Contains reports on three research projects.Joint Services Electronics Program (Contract DAAB07-71-C-0300)National Institutes of Health (Grant 5 PO1 HL14332-03)Environmental Measurements Project Laboratory grant from the Dean of Science, M.I.T

Dylla, H. Frederick

Jarrell, Joseph A.

Weaver, James C.

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GENERAL PHYSICSI. MOLECULE MICROSCOPYAcademic and Research StaffProf. John G. King Dr. James C. WeaverDr. John W. Peterson Frank J. O'BrienGraduate StudentsH. Frederick Dylla Steven R. Jost Bruce R. SilverJoseph A. Jarrell Dusan G. Lysy Peter W. StevensA. NEUTRAL-ION RATIO MICROSCOPY JSJoint Services Electronics Program (Contract DAAB07-71-C-0300)James C. Weaver, H. Frederick DyllaRecent work 1 ' 2 in the Molecular Beams group has been directed toward developingseveral types of molecule microscopy. One of these, scanning desorption moleculemicroscopy (SDMM) using electron- stimulated desorption (ESD) offers particularpromise in materials science applications. The basic idea is first to expose a surfaceto a flux of neutral, surface-stain molecules in such a way that molecules stick inregions of preferred binding and not in others. Subsequently, molecules are desorbedwith a scanning electron beam and detected with an ionizer-mass spectrometer in orderto reveal where the applied molecules bind. Thus we seek a pictorial mapping of bindingsites for neutral molecules, and in so doing utilize surface chemistry interactions(defining the binding sites) to provide contrast.We propose now neutral-ion ratio microscopy, an extension of this technique whichshould also provide another form of contrast, one based on electron tunneling (which isitself a convolution of electronic levels), density of states, matrix elements, etc. of thesample surface. Conceptually, the experimental arrangement is simple: surface-stainmolecules are desorbed from the sample surface, but the number of neutrals, n 0 , andions, n , are detected separately (for example, with a 50% duty cycle). The quantity(n0 +n ) provides a measure of the binding-site density while, as we shall argue qualita-tively, the ratio nO/n+ should depend on surface parameters such as variations in elec-tron tunneling probability, and so forth.A recent experiment 3 has shown that there is approximately a tenfold intensity vari-ation as a function of desorption (emission) angle for desorbed 0+ ions (neutrals werenot measured) from a clean W (100) surface. This angular variation in desorbed ionintensity is independent of the angle of incident (desorbing) electrons. A possibleexplanation of this striking experimental result stimulated the idea of Neutral-Ion RatioMicroscopy (NIRM). Although the ESD desorption process is complicated, a simplifiedqualitative explanation 4 - 6 can be broken down into the following components: (i) primary JSQPR No. 114SEMICONDUCTORZzwz.0u0 GAPW_jWu,4_j(b) tiW , 4 INCREASESI-ION FORMEON0) METASTABLESA~ NTIBONDINGS GROUNDo(d) t. ION FORMEDI.-41 ION0, METASTABLENTIBONDINGCwa GROUNDW Wo(d)IL 4INCREASESIONIr0). METASTABLEAN=TIBONDING0a GROUND0U)o) SEMIC(WZzWZ0 GAPW._wz4_j0C,ZU))NDUCTOISEMICONDUCTOG AIGAPMSEMICONDUCTOFig. I-1.Energy-level diagrams for energy ofS adsorbate-substrate interaction andfor single electrons of the same sys-tem, with A the adsorbate- substrateseparation. A hypothetical desorptionprocess might include: (a) PrimaryINCREASES excitation from the adsorbed groundstate to a metastable state. (b) Initialoutward movement of the metastablecurve because, by the Frank-Condonprinciple, the repulsive part of theR metastable curve is reached. As Aincreases the energy levels of thesingle-e lect ron representation de-RESONANT crease and the adsorbate acquires ki-ONIZATION netic energy. (c) Formation of an ionOFETASTABLE by resonant ionization. (d) Furtherincrease in A because, by assump-tion, sufficient kinetic energy hasalready been acquired by the outward-moving adsorbate. The single-electron levels decrease further.(e) A neutralizing transition to anR antibonding state, with either reso-nant or Auger processes possible, sothat in this hypothetical sequence adesorbed neutral results. It shouldbe emphasized that this hypotheticalsequence is intended to represent thevariety of possible transitions thatINCREASES govern the desorption of neutrals andions, and hence the ratio of neutralsto ions.(e) tcaWWI-40inisSEMICONDUCTORAUGER- - --I RESONANTGAPAUGER .RESONANTQPR No. 114(a) tw,II--U) cWW00WU)C4,"(I. MOLECULE MICROSCOPY)excitation of the adsorbate-substrate (adsorbed molecule-surface) system by a single JSincident electron to the repulsive portion of a metastable state, (ii) deexcitation beforedesorption with high probability (which accounts for the low overall ESD cross sections),but in some cases a transition to either an ionic or a neutral state, including remainingin the metastable state, after sufficient kinetic energy has been accumulated for desorp-tion. For reasons of experimental convenience, most workers have studied only ions,although a few have measured metastable 7 or ground-state neutrals. 86As Madey and Yates pointed out in their review, the considerable work on ion neu-tralization spectroscopy (INS) of Hagstrum and his co-workers 9 should be relevant tounderstanding the types of transition between the metastable state and the ionic or otherelectrically neutral states. For example, depending on the electronic states of the sur-face and the perturbed ionization potential of the molecule, a molecule near a surfacemay be ionized or an ion neutralized by resonant or Auger electron tunneling pro-cesses. Since tunneling processes in general are sensitive to density of states, matrixelements, etc., the ratio n0/n + of desorbed molecules should be sensitive to variationsin these properties of the surface. Furthermore, tunneling processes occur readilyonly over short distances so that nO/n+ should mainly be sensitive to the outermostmonolayer of the surface. This uniquely high sensitivity to surface rather than bulkproperties has been found and emphasized by Hagstrum for INS.Naively, then, we may think of ESD as an excitation to a primary state followedusually by deexcitation without desorption but occasionally by desorption in which theratio n/n + is governed by time-reversed INS. A possible sequence of events is shownin Fig. I-1 where two different approximate representations are used.By varying the choice of stain molecule, we can hope to vary both the affinity forbinding sites (relevant to the nO + n signal) and the ionization potential (relevant to thenO/n + signal), so that both signals should respond to spatial variations of the sample sur-face. Again speaking simply, the application of SDMM with ESD should provide contrastwith surface specificity revealing variations in the chemical nature of the surface(binding site distribution by nO + n+ ) and, in a complicated way, in the electronic sur-face properties (via nO/n+). We are beginning experiments in which two materials (goldand carbon) are deposited to provide a test surface with large-scale (10-1 cm) spatialvariation in composition, and we shall examine both nO + n+ and nO/n+ signals in orderto see if the expected contrast exists.Stimulating conversations with P. W. Stephens and J. G. King are gratefullyacknowledged.References1. J. C. Weaver and J. G. King, Proc. Natl. Acad. Sci. U.S. 70, 2781 (1973). iSQPR No. 114(I. MOLECULE MICROSCOPY)JS 2. J. C. Weaver, Quarterly Progress Report No. 113, Research Laboratory of Elec-tronics, M. I. T., April 15, 1974, pp. 1-7; H. F. Dylla and G. A. Herzlinger,Quarterly Progress Report No. 108, Research Laboratory of Electronics, M. I. T.,January 15, 1973, pp. 2-5.3. J. J. Czyzewski, T. E. Madey, and J. T. Yates, Phys. Rev. Letters 32, 777 (1974).4. P. A. Redhead, Can. J. Phys. 42, 886 (1964).5. D. Menzel and R. Gomer, J. Chem. Phys. 41, 3311 (1964).6. T. E. Madey and J. T. Yates, J. Vac. Sci. Technol. 8, 525 (1971).7. I. G. Newsham, J. V. Hogue, and D. R. Sandstrom, J. Vac. Sci. Technol. 9, 596(1972).8. D. Menzel, Ber. Bunsenges. Physik. Chem. 72, 591 (1968).9. H. D. Hagstrum, Science 178, 275 (1972).B. TRANSMISSION-RESOLUTION CURVES FOR THE QUADRUPOLEMASS SPECTROMETERJoint Services Electronics Program (Contract DAAB07-71-C-0300)National Institutes of Health (Grant 5 PO1 HL14332-03)H. Frederick Dylla, Joseph A. JarrellThe electric quadrupole mass filter developed by Paul et al. has recently becomethe basis of an important piece of laboratory instrumentation 2 and is now available fromnumerous manufacturers of commercial mass spectrometer systems. An importantmeasure of the performance of a quadrupole mass filter as a mass spectrometer isobtained as a plot of the transmission through the quadrupole for ions of a particularmass m as a function of the resolving power m/Am, where Am is the half-width of themass line.According to the usual trade-off in mass spectrometers, the resolution increaseswith decreasing transmission. The transmission-resolution curve for a quadrupole massspectrometer is characterized by two regions of differing slope. In the first region ofthe curve, near unity relative transmission, the decrease in transmission with resolu-tion is small. A figure of merit obtained from the curve in this region is the highestattainable resolution at near unity transmission. Theoretically, this number dependson the size of the entrance aperture relative to the size of the quadrupole (or the effec-tive size of the aperture if ion lenses are employed to focus the entering ion beam), andon the transverse momentum of the ions relative to the applied quadrupole voltages. 3The mass peaks in this region have flat-topped trapezoidal shapes.In the second region of the curve, the decrease in transmission with increasing reso-lution is much more drastic. In this region, a figure of merit is the ultimate resolutionJS obtained from a distinguishable mass line and the relative transmission at this point.QPR No. 114Fig. 1-2.Relative transmission vs reso-lution for the 28 amu peak. Datarelevant to the mass spectrom-eter operation for these curves:emission current = 33 mA, elec-tron energy = 100 V, ion energy= +7 V, extractor voltage = +7 V,ion lens #1 = +4 V, ion lens #2 =+4 V, ion lens #3 = -68 V, AM(half-width) control = 0 V, elec-tron multiplier voltage = -4 kV,electron multiplier gain = 1. 5 X410 . For the ELFS curve PN = 1.5-6X 10 Torr; for the non-ELFS-6curve P = 4.7 X 10 Torr.N21000 100RESOLUTION m/AmCHO + C2H+S 0.036 amuFig. I-3.Best resolved multiple peak(29 amu) with ELFS quadru-pole.QPR No. 114(I. MOLECULE MICROSCOPY)JS As in the first region, this figure of merit depends theoretically on the entrance con-ditions; the ultimate resolution can also be limited by the precision of the quadrupoledimensions and applied voltages.4 The mass peaks in this region have triangularshapes.We have measured the transmission-resolution curve for a quadrupole mass spec-trometer that was one of several purchased from Extranuclear Laboratories,5 for thepurpose of molecular-beam detection and residual gas analysis. The transmissionresolution curves were measured with and without a design change for the quadrupoleentrance aperture that has recently been developed by Extranuclear Laboratories. Thenew design has been designated ELFS (Extranuclear Laboratories Field Separation) andpatents are pending. We have confirmed that the design change improves the resolu-tion at near unity transmission and the ultimate attainable resolution.1. Experimental ApparatusThe quadrupole (#324-9) had a field radius of .95 cm, a length of 22 cm, and wasoperated at a frequency of 3.7 MHz for a nominal mass range of 0-32 amu by the Extra-nuclear Laboratories power system (#011-1B). An axial electron bombardment ionizer(#041-1) and an off-axis 14-stage Cu-Be electron multiplier (EMI 9603B) were used withthe quadrupole. The output current of the electron multiplier was detected with a FEToperational amplifier (AD 540J) and displayed on a strip chart recorder or storageoscilloscope. The entire quadrupole assembly was flange-mounted in a 70-literstainless-steel, Viton and lead O-ring sealed, vacuum system. The vacuum system waspumped by a liquid nitrogen trapped Varian NHS-4 diffusion pump with DC 705 as thepump fluid.2. Procedure and ResultsMeasurements were made with the vacuum system containing a partial pressure of-6N2 in the 10- 6 Torr range as determined by a nude Bayard-Alpert gauge. The ionizerand quadrupole electronics were adjusted for maximum transmission and a smoothtrapezoidal shape for the 28 amu peak, with a resolution that just resolved the peakfrom the adjacent 27 amu and 29 amu peaks. The height and half-width of this peakestablished the point of unity transmission and minimum resolution on the transmission-resolution curve. The resolution was incrementally increased subsequently and thecorresponding trace of the 28 amu peak was recorded on the strip chart recorder andoscilloscope.The procedure was repeated with the ELFS entrance aperture in place of the usualgrounded entrance aperture. Relative transmission vs resolution of the 28 amu peak isplotted in Fig. I-2 for the ELFS and non-ELFS quadrupoles. The plot shows that bothJS quadrupoles are capable of resolving adjacent mass peaks over the entire mass rangeQPR No. 114(I. MOLECULE MICROSCOPY)with near unity transmission (m/Am = 70 at a relative transmission of . 90). It also JSshows that the ELFS quadrupole attains a significantly greater resolution than the non-ELFS quadrupole for all values of relative transmission below the trapezoidal peak(nearunity transmission) region. The ultimate attainable resolution for ELFS is 1500, com-pared with 500 for the non-ELFS.Figure I-3 shows a tracing of the best resolved multiple peak (29 amu) in the back-ground spectrum that was obtained with the ELFS quadrupole. With the 29 amu doubletidentified as CHO +-C2H after Fite and Irving, the resolved mass difference is+ +.036 amu. We were unable to split the 28 amu peak into its three constituents (NZ , COC 2 H4 ) during our measurements, since this would have required a resolvable mass dif-ferenceof the order of . 010 (or m/Am = 3000).Oscilloscope traces showing the peak shapes obtained for the first 11 points of thetransmission-resolution curve are shown in Fig. I-4a for the ELFS quadrupole and inELFSQUADRUPOLE4.5X10 A/Torr N 2- ~ 0.2 omu/cm(a)NON-ELFSQUADRUPOLE3.3x10 - 5 A/Torr N 20.2 omu/cm(b)* ION CURRENT AT THE INPUT OF THE ELECTRON MULTIPLIERFig. I-4. Mass 28 peak shape for transmission vs resolutiondata of Fig. 1-2. JSQPR No. 114__(I. MOLECULE MICROSCOPY)JS Fig. I-4b for the non-ELFS quadrupole. The ordinate of Fig. I-4a gives the approxi-mate absolute transmission for background gas (in A/Torr N2 before electron multipliergain) based on the ion gauge pressure measurement. Note that the absolute sensitivityof the ELFS quadrupole is an order of magnitude larger than that of the non-ELFS.References1. W. Paul and H. Steinwedel, Z. Naturforsch. 8A, 448 (1953); W. Paul and M. Raether,Z. Physik 140, 262 (1955).2. For a recent review see P. H. Dawson and N. R. Whetten, "The Monopole MassSpectrometer," Advances in Electronics and Electron Physics (Academic Press,Inc., New York, 1969), Vol. 21, pp. 59-90.3. Ibid., p. 88.4. Ibid., p. 89.5. The quadrupole mass spectrometer manufactured by Extranuclear Laboratories, Inc.was chosen as the result of a comparative study of 9 commercially available instru-ments, Internal Report, Molecular Beam Laboratory, Research Laboratory of Elec-tronics, M. I. T., October 25, 1972.JS 6. W. L. Fite and P. Irving, J. Vac. Sci. Technol. 11, 351 (1974).C. ASSAY BY VOLATILE ENZYME PRODUCTSNational Institutes of Health (Grant 5 PO1 HL14322-03)Environmental Measurements Project Laboratory grant from theDean of Science, M. I. T.James C. WeaverDuring the past year we have developed plans for a highly sensitive biochemicalassay. The technique is based on the detection of volatile enzyme products (VEP), andthereby combines enzyme specificity and the ability to count individual molecules in avacuum system with an expected efficiency ET 1 0 - 4  One basic configuration is shownin Fig. I-5.The potential sensitivity and time response of the VEP technique can be estimatedin the following way. Three effective permeabilities act in series to determine the rateat which a nonvolatile substrate, S, is converted into a flux of neutral molecules in thevacuum system. The first is the diffusion of S through the unstirred layer. The secondis the enzyme, E, reaction rate for the conversion of S into products, at least oneof which must be a volatile, V, and must also be permeable to the membrane. The thirdis the permeability of the membrane. If DS is the substrate diffusion constant, the first- 3  - 4  -1effective permeability is DS/x = 10 -10 cm-s . Also, if the amount of immobilizedenzyme is expressed as the number of monolayers, nmL, and aE - 10 moleculesE-cm2 is the monolayer density, then the effective permeability isQPR No. 114(I. MOLECULE MICROSCOPY)ENZYME REGION M BRANEVSAMPLE SOLUTION i Vl QMSSx I-Fig. 1-5. Schematic of a VEP configuration. The sample molecules are the substrate,S, for the enzyme which is immobilized near or on one side of a semiper-meable membrane. An unstirred boundary layer of thickness ~10 - 2 cm pre-sents a diffusive barrier to S, and, in many cases, also serves to keep mostof the volatile product from diffusing back into the sample solution, since themembrane permeability, PM' can be much greater than D /x, where Dv isthe volatile's diffusion constant. If the membrane area =1 cm2 and a mini-mum count rate of A = 1 cps is taken to represent an ultimate limit of detec-tability, then the corresponding minimum detectable substrate concentration-14is estimated to be [S]min = 5 X 10 14 M for systems for which a suitableenzyme and volatile product can be obtained. Here UI is a universal ionizerand QMS is a quadrupole mass spectrometer for detection and identificationof the volatile products.Vm nmL'EVTflKm K mwhere vT is the turnover number in molecules S per molecule E per second, fl is thefraction of active enzyme, and K is the Michaelis constant. Depending on the type ofmimmobilization and the enzyme reaction that is used, the range 10-4 < V /K m1 -110 cm-s is expected. Finally, by suitable choice, the membrane permeability canbe PM m 10- 2 cm. Thus, the total effective permeability is determined from-1 S m -1P + +PPeff l Mand typically will be in the range 10-4 < Pf 10 cm-s . Of the volatile productsgenerated, a fraction, PM/[PM+Dv/x], will permeate the membrane and evaporate intothe vacuum. The technical problems of detecting small fluxes of-neutral molecules invacuum are identical to those encountered in molecule microscopy. We expect the totalcounting efficiency, ET, to be ET = E(Q2/w), where E = 10- 3 is the universal ionizer effi-ciency, and Q/Tr is the solid angle fraction for effusion which is subtended by the ion-izer. Finally, if the membrane area is A m , the expected count rate isQPR No. 114(I. 'MOLECULE MICROSCOPY)PS Peff pM+ (/x) Am [S],where [S] is the substrate concentration in molecules-cm-3The time response of the VEP technique should usually be limited by the substratediffusion time, TD =x /2D 1 0 s. Thus, for example, at a concentration [S] = 10 -1 2 M1the expected count rate is ~2 X 10 with an accumulated count in 10 s to get a signal-to-noise ratio of ~15.In another configuration it should be possible to react all of the sbustrate and,furthermore, to arrange for all of the product to permeate the membrane into thevacuum. With the same criterion in this mode the ultimate limit of detectability shouldbe "10 - 2 0 mole or 104 molecules.Preliminary experiments in collaboration with M. K. Mason and J. A. Jarrell arein progress and are yielding very encouraging results which suggest that the highexpected sensitivity will be realized. The author also acknowledges helpful conversa-tions with J. W. Peterson, Professor C. L. Cooney, of the Department of Nutrition,M. I. T., and J. G. King.QPR No. 114

Molecule Microscopy

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Molecule Microscopy

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