AUTOIONIZATION ELECTRON SPECTRA RESULTING from COLLISIONS BETWEEN Li2 MOLECULES and CORE PHOTOIONIZED Li+ ATOMIC ION P

AUTOIONIZATION ELECTRON SPECTRA RESULTING FROM COLLISIONS BETWEEN Li2 MOLECULES AND CORE PHOTOIONIZED Li+ ATOMIC ION P. Gerard, D. Cubaynes, J. Bizau, F. Wuilleumier

To cite this version:

P. Gerard, D. Cubaynes, J. Bizau, F. Wuilleumier. AUTOIONIZATION ELECTRON SPEC- TRA RESULTING FROM COLLISIONS BETWEEN Li2 MOLECULES AND CORE PHOTOION- IZED Li+ ATOMIC ION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-719-C9-724. 10.1051/jphyscol:19879122. jpa-00227231

HAL Id: jpa-00227231 https://hal.archives-ouvertes.fr/jpa-00227231 Submitted on 1 Jan 1987

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AUTOIONIZATION ELECTRON SPECTRA RESULTING FROM COLLISIONS BETWEEN Li, MOLECULES AND CORE PHOTOIONIZED Li+ ATOMIC ION

P. GERARD, D. CUBAYNES, J.M. BIZAU and F.J. WUILLEUMIER

Laboratoire de Spectroscopic Atomique et Ionique and LURE, Universite Paris-Sud, Bdt. 350, F-91405 Orsay Cedex, France

RESUME

Les resultats obtenus dans une rscente @tude de la photoionisation du lithium atomique et moleculaire sont prssentes. Une nouvelle interpretation est proposse.

ABSTRACT

The results obtained in a recent photoemission studies of atomic and molecular ?i vapor, using electron spectrometry, are summarized. Suggestions for the explana- tion of the experimental observations are presented.

I. INTRODUCTION In earlier studies of photoionization in Li vapory1, using electron spectrometry, additional lines that could not be attributed to the photoionization of atoms, were observed. The kinettc energy of this group of electrons was found to be constant v~hanthe photon energy was varied between 75 eV and 150 eV. These kinetic energies were measured, at that time, to be between 48 eV and 50 eV. Their intensities were comparable to the intensity of the atomic photoelectron lines, as It can be seen in figure 1 of reference 1. Two main reasons lead to the conclusion that these additio- !?a7 lines should be attributed to Auger transitions in a lithium molecule: i) the corlstant value of their kinetic energy as a function of photon energy; ii) the fact rhat Auger transitions are not possible after ionization of Li-atoms in the 1s sub- she1 1. In addition, in the energy range measured for these electrons, previous experiments, involving electron impact 2 or ion impact 3 ,had not revealed the existence of electron lines at energies lower than 50 eV. Only electrons emitted in the autoionization of molecular excited states had been observed ?n these experiments, above 53 eV. The fact that no lines had been observed below 50 eV was attributed to the low values of ionization crcss section by electron impact.

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However, the attribution of these lines to molecular Auger decay left several questions open: First, the average for the lso- and lso; binding energies in Liz had 2 9 been estimated to be between 61.73 eV and 62.5 ev4. None of these values were com- patible with the spectra observed at that time: assuming a pure Coulombic potential curve4 for Lizu , the estimated values for the molecular Is binding energies were in the range from 65 eV to 66.5 eV, i .e. in the same binding energy region as the 3 IS-electron binding energies in atomic lithium (64.41 eV for the ls2s S ionic state, 66.31 eV for the ls2s 1S state, according to Ref.5). These values were far from the theoretical estimates. Second, at the temperature used in the photoemission experiments, the contribution of dimers to the vapor pressure was supposed to be in the order of 1xS6The intensity of the observed molecular structure would have required molecular photoionitation cross sections considerably greater than the atomic cross sections. No reason was found for a possible concentration of Liz dimer higher in the source volume of the electron analyzer than in the oven.

I I. EXPERIMENT Under better experimental conditions, we repeated and we extended the photoelectron spectrometry experiments on Li, using basically the same experimental set up, i .e. a furnace mounted on the axis of a cylindrical mirror electron analyzer (CMA). However, higher photon flux were available in these new experiments, because of the routine use of new toroidal grating monochromator. Thus, data could be obtained at better resolution and/or at lower atomic densities. In particular, we were able to make study of the intensity variation of the electron lines as a function of several parameters such as the temperature in the lithium furnace or the photon energy near threshold and in the vicinity of atomic resonance lines. 111. RESULTS

Fig.l- Electron spectrum following ionization of a Li va- 2 por by 91.50 eV. Peak noted 0 (1) corresponds to photoioni- Q) I zation of the 2s-electrons in U) I Li atoms (Li+ ls2 IS final \ 800- state). Peaks noted 2,3,4 and V) 5 corresponds to photoioniza- -C 1 tion of the Is-electrons in Li a atoms, the ion being left in 3 A 0 ls2s 3s (peak 2), ls2s (3), ls2p 3~ (4) and ls3s and ls3p 0) states, respectively. Peaks noted A and a are of molecular origin. (From Ref. 7) .

200 400 600 channel number The results obtained in this new series of experiments are summarized in Figu- res 1 to 4. Fig.1 shows a complete electron spectrum of Li vapor taken at 91.50 eV photon energy. Peaks noted 1, 2, 3, 4, and 5 are electron lines due to photoionization of Li atoms in the outer- and inner-shells, as explained in the caption. Peaks noted A and a are the additional lines previously attributed to Auger-decay of core- ionized Li molecules. As in the previous experiments ,' the kinetic energy of these lines has been found to be constant when the photon energy was varied over an even wider energy range, including the Is-atomic ionization thresholds around 64 eV. However, the absolute values measured for the kinetic energies of the lines were different.7 The main structures in the electron spectrum were observed at 51.56 (6) eV, 52.8(1) and 53.51 eV, about 3 eV higher than in the earlier measurements.' A reason for the discrepancy between the two sets of values was found in the calibration of the absolute kinetic energy scale. Our values are in excellent agreement with the energies of the molecular autoionizing electrons observed by electron impact ( 51.5, 51.6, 52.9 and 53.9 eV). 2 Because of the higher photon flux available, we were able to work at a much better resolution of the monochromator (0.3 eV compared to 0.9 eV) and to scan the photon energy range in which lines A and a appear. Fig.2 presents spectra obtained for these 1ines in their threshold region, around 64 eV photon energy. Below 64 eV, they could not be detected. When one increases the photon energy by small steps, they appear suddenly with a large intensity, fl'rst the A1 lines ( 51.56 eV kinetic energy), and second, line A2 (52.8 eV) and a, above 66 eV. The excitation function we obtained for their intensity between 64 eV and 68 eV photon energy is shown in Fig.3. The atomic-ionization thresholds are marked in the Figure. The two arrows on the top of the figure indicate the position of some resonant atomic lines involving

KINETIC ENERGY (eV)

hr - 61.22 *) nv - s.n *r Fig.2- Electron spectra showing the photon-energy dependence of the A-lines for various photon energies in the region of the Is- atomic ionization thresholds . From Ief t to Eight and from top to bottom, the photon energies are: 64.22eVY 64.33 eV, 66.15 hv - 66.15 N eV and 66.34 eV, respecti? vely . C9-722 JOURNAL DE PHYSIQUE

Fig.3- Excitation function of the A-electron lines as a function of photon energy. See text for detailed comments.

PHOTON ENERGY in eV 2 2 12 -two-electron core.-excited states: 1s 2s S -+(ls2p P)3s P transition at 65.29 eV, 2 2 12 1s 2s S 4 (ls2p P)4s P transition at 66.44 eV. From this figure, one can deter- mine a threshold energy of 64.4(2) eV for the A1 line, which coincides , within the error bars, with the first 1s-atomic ionization thresh01 d. Finally, in Fig.4, we. present some qua1 itative results, showing the temperature dependence of the A-lines. The three spectra, on the right part, are the atomic inner-she1 1 photoionization 1ines, measured at three different temperatures in the oven. Their intensity has been normalized to the same arbitrary units. The three spectra on the left part of the figure are the A-lines taken at the same temperatures. The relative variation of the intensity of these A-lines differs greatly from the intensity of the group of atomic lines, which suggests that their origin is not due to molecular photoioni zation .

IV. INTERPRETATION The new sets of results presented above lead to a reinterpretation of the Li data. A sumnary of the. experimental observations can be made as follows. The observed lines are not Auger lines, because their actual kinetic energy is definitely too high, and because no enhancement was found in the molecular photoionization cross section recently cal~ulated~'~which would have helped to explain the intensities measured with such a low relative abundance of Lip molecule (1 to 2%). The position of the lines is now consistent with them being assigned to molecular autoionization 3 2 lines associated with the Liz (1r IT n+ ) neutral excited species Autoioniza- 9 .'" tion lines corresponding to n@ = ll~,,lo;, 3u and l-rr have been observed at the 9 9 same kinetic energies under electron impact. (Ref. 2) Following the analysis of Larkins and ~ichards,~we come to the conclusion that the lines A, observed in our experiment between 50 eV and 53 eV kinetic energy, are the consequence of ion-molecule collision processes. Among. several possible mecha- nisms suggested to account for the observation^,^ we think that the following se- quence of processes explains reasonably we1 1 the experimental results, in particular the fact that the threshold for the A-lines is equal to the atomic inner-shell threshold. 1. Atomic photoionization: ~i(ls22s) + hJ -+ Li'(lsn1) + e- * 2. on-mlecule collision: ~i+(lml)+ ~i~(1r~2~~)+ ~i+(ls~)+ ~i~(l22%~n+ 32 4 3. Molecular autoionization: Liz* (lo- 2v nf ) -4 Li; (lo- 20- ) + e- 9 9 Since the molecular structure has been found to resonate at some photon energies corresponding to the excitation energies of two-electron atomic transitions to lsnln'l ' excited states of atomic lithium, collision process 2 could also involve 9 core-exci ted atomic 1i thi um. A qualitative analysis of this phenomenon would require accurate measurements of the teaErature in the interaction volume. However, when one takes into account the relativs abundance of Li+ ions produced by photoionization to the relative abundance of Li, molecule (I to 2%) and the density of atomic Li in the 3 KINETIC EN~RGY(e~) beam (1312 to 1013 atomslcm ), one ;en ends up with cross section for process 2 that should have values in the boa 10-15to cm2 range to explain the data. This does not seem to be un- zrr reasonable, since it is established that ion-atom cross sections for inner-shell excitation and ionization can be greater than photodonization ,, cross sections by several orders of 10,ll W 2w magnitude.

158 a 2w 1- Fig.4- Electron spectra following sr ionization of a Li vapor at 81 eV photon energy, for three different temperatures in the oven. The left colunm shows the intensity of the A and a electron lines. The right 2% column presents the intensity of the atomic photoionization lines to which all spectra have been normalized. The spectra are not corrected I for the energy transmission of the I I' tw CMA.

I: I I ,',I I so I I C%.&,c:'i" 't' '111t%,,,

BINDING ENERGY (eV) (39-724 JOURNAL DE PHYSIQUE

One should note that tl.lis interpretation of the spectrun~does not change much the values of the partial photoionization cross sections that have been measured for atomic lithi~m.~In the analysis of the data, only the spectra have been kept that shows a relative intensity for the intensity of the A-lines lower than lo%, in com- parison to the intensity of the atomic lines. As for the Auger lines, their intensity under our experimental conditions,is probably to weak to allow their detection.

V. ACKOWLEDGEMENTS The authors are extremely grateful to F. Larkins for helpful discussion and suggestions in the interpretation of the data.

1. S. Krummacher, V. Schmidt, J.M. Bizau, D.L. Ederer, P. Dhez and F.J. Wuilleumier, J. Phys. B -15,4363(1982). 2. W.H.E. Schwarz, W. Butscher, D.L. Ederer, T.B. Lucatorto, B. Ziegenbein, W. Meh- lhorn and H. Prompeler, J. Phys. B -11,591(1978;. 3. P. liem, R. Bruch and N. Stolterfoht, J. Phys. B 8, L480(1975). 4. W.H.E. Schwarz and T.C. Chang, Int. J. Quantum Chem. =,91(1976). 5. C. Moore, Table of Energy Levels, National Bureau of Standards, Washington,DC, 1971. 6. A.N. Nesmeyanov, in:"Vapor Pressure of the Chemical Elements", Elsevier, Amster- dam, 1963, p.122. 7 .P .Gerard,Thesis 3Sme Cycle, University Paris-Sud, Orsay, 1984 (unpublished). 8. G.B. Backsay, G. Bryant and N.S. Hush, 1nt.J. Quant. Chem.,1987. 9. F.P. Larkins and J.A. Richards, Aust. J. Phys. -39,1(1987). 10. U. Fano and W. Lichten, Phys. Rev. Lett. %,627(1965). 11. F.P. Larkins, J. Phys. B -5,571(1972).