26 ROBERT A. MAPLETON It is seen that the two experimental values of Berkner, et al. are less than the corresponding OBK values, which fact suggests that the asymptotic energy region may not yet be reached. Of course, it is not known whether OBK cross sections are the asymptotic values of a correct theory, and at what energy the onset of the asymptotic value would occur. Little is also known how well electron capture from N~ can be described in terms of N atoms' (factor of 2) or how much the present OBK values would be altered by a recalcula- tion with improved wave functions. In the author' s ' T. F. Tuan and K. Gerjuoy, Phys. Rev. 117, 756 (1960). opinion, it is very difBcult to decide theoretically what the asymptotic cross section is for a target as compli- cated as N2. An atomic system much more tractable to theoretical analysis is helium, and the energy range of the OBK calculations9 using the 6-parameter helium wave func- tion have also been extended to 100 MeV. The OBK cross sections for capture from He, N, and 0, described in this paper, can be obtained from the author. Gratitude is expressed to Professor A. Dalgarno for informing the author of these measurements. s R. A. Mapleton, Phys. Rev. 130, 1839 (1963). PH YSI CAI REVIEW VOI UME 145, NUMBER f 6 MA Y 1966 Radiative Lifetime of the First P, t& State of Ionized Calcium and Magnesium by the Hanle Effect* WINTHROP W. SHITHj'$ AND ALAN GALLAGHER Joint Institstte for Laboratory Astrophysics, ) Botttder, Colorado (Received 1 November 1965) The lifetime of the Ca+ 4'Pg2 state and that of the Mg+ 3'P3g2 state have been measured by the Hanle- eBect method with optical excitation from the ground states of the ions. The lifetimes are, respectively, (6. 72+0.20) &10~ and (3. 67%0. 18) X1M sec. The iona were produced by introducing traces of calcium or magnesium into an argon discharge. "Alignment" depolarization cross sections 0. were obtained for the collisional depolarization of the following states of calcium and magnesium due to collisions with argon: o (Ca 4'Pi state) = (1. 9+0. 3) && 10 ' cm', a(Ca+ 4 Pets state) = (1. 4&0. 2) && 10 I cm ' o (Mg 3'Pi state) = (1. 9+0. 3)10 '4 cm' o (Mg+ 3'Pets state) = (1. 3 ~0. 25) )& 10 '4 cm'. The magnitudes of these cross sections are in accord with the theories of Byron and Foley and of Omont; we give a discussion of the extension of these theories to include ion-atom collisions. I. INTRODUCTIOH K report here the application of the Hanle-effect technique to the measurement of lifetimes of excited states of ions. Ca+ and Mg+ were chosen, since their absolute oscillator strengths are of astrophysical importance and are useful for testing various theoretical methods of calculating oscillator strengths. ' The Hanle effect has been known for many years as a technique for measuring lifetimes or transition probabilities for optical resonance lines in atoms. ' The method has been *Work was supported by the National Bureau of Standards and by the Advanced Research Projects Agency (Project DEFENDER), monitored by the U. S. Army Research OKce- Durham, under Contract DA-31-124-ARO-D-139. t Present address: Department of Physics, Columbia Uni- versity, New York, New York. f National Academy of Sciences-National Research Council Postdoctoral Fellow assigned to National Bureau of Standards (1963 — 65). $ Of the University of Colorado and the National Bureau of Standards, Boulder, Colorado. ' See, for example, A. S. Douglas and R. H. Garstang, Proc. Cambridge Phil. Soc. 58, 377 (1962). ' A. C. G. Mitchell and M. W. Zemansky, Resonance Radiutioe used frequently in recent years, primarily by the group at Columbia. ' This recent work has demonstrated the accuracy of the method for measuring atomic state lifetimes (a typical uncertainty is 3%%u~), and it is an aim of the present paper to demonstrate that the method can be used to measure ionic state lifetimes with comparable accuracy. Although the results reported here are only for Group II ions, we think that the method should have wide application for precision determinations of transi- tion probabilities of resonance lines of other ions. Methods for measuring transition probabilities of atoms or ions may be divided into two classes: those which measure oscillator strengths directly and those which measure excited-state lifetimes. Typical of the first class are the methods of anomalous dispersion (the "hook" method), ' emission from thermal arcs, ' and and Excited AIoms (Cambridge University Press, New York, 1961), Chap. P. ' For a review of the theory and experimental techniques, see A. Lurio, R. L. deZafra, and R. J. Goshen, Phys. Rev. 134, A1198 (1964), and references contained therein. See, for example, C. H. Corliss and 9/. R. Bozman, Natl. Bur. Std. (U. S. ) Monograph 53 (1962).