Meetings

Chemical Accelerators

The phrase "chemical accelerators"is scarcely older than the conferencebearing that name held at the Univer-sity of Colorado, Boulder, 28-29March 1968. However, interest in thesubject was so great that what wasoriginally intended to be an unpubli-cized workshop for one or two dozenpeople grew to include nearly ahundred.

Chemical accelerators is a name sug-gested by one of us for devices thatproduce beams of chemically interestingspecies at relative kinetic energies of afew electron volts. Most studies ofchemical kinetics made by traditionalthermochemical methods are limitedby the properties of container materialsto reactions occurring near the classicalArrhenius activation energies. In thepast decade or so, however, it hasbecome evident that there is a wealthof important chemistry occurring onlyat higher energies (of the order of thestrength of chemical bonds, 2 to 10ev). The methods that have revealedthis richness and order of medium- andhigh-energy chemistry have been hot-atom techniques, largely nuclear andalso photochemical, and in the case ofions, mass spectrometry.

While hot-atom studies overcome theenergy limitations of thermochemicalmethods, they also share some of thelimitations of these classical techniques.Energies of reactive collisions are usu-ally not well defined, and products aregenerally identified only after under-going further collisions which modifytheir original properties. The molecular-beam method can, to a large extent,avoid these ambiguities and also providevaluable information on angular andvelocity distributions. It therefore nowseems clear that it is desirable to studythis new hot chemistry using molecularbeams in the electron-volt region. Thuschemical accelerators can provide thesame type of information for elemen-tary chemical kinetics that nuclear ac-celerators have yielded so successfullyfor nuclear reactions.

Although the conference revealed a

818

surprising number of different conceptsfor building chemical accelerators, rela-tively few have been made operational.These methods have been largely con-fined, because of technical limitations,to a somewhat special class of reactionstudies. As a result, the potential powerof these methods applied in a con-certed way to the broad problem ofchemical dynamics has not been givensufficient attention. Accordingly, theconference focused on methods, ratherthan on results, in order to explore theextent to which technical limitationscan be removed, thus permitting thelarge-scale exploitation of chemicalaccelerators.

After some brief welcoming remarks,John Willard (University of Wisconsin)conducted the first session. This wasa review of what is already known abouthot chemistry and provided a back-ground to the problems remaining forchemical accelerators to solve. F. S.Rowland (University of California,Irvine) presented a critical discussion ofachievements of hot-atom chemistryusing the nuclear and photochemicalmethods. V. L. Talroze (Institute ofChemical Physics, Moscow) discussedmass spectrometric studies of ion-mole-cule reactions. John Polanyi (Universityof Toronto) and Martin Karplus (Har-vard University) presented some resultsof their trajectory calculations of hot-atom reactions. It is obvious that whilesome methods of theoretical chemicalkinetics (for instance, "absolute" ratetheory) are based on assumptions whichlimit them to thermal-equilibrium sys-tems, the more fundamental trajectory-potential-energy-surface approach is asapplicable to "hot" as it is to "thresh-old" reactions. Indeed, studies of hotreactions are essential to gain informa-tion on potential energy surfaces overthe whole region and not just near thesaddle point.

In a discussion of ion accelerators,chaired by Wade Fite (University ofPittsburgh), Arnim Henglein (Hahn-Meirner-Institut, Berlin) gave the in-troductory talk. His research grouphas built a device in which a beam ofions strikes a gas target and products

recoiling in the forward direction are

velocity analyzed. Work with this de-vice has led to Henglein's postulate ofthe spectator stripping model of ion-molecule reactions. Subsequently, anumber of other ion accelerators havebeen built. In the relatively high-en-ergy region, B. Mahan (University ofCalifornia, Berkeley) and T. Bailey(University of Florida) have addedangular discrimination, thus providingmore detail on the validity of the spec-tator stripping approximation. R. Wolf-gang reported on an accelerator,named EVAtron, which he built withZ. Herman and other collaborators.The EVAtron covers the low as well asthe higher energy range. By employinga crossed-beam rather than a gas cham-ber target and using angular as well as

velocity discrimination, it has beenshown that formation of an intermedi-ate complex does not always occureven at lowest energies. A new andgeneral model for direct reactions oc-

curring at low energies was reported.Another approach to the somewhat

difficult problem of studying ion-mole-cule reactions in the relatively low-energy (1 to 2 ev) region was discussedby R. Neynaber (General Dynamics/Convair, San Diego) in a sessionchaired by R. S. Berry (University ofChicago). This is the "merging beam"technique, whereby two beams-ionic,or neutralized-are superimposed sothat reaction occurs at energies corre-sponding to the difference in theirvelocities. This method provides excel-lent fine-energy control and allowsbeam methods to be extended down tothe milli-electron-volt region. While itshould be useful for measuring crosssections, the method appears to holdless promise for detailed mechanisticstudies. This is caused by the largecenter-of-mass motion which makes itdifficult to measure angular distribu-tions. Two other merging beam ma-chines currently in operation weredescribed by E. Entemann (GeneralDynamics/Convair, San Diego) andW. Aberth (Stanford Research Insti-tute, Menlo Park). The discussion atthis session left the feeling that "partialmerging" (making beams cross at asmall angle) may be a promising ad-junctive technique.

Chemical accelerators for neutralspecies are far more difficult to oper-ate than those for ions, not only be-cause neutrals are harder to accelerate,but also because they are harder todetect. Perhaps the most successfulmethod so far devised for producing

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neutral beams is that of neutralizingthe charge from ionic beams withoutdisturbing the momentum of the travel-ing species. N. Utterback (DefenseResearch Laboratory, General MotorsCorp., Santa Barbara) reviewed thistopic in a session chaired by E. Greene(Brown University). Using chargetransfer, Utterback and his collabora-tors have produced neutral beams fromthe high-energy range down to a fewelectron volts. They have successfullystudied several reactions, avoiding theproblem of detecting neutrals by choos-ing processes yielding ionic products.If resonant charge transfer is used, itappears that the internal energy stateof the resulting neutral beam can bereasonably well controlled. (Talroze'stalk had already made it clear thatresonant transfer processes were com-mon and occurred with large cross sec-tions.) L. M. Branscomb (Joint Insti-tute for Laboratory Astrophysics)discussed possibilities of producing neu-tral beams by photoelectron detach-ment from negative ions. This tech-nique should become useful whenmore intense lasers and negative ionbeams are developed. In competitionwith positive-ion-neutralization methodsthis technique has potential advantagesat low energies, at high angular resolu-tion and when excited-state control inthe initial beam is needed.A partial solution to the detection

problem is inherent in experimentsbeing performed by J. Paulus (Centrede Recherches Nucleaire, Strasbourg),S. Wexler (Argonne National Labora-tory), and M. Menzinger and R. Wolf-gang (University of Colorado). Theyhave used beams labeled with radio-isotopes to detect labeled products byradioactivity counting. The Coloradogroup reported experiments in which atritium ion beam was charge exchangedand reacted with a solid target. By thismeans they found it possible to makethe first determinations of the thresh-olds of hot-atom displacement reactions.Given the availability of radioisotopesof useful half-life, this detection meth-od could have wide applicability. Aneven more general, indeed universal,method of detection is reionization ofneutrals by electrons or protons. So far,however, such reionizers only operatewith low efficiency.

J. Ross (MIT) was chairman of a

session in which nozzle expansiontechniques were discussed. This meth-od, growing out of a suggestion byKantrowitz and Grey, has been more

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for the production of neutral beams.J. Fenn (Yale University) reviewed thetopic, emphasizing "seeding" methodsin which a heavy species is swept alongby lighter "driver" molecules, thus in-creasing its translational energy. E.Knuth (UCLA) discussed means ofreaching higher energies by plasmaheating the gas before expansion. Ingeneral, nozzle expansion seems an ex-cellent approach to the low- and medi-um-energy range (1 to 10 ev). It shouldprovide intense beams and excellentenergy control. However, the seedingtechnique is limited to heavier species,and high-temperature heating beforeexpansion leaves some question as tothe internal state of the beam mole-cules. At the time of the conference,no hot reaction appears to have beenidentified by using nozzle techniques.However, J. Deckers (University ofToronto) and J. Anderson (PrincetonUniversity) discussed some interestingnegative results.

In a session conducted by E. Fer-guson (ESSA, Boulder), L. Wharton(University of Chicago) first reviewedthe historic device of Bull and Moon(University of Birmingham) whichcan claim the distinction of being thefirst chemical accelerator. Regretfully,this charming method of accelerationby slapping molecules with a spinningrotor seems limited to the low energiesachieved by these pioneers. Whartonthen reviewed the building of his accel-erator for neutral species having per-manent dipole moments or which arehighly polarizable. Though expensiveand time consuming in its construction,this is a most interesting machine. Costconsiderations tend to limit the deviceto the acceleration of permanent dipolarmolecules at low energies. However,it should have excellent energy control.Moreover it was the only concept dis-cussed which inherently yields a polar-ized beam.

C. Schlier (University of Freiburg)and J. Los (FOM Institute, Amster-dam) discussed sputtering methods inwhich atoms are dislodged from sur-faces by impact of high-energy ions.The atoms in the resulting spray haveelectron-volt energies, but their veloc-ity distribution is so wide that a high-speed chopper is required to select areasonably homogeneous beam. Todate, sputtering has been used largelyto study nonreactive scattering, but itsfuture as a means of studying chemicalreactions appears bright.

This does not exhaust the catalog ofmethods discussed at the meeting. For

15 NOVEMBER 1968

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instance, D. Hansen (Thompson-Ramo-Woolridge) mentioned experimentsaimed at producing bursts of fastatoms by volatilizing electrostaticallyaccelerated dust particles with a laserpulse. And it seems certain that addi-tional techniques will be developed inthe future.The meeting closed with a panel dis-

cussion led by I. Amdur (MIT). Thepanelists, R. Bernstein (University ofWisconsin), S. Datz (Oak Ridge Na-tional Laboratories), M. Karplus, A.Kupperman (California Institute ofTechnology), B. Mahan, and the under-signed organizing committee, discussedthe problems and the prospects ofthe field. Much of the critical ap-praisal of the comparative merits ofchemical accelerators, which has al-ready been mentioned in this report,resulted from this session. Quite obvi-ously the field is in its early infancyand the direction of its development isstill far from evident. However, it wCasgenerally agreed that in the more dis-tant future the production of beamshaving not only a high kinetic energy,but also in selected states of internalexcitation, would be important. Justwhat combination of techniques willprove to be most useful in the next 10years is something about which few par-ticipants were willing to speculate. Onegeneral conclusion seems quite clear;the field will grow rapidly now that itsfuLndamental importance to elementarychemical kinetics has been recognized.

This conference was stupportedthrough a grant from the AdvancedResearch Projects Agency (ProjectDEFENDER), monitored by the U.S.Army Research Office, DuLrham, NorthCarolina, under contract DA-31-124-ARO-D- 1 39.

R. WOLFGANGR. N. ZARE

L. M. BRANSCOMBDeparuti en ts of Chem istry, Physicsand Astrophysics, and JoinitInistitite for Labooratory Astrophysics,Untiversityi of Color-ado, Boiulder

Calendar of EventsNational Meetings

December

1. Medical Aspects of Sports, 10th.Miami Beach, Fla. (F. Hemin AmericanMedical Assoc., Dept. of Health Educa-tion, 535 N. Dearborn St., Chicago, 111.)

1. American Acad. of Oral Medicine,New York, N.Y. (S. Conrad, 133-28228th St., Laurelton, N.Y. 11413)

1-4. American Medical Assoc., MiamiBeach, Fla. (F. J. L. Blasingame, 535N. Dearborn St., Chicago, Ill. 60610)

1-4. Reticuloendothelial Soc., 5th, NewYork, N.Y. (F. J. DiCarlo, Warner-Lam-bert Research Inst., Morris Plains, N.J.07950)

1-5. American Inst. of Chemical Engi-neers, 61st, Los Angeles, Calif. (Secre-tary, 345 E. 47 St., New York 10017)

1-6. Radiological Soc. of North Amer-ica, Chicago, Ill. (M. D. Frazer, 1744 S.58 St., Lincoln, Neb. 68506)

2. Quantum Chemistry, 9th winter inst.,Gainesville, Fla. (Winter Institute. 525Nuclear Sciences Bldg., Univ. of Florida.Gainesville 32601)

2-3. Applications of Simulation, 2ndconf., New York, N.Y. (A. Ockene, IBMCorporation, 112 E. Post Road, WhitePlains, N.Y. 10601)

2-4. New England Conf. on Air Pollu-tion, Waterville, Maine. (Director, ColbyNews Bureaau, Colby College, Waterville04901)

2-4. Western National GeophysicalUnion, San Francisco, Calif. (J. C. Harri-son, Dept. of Geophysical Sciences, Univ.of Colorado, Boulder 80302)

2-6. Greater New York Dental Mtg.,44th, New York, N.Y. (M. Purdy, Room106A, Statler-Hilton, New York 10001)

3. American Soc. of Therapeutic Radiol-ogists, Chicago, 111. (J. A. de] Regato,Penrose Cancer Hospital, 2215 N. Cas-cade. Colorado Springs, Colo. 80907)

3-4. Vehicular Technology Conf., SanFrancisco, Calif. (W. G. Chaney, LenkurtElectric, 1105 Country Rd., San Carlos,Calif. 94070)

3-5. Entry Vehicle Systems and Tech-nology Conf., Williamsburg. Va. (M. H.Bloom, Polytechnic Inst. of Brooklyn,Graduate Center, Route 110, Farming-dale, N.Y. 11735)

4-6. Optical Character Recognition inComputerized Management of Informationin the Next Decade, Hollywood, Fla. (In-ternational Business Forms Industries, 20Chevy Chase Circle, NW, Washington,D.C. 20015)

4-6. Academy of Psychosomatic Medi-cine, Miami Beach, Fla. (E. Dunlop, 150Emory St., Attleboro, Mass. 02703)

4-7. American Assoc. of Physicists inMedicine, Chicago, 111. (R. 0. Gorsop,Stein Research Center, Jefferson MedicalCollege. 920 Chancellor St., Philadelphia,Pa. 19107)

5-6. American Rheumatisnm Assoc.,Ttucson. Ariz. (M. M. Walsh, 1212 Ave-nute of the Americas, New York 10036)

6-7. American Federation for ClinicalResearch, Boston, Mass. (H. J. Levine.New England Medical Center Hospitals.17 1 Harrison Ave., Boston 02 111)7- 12. American Acad. of Dermatology

and Syphilology, 27th, Chicago, Ill. (S. E.Huff. 1636 ChuLrch St., Evanston, 111.)

8-13. American Soc. of AgriculturalEngineers, Chicago, Ill. (P. L. Bellinger,Technical Coordinator, 420 Main St., St.Joseph, Mich. 49085)

8-15. Symposium of Analogue and Digi-tal Computers in Hydrology, Tucson, Ariz.(American Federation of InformationProcessing Societies, 211 E. 43 St., NewYork 100 17)

9-11. Computer Conf., San Francisco,SCIENCE, VOL. 162

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