We describe the techniques used in calibrating a monochromator/spectrometer system for gas-phase photoelectron angular distribution and branching ratio measurements. We report a self-consistent set of values for the Ne 2p, Ar 3p, Kr 4p/sub 3/2/ and 4p/sub 1/2/, and Xe 5p/sub 3/2/ and 5p/sub 1/2/ photoelectron asymmetry parameters and for the Kr 4p/sub 3/2/:4p/sub 1/2/ and Xe 5p/sub 3/2/:5p/sub 1/2/ branching ratios for the energy regions from threshold to approximately 15 iV. 22 refs., 7 figs

Dehmer, J.L.

Hardis, J.E.

Holland, D.M.P.

Parr, A.C.

Southworth, S.H.

Southworth, S. H.

Parr, A. C.

Hardis, J. E.

Dehmer, J. L.

Holland, D. M. P.

UNT Digital Library

‘ LA-UR -85-3113Loa AIww09 National Labwatory b opamM by IIW Unlvw81ry of C41fwnk to? tlu Unti Sm~ ~monl of En6rgy under contraci W-740S EN(X14.LA-iJR--85-3ll3DE86 000789TITLE: CALIBRATION OF A MONOCHROMATOR/SPECTROMETERSYSTEM FORTHE MEASUREMENT OF PHOTOELECTRON ANGULAR DISTRIWTIONSAND BRANCHING RATIOSAUTHOR(S): S. H. SouthworthA. C. ParrJ. E. HardisJ. L. DehmerD. M. P. HollandSUBMlrTED TO: InternationalConference on SynchrotronsInstrumentationand RadiationStanford UniversityStanford, CaliforniaTo be published in Nuclear Instruments & MethodsThlI raPrI wu prapmrd m ● ncoounl of work IpIImrd by ●n qwrcy of tha Urrltai StttwOovwnment, Nelllnr the Unlld Stal*@vernmenl nornny~nq lhe~f, ~rnnyoftklremployw. makm any wnrrnnty, cxprusor Implkd, orauurnu wry loEal Ilnbilltyor rMporlIl-blllly for theaccurt,cy, completanou, or umfuln~of anylnfomntlon, ●ppsratuo, product, or~dlwlti, or I@pr=enhlhal ha urn would no! lrrfrinr prlvatelyownd rights, Reror-erox horelo to my spdfic cornmwclol prcduct, procas, or Mda by trade name, trndammk,mtnuflcturw, or othor-wlm doc.i not rrecasarlly cimtltute or imply III endormmwrt, reoom.menddon, w boring by the United StNos Gwcrnmonl or my agorrcy (herwf, TIM vlavmBy accapla,)c- of Ihla arllclo, md opinlom of nuthon oaprwd heraht do not n.qrily mt~te or mfled them of the ,froproducoIhopublbhcd form 01 Ihls tUnlld Slntcn Oovornmont or ●ny •~rrcy thamof,Th. LOS Alamo@ Nallnnal Labofslory fwuoato lhal Iho publlohor Idonllfy ttrla arllclo aaworhparformod undortho ausplcti ollho US. Mpnrlmonlol EnergylLOMhTiiilOSLosAlamos,NewMexico875.gLos Alamos National LaboratorCALIBRATION OF A MONCCHROMATOR/SPECTROMETERSYST~ FOR THE MEASUREMENTOF PHOTOELECTRON AIWJLAR DISTRIBUTIONS AND BRANCHING FUTIOSS. H. SOUTHWORTHLos Alams National laboratory, Los Alarms, NM 87545, USAA. C. PARR and J. E. HARDISSynchrotronsUltraviolet Radiation Facility~ National Bureau of Standards, Gaithersburg, MD 20899, USAJ. L. MHM13RArgome National Laboratory, Argonne, IL 60439, USAandD. M. P. HOLLANDDaresbury Laboratory, Darasbury, Wa?rington WA4 4AD, UKABSTRACTWe describe the tecbrlquas used iI~calibrating a mnochromator/spectrometersystem for gas-phase photoelectron angular distribution and branching ratiomeasurements. We report a self-consietant set of vmlws for the Ne 2p, Ar 3p,~.r4P3/2 and 4P1/2~ ●nd fi 5p3/2 and %311z photoelectron -y-try parameterand for the Kr 4p312:4p112 ●nd Xe 5p3,2:5p112 branching ratio- for the energyre~ions from threshold to approximately 15 eV.-2-1. IntroductionThe measurement of wavelength depende~tangular distribucione, branching ratioa, andphotoelectronpartial crossaectione ie one of the important applications of synchrotronsradiation In atomic and mlecular DhySiC!3. In combination withtheoretical studies, photoelectron meamrements reveal thestructural ●nd d~amical factors which underlie photolonizationprocesses. For example, the tunable continuum property ofnynchrotron radiation hae allowed the study of resonant processessuch ae those due to shape resonances and autoiGnization [1].Quantitative photoelectron maaltre~nts using synchrotronsradiation require knowledge of the wavelength dependent intensity●nd polarization of the light beam and of the transmission of the●lectron analyzer(s) ●s a function of photoelectron kinetic●nergy and ejection angle. Eere w describe the techniques usadin callbracing a monochromator/electrometer system formeasurements of photoelectron angular distributions and branchingratios. The apectrometar [2] was ● second-generation fnatrumemtdaaigned for high resolution molecular mtudioa. Themonochromator [3] wee the high throughput, 2 m, normal incidenceInstrument locatad at the Synchrotrons Ultraviolet RadiationFacility.Ne, Ar, Kr, ●nd Xe were used for calibration studies,because their ●ngular ●symmetry paramatars, B’s [4-10], and totalcrooe sections [6,7,11] ma known fairly well. As diacuceedfully below, COM diocrapmcieo occurr~d ●t certain ●n~rgiee inthe calibration resulto basad on tha differant rara gaostandards. Our ●pproach was to darive sp~ctrometar calibration-3-funccions which were mst consistent among all of the rare gasdata. We present here the resulting self% onsiatent set of S’Sfor the Ne 2p, & 3p, * *3/2 an< 4PI/2s and ~ 5P3/2 and5pl/2 eubshells and of the branching ratios Kr 4P3/2:4pl/2and & 5p3/2:5Pl;z*2. Light beam calibrationThe monochromator [3] was used with an osmium coated foldingmfrror and a 2400 l/mm osmium coated grating blazed for 500A infirec order. Tungsten photodiodes were used to monitor theintensity of the light beam. With the prasent ~ating, themaxiwm l]ght intensity occurred near 20 eV, and the intensityfell to below 10% of maximum for hv > 30 eV. However, it weepossible to record photoelectron dat. ●bove 30 eV with Ligh beamcl~rronteand long collect.tontimes. The elactron epectro~terwas used to observe photoelectron produced by eacond-rder lightand to tharaby determine that the second~rder likht intensitywarnnegligible for thim grating. Hcwaver there appeared to be ●acat~nrd light componant which causmd difficulty ir.determiningthe tiranamirns!onfunctions of the electton ●nalyzcrm. This pointis dlacunaed further below.Although synchrotronsradiation la ●l”’Opticallypolarizad, ithas been shown [12] thaL thm form of tha photoalmctron •n~ulkrdistribution if the ●alm ●o that L’orpartially lin,a:l) polariuedlight. Ther@fora it haa becoma common to daacriba the photonbeam ●s having a degree of polarizationP “ (Ix - xy)/(q + $?),whera Ix ●nd $ ara the tn~ansitims, rasptcLivaly, of themajor ●nd minor componant~ of linaar polarisation. Tho-4-polarizat~on p was measured using triple-reflectionanalyzers andfound to be typically 70 - 75 % over the energy range hv - 12 -33 eV. The ~asured values of p are cloee Co those =lculacedfor the Inherent polarization of the synchrotronslight in this●nergy range, confirming that the normal incidencefrom the beam lime optics did not greatly modify p.IK is worth noting that Schmidt and coworkersreflections[5,13] haveretained the point of view of an elliptically polarized lightbeam and have treasured for their beam line a tilt of thepolarization ●nip”: with respect to the synchrotronsorbit. Inthe preeent work we have assumed that the polarization ellipseremains parallel to the synchrotronsorbit, which seems reasonableconeiderlng the simple geometry of our beam line optics.3. Electron spectrometer calibrationThe spectrometer [2] contains two 10.2 cm mean-radiushemispherical electron analyzers which &repositioned to detectphotoelectrons ejerted in a plar~ewhich is perpendicular to thepropagation direction of the light beam. Wlch this geometry thedifferential croee section appropriate for electric dipolephotoionizntion of uandomly oriented (typical gas phase) targetscan be expressed aurdu/dn = (u/4n) [1 +(’B/4)(1 + 3P coo Ze)]where,u - totkl (angla-intagra~ti+lcross oec~ion,B- photo~lectron asymmetry parameter,p = degree of polarizstiol,of light bsmm, ●dO - photoalectcou ejection angle wi~h raspact to the majorpolarization QX1O of the light beam.-5-One of the analyzers ia fixed to detect photoelectronsejected at a = 0° while the other analyzer is rotatable over therange 6 = 0° - 90°. However in the present set of =asuremntsthe rotatable analyzer was also held at a fixed angle of 0 =90°. Apertures of 1.5 cmadiameter were used on the entrance andexit lenses, yialding an angular resolution of 4“. For the raregas measurements reported here the electron analyzers wereoperated at 10 eV pass energy for which the intrinsic resolutionof the electron analyzers was measured to be 90 - 100 rmV. Theoverall energy resolution wae given by the spectrometerresolution convoluted with the photon bandwidth, which was 2.2 AFWHM (Full Width at Half Maximum) for the present data.Studios were made to check for pressure dependent effectswhich can urise due to photoelectron scattering from the emaplega@a8. We observed, Sor example, preseure+epe,tdent angulardistributions in Kr and M at kinetic energies where those gaseshave strong maxima in their electron acatterlng crose sections[14]. To ●void these effects, the photoelectron intensity ineach enalyzer vaa masured aa a function of praasure for ●ach●ampla gas at or naar the mximum of the scattering croee cection[14]. From such ~aauramante ● prmosure range was determinedovar which tha angular distribution results ● re free ofSiqnificnnc acatcaring ●ffecte. 0 and branching ratiomeasurements were typically obtained with ● background preesureof 1 x 10-3 torr in the spectrometer chambgr.Tha Ar 3p nubshell was uoed for the primary calibration●tandard. A typical calibration data set consisted of the 0° mnd90° photoalqctron spectra of #r 3P recordad over ● range of-6-kinetic energies. Using literature values for the Ar 3p B’S[7-9] and cross section [11] along with the measured relativeIntewity and polarization of the light beam, the data werereduced to obtain the relacfve tranemfssiona of the 0° and 90°analyzers as a function of kinetic energy.programs was developed to automate the dataso that the stability of the system could beA set of computerreduccion procedurechecked quickly andfrequently. Based on regular calibration Nns with Ar 3p, therelative transmissions of the analyzers were found to be stableover a period of several months. Nring this time, ~asuremencswere made on the other rare gases and on wlecular sauples. Thetransmission functions of the electron analyzers wereincorporated in the fitting procedure for new datu which correctbthe raw spectra and derivesracioe.4. Results for che rara gasesNote that to determineaced to know only thm ratioasymmetry parameters and branching6’s wiCh the present technique weof t’e transmissions (the “angularratio”) of the 0° and 90” analyzars as a funccion of kineticenergy, in addition to the polarizecion p. In turn, thecalibrated angular ratlc dopendo only on che values of B assumedto be corract for the callbratian gae, along with p. However,tha branching ratio re~ulcs depend on the -ariacion ‘ftha analyzer tranemisaio,m with kinetic anargy. Calibration for*:habranching ratio rnsults charefora requiras knowledge of therelative intensity of the first-order compmanl: of the light beamand of the total cross section for the calibration gae. First wediacuse rasultm for the rara gae 0’s and then diecllss thabranching ratios.-7-.. .Based onwith previousrare gases fordiscrepanciesattributed tothe Ar 3p calibration, we obtained good agreementmasureuents and theorykinetic energies in theoccurred for energiestwo factors: (1)structure in Ar, Kr, and Xe, and (2)for the B’S of the otherrange O - 10 eV. However,above 1.0 eV. This wasu~reaolved autoionlzationthe rare gas 13’sare lesswell knowo for energies!above 10 ●V. The photoabsorption spectraof the rare gases all display prominent Rydberg series aeaociatedwith the thresholds fo: ionization of the valence s~rbftals eswell as duublyaxcited resonant states [15-17]. It is known thatthese states aucoionize, resulting in strong, resonant variatj.onof the B’S and branching ratios for the valence parbitals [18].Thus, we cculd infer that, for example, in the kinetic energyregion just above 10 ●V, our original angular ratio calibrationwas in error due CO autoionizatlon ●ffects on the Ar 3p e~s.Kaepiug in mind which energy regions for each rare gss weresusceptible t.o autoionization effects, we then determined amodified angular ratio calibration based on an optimum tit to allof the rare gas eta. The optimized calibration ratio was thenapplied to all of the raw data to generate a self%onsietant setof E’s for the cars gas standards.Thevery goodin F.gs.resultlng 6’s are plotted in Figs. 1 - 4. We obtainagreement with earlier measurements. The hatched areas2- 4 denote the energy regions of the prominent e + pautoionizingregions theresults forresonances in Ar, X.r, and Xa, In thoaa energy-asured B’O show deviation frcm the theoreticalwhich autoioniza~ion eflecta wmre not explicitly. ,,-8-Included. A new result of our data is that for Ns 2p we observea minimum in B near threshold as had been predicted theoreticallyby lGmnedy and Manson [6].The Kr 4P3/2:4Pl/2 and Xe 5P3/2:5PI/2 branchingratios also originally displayed deviations at higher kineticLma&d;+;.~●nergies from previous measurements and theory.AThe ap~arenttransmissions of the electron analyzers fell off with increasingenergy more rapidly then expected, based on theoreticalconsiderations and previous experience with analyzers of thistype. These discrepancies were attributed to lack of knowledgeof the relative Inteneity of the first-rder light beam,resulting from a scattered light component and use of a tungstenwire tish photodiode which had not been directly calibrated.Consequently, we modified the energy variation of the analyzertransmissions to best fit previously measured and calculatedbranching ratios for Kr and Xe. The results are plotted in Fig.5. We obtain good agreement with earlier ~asurements andtheory. Note that we observe some sharp variation of the Xebranching ratio in the energy region of the o + p autoionizingresonances [17,18].In summary, we have described our techniques in performingquantitative photoelectron ❑easurements using synchrotronsradiation. We have based the calibration of our apparatus on p’sand branching ratios for the rare gaeea and have presented aself-consistent set of values for those standarda. We believethese results give a measure of confidence in our results formolecular gases reported elsewhere.-9-Ack.lowledgementgWe thank Dr. D. L. Ederer for helpful discussions andassistance. We are grateful to the staff of the SynchrotronsUltraviolet Radiation Facility for their cooperation during thecourse of this work. This wcrk was supported in part by theOffice of Naval Research and the Department of Energy.-1o-References[1][2][3][4][5][6][7][8][91[10][11][12]B. Sonntag and F. Wuilleumier, Nucl. Instr. and Meth. 208(1983) 735; J. L. Deh.mer,D. Dill, and A. C. Parr, inPhotophysics and Photochem.lstryin the Vacuum Ultraviolet,edited by S. McGlynn, G. Findley, and R. Euebner (~idel,Dordrecht, 1985), pp. 341.A. C. Parr, S. E. Southworth, J. L. Ihshmer,and D. M. P.Holland, Nucl. Inscr. and Heth. 222 (1984) 221.D. L. Ederer, E. E. @le, and J. B. West, I’&cl.Instr. andMath. 172 (1980) 185.K.J.H.Codling, R. G. Houlgate, J. B. West, and P. R. Woodruff,Phys. B9 (1976) L83.Darenbach, R. Malutzki, and V. Schmidt, Mel. Instr. andHeth.D. J.W. R.K.-N.208 (1983) 845.Kennedy and S. T. Hanson, “fiys.Rev. A5 (1972) 227.Johnson and K. T. Cheng, Phys. Rev. A20 (1979) 978;Huang, W. R. Johnson, and K. T. Cheng, At. Data Nucl.Data Tables 26 (1981) 33.D. M. P. Holland, A. C. Parr, D. L. Ederer, J. L. Dehmer,and J. B. West, Nucl. Instr. and Meth. 195 (1982) 331.R. G. Houlgate, J. B. West, K. Codling, and G. V. Marr, J.Electron Spectrosc. 9 (1976) 205.M. O. Krause, T. A. Carlson, and P. R. Woodruff, Phys. Rev.A24 (1981) 1374.G. V. Marr and J. B. West, At. Data Nucl. Data Tables 18(1976) 497; J. B. West and J. Morton, ibid. 22 (1978) 103.J. A. R. Samson and A. F. Starace, J. Phys. B8 (1975) 1806,Corrigendum 12 (1979) 3993.-11-[]3] V. Schmidt, Phys. Lett. 45A (1973) 63.[14] L. J. Kieffer, Atom. Data 2 (1971) 293.[15] K. Codling, R. P. Madden, and D. L. Ederer, Phvs. Rev. 155(1967) 26.[16] R. P. Madden, D, L. Ederer, and K. Codling, Phys. Rev. 177(1969) 136.[17] K. Codling and R. P. Madden, Phys. Rev. A4 (1971) 2261; J.Res. Natl. Eur. Std. 76A (1972) 1.[18] D. L. Ederer, A. C. Parr, J. B. West, D. Ulland, and J. L.Dehmer, Phys. Rev. A25 (1982) 2006 and references citedtherein.[19] H. J. Levinson, 1. T. McGovern, and T. Gustafsson, J. Phys.B13 (1980) 253.[20] M. Y. Adam, F. Wuilleumier, S. Krummacher, N. Sandner, V.Schmidt, and W. Mehlhorn, J. Electron Spectrosc. 15 (1978)211.[21] J. A. R. Samaon, J. L. Gardner, and A. F. Starace, Phys.Rev. A12 (1975) 1459.[22] W. Ong and S. T. Lanson, J. Phys. Bll (1978) L163.-12-Figure captionsFig. 1.Fig. 2.Fig. 3.Fig, 4.Photoelectron asy~etry parameter for theFilled circles: pre9ent measurements;Ne 2p subshell.open circles:measurements from Ref. [5]; X’9: measurements from Ref.[4]; dashed line: HP (dipole-length form) calculationfrom Ref. [6]; solid line: RRPA calculation from Ref.[7].Photoelectron asymmetry parameter for the Ar 3p subshell.Filled circles: present measurements; open circles:❑easurements from Ref. [8] (2:1 weighted average of Ar3p3/~ and 3pi/2 results);Xts: measurements fromRef. [9];solid line: RRPAY-e hatched area denotes theautoicnizing Resonances (seecalculation from Raf. [7].energy region of Ar 3s + npRef. [161).Photoelectron asymmetry parameters for the Kr 4p3/2 and4PJ/2 subshclls. Filled circles: presentmt:a9urementF; open circles: measurements from Raft [8];~olid lines: RRPA calculations from Ref. g7]. Thehatched area denotes the energy region of Kr 4s + npautoiontzing resonances (see Ref. [17]).FhotoelectEon asymmetry parameters for the Xe 5p3/2 and5pl/2 subshells. Filled Circles: presentmeasurement ;solid lines:hatched areaautolonizing raaonancesFig, 5. The photoiontzationopen circles: measurements from Ref [8];RRPA calculations from Ref. [7]. Thedenotes the energy region of Xe 5s + npnnrl Xe 5P3/2:5PI/2s(see Ref. [17]).branching ratios K 4P3/2:f’~I;&Filled clrcloa: present.,-13-msasurements; open circles: ~asurements from Ref. [19];x’s: measurements from Ref. [10]; triangle9:measurements from Ref. [20]; dotted lines: measurementsfrom Ref. [21]; solid lines: RRPA calculation from Ref.[7]; dashed line: DF calculation from Ref. 122]. Thehatched areas denote the energy regions of the Kr 4.9+ npand the Xe 5s + np autoionizing resonances (see Ref.[17])..10.05.PG-05.NE 2p\\I I I I 1 I I [ I I i I I I I I I22 26 30 34 38hv (eV)Fig.1.—. ——. .-0L.I.5I.0B0.5081’ -?-tIIII i I I I I I I I —~I16 20 24 28 - 32 36hv (eV)Hg. 2..2.0 1 1 1I 1 I I I 1KR 4pv2i.5-l.o -#0.5-1B“+ a 1 # I I 1 1 a 12.0-KR 4P,/2+!t1.5- t-r+?I1.o-- ~~10.5+to- +I& 20 24 28 32hv (eV)Fig. 3.-17-2.0’1.5’loci0.5”2.0’I .5”1.0”0.5”0“XE5?,/2+13 17 21 25 29hv (eV)Fi#, 4.. .“..02.21.62.0I.4KRI a n I I 1+T‘+~+b+...... ............... .XEt L~ ~._.________“}+.~.+-~-+- J{! t{ +.i-l;;~~ x , a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -x+ o Nxb13 17 21 25 29 33hl~(eV)Fig . 5

Calibration of a monochromator/spectrometer system for the measurement of photoelectron angular distributions and branching ratios

Calibration of a monochromator/spectrometer system for the measurement of photoelectron angular distributions and branching ratios

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