Double photoionization of CO2, OCS, C2H2, CF4, and C6H6 studied by threshold photoelectrons coincidence (TPEsCO) spectroscopy

Chemical Physics ELSEVIER

ChemicalPhysics187 (1994) 125-135

Double photoionization of C02, OCS, C2H2, CF,, and C6H6 studied by threshold photoelectrons coincidence (TPEsCO) spectroscopy R.I. Hall a, L. Avaldi b, G. Dawber ‘, A.G. McConkey ‘, M.A. MacDonald d, G.C. King a aLaboratoire de Dynamique Moleculaire et Atomique (CNRS VRA 774), Vniversit6 P et U Curie, 4place de Jussieu TlZ-B75. 75252 Paris Cedex 05, France b IMAl de1 CNR, Area delle Ricerca a?Roma, CPIO-ooO16Monterotondo.Iraly c Department of Physics, Schuster Lubomtory, Manchester University, Manchester U13 9PL.,UK d SERC, Daresbury Labomtov, Daresbury. Warrington WA4 4AD. UK Received1 February1994

Abstract Double photoionization of COZ, OCS, CzHz,CF, and C,H, molecules has been studied in an electron4ectron coincidence experiments, where nearly zero kinetic energy ( < 20 meV) electrons are detected. This first application of TPEsCO spectroscopy to polyatomic molecules confirms the ability of this experimental technique to determine accurate double ionization potentials and locate excited electronic states. The spectrum of benzene reveals what appears to be the first observation of vibrational structure in a polyatomic dication.

1. Introduction The study of the spectroscopy and dynamics of molecular dications has been recently boosted by the advent of synchrotron radiation. New experimental techniques based on direct double photoionization hv+M+M2++el

+e

2

followed by the coincidence detection of photoelectrons and/or photoions, have been developed. These different coincidence techniques are referred to by acronyms, that show the types of particles detected. Thus, for example, PEPICO is a photoelectron-photoion coincidence experiment, while PEPECO and PIPICO indicates photoelectron-photoelectron and photoion-photoion coincidence experiments, ElsevierScienceB.V. SSD10301-0104(94)00188-G

respectively. A detailed review of all these techniques has recently been made by Eland [ 11. These techniques add to another class of experimental methods based on Auger electron spectroscopy [ 21 and double charge transfer (DCT) spectroscopy [ 31. Among the coincidence spectroscopy techniques PEPECO has the advantage that it can tackle the investigation of dications directly, independently of the subsequent fate of the dication. Recently Hall et al. [ 4,5] have added to this collection of techniques a new one, which combines the selectivity of coincidence experiments to the high sensitivity of threshold spectroscopy. In this coincidence spectroscopy photodouble ionization processes are detected by recording the coincidence between two essentially zero energy electrons. This technique that has been named threshold photo-

R.I. Hall et al. /Chemical Physics 187 (1994) 125-i-135

126

electron(s) coincidence (TPEsCO) spectroscopy, represents in the case of dications the analogue of threshold photoelectron (TPE) spectroscopy for singly charged cations [ 61. In a TPE spectrum a peak occurs any time the scanned photon energy goes through the threshold of a singly charged ion state. In a TPEsCO spectrum a true coincidence signal is the signature of a double ionization event and will be recorded each time the photon energy crosses the threshold of a double ion State.

TPEsCO has proved itself to be a powerful technique for the investigation of the dynamics of the photodouble ionization process in atoms [ 71. Its application to the spectroscopy of molecular dications enabled us to establish accurate values of the double ionization potentials (DIP) of the ground and several excited states of diatomic molecules [4,5,8,9]. Moreover the energy resolution of these experiments was sufficient to resolve vibrational structure of electronic states and determine spectroscopic constants. A TPEsCO spectrum also provides valuable informations on the mechanism of formation of the doubly charged ion. The absorption of an energetic photon can result in the direct emission of two electrons, or in ionization to an excited state of the singly charged ion, that subsequently autoionizes to a lower lying dication

state. This latter two-step process has been observed to play a major role in double ionization of rare gases near threshold in TPEPICO experiments [ lo]. In aTPEsC0 spectrum, the indirect mechanism will only contribute if the single ion state is first excited at its threshold and then if it is degenerate, within the experimental resolution, with the dication state (Fig. 1). The intensities of the experimental peaks give indications of the relative importance of the two mechanisms. If only the direct mechanism were to occur, the peaks in the various vibrational series would have intensities governed by the Franck-Condon principle. The indirect or twostep mechanism, on the other hand, results in erratic peak envelopes with intensities governed by a combination of the Franck-Condon principle for the excitation of the single ion state and the cross section for the autoionization process. In this work TPEsCO spectroscopy has been applied to the investigation of dications of the polyatomic molecules: COZ, OCS, C2HZ,CF, and C&,. Section 2 is devoted to a brief presentation of the experimental setup and procedure, while the experimental results are presented and discussed in Section 3. Finally Section 4 is devoted to some conclusions.

15

-10 ’

0.0

1 0.5

1.0

2.0

1.5

2.5

3.0

3.5

R 64) Fig. 1. Schematicrepresentationof single photondouble ionizationof an arbitrarymoleculeM. The dii

mechanismresultingin the simukanecu ejection of two electrws is representedby process (a), while process (b), which proceedsvia a near-degenerate. state of the singly chargedion, is aa example of the two-step mechanism.

R.I. Hall et al. /Chemical Physics 187 (1994) 125-135

2. Experimental method The main elements of the apparatus are a photon beam, an effusive gas beam and two electron energy analyzers, which are mounted on the opposite sides of a turntable, that can be rotated in a plane perpendicular to the photon beam. Two different versions of the coincidence spectrometers have been used in these experiments. The former one, used for the investigation of CO*, &HZ and CF,, is equipped with two different analyzers, namely a 127” electrostatic cylindrical deflector analyzer (CDA) and a time-of-flight (TOF) analyzer, while in the latter version the TOF analyzer has been replaced by a second 127 ’ CDA. These coincidence spectrometers have been described previously by Hall et al. [ 11,7 1, thus here only the main elements relevant to the present measurements will be repeated. The collection of zero energy electrons is realized using the penetrating field technique, developed by Cvejanovfc and Read [ 121. Briefly, a potential well is formed in the interaction region by a field penetration of an extracting electrode into the interaction region through a 5 mm aperture in a screening electrode. This technique provides a large collection angle and, furthermore, forms a crossover point in the particle trajectories [ 111, which ensures efficient transmission of the electrons through the subsequent electron optics of the analyzer. The performances and efficiencies of each analyzer were determined by collecting spectra of nearzero energy electrons in each analyzer, in the absence of the penetrating field of the other, in the region of the Ar+ 2P,12,3,2states. These spectra showed that the TOF analyzer operating in the threshold mode has a response function characterized by a high energy tail, due to the finite energy electrons directly emitted into the lens system from the interaction region. This fact, which prevented a complete resolution of the vibrational progression in the case of the first ever TPEsCO measurement of the ground and first excited states of CO*+ and N:+ [ 41, does not affect the present measurements on polyatomic molecules, where the energy resolution of the photon beam (S&l00 meV) does not allow the resolution of the vibrational structure of the dication states. Standard NIM electronics is used to measure coincidences between electrons detected by the two analyzers. True and random coincidences and also the single counts, i.e. the photoelectron yield measured by each analyzer, are accumulated using a LSIll mini-

121

computer, that also controls the photon energy scan. The tunable photon beam is provided by the Synchrotron Radiation Source (SRS) at the SERC Daresbury Laboratory either via a toroidal grating monochromator (TGM) for the CO*, C2H2 and CF, experiments, or via a 5 m, vertically dispersing, normal incident monochromator for the OCS and &I-& experiments. The photon beam enters the coincidence spectrometer via a 1 mm diameter glass capillary, where it intersects the gas emanating from a hypodermic needle. The photon beam intensity during the energy scan is monitored by an A1203 photodiode sitting on the axis of the photon beam approximately 30 cm from the interaction region. The apparatus was maintained at approximately 100°C in order to eliminate and compensate for local potential variation in the scattering region. The collection modes of the analyzer were set up by balancing the extractor potentials in such a way that at the threshold of a photoionization process the electron yield was divided between the two analyzers in the ratio corresponding to their efficiencies, determined as described above. Under these conditions the electrostatic potential between the analyzers has the form of a symmetric saddle. In close vicinity of a threshold, photo-double ionization yields two near zero energy electrons, which when emitted in approximately opposite directions, can drift to different sides of the potential ridge and be detected in different analyzers. A true coincidence signal is then the signature of a double ionization event and will be recorded each time the scanned photon energy goes through the threshold of a double ion state. The threshold energy resolution of the two analyzers is less than 20 meV and the two analyzers collect threshold photoelectrons over a solid angle of approximately 2n steradians. The energy scale of the spectra was calibrated by using an admixture of the studied gas and argon or xenon. In the experiments on the TGM beam line the spectra were calibrated against the ti’ ‘P, threshold, while the Ar+ 3s-’ and Xe+ 5s- ’ thresholds were used in the experiments on the other beam line. At a typical gas pressure in the interaction region of lo-* mbar and with a photon flux of a few 10” photons/s the coincidence count rate varied from 0.1 to 9 coincl s in C,I& and CF,, respectively.

Table 1

31.5f0.5

‘A

‘A, ‘% ‘A,,

26.0f0.1

[ 131

[16]

[ 161

31.7a 1431

[39]

[22] [23]

26.0 [38]

33.7 [37]

31.8f0.2 32.4f0.2

40.6*0.3

38.9f0.2

H+

ion yield [43].

27.3 f 0.5 [56]

24.6kO.2 [56]

32.7f0.3

30.7f0.5 30.3 f0.3

40.9 f 0.5 [22] 40.5kO.3 [23]

37.7f0.5 [22] 37.8rtO.3 [23]

OH+

DCT projectile ion

’ This value has been obtained by extrapolation of the measured &Hi+

Cd&

‘A,

32.7 [65]

30.0 f 0.5

36.2k0.4 [19] 37.7 f 0.3 [21] 37.9 f 0.4 [ 201

36.4 f 0.3 [ 131 38.0 f 0.2 [ 141 37.2 f 0.5 [ 151 38.1 f 0.5 [ 161 37.0*0.5 [17] 38.6fO.l [18]

40.5 f 1.0 [20]

Photoionization

Electron ionization data

3s+

‘4 ‘2: ‘P-”

‘I;;

a% %s

ocs

co*

Assig.

[21] [21]

[21]

[21]

Experimentaldeterminationof the energies (in eV) of the electronic states of the CO,, OCS, Cp,

[23]

30.4rtO.3 [23]

40.5 rtO.3 [23]

37.5 f0.3

F+

and C,& dication

[24]

26.1 kO.8 [55]

39.0f0.3

Auger electr. spar.

31.4kO.3 [25] 32.4*0.1 [25]

39.0f0.2 [25] 39.6f0.3 [25] 40.4kO.2 [25]

SEC

24.6kO.l

31.7rtO.l

3O.lIkO.l

38.5kO.l 39.2fO.l

37.2kO.l

Present work

R.I. Hall et al. /Chemical Physics 187 (1994) 125-135

3. Results The TPEsCO spectra of CO*, OCS, C,H,, CF, and C,H, are shown in Figs. 2-6. The spectra have been put on an absolute photon energy scale as described in Section 2, and have been corrected for variations in photon intensity as a function of photon energy by normalizing to the photodiode output. The spectra have also had a smooth background removed corresponding to the random coincidences [ 81. 3.1. co, The CO* molecule is linear in its ground state and its electronic valence shell configuration is (4u,)*(3cr”)*( llr”)4( lQ4

x ix;

.

The main difficulty in the study of CO2 arises from the degeneracy of the two outermost orbitals, which is expected to result in a high density of states in the molecular dication. The CO’,+ dication has been experimentally investigated in several electron impact experiments [ 13181, photoionization [ 19,201 and PIPICO experiments [ 211 as well as in DCT [ 21-231 experiments. Other information has been provided by Auger electron spectroscopy (24) and single-electron capture reaction (SEC) [ 251. The experimentally determined energies of the different states of the CO:+ dication in the

36.5

37.5

129

energy region investigated in the present work are summarized in Table 1. The CO;+ TPEsCO spectrum over the region 36% 39.5 eV is shown in Fig. 2. Two broad features can be clearly identified and the start of a third can be suspected at the extreme high energy side of the spectrum. The onset for CO;” formation has been derived from the energy calibration to be at 37.2 f 0.1 eV, while the other two features occur at about 38.5 and 39.2 eV, respectively. This observation shows that in the investigated region at least three electronic states of the dication exist: the ground state and two excited states lying 1.3 and 2 eV above, respectively. The measured energy of the ground state is lower than most of the previous determinations, although it is not inconsistent with them due to their large uncertainties (see Table 1). On the other hand the spacing between the ground and excited states is in very good agreement with the highresolution investigation of the translational energy loss incurred in a SEC reaction by Jonathan et al. [ 251. Theoretical calculations of the CO’,+ structure based on one-electron theory have been done by Kelber et al. [ 261 in order to reproduce the CO2 Auger spectrum, while Laramore [27] and Agren [28] used SCF Xcr and ab initio computational methods, respectively. More recently Millie et al. [ 211 have calculated the term scheme of CO;+ with ab initio SCF-CI calculations, by using the algorithm of the CIPSI method [ 29 1. This latter calculation set the vertical ionization poten-

38.0

38.5

Photon energy (eV) Fig. 2. TRW0 spectrumof COz. The photon energy step is 30 mcV, the collection time per point is 4.5 min and the coincidence count rate is 1.5 co&/s at 39 eV. The ticks show the position of the dication groundand excited states determinedin the presentwork.

R.I. Hall et al. /Chemical Physics 187 (1994) 125-135

130

“L.”

29.0

29.5

30.0

30.5

31.0

31.5

Photon energy (eV) Fig. 3. TPESCOspectrumofOCS. The photon energy step is 15 meV, the collection time per point is 7 mio and the coincidence count rate is 0.3 coin& at 30.7 eV. Tk ticks show the position of the dieationgroundstate determinedin the presentwork and of tiie lowest excited state accordingprevious experimentaland theon&aI data f see text). tials at 36.7 eV and estimated the uncertainty of the relative energies of the lower states to be of the order of 0.1-0.2 eV. In the region investigated in this work this calculation locates the “S: ground state and the two excited ‘Ag and ‘2: states at 1.2 and 1.83 eV above it, respectively. These values are in very good agreement with the present obviations, but for a shift of the 0.5 eV of the absolute scale, and allow a definite assignment of the features in the TPEsCO spectrum as the first three electronic states of CO $+ .

3.2. ocs The valence shell confi~tion electron state is (60)*(5a)Z( ln)4(2n)4

of OCS in its ground

X’C + .

As for the case of COz, the degeneracy of the two outermost orbitals leads to a high density of states in the molecular dication, that in OCS is further enhanced due to its lower s~rne~. ~~rnen~l info~ation on OCS’+ electronic states have been obtained via electron ionization experiments [ 161, DCT spectroscopy [21-231, SEC [ 251 and Auger electron spectroscopy [ 301. The experimentally determined energies of the different OCS’+ states in the region of

interest of the present work are summarized in Table 1. The TPEsCO spectrum of OCS over the 29-3 1S eV region is shown in Fig. 3, where a coincidence yield, that starts at about 29.2 eV, is observed to develop into a broad feature centered around 30.5 eV. The yield then shows a local mi~mum rising again above 3 1 eV. From an extrapolation of the rising front of the main feature we have put the onset for OCS2’ formation at 30.0 f 0.1 eV. This value is in very good agreement with the OCS’+ appearance potential measured in the electron impact experiment by Cook et al. [ 161 and with the value of the OCS’+ ground state measured in the more recent DCT spectroscopy ex~~men~ using OH+ and F+ projectile ions [22,23]. DCT spectroscopy data with an H + projectile ion 1211 and SEC [ 51 experiments locate the first excited state of the dication at 31.8 f0.2 and 31.4 f0.3 eV, respectively. These previous observations support the hypothesis that the observed rise at the high energy side of the TPEsCO spectrum can be attributed to the first excited state of OCs*+. OCS2+ electronic states have been calculated with the SCF-CI method, using the CIPSI algorithm 1291, by Millie et al. [ 211 and using the Moller-Plesset perturbation theory with a split valence basis set by Lang-

131

R.I. Hall et al. /Chemical Physics 187 (1994) 125-135

ford et al. [ 231. The SCF calculation [ 211 locates two OCS2+ states in the investigated region: the 3ZZ+ ground state at 29.1 eV and the first excited ‘A state 1.1 eV above it. As already observed in CO2 this calculation results in a ground state energy that is lower than the observed one, the difference being about 1 eV in this case, but the spacing between the states is in very good agreement with the measured value. A similar sort of agreement is obtained by comparing the present results with the values predicted by the other theory [23]. The non-vanishing coincidence yield observed below what is considered to be the 0CS2+ ground state, may be evidence of a precursor state. This state is a singly charged state, belonging to a series converging either to the ground or to an excited state of the dication. On being excited at its threshold this singly charged state would yield a zero-energy electron and could then produce a second one as the potential curve crosses into the doubly charged ion continuum. Such a process was first observed in H20 by the detection in coincidence of positive ion fragments [ 3 11. A two-step mechanism, via an intermediate singly charged ion state, has been observed to play a major role in the population of vibrational levels of dication states of diatomic molecules outside the Franck-Condon region [5,9] and to produce a continuous coincidence yield above 38.5 eV in the case of O2 [5]. A similar mechanism, via an

-100

I

31

32

OCs+* precursor state which, crossing into the OCs2+ continuum, can dissociate to the lowest CO+ +!I+ dissociation limit at 27.5 eV [32], may therefore account for the present observation below 30 eV. 3.3. C,H, C,H, in its linear ground state has the valence orbital configuration (2o”)2( 30,)2( llr”)2 x ‘2; . C2Hi+ is one of the smallest stable polyatomic dicalions and it has been found in the mass spectra of hydrocarbons and other classes of compound [ 33,341. The spectrum of the C2G+ electronic states has been investigated experimentally via Auger spectroscopy [ 35,361, DCT spectroscopy [ 37-391 and the PIPICO [ 401 method. The previous experimental observations of the C2H$+ electronic states are summart‘red in Table 1. Theoretical calculations of its electronic structure have been made using semiempirical methods [ 4 1,421, SCF [38,43], ab initio [44] and Green’s function methods [ 45,461. The dissociation of the C2Hg+ dication in the 32-65 eV energy range has also been recently investigated via PEPIPICOcoincidences by Thissen et al. [43], who also reviewed all the previous work.

I 35

36

Fig. 4. TPFXO spectrumof &Hz. The photon energy step is 30 meV, the collection time per point is 7 min and the coincidence count mte is 0.9 c&c/s at 34 eV. The ticks show the position of the dication groundstate determinedin the presentwork and of the lowest excited states accordingpreviousexperimentaland theoreticaldata (see text).

132

R.I. Hall et al. /Chemical Physics 187 (1994) 125435

The measured C,H$+ TPEsCO spectrum over the energy 3 l-36 eV region is shown in Fig. 4. The coincidence yield shows a broad feature slowly rising up to a short plateau between 33 and 34 eV prior to a further rise above 34 eV. From an extrapolation of the measured yield we have derived a value for the onset of C,@ + formation of 3 1.7 f 0.1 eV. This value is about 1 eV lower than the double ionization energy determined both by electron impact mass spectrometry [ 471 and by DCT spectroscopy [ 391. It is in agreement with the value proposed by Thissen et al. [ 431 on the basis of the extrapolation to zero of the C&+ yield measured in their photoionization experiment. According to theoretical calculations, in the studied region there are three different electronic states, with only the ‘S: state being a pure two hole state ( 1~; *), while both the ‘Ag and 32; states are expected to be a combination of two main configurations ( 1~;’ and 1~; 32~,,). The vertical double ionization energy calculated by Ohrendorf et al. [46] and Thissen et al. [43] are 31.35 and 32.0 eV, respectively. Both the values are in reasonable agreement with the present observations. The two excited states are expected to be located at 32.47 and 33.24 eV according to Ohrendorf et al. [ 461 and at 32.9 and 33.5 eV according Thissen et al. [ 43 1, but are not observed in our spectrum. 3.4. CF, The inner- and outer-valence configuration of the pentatomic CF, molecule is

(3ai)2(2t#(4ai)2(3t2)6(1e)4(4t#(lt1)6

‘Ai .

Experimental information on the Cc+ dication has been obtained via Auger spectroscopy [48], PIPICO [ 321 and PEPIPICO [ 491 experiments. None of these techniques, however, provide direct information on the two hole states of CF,. The CF, TPEsCO spectrum over the 30-50 eV region is shown in Fig. 5. The spectrum reveals a broad feature, with a sharp rise and an irregulartail. By extrapolation of the rising front of the spectrum, the onset for CG+ formation has been set at 37.5 f 0.5 eV. Larkins and Tulea [50] have calculated by a semiempirical molecular orbital model the energy of 107 two-hole states associated with the seven CF, outermost orbitals. Several of these states are located in the region studied in this work and can contribute to

the TPEsCO yield. Among them in Fig. 5 we have indicated the ones that have been identified by Codling et al. [49] to be the precursor states for the fragmentation channels CFZ + F+, CF: +F+ and CF+ + F+. The vertical ionization energies of the inner-valence 3a, and 2h CF, orbitals are at 40.3 and 43.8 eV [ 5 1,521. Thus one can expect that double ionization via an intermediate singly charged ion could play some role in this energy region. The comparison of the TPEsCO spectrum and the threshold spectra, independently measured by the two analyzers, does not provide any evidence for ionization of the inner valence states at their threshold that could enhance the TPEsCO yield. Therefore the present data seems to indicate that CF, double ionization mainly occurs via a direct process. 3.5. cd& The benzene ground state configuration is (a2U)2(e1,)4 ‘Ai, . The first observation of doubly charged cations of aromatic molecules is due to Hustrulid et al. [ 531 who in 1937 observed the benzene cation [ ‘*C, ‘3CH,J *+ in a mass spectrometer. Since that time experimental information on the appearance energies of the longlived doubly charged ions from benzene has been obtained by electron impact mass spectrometry [ 13,18,54], while information on the excited states has been derived from Auger [ 551, DCT [ 38,561 and PIPICO [ 571 experiments. The experimentally determined energieS Of the.different&@+ dication StateS in the energy region investigated in the present work are summarized in Table 1. The benzene TPEsCO spectrum over the 24-27 eV energy region is shown in Fig. 6. This spectrum reveals a series of three peaks, with a separation of = 120 meV with the first one centered at 24.6fO.l eV, followed by a broad feature with a maximum at = 25.3 eV and by a slowly decreasing tail. The position of the first peak in our spectrum is in good agreement with the lowest double ionization energy measured by DCT spectroscopy using OH+ [ 561 as a projectile ion. The series of three peaks can be interpreted as a vibrational progression belonging to the lowest dication state of benzene. The energy spacing is very close to the vibrational quantum of the y mode, which corresponds to the symmetric breathing of the C6 ring, observed in

133

R.I. Hall et al. /Chemical Physics 187 (19%) 125-135

45

40

35

50

Photon energy (eV) Fig. 5. TF%sCOspectrumof CF.+The photon energy step is 0.5 eV, the collection time per point is 3 min and the coincidence count rate is 9 c&c/s at 43 eV. The ticks show the position of the dication ground state determinedin the present work and of the lowest excited states accordiig previousexperimentaland theoreticaldata (see text).

both the Rydberg series [ 581 and in the C,G ionic states at 9.4, 11.4 and 16.9 eV [59]. The previous experimental investigations agree in locating another electronic states at about 26 eV. A number of theoretical studies of the electronic states of the benzene dication has been reported [ 60641. Tarantelli et al. [ 631, who used the two-particle Green’s function method and the second order alge-

-50

’

24.0

24.5

25.0

braic diagrammatic construction formalism, computed 226 doubly ionized states of benzene in the 2340 eV energy region. The 3A2.sground state is located by this calculation at 23.34 eV, which is about 1.3 eV lower than that measured. More recently Krogh-Jesperen [ 641 has calculated a vertical double ionization energy for the triplet ground state of 23.83 eV, which is in slightly better agreement with the experiments. All the

25.5

26.0

I 26.5

Photon energy (eV) Fig. 6. TPEsCO spectrumof C,& The photon energy step is 20 meV. the collection time per point is 30 min and the coincidence count mte is 0.1 coinc/s at 25.5 eV. The ticks show the position of the dicationgroundstateand of a vibrationalpmgmssionbelongingto this statedetermined in the presentworicand of the lowest excited states accordingpreviousexperimentalaad theoreticaldata (see text).

134

R.I. Hall et al. /Chemical Physics 187 (1994) 125435

calculations locate two other electronic states, the ‘&s and ‘Ai, states, in the studied region. Thus the broad structure observed in the TPEsCO spectrum can be attributed to the overlap of what appears to be several c&+ states.

4. ConeIusions In this work TPEsCO spectroscopy, i.e. an experimental technique that combines the selectivity of the electron-electron coincidence method and the sensitivity of threshold spectroscopy, is shown to be a valuable experimental tool for the investigation of doubly charged ions of polyatomic molecules, too. The coincidence detection of the two ejected electrons provides direct access to all the dication electronic states and does not suffer from any constraint due to their lifetime. Thus accurate double ionization potentials have been determined for the lowest electronic levels of C02, OCS, C,H,, CF, and C,& dications. Moreover TPEsCO, like all the techniques based on photoionization, allows the observation of both singlet and triplet states, as clearly shown by this work and by the previous studies of dications of diatomic molecules [ 4,5,8,9]. This is an advantage with respect to other experimental techniques that either give information only on singlet states, as Auger electron spectroscopy, or need different types of projectile ions, as in DCT experiments, to access to the electronic states with different spin multiplicity. So far the information provided by TPEsCO spectroscopy is the front-edge of our knowledge on dication states and represents an engaging challenge to the theoretical study of the dications. The comparison of the coincidence and single electron yields in a TREsCO measurement allows the investigation of the mechanism of formation of the dication states. From the present observations it appears that the main process, occurring in the studied polyatomic molecules, is the direct formation of the doubly charged ion, except for the case of OCS. In the latter a nonvanishing coincidence yield has been observed below that corresponding to what is believed to be the OCS’+ ground state. This may be considered as evidence of a two-step mechanism, which forms the doubly charged ion via a precursor OCS + * state. The existence of such a process has to be confirmed by further photoelectron

experiments which assess what are the OCS+ satellite states that can be populated in this energy region. In the C& TPEsCO spectrum a vibrational progression belonging to the lowest dication state has been clearly observed. To our knowledge this is the first observation of vibrational structure in a polyatomic dication. At present TPEsCO spectroscopy is limited only by the available photon energy resolution and the strength of the signal (i.e. realistic dam collection times). The combination of TREsCO and third generation synchrotron radiation facilities with higher photon flux and better energy resolution is expected to result in a further step towards a more accurate definition of the electronic and vibrational structure of molecular dications.

Acknowledgement We are most grateful to the staff of the Daresbury SRS facility for the excellent working conditions. Work partially supported by SERC and by the Royal Society (UK)Consiglio Nazionale delle Ricerche (Italy) exchange programme.

References [ 1] J.H.D. Eland, Coincidence studies of multiionized molecules, in: Vacuum ultraviolet photoionization and photodissociation of molecules and clusters, ed. C.Y. Ng (World Scientific, Singapore, 1991) p. 297. [ 21 M. Thompson, M.D. Baker, A. Christie and J.F. Tyson, Auger electron spectroscopy (Wiley, New York, 1985). [3] J. Appell, J. Dump, F.C. Fehsentkl and P.G. Fournier, J. Phys. B 6 (1973) 197. [41 R.I. Hall, A. McConkey, L. Avaldi, M.A. MacDonald and G.C. King, J. Phys. B 25 (1992) 411. [5] R.I. Hall, G. Dawber, A. McConkey, M.A. MacDonald and G.C. King, Phys. Rev. I.&tens 68 (1992) 2751. [6] W.B. Peatman, G.B. Kasting and D. Wilson, J. Electron Spectry. Relat. Phenom. 7 (1975) 233. 171 RI. Hall, G. Dawber, A.G. McConkey, M.A. MacDonald and G.C. King, Z. Physik D 23 (1992) 377. [ 81A.G. McConkey, G. Dawber, L. Avaldi, M.A. MacDonald, G.C. King and RI. Hall, J. Phys. B 27 (1994) 271. 191 A.G. McConkey, G. Dawber, L. Avaldi, M.A. MacDonald, G.C. King and RI. Hall, J. Phys. B 27 (1994) 2191. [lo] RI. Hall, K. Ellis, A.G. McConkey, G. Dawber, L. Avaldi, M.A. MacDonald and G.C. King, J. Phys. B 25 (1992) 377. Ill] RI. Hall, A. McConkey, K. Ellis, G. Dawber, L. Avaldi, MA. MacDonald and G.C. King, Meas. Sci. Technol. 3 ( 1992) 3 16.

R.I. Hall et al. /Chemical Physics 187 (1994) 125-135 [ 121 S. Cvejanovic and F.H. Read, J. Phys. B 7 (1974) 180. [13] F.H. Dormann and J.D. Morrison, J. Chem. Phys. 35 (l%l) 575. [ 141 AS. Newton and A.F. Sciamanna, J. Chem. Phys. 40 (1964) 718. [ 151 T.D. M&k and E. Hille, J. Chem. Phys. 69 (1978) 2492. [ 161 R.G. Cooks, D.T. Tenvilligerand J.H. Beynon, J. Chem. Phys. 61 (1974) 1208. [ 171 J.H. Agee, J.B. Wilcox, L.E. Abbey and T.F. Moran, Chem. Phys. 61 (1981) 171. [ 1 81 B. Bmhm, U. Probe and H.P. Neitzke, Intern. J. Mass Spectrom. Ion Phys. 57 (1984) 91. [ 191 J.A.R. Samson, P.C. Kemeny and G.N. Hadad, Chem. Phys. Letters 51 (1977) 75. [20] G. Dujardin and D. Winkoun, J. Chem. Phys. 83 ( 1985) 6222. [21] P. Millie, I. Nenner, P. Archiml, P. Lablanquie, PG. Foumier and J.H.D. Eland, J. Chem. Phys. 84 (1986) 1259. [22] W.G. Griffiths and F.M. Harris, Intern. J. Mass Spectrom. Ion Processes 87 (1989) 349. [23] M.L.Langford,F.M.Harris,C.J.Reid,J.A.BallantineandD.E. Party, Che.m. Phys. 149 (1991) 445. [24] W.E. Moddeman, T.A. Carlson, M.O. Krause, B.P. Pullen, W.E. Bull and G.K. Schweitzer, J. Chem. Phys. 55 (1971) 2317. [ 251P. Jonathan, M. Hamdam and A.G. Brenton, Chem. Phys. 119 (1988) 159. [26] J.A. Kelber, D.R. Jennison and R.R. Rye, J. Chem. Phys. 75 (1981) 652. ]27] G.E. Laramore, Phys. Rev. A 29 (1984) 23. [28] H. &ten, J. Chem. Phys. 75 (1981) 1267. [29] B. Huron, J.P. Malrieu and P. Rancuml, J. Chem. Phys. 58 (1973) 5745. [30] L. Asplund, P. Kelfve, H. Siegbahn, 0. Goscinski, H. FellnerFeldegg, K. Hamrhi, B. Blomster and K. Siegbahn, Chem. Phys. Letters 40 (1976) 353. [ 3 11 D. Winkoun, G. Dujardin, L. Hellner and M.J. Besnard, J. Phys. B 21 (1988) 1385. [32] D.M. Curtis and J.H.D. Eland, Intern. J. Mass Spectrom. Ion Processes63 (1985) 241. [33] J.H. Beynon, A. Mathias and A.E. Williams, Org. Mass. Spectrom. 5 (1971) 303. [34] BE. Jones, L.E. Abbey, H.L. Chatam, A.W. Hanner, L.A. Teleshefsky, E.M. Burgess and T.F. Moran, Org. Mass Spectrom. 17 (1982) 10. [ 35 I M. Thompson, P.A. Hewitt and D.S. Wcoliscroft, Anal. Chem. 48 (1976) 1336. [36] R.R. Rye, TX!. Madley, Chem. Phys. 69 (1978) [37] J.H.D. Eland, S.D. Price, and P.G. Foumier, Phil.

J.E. Houston and P.H. Holloway, J. 1504. J.C. Cheney, P. Lablanquie, I. Nenner Trans. Roy. Sot. A 324 (1988) 247

135

[ 381 J. Appell, J. Dump, F.C. Fehsenfeld and P.G. Foumier, J. Phys. B 7 ( 1974) 406. [39] S.R. Andmws, FM. Harris and D.E. Parry, Chem. Phys. 166 (1992) 69. [40] J.H.D.Eland,F.S. Wart, P.LablanquieandI. Nenner,Z.Physik D4 (1986) 31. [41] J.R. Applmg and T.F. Moran, Chem. Phys. Letters 118 (1985) 188. [42] G.W. Burdick, G.C. Shields, J.R. Appling and T. F. Moran, Intern. J. Mass Spectrom. Ion Phys. 64 (1985) 315. [43] R. Thissen. J. Deb&he. J.M. Robbe. D. DuIIot, J.P. Flament and JHD. Eland, J. Chem. Phys. 99 (1993) 6590. [44] J.A. Pople, M.J. Frisch, K. Raghavachari and P. von Schleyer, J. Comput. Chem. 3 (1982) 468. [45] C.M. Liegener. Chem. Phys. 92 (1985) 97. [46] E.M.-L.Ohrendorf.F.TarantelliandL.S.Cedetbaum,J.Chem. Phys. 92 (1990) 2924. [47] J.R. Appling, BE. Jones, LE. Abbey, D.E. Bostwick and T.F. Moran, Org. Mass Speetrom 18 (1983) 282. [48] R.R. Rye and J.E. Houston, J. Chem. Phys. 78 ( 1983) 4321. [49] K. Codling, L.J. Frasinski, P.A. Hatherly, M. Stankiewicz and F.P. Larkins. J. Phys. B 24 (1991) 951. [50] P.P. Larkins and L.C. Tulea, J. Phys. Coil. (Paris) C9( 12) 48 (1987) 725. [51] K. Siegbabn, ed., ESCA applied to fme molecules (Elsevier, Amsterdam, 1969). [52] M.S. Banna,B.E.Mills,D.W. DavisandD.A. Shirley, J.Chem. Phys. 61 (1974) 4780. [53] A. Hustrulid, P. Kusch and J.T. Tate, Phys. Rev. 52 (1937) 843. [54] B.P. Mathur, E.M. Burgess, D.E. Bostwick and T.F. Moran, Org. Mass Spectrom. 16 (1981) 92. [55] R. Spohr, T. Beckmark, L.O. Werme, L. Karlsson, C. Nordling and K.K. Siegbahn, Physica Scripta 2 (1970) 31. [56] W.J. Griffiths and F.M. Harris, Chem. Phys. 157 (1991) 299. [ 571 P.J. Richardson, J.H.D. Eland and P. Lablanquie, Org. Mass Spzctrom. 21 (1986) 289. [58] G. He&erg, Molecular spectra and molecular structme. III. Electronic spectra and electronic stntctum of polyatomic molecules (Van Nostrand, Princeton. 1966). [59] L. Karlsson, L. Mat&ton, R. Jadrny, T. Bergmark and K. Siegbahn, Physica Scripta 14 (1976) 230. [60] T.W. Bentley and C.A. Wellington, Org. Mass Spectrom. 16 (1981) 523. [61] K. Lammertsma and P. Schleyer, J. Am. Chem. Sot. 105 (1983) 1049. [62] M.J.S. Dewar and M.K. Holloway, J. Am. Chem Sot. 106 (1984) 6619. [63] F. Tarantelli, A. Sgamellotti, L.S. Cede&mm and J. Schirmer, J. Cl-tern. Phys. 86 (1987) 2201. [64] K. Krogh-Jesperen, J. Am. Chem. Sot. 113 (1991) 417. [ 651 T. Ast, J.H. Beynon and R.G. Cooks, Chg. Mass Spectmm. 6 (1972) 749.