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The measurement of absolute oscillator strengths for lines of neutral atoms
J. Quant. Spectrosc. Radiat. Transfer. Vol. 3, pp. 299-303. Pergamon Press Ltd. 1963. Printed in Great Britain
THE M E A S U R E M E N T OF ABSOLUTE OSCILLATOR STRENGTHS F O R LINES OF N E U T R A L ATOMS ROBERT B. KING California Institute of Technology, Pasadena, California Abstract--Comments are made on the present status of absolute oscillator strengths of lines in atomic spectra and on methods of measurement of these quantities. The measurement of absolute oscillator strengths of lines of neutral metals by means of an atomic beam experiment is described and recent results are discussed. I. I N T R O D U C T I O N CONSIDERABLE progress has been made in the past few years in the determination of the absolute oscillator strengths (f-;~alues) of spectral lines of neutral atoms by both experimental and theoretical methods. These data, which are required for determinations of abundances of the elements in the sun and stars and are useful in studies o f terrestial sources, have been (and still are) perhaps the least well known of any of the basic spectroscopic quantities. However, owing to an increase in the number of workers in the field and to improvements in techniques, both experimental and theoretical, absolute f-values are gradually becoming available for elements for which no data previously existed and more precise and reliable values are being determined for others. For lines o f ionized atoms, promising experimental methods are beginning to be developed, but most o f the absolute f-value data available at present for lines of ions are from theoretical calculations. Broadly speaking, there are two basic classes o f experimental methods o f measurement of the absolute f-values or transition probabilities o f atomic lines. The first class includes methods whereby the strength o f the line absorbed or emitted by a known number of atoms is measured. I f conditions are such as to permit the calculation of the population of the initial energy level involved in the transition, then the absolute probability of the transition can be derived from the observed strength o f the line. Many completely different types of experimental techniques fall into this classification depending upon how the atoms are excited (e.g. absorption tube, electric arc, atomic beam, etc.) and their absolute numbers determined and how the absolute strength o f the line is measured (e.g., total absorption, emission intensity, anomalous dispersion, magneto-rotation, etc.). The second class of experiments includes those in which the life-time of the upper energy level involved in the transition is measured. Then, if the relative probabilities of all downward transitions from this state are known, the absolute transition probabilities o f these lines are obtained since, under the proper conditions, the life time of the excited state depends only on the sum of all the transitions from it to lower energy states. Knowledge of the absolute number of atoms involved is not required. Thus the most difficult quantity to determine and the source o f the largest systematic errors in method of the 299
300
ROBERT B. lONG
first class is avoided. Recent applications of modern techniques of life-time measurements, notably by OTTINGERand ZIOCK Ct) and by DEMTRSDER(2) in Bonn, appear to have greatly increased the precision of this method and it offers great promise for future applications. There are very good reasons why new techniques of measurement should be encouraged and why established ones should be continued. First, because of the enormous range in the chemical and physical properties of the elements and the characteristics o f their spectra, no single method is applicable to all or even well adapted to a very large number with maximum efficiency. Second, measurements of absolute f-values by whatever method, in c o m m o n with other absolute physical measurements, are always subject to the possibility of the existence of undetected systematic errors. The theorists in the field are faced with the same type of problem arising from the approximate methods that must be used in calculating absolute f-values, particularly in complex spectra. Absolute f-value determinations by different methods, experimental and theoretical, have been known to differ by factors o f 2 or 3, and even much more in a few instances, as those who have had occasion to use these data are well aware*. Therefore, it is essential that measurements be repeated using different techniques. A good rule would be to consider no absolute f-value as definitive until at least two, or preferably more, measurements by quite different and reliable methods give results which agree within the internal uncertainties of the experiments. The internal errors should not be greater than 10-15 per Cent with techniques now available and should become smaller in the future.
II. D E S C R I P T I O N OF A USEFUL E X P E R I M E N T A L P R O C E D U R E We shall now describe the present status of an experimenL designed to measure f-values, t This work has been in progress for several years at the California Institute of Technology and is sponsored by the Office of Naval Research. In our experiment, the absolute f-values are determined by measuring the total absorption of very weak lines absorbed from a light beam passing through a beam of atoms. The concentration o f absorbing atoms is measured as part of the experiment. Thus the method falls in the first class of experiments described above. The principles of this method were first applied by KOPFERMANN and WESSEL¢SIin G0ttingen. The technique is applicable to most o f the metals which evaporate as monatomic gases. The advantage over other methods in its general class lies in the fact that the number of absorbing atoms is determined rather directly in the experiment, thereby avoiding the necessity o f relying on vapor pressure or other thermo-chemical data whose quality in the past, at least, has been highly variable. The general theory and procedures of the atomic beam experiment have been described by BELL et al. (4) and the details will not be repeated here. Absolute f-values have been published previously for the strongest ground state lines in the spectra o f Cu ~4), Fe ~4) MnC5 ~ and Pb. ~6~ Recent unpublished results will be mentioned later in this paper. * A critical survey of the experimental and theoretical f-value data available in 1960, both absolut and relative, for many atoms and ions is given by L. GOLDBERa,E. A. MULLER,and L. H. ALLERin their paper: The Abundances of the Elements in the Solar Atmosphere, Astrophys. J. Suppl. 5, No. 45 (1960). t A comprehensive bibliography of papers on absolute and relative f-values of atoms and ions has been recently published: Bibliography on Atomic Transition Probabilities by B. M. GLENNONand W. L. WIESE, National Bureau of Standards, Washington; Monograph 50 (1962).
The measurement of absolute oscillator strengths for lines of neutral atoms
301
Referring to a schematic diagram of the apparatus shown in Fig. 1, the atomic beam method may be described very briefly as follows. A beam of atoms (1) of the metal being studied is effused upward through a small orifice in a crucible (2) of suitable material which
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FIG. 1. Schematic diagram of atomic beam apparatus. is electrically heated by a tubular furnace (3). The atomic beam is restricted in angular spread by a rectangular opening (4). The beam flux is measured by allowing the central part of the beam to pass through a hole (5) and deposit on the inner surface of a thin, conical aluminum pan (6) that is attached to the arm of an electro-micro-balance. A shutter (7) prevents atoms from depositing on the pan until desired. The temperature inside the crucible is measured with an optical pyrometer by viewing the crucible orifice through the window (8). The light beam, consisting essentially of a thin sheet of light from a high pressure, quartz capillary mercury arc, passes through the atomic beam horizontally at a determined position. The instantaneous number of atoms per cm~ in the path of the light beam is determined from the measurement of atomic beam flux, knowledge of the angular distribution of atoms in the beam, and their mean velocity as calculated from the temperature of the crucible. The light beam is brought to a focus on the slit of a 21-ft concave grating Rowland-type spectrograph, with the aid of which the total absorption of the spectral lines are measured by either photographic or photoelectric photometry. Several major improvements in the apparatus and in the techniques of recording and measurement have been introduced recently by George Lawrence and John Link, aided by Robert Ashenfelter. These improvements will be described in detail in forthcoming publications, but three of them may be briefly mentioned here: (a) A new electrobalance, shown schematically in Fig. 1, has been constructed. The current required to keep the balance arm horizontal as the atomic beam deposits in the
302
ROBERT B. KING
conical balance pan is continuously recorded on a strip chart. The "noise" inherent in the system corresponds to approximately ½t~g. The slope of the line recorded on the chart yields the deposit rate in tzg per see. A minimum of 50-100 fig are usually deposited during a run. This balance also permits the accurate measurement of the impulse force imparted by the atoms in the beam to the balance pan when the shutter is opened or dosed. This measured force may not only be accurately compared with the calculated impulse force, (4) but may also be used to make a determination that all the atoms entering the conical pan are captured by it. The latter follows if the measured impulse force is equal to the product of the obsei-ved deposit rate and the time required for the balance current to return to its value prior to the time the shutter was opened. For all metals studied so far, with the exception of Au and probably T1, all atoms appear to be captured by the pan, within the accuracy of measurement, which is about 1 per cent. It should be noted that this is not a measurement of the accommodation coefficient, since most of the atoms that may be reflected on first impact with the conical pan are probably captured by it eventually. (b) An ionization gage (not shown in Fig. 1) within the vacuum chamber has been developed to monitor the atomic beam flux. A negligible fraction of the atoms in the beam is ionized by a constant electron current directed horizontally across the beam. Measurement of the ion current produced in the atomic beam, and collected by a charged ring concentric with the hole in the top plate (5), monitors the constancy of the atomic beam flux independently of the deposit rate measured by the electrobalance. (c) A photoelectric recording system has been developed to replace the photographic plate as a means of measuring the total absorption, or equivalent width, of lines absorbed by the atomic beam from the continuous spectrum of the light beam. A single photomultiplier tube is located behind two nearby slits on the focal curve of the spectrograph. As a shutter alternately exposes each slit at the rate of 4 times per sec, a continuous measurement of the difference in the d.c. amplified signals is recorded while one slit slowly scans across the line and the other slit views the nearby continuum. To avoid, as much as possible, uncertainties introduced by line broadening effects, including hyperfine structure, conditions must be obtained such that the absorption lines are very weak, that is, the lines must lie on the linear part of the "curve of growth" where the relationship between equivalent width and numbers of absorbing atoms is truly linear regardless of the broadening processes present. This generally means that the measurements must be made on lines the equivalent widths of which are less than about 0-010 A. With the new recorder, equivalent widths of less than 0.001 A can be measured with reasonable precision. As mentioned before, absolute f-values for lines in the spectra of Cu, Fe, Mn, and Pb measured with the atomic beam apparatus, prior to introduction of the specified improvements, have been published. (4-6) With the improved apparatus, George Lawrence and John Link have recently completed measurements of absolute f-values of one or more lines in the neutral spectra of ten additional metals. Lawrence has measured lines of Co, Ni, Ag, and Au. Link has measured Cr, Ga, Pd, In, Sn, and T1. Their results are now being prepared for detailed publication elsewhere. For all of these elements, with the exception of Pd, absolute f-values for at least one of the lines measured by Lawrence and Link have been measured by other investigators by other methods. These are cited below to show a sampling of the variety of methods that have been used to measure absolute f-values.
The measurement of absolute oscillator strengths for lines of neutral atoms
303
OSTROVSKY and PENKIN*, using absorption tubes and anomalous dispersion, have measured absolute f-values for lines of.Cr, (7) Ga(S) and In.(Sl ESTABROOK, using an absorption tube and measuring the total absorption of weak lines, obtained values for Ni (9) and Cr. (10) HINNOV KOHN (11) derived f-values for Ag lines from measurements in flame spectra. OTTINGER and ZtOCK (1), from measurements of the life times of the excited states, have obtained absolute f-values for Ga, and DEMTRODER(9), using this method, has reported values for G a and T1. In addition, transition probabilities or f-values derived from arc intensity measurements, and placed on an absolute scale with the aid of absolute f-values from other experiments or from theory, have been published for Cr, Co, Ni, Ga, and Ag by ALLEN and ASAAD(19), and for Sn by PROKOF'EV et al. (13) Absolute f-values also derived from arc spectra, are given for all of the lines measured by Lawrence and Link in the recently published monumental work E x p e r i m e n t a l Transition Probabilities for S e v e n t y E l e m e n t s by CORLISS and BOZMAN(14). The results obtained by Lawrence and Link with the atomic beam are not in very good agreement with most of the previous work by others cited above. However, good agreement was obtained for lines of Ga and TI with the results obtained by the life-time method by OTTINGER and ZIOCK and by DEMTRODER. It is also noteworthy that OTTINGER and ZIOCK'S measurement of the absolute f-value of the Fe line A3720 yielded f = 0.035 (1) with a n estimated uncertainty of 10 per cent. This new value is significantly different from ZIOCK'S(15) first published value o f f = 0.046 + 30 per cent and is in good agreement with the v a l u e f = 0.032 obtained by BELL et aL (4), by the atomic beam method.
REFERENCES 1. C. OTTINGERand K. ZIOCK,Z. Naturf 16a, 720 (1961). 2. W. DEMTRODER,Z. Physik, 166, 42 (1962). 3. H. KOPFERMANNand G. WESSEL,Z. Physik 130, 100 (1951). 4. G. D. BELL,M. H. DAVIS,R. B. KINGand P. M. ROUTLY,,4strophys. J. 127, 775 (1956). 5. G. D. BELL, M. H. DAVIS,R. B. KING and P. M. ROUTLY,,4strophys. J. 129, 437 (1959). 6. G. D. BELLand R. B. KING, ,4strophys. J. 133, 718 (1961). 7. Yu. OSTROVSKIIand N. P. PENKIN,Opt. Spectrosk. 3, 193 (1957). 8. Yu. OSTROVSKnand N. P. PENKIN,Opt. Spectrosk. 4, 719 (1958). 9. F. B. ESTABROOK,,4strophys. J. 113, 684 (1951). 10. F. B. ESTABROOK,,4strophys. J. 115, 571 (1952). 11. F. HINNOVand H. KOHN,J. Opt. Soc. ,4mer. 47, 156 (1957). 12. C. W. ALLENand A. S. ASAAD,Men. Not. R. ,4str. Soc. 117, 35 (1957). 13. V. K. PROKOE'E~¢,I. M. NAGIBINA,and G. P. PETROVA,Opt. Spectrosc. 8, 195 (1960). 14. C. H. CORLISSand W. R. BOZMAN,National Bur. Stand. Wash., Monograph 53 (1962). 15. K. ZIOCK,Z. Physik 147, 99 (1957).
* In connexion with the extensive Russian work on relative and absolute fivalues, attention should be called to a recent valuable addition to the literature: Optical Transition Probabilities..4 collection o f Russian Articles, !924-1960, translated from the Russian and published for the National Science Foundation, Washington, D.C., and the Department of Commerce, U.S.A., by the Israel Program for Scientific Translations. This work is available from the Office of Technical Services, U.S. Department of Commerce, Washington 25, D.C. (Price $4.75).
THE M E A S U R E M E N T OF ABSOLUTE OSCILLATOR STRENGTHS F O R LINES OF N E U T R A L ATOMS ROBERT B. KING California Institute of Technology, Pasadena, California Abstract--Comments are made on the present status of absolute oscillator strengths of lines in atomic spectra and on methods of measurement of these quantities. The measurement of absolute oscillator strengths of lines of neutral metals by means of an atomic beam experiment is described and recent results are discussed. I. I N T R O D U C T I O N CONSIDERABLE progress has been made in the past few years in the determination of the absolute oscillator strengths (f-;~alues) of spectral lines of neutral atoms by both experimental and theoretical methods. These data, which are required for determinations of abundances of the elements in the sun and stars and are useful in studies o f terrestial sources, have been (and still are) perhaps the least well known of any of the basic spectroscopic quantities. However, owing to an increase in the number of workers in the field and to improvements in techniques, both experimental and theoretical, absolute f-values are gradually becoming available for elements for which no data previously existed and more precise and reliable values are being determined for others. For lines o f ionized atoms, promising experimental methods are beginning to be developed, but most o f the absolute f-value data available at present for lines of ions are from theoretical calculations. Broadly speaking, there are two basic classes o f experimental methods o f measurement of the absolute f-values or transition probabilities o f atomic lines. The first class includes methods whereby the strength o f the line absorbed or emitted by a known number of atoms is measured. I f conditions are such as to permit the calculation of the population of the initial energy level involved in the transition, then the absolute probability of the transition can be derived from the observed strength o f the line. Many completely different types of experimental techniques fall into this classification depending upon how the atoms are excited (e.g. absorption tube, electric arc, atomic beam, etc.) and their absolute numbers determined and how the absolute strength o f the line is measured (e.g., total absorption, emission intensity, anomalous dispersion, magneto-rotation, etc.). The second class of experiments includes those in which the life-time of the upper energy level involved in the transition is measured. Then, if the relative probabilities of all downward transitions from this state are known, the absolute transition probabilities o f these lines are obtained since, under the proper conditions, the life time of the excited state depends only on the sum of all the transitions from it to lower energy states. Knowledge of the absolute number of atoms involved is not required. Thus the most difficult quantity to determine and the source o f the largest systematic errors in method of the 299
300
ROBERT B. lONG
first class is avoided. Recent applications of modern techniques of life-time measurements, notably by OTTINGERand ZIOCK Ct) and by DEMTRSDER(2) in Bonn, appear to have greatly increased the precision of this method and it offers great promise for future applications. There are very good reasons why new techniques of measurement should be encouraged and why established ones should be continued. First, because of the enormous range in the chemical and physical properties of the elements and the characteristics o f their spectra, no single method is applicable to all or even well adapted to a very large number with maximum efficiency. Second, measurements of absolute f-values by whatever method, in c o m m o n with other absolute physical measurements, are always subject to the possibility of the existence of undetected systematic errors. The theorists in the field are faced with the same type of problem arising from the approximate methods that must be used in calculating absolute f-values, particularly in complex spectra. Absolute f-value determinations by different methods, experimental and theoretical, have been known to differ by factors o f 2 or 3, and even much more in a few instances, as those who have had occasion to use these data are well aware*. Therefore, it is essential that measurements be repeated using different techniques. A good rule would be to consider no absolute f-value as definitive until at least two, or preferably more, measurements by quite different and reliable methods give results which agree within the internal uncertainties of the experiments. The internal errors should not be greater than 10-15 per Cent with techniques now available and should become smaller in the future.
II. D E S C R I P T I O N OF A USEFUL E X P E R I M E N T A L P R O C E D U R E We shall now describe the present status of an experimenL designed to measure f-values, t This work has been in progress for several years at the California Institute of Technology and is sponsored by the Office of Naval Research. In our experiment, the absolute f-values are determined by measuring the total absorption of very weak lines absorbed from a light beam passing through a beam of atoms. The concentration o f absorbing atoms is measured as part of the experiment. Thus the method falls in the first class of experiments described above. The principles of this method were first applied by KOPFERMANN and WESSEL¢SIin G0ttingen. The technique is applicable to most o f the metals which evaporate as monatomic gases. The advantage over other methods in its general class lies in the fact that the number of absorbing atoms is determined rather directly in the experiment, thereby avoiding the necessity o f relying on vapor pressure or other thermo-chemical data whose quality in the past, at least, has been highly variable. The general theory and procedures of the atomic beam experiment have been described by BELL et al. (4) and the details will not be repeated here. Absolute f-values have been published previously for the strongest ground state lines in the spectra o f Cu ~4), Fe ~4) MnC5 ~ and Pb. ~6~ Recent unpublished results will be mentioned later in this paper. * A critical survey of the experimental and theoretical f-value data available in 1960, both absolut and relative, for many atoms and ions is given by L. GOLDBERa,E. A. MULLER,and L. H. ALLERin their paper: The Abundances of the Elements in the Solar Atmosphere, Astrophys. J. Suppl. 5, No. 45 (1960). t A comprehensive bibliography of papers on absolute and relative f-values of atoms and ions has been recently published: Bibliography on Atomic Transition Probabilities by B. M. GLENNONand W. L. WIESE, National Bureau of Standards, Washington; Monograph 50 (1962).
The measurement of absolute oscillator strengths for lines of neutral atoms
301
Referring to a schematic diagram of the apparatus shown in Fig. 1, the atomic beam method may be described very briefly as follows. A beam of atoms (1) of the metal being studied is effused upward through a small orifice in a crucible (2) of suitable material which
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ce11$~
~'~ B
~
~//e....i onc~rr~t
/
~oghrtce
~
Spectr
"-'r---Z3-,-a
FIG. 1. Schematic diagram of atomic beam apparatus. is electrically heated by a tubular furnace (3). The atomic beam is restricted in angular spread by a rectangular opening (4). The beam flux is measured by allowing the central part of the beam to pass through a hole (5) and deposit on the inner surface of a thin, conical aluminum pan (6) that is attached to the arm of an electro-micro-balance. A shutter (7) prevents atoms from depositing on the pan until desired. The temperature inside the crucible is measured with an optical pyrometer by viewing the crucible orifice through the window (8). The light beam, consisting essentially of a thin sheet of light from a high pressure, quartz capillary mercury arc, passes through the atomic beam horizontally at a determined position. The instantaneous number of atoms per cm~ in the path of the light beam is determined from the measurement of atomic beam flux, knowledge of the angular distribution of atoms in the beam, and their mean velocity as calculated from the temperature of the crucible. The light beam is brought to a focus on the slit of a 21-ft concave grating Rowland-type spectrograph, with the aid of which the total absorption of the spectral lines are measured by either photographic or photoelectric photometry. Several major improvements in the apparatus and in the techniques of recording and measurement have been introduced recently by George Lawrence and John Link, aided by Robert Ashenfelter. These improvements will be described in detail in forthcoming publications, but three of them may be briefly mentioned here: (a) A new electrobalance, shown schematically in Fig. 1, has been constructed. The current required to keep the balance arm horizontal as the atomic beam deposits in the
302
ROBERT B. KING
conical balance pan is continuously recorded on a strip chart. The "noise" inherent in the system corresponds to approximately ½t~g. The slope of the line recorded on the chart yields the deposit rate in tzg per see. A minimum of 50-100 fig are usually deposited during a run. This balance also permits the accurate measurement of the impulse force imparted by the atoms in the beam to the balance pan when the shutter is opened or dosed. This measured force may not only be accurately compared with the calculated impulse force, (4) but may also be used to make a determination that all the atoms entering the conical pan are captured by it. The latter follows if the measured impulse force is equal to the product of the obsei-ved deposit rate and the time required for the balance current to return to its value prior to the time the shutter was opened. For all metals studied so far, with the exception of Au and probably T1, all atoms appear to be captured by the pan, within the accuracy of measurement, which is about 1 per cent. It should be noted that this is not a measurement of the accommodation coefficient, since most of the atoms that may be reflected on first impact with the conical pan are probably captured by it eventually. (b) An ionization gage (not shown in Fig. 1) within the vacuum chamber has been developed to monitor the atomic beam flux. A negligible fraction of the atoms in the beam is ionized by a constant electron current directed horizontally across the beam. Measurement of the ion current produced in the atomic beam, and collected by a charged ring concentric with the hole in the top plate (5), monitors the constancy of the atomic beam flux independently of the deposit rate measured by the electrobalance. (c) A photoelectric recording system has been developed to replace the photographic plate as a means of measuring the total absorption, or equivalent width, of lines absorbed by the atomic beam from the continuous spectrum of the light beam. A single photomultiplier tube is located behind two nearby slits on the focal curve of the spectrograph. As a shutter alternately exposes each slit at the rate of 4 times per sec, a continuous measurement of the difference in the d.c. amplified signals is recorded while one slit slowly scans across the line and the other slit views the nearby continuum. To avoid, as much as possible, uncertainties introduced by line broadening effects, including hyperfine structure, conditions must be obtained such that the absorption lines are very weak, that is, the lines must lie on the linear part of the "curve of growth" where the relationship between equivalent width and numbers of absorbing atoms is truly linear regardless of the broadening processes present. This generally means that the measurements must be made on lines the equivalent widths of which are less than about 0-010 A. With the new recorder, equivalent widths of less than 0.001 A can be measured with reasonable precision. As mentioned before, absolute f-values for lines in the spectra of Cu, Fe, Mn, and Pb measured with the atomic beam apparatus, prior to introduction of the specified improvements, have been published. (4-6) With the improved apparatus, George Lawrence and John Link have recently completed measurements of absolute f-values of one or more lines in the neutral spectra of ten additional metals. Lawrence has measured lines of Co, Ni, Ag, and Au. Link has measured Cr, Ga, Pd, In, Sn, and T1. Their results are now being prepared for detailed publication elsewhere. For all of these elements, with the exception of Pd, absolute f-values for at least one of the lines measured by Lawrence and Link have been measured by other investigators by other methods. These are cited below to show a sampling of the variety of methods that have been used to measure absolute f-values.
The measurement of absolute oscillator strengths for lines of neutral atoms
303
OSTROVSKY and PENKIN*, using absorption tubes and anomalous dispersion, have measured absolute f-values for lines of.Cr, (7) Ga(S) and In.(Sl ESTABROOK, using an absorption tube and measuring the total absorption of weak lines, obtained values for Ni (9) and Cr. (10) HINNOV KOHN (11) derived f-values for Ag lines from measurements in flame spectra. OTTINGER and ZtOCK (1), from measurements of the life times of the excited states, have obtained absolute f-values for Ga, and DEMTRODER(9), using this method, has reported values for G a and T1. In addition, transition probabilities or f-values derived from arc intensity measurements, and placed on an absolute scale with the aid of absolute f-values from other experiments or from theory, have been published for Cr, Co, Ni, Ga, and Ag by ALLEN and ASAAD(19), and for Sn by PROKOF'EV et al. (13) Absolute f-values also derived from arc spectra, are given for all of the lines measured by Lawrence and Link in the recently published monumental work E x p e r i m e n t a l Transition Probabilities for S e v e n t y E l e m e n t s by CORLISS and BOZMAN(14). The results obtained by Lawrence and Link with the atomic beam are not in very good agreement with most of the previous work by others cited above. However, good agreement was obtained for lines of Ga and TI with the results obtained by the life-time method by OTTINGER and ZIOCK and by DEMTRODER. It is also noteworthy that OTTINGER and ZIOCK'S measurement of the absolute f-value of the Fe line A3720 yielded f = 0.035 (1) with a n estimated uncertainty of 10 per cent. This new value is significantly different from ZIOCK'S(15) first published value o f f = 0.046 + 30 per cent and is in good agreement with the v a l u e f = 0.032 obtained by BELL et aL (4), by the atomic beam method.
REFERENCES 1. C. OTTINGERand K. ZIOCK,Z. Naturf 16a, 720 (1961). 2. W. DEMTRODER,Z. Physik, 166, 42 (1962). 3. H. KOPFERMANNand G. WESSEL,Z. Physik 130, 100 (1951). 4. G. D. BELL,M. H. DAVIS,R. B. KINGand P. M. ROUTLY,,4strophys. J. 127, 775 (1956). 5. G. D. BELL, M. H. DAVIS,R. B. KING and P. M. ROUTLY,,4strophys. J. 129, 437 (1959). 6. G. D. BELLand R. B. KING, ,4strophys. J. 133, 718 (1961). 7. Yu. OSTROVSKIIand N. P. PENKIN,Opt. Spectrosk. 3, 193 (1957). 8. Yu. OSTROVSKnand N. P. PENKIN,Opt. Spectrosk. 4, 719 (1958). 9. F. B. ESTABROOK,,4strophys. J. 113, 684 (1951). 10. F. B. ESTABROOK,,4strophys. J. 115, 571 (1952). 11. F. HINNOVand H. KOHN,J. Opt. Soc. ,4mer. 47, 156 (1957). 12. C. W. ALLENand A. S. ASAAD,Men. Not. R. ,4str. Soc. 117, 35 (1957). 13. V. K. PROKOE'E~¢,I. M. NAGIBINA,and G. P. PETROVA,Opt. Spectrosc. 8, 195 (1960). 14. C. H. CORLISSand W. R. BOZMAN,National Bur. Stand. Wash., Monograph 53 (1962). 15. K. ZIOCK,Z. Physik 147, 99 (1957).
* In connexion with the extensive Russian work on relative and absolute fivalues, attention should be called to a recent valuable addition to the literature: Optical Transition Probabilities..4 collection o f Russian Articles, !924-1960, translated from the Russian and published for the National Science Foundation, Washington, D.C., and the Department of Commerce, U.S.A., by the Israel Program for Scientific Translations. This work is available from the Office of Technical Services, U.S. Department of Commerce, Washington 25, D.C. (Price $4.75).