The dynamics of electronic autoionization in O2 has been studied using a new apparatus which combines a free-jet supersonic expansion with synchrotron radiation. Ions and electrons were analyzed by a double time-of-flight spectrometer. The spin-orbit sublevels of the 3Πu (v=0 and 2) Rydberg states in O2 were selectively excited and the resulting O+2 final states were determined by time-of-flight photoelectron spectros copy. A strong variation of the 2Π1/2g :2Π3/2g branching ratio was observed. This variation results from the selection of a single continuum wave function in the autoionization process

Guyon, P.

Heiser, F.

Hepburn, J.

Reynolds, D.

Weng, T.

English

MPG.PuRe

VOLUME 67, NUMBER 6 PHYSICAL REVIEW LETTERS 5 AUGUST 1991Highly Selective Population of Spin-Orbit Levels in Electronic Autoionization of 02Paul-Marie Guyon and John W. Hepburn "'Laboratoire des Collisions Atomiques et Moleculaires, Universite Paris AI, Batiment 351, 91405 Orsay, FranceTaimeng WengLaboratoire pour l'Utilisation du Rayonnement Electromagnetique, Universite Paris XI,Batiment 209D, 91405 Orsay, FranceFranz HeiserFritz Haber Institut der Max Plank Gesellshaft, Faradayvveg, 1000 Berlin, GermanyDavid ReynoldsDepartment of Chemistry, University of Nottingham, Nottingham, United Kingdom(Received 25 March 1991)The dynamics of electronic autoionization in 02 has been studied using a new apparatus which com-bines a free-jet supersonic expansion with synchrotron radiation. Ions and electrons were analyzed by adouble time-of-Aight spectrometer. The spin-orbit sublevels of the H '1I, (v =0 and 2) Rydberg states in02 were selectively excited and the resulting Oz+ final states were determined by time-of-Right photo-electron spectroscopy. A strong variation of the Hip2~. H3~2g branching ratio was observed. This varia-tion results from the selection of a single continuum wave function in the autoionization process.PACS numbers: 33.20.Ni, 33.60.Cv, 33.80.EhPhotoionization of molecular oxygen near the ioniza-tion threshold shows intense autoionization peaks super-imposed on a weak continuum. Some of these reso-nances, assigned to Rydberg series converging to 02+a H„, were seen to be split into several components in thehigh-resolution photoionization spectrum of Dehmer andChupka [1]. Recently, Tonkyn, Winniczek, and Whiteobserved a small portion of the spectrum comprising theH II„Zg (2,0), (1,0), and (0,0) bands at still higherresolution using a vacuum-ultraviolet (VUV) laser for ex-citation of a supersonically cooled beam of oxygen mole-cules [2,3]. These authors also interpreted the fine struc-ture and determined the spin-orbit constant from a fit oftheir photoionization spectrum using a pure IT„Zgtransition within the Hund's case a coupling scheme.These series autoionize to the 02+ ground state and theobserved vibrational branching ratio is consistent with themodel of Bardsley and Smith for electronic autoioniza-tion [4-7]. The most recent photoelectron spectroscopicstudies of this system by Holland and West [8] and byTonkyn, Winniczek, and White [3] did not resolve thetwo 0+ = 2 and 0+ = 2 spin-orbit components of theHg ground-state vibrational levels.For these reasons, the investigation of the variation ofthe H]y2g. Hy2g branching ratio upon selective excita-tion of the F~, F2, and F3 spin-orbit components of theH(v =0) and H(v =2) excited states was undertaken.Experiments were carried out using a 3-m normal-incidence monochromator installed on SuperACO, thenew synchrotron radiation source in Orsay, and a free-expansion jet adapted to a double electron-ion time-of-flight coincidence analyzer [9]. This apparatus made itpossible to resolve low-energy photoelectron peaks sepa-rated by about 5 meV at energies as low as 10 meV, al-lowing us to resolve the spin-orbit components of the 02+Hg state. The results presented here show a strong vari-ation of this branching ratio across the H, resonanceprofiles which is interpreted as due to a selection rule inthe choice of the ionization continuum to which the reso-nances are coupled.The details of the experimental setup wi11 be given in aforthcoming paper and only the essential features wi11 beoutlined here. Photoelectrons and ions were extracted inopposite directions at right angles to both the jet and thephoton beam. Photoelectrons were selected by both theirtime of flight (TOF), measured with respect to the pho-ton pulse arrival time, and by angular discrimination[10]. The threshold electrons reached the detector inabout 100 ns, less than the 120-ns photon pulse periodwhen the ring was operated in the two-bunch mode. En-ergetic e1ectrons arrived sooner or later depending onwhether they were emitted forward or backward withrespect to the detector direction. In the present experi-ment, the TOF analyzer was mounted perpendicular tothe orbit of the synchrotron ring, at 90' from the VUVpolarization vector.The monochromator was operated at a bandwidth of0.02 nm, i.e., 2.5 meV around 102.65 nm, the ionizationthreshold of 02. The target beam was formed by expand-ing pure oxygen gas from a 20-pm pin hole, with the noz-zle about 1 cm from the VUV beam. The actual temper-ature of the jet could not be determined.The photoionization e%ciency curve of the 02H 3II„(v =0) Rydberg state is shown in Fig. 1. Thisspectrum closely resembles that obtained by Tonkyn,Winniczek, and White [3] using a pure beam of oxygenso that the peak maxima may be assigned to the rotation-al bandheads of the three spin-orbit components. Wechose these maxima, namely, A, =100.38, 100.31, and100.26 nm for H(v =0) and X =98.31, 98.26, and 98.201991 The American Physical Society 675VOLUME 67, NUMBER 6 PHYSICAL REVIEW LETTERS 5 AUGUST 1991X iO4),2. 5 .21003.11002.6 1003.8I(c) 3/2 1/2V+=1 3/2 1/2rF @=031.5 „Z0 1OF3 F2 F,'TOF (0)01000 I10081006I I1002 1004WAVELENGTH (A)FIG. 1. Photoionization efficiency curve of 02 in the regionof the H X(0,0) band. Underlying continuum signal islargely due to second-order light.3/2 1/2V =1TOF (0)3/2 1/20F2&=1nm for H(v =2), respectively, to excite selectively the F;(i =1,2, 3) sublevels. The H(v =0) and H(v =2) peakswere superimposed upon a continuum due to direct ion-ization and ionization by light at the second-order wave-length. The latter, which results primarily in fast elec-trons, did not affect the present TOF spectra signif-icantly.The electron time-of-flight spectra obtained upon exci-tation of the H(v =0) and H(v =2) F~, Fq, and F3 com-ponents are shown in Figs. 2 and 3, respectively. Theywere roughly symmetrical with respect to the time offlight of the zero-energy electrons because of the contri-bution of both the forward and backward ejected elec-trons produced in the excitation region. One observesthat the lowest-energy peaks are split into their two spin-orbit components 0+ = 2 and 2 . They are associatedwith 0 + v 1 and 2, respectively. The variation in theH H branching ratio with excitation energy is1/2g: 3/2gdramatic and clearly indicates a strong correlation be-tween 0 and 0+, the projections of the total angularmomentum on the molecular axis.To interpret the TOF intensities, a direct measurementof the transmission function of the electron analyzer wasperformed by exciting Ar gas at the P1/2 threshold andabove. Over the energy range of interest, the transmis-sion function was given by the electron signal as a func-tion of electron energy, since the asymmetry parameter Pis known to be constant and close to 0, and the ionizationcross section is essentially wavelength independent [11 .The resulting transmission function is shown in Fig. 4.The intensity of the 02+ 0+ lines was then obtained bydividing the peak areas from the time-of-flight spectrumby the transmission at the electron energy correspondingto the peak maxima. The slight variation of the electronangular distribution over the resonance profile as mea-sured by Tonkyn, Winniczek, and White [3] does notsignificantly affect the relative peak intensities and wasA second method used to analyze the time-of-flightspectra was a Monte Carlo simulation, the details ofwhich will be published later. The coefTicients for the two676UJX,Vl(a) 3/2 1/20F, 0=20 50 100 150TIME OF FLIGHT CHANNEL NUMBERFIG. 2. TOF photoelectron spectra of 02 obtained upon exci-tation of the H X(0,0) band at (a) k =100.38 nm, bX=100.31 nm, and (c) X=100.26 nm. Histogram bars repre-sent the peak area divided by transmission at the correspondingphotoelectron energy (see Table I for values). Smooth curvesshaded by dots are the result of the simulation described in t etext. TOF(0) indicates the time of flight for zero-energy elec-trons.0+ components of each vibrational transition were ad-justed for the best fit with the experimental spectrum; theresults of such fits for the H(v =0) state are shown inFig. 2. The branching ratios obtained by both methodsare given in Table I. Because of the poorer resolutionand statistics, the fine structure of the highest-energyphotoelectron peaks was not analyzed, but one can seefrom the spectra that the branching ratios observed forthe lowest-energy photoelectron peaks are consistent withthe shapes of the higher-energy peaks.The good agreement between the two analyses of thespectra shows with no ambiguity the following correla-tions:2F1 +3/2 F2 +l/2+ +3/2 F3 + I/2 ~It should be stressed that the present measurement givesonly a lower limit for the selectivity for two reasons.First, pure F; states were not excited because of the finiteVO1 UME 67, NUMBER 6 PH YSICA'L R EV1 EW LETTERS 5 AUoUsT 19913/2 1/2 3/2 1/2 3/2 1/2V=2 ~ 1 ~ 0 n1000ENERGV (V).01 .02 .04 .06.08.1(.2 .3I I.4 .5 .6 .7 .8 .9 .10l 1 I I I ITOF (0)XQV)M&50XtX'.'l. 2 8 ne /c ha nn el3/2 1/2V =23/2 1/2 3/2 1/2g80 100 120 140 160 180 200UJV)OTOF (0)3/2 1/2V =2F2 Q=13/2 1/2 3/2 1/2o l1CHANNEL NUMBERFIG. 4. Transmission function for electron time-of-flightspectrometer.were incorrect, the evidence for a negative 2 is strong.However, they erroneously assigned the value 0 =0 toF~, the lowest-energy component of the triplet, and 0 =2to F3 which is inconsistent with a negative A value for aninverted state. The assignment we have used in our selec-tive excitation is the correct one for negative A (F~ is0 =2, Fq is 0 =1, and F3 is 0 =0). A negative valuefor A for a H„ is consistent with the presence of an elec-tron hole, i.e., (tr„) or (tr„) ' configuration, as 8 =200cm is positive for the ground-state ion with (trg )configuration.According to Kronig's selection rules the nonradiativehomogeneous transitions are the following:0 50 100 150TIME OF FLIGHT CHANNEL NUMBERFIG. 3. TOF photoelectron spectra of 02 obtained upon exci-tation of the H X(2,0) band at (a) K=98.31 nm, (b)=98.26 nm, and (c) A, =98.195 nm. Bars indicate peak area di-vided by transmission.width of the rotational envelope. Second, there is a con-tribution to the signal, albeit small, from the direct ion-ization process, which will populate both states with thesame probability.The assignment of the H H„ lines to a Rydberg seriesconverging to the a H„state of the ion proposed byNishitani et al. [12l can easily be obtained from con-sideration of the effective principal quantum number n*which is found equal to 1.94 for H(U =0), thus yieldingn =3 and a quantum defect 6 = 1.06 which correspondsfor first-row elements to a 3s Rydberg orbital. The H(0)state thus has a (tr„) (trs) (3sag) configuration. Beingthe lowest member of the Rydberg series and having alarge spin-orbit coupling, it is appropriate to describe itwithin the Hund's case a coupling scheme A = 1,=0, 1,2. Tonkyn, Winniczek, and White [3] have es-timated the spin-orbit constant from a fit of the photoion-ization spectrum using the known value A = —47.8 cmfor the a H„state as the initial guess. Their value2 = —60 cm ' is confirmed by calculation [131. Sincethe simulation would not fit the spectrum if the sign of ATABLE I. Hl/2. H3/2 branching ratio. In each entry, thefirst number given is the result of dividing the peak area by thetransmission, and the second number was determined by MonteCarlo simulation.TransitionH(O)- r+=1H(2) ~ ir =3Fi (n=2)0.22; 0.30.19; 0.24F, (n =1)1.35; 1.430.47; 0.45F, (n =O)4.3; 52.7; 2.5677aA =0, aS =0, an =0, aJ =0.The continuum states must be H, for a pure Coulombinteraction, so that the two accessible continuum wavefunctions are a„or 6„. The possible states that one canconstruct for (xs)(o„) and (trg)(B„) configurations areshown in Table II.%e see that the conservation rules lead to opposite con-clusions concerning the final spin-orbit component 0+which is populated by autoionization of a given sublevelF; of the H state according to the cr„or the B„characterof the continuum electron wave function.The correlation between 0 and 0+ observed experi-mentally [expression (1)l and the negative A for theH H, state signify that among two continua accessibleaccording to selection rules for electronic autoionizationthe H H„Rydberg series of the oxygen molecule decayalmost entirely into o.„. This in turn means that thebielectronic integrals of the type (tr„os(1/r~qiitrg ka„) areVOLUME 67, NUMBER 6 PH YSICAL REVIEW LETTERS 5 AUr. UST 1991TABLE II. Correlations between Rydberg and ion states.Configuration State (B,Z~ =(X+,cr+~(k, , o., ~ Ion state(n, )(~.)H2H]'H2&2, i/ =&i, —,' /&0, —,' /&—2, —I/=&—i, ——,'/&o, ——,'/&1,0I =(It~»[&I, l I&0, —I I+&I, —I I&0, l I]&—I,0 I = (I/~2) [& —i, ——,' 1 &0, —,' I+ & —i, —,' 1 &0, ——,' I]&o, i/=& —1, —,' /&0, —,' /&o, —i/=& i, ——,' /&0, ——,'/2H3l2H3g+ H[lg2H]l22H](2&—1,0I =(It'~2) {&I, l I& —2, I I+&I, l I& —2, —l I] '113t2+'11 t&1,0I =(It~2){&—I, l I&2, l I+& —I, l 1&2, —I I]Ho &0, 1( =& i, —,' /& —2, —,' /&0, —I/=&—1, ——,'/&2, ——,'/2H3(2much greater than &tr„crs] I/r ~2~trs k6'„). Two cautionarynotes should be added at this point. Since the analyzerused had a strong angular discrimination, the selectivityobserved could have been affected by this (i.e., if the o„continuum was favored at 90', and the 8, at 0'). Thereis no evidence for such an eff'ect, so it was not consideredhere. As well, although the F; assignments are certainfor the H(v =0) state, strong perturbations in theH(v =2) state make them slightly less sure [3]. Thiscould be why the selectivity observed was less strong inH(v =2), but the overall effect still persists for this state,as should the F; assignments.The present work illustrates how high-resolution time-of-Aight photoelectron spectroscopy combined with thecontinuous wavelength tuning provided by new synchro-tron radiation sources can bring new information on thecoupling of autoionized resonances with the ionizationcontinuum. In the case of 02 the experiment revealed astrong selection rule for the decay of the H H„Rydbergseries into the H~ ionization continuum which is illus-trated by the variation of the 0+( 2 . 2 ) branching ratioby more than an order of magnitude along the line pro-files. The method outlined here can be applied to othermolecular systems provided that the combination of jet-cooled molecules with high-resolution photon source al-lows selective excitation of isolated resonances. It will beof interest in future work to study the 0+( & . —', ) branch-ing ratio for higher members of the series when the cou-pling scheme appropriate to describe the Rydberg stateswill gradually move from a to e [14], and for higher vi-brational members of the same series because the cou-pling with dissociation continua, which is expected tovary strongly with U, may alter the pure H„character ofthe resonance.We are particularly thankful to P. Archirel for his helpin the description of the quantum states, and to H.Lefebvre-Brion for her calculation of the spin-orbit con-stant. The help of the technical staA' at Laboratoire pour6781'Utilisation du Rayonnement Electromagnetique is alsoacknowledged. J.W.H. acknowledges the support of theNatural Sciences and Engineering Research Council,through the International Exchange Programme, and theAlfred P. Sloan Foundation; NATO is thanked for agrant (No. 04 0784 86)."' Permanent address: Department of Chemistry, Universi-ty of Waterloo, Waterloo, Ontario, Canada N2L 3G1.[1] P. M. Dehmer and W. A. Chupka, J. Chem. Phys. 62,4525 (1975).[2] R. G. Tonkyn, J. W. Winniczek, and M. G. White, Chem.Phys. Lett. 164, 137 (1989).[3] R. G. Tonkyn, J. W. Winniczek, and M. G. White, J.Chem. Phys. 91, 6632 (1989).[4] K. Tanaka and I. Tanaka, J. Chem. Phys. 59, 5042(1973).[5] J. H. Eland, J. Chem. Phys. 72, 6015 (1980).[6] L. F. A. Ferreira, these d' etat, Universite Paris XI, Orsay,1984 (unpublished); P. M. Guyon and L. F. A. Ferreira,in Proceedings of the Workshop on Autoionization ofMolecules (ANL Report No. ANL-PHY-85-3, 1985), p.129.[7] A. L. Smith, Philos. Trans. Roy. Soc. London A 268, 169(1970).[8] D. M. P. Holland and J. B. West, Z. Phys. D 4, 367(1987).[9] M. R. Viard, O. Atabek, O. Dutuit, and P. M. Guyon, J.Chem. Phys. (to be published); M. R. Viard, these d' etat,Universite Paris XI, Orsay, 1987 (unpublished).[10] T. Bear, P. M. Guyon, I. Nenner, A. Tabouche-Fouhaille,R. Botter, L. F. A. Ferreira, and T. R. Govers, J. Chem.Phys. 70, 1585 (1979).[11]D. M. P. Holland, J. L. Dehmer, and J. B. West, Nucl.Instrum. Methods 195, 331 (1982).[12] E. Nishitani, I. Tanaka, T. Kato, and I. Koyano, J. Chem.Phys. 81, 3429 (1984).[13] H. Lefebvre-Brion (private communication).[14] H. Frolich, M. Glass-Maujean, and P. M. Guyon, J.Chem. Phys. 94, 1102 (1991).

Highly selective population of spin-orbit levels in electronic autoionization of O<sub>2</sub>

http://hdl.handle.net/21.11116/0000-000A-12E9-B

Highly selective population of spin-orbit levels in electronic autoionization of O<sub>2</sub>

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