mass spectrometric and coincidence studies of double photoionization of small molecules

Z. Phys. D - Atoms, Molecules and Clusters 4, 31-42 (1986) Atoms, Molecules zo,oo, and Clusters fLir Physik D

@ Springer-Verlag 1986

Mass Spectrometric and Coincidence Studies of Double Photoionization of Small Molecules

J.H.D. Eiand and F.S. Wort Physical Chemistry Laboratory, South Parks Road, Oxford, United Kingdom

P. Lablanquie LURE, Laboratoire CNRS, CEA et MEN, Universit6 de Paris-Sud, Orsay, France

I. Nenner LURE, Laboratoire CNRS, CEA et MEN, Universit6 de Paris-Sud, Orsay, France D6partement de Physico-chimie, CEA, Centre d'Etudes Nucl6aires de Saclay, Gif sur Yvette, France

Received March 28, 1986

Current and future coincidence techniques in the study of double and multiple ionization by photon impact are reviewed. New results are presented on the formation of Xe +, Xe 2 + and Xe 3 + in the region of 4d-a ionization and the triple ionization mechanism is discussed. The thresholds for X e 2+ and Xe 3+ are determined as 33.05-t-0.3 and 64.1 _+0.3 eV respectively. Triple ionization of a molecule (OCS) followed by frag- mentation into three cations is demonstrated for the first time. The formation and charge separation reactions of several molecular double cations are examined by coincidence techniques: intramolecular isotope effects, rearrangement reactions and slow dissociations are shown to occur in triatomic and other small doubly charged molecular ions.

PACS: 33.20.Rm; 33.80.Eh; 33.80.Gj

1. Introduction

Direct double ionization by single photons is forbidden in the frozen orbital independent particle model for both atoms and molecules. Because of correlation effects this process is nevertheless observed with significant intensity for all species, even in the energy range where indirect channels such as the Auger effect are excluded on energy grounds. The main focus of interest for many years was on threshold behaviour, mainly on how the cross-section for direct multiple ionization should vary as a function of the excess energy above threshold [1, 2]. A recent advance has been the realization that threshold behaviour can be strongly modified when the outgoing two-electron wave is restricted to certain spe- cial symmetries [3]. Experimental verification of many theoretical predictions is still lacking, however, and little is known of the range of energies over

which the predicted cross-section law, electron spectrum and angular correlation between electrons hold true. The distributions of the two photoelectrons from double ionization in relative velocity and included angle are expected to change markedly between the threshold region and the high energy region, but experimental difficulties have hitherto pre- vented the measurement of these distributions.

Because the single-photon two-electron ionization process derives its intensity mainly from multi- body effects, it provides an important testing ground for the recent theoretical models of atomic and molecular structure which include such effects. Another major impetus for experimental investigation is that both the spectra of the doubly charged species formed, and the propensity rules describing their formation are hitherto largely unknown, particularly for molecules. When a molecule is doubly ionized it usually dissociates into singly charged fragments,

32 J.H.D. Eland et al.: Double Photoionization of Small Molecules

and the dynamics of these coulomb repulsion-driven dissociations are interesting, particularly as proto- typical reactions on repulsive surfaces.

If multiple ionization is brought about by photons of sufficient energy to excite inner shells, the Auger effect dominates. A new and surprising result of recent studies is that the excess energy deposited in the resulting species remains localized at an atomic site sufficiently long to produce site-specific frag- mentation behaviour [~6] . The study of such effects at innner shell absorption edges and at near-edge resonances absolutely requires the use of tunable synchrotron radiation.

In this paper we review the experimental techniques currently used in multiple photoionization work, and we present recent results on the double ionization process itself in atoms and molecules, and on the dynamics of dissociation of multiply-charged molecular ions.

2. Experimental Techniques

2.1. Photon Sources

Two types of light source have been used in recent work on double photoionization: monochromatized synchrotron radiation and filtered atomic lines from gas discharge lamps. Both have serious deficiencies in their usual forms.

Synchrotron radiation from a storage ring monochromatized by a grazing-incidence grating monochromator offers the tunability essential for measurement of energetic thresholds, together with more than ample intensity. Unfortunately it is usually con- taminated by light from higher diffraction orders, and by scattered light of many wavelengths, both of which can have a crucial effect in double ionization work. Cross-sections for double ionization by the Auger effect from inner-shell ionization are larger by several orders of magnitude than those for direct double-ionization near threshold. Even a small second order contribution can introduce spurious struc- tures into yield curves by mirroring edges and near- edge resonances of higher energy. Even second-order light of energy well below Auger thresholds has a confusing effect in the critical region of the double ionization threshold because the cross-section for direct double ionization rises rapidly with energy, and the very weak threshold process is easily lost in the stronger background of second-order light effects. Similar reasoning applies to scattered light. Use of a normal-incidence monochromator which can bare- ly scan photon energies up to 50 eV and which re- flects no higher energy photons has some advantage here.

We have recently partially solved this problem by using thin metallic foil filters [7] in addition to a grazing incidence monochromator [8] equipped with a custom made ion-etched grating designed to concentrate intensity into the first order. The combi- nation effectively eliminates higher orders and re- duces scattered light. We would urge that either filters or a double monochromator are essential if a wide energy range is to be covered.

The most commonly used gas discharge source is a low-pressure He discharge producing HeI and HeII light together with various impurity lines. Hith- erto we have simply used carbon foils to absorb HeI, and leave the HeII light together with visible light and Lyc~ [7, 9]. Unfortunately the slope of the filter cut-off tends to emphasize the higher energy lines, passing HeIIfl, 7, for instance, better than HeIIc(. Because of the rise in double ionization cross-section with photon energy, alluded to above, this causes great uncertainty as to which atomic line actually produces the effects observed. The solution is ob- viously to use a monochromator in place of the filter and this is now being tried.

2.2. Photoionization Mass Spectrometry

Because of the simplicity of construction and the multiplex advantage of observing ions of all masses simultaneously with high collection efficiency, time- of-flight (TOF) mass analysis is frequently used. This technique has the further advantage in multiple ionization work that the delay time before detection of possibly unstable ions is minimized, so that doubly- charged molecular ions can be observed with maximum sensitivity. Simple TOF mass spectrometers using the Wiley and McLaren focussing conditions [10] offer unit mass resolution for thermal ions up to approximately mass 100: the higher resolution offered by reflectron techniques [11] has not yet been considered worth the consequent loss in sensitivity. The yield of ions of each mass as a function of wavelength is normalized to constant target gas pressure and incident photon flux 'off line', that is, after the computer-controlled experiments.

The most important results from photoionization mass spectrometry in this context are the appearance potentials of doubly-charged ions, and the yield curves giving cross-sections for their formation as a function of wavelength.

2.3. Coincidence Techniques: PIPICO and Congeners

Molecular double photoionization produces, typi- cally, four charged particles in the final state:

m+hv ~ [m 2+ + 2 e - ] -~m + +m + +e~ --k e2-.

J.H.D. Eland et al.: Double Photoionization of Small Molecules 33

In order to characterize the process it is evidently desirable to detect as many of the final-state species as possible. Furthermore, it is almost essential to detect at least two of them in coincidence, in order to distinguish double ionization from the more abundant single ionization which always accompan- ies it. The energy balance (neglecting heavy-particle kinetic energies) in the initial double ionization step is:

E(rn 2 +) = h v - K E ( e ; ) - - K E(e; ).

To determine the spectrum of the doubly- charged species one must detect two electrons in coincidence, each being energy analysed. One electron could be chosen to be of zero kinetic energy (a threshold electron), as this choice brings a great advantage in collection efficiency, though at the cost of losing some information on the ionization process.

With a choice among five charged particles (two electrons, two fragment ions or stable doubly- charged ions) many coincidence experiments can be conceived. In fact there are four possible double coincidence experiments, of which three are being ac- tively pursued, three possible triple coincidence choices, of which only one is being carried out so far, and one quadruple coincidence experiment. The practicable combinations are discussed briefly below; few experimental details are given here, but references to the original papers are given where possible.

a. Double Coincidence Techniques

m 2+ . . . e - .

The spectrum of electrons in coincidence with stable doubly-charged ions could be determined in any conventional PEPICO (photoelectron-photoion coincidence) apparatus [12, 13] with a suitable light source. This is an extremely promising experiment which could tell a great deal about electron-electron correlation in the ionization process. Preliminary experiments are currently being attempted on rare gas targets.

An alternative form of this experiment would register coincidences between doubly-charged ions and threshold electrons as a function of photon wavelength, using a TPEPICO (threshold PEPICO) apparatus [14]. This experiment may be made difficult by the abundance of low energy electrons found at all the relevant short wavelengths, apparently emanating from various forms of indirect multiioni- zation.

e[ ... e2-.

The ultimate experiment to characterize the double ionization process would register both photoelectrons, with energy analysis of each. An important objective is to discover how excess energy above the double ionization threshold is distributed between the two outgoing electrons. Current doctrine, already confirmed to some extent in electron impact ionization [15-17] is that the electron spectrum should be flat in the region of threshold, but become peaked near zero and near the maximum possible kinetic energy, at high photon energies [18]. The extent of the "near threshold" region must also be determined experimentally.

New apparatus is being built for a frontal attack on this difficult technique, which is limited by the problem of sensitivity. Several forms of the experiment have been envisaged (and some tried) to overcome the problem of low collection efficiency in conventional electron energy analysers. Both electrons could be detected and simultaneously energy-analysed in full multiplex by using a time-of-flight (TOF) analysis along two opposed flight paths, following ionization by pulsed light from an electron storage ring in single-bunch mode. This idea is very promising, though the energy resolution would be limited: high efficiency would be possible, especially with a magnetic paralMizer [19]. Alternatively, one threshold electron could be selected with high efficiency by TOF + steradiancy in a weak electric field, and the second electron analysed by TOF or by a conventional energy analyser.

m ~ . . . e - ,

The singly-charged fragments from coulomb explosions are characterized by their high kinetic energies, often exceeding 3 eV. A useful experiment would therefore be to record the spectrum of electrons in coincidence with high kinetic energy singly- charged ions, thereby discriminating against ordi- nary single ionization. This has not been tried, though it could also be done in standard PEPICO apparatus.

- +

e . . . / 'n 1 .

The Auger electron produced after an inner shell ionization has a well defined kinetic energy and cor- responds to the formation of a specific doubly charged ion state m 2+. The detection of all singly charged fragments in coincidence with an Auger electron would give the same information as triple coincidence (threshold) e - . . . m~-.., m~- (see below).


m ~ . . + . m z .

Because positive ions can be gathered efficiently by the use of electric fields, and can be mass- and energy-analysed by TOF, the photoion-photoion coincidence experiment (PIPICO) is relatively easy, and has enjoyed wide success [20-23]. It is closely related to - indeed a successor o f - similar experiments which used charged particle projectiles to produce double ionization [24]. The use of photons has the advantage of limiting the maximum energy transfer, and tunable photon sources offer additional advantages in searching for resonances, and in the determination of thresholds.

The recent realizations of ion-ion coincidence experiments have used a TOF mass spectrometer apparatus with an extended multichannel electron multiplier detector. All ions are drawn to the same detector by strong electric fields (30-1000Vcm -~) where they arrive at times after formation which are proportional to the square roots of their masses. The signature of a dissociative double ionization event is the arrival at this detector of a pair of ions within a definite delay of a few tens to hundreds of nanose- conds of each other. The PIPICO spectrum, repre- senting the number of events against the pair delay time, is equivalent to the autocorrelation function of signals from the detector. Because pulse amplifiers and discriminators have dead times, closely spaced pulses from a single detector are difficult to record, hindering the study of ion pairs of equal mass. This difficulty can be overcome, however, if two or more distinct anodes are mounted after the multichannel electron multiplier detector, each with its own ampli- fier and discriminator. The scheme shown in Fig. 1 then represents a ' s tandard ' set-up.

The PIPICO apparatus in Fig. 1 is capable of recording all dissociative double ionization processes leading to cation pairs. The most useful results from PIPICO measurements are the yields of each pair- formation reaction, and the kinetic energy releases. The yields for specific reactions identified by the peak positions are determined as peak areas, while kinetic energy releases are deduced from the peak widths [21, 22, 25]. In favourable cases the doubly- charged ion lifetimes can also be determined from the examination of tails to PIPICO peaks [23].

b. Triple Coincidence Experiments

e l . . . e 2 . . . m +.

As soon as electron-electron coincidence measurements can be done with energy analysis, an obvi- ous extension will be electron-electron-ion coincidences, to determine how individual states of dou-

Channel plate multipliers

Drift tube

MCA

-V ----~w ~L~ ~j

+0 ~ Source

Fig. 1. Cut-away view of a simple PIPICO apparatus. The apertures between the source and drift tube are normally covered with grids for field uniformity. TAC: time-to-amplitude converter, MCA: multi-channel analyser

bly-charged ions react. This is still a distant possibil- ity, however.

e - . . . m ~ . . . m ~ .

The electron-ion-ion triple coincidence experiment is both feasible and highly attractive. In the simplest form, which is already being done [26], electrons are not energy analysed, but are used as a time reference for measurement of the flight times of two or more time-correlated positive ions. An electronic system of time-to-digital conversion with 'multi-hit ' capability is needed to allow this, with direct computer readout (unless a two-dimensional multichannel analysis system is available). The flight times of both members of a cation pair are measured absolutely: the sum of the two flight times, t l+ t z is independent of any kinetic energy release to first order, and so gives an accurate identification of the reaction channel. The difference t l - t 2 spectrum gives all the same information as is available from conventional PIPICO experiments, while the correlation between t l - t o and t 2 - t °, where the zero su- perscripts indicate thermal ions, gives information on the correlation of the initial moments. The correlations will distinguish between simultaneous charge separation (Coulomb explosions), and sequential charge separation reactions [26].

In a more advanced form of this experiment the electrons would be energy analysed, the object of the experiment being to obtain the spectrum of electrons coincident with cation pairs. Since dissociative double ionization is far more common than formation of stable doubly-charged ions, this experiment may prove even more useful than the m 2 + - e - coincidence measurements already under way.

J.H,D. Eland et al.: Double Photoionization of Small Molecules 35

260

OCS r-C%S +

s++ j/'ll °+

460 860 Time difference/ns

Fig. 2. Demonstration of C + - - O + - S + triple coincidences from triple photoionization of OCS. The upper curve is a partial PIPICO spectrum of OCS showing the correlations of all pairs. The lower curve shows coincidences between two subsequent ions after a first ion has already been registered. Accumulation times were 15 and 180 rain respectively. The shape of PIPICO peaks is explained in Sect. 3

m1 ~ . . .m~ . . .m~ .

Just as doubly charged molecular ions dissociate into pairs of singly-charged cations, so we expect triply-charged molecular ions to dissociate mainly into triplets of cations. The formation of stable triply-charged molecular ions of small molecules is rare, but a few cases are known [27, 28], and large organic species, particularly aromatic compounds, often exhibit triply-charged ions in their mass spectra.

We have now demonstrated the formation of ion triplets in COS at energies higher than 65 eV by using a modified PIPICO experiment. The detection of a first ion generated a gate pulse which allowed subsequent pulses to start the time-to-amplitude converter after an interval corresponding to the difference in flight time between C ÷ and O +. The spectrum of coincidences between subsequent ions and the second ion of the C + - O + pair selected by the choice of gate time interval, is shown in Fig. 2, where it is compared with a normal PIPICO spectrum under the same extraction conditions. The fact that only S+/O ÷ pairs are seen in the triple coincidence spectrum whereas the pairs C+/S + and CS+/O ÷ are most abundant in the normal PIPICO spectrum, proves that real C + - O + - S ÷ coincidences have been recorded. As explained later in the article, the double peaked spectrum is a consequence of a large energy release. The triple coincidence spectrum shows S + and O + ions with more initial kinetic energy than in the PIPICO spectrum: this is expected, because the Coulomb repulsion is greater in a OCS 3 + precursor than in OCS 24.

3. Results

3.1. On the Ionization Process

a. On the Double Ionization Process in Atoms. Xe: We have recently remeasured the yields of Xe ÷, Xe z+ and Xe 3+ ions from photoionization of Xe in the region near the 4d threshold, using selected synchrotron radiation with filters to eliminate higher orders, and with better statistics than in previous work. The results shown in Fig. 3 and 4 may be compared favourably with the results of Hayaishi et el. [29]. We confirm that the major part of the resonance intensity goes into Xe 2÷ formation and concur provisionally with Hayashi et al. [29] that the mechanism is a two-step process via excited states of Xe ÷. We also find, contrary to Hayaishi et al. [29] that the resonances are distinctly stronger than the continuum in the Xe 3 ÷ channel, where we observe a smooth rise, without any steps or any change at the 4da/z threshold, underlying the resonances. Even the first resonance ( 4 d - 6 p ) is clearly visible in the Xe 3 + cross section (Fig. 4). The branching of the resonance contribution into Xe 3+ compared with Xe 2 + rises steadily with increasing excess energy above the Xe a+ ionization threshold; for the first four resonances visible in Fig. 3, the ratios, with excess energies in parentheses are: 0.008 (1.01 eV), 0.029 (2.27 eV), 0.033 (3.0eV) and 0.069 (4.2eV).

12

3 .£3

x 0

o 10

7.5 I

5 c..

I._J

2"5

6t,

~ Tofa[

' A Me2+

66 68 70 Phofon energy/eV

2"8

2"t

-4.

+7 0

0.6

-0.4

0.2

0

Fig. 3. Ionization cross-section curves for Xenon in the region of the 4d threshold

36 J,H.D. Eland et al.: Double Photoionization of Small Molecules

0 -

Photon energy/eV Fig. 4. Detail of the region of the appearance of triply-charged xenon, showing the different branching of the 4d-6p resonance into the doubly and triply ionized channels

These ratios could be fitted to a power law in the excess energy with an exponent between 1.5 and 2. We therefore propose that branching from the 4d- hole resonance of Xe into Xe 3+ is mainly a direct process, with a rate related to the available phase- space, rather than an indirect process as in Xe 2 ÷ formation.

The Xe 3 ÷ threshold used above was determined by extrapolating a plot of the square root of the Xe 3 + intensity outside the peaks, against energy. We find a threshold of 64.1_+0.3 eV in good agreement with the threshold of Dutil and Marmet [30] of 64.35 eV, but in surprising disagreement with the standard compilation [31]. We have also made a measurement of the threshold for Xe 2÷ production (not shown as a figure) and find an appearance potential of 33.05_+0.35 eV, again in agreement with Dutil and Marmet's value (33.11+0.04 eV) [30], but in disagreement with the long extrapolation of Ryd- berg series (31). Although the measurement of thresholds rather than series limits can be falsified by field ionization or by collisional ionization of excited states just below threshold, we estimate that any such effects should be too small under our experimental conditions to affect the results signifi- cantly.

b. On the Double Ionization Process in Molecules. The PIPICO technique and TOF mass spectrometry with synchrotron radiation have allowed the cross- sections for double ionization of several molecules to be measured as functions of wavelength. Yield curves for SO2 [20], CS2 [22], CO2 and COS [32], CH4 [33] have recently been measured by these techniques.

CO ++ • , " . , . . . " ' ' . . , , '

, . . . " . - . . •

<::.i.:::i

40 60 80 Photon energy / eV

Fig. 5. Metastable CO 2+ yield as a function of photon energy

The cross-section curves both for stable states observable as m 2÷ and for dissociative states, show a near-linear onset in accordance with the Wannier law. There are inflexions and maxima at higher energies, however, and these seem to hint at indirect ionization processes. Because many final electronic states of the doubly-charged ions are accessible, at least a complex overlay of linear onsets might be expected. A typical experimental result of this sort is presented in Fig. 5, which shows the yield of CO 2+ from threshold to 100 eV photon energy.

When several electronic states of a singly charged molecular ion lie adjacent to one another, the interaction of discrete levels converging on upper limits with lower continua gives rise to resonances and autoionization. We must surely expect similar phe- nomena in double ionization. As already suggested by Masuoka [34] in a mass spectrometric study of CO with which Fig. 5 should be compared.

More pronounced resonance effects producing m 2+ are found in the vicinity of inner-shell thresholds. Morin et al. [6] have shown that in tetramethyl silane, photoexcited near the silicon 2p edge, an in- tense resonance (core excited neutral molecule) decays very efficiently into double or multiple ionization continua, via a multiple Auger process. Similar observations have been made by Dujardin et al. [35] in CH3I, photoexcited in the vicinity of the I4d edge.

c. Singlet and Triplet Formation in Double Photoioni- zation. The overall process of double photoionization is dominated by the electric dipole interaction, at least the vuv region. Nevertheless, the presence of two unbound electrons in the final state prevents the dipole selection rules from restricting the states of doubly-charged ions populated in the process. In particular, there is no a priori expectation that when closed shell molecules are doubly photoionized the resulting ions, should be predominantly triplets or predominantly singlets, as both are allowed.


÷

exp. fheory

~ 1 E ~ 3 E

~ 1 A

-40 eV I

I1t ---9/t' j t T -35

4o 9&o Time difference/ns

Fig. 6. Evidence that the main dissociation of NH3 z+ occurs from the singlet ground state, based on the assumption that the products are formed in their ground electronic states. The experimental points are represented as error bars, while the solid line is a com- puted simulation for 5.4 eV energy release. The theoretical values are from Tarantelli et al. [54], calibrated on the Auger spectrum of Shaw et al. [55]

Photoion-photoion coincidence experiments, when compared with theoretical calculations and with earlier results from Auger spectroscopy have now demonstrated that in the cases of C02 , COS, CS2 [32], H 2 0 [25] and probably CH4 [36], and C2Hz [37], photoionization strongly populates the triplet ground states of the molecular double cations. This seems to be the most usual behaviour. Similar experiments demonstrate equally clearly, however, that double photoionizat ion of NH3 populates a singlet ground state of N H ~ + [38, 37]. There is also less clear evidence from P I P I C O that higher states are also populated, including both singlets and triplets.

The nature of the P IP IC O evidence in the case of NH3 is indicated in Fig. 6 as an example of the experimental results and of the comparison with theory. I t is not possible at this time to find a justification for the populat ion of triplet states in double photoionization, nor to enunciate a propensity rule. The fact that triplets are populated, in contrast to the restriction of the Auger effect [39] and double charge exchange [40] to singlets, has been very useful, however, in establishing an absolute energy scale by means of which theory and these other experiments can be compared [32, 36].

3.2. On Dissociations of Doubly Charged Ions

In P IP ICO spectra, peak positions identify dissociation channels, peak areas correspond to branching ratios, and peak shapes contain detailed information on kinetic energy releases, angular distributions and rate constants. We now examine in turn recent

Table 1. Examples of simple bond-breakage reactions of doubly charged ions formed by HelI ionization

Molecule Products Branching KE Precursor m 2+ ratio" in eV energy/eV energy

HCN H++CN + 0.95 5.3 38.3 (40) [35] HC~CH H++C2H + 0.56 4.8 35.3 32.7[44] COS CO + +S + 0.94 5.5 33.0 30.0 [48] N20 NO + +N ÷ 0.75 6.3 35.0 36.4 [51]

N~- +O + 0.25 5.5 36.3 CF4 CF + +F + 0.63 5.0 37.2

a Fraction of all pair-forming dissociations

results on these aspects of doubly-charged ion reactions.

a. Dissociation Reaction Channels and Isotope Effects. Competi t ion and isotope effects.

The simplest charge separation reactions are breakages of single bonds producing two charged fragments. These are the dominant decay channels of almost all the small doubly-charged ions investigated so far, examples of which are given in Table 1.

The two reactions quoted for nitrous oxide are of relative abundance 3:1, and raise the question of competi t ion in the decays. It is often assumed as a working hypothesis that each different dissociation reaction of a doubly-charged ion is the result of exci- tation to a different electronic state, and so all such reactions are non-competing 1-20, 21]. Some experimental justification of the assumption may be claimed if the measured thresholds of two reactions are very different, or if the precursor states located by adding the observed kinetic energy releases to the minimum product energies, are widely separated. In the case of the two main reactions of N 2 0 2+ the thresholds have not yet been measured, but the precursor state energies are quite close, so there is less reason to abandon the idea of competing reactions. On general grounds, it can be argued that the repulsion-driven charge separations are likely to be so rapid that no competi t ion is possible, but rather that the wave-packet simply expands irreversible on the repulsive surface into whatever final channels are accessible. Unfortunately, no detailed calculations of such processes have yet been completed.

Where two or more reaction channels are distin- guished only by the presence of different isotopes, the existence of competi t ion is undeniable, so intramolecular isotope effects provide a diagnostic tool for the reaction mechanism. We have recently under- taken the measurement of several intramolecular H / D isotope effects with this objective in mind.

The experimental observations, results of which are presented in Table 2, were relatively straighffor-


Table 2. Intramolecular isotope effects in doubly-charged ion decay after HelI ionization

Molecule Ratio Value Comment

HDO

HDS NH2D

H+ +OD+/D+ +OH + 5.0

H+ +DS+/D+ +HS + 1.8 NHD + + H+/NH~ +D + 1.3

NHD2 ND~ +H+/NHD + +D + t.5

HC~-CD CzD+ +H+/ C2 H+ + D ÷ 2.9

CHsD CH2D + +H+/CH~ +D + 1.7

No stable H202+ exists [25]

H2S 2+ is stable

NH~ + is stable [52], [533

C2H~ + is stable [45]

CH] + stable only outside the FC zone [36]

ward for the stable compounds CHsD and C2HD, but involved measurements of several different isotopic mixtures for the compound HDS, HDO and the ammonia isotopes, which exchange readily. The ratios given are believed to be accurate within 10%, and refer strictly to double ionization by HeII light provided by a filtered lamp.

We note first that in all cases, the heavier isotope remains preferentially in the molecular rather than the atomic fragment. This is the normal direction of the effect, and it could be explained in four possible ways:

(i) The density of states in the products, and likewise in any relevant transition state is higher if the product is deuterated;

(ii) If tunnelling through a barrier is involved, H ÷ will escape much faster than D+;

(iii) The vibrational wavefunction in the molecule before ionization is asymmetric, extending further in the direction of H than of D motion. The initial wavepacket on the m 2+ surface is therefore asymmetric also, and even on a symmetric, repulsive surface its evolution will result in more H cleavage than D cleavage.

(iv) If any barrier to dissociation has to be overcome, its height will be slightly different for the isotopicatly different pathways because of zero-point energy differences.

In thermal reactions of neutral molecules (iv) (zero point energies) is dominant, but we believe that in doubly-charged ion decay its effect is negligible, because the energy difference involved (order of t0 meV) is small compared with the spread of excita- tion energies within the dissociating ions (order of i eV). The relevance of densities of states and tunnelling (i) and (ii) is hard to assess as it depends criti-

cally on the presence or absence of a barrier to dissociation. The existence of a barrier can be inferred where 'stable' m 2 ÷ ions exist (as noted in Table 1), but even so, the dissociative state may lie well above the barrier. In most of the cases investigated hither- to, including those in Table 1, the precursor state energy is higher than the appearance potential of any stable m 2+ ions. We are forced to the conclusion that the asymmetry of the wavepacket, explanation (iii) above, is the only general explanation of the observed isotope effects in doubly-charged ion dissociations. This explanation has another aspect, put forward quantitatively by Fiquet-Fayard et aI. [41]. The motion of the centre of the wavepacket on a surface is classical [42], and on a symmetric repulsive surface does not distinguish isotopes, but the expansion of the packet as it moves is mass dependent. It expands more rapidly towards H loss than D loss, hence in addition to the initial asymmetry of the wavepacket on the surface, which favours H loss, this other mass-dependent spreading effect also produces an effect in the same direction. From the original calculations of Fiquet-Fayard et al. [41] it is already clear that the magnitudes of the isotope effects reported in Table 1 can be understood in this way. Detailed calculations of the relevant surfaces are under way for water [43], with a view to dynam- ical calculations.

In the case of C2H 2+ it is very likely that a barrier exists: there is a stable CzH~ + ion known [44] and semi-empirical quantum mechanical calculations [45] have predicted a barrier height of 2.7 eV for H ÷ loss. The rather large intramolecular isotope effect in this case may, therefore, have its origin partly in tunneling. In the case of H202 +, on the other hand, it is virtually certain that there is no barrier at the energy of the Franck-Condon zone [25]. We therefore attribute the very large intramolecular isotope effect in this case to the asymmetry of the wavepacket and the asymmetric spreading effect.

b. Rearrangement Reactions. Because of the strong coulomb repulsion and the general absence of m 2 + ions we initially assumed that dissociations of doubly-charged ions would be so rapid as to preclude rearrangements, at least in small species. We can now demonstrate that this was an over-simplified view.

Figure 7 shows the PIPICO spectrum of acetylene, demonstrating the reaction:

C H - C H 2+ -~CH + + C +

which occurs to the extent of 6% of all charge separation reactions following HeII ionization. Other rearrangement reactions are collected in Table 3.


t2n

g

C2D2

600V

C2H2

-100 300 700 1100 I

1500 Time of flight diffecence/ns

Fig. 7. PIPICO spectra of acetylene and acetylene-d2, showing the rearrangement reaction. The peaks at large time difference correspond to C - H cleavage, while the peak around zero time represents C - C bond breakage

Table 3. Rearrangements in charge separation reactions of doubly- charged ions formed by HeII ionization

Molecule P roduc t s Branching Comments ratio

HC=CH CH~-+C + 0.06 S02 S ++ O~ 0.03 More abundant than

0 + + 0 + CH4 CH~- + H~- 0.13 Ratio 0.15 in CD4 CH3OH CHO + + H~ 0.10 Origin of H~ in

mass spec. C 6 H 6 CH~ + CsH~ 0.25 Stow reaction:

z > 200 ns

The rearrangement reaction of the benzene double cation has been known for a long time as a slow ( 'metastable ' ) reaction [46]; we have now demonstrated [23] that it actually occurs on a time scale of a few hundred ns, and is only one of a number of similar reactions. The other rearrangements involve only single hydrogen a tom migrations, and are considerably faster.

The existence of the reactions tabulated in triatomic, tetra-atomic and penta-atomic dications demonstrates that at least some parent ions with energies above the lowest dissociation limits live long enough for the a tom transfers to take place, which must be several vibration periods. We do not under- stand why, in contrast to the cases mentioned other similar reactions do not occur, or are very rare. The detailed form of the individual potential surfaces evidently play a controlling role, even when coulomb repulsion is the dominant driving force.

c. Multiple Bond Cleavages. Very many doubly- charged ion dissociations produce unobserved neutral fragments, as well as two singly-charged cations. At relatively low photon energies, such as 40.8 eV (HelI), these pathways are less abundant than simple bond cleavage, but at higher energies they become more important. In some exceptional cases the major process even at low energy is of this type: for instance the major dissociation of SF 2+ after HeI I ionization is

• 2 + SF 6 -+ SF~- + F + + F 2 .

Before these many-body fragmentations can be interpreted in detail it is necessary to ,establish the chemical reaction sequence. Taking SF6 as an example, there are three major possibilities:

( i ) Simultaneous explosion:

SF 2+ ~SF~- -+F+ + F 2 .

(ii) Sequential via SF 2+ :

SF 2 + _+ SF 2 + + F2

SF 2+ --+ SF~- + F v.

(iii) Sequential via SF~-:

+ S F ; + V +

SF + -+ SF2 + F2.

Hitherto it has normally been assumed that reactions of this sort take place either as simultaneous explosions, or via doubly-charged intermediates. Dujardin et al. have recently demonstrated, by the study of energy releases and thresholds, that the formation of H + + I + from CH3 I2+ goes via a singly- charged intermediate route [35]. As already mentioned, the electron-ion-ion triple coincidence technique also makes it possible to distinguish such routes directly [26]; preliminary results indicate that in SF6 the doubly-charged intermediate SF] + is indeed involved.

A parametric model of the possible sequential mechanisms has been provided for the case of linear dissociation by Lablanquie et al. [22]. We have also shown elsewhere [47] that two ion pairs, O+/S + and O+/CS ÷ from OCS 2+ appear at almost the same photon energy; this suggests that the CS + ion observed in the second pair may also partially dissociate, forming the S ÷ observed in the first pair. Such a correspondence of thresholds provides another experimental clue to the occurrence of sequential dis- sodat ion reactions.

d. Kinetic Energy Releases and Kinetic Energy Release Distributions. In the simple P IP ICO method the coincidence spectrum represents intensity versus


arrival time difference in a correlated pair of ions. To first order, if and only if the Wiley and McLaren time-focus conditions are fulfilled [10], the time of flight for an ion is determined by its mass and the component of its initial momentum along the spectrometer axis

t = t o + p°/E cos 0

where E is the accelerating field, t o is the flight time for a thermal ion, p0 is the initial momentum and 0 is the angle from the axis.

In a simple bond cleavage:

A B 2+ ~ A + + B +

the conservation of linear momentum requires pO + p ° = 0 and Oa=lSO+O B. The time difference becomes

A tAB--- tA -- t~ = A t°B + 2p°/E cos 0.

Since 0 can range freely the maximum and minimum values of A tAB, corresponding to 0 = 0 and 180 ° are

(A tAB)m,x = A tOA~ + 2p°/E,

(A taB)rain = A t ° e - 2 p°/E.

The width of W of a PIPICO peak is the difference between these maximum and minimum values, namely

W = A tma x - - A tmi n = 4p°/E.

The total kinetic energy released in the dissociation can be deduced from W because:

0 2 / - - 0 2 . . . . p 0 2 [ MA~ ] Uo=PA / Z M A + p , / Z X I , = - ~ - [ M ~ B ] "

Examples of energy releases deduced in this way have been given in Tables 1, 2 and 3. The usual transformation from CM to LAB coordinates shows that if all initial orientations of the momentum vec- tor are equally likely, and if all ions are detected, a flat-topped PIPICO peak should result from a single-valued energy release.

By placing apertures in the ion flight path or restricting the size of the ion detector, or by using only a weak accelerating field it is possible to discriminate against ions which start with large sideways components of momentum. If such discrimination is so strong that only initial angles 0 near 0 and 180 ° lead to detection, then each cation pair process gives rise to a double peak, the two components being separated, for a single-valued energy release, exactly by the width Wcalcutated above. That this is a realistic model in practice is illustrated in Fig. 8 which shows the two main dissociation processes of N202+. From the gap between the two peaks for each pro-

NO++N + . . . . . . KE= 6.3~V ,o {

.N;_t+ o + ,

I ' 6bo ' a 6 o

,'--"-" FWHH =O,TeV

1000ns

16 10 64 21 0 12 4610 16~

0, /! , _ , . . / % J .....

-I000 6 I0"00 ns Time difference

Fig. 8. Detail of Hell PIPICO spectra of nitrous oxide and oxygen, under conditions of very strong angular discrimination against off- axis ions. Notice the very narrow peaks in the main dissociation reaction of N202+, and the multiple energy releases in oxygen doubly-charged ion decay

cess the energy release can evidently be obtained. If the energy release is not single valued, each wing of the double peak should evidently be structured accordingly. Such multi-valued energy releases have been clearly recognised in a few cases, such as oxygen (Fig. 8). In general each energy release will have a distribution centred on a mean value, and the width and form of the kinetic energy release distribution (KERD) can be deduced from the PIPICO peak shape, but this is not entirely straightforward. The peak wings in a spectrum like the one shown in Fig. 8 are broadened by at least three effects, in addition to the actual spread of energies released. These are the apparatus time resolution, the thermal velocity of the molecules before ionization, and geometri- cal factors determining the acceptance angle, which include the actual size of the ionization volume. When great care is taken to account for all these effects, particularly by examining the PIPICO peak shapes as a function of the electric field strength, widths of the KERDs can sometimes be derived. The dissociation of N202+ has been very thoroughly examined: the most important reaction releases 6.3 eV in a distribution with a width (FWHM) of 0.7 eV.

Once the kinetic energy release distribution is known, the shape of a PIPICO peak taken without angular discrimination reveals the distribution of velocity components along the spectrometer axis, and


S N Q~C. 0++ N 0 +

g

§ ---'i 1 "-KE=B6ev ~ / ÷ ÷ '

0 260 460 600 Time difference / ns

Fig. 9. HeII PIPICO spectrum of nitrogen dioxide at a drawout field of 1000 V/cm. Both the tail towards low times on the main peak and the build-up near zero time are due to slow (50 ns) dissociation, and are designated as metastable, m*, in the Figure

hence gives the angular distribution of dissociation vectors relative to that axis. In work with synchrotron radiation the ionizing light may be polarized either along or perpendicular to the spectrometer axis; hence differences of PIPICO peak shape should reveal any anisotropy in the double ionization transition moment. Results so far on N 2 0 and other molecules have shown only that any anisotropy is relatively small.

e. Dissociation Rates. The existence of 'metastable peaks' in mass spectra corresponding to both charge-separating and charge-retaining fragmentations of doubly charged ions [48] demonstrates that some of these ions have lifetimes in the microsecond range. Dissociation on a time scale comparable with the acceleration time affects the total flight time for an ion because it may be accelerated partly as an undissociated species, and partly as a fragment. In PIPICO experiments the undissociated doubly- charged parent ion either has a mass to charge ratio greater than that of one fragment and less than that of the other, or has the same mass to charge ratio as both fragments. The time-of-flight diffference between two unequal ions of a pair is always reduced by slow decay, however, since the heavier ion is relatively slowed and the lighter one is speeded up. The greater the mass difference between two ions, the easier it is to observe slow decay as a 'tailing' towards low time differences in PIPICO spectra.

The first slow dissociation of a doubly-charged ion searched for in this way by the PIPICO technique [21] was the reaction already discussed:

N 2 O 2 + -+NO + + N +

which was already known from electron-impact mass spectra to have a metastable component [49]. Although an analytical relationship between ion lifetime and PIPICO time difference can be derived in

the absence of kinetic energy release, it is of little use, since kinetic energy release is always significant and cannot be included analytically. In practice lifetimes can be estimated by comparing observed PIPICO peak shapes with computer simulations which include kinetic energy release. This procedure yields a mean lifetime of about 100 ns for the N 2 0 decay, considerably shorter than the mean lifetime determined from electron impact data [49]. In reali- ty, the spread of internal energies probably leads to a range of decay rate constants, from which each technique selects the most measurable value. The second case found was the well-known dissociation of benzene doubly-charged ions:

C6H 2 + - , c s H ; + CH2

which was fitted by a mean lifetime of 200 ns [23]. This is again shorter than would be expected fi'om the intensity of the dectron-impact metastable peak.

A third example is shown in Fig. 9. The PIPICO spectrum of NO2 has not been reported before, so the essential details of the results are included here.

The main dissociation reaction is

NO22+-+NO++O +, Uo=8.6eV.

The large kinetic energy release, if added to the energy of ground state products, indicates a precursor state at 34.6 eV which can be compared with the electron-impact appearance potential of NO 2+ at 35.0 eV [50]. It seems possible that the dissociation occurs from the ground state of NO2 z+ , which should be of 2I/3 symmetry. The existence of rather slow decay may then be connected with the spin- forbidden character of the dissociation, since the product symmetry is 4Z- [48].

We particularly want to thank P. Morin for help with the experiments and for stimulating discussions. JHDE thanks the SERC for a grant to construct the apparatus at Oxford. We all thank the staff of LURE for help and support and G. Dujardin for com- municating his results before publication.

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J.H.D. Eland F.S. Wort Physical Chemistry Laboratory Sourth Parks Road Oxford, OX 1 3 QZ United Kingdom

I. Nenner P. Lablanquie LURE Laboratoire CNRS, CEA et MEN BM. 209c Universit~ de Paris-Sud F-9t405 Orsay C6dex France

mass spectrometric and coincidence studies of double photoionization of small molecules

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