ii
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`ASML 1119
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`ASML 1119
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`

`
`CODEN: JOSAAM
`
`ISSN: 003039,,
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`0
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`Qouanai. on "res Cancer. Society or Aussies
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`Joseph W. Goodman, Editor
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`S
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`

`
`Eleventh International Quantum Electronics Conference
`
`Session A
`
`8:30 A.M.
`
`Monday, June 23, 1980
`Independence Room
`
`Laser Spectroscopy l
`Chairman: P. F. Liao
`
`A.l. Doubly Excited Alkaline Earth
`Atoms* (Invited), T. F. GALLAGHER, K. A.
`SAFINYA, AND w. SANDNER, SR1 Inter-
`national, Menlo Park, CA, AND W. E.
`COOKE, University of Southern California,
`Los Angeles, California.
`(30 min.)
`
`We describe recent experiments to probe
`the properties of doubly excited autoionizing
`atoms using _a laser spectroscopic technique;
`The physical basis of the excitation process
`as well as experiments to probe the autoion-
`ization process are described.
`*This work supported by AFOSR, NSF, and
`DOE.
`
`A.2. Rydberg Atom Masers (Invited), S.
`HAROCH/E, C. FABRE, P. GOY, M. GROSS,
`AND B. MOI, Laboratoire de Physique de
`l’Ecole Normale Supérieure, Paris,
`France.
`(30 min.)
`
`Using optically pumped alkali Rydberg
`atoms as the active medium, we have recently
`developed new types of pulsed maser sources
`with unusual characteristics. Due to the
`giant electric dipole matrix elements of the
`Rydberg states, these masers have extremely
`low inversion thresholds—several orders of
`magnitude smaller than those of conventional
`masers-—and “microscopic” energy outputs.
`Very sensitive detection procedures have to
`be used in order to observe their emission.
`An indirect detection method consists in
`studying the field ionization characteristics
`of the atoms, which is strongly modified when
`maser action occurs. Direct detection of the
`tiny microwave bursts has also been achieved
`by a heterodyne technique with a very sen-
`sitive Schottky diode mixer. Using both
`methods, we have made a detailed study of
`the emission characteristics (typical number
`of inverted atoms at threshold ~10‘; pulse
`energy «-1 to 10 eV; peak emission power
`~10‘12 W; pulse duration ~0.5 us).
`Owing to the large number of energy levels
`near the ionization limit, these new masers
`can be operated at many wavelengths ranging
`from the centimeter to the submillimeter
`range;
`In fact, the source should contin-
`uously evolve from the maser to the laser case
`when the binding energy of the levels in-
`volved in the emission is increased. These
`new coherent sources are bound to have very
`interesting applications in fundamental
`physics (study of superradiance of very small
`systems of atoms) and for the technology of
`millimeter and submillimeter wave detectors
`as well.
`
`577
`
`A.3. Singlet-Triplet Mixing by Hyper-
`fine Interactions in 3He, R. R. FREEMAN,
`P. F. LIAO, R. PANOCK, AND L. M. HUM-
`PHREY, Bell Telephone Laboratories,
`Holmdel, NJ 07733 (15 min.)
`
`A complete determination of the hyperfine
`structure of the 23P and 33D states of 3He
`through analysis of Doppler-free intermo-
`dulated fluorescence spectra is reported. We
`find the structure of the 33D state to be sig-
`nificantly modified by singlet-triplet mixing
`which is induced by hyperfine interactions.
`The hyperfine interaction is dominated by
`the Fermi contact interaction of the inner 13
`open shell electron with the nucleus and
`therefore does not decrease for higher lying
`states. Hence, unlike the case of one elec-
`tronlike spectra (e.g., alkali atoms) or mul-
`tielectron atoms with zero spin (e.g., 4He) the
`high Rydberg states of 3He will have their
`electronic structures completely dominated
`by the hyperfine interaction.
`In particular,
`the hyperfine induced singlet-triplet mixing
`for 3He will increase rapidly with increasing
`principal quantum number n. Our results
`are in good agreement with theoretical cal-
`culations of the hyperfine interaction.
`In Fig. 1 we show-a portion of our spectrum
`which contains transitions associated with
`the 23P1,2 levels to the 33D1,2,3 states of 3He.
`These states were obtained in a dc discharge
`tube operated with 0.8 Torr of 3He. The tube
`is probed with two counterpropagating tun-
`able laser beams which are chopped at dif-
`ferent frequencies. By monitoring fluores-
`
`cence at the difference frequency we obtain
`the Doppler-free spectrum shown in Fig. 1.
`This spectrum is fit to a parametrized hy-
`perfine Hamiltonian and the calculated res-
`onance positions and line strength from the
`fit are shown in the figure. We find the ma-
`jority of the interaction is due to the Fermi
`contact term of the Is electron and that this
`term is nearly the same for the 2P and 3D
`states as expected. This term produces
`sizeable singlet-triplet mixing which must be
`included to correctly give the structure. The
`dotted lines show resonance positions if one
`neglects this mixing.
`Because the hyperfine interaction is es-
`sentially constant, our results, along with
`published fine structure measurements allow
`a precise determination of the structure of all
`higher lying states in 3He. We find, for ex-
`ample, we can reproduce the two-photon
`spectra recently obtained by Giacobino et al. 1
`and also predict the hyperfine splittings of
`the n ‘D2 states observed in level crossing
`experiments? In Table I we give our calcu-
`lated values for these splittings and the
`measured experimental values. There is
`excellent agreement. As one of the simplest
`atoms, helium is amenable to accurate cal-
`culations.
`In Table I we also include the re-
`sults of a theory based on hydrogenic elec-
`tronic wave functions and good agreement
`with our calculations is again found.
`In conclusion we have made a determina-
`tion of the hyperfine interaction in 3He. This
`determination shows important singlet-
`triplet mixing effects which will dominate the
`
`lllltl
`
`5%
`>
`‘35 N
`3
`3
`3
`23 n
`“U I
`NN N
`?> B
`v-1-
`N
`
`2:
`">""
`5
`°>
`tuna
`§
`3
`3 3 3
`m
`3 ~““n
`"
`I‘-"‘."i'
`N NNNN
`\
`\
`-o
`-:3.-‘-3
`N NNN~r-
`
`13/2-CROSSOVER
`
`23/2-‘I1/2
`
`25/2-37/2 25/2-35/2 13/2-13/2
`
`--.+LJ
`25/2-CROSSOVER
`
`25/2‘!3/2
`
`11/2-‘I3/2
`
`FIG. 1.
`
`Portion of Doppler-free spectrum of 230-330 transitions in 3He. The levels are marked by (23HF—
`(23D)F’. The upper trace is the transmission of an interferometer having an FSR = 122.4 MHz. The solid
`lines show the calculated positions and intensities including singlet-triplet mixing. The dotted lines show
`the calculated positions if singlet-triplet mixing is ignored.
`
`577
`
`

`
`cm“1. The magnitude of the cross section for
`collisional deexcitation by spontaneous
`emissions, as, was determined by measuring
`the total number of signal photons integrated
`over the emission bandwidth and using, for
`the number of detected photons,
`
`N[Ba(5d 1D2)]N[Tl(6p 2P3/2)]osl_/‘r Vgf‘,
`
`where N[Ba(5d 1D2)] and N [Tl(6p 2P3/2)] are
`the number densities for the initial storage
`levels, V is the mean velocity of collision, 7 is
`the effective radiating time, V0 is the effective
`radiating volume, and §' is the ratio of de-
`tected to generated photons.
`In this manner
`we obtain a measured value for the cross
`section for dipole-quadrupole collisional
`deexcitation of as = 1.5 X 10”” cm2, with an
`overall experimental uncertainty of ap-
`proximately a factor of 7.
`The results of this experiment have ap-
`plication to the construction of low-gain,
`high-energy storage media and to the spec-
`troscopic study of the interaction potentials
`of colliding atoms.
`1S. E. Harris and J. C. White, IEEE J. Quantum
`Electron. 13, 972 (1977).
`2J. C. White, G. A. Zdasiuk, J. F. Young, and S. E.
`Harris, Phys. Rev. Lett. 41, 1709 (1978); 42, 480(E)
`(1979).
`3W. R. Green, M. D. Wright, J. Lukasik, J. F. Young,
`and S. E. Harris, Opt. Lett. 4, 265 (1979).
`
`J.8. Sodium Plasmas Produced by Mil-
`liwatt cw Laser Irradiation,* M. E.
`KOCH, K. K. VERMA, AND W. C. STWALLEY,
`Iowa Laser Facility and Departments of
`Chemistry and Physics, University of
`Iowa, Iowa City, IA 52242.
`(15 min.)
`
`There are a variety of reports of significant
`laser-produced ionization of alkali metal
`vapors using pulsed lasers (in Lil and in Na2)
`and using cw lasers (in Cs3 and in Na‘).
`(See
`also Ref. 5 for additional background.)
`While all of this work is quite interesting,
`much of it involved resonance lines1’2’4 and so
`is perhaps not terribly surprising. The other
`work,3 on the other hand, involves transitions
`between a radiatively trapped upper level of
`an alkali-metal resonance line and a more
`highly excited level which can associatively
`ionize to form M5*.
`In principle, then, this cw
`plasma may contain a concentration of MQ’
`which is quite nonequilibrium. Moreover, in
`contrast to a discharge where M; may be
`rapidly destroyed by photodissociation, the
`M; may be stable with respect to laser and
`other light (e.g., near-resonance lines) found
`in the laser-produced plasma.
`With this in mind, we have irradiated a
`sodium heat pipe (typically at 10 Torr), usin
`a focused cw dye laser at 5688.2 or 5682.6
`(313 ~> 4d), and also reproduced the Cs result3
`at 6010 A. Unlike Cs,3 where a strong atomic
`ion-electron radiative recombination ‘con-
`tinuum is seen, we see no significant spec-
`troscopic evidence for atomc ions in our Na
`plasma. We feel this is because in Cs, at the
`upper level of transition studied by Tam and
`Happer, the channel of ion pair formations
`(Cs** + Cs —> Cs+ + Cs‘) is available in ad-
`dition to associative ionization (Cs** + Cs —>
`Cs; + e‘). However, for Na** = Na(4d),
`only associative ionization can occur ener-
`getically, so we have produced essentially a
`
`627
`
`molecular ion plasma. Also we note that this
`plasma can be produced at quite low power
`(~2 mW focused broadband laser light at
`5688.2 A!) and we are currently examining the
`energy balance in detail.
`We have obtained spectra of this plasma in
`the 2000-9000 A region. The interpretation
`of this spectrum is still not completely clear.
`The various atomic lines seen can be under-
`stood in terms of Na(4d)—Na and Na(3p)-
`Na(3p) collisions“ and the process (disso-
`ciative recombination): Na§ + e‘ —> Na**
`+ Na, where Na** is a highly excited Na atom
`(e.g., 4d or 5.9). The structure seen near the
`exciting line and to the red is presumably
`molecular fluorescence and D line absorption.
`We see five broad features at ~3650, 3780,
`4350, 4520, and 8000 A which remain to be
`explained.
`We have examined the 420041700 A region
`under high resolution and find the structure
`in that region to be a continuum, not densely
`spaced lines. A possible explanation is that
`these continua represent the processes Na;
`+ e‘ —> Na; + hi/, where Na; is an excited
`state of Nag. The occurrence of such mo-
`lecular ion-electron radiative recombination
`has never been previously established, al-
`though the atomic form is well known. The
`features we see peaking at 4350 and 4520 A
`have been observed in other ways, e.g., in
`discharges,'7‘9 in Ar‘ laser UV-line irradiation
`of the Nag C <—X bands,” in two-photon N32
`excitation,“ and in cw and pulsed dye-laser
`excitation at the Na D lines.12“1“ Similar
`features occur in K, Rb, and Cs.3'15-16 Several
`explanations have been proposed involving
`free-free, free-bound, or bound-free pro-
`cesses. Note that the radiative recombina-
`tion discussed above can be cast in “bound-
`free” form when a high molecular Rydberg
`state Nag’ is formed as a resonance in e‘—Na§'
`scattering. Since many of the potential en-
`ergy curves of Nag are fairly well known, e.g.,
`from high quality ab initio calculations and
`a variety of recent experiments, we are
`carrying out explicit calculations of a number
`of these alternatives. We also have and will
`continue to carry out simultaneous ionization
`detection to attempt to resolve the origin of
`the 4350 and 4520 A continua. Finally we
`note that some mechanisms suggest these
`bands might be made into a powerful violet
`laser with limited tunability.
`The 8000 A feature almost certainly cor-
`responds to the
`
`Ali — xii
`11
`g
`
`satellite band‘7*18 and has previously been
`observed ‘in a Na (or other alkali) dis-
`charge.8’13=19 There is a continuum overlaid
`with many discrete lines. The discrete lines,
`however, extend through all parts of the laser
`path while the continuum is concentrated in
`the central “white” region near the focus
`where the 4350 and 4520 A features appear.
`The 3650 and 3780 A features have ap-
`parently not been previously reported. They
`also appear continuous and possible expla-
`nations for them are similar to those men-
`tioned above for
`the 4350 and 4520 A
`hands.
`
`We are currently examining these spectra
`and extending them in a variety of ways, with
`
`emphasis on obtaining microscopic under-
`standing of the plasma formation process.
`*Supported by the National Aeronautics and Space
`Administration and the National Science Founda-
`tion.
`1T. J. Mcllrath and T. B. Lucatorto, Phys. Rev.
`Lett. 38, 1390 (1977). I
`2T. B. Lucatorto and T. J. Mcllrath, Phys. Rev.
`Lett. 37, 428 (1976).
`3A. Tarn and W. Happer, Opt. Commun. 21, 403
`(1977).
`‘G. H. Bearman and J. J. Leventhal, Phys. Rev.
`Lett. 41, 1227 (1978); 41, 1759(E) (1978).
`5G. S. Hurst, M. G. Payne, S. D. Kramer, and J. P.
`Young, Rev. Mod. Phys. 51, 767 (1979).
`GM. Allegrini, G. Alzetta, A. Kopystynska, L. Moi,
`and G. Orriols, Opt. Commun. 19, 96 (1966).
`7H. Bartels, Z. Physik 73, 203 (1932).
`3K. Schmidt Proceedings of the Sixth International
`Conference on Ionization Phenomena in Gases
`(Paris, 1963), Vol. 3, p. 323.
`9J. J. de Groot and J. A. J. M. van Vliet, J. Phys. D
`8, 651 (1975).
`NJ. P. Woerdrnan, Opt. Commun. 26, 216 (1978).
`“J. P. Woerdman, Chem. Phys. Lett. 43, 279
`(1976).
`12M. Allegrini, G. Alzetta, A. Kopystynska, L. Moi,
`and G. Orriols, Opt. Commun.22, 329 (1977).
`13A. Kopystynska and P. Kowalczyk, Opt. Commun.
`25, 351 (1978).
`“A. Kopystynska and P. Kowalczyk, Opt. Commun.
`28, 78 (1979).
`15M. M. Rebbeck and J. M. Vaughan, J. Phys. B 4,
`258 (1971).
`‘SJ. M. Brom and H. P. Broida, J. Chem. Phys. 61,
`982 (1974).
`17W. C. Stwalley, “Laser Manipulation of Metallic
`Vapors", Radiation Energy Conversion in Space,
`edited by K. W. Billman, Vol. 61 of Progress in As-
`tronautics and Aeronautics (1978), p. 593-601.
`13L. K. Lam, A. Gallagher, and M. M. Hessel, J.
`Chem. Phys. 66, 3550 (1977).
`19P. P. Sorokin and J. R. Lankard, J. Chem. Phys.
`55, 3810 (1971).
`
`J.9. Laser-Induced Penning/Associative
`Ionization in Crossed Atomic Beams, P.
`POLAK-DINGELS, J.-F. DELPECH, AND J.
`WEINER, Department of Chemistry,
`University of Maryland, College Park, MD
`20742.
`(15 min.)
`
`Laser-switched or laser-modified collisions
`are the object ‘of intensive theoretical and
`experimental study because they offer the '
`possibility of controlling the relative proba-
`bilities of competing inelastic and reactive
`exit channels. The influence of the laser field
`is to modify the electronic states of the sys-
`tem during a collisional encounter. Laser-
`induced collisions are characterized by
`atomic-field interactions which are nonre-
`sonant with respect to dipole-allowed tran-
`sitions of the separated collision partners.
`We discuss here new results on Penning/
`associative ionization of Na/Na collisions in
`the presence of optical field power densities .
`of 2107 W/cmz.
`The experimental set-up is as follows.
`Two alkali atomic-beam sources are mounted
`on a multiported vacuum chamber at right
`angles in the horizontal plane. Two laser
`beams enter from opposite ports and overlap
`at the interaction region with an angle of
`nearly 180°. The light sources are flash-
`lamp pumped tunable dye lasers synchro-
`nized together and with a box car integra-
`tor/amplifier used to record the ion signal. A
`quadrupole mass filter, mounted above the
`
`627