies from nearly one for higher states to 0.003 for the 4 P state. Results are consistent with a theory of Franck's that ionization results from the combination of excited and normal atoms to form molecule ions. CONTENTS Page I. Introduction 52 1. Theproblem 52 2. Caesium absorption 52 II. Experimental procedure 54 1. Method 54 2. Radiation flux 55 3. Properties of the space charge tube 55 ' III. Ionization by a continuous spectrum 56 1. Intensity relation 56 2. Effect of electron current and voltage 56 3. Temperature effect 56 4. Absorption effect 57 5. Pressure effect 59 6. Quenching by foreign gases 60 IV. Ionization by helium lines 61 1. The helium line at 3,888 A 61 2. Ionization by 3,188 A of helium 62 3. Ionization by other line sources 62 V. Discussion 63 1 Restriction of the problem 63 2. Correction for absorption 63 3. The pressure variation of efficiency 66 4. The hypothesis of molecule ion formation 69 i VI. Summary 71 SI 52 Bureau of Standards Journal of Research [voi.s I. INTRODUCTION 1. THE PROBLEM Experiments on the photoionization of caesium vapor by Mohler, Foote, and Chenault 1 have shown that in addition to the predicted ionization beyond the absorption series limit resulting from con- tinuous absorption there was some ionization by line absorption. Illumination with light from an incandescent lamp analyzed by a monochromator showed separate sharp peaks in the ionization at the fourth, fifth, sixth, and seventh lines of the principal series, while the partially or completely unresolved higher lines gave an effect which increased to a maximum at the limit (3,184 A). The third line gave a barely measurable effect and the second line no effect. Kecent results by Lawrence and Edlefsen 2 show a similar phenom- enon in rubidium vapor with peaks at the fourth, fifth, and sixth lines, though one other peak at 3,490 A does not coincide with an absorption line. These measurements have been made by observing the neutralizing effect of the ions on a thermionic current limited by electron space charge. The method is extremely sensitive and unaffected by surface photo-electric emission though it does not give an absolute measure of the ion current. Mohler, Foote, and Chenault proposed the theory that the excited atoms produced by line absorption were ionized by atomic collisions in which the ionization energy (maximum 0.7 volt) came from thermal agitation. It has been pointed out in criticism 34 that the probability of such thermal ionization is too small to account for the observed effect, and Franck and Jordan have suggested that electron collisions with excited atoms might explain the ionization, since meas- urements were made in the presence of an electron current. Another suggestion of Franck's is that the excited atoms on collision with normal atoms form molecule ions. The criticism that the ionization by line absorption is too great to be explained by thermal agitation is borne out by the following experiments, but a quantitative estimate of the probability of ionization involves a knowledge of the ratio of the number of ions produced to the number of quanta absorbed in a fine line of unknown width. For this reason Mohler, Foote, and Chenault limited their discussion to the relative heights of the peaks at the separate lines. Recent investigations of the absorption spectrum of caesium and of the absolute value of the photo-ionization beyond the series limit offer a basis for further investigation of the ionization by line absorption. 2. CESIUM ABSORPTION A recent paper by Waibel 5 gives quantitative measurements of the absorption by the higher series lines of caesium. The significant datum in line absorption is the integral of the absorption coefficient across the line and to make the magnitudes measurable he used thin layers of vapor at high pressure so that lines were much broadened. The third column of Table 1 gives WaibePs observed values of fkdv in -1 terms of atomic absorption coefficient and cm for the doublet 1 Mohler, Foote, and Chenault, Phys. Rev., 27, p. 37; 1926. J Lawrence and Edlefsen, Phys. Rev., 34, p. 233; 1929. 3 Franck and Jordan, Anregnung voq Quanten Spriingen durch Stosse, p. 126, Julius Springer, Berlin. * Qudden, Lichtelektrische Erscheinungen, p. 224, Julius Springer, Berlin. ' Waibel, Zeits. f. Phys., 53, p. 459; 1929. — Mohler 1 Boeckner] Photo-Ionization by Line Absorption 53 IS— mPi i2 - The fourth column gives this integral divided by the mean frequency interval between lines Av= (vm+i~ vm _i)/2. This constant ratio should equal the atomic absorption coefficient at the 6 series limit, k . The authors have published absolute measurements of the ionization produced by continuous absorption in caesium vapor 10" 19 from which they compute a value of k = 2.SX . This is not in satisfactory agreement with the value from WaibePs data, and other measurements give still higher values. For present purposes it has seemed best to use our value of k , together with values of fkdv obtained by multiplying WaibeFs values by 2.3/0.48 as given in column 5 of Table 1. It will be shown later that the lower value of Waibel would lead to serious inconsistencies in the present results. Table 1. Line absorption in caesium lS-mPi fkdv fkdv fkdv Transmis- m sion of 4 cm in A observed ^ o =2.3X10-i 9 Ay at 0.01 mm 8 4 3877 (550X10-1 ) 0.012 5. 3612 (288) .100 6 3477 35X10-18 0.40X10-i» 170 .26 3398 20.6 .37 100 .45 8. 3349 16.2 .43 79 .53 9 . 3313 12.7 .48 62 .61 10 - 3288 9.2 .47 45 .70 11. 3270 7.3 .49 35 .76 12 3257 5.6 .49 27 .80 13 3246 4.3 .47 21 .84 14 3247 3.4 .47 16 .88 15. _. 3231 2.7 .45 13 .90 16 3225 (11) .91 WaibePs paper includes values for the width of the lines at various pressures. Results are expressed in terms of the "half width" here denoted by b\ or bv which is the interval between points where k has half the maximum value. The different lines of the series have nearly the same half width which varies as the square root of the pressure. For lS—mP2 b\ varied from 0.55 A at 12.3 mm pressure to 0.30 A at 4.4 mm. From thisrate of variation one computes values of 0.05 A at 0.1 mm, 0.015 A at 0.01 mm, and 0.005 A at 0.001 mm. (Pressures reduced to 0° C.) But the Doppler width for caesium atoms at 500° K. is 0.0055 A so that below 0.001 mm the Doppler broadening will be predominant. Since the lines are doublets in which each component has a hyperfine structure it becomes very complicated to compute the line shapes or widths to be expected at low pressure. If lines have the shape theoretically predicted for collision broad- ening 7 then fkdv = k(msix.) bv (1) The transmission of a layer of vapor of length L containing N atoms per cm3 for light coincident with an absorption line is then NLfkdv/Sv trans. (2) 8 Mohler and Boeckner, B. S. Jour. Research, 3, p. 303; 1929. » Handbuch der Physik., 20, p. 496. — 54 Bureau of Standards Journal of Research [Vol. 6 II. EXPERIMENTAL PROCEDURE 1. METHOD Figure 1 illustrates the type of space-charge tube used in most of this work. This is a quartz tube containing a platinum cylinder anode 4 cm long and 3 cm in diameter. In the ends of the anode are slits about 8 mm wide to admit light and for the cathode leads. The cathode is a hairpin platinum wire coated with oxides. The tube was usually sealed off from the pumps and heated by two inde- pendent heating units, one around the body of the tube and one around the appendix so that effects of vapor pressure and temperature could be studied independently.
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