AD-A129 958 PRODUCTION OF SHORT-WAVELENGTH (XUV) PHOTONS FROMION LASER-EXCITED-SURFA..U) ROCHESTER UNIV NY DEPT DFCHEMISTRY H LEE ET AL JUN 83 UROCHESTER/DC/83/YR-35

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(2J TECHNICAL REPORT No. 35

Production of Short-Wavelength (XUV) Photons from Ion-Laser-

Excited-Surface Charge Exchange: Li '+,He + + Si(111) Systems

by

Hai-Woong Lee and Thomas F. George

Prepared for Publication

in

Coherence and Quantum Optics V,Proceedings of the Fifth Rochester Conference on Coherenceand Quantum Optics, ed. by L. Mandel and E. Wolf (Plenum, New York)

Department of ChemistryUniversity of RochesterRochester, New York 14627

June 1983

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Production of Short-Wavelength (XUV) Photons from Interim Technical ReportIon-Laser-Excited-Surface Charge Exchange:Li 3+,He + + Si (1i) Systems 6. PERFORMING ORG. REPORT NUMBER

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Hai-Woong Lee and Thomas F. George N00014-80-C-0472

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Prepared for oublication in Coherence and Quantum Optics V, Proceedings ofthe Fifth Rochester Conference on Coherence and Quantum uptics, ed. byL. Mandel and E. Wolf (Plenum, New York)

19. KEY WORDS (Conmtinue an feere aide if nec.eeoa aid idenuif7 book umbe)PRODUCTION OF SHORT-WAVELENGTH LIGHT 3+ION-SURFACE CHARGE EXCHANGE LiaHe+ + Si(111)LASER-EXCITED SURFACE STATES SEMICLASSICAL THEORY

HIGH INVERSION DENSITIESMODERATE, ISER POWERS

20. ABSTRACT (Comnlnu an revemre side It neooemyf din Identli 6 We&Tek "OMbemi ClI asslIca]I1culationsare carried out for the probabilities of electron transfer for LCI and Hetionscolliding with a Si(111) surface, where a laser is used to excite electrons insilicon from the valence band to surface states. It is shown that with amoderate-power laser, high inversion densities of Li2+ and He+ can be obtained,as necessary for high gain.&-

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To appear in the Ptoceq4nZn o6 theFidth RocheAtet Conde .nce cn CoheAenceand Quantum Optic, ed. by L. Mandel andE. Wolf (Plenum, New York).

PRODUCTION OF SHORT-WAVELENGTH (XUV) PROTONS FROM ION-LASER-

EXCITED-SURFACE CHARGE EXCHANGE: Li 3+,He+ + Si(1ll) SYSTEMS

Hai-Woong Lee

Department of PhysicsOakland UniversityRochester, Michigan 48063 .....

and

Thomas F. George

Department of Chemistry "10 000University of RochesterRochester, New York 14627

ABSTRACT

Semiclassical calculations are carried out for the probabilitiesof electron transfer for Li3+ and He+ ions colliding with a Si(ll)surface, where a laser is used to excite electrons in silicon fromthe valence band to surface states. It is shown that with amoderate-power laser, high inversion densities of Li2+ and He+ canbe obtained, as necessary for high gain.

INTRODUCTION

It has been proposed1 2 that some selected charge-exchangeprocesses may serve as a means of achieving population inversionfor short-wavelength (VUV and soft X-ray) lasers. In a recentstudy,2.3 we have analyzed the possibility of obtaining coherentshort-wavelength radiation based on neutralization of positiveions Am+ at a semiconductor surface S.

A!+ + S - A(m-1 ) + (1)

Lee and George

In particular, we have proposed that significant enhancement of gaincan be achieved be electronically exciting the surface exposed toimpinging ions. This is based on the observation that the captureprobability is significantly higher for a surface electron(especially for an electron in normally unoccupied surface bandsin the band gap region) than for a bulk electron.2 If a largenumber of bulk electrons can be excited to surface bands byirradiating a surface with a source of appropriate power and wave-length, a significant enhancement of gain results. In Reference 3,cross-section and gain calculations on the system

He 2+ + si(111) -H+(3L) + Si+ (2)

have been carried out.

Here we consider the following two processes:

Li3+ + Si(11l) -b- Li2+(4L) + Si, (3)

He+ + Si(111) He(2 3S) + Si+ . (4)

Process (3) produces Li2+ predominantly in the third excitedlevel, Li2+(4t). Process (4) is not a short-wavelength lasercandidate, but it may represent an efficient way of producingmetastable helium atoms, He(23S).

CALCULATIONS AND RESULTS

The probability P for capture of a surface electron by theincoming ion (LI3+ or He+) is calculated using the semiclassicalformula2 ,

3

P - -exp[- fods rz) (5)

whererc ) - ~(o) Iz I O()12 (6)

s is the ion-surface separation, v is the ion velocity assumedto be constant, p denotes th# density of surface states, E. isthe resonance energy, and HIPO is the coupling matrix elementfor an electron of energy Eo. [The transfer of a surface electronof energy Zo to the ion is an energy conserving process. Notethat Ko changes with time because the initial and final potentialenergy curves very with x.] We assume that charge exchange occursmainly as a result of a repulsive force between the ion and thesurface, and evaluate the coupling matrix element according to theformula

Lee and George

H E OW ~IEo+EAIF(z).(7IF ) 2 o 7)

F is the overlap between the initial state (i.e., surface state)and the final state (i.e.. atomic state into which the electronis captured) of the electron, and EA is the ef ective ionizationenergy of the final state [EA a -7.6 eV for Li +(4t) and EA a-4.8 eV for He(23 S), measured from the ionization level]. Theresonance energy Eo is calculated by assuming that the potentialenergy curves are determined mainly by image forces, which yields

K - E +N(K-1)e (8E 0 + 4(K+i)z, - (8)

where N - 5 for Li3-Si and N - I for He+-Si. and K is thedielectric constant of the solid (K - 11.8 for silicon). Thedensity of surface states of silicon is taken to be4 p/area =4 x 1014/eV.cm2 . The integration in Equation (5) can now beperformed numerically.

For Process (3) we obtain P a I-exp(-0.00645/v), where thevelocity v is to be expressed in atomic units. At v n 0.1 a.u. -2.2 x 107 cm/sec we have P a 0.063, which yields the charge-exchange

cross section a a 2.7 12. [The cross section a was estimated usinga simple formula a x vz2P, where zo is the ion-surface separation atwhich electron capture occurs. zo a 3.7 - 5.0 X for Process (3).]This value of cros +section is large enough to give a high inver-sion density of Li necessary for high gain, provided that highdensities of Li3+ and surface electrons are provided. The reguireddensity of surface electrons3 is typically on the order of 10 6 1018/cm3, which corresponds to the area density 109 - I011/cm2.This value of the area density of surface electrons appears to bewell within the reach of a moderate-power infrared laser.5

For Process (2) we obtain p a 1-exp(-0.0695/v), where thevelocity v again is to be expressed in atomic units. At v - 0.1a.u. a 2.2 x 107cm/sec, we have P a 0.50, which yields a a 11 12(so a 2.6 - 4.5 1). For production of high-density metastablehelium atoms one must pump a sufficient number of bulk electronsinto surface bands. In view of the fact that this can be achievedwith the use of a moderate-power laser, the density of metastablehelium produced by the Process (2) may well be limited by theavailable density of He+.

Finally, it should be mentioned that our analysis is based ona one-dimensional nearly-free-electron model of a surfaceS,6according to which a semiconductor has a direct gap. In reality,however, semiconductors like silicon have an indirect gap and theexcitation of a surface by radiation may have to be accompaniedby photon excitations.

Lee and George

ACKNOWLEDGMENTS

This research was supported by the U.S. Army Research Office,the Air Force Office of Scientific Research (AFSC), United StatesAir Force, under Grant AFOSR-82-0046, the Office of Naval Research,the donors of the Petroleum Research Fund, administered by theAmerican Chemical Society,and the Research Corporation. The UnitedStates Government is authorized to reproduce and distribute reprintsfor governmental purposes notwithstanding any copyright notationhereon. HWL acknowledges Oakland University for a Research Fellow-ship, and TFG acknowledges the Camille and Henry Dreyfus Foundationfor a Teacher Scholar Award (1975-84) and the John Simon GuggenheimMemorial Foundation for a Fellowship (1983-84).

REFERENCES

1. A. V. Vinogradov and I. I. Sobel'man, The Problem of LaserRadiation Sources in the Far Ultraviolet and X-Ray Regions,Soy. Phys. JETP 36: 1115 (1973); M. 0. Scully, W. H. Louiselland W. B. McKnight, A Soft X-Ray Laser Utilizing Charge Ex-change, Opt. Commun. 9: 246 (1973); J. S. Helman, C. Rau andC. F. Bunge, X-Ray Laser Implementation by Means of a StrongSource of High-Spin Metastable Atoms, Phys. Rev. A 27: 262(1983).

2. H. W. Lee, W. C. Murphy and T. F. George, Neutralization ofIons at an Electronically-Excited Semiconductor Surface, Chem.Phys. Lett. 93: 221 (1982).

3. H. W. Lee and T. F. George, Emission of Short-Wavelength Photonsfrom Ion-Surface Charge Exchange, IEEE J. Quantum Electron.,submitted.

4. S. G. Davison and J. D. Levine, Surface States, in "Solid StatePhysics," Vol. 25, H. Ehrenreich, F. Seitz and D. Turnbull,eds., Academic Press, New York (1970), and references therein.

5. W. C. Murphy, A. C. Beri, T. F. George and J. Lin, Analysis ofLaser-Enhanced Adsorption/Desorption Processes on SemiconductorSurfaces via Electronic Surface State Excitation, Mat. Res.Soc. Smp. Proc. 17: 273 (1983).

6. . C. Murphy and T. F. George, Laser-Induced Electron-PhononProcesses at Metal Surfaces, Surface Sci., submitted.

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