Journal of Electron Spectroscopy and Related Phenomena, 41(1986) 421-438 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE FORMATION AND DESTRUCTION OF DOUBLY-CHARGED POLYCYCLIC AROMATIC HYDROCARBON CATIONS IN THE INTERSTELLAR MEDIUti*

SYDNEY LEACH

D&artement d’rlstrophysique Fondrrmentak, Observatoire de Paris-Meudon, 92190 - Meudon (France) and Laboratoire o!e Photophysique Molkculaire du C.N.R.S., Bdtiment 213, Universiti Park&d. 9140~Orsay (France)

(Received 11 August 1936)

ABSTRACT

Large polycyclic aromatic hydrocarbons (PAH) have recently been held responsible for certain IR emission bands and visible region diffuse absorption bands observed in the interstellar medium (ISM). A mechanism for the fragmentation and eventual destruction of PAH species in the ISM is suggested. This involves a number of excitation processes that could lead to their double ionization in the ISM. The poesibility of fragmentation of the doubly-charged PAHs by Coulomb explosion processes involving rapid direct dissociation or by tunnelling processes is examined in the context of the relevant cosmic timescale and evente in the ISM.

INTRODUCTION

Large polycyclic aromatic hydrocarbons (PAH), containing 50 or more atoms, have recently been proposed as existing in the interstellar medium (ISM). They have been assigned as the carriers of the so-called “unidentified infrared emission” (IlIFt) bands observed in the line-of-sight of a number of stellar sources, e.g. NGC 7027 and HD 44179 [l, 2].This proposition is based on the similarity of the observed emission spectra in the 3-12 pm range to known lR absorption spectra of species such as coronene [l, 21. More recently, and more tenuously from the observational evidence, PAHs have been considered to be responsible for the Diffuse Interstellar Bands (DIB) observed in absorp- tion in the 430-700 nm region [3-51.

The PAH species in the ISM probably exist as a range of compounds and are considered to be in the form of neutral closed shell, molecular radicals and/or singly-ionized species [4]. Some of the infrared signatures indicate that some of the PAHs are in partially dehydrogenated form [6]. The PAHs have been estimated to exist in cationic form to more than 60% in extent [2, 41.

This study proposes possible mechanisms for the fragmentation and even-

* Dedicated to Professor E. Heilbronner on the occasion of his 95th birthday.

03W2043/36/$03.50 0 1939 Elsevier Science Publishers B.V.

tual destruction of PAH species in the ISM. These mechanisms invoke the dissociative double ionization of PAHs by sequential two or three-stage excita- tion processes involving photons and/or cosmic ray and other energetic par- ticles in the interstellar medium.

Although multiply-charged atoms are known to exist in various astrophysi- cal situations, including shock regions of the ISM [7,8], doubly-charged mole- cular ions have not generally been considered as taking part in astrophysical processes. This is due to the paucity of laboratory-based information on these species as well as to the difficulty in distinguishing between processes originat- ing in doubly and in singly-charged ions [9]. The general properties of doubly- charged molecular ions have been discussed recently in the context of their possible formation and detection under astrophysical conditions [9]. Some of these properties will be recalled here in relation to their possible significance to the existence of PAHs in the interstellar medium.

The next two sections discuss processes of formation of doubly-charged molecular ions and the transition probabilities for double ionization by single excitation and by sequential multi-excitation processes. They are followed by considerations of the stability of doubly-charged molecular ions and their propensity for dissociation. The formation of doubly-charged polycyclic aro- matic hydrocarbons in the ISM is then examined. Finally, the destruction of these species by Coulomb explosion processes involving rapid direct dissocia- tion or by tunnelling processes is considered on the timescale relevant to both the molecular and the cosmic events occurring in the interstellar medium.

It is a pleasure to dedicate this paper to Edgar Heilbronner who has domes- ticated the singly-charged polycyclic aromatic hydrocarbons in a number of elegant studies of the photoelectron spectra of PAHs.

DOUBLY-CHARGED MOLECULAR IONS: FORMATION PROCESSES

The formation of doubly-charged molecular ions require minimum energies of the order of 20-40 eV with respect to the neutral ground state. A compilation by Tsai and Eland [lo] shows that for many molecules the ratio of double to single ionization potentials I + +/I+ x 2.8. The usual methods of formation of doubly-charged ions in the laboratory are by electron impact, ion impact, double-charge transfer, charge stripping and photon impact [9, 111. Energy deposition is well defined in photon impact in contrast to most of the other techniques.

Besides the formation of doubly-charged ions in the 20-40eV range by ejection of two valence electrons, core ionization followed by Auger radiation- less transition processes result in the creation of these species at higher energies [12].

SIMULTANEOUS AND SEQUENTIAL DOUBLE IONIZATION: TRANSITION PROBA- BILITIES

In the low excitation energy range, the cross-sections for double ionization by various processes are given in Table 1 for representative ranges of incident

426

TABLE 1

Cross-sections for formation of doubly-charged molecular cations by ejection of valence electrons (excluding Auger processes)

Process Cross-section Reference

aAB+hv-+AB+++2e- s 0.1-2 Mb 11 bAB+ + hv+AB++ + e- =lOOMb 13 cAB+H+-+AB++ +H- x0.01-1 Mb 14 dAB+ + H+ +AB++ + H z 0.1-l Mb 14

eAB+e-+AB” +3e- = l-10 Mb 15 fAB++e-+AB’++2e- x0.1-10Mb 16, 17

particle energies [9]. The values given for o(b) and a(f) are for atoms since these processes have been little investigated in quantitative fashion for mole-

cules. Double ionization by single photon or single particle excitation results from

multi-electron interactions, i.e. electron correlation effects, and would have zero probability if these interactions did not occur. If two isoelectronic species of different volume at comparable excitation energy excess above the double ionization threshold are compared, it is found that the double photoionization cross-section (and presumably also for processes c and e) is much greater for the species whose electron configurational space is smaller. In the case of neon and methane the values are a’+(Ne) = 0.25Mb [18], b++(CHI) = 0.034Mb [19]. The smaller volume of Ne would give rise to greater electron correlation effects responsible for ejection of two valence electrons by the electric dipole tran- sition. This view is supported by the large difference in the electron correlation energies E,, for neutral Ne and CH, whose values are respectively - 8.60 eV [20] and - 3.9OeV [21], where EC, is defined as the difference between experi- mental binding energies and accurate HartreeFock values.

It is clear that when the total number of electrons in a related family of molecules, e.g. CO, --+ COS + CS, [22-241 is increased, the cross-section for double photoionization also increases, no doubt indicating that the greater possibilities for many-particle interactions more than offset the increased electron configurational space. This is mirrored in the increasing importance of satellite bands in photoelectron spectra in such molecular series. It is thus to be expected that for polycyclic aromatic hydrocarbons, increasing the num- ber of electrons in a related series will also give rise to augmented double- ionization cross-sections. There is no direct information in this respect for PAHs but it is striking that electron-impact mass spectral studies have shown that the percentage ratio of doubly-charged to singly-charged non-dissociated ions of polycylic aromatic hydrocarbons increases rapidly, and quasi-linearly, with increasing number of x electrons [25]. The quasi-linear relation is pertur- bed by structural effects but it is of interest that for the largest PAH studied in this context, hexabenzocoronene, which has 42x electrons, the ratio of doubly to singly-charged ions is quite high, about 60% [25].

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Removal of two valence electrons by double photoionization of a closed shell species AB can give rise to both singlet and triplet states of the doubly-ionized AB++. No restriction is expected on the formation of triplet final state AB’ + ions, since the electric dipole transition transports two electrons to the ioniza- tion continuum. This is not the case for the double charge transfer process c, where a spin conservation rule holds for light incident ion projectiles [14]. In Auger transitions it is expected that the probability of forming triplet states is extremely small among the doubly-charged ion final states corresponding to loss of two electrons from the same molecular orbital. The Auger transition probability can become non-negligible if the final triplet state corresponds to ejection of electrons from two different orbitals [26]. For example, the experi- mental observations on double photoionization [19], double charge transfer [27] and Auger transitions [26,28] in methane are in good agreement with the above considerations.

In the two-stage sequential processes leading to doubly-charged molecular ions, the initial process AB + AB+ for a closed shell species forms doublet AB’ electronic states, with a cross-section of the order of 100 Mb. The second step, AB’(2X) + ABC +, generally involving the doublet ground state of the singly- charged ion, can give rise to either singlet or triplet electronic states of AB+ + since the initial state in this step is an open-shell species.

Thus double ionization by a single particle (hv, H+, e-) incident on polycy- clic aromatic hydrocarbons, (except for the double charge transfer process c) should lead to formation of both singlet and triplet electronic states of PAH + +. From Hund’s rule, the ground state of PAH+ + is expected to be a triplet state. For PAH++ having symmetry axes of order 3 or more, the ground state is expected to be also orbitally degenerate, and so exhibit Jahn-Teller distortion effects [29]. For example, for the prototype species benzene+ +, of &, symmetry, the electron configuration will give rise to ‘a3Alg, ‘*‘A,, and ‘s3EW states. The ground state should be the (orbitally) doubly-degenerate i!?E,, state.

STABILITY OF DOUBLY-CHARGED MOLECULAR IONS

In general, removal of one valence electron from AB will give rise to a singly-charged ion AB+ of different equilibrium structure, if the electron re- moved has bonding or anti-bonding properties (note that ejection of an atom- atom non-bonding electron can have significant effects on the nuclear force field and even on the angular geometry [30]). Likewise, removal of a second valence electron to form AB++ can produce a doubly-charged cation of dif- ferent nuclear structure. It should be remembered, however, that removal of two II electrons from a large PAH (which has a large pool of x electrons) should not create very marked structural modifications except if the lability of the a electrons facilitates isomerization of the doubly-charged ion.

Figure 1 schematically illustrates the possibility of different equilibrium structures for the AB, AB+ and AB+ + species. The lowest dissociation limit of AB+ + will in general lie below the double ionization threshold, due to Coulomb renulsion forces between the two singly-charged product ions A+ and B+. The

431

AB

w

r(A-8)

Fig. 1. Schematic representation of potential energy surfaces of a neutral (AB), singly-ionized (AB+) and doubly-ionized (AB+ +) molecular species.

attractive covalent curve of AB’ + , corresponding to the strongly-bound charge polarization part of the potential energy surface, and whose endothermic dissociation is to A+ + + B, will be crossed by the Coulomb repulsion curve A+ + B+. Interaction betwen the two potential curves will lead to electron transfer in crossing from one to the other curve [31]. The exothermicity of the reaction to A+ + B’ can be associated with rapid dissociation or to a slower fragmentation rate if tunnelling from the metastable potential well is involved. Metastable doubly-charged molecular ions, whose lifetimes are greater than FZ 1~8, have been detected by electron-impact mass spectrometry [32] and fkom mass spectrometry with soft X-ray excitation [33] and vacuum ultraviolet photon excitation [lo, 341. However, if the covalent and Coulomb potential energy surfaces interact close to the covalent surface minimum, or if there is a significant geometry change in the AB(AB+) --) AB++ process, the vertical transition could lead directly to the dissociative part of the doubly-charged ion potential surface. The propensity for dissociation by direct or by tunnelling processes has been used, via laser-ion beam interaction studies [35] and the photoion-photoion coincidence (PIPICO) technique [ll, 19,22,23,27] as specif- ic tools for the investigation of the spectroscopy and dissociative relaxation of doubly-charged molecular cations.

It should be noted that the fragmentation pathway AB’ + + A+ + + B has also been observed, but that dissociation to two singly-charged products. is expected to be more probable than to one doubly-charged fragment, at least for the lower electronic states of AB” [ll].

432

FORMATION AND DESTRUCTION OF DOUBLY-CHARGED PAHs IN THE INTERSTELLAR MEDIUM

Single excitation formation processes

Doubly-charged molecular ions could be formed in the interstellar medium by interaction of molecules with high energy photons, cosmic ray particles or fast particles in shock regions [9]. Ultraviolet radiation emitted by a star embedded in a dense cloud is absorbed in the immediate neighbourhood of the star by the H atom ionization continue at L < 912A (13.6eV). However, the H atom ionization cross-section at It < 199A (124eV) is sufficiently small for photons having these energies to penetrate great depths of interstellar space [36]. These soft X-rays, if of sufficient flux, as well as the low energy components of cosmic rays, and incident shock-heated particles, could play a role in creating doubly-charged molecular ions in the ISM, as they do in the formation of multiply-charged atoms 17, 371.

Sequential two-stage excitation formation processes

A more efficient way of forming PAH+ ’ in the ISM is a sequential two-stage excitation process, The efficiency of the sequential processes results from the expectation that PAHs are readily singly-ionized with radiation of less than 13.6 eV energy [29] and that for larger members of these species the ionization energy to form PAN + + from PAH+ could also be less than 13.6eV.

TABLE 2

Ionization energies (ev) for formation of eingly and doubly-charged polycyclic aromatic hydro- carbon cations

Species x I+ -x-+x+ I++ =x.+x++ I++ - I+ I x+ -t x++

B-at? GAl 9.24 26.4 17.16

Nebulae Cl04 8.16 22.7 14.65

#ElIwe Cd+0 1.47 21.1 13.63

Phenanthrsne Cl& 7.36 23.1 16.24

Pyme WI0 7.41 24.0 16.69

Tetraeene Cl&a 1.04 22.14 15.10

Tetraphene Cdl2 7.41 22.03 14.56

chrya- GA2 7.60 23.33 16.73

Triphenylene Wk? 7.89 24.10 16.21

Perylena C&b 7.06 26.0 13AXl

Pentacene cz2Hl4 6.74 19.6 12.36

1,2,6,EDibmmnthracene C&l4 7.33 26.3 13.42

Picene C&l4 7.64 21.6 Lv.SS

caonene Cadb 7.38 21.0 13.64

1.2.3$-Diberuopentacene C&G,2 6.66 19.0 12.05

OV&ZW! wb4 6.36 19.6 12.74

Decacyclene G#Q 7.27 aO.1 12.33

IiWA?W,iXXXO~~~% wbs 7.65 lS& 12.W

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These expectations are examined in Table 2 which gives the first (I+) and second (I+ +) ionization potentials of a number of polycyclic hydrocarbons, as well as the difference f’ + - I + which represents the energy needed for forma- tion of a doubly-charged PAH’ + cation from PAN’. The data show that the first ionization potentials are all well below 13.6eV and that for the larger PAHs the difference I++ - I+ is indeed also less than 13.6eV. Data on the adiabatic first ionization potentials are taken from photoelectron spectroscopy [38]. The double-ionization values are mass spectrometric appearance poten- tials [lo, 251 and so correspond rather to PAH + PAH+ + vertical energies. The true PAH+ + PAH+ + vertical energies could therefore be smaller than the values given in column I+ + - I+ of Table 2 which are thus to be considered as upper limiting values.

The ionization cross-sections for each step, PAH + PAH+ and PAH+ + PAH++ , in the sequential process should be quite high, of the order of 190 Mb. If the energy of the incident photon (or particle) absorbed is less than 1 eV above the threshold energy for the process PAH’ + PAH+ +, the cross-section for this second step could be somewhat diminished. This could occur because of the possibility of competing non-ionization processes similar to those found close to threshold for the ionization process AB 3 AB+. In the latter case, molecular ionization yields less than unity are common although they gener- ally rise rapidly to above 50% within x 1 eV above threshold [39, 401.

The data in Table 2 indicate a general tendency for the I+ + - I+ difference to decrease with increasing size of the PAH. For large PAHs this energy difference is probably less than 12.5eV, so that for a maximum excitation energy of 13.6 eV the maximum excess internal energy in PAH’ + is 1 eV or greater. Thus the average ionization yield in the step PAH+ + PAH + + should be at least 50%.

Singly-charged PAH+, as also neutral PAH, should be radiation resistant to fragmentation when the number of constituent atoms is large. Dissociation of the ground state of PAH or PAH+ is an endothermic process. Assuming the average internal energy of a PAH or PAH+ in the ISM to be 10 eV on excitation by an incident photon or particle, the probability of breaking one bond of x 4.7 eV energy of the 50-atom species is extremely small. This has been demon- strated by recent calculations of the unimolecular dissociation rate for PAH species containing various numbers of atoms. The models used are either of the solid state type [l, 41 or based on the RRKM [41] procedure in which it is assumed that total redistribution of the internal energy will occur among all vibrational modes of the ground state, after complete internal conversion. The results [l, 4,421 predict that PAH and PAH+ species should be extremely stable, even on cosmic timescales, when they contain several tens of atoms.

In the context of the present study we conclude that the first stage of the sequential two-excitation process should produce stable PAH+, ready to re- ceive further excitation to form the doubly-ionized ion PAH’ + .

Although the values depend strongly on particular regions of the inter- stellar medium, the interval between photon absorption in the ISM will be

434

taken here as between 3 x 10e4 years and 3 years, and the interval between H or Hz collisions with a PAH species to be of the order of 3 x lo-’ years or greater [36]. Previously estimated lifetimes of PAH species in the ISM are of the order of l@ years and greater [1,4,42]. Thus it appears possible for doubly-~har- ged PAH+ + cations to be formed on this cosmic timescale by a sequential double-excitation process.

Destruction (fragmentation) of PAW++

Direct d~s~~tio~ by Confab explosion As discussed earlier, doubly-charged molecular cations are fragile and easily

subject to dissociation via direct or tunnelling processes. By photoion- photoion coincidence (PIPICO) experiments on triple product fragmentation of small doubly-charged ions, e.g. CH: ’ [19], it was shown that Coulomb dissocia- tion occurs at such a fast rate that there is ins~cient time for the excess energy to be statistically r~ist~but~ among all the degrees of freedom of the products [19, 311. Even the more restricted distribution associated with an impulse model fails to account for the experimental PIPICO results. In the impulse model, bonds are broken in the bond axis directions and no distinction is made between the dynamics of charged and uncharged fragments of the same mass. The Coulomb repulsion model, in which the two charged fragments fly apart at 1800 (Coulomb explosion), and the third, uncharged, fragment takes up virtually no kinetic energy, was found to be the only dynamic model to account for the observations.

It is not known how rapid would be the Coulomb explosion of a large PAH’ + species. A PIPICO study of the prototype aromatic hydrocarbon, benzene 1433, indicates that the mean rate for the dissociation process CJIs+ * -_) C,H,’ + CH,+ is 5 x 10-gs-l. This rate may indicate a slow direct dissociation, i.e. involving partial redistribution of internal energy in the initial CB He’ + state, or the existence of a tunnelling process. Extension of such studies to higher members of the PAH series is required.

Because of the nature of the Coulomb repulsion process, if some redistribu- tion of internal energy occurs before association, the number of effective oscillators taking part in an RRKM-type redistribution process leading to dissociation could be considerably less than the full 3N - 6 oscillators of an N-atomic species. The unimolecular dissociation rate could therefore be enhan- ced in these circumstances.

Dissociation via tu~~l~i~g If direct dissociation (via d,, Fig. 2) of the PAH++ ion newly-formed in the

ISM does not occur, the doubly-charged cation would be trapped in a meta- stable potential well. It could lose its rovibrational energy by infrared radi- ative emission processes, eventually down to the v = 0 vibrationless level, in competition with dissociation by tunnelling. The infrared emission rates are probably in the range l-loOs_’ @4, 451 or greater [46]. Unit probability of

436

P++tAH

PAH+

‘L e0

PAH

Fig. 2. Schematic representation of physical prowssea relevant to the formation, excitation, rapid dissociation end tunnel&g dieeociation of doubly~h~g~ polycyelic aromatic hydrocarbon ca- tions.

fragmentation by tunnelling would exist within the collision interval of 3 x 10Va years for a tunnelling rate of the order of O,l s-l. Thus dissociation by a tunnelling process, even at very low rates (e.g. in the vibrationless state (tl, Fig. Z)), could provide si~~cant mechanisms for destruction of these species on cosmic timescales.

Dissociation via additional excitatian processes If the PAH+ + ions prove to be excessively stable, on cosmic timescales, with

respect to the fragmentation mechanisms discussed above, faster dissociative rates could be achieved by PAH+ + abaorption of photons from stellar sources, the cosmic background radiation or by particle excitation in the ISM. Such excitations could bring the PAH++ to dissociative regions above fragmenta- tion barriers (e,, e2, Fig. 2) or to higher regions of the metastable well where tunnelling rates are fast enough to compete efficiently with IR radiative loss (&, Fig. 2). The tz process may be not very efRcient if ground state excitation to high vibrational levels has to occur by a single excitation process (ea, Pig. 2). An

436

effective process is nevertheless conceivable via excitation (e4) to a fluorescing (fi) higher electronic state. Fluorescing diatomic doubly-charged ions are known 147, 461; the relative stability of PAH” states may be conducive to fluorescence, but this has not yet been tested.

Fragmentation of PAH+ + could lead to deprotonation or to the loss of larger fragments. Deprotonation would be favoured in tunnelling processes but not necessarily in direct fragmentation pathways. Following the suggesflon that some of the IR emission band (DIR) signatures relate to partially dehyd- rogenated species [l, 61, it is conceivable that the latter might occur in the form of singly-charged deprotonated PAW ions resulting from PAH+ + fragmenta- tion. The IR signatures of deprotonated PAH’ ions and partially-dehyd- rogenated neutral PAHs should be very similar.

CONCLUSION

In this work it has been shown that if polycyclic aromatic hydrocarbons are present in the interstellar medium, conditions could exist for the formation of their doubly-charged cations. However, the fragility of doubly-charged polycy clic hydrocarbons, arising from Coulomb explosion processes involving direct dissociation and/or tunnelling effects, could also lead to destruction of these species or at least to deprotonation and loss of other charged or neutral fragments.

There exists very little laboratory work of relevance to the fragmentation processes and yields of the doubly-charged cations of polycyclic aromatic hydrocarbons. Various types of experiment could provide much useful informa- tion in this respect: (i) laser-induced dissociation of PAH++ ion beams could lead to dete~nation of tunnelling rates, and identification of fragments, as well as providing excitation spectra of dissociation channels; (ii) similar ex- periments with other photon sources, eventually using ion traps; (iii) photo- ion-photoion coincidence experiments on polycyclic aromatic hydrocarbons, which can provide information on the ejected fragments, in particular their nature, yields, internal and kinetic energies, and on the state-to-state dissocia- tion dynamics 119, 311.

The above considerations on the formation and destruction of doubly-char- ged polycyclic aromatic hydrocarbons have been made in the context of recent proposals concerning the identification of the so-called “unidentified infrared emission” bands and the visible region diffuse absorption bands observed in the interstellar medium. Determination of their validity requires further astrono- mical observation of relevant spectra and more detailed knowledge of the ambient physico-chemical conditions in the interstellar medium. It also re- quires extensive laboratory studies following a number of suggestions made in this presentation.

ACKNOWLEDGEMENT

This work has benefited from a CNRS/ATP contract “Physico-chimie des Molecules Interstellaires”.

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