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The benzene dication: Energies of excited states determined by double-charge-transfer spectroscopy using OH+ and F+ projectile ions
Chemical Physics I57 ( 1991) 299-304 North-Holland
The benzene dication: energies of excited states determined by double-charge-transfer spectroscopy using OH+ and F+ projectile ions W.J. Griffrths
’ and F.M. Harris ’
Mass Specirometry Research Unit, University College of Swansea. Singleton Park, Swansea, SA2 SPP. UK Received 4 March 199 1
Double-charge-transfer spectroscopy has been used to measure double-ionization energies of C6H6 to electronically excited states of the dication. Since OH+ and F+ projectile ions were used, it is likely, because of the Wigner spin-conservation rule, that triplet states of the dication were populated. Previously computed energies of certain groupings of triplet states of C,H:+ are in good agreement with the measured double ionization energies of 24.6+0.5,27.3 t0.5,29.8~0.5,31.1 f0.5,32.7f0.8,34.5?0.8 and 37.2 ?I 1.OeV. Because of the high density of singlet and triplet states, it is only certain that the spin-conservation rule applies to reactions in the present investigation which populate the states with the two lowest energies. Comparison with the calculated data suggests that 24.6 eV is the energy of the ‘A, state whilst 27.3 eV corresponds to the group of states ‘E,,, ‘B,r, ‘El, and 3B21 which have been calculated to be separated by no more than 0.34 eV.
1. Introduction In a theoretical study of benzene, Tarantelli et al. [ 1 ] applied the two-particle Green’s function method
and the second-order algebraic diagrammatic construction formalism (ADC (2 ) ) to compute doubleionization energies to ground and excited states of the dication with the aim of interpreting the Auger spectrum [ 2,3] of the molecule. Although only eight distinct peaks (the lowest at a double-ionization energy of 26.1 eV and the highest at 42.7 eV) were evident in the spectrum, the calculations showed that in the range 23-40 eV as many as 226 doubly ionized states of benzene exist. Seventy-seven computed states (4 1 singlets and 36 triplets) were classified as “main” states, having a two-hole contribution larger than 50%, and these, therefore, are responsible for the large part of the Auger-spectrum intensity. Because of the larger number of states, Tarantelli et al. [ 1 ] discussed the spectrum in terms of groupings of the states. Two Gaussian fits, one of the computed sin’ Present address: Department of Physics, Uppsala University, S-75 1 2 1 Uppsala, Sweden. 2 Author to whom correspondence should be addressed.
glet states and one of the computed triplet states, were illustrated diagrammatically. The first theoretical maximum of the singlet distribution occurs at a double-ionization energy of 24.3 eV and the first triplet maximum at 23.3 eV. On the basis of previous evidence [ 41, it is to be expected that the lowest triplet states have very little Auger intensity. The lowest calculated singlet band is 1.8 eV lower than the first peak in the Auger spectrum with reflects the extent of the unbalanced account of correlation energy in the ground and dicationic states given by the second-order ADC scheme. Thus, the calculated peak energies were corrected by adding 1.8 eV to them in order to allow for this effect. The Gaussian fit for the singlet states reveals seven peaks, the spacings of which are in good agreement with those measured for the first seven Auger peaks. The first computed singlet peak is a combination of two states, the ‘EZgand ‘Ai, having calculated energies of 23.96 eV and 24.59 eV, respectively. The first triplet peak, on the other hand, is due to one state ( 3A2g)only, and is at 23.34 eV. In the present investigation, an attempt has been made to measure the double-ionization energy to this triplet state and also to others at higher energies. Double-charge-transfer
030 1-O104/9 I/% 03.50 0 199 I Elsevier Science Publishers B.V. All rights reserved.
300
WJ. Gri&ths, FM Harris I The benzene dilation
(DCT) spectroscopy [ 5 ] has been used which is a technique in which fast-moving singly charged positive ions undergo double-electron-capture (DEC) reactions with the molecules under investigation. Such reactions are endoergic, and the energy to drive them comes from the translational energy of the projectile ion. By measuring the translational-energy spectrum of the negative ions formed, and determining the energy losses corresponding to the peaks in it, double-ionization energies to ground and excited dicationic states can be evaluated. The choice of projectile ion is important for two reasons. ( 1) A consequence of the Wigner spin-conservation rule [ 61 (total spin angular momentum must not change in the course of the collision) is that in DCT spectroscopy triplet or singlet states of the dications are populated depending on which projectile ion is employed. In most of the DCT spectroscopy studies carried out by Fournier and his colleagues [ $71, H+ has been used as the projectile ion. Both H+ and Hhave an electronic spin equal to zero. By conservation of spin, therefore, the doubly ionized ion will have the same spin state as the neutral molecule. Most of the DCT spectroscopy studies carried out at Swansea have been with the OH+ projectile ion. Since OH+ has a triplet ground state (%- ), and the resulting OH- ion generated in a DCT spectroscopy experiment is in a singlet state ( ‘C+ ), it follows from the spin-conservation rule that collisions with molecules which are in singlet states should populate triplet states of the dication. This has been shown to be the case for CO*, OCS and CS2 [ 8,9 1, NH, [ 10,111, CCL, [ 12 ] and N20 [ 13 1. In the present DCT spectroscopy study of benzene, therefore, OH+ has been used as one of the projectile ions in the hope that, even for such a large molecule, the spin-conservation rule will apply, and the double-ionization energies to dicationic triplet states can be measured. (2) On the basis of theoretical [ 14,lS ] and experimental [ 16,17 ] studies, it has been established that a reaction window exists for DEC reactions, i.e. that the endoergicity of the reaction must sit within a window of values and, in fact, maximizes within that window. An important consequence of this is that the use of projectile positive ions which give a high release of energy when they charge invert in a DEC reaction tends to result in the population of higher-lying dicationic states. Such an ion is F+ because of the
high values of single-ionization energy and electron affinity of F. Thus, F+ has been used in previous studies to investigate excited states of COZ, OCS and CSz [9], NH3 [ll], N20 [13J, CH3Br [17] and CHsI [ 18 ] which are higher in energy than the states investigated using OH+. It has also been used in the present study with the same purpose in mind.
2. Ex~rimen~ If the projectile ions OH+ and F+ are denoted by A+, the DEC reactions of interest in the present investigation can be represented by A’+C6H6+A-
+C,Hif+ .
(11
In practice, A+ has a translational energy of 6 keV and the DEC reactions are thus sufficiently rapid to ensure vertical double ionization of C6H6. The ion A- can also be generated in sequential reactions involving two C6H6 molecules, i.e. A+ +C6H6-+A+C6H$
,
(2)
A+C6H6-+A-+CgHgf .
(3)
Peaks in the A- translational-energy spectrum due to reaction ( 1) increase in height linearly with C6H6 pressure whilst those due to reactions (2) and (3) show a quadratic dependency. Another difference is that, in general, their positions on the A- translational-energy scale are different. If the translational energy of A+ is denoted by E,, and that of A-, when C,H$+ is formed in its ground state in reaction ( I), by E,, then E,,-E,
=IE,(C,H,)-E(A++A-)
,
(4)
where IE2 ( C6H6) is the double-ionization energy of C,H, to the ground dicationic state, and E( A++A- ) is the energy released due to charge inversion of A+. If an excited state rather than the ground state of C,Ha+ is populated in reaction ( 1 ), the A- ions have a lower translational energy E’, which is related to the double-ionization energy IE; ( ChHC) to that state through eq. (4) in which E, and IEz(CaHs) are replaced by E; and IE; ( C6Hs ) , respectively. The translational energy, E,, of the A- ions formed by reactions (2 ) and (3) depends on the single-ion-
W J. Grifiths, FM
301
Harris / The benzene dication
ization energies IE,,(C6H6) and IE,,(C,H,) to the states x and y of C6H,f populated in the reactions. Thus,
lowing through to the detector in turn negative ions having different translational energies.
& -& =IE,,(W%)
3. Results
+IE,y(G&)
-E(A+-+A-).
(5)
In order to calibrate the translational-energy-loss scale, a DCT spectrum was obtained with xenon atoms for which the relevant single- and double-ionization energies are known [ 191. For the single-coliision reaction A++Xe+A-+Xe’+
A typical calibration spectrum obtained with the OH’ projectile ion is shown in fig. 1a. By varying the 100
(a)
(6)
the translational energy of A- is E,,, and E,-E,,=IE,(Xe)-E(A+~A-).
(7)
5C
By subtracting eq. (7) from eq. (4) and from eq. ( 5 ) the equations L -E, =IE,(C6H6)-IE,(Xe)
(8)
and &, -E;!=IE,,(CaH6)+IEI,(Xe)-IE,(Xe)
(9)
are obtained from which the double-ionization energy and sum of the since-ioni~tion energies of C,H, are readily determined once E,,, E, and E2 have been measured. The relevant translational energies were measured using a ZAB-2F mass spectrometer #’ operated with the source at a constant voltage, nominally 6 kV. This voltage need not be known, but it governs the E0 in eqs. (4) and (5 ) and was maintained at one value in order to justify the use of eqs. (8) and (9). The OH+ and F+ ions were generated by dissociative ionization of Hz0 and CF4, respectively, and, after acceleration and mass selection, the projectile ions interacted with C,H, molecules contained in the collisiongas cell of the second field-free region of the spectrometer. Negative ions were transmitted to the detector by reversing the polarities of the voltages normally applied to the plates of the electric sector. DCT spectra were obtained by scanning these voltages al” Manufactured by VG Analytical Limited, Manchester, Gt. Brittain.
C
5970
I
I
5960
Translational
I
6000
5990
energy
(eV)
Fig. 1. Negative-ion spectra resulting from doubie-electron-capture reactions of 6 keV OH’ ions with (a) xenon atoms and (bf benzene molecules.
302
W.J. Griffiths, FM. Harris /The benzene dication
xenon-gas pressure, it was established that peaks i and ii are due to OH- ions formed in the sequential single-electron-capture reactions whilst I and II are due to OH- ions formed by DEC reactions in single collisions. These peaks are interpreted in the following way. For peak i, the sequential reactions result in both Xe+ ions being in the 2Ps,2 state, but for peak ii one of the Xe+ ions is in the 2PJ,2 state, the other in the 2P ,,2. For peak I, the Xe2+ ion is formed in the ‘P2 state, and for II it is in the ‘P,-, and/or the ‘P, state which are separated by 0.2 12 eV. All these single- and double-ionization energies are known [ 191, and the relative spacings of the peaks confirm the above interpretation. Thus, the translational-energy positions of peak I or II can be used as E,, together with the corresponding IE2 (Xe) in eqs. (8 ) and (9 ). It is known [ 201 that beams of OH+ ions formed following dissociative electron ionization of Hz0 contain small components which are in long-lived (lifetimes greater than several ps) electronically excited states. These are the a ‘A and b ‘E+ at 2.15 eV and 3.55 eV [ 2 11, respectively, above the 3E- ground state. Reactions of ions in these excited states could lead to minor peaks in the spectra. Since for such ions E(A+ +A-) is larger than for reactions of groundstate OH+ ions, the energy loss Eo- E2 (eq. (5 ) ) would be smaller, i.e. minor peaks would be observed to the right of peak i in fig. la if reactions of the excited-state ions were significant. Since no such peaks are observed, the conclusion can be drawn that the concentrations of excited-state ions in the OH+ beam are sufficiently low as not to affect the spectra of the present investigation. A typical spectrum obtained when OH+ projectile ions interacted with C6Hb is shown in fig. lb. From the pressure dependencies of the heights of the peaks it was shown that peaks a and b were due to the sequential reactions ( 2 ) and ( 3 ) whilst A, B and C were due to the formation of OH- ions by reaction ( 1). The translational-energy positions of the maxima of A, B and C correspond to E, and E; values which, when inserted in eq. (8), give double-ionization energies for C6H, of 24.6 rt 0.5 eV, 21.3 & 0.5 eV and 29.8 f 0.5 eV. Typical spectra obtained when using F+ as the projectile ion are shown in fig. 2. The spectrum (a) obtained with Xe showed no peaks due to the sequential single-electron-capture reactions. Peaks I and II are analogous to I and II of fig. 1a and are due to Xe2+
(b)
‘L
o-l- . 59’70
'
59'80
Translational
’
59’90
energy
r
so'00
(ev)
Fig. 2. Negative-ion spectra resulting from doubleelectron-capture reactions of 6 keV F+ ions with (a) xenon atoms and (b) benzene molecules.
being formed in the 3P2 and the 3P0 (and/or ‘P,) states, respectively. To check the possibility that longlived excited-state components in the F+ ion beam are interacting with Xe is more difficult in the absence of the sequential, single-electron-capture reactions. Peak I is due to F+ (3P) reacting with Xe to populate the 3P2 state of Xe2+ having an energy of 33.327 eV [ 191. The next state of F+ is the ‘D2. It has been shown [ 221 that ions in this state are pres-
W.J. Griffhs, FM. Harris / The benzene dication
ent in a beam formed following dissociative electron . ionization of CF,. Because of spin conservation, the lowest-energy DEC reaction of F+ ( ‘D2) will populate the ID2 state of Xe2+ at 35.447 eV. Since the separation between the ground and ID2 states of F+ is 2.59 eV [ 191 the lowest-energy DEC reaction of F+ ( ‘D2) with Xe would give a peak 0.47 eV to the right of peak I in fig. 2a. No such peak is evident but it could rightly be argued that the resolving power of the spectrometer used in the present investigation is not high enough to reveal it. However, if the ‘D2 ions in the F+ beam were a significant constituent a shoulder would be observed on the right-hand side of peak I. Its absence suggests that F+ ( ‘D2) ions are not sufficiently numerous to affect the positions of the peaks in the spectra recorded in this investigation. The heights of the peaks D, E, F and G in fig. 2b all increased approximately linearly with C6H6 pressure and were due to F- ions formed by reaction ( 1) . Peak G is just discernible but is real since it was found to be present in all the spectra recorded. From the positions of their maxima, the double-ionization energies calculated using eq. (8) are 3 1. 1 + 0.5 eV, 32.7f0.8 eV, 34.5kO.8 eV and 37.2fl.O eV. All these energies are higher than those determined from the 0H+/C6H6 spectra and this work confirms the value of using F+ to investigate higher-lying dicationic states.
4. Discussion The lowest double-ionization energy measured in the present investigation is 24.6 eV k 0.5 eV. This is significantly lower than 26. I f 0.8 eV, the lowest energy determined from an Auger spectrum [ 31. It is likely that the difference is due to the populating of a triplet state (or states) in the DCT spectroscopy experiment using OH+ as a projectile ion and the populating of a singlet state (or states) in the Auger spectroscopy experiment. In their constructed spectra, Tarantelli et al. [ 1] assume that the first triplet peak is due to one state only, the 3A2, at 23.34 eV whilst the first singlet peak has contributions from the ‘Elg and ‘A,, states at a mean energy of 24.3 eV. Thus, from their theoretical study, Tarantelli et al. [ 11 predict that the first triplet peak and first singlet peak are separated by about 1 eV, a value which is in agreement (within experimental uncertainty) with
303
that determined from the Auger and present DCT spectra. Vertical double-ionization energies calculated recently by Krogh-Jespersen [24] are 23.83 eV to the ground triplet state and 24.97 eV to the ground singlet state i.e. a difference of 1.14 eV which agrees with that of the previous calculations [ 1 ] and with that determined from the present and previous [ 31 measurements. Further evidence that a ground triplet state is populated in the present investigation is obtained from a previous DCT spectroscopy experiment [23] in which H+ was used as a projectile ion. As mentioned earlier, the use of H+ should result in the populating of singlet states and it is interesting that the lowest double-ionization energy determined by Appell et al. [ 23 ] is 26.0 + 0.5 eV i.e. in agreement with the Auger result of 26.1 eV, but significantly higher than the lowest double-ionization energy determined in the present investigation. The double-ionization energy of C6H6 determined in an electron-impact ionization investigation [ 25 ] is 26.02 0.2 eV which suggests, in the light of the above information, that the lowest state accessed in such an experiment is the ground singlet state. The experimental procedure requires, however, that the dication is stable for at least several p in order to be detected following formation in the source and passage through a magnet. Such a requirement is not necessary in Auger spectroscopy nor in DCT spectroscopy, and thus the results from them may not be directly comparable with that from an appearance energy measurement in an electron-impact ionization experiment. It would appear from the above facts that spin is conserved in the DEC reaction of OH+ with C6H, which leads to the populating of the lowest-energy dicationic state. To see if this is the case for higher-lying states, the present data are compared with the energies of the peaks deduced from the groupings of calculated triplet states [ 11. These groupings were presented graphically in the previous publication [ 1 ] but the energies of the peak maxima have been made available [ 26 ] to the present authors. They are listed, together with the present results, in table 1. It can be seen that the theoretical data (normalized as described in the footnote to the table) agree well with the experimental data except that a peak at 3 1.1 eV is not predicted. It is possible, however, that the energies of two of the computed states when averaged
W.J. Griflths. FM. Harris / The benzene dication
304
Table I Double-ionization energies from the present investigation and the corresponding energies of peaks in a simulated spectrum of groupings of calculated triplet states [ 1,251 Double-ionization
energies
(eV)
Experimental (DCT spectroscopy using OH+ and F+ projectile ions p)
Theoretical
Peak Peak Peak Peak Peak Peak Peak
24.6 27.6 30.1
A B C D E F G
24.6f0.5 27.320.5 29.8kO.5 31.1kO.5 32.7fO.S 34.5kO.8 37.2f I.0
b,
32.4 34.9 37.6
’ Peaks A, B and C were observed with the OH+ projectile ion and peaks D, E, F and G with the F+ projectile ion. ’ Data obtained by the addition of 1.3 eV to the calculated values in order to bring the lowest calculated and experimental energies into agreement.
could give rise to peak D. These are the 3B,U and 3E2U having energies of 29.70 and 30.02. Adding 1.3 eV to the average value of 29.86 eV gives 31.16 eV which is in good agreement with the value determined from the position of peak D. It is thus tempting to conclude that spin conservation is valid for all reactions in the energy range investigated. However, from the H+ DCT spectrum [ 23 ] double-ionization energies of 30.3kO.5 eV and 33.lkO.5 eV were obtained which agree, within experimental uncertainties, with the values 29.8 k 0.5 and 32.7 + 0.8 eV, respectively, of the present investigation. Further, Tarantelli et al. [ 1 ] deduced that the “best” compiled Auger spectrum from theoretical data included a mixture of singlet and triplet states with the triplets being given a low weighting of 0.3. The present spectra may also be due to a combination of triplet- and singlet-state energies with perhaps a higher weight being given to the triplets states. The first two peaks, however, which give double-ionization energies of 24.6 and 27.3 eV, seems to be mainly triplet. The nearest Auger peaks are at 26.1 and 28.2 eV and the nearest H+ DCT peaks are at 26.0 and 30.3 eV. Thus, the first peak in the present work is probably due to population of the 3A28 state whilst the second is probably due to the population of one or more of the states 3EiU, ‘B,,, 3E,, and 3B2, which are calculated [ 1 ] to be separated by no more than 0.34 eV.
References [ I ] F. Tarantelli,
A. Sgamellotti, L.S. Cederbaum and J. Schirmer, J. Chem. Phys. 86 ( 1987) 220 I. [2] K. Sieghbahn, C. Nordling, G. Johansson, J. Hedman, P.F. Heden, K. Hamrin, U. Gel&, T. Bergmark, L.O. Werme, R. Manne and Y. Baer, ESCA Applied to Free Molecules (North-Holland, Amsterdam, 1969). [3] R. Spohr, T. Bergmark, N. Magnusson, L.O. Werme, C. Nordling and K. Sieghbahn, Physica Scripta 2 ( 1970) 3 I. [4] H. &ren, J. Chem. Phys. 75 (1981) 1267. [ 5 ] J. Appell, Collision Spectroscopy, ed. R.G. Cooks (Ebevier, Amsterdam, 1978) p. 244. [6] E. Wigner, Nachr. Ges. Wiss. Goettingen Math. Phys. Kl 2A (1927) 325; H.S.W. Massey, Rept. Progr. Phys. 12 (1949) 248. [7] P.G. Foumier, J. Foumier, F. Salama, S. Stark, S.D. Peyerimhoff and J.H.D. Eland, Phys. Rev. A 34 ( 1986) 1657, and references therein. [ 81 W.J. Grifliths and F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 87 ( 1989) 349. [9] M.L. Langford, F.M. Harris, C.J. Reid, J.A. Ballantine and D.E. Parry, Chem. Phys. I49 ( 199 1) 445. [IO] W.J. Griffiths and F.M. Harris, Rapid Commun. Mass Spectrom. 4 ( 1990) 366. [ 111 M.L. Langford, F.M. Harris, P.G. Fournier and J. Fournier, J. Chem. Phys., submitted. [ 121 P.G. Foumier, G. Comtet, J. Foumier, S. Svensson, L. Karlsson, M.P. Keane and A. Naves de Brito, Phys. Rev. A 40 (1989) 163. [ 13 ] F.M. Harris, C.J. Reid, J.A. Ballantine and D.E. Parry, J. Chem. Sot. Faraday Trans. 87 ( 199 1) 168 I. [ 141 D. Mathur, Intern. J. Mass Spectrom. Ion Processes 83 (1988) 203. [ 15 ] D.P. Almeida and M.L. Langford, Intern. J. Mass Spectrom. Ion Processes 96 (1990) 331. [ 161 W.J. Griffiths, D. Mathur and F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 87 (1989) RI. [ 171 M.L. Langford and F.M. Harris, Rapid Commun. Mass Spectrom. 4 ( 1990) 125. [ 18 ] W.J. GrifIiths, F.M. Harris and D.E. Parry, J. Chem. Sot. Faraday Trans. 86 ( 1990) 280 1. [ 191 C. Moore, Atomic Energy Levels (Natl. Bur. Stand,, Washington, DC, 1949). [20] M. Hamdan, S. Mazumdar, V.R. Marthe, C. Badrinathan, A.G. Brenton and D. Mathur, J. Phys B 21 (1988) 2571. [21] A.J. Merer, D.N. Malm, R.W. Martin, M. Horani and J. Rostas, Can. J. Phys. 53 (1975) 25 I. [22] C.S. Enos and A.G. Brenton, private communication (1990). [ 231 J. Appell, J. Durup, EC. Fehsenfeld and P. Foumier, J. Phys. B7 (1974)406. [24] K. Krogh-Jespersen, J. Am. Chem. Sot. 113 ( 1991) 417. [25] F.H. Dorman and J.D. Morrison, J. Chem. Phys. 35 (1961) 575. 1261 F. Tarantelli, private communication (1990).
The benzene dication: energies of excited states determined by double-charge-transfer spectroscopy using OH+ and F+ projectile ions W.J. Griffrths
’ and F.M. Harris ’
Mass Specirometry Research Unit, University College of Swansea. Singleton Park, Swansea, SA2 SPP. UK Received 4 March 199 1
Double-charge-transfer spectroscopy has been used to measure double-ionization energies of C6H6 to electronically excited states of the dication. Since OH+ and F+ projectile ions were used, it is likely, because of the Wigner spin-conservation rule, that triplet states of the dication were populated. Previously computed energies of certain groupings of triplet states of C,H:+ are in good agreement with the measured double ionization energies of 24.6+0.5,27.3 t0.5,29.8~0.5,31.1 f0.5,32.7f0.8,34.5?0.8 and 37.2 ?I 1.OeV. Because of the high density of singlet and triplet states, it is only certain that the spin-conservation rule applies to reactions in the present investigation which populate the states with the two lowest energies. Comparison with the calculated data suggests that 24.6 eV is the energy of the ‘A, state whilst 27.3 eV corresponds to the group of states ‘E,,, ‘B,r, ‘El, and 3B21 which have been calculated to be separated by no more than 0.34 eV.
1. Introduction In a theoretical study of benzene, Tarantelli et al. [ 1 ] applied the two-particle Green’s function method
and the second-order algebraic diagrammatic construction formalism (ADC (2 ) ) to compute doubleionization energies to ground and excited states of the dication with the aim of interpreting the Auger spectrum [ 2,3] of the molecule. Although only eight distinct peaks (the lowest at a double-ionization energy of 26.1 eV and the highest at 42.7 eV) were evident in the spectrum, the calculations showed that in the range 23-40 eV as many as 226 doubly ionized states of benzene exist. Seventy-seven computed states (4 1 singlets and 36 triplets) were classified as “main” states, having a two-hole contribution larger than 50%, and these, therefore, are responsible for the large part of the Auger-spectrum intensity. Because of the larger number of states, Tarantelli et al. [ 1 ] discussed the spectrum in terms of groupings of the states. Two Gaussian fits, one of the computed sin’ Present address: Department of Physics, Uppsala University, S-75 1 2 1 Uppsala, Sweden. 2 Author to whom correspondence should be addressed.
glet states and one of the computed triplet states, were illustrated diagrammatically. The first theoretical maximum of the singlet distribution occurs at a double-ionization energy of 24.3 eV and the first triplet maximum at 23.3 eV. On the basis of previous evidence [ 41, it is to be expected that the lowest triplet states have very little Auger intensity. The lowest calculated singlet band is 1.8 eV lower than the first peak in the Auger spectrum with reflects the extent of the unbalanced account of correlation energy in the ground and dicationic states given by the second-order ADC scheme. Thus, the calculated peak energies were corrected by adding 1.8 eV to them in order to allow for this effect. The Gaussian fit for the singlet states reveals seven peaks, the spacings of which are in good agreement with those measured for the first seven Auger peaks. The first computed singlet peak is a combination of two states, the ‘EZgand ‘Ai, having calculated energies of 23.96 eV and 24.59 eV, respectively. The first triplet peak, on the other hand, is due to one state ( 3A2g)only, and is at 23.34 eV. In the present investigation, an attempt has been made to measure the double-ionization energy to this triplet state and also to others at higher energies. Double-charge-transfer
030 1-O104/9 I/% 03.50 0 199 I Elsevier Science Publishers B.V. All rights reserved.
300
WJ. Gri&ths, FM Harris I The benzene dilation
(DCT) spectroscopy [ 5 ] has been used which is a technique in which fast-moving singly charged positive ions undergo double-electron-capture (DEC) reactions with the molecules under investigation. Such reactions are endoergic, and the energy to drive them comes from the translational energy of the projectile ion. By measuring the translational-energy spectrum of the negative ions formed, and determining the energy losses corresponding to the peaks in it, double-ionization energies to ground and excited dicationic states can be evaluated. The choice of projectile ion is important for two reasons. ( 1) A consequence of the Wigner spin-conservation rule [ 61 (total spin angular momentum must not change in the course of the collision) is that in DCT spectroscopy triplet or singlet states of the dications are populated depending on which projectile ion is employed. In most of the DCT spectroscopy studies carried out by Fournier and his colleagues [ $71, H+ has been used as the projectile ion. Both H+ and Hhave an electronic spin equal to zero. By conservation of spin, therefore, the doubly ionized ion will have the same spin state as the neutral molecule. Most of the DCT spectroscopy studies carried out at Swansea have been with the OH+ projectile ion. Since OH+ has a triplet ground state (%- ), and the resulting OH- ion generated in a DCT spectroscopy experiment is in a singlet state ( ‘C+ ), it follows from the spin-conservation rule that collisions with molecules which are in singlet states should populate triplet states of the dication. This has been shown to be the case for CO*, OCS and CS2 [ 8,9 1, NH, [ 10,111, CCL, [ 12 ] and N20 [ 13 1. In the present DCT spectroscopy study of benzene, therefore, OH+ has been used as one of the projectile ions in the hope that, even for such a large molecule, the spin-conservation rule will apply, and the double-ionization energies to dicationic triplet states can be measured. (2) On the basis of theoretical [ 14,lS ] and experimental [ 16,17 ] studies, it has been established that a reaction window exists for DEC reactions, i.e. that the endoergicity of the reaction must sit within a window of values and, in fact, maximizes within that window. An important consequence of this is that the use of projectile positive ions which give a high release of energy when they charge invert in a DEC reaction tends to result in the population of higher-lying dicationic states. Such an ion is F+ because of the
high values of single-ionization energy and electron affinity of F. Thus, F+ has been used in previous studies to investigate excited states of COZ, OCS and CSz [9], NH3 [ll], N20 [13J, CH3Br [17] and CHsI [ 18 ] which are higher in energy than the states investigated using OH+. It has also been used in the present study with the same purpose in mind.
2. Ex~rimen~ If the projectile ions OH+ and F+ are denoted by A+, the DEC reactions of interest in the present investigation can be represented by A’+C6H6+A-
+C,Hif+ .
(11
In practice, A+ has a translational energy of 6 keV and the DEC reactions are thus sufficiently rapid to ensure vertical double ionization of C6H6. The ion A- can also be generated in sequential reactions involving two C6H6 molecules, i.e. A+ +C6H6-+A+C6H$
,
(2)
A+C6H6-+A-+CgHgf .
(3)
Peaks in the A- translational-energy spectrum due to reaction ( 1) increase in height linearly with C6H6 pressure whilst those due to reactions (2) and (3) show a quadratic dependency. Another difference is that, in general, their positions on the A- translational-energy scale are different. If the translational energy of A+ is denoted by E,, and that of A-, when C,H$+ is formed in its ground state in reaction ( I), by E,, then E,,-E,
=IE,(C,H,)-E(A++A-)
,
(4)
where IE2 ( C6H6) is the double-ionization energy of C,H, to the ground dicationic state, and E( A++A- ) is the energy released due to charge inversion of A+. If an excited state rather than the ground state of C,Ha+ is populated in reaction ( 1 ), the A- ions have a lower translational energy E’, which is related to the double-ionization energy IE; ( ChHC) to that state through eq. (4) in which E, and IEz(CaHs) are replaced by E; and IE; ( C6Hs ) , respectively. The translational energy, E,, of the A- ions formed by reactions (2 ) and (3) depends on the single-ion-
W J. Grifiths, FM
301
Harris / The benzene dication
ization energies IE,,(C6H6) and IE,,(C,H,) to the states x and y of C6H,f populated in the reactions. Thus,
lowing through to the detector in turn negative ions having different translational energies.
& -& =IE,,(W%)
3. Results
+IE,y(G&)
-E(A+-+A-).
(5)
In order to calibrate the translational-energy-loss scale, a DCT spectrum was obtained with xenon atoms for which the relevant single- and double-ionization energies are known [ 191. For the single-coliision reaction A++Xe+A-+Xe’+
A typical calibration spectrum obtained with the OH’ projectile ion is shown in fig. 1a. By varying the 100
(a)
(6)
the translational energy of A- is E,,, and E,-E,,=IE,(Xe)-E(A+~A-).
(7)
5C
By subtracting eq. (7) from eq. (4) and from eq. ( 5 ) the equations L -E, =IE,(C6H6)-IE,(Xe)
(8)
and &, -E;!=IE,,(CaH6)+IEI,(Xe)-IE,(Xe)
(9)
are obtained from which the double-ionization energy and sum of the since-ioni~tion energies of C,H, are readily determined once E,,, E, and E2 have been measured. The relevant translational energies were measured using a ZAB-2F mass spectrometer #’ operated with the source at a constant voltage, nominally 6 kV. This voltage need not be known, but it governs the E0 in eqs. (4) and (5 ) and was maintained at one value in order to justify the use of eqs. (8) and (9). The OH+ and F+ ions were generated by dissociative ionization of Hz0 and CF4, respectively, and, after acceleration and mass selection, the projectile ions interacted with C,H, molecules contained in the collisiongas cell of the second field-free region of the spectrometer. Negative ions were transmitted to the detector by reversing the polarities of the voltages normally applied to the plates of the electric sector. DCT spectra were obtained by scanning these voltages al” Manufactured by VG Analytical Limited, Manchester, Gt. Brittain.
C
5970
I
I
5960
Translational
I
6000
5990
energy
(eV)
Fig. 1. Negative-ion spectra resulting from doubie-electron-capture reactions of 6 keV OH’ ions with (a) xenon atoms and (bf benzene molecules.
302
W.J. Griffiths, FM. Harris /The benzene dication
xenon-gas pressure, it was established that peaks i and ii are due to OH- ions formed in the sequential single-electron-capture reactions whilst I and II are due to OH- ions formed by DEC reactions in single collisions. These peaks are interpreted in the following way. For peak i, the sequential reactions result in both Xe+ ions being in the 2Ps,2 state, but for peak ii one of the Xe+ ions is in the 2PJ,2 state, the other in the 2P ,,2. For peak I, the Xe2+ ion is formed in the ‘P2 state, and for II it is in the ‘P,-, and/or the ‘P, state which are separated by 0.2 12 eV. All these single- and double-ionization energies are known [ 191, and the relative spacings of the peaks confirm the above interpretation. Thus, the translational-energy positions of peak I or II can be used as E,, together with the corresponding IE2 (Xe) in eqs. (8 ) and (9 ). It is known [ 201 that beams of OH+ ions formed following dissociative electron ionization of Hz0 contain small components which are in long-lived (lifetimes greater than several ps) electronically excited states. These are the a ‘A and b ‘E+ at 2.15 eV and 3.55 eV [ 2 11, respectively, above the 3E- ground state. Reactions of ions in these excited states could lead to minor peaks in the spectra. Since for such ions E(A+ +A-) is larger than for reactions of groundstate OH+ ions, the energy loss Eo- E2 (eq. (5 ) ) would be smaller, i.e. minor peaks would be observed to the right of peak i in fig. la if reactions of the excited-state ions were significant. Since no such peaks are observed, the conclusion can be drawn that the concentrations of excited-state ions in the OH+ beam are sufficiently low as not to affect the spectra of the present investigation. A typical spectrum obtained when OH+ projectile ions interacted with C6Hb is shown in fig. lb. From the pressure dependencies of the heights of the peaks it was shown that peaks a and b were due to the sequential reactions ( 2 ) and ( 3 ) whilst A, B and C were due to the formation of OH- ions by reaction ( 1). The translational-energy positions of the maxima of A, B and C correspond to E, and E; values which, when inserted in eq. (8), give double-ionization energies for C6H, of 24.6 rt 0.5 eV, 21.3 & 0.5 eV and 29.8 f 0.5 eV. Typical spectra obtained when using F+ as the projectile ion are shown in fig. 2. The spectrum (a) obtained with Xe showed no peaks due to the sequential single-electron-capture reactions. Peaks I and II are analogous to I and II of fig. 1a and are due to Xe2+
(b)
‘L
o-l- . 59’70
'
59'80
Translational
’
59’90
energy
r
so'00
(ev)
Fig. 2. Negative-ion spectra resulting from doubleelectron-capture reactions of 6 keV F+ ions with (a) xenon atoms and (b) benzene molecules.
being formed in the 3P2 and the 3P0 (and/or ‘P,) states, respectively. To check the possibility that longlived excited-state components in the F+ ion beam are interacting with Xe is more difficult in the absence of the sequential, single-electron-capture reactions. Peak I is due to F+ (3P) reacting with Xe to populate the 3P2 state of Xe2+ having an energy of 33.327 eV [ 191. The next state of F+ is the ‘D2. It has been shown [ 221 that ions in this state are pres-
W.J. Griffhs, FM. Harris / The benzene dication
ent in a beam formed following dissociative electron . ionization of CF,. Because of spin conservation, the lowest-energy DEC reaction of F+ ( ‘D2) will populate the ID2 state of Xe2+ at 35.447 eV. Since the separation between the ground and ID2 states of F+ is 2.59 eV [ 191 the lowest-energy DEC reaction of F+ ( ‘D2) with Xe would give a peak 0.47 eV to the right of peak I in fig. 2a. No such peak is evident but it could rightly be argued that the resolving power of the spectrometer used in the present investigation is not high enough to reveal it. However, if the ‘D2 ions in the F+ beam were a significant constituent a shoulder would be observed on the right-hand side of peak I. Its absence suggests that F+ ( ‘D2) ions are not sufficiently numerous to affect the positions of the peaks in the spectra recorded in this investigation. The heights of the peaks D, E, F and G in fig. 2b all increased approximately linearly with C6H6 pressure and were due to F- ions formed by reaction ( 1) . Peak G is just discernible but is real since it was found to be present in all the spectra recorded. From the positions of their maxima, the double-ionization energies calculated using eq. (8) are 3 1. 1 + 0.5 eV, 32.7f0.8 eV, 34.5kO.8 eV and 37.2fl.O eV. All these energies are higher than those determined from the 0H+/C6H6 spectra and this work confirms the value of using F+ to investigate higher-lying dicationic states.
4. Discussion The lowest double-ionization energy measured in the present investigation is 24.6 eV k 0.5 eV. This is significantly lower than 26. I f 0.8 eV, the lowest energy determined from an Auger spectrum [ 31. It is likely that the difference is due to the populating of a triplet state (or states) in the DCT spectroscopy experiment using OH+ as a projectile ion and the populating of a singlet state (or states) in the Auger spectroscopy experiment. In their constructed spectra, Tarantelli et al. [ 1] assume that the first triplet peak is due to one state only, the 3A2, at 23.34 eV whilst the first singlet peak has contributions from the ‘Elg and ‘A,, states at a mean energy of 24.3 eV. Thus, from their theoretical study, Tarantelli et al. [ 11 predict that the first triplet peak and first singlet peak are separated by about 1 eV, a value which is in agreement (within experimental uncertainty) with
303
that determined from the Auger and present DCT spectra. Vertical double-ionization energies calculated recently by Krogh-Jespersen [24] are 23.83 eV to the ground triplet state and 24.97 eV to the ground singlet state i.e. a difference of 1.14 eV which agrees with that of the previous calculations [ 1 ] and with that determined from the present and previous [ 31 measurements. Further evidence that a ground triplet state is populated in the present investigation is obtained from a previous DCT spectroscopy experiment [23] in which H+ was used as a projectile ion. As mentioned earlier, the use of H+ should result in the populating of singlet states and it is interesting that the lowest double-ionization energy determined by Appell et al. [ 23 ] is 26.0 + 0.5 eV i.e. in agreement with the Auger result of 26.1 eV, but significantly higher than the lowest double-ionization energy determined in the present investigation. The double-ionization energy of C6H6 determined in an electron-impact ionization investigation [ 25 ] is 26.02 0.2 eV which suggests, in the light of the above information, that the lowest state accessed in such an experiment is the ground singlet state. The experimental procedure requires, however, that the dication is stable for at least several p in order to be detected following formation in the source and passage through a magnet. Such a requirement is not necessary in Auger spectroscopy nor in DCT spectroscopy, and thus the results from them may not be directly comparable with that from an appearance energy measurement in an electron-impact ionization experiment. It would appear from the above facts that spin is conserved in the DEC reaction of OH+ with C6H, which leads to the populating of the lowest-energy dicationic state. To see if this is the case for higher-lying states, the present data are compared with the energies of the peaks deduced from the groupings of calculated triplet states [ 11. These groupings were presented graphically in the previous publication [ 1 ] but the energies of the peak maxima have been made available [ 26 ] to the present authors. They are listed, together with the present results, in table 1. It can be seen that the theoretical data (normalized as described in the footnote to the table) agree well with the experimental data except that a peak at 3 1.1 eV is not predicted. It is possible, however, that the energies of two of the computed states when averaged
W.J. Griflths. FM. Harris / The benzene dication
304
Table I Double-ionization energies from the present investigation and the corresponding energies of peaks in a simulated spectrum of groupings of calculated triplet states [ 1,251 Double-ionization
energies
(eV)
Experimental (DCT spectroscopy using OH+ and F+ projectile ions p)
Theoretical
Peak Peak Peak Peak Peak Peak Peak
24.6 27.6 30.1
A B C D E F G
24.6f0.5 27.320.5 29.8kO.5 31.1kO.5 32.7fO.S 34.5kO.8 37.2f I.0
b,
32.4 34.9 37.6
’ Peaks A, B and C were observed with the OH+ projectile ion and peaks D, E, F and G with the F+ projectile ion. ’ Data obtained by the addition of 1.3 eV to the calculated values in order to bring the lowest calculated and experimental energies into agreement.
could give rise to peak D. These are the 3B,U and 3E2U having energies of 29.70 and 30.02. Adding 1.3 eV to the average value of 29.86 eV gives 31.16 eV which is in good agreement with the value determined from the position of peak D. It is thus tempting to conclude that spin conservation is valid for all reactions in the energy range investigated. However, from the H+ DCT spectrum [ 23 ] double-ionization energies of 30.3kO.5 eV and 33.lkO.5 eV were obtained which agree, within experimental uncertainties, with the values 29.8 k 0.5 and 32.7 + 0.8 eV, respectively, of the present investigation. Further, Tarantelli et al. [ 1 ] deduced that the “best” compiled Auger spectrum from theoretical data included a mixture of singlet and triplet states with the triplets being given a low weighting of 0.3. The present spectra may also be due to a combination of triplet- and singlet-state energies with perhaps a higher weight being given to the triplets states. The first two peaks, however, which give double-ionization energies of 24.6 and 27.3 eV, seems to be mainly triplet. The nearest Auger peaks are at 26.1 and 28.2 eV and the nearest H+ DCT peaks are at 26.0 and 30.3 eV. Thus, the first peak in the present work is probably due to population of the 3A28 state whilst the second is probably due to the population of one or more of the states 3EiU, ‘B,,, 3E,, and 3B2, which are calculated [ 1 ] to be separated by no more than 0.34 eV.
References [ I ] F. Tarantelli,
A. Sgamellotti, L.S. Cederbaum and J. Schirmer, J. Chem. Phys. 86 ( 1987) 220 I. [2] K. Sieghbahn, C. Nordling, G. Johansson, J. Hedman, P.F. Heden, K. Hamrin, U. Gel&, T. Bergmark, L.O. Werme, R. Manne and Y. Baer, ESCA Applied to Free Molecules (North-Holland, Amsterdam, 1969). [3] R. Spohr, T. Bergmark, N. Magnusson, L.O. Werme, C. Nordling and K. Sieghbahn, Physica Scripta 2 ( 1970) 3 I. [4] H. &ren, J. Chem. Phys. 75 (1981) 1267. [ 5 ] J. Appell, Collision Spectroscopy, ed. R.G. Cooks (Ebevier, Amsterdam, 1978) p. 244. [6] E. Wigner, Nachr. Ges. Wiss. Goettingen Math. Phys. Kl 2A (1927) 325; H.S.W. Massey, Rept. Progr. Phys. 12 (1949) 248. [7] P.G. Foumier, J. Foumier, F. Salama, S. Stark, S.D. Peyerimhoff and J.H.D. Eland, Phys. Rev. A 34 ( 1986) 1657, and references therein. [ 81 W.J. Grifliths and F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 87 ( 1989) 349. [9] M.L. Langford, F.M. Harris, C.J. Reid, J.A. Ballantine and D.E. Parry, Chem. Phys. I49 ( 199 1) 445. [IO] W.J. Griffiths and F.M. Harris, Rapid Commun. Mass Spectrom. 4 ( 1990) 366. [ 111 M.L. Langford, F.M. Harris, P.G. Fournier and J. Fournier, J. Chem. Phys., submitted. [ 121 P.G. Foumier, G. Comtet, J. Foumier, S. Svensson, L. Karlsson, M.P. Keane and A. Naves de Brito, Phys. Rev. A 40 (1989) 163. [ 13 ] F.M. Harris, C.J. Reid, J.A. Ballantine and D.E. Parry, J. Chem. Sot. Faraday Trans. 87 ( 199 1) 168 I. [ 141 D. Mathur, Intern. J. Mass Spectrom. Ion Processes 83 (1988) 203. [ 15 ] D.P. Almeida and M.L. Langford, Intern. J. Mass Spectrom. Ion Processes 96 (1990) 331. [ 161 W.J. Griffiths, D. Mathur and F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 87 (1989) RI. [ 171 M.L. Langford and F.M. Harris, Rapid Commun. Mass Spectrom. 4 ( 1990) 125. [ 18 ] W.J. GrifIiths, F.M. Harris and D.E. Parry, J. Chem. Sot. Faraday Trans. 86 ( 1990) 280 1. [ 191 C. Moore, Atomic Energy Levels (Natl. Bur. Stand,, Washington, DC, 1949). [20] M. Hamdan, S. Mazumdar, V.R. Marthe, C. Badrinathan, A.G. Brenton and D. Mathur, J. Phys B 21 (1988) 2571. [21] A.J. Merer, D.N. Malm, R.W. Martin, M. Horani and J. Rostas, Can. J. Phys. 53 (1975) 25 I. [22] C.S. Enos and A.G. Brenton, private communication (1990). [ 231 J. Appell, J. Durup, EC. Fehsenfeld and P. Foumier, J. Phys. B7 (1974)406. [24] K. Krogh-Jespersen, J. Am. Chem. Sot. 113 ( 1991) 417. [25] F.H. Dorman and J.D. Morrison, J. Chem. Phys. 35 (1961) 575. 1261 F. Tarantelli, private communication (1990).