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Chemical Physics 315 (2005) 8–16

Light emissions accompanying molecular ionization foundby a new triple coincidence technique

Stuart Taylor *, John H.D. Eland

Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK

Received 17 January 2005; accepted 31 March 2005Available online 27 April 2005

Abstract

A new electron–photon–ion(s) coincidence technique has enabled the discovery of new emissions from photoionization of N2,NO, CO, CO2, CS2, OCS, N2O, SO2 and CF4. For N2O, we see evidence of fluorescence from the doubly charged ion to a disso-ciative state. For CS2, an emission is found in coincidence with the stable doubly charged ion.� 2005 Elsevier B.V. All rights reserved.

Keywords: Coincidence spectroscopy; Photoionization; Fluorescence; Molecules; Spectra; Lifetimes; Mass spectrometry; Double ionization; CS2;N2O

1. Introduction

Fluorescence from molecular ions and their frag-ments is rare when compared with fluorescence fromneutral species. Most examples are from a few groupsof molecules such as diatomics, triatomics, halogenatedbenzene compounds and unsaturated alkyne derivatives.Fluorescence from doubly charged ions is even less com-mon, with N2 [1–4], NO [5,6] and possibly CO [7] beingthe only known examples.

Investigations into ionic emissions were originallydominated by emission spectroscopy using electron im-pact or discharge sources [8]. Photoelectron spectra werehelpful in predicting radiative decay and in particularthe wavelength, thus providing confirmation for the des-ignation of observed electronic transitions. The develop-ment of the photoion-fluorescence coincidencetechnique (PIFCO) [9] allowed quantum yields to bedetermined and allowed fragment emissions to be distin-

0301-0104/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemphys.2005.03.018

* Corresponding author. Tel.: +44 1865 275478; fax: +44 1865275410.

E-mail address: stuart.taylor@chem.ox.ac.uk (S. Taylor).

guished. The major disadvantage was the lack of infor-mation on the electronic and vibrational states of theion involved in the fluorescence. This can be elucidatedby the photoelectron-fluorescence coincidence (PEFCO)technique [10], where energy-analysed electrons are de-tected in coincidence with photons.

In this study, we focus on improving the photon col-lection efficiency of PIFCO, in order to increase sensitiv-ity. A further advance has been the introduction of anelectron detector to be used in coincidence with bothphotons and ions. Measuring the ion time-of-flight rela-tive to electrons, but in coincidence with photons, re-moves the decay curve and leaves a conspicuousnarrow peak. Benefits include an improved signal tonoise ratio, and the ability to resolve otherwise overlap-ping decay curves. The photon arrival times are also re-corded with respect to the electrons and can be used forlifetime analysis. Experiments were performed withVUV light beyond double ionization thresholds. Manynew emissions have been discovered including severalfrom double ionization processes. Limits for quantumyields have been set for those emissions that were notobserved.

S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16 9

2. Experimental

A schematic diagram of the apparatus is shown inFig. 1. Molecules in an effusive jet are ionized byVUV light from an Electron Cyclotron ResonanceLamp. Wavelengths are selected by a grazing incidencemonochromator and refocused by a toroidal mirror. Anelectric field of between 200 and 400 V cm�1 accelerateselectrons towards a channel electron multiplier of10 mm diameter, and ions into a time-of-flight massspectrometer. Collection efficiencies for electrons wereabout 40% and for ions, about 15%. Light was gatheredusing a polished aluminium ellipsoidal mirror, with twofocal points coinciding with the light-gas intersectionand the photomultiplier window. A photon collectionefficiency of 6.5% was obtained, with most lossesthought to arise from the finite ionization volume, andfrom light scattering due to surface roughness. TheHamamatsu R585 photomultiplier has a relatively flatquantum efficiency of about 10–13%, in the wavelengthrange of 200–500 nm. The drop in quantum efficiency ismore gradual at longer wavelengths than at shorterwavelengths.

During each experimental run, six data files weresimultaneously produced to record different coincidencecombinations:

1. Electron–ion12. Electron–ion1–ion23. Photon–ion14. Photon–ion1–ion25. Electron–photon–ion16. Electron–photon–ion1–ion2

The detection of an electron, or if absent, a photon, actsas a start signal for each coincidence set. If an electron isthe start, then the photon is required to arrive within500 ns, if it is to be registered. All ions detected withina preset period of a few microseconds, are recorded as atime-of-flight spectrum measured relative to the startsignal.

Fig. 1. Experimental set-up.

3. Data treatment

Correlated particles are those that originate from thesame ionization event. When detected in coincidence, atime-of-flight will be recorded that corresponds to a�real� peak. In addition, accidental coincidences betweenuncorrelated particles are also detected and form abackground.

3.1. Electron–ion and photon–ion coincidence data

In a mass spectrum, false coincidences may be re-corded when one photoionization event (Event 1) is fol-lowed rapidly by another photoionization event (Event2). The detection of a �true� or �false� coincidence de-pends on whether an electron is detected in coincidencewith a correlated or non-correlated ion, respectively.Since the particles detected in a false coincidence arenot correlated, they form a background that is generallyflat. However, the probability of recording a false coin-cidence before (Coincidence A) and after (CoincidenceB) the peak corresponding the ion from Event 1, areslightly different. This is because when one electron isdetected, no others will be registered for a predeter-mined time window of several microseconds.

Event 1 E1-------------------I1

Event 2 E2--------------------I2

Time !

Coincidence A ¼ E2 þ I1

Coincidence B ¼ E1 þ I2

Note: If E1 + I1 or E2 + I2 are detected, a �true� coinci-dence will be recorded at a time interval correspondingto a real peak. If both ions are detected then the coinci-dence will be recorded in the ion pair spectrum.

The probabilities of coincidences A and B are:

A ¼ Pðnot E1Þ � PðE2Þ � PðI1Þ � Pðnot I2ÞB ¼ PðE1Þ � Pðnot I1Þ � PðI2Þ

Coincidence A requires that that E1 is not detectedand so has a lower probability of occurring than Coin-cidence B. The background therefore increases in a step-wise manner after every peak, by an amount that isroughly proportional to the area of the peak. This anal-ysis also applies to photon–ion spectra. However, thestepwise increase in background is not observed becausethe probability of not detecting a preceding photon is�1.

3.2. Electron–ion (+ photon) spectra

This type of coincidence is displayed as a mass spec-trum, with the requisite that a photon arrived within500 ns of the electron. There are four types of false

10 S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16

coincidence contributing to the background, as de-scribed below. The subscripts indicate whether or notthe particles are correlated.

(1) e1 + hm2 + I3All particles are uncorrelated, so a flat backgroundis produced. The contribution of this type of falsecoincidence is negligible.

(2) e1 + hm1 + I2Ions are uncorrelated with electrons so a flat back-ground is produced. This is analogous to the back-ground produced in electron–ion and photon–ionspectra (Section 3.1) with the start represented bytwo correlated particles. In this case, the probabil-ity of not detecting e1 + hm1 is �1, so a stepwiseincrease in background is not observed. Often itis useful to view the time differences between elec-trons and photons for a particular mass peak. Thistype of false coincidence will contribute to thedecay curve as a weighted average of all detectedion fluorescence processes. It is thus necessary tosubtract the decay curve arising from the flatbackground.

(3) e1 + hm2 + I1This type of false coincidence appears as a scaleddown mass spectrum. Subtraction of this back-ground is particularly important because it canlead to �false� peaks. Fortunately, because the massspectrum is collected simultaneously, only a mea-surement of the average photon detection rate isrequired to normalise the mass spectrumsubtraction.

(4) e1 + hm2 + I2Some correlation between the ions and electrons isretained because of the requirement that the pho-ton arrives within 500 ns of the electron. The effectis to replicate a scaled down photon–ion spectrumwith a 500 ns broadening. This will be further com-plicated by the non-uniform competition betweene1 and e2 as the photon–ion peak is traversed.The effect is less important than (3) because sharpfalse peaks are not created. Since its accurate sub-traction requires a detailed knowledge of collec-tion efficiencies and lifetimes, it is not attempted.

The most important contributor to the background isfrom (2), because all other false coincidences require aphoton to arrive within a short time period of an uncor-related electron. However, when there are no strongemissions or the ion count rate is low, the abundanceof even this type of false coincidence is much lower thanthose between photons and ions in PIFCO experiments.This is because in this circumstance, the majority of reg-istered photons are due to the dark count of the photo-multiplier. Under triple coincidence conditions, thesephotons are very unlikely to be coincident with electrons

and so are of negligible importance. The sensitivity ofour apparatus therefore increases as the intensity offluorescence decreases.

In the two-dimensional spectra of ion pairs, the rela-tive contribution of background towards a peak is muchlower than for single ions. For quantum yield calcula-tions, only subtractions due to �false� peaks are made.

4. Results and discussion

Lifetime analysis is performed when justified by thestatistics. Accurate measurements of short lifetimes arelimited by a minimum spread for electron–photon timedifferences of about 10 ns (FWHM). The limiting factorfor long lifetime measurement depends on whether theemitting species is charged. Ions are rapidly acceleratedby the electric field away from the focal point of theellipsoid. Since this region of sensitive photon detectionis small, a strong discrimination against the exponentialtail is seen for lifetimes longer than about 100 ns. Theresidence time for neutral fragments is much longer, sothe 500 ns photon detection period becomes thelimitation.

The discrimination effect can often be useful for distin-guishing between neutral and ionic emissions.However, itcomplicates the lifetime analysis and quantumyield deter-mination. For this reason, it was quantified by comparingthe observed decay of N2O

+, against the known lifetime.The N2O

+ emission is a good candidate because it mainlyinvolves a single vibrational level from the A state [11].The lifetime (245 ns) is also of an appropriate length to ob-serve the majority of the decay within 500 ns, but stillshow a strong discrimination. Where appropriate, simu-lations are performed to match proposed lifetimes withexperimental data, and to calculate discrimination factorsthat are used for quantum yield determination. The reli-ability of this method is strongly dependant on the statis-tics, and also steadily deteriorates for lifetimes above afew hundred nanoseconds. The model successfullypredicted decay curves for Nþ

2 and COþ2 .

The quantum yield (/tot) is defined as the fraction ofionization events (Itot) leading to each fluorescent prod-uct, where the product is defined by the one or more ionsproduced. However, its value is affected by the non-uni-form ion and electron collection efficiencies, which willbe important for the high photon energies used in thisstudy. We therefore also quote yields (/f) that describethe fraction of each ionic product that is coincident withfluorescence. /f is determined for all processes where theproduct is unambiguous, which is the case for all parention emissions and all ion pair emissions. However, sincethe ion collection efficiency is well below unity, themajority of ion pairs from double photoionization aredetected as single ions. Therefore, to calculate /f forsingle ions often requires a subtraction of this

Table 1

Quantum Yields for ions coincident with photons

Molecule k (nm) Fragment /f (%) /tot (%)

N2 30.4 Nþ2a 16.23 7.70

N+ 0.20 0.11

NO 25.6 NO+ 0.133 0.072

O+ 0.202

N+ 0.335 0.126

NO2+ 0.753 0.026

CO 30.4 CO+ 13.27 5.01

O+ 0.36

C+ 0.05

25.6 CO+ 11.93 3.18

CO2+ <0.086 <0.00077

CO2 25.6 COþ2b 43.345 23.573

CO+ 14.283 0.695

O+ 1.250 0.093

CO+ + O+ 1.026 0.159

CO2þ2 <0.191 <0.003

OCS 30.4 OCS+ 0.913 0.030

CS+ 5.581 0.167

S+ 0.709 0.276

CO+ 5.953 0.534

CO+ + S+ 1.079 0.324

OCS2+ <0.067 <0.002

25.6 OCS+ 0.581 0.025

CS+ 7.855 0.076

S+ 1.175 0.429

CO+ 8.063 0.661

CO+ + S+ 1.482 0.443

CS+ + O+ 1.336 0.033

OCS2+ <0.163 <0.004

SO2 58.4 SOþ2 <0.0016 <0.00067

SO+c 0.60 0.35

25.6 SO+c 0.437 0.145

S+ 0.136

SO+ + O+c 0.213 0.006

N2O 58.4 N2O+d 48.7 10.63

30.4 N2O+d 39.15 5.78

Nþ2 15.20 2.032

NO+ + N+ 0.570 0.062

N2O2+ <0.005

25.6 N2O+c 36.04 4.887

Nþ2 15.829 1.361

NO+ + N+ 0.792 0.144

Nþ2 þOþ 4.462 0.397

N2O2+ <0.0013

CS2 25.6 CSþ2 8.49 1.13

CS+ 1.93 0.23

S+ 1.45 0.24

C+ 1.77 0.05

CS+ + S+ 1.61 0.52

CS2þ2 1.65 0.11

CF4 25.6 CFþ3 11.85 8.29

CFþ2 2.41 0.09

F+ 22.72 0.04

The fragment column refers to the ion(s) detected in the mass spectrometer and

is not necessarily the emitting species. /f and /tot are defined early in Section 4.

< Upper limit for emission.a Discrimination factor = 0.964, based on lifetime of 60 ns.b Discrimination factor = 0.888, based on lifetime of 130 ns.c Discrimination factor = 0.394, based on lifetime of 2400 ns.d Discrimination factor = 0.728, based on lifetime of 245 ns.

S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16 11

contamination. This subtraction, with correction for thereduced collection efficiency of ion pairs, also needs tobe applied to Itot to enable the determination of /tot.

In mass spectra recording single ion arrivals, the dif-fering kinetic energy releases sometimes allow single ionsand ion pairs to be distinguished in electron–ion spectra.The number of single ionization counts is thus extractedwhen possible. If the ion pair process is coincident withphotons, then a subtraction on the electron–photon–ioncoincidences is performed using the contamination ratioobtained from the mass spectrum (where the statisticsare far better). Therefore, for some fragment-photoncoincidences with ion pair contamination, /f is tenta-tively given. A calculation of Itot is required for any sub-sequent calculation of /tot, but the ion paircontamination cannot always be quantified. An estimateof the contamination is therefore made, based on thenumber of ion pairs and a typical ion collection effi-ciency of 0.15. This treatment is considered good en-ough for the determination of Itot and thus values of/tot, but not good enough for the determination of /f

for the corresponding fragment. Values of /tot and /f

are listed in Table 1.Photon collection efficiencies were calibrated by tests

on N2 at 58.4 nm, for which the fraction of the emittingB2Rþ

u is known [12]. The wavelength for this emissioncoincides with the maximum for the photomultipliersensitivity (�400 nm), so quantum yields for emissionswith wavelengths significantly different from Nþ

2 willbe underestimated. Further underestimation occurs inmost cases, due to an unquantifiable discrimination fac-tor. Our quantum yields should therefore be consideredas minimum values. For doubly charged ions where noemission was observed, maximum values are given thatcorrespond to a signal-to-noise ratio of 2. The validityof these limits is based on an assumption that the hypo-thetical lifetime is short enough to exclude any discrim-ination effect.

4.1. Nitric oxide (NO)

The electron–photon–ion spectrum shown in Fig. 2,illustrates known emissions from NO+ and NO2+, andadditional emissions coincident with the two ionic frag-ments. Of the two known emitting states from NO2+

(B2R+ [5] and A2P [6]), only the �B� state should be de-tected by our photomultiplier due to the long wave-length of the emission from the �A� state. The lifetimeis measured to be 13 ± 5 ns, significantly shorter thanthe calculated radiative lifetime of 45 ns (v = 0) [13]. Ifit is assumed that the fluorescence is dominated by thev = 0 level, then the lifetime for competitive predissocia-tion can be estimated to be 18 ns. Based on the relativepopulations of the v = 0 B2R+ state to the quasistableX2R+ and A2P states, and the relative rates of radiativedecay and predissociation, /f can be estimated to be

1.3% for NO2+. This is somewhat larger than our mea-sured value (0.75%), but a lot smaller than that of Bes-nard et al. [5] (about 4.5% at our ionizing wavelength).

Fig. 2. Electron–ion time difference (+hm) spectrum of NO at 25.6 nm.

12 S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16

Our underestimation can be explained in part by thedrop in photomultiplier sensitivity at the wavelengthof this emission.

The N+ peak has a lifetime of around 200 ns. The O+

peak comprises at least three distinct lifetimes. A goodfit was produced with a simulation of the following threelifetimes:

s1 ¼ 10 ns ð20%Þ; s2 ¼ 60 ns ð40%Þ;s3 ¼ 1000 ns ð40%Þ.

These fragment emissions can be either from the ion orfrom the neutral partner fragment. A detectable emis-sion from N+ [14,15] or O+ [16] requires far more inter-nal energy than an emission their corresponding partnerfragment, but both are possible at this photon energy.

Fig. 3. Electron–ion time difference (+hm) spectrum of CO2 at 25.6 nm.

4.2. Carbon monoxide (CO)

In addition to the known CO+ emission we find coin-cidences for both C+ and O+ at both 30.4 and 25.6 nm.At both wavelengths, the decay curves for O+ coinci-dences were simulated well using two lifetimes of 10and 100 ns in a 1:1 ratio. A lifetime of a few hundrednanoseconds was observed for C+, with an additionalshort lifetime (<10 ns) at 25.6 nm, limited to low kineticenergy fragments. A possible candidate for this shortlifetime is the persistent C+ 2F (2s24f)–2D (2s23d) emis-sion, whose threshold is at 43.4 eV. It could be accessedfrom a state of CO+ seen at 45.6 eV in our own unpub-lished data.

Peaks in the emission spectra of CO have been attrib-uted to the 13R+–X3P (0,0) (0,1) (0,2) transitions ofCO2+ [7]. They involve only the lowest rotational levelof the emitting state, which was excited in a supersonicexpansion where the low rotational levels are preferen-tially populated. The v = 0 and v = 1 levels of the X3Pstate do not dissociate on our mass spectrometer time-scale (unlike v = 2) [17] and transitions to them shouldcorrespond to 62% of the total emission intensityaccording to intensity factors calculated in [7]. If as sug-

gested, no other states significantly contribute to thefluorescence process, our upper limit for CO2+–photoncoincidences ((/tot) = 0.00077%) can be modifiedaccordingly to give (0.00077% · 1/0.62 = 0.0012%) as amaximum quantum yield for CO2+ fluorescence under25.6 nm excitation at room temperature.

4.3. Carbon dioxide (CO2)

Photons in coincidence with the ionic fragments ofCO2 are shown in Fig. 3. CO+–photon coincidencesare known from CO2 [18]. Our data show at least twodifferent lifetimes at 30.4 and 25.6 nm, consistent withthose for the A2P and B2R states of CO+. For the longerlifetime we see a discrimination against the exponentialtail that should only occur for ionic emissions. The shortlifetime emission is the minor contributor at both wave-lengths, although by a less extent at 25.6 nm.

New emissions are observed in coincidence with O+

at both wavelengths and with CO+ + O+ at 25.6 nm.The O+ peak involves one short lifetime (<10 ns) anda very long one with no discrimination against the expo-nential tail. The long lifetime of the ion pair emission isconsistent with the A2P state of CO+. Direct evidence ofa CO+ fragment emission is seen in the two dimensionaldecay (Fig. 4(a)). Counts recorded for �slow� photonemission (a large time difference between photoioniza-tion and fluorescence), reveal a discrimination againstdissociations where the CO+ ion is accelerated towardsthe ion detector. More simply, the sensitivity towards�slow� photon emission correlates with the residence timeof the CO+ fragment.

4.4. Carbonyl sulphide (OCS)

Apart from SO+, which is formed in very low abun-dance, all singly charged ions are found in coincidencewith photons at both 30.4 and 25.6 nm (Fig. 5). At30.4 nm, photons are found in coincidence withCO+ + S+ and at 25.6 nm, with O+ + CS+ also. Both

Fig. 4. Electron–photon–ion–ion coincidences displayed as photon–ion–ion coincidences: (a) from CO2 ionized at 25.6 nm; (b) from SO2 ionized at25.6 nm; (c) and (d) from N2O ionized at 25.6 nm.

Fig. 5. Electron–ion time difference (+hm) spectrum of OCS at 25.6nm.

Fig. 6. Photon–ion time difference spectrum of SO2 at 25.6 nm.

S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16 13

lifetimes are long, and are consistent with molecularfragment emission.

The weak parent ion emission is due to predissocia-tion of all, but the v = 0 level of the A2P state. TheC+ peak involves a lifetime of a few tens of nanosec-onds, plus at 25.6 nm, a short lifetime of <10 ns. ForO+, a similar short lifetime is observed plus a very longone. The CO+ decay curve can be accounted for by CO+

fluorescence alone, where �B� state emission occurs onlyat 25.6 nm. The S+ peak involves at least two lifetimes inaddition to the ion pair process. The overall shape isvery similar to that seen for O+ from CO2 where themain partner fragment is also CO.

Interestingly, Field and Eland [18], who did observeCO+ and CS+–photon coincidences, did not see the S+

peak. The prominence of peaks in photon–ion spectradepends on the lifetime. In our photon–ion spectrum,

we still observe the S+ peak strongly, since it is associ-ated in part with a short lifetime. We therefore speculatethat the emission responsible for the short lifetime is be-low their minimum detectable wavelength of 250 nm.

4.5. Sulphur dioxide (SO2)

At 25.6 nm, emissions are observed in coincidencewith O+, S+, SO+ (Fig. 6) and O+ + SO+. A weak coin-cidence signal is observed for SOþ

2 , as had been reportedbefore by Dujardin and Leach [19]. However, we hadalso observed coincidences of a similar weak magnitudefrom ions whose emission was not expectedðNHþ

3 ; CH3CNþ and Cþ þOþÞ. In these cases, the

unusual electron–photon time distribution and contra-dictory evidence from the literature had led us to dismissthese results as an experimental artefact. The cause isstill unknown, but our observations show that artificial

14 S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16

photons are being registered in coincidence with about 1in 106 electrons, equivalent to /f = 1–3 · 10�4. In anexperiment performed at 58.4 nm, with the electrondetector turned off, the SOþ

2 emission was not observedand we set an upper limit of /tot = 6.7 · 10�6 assuminga lifetime <10 ns. This is about a third of the valuequoted by Dujardin and Leach (2–2.2 · 10�5) for theirpositive result. The difference between our yields forthe SO+ emission (/tot = 3.5 · 10�3 [this paper] and6 · 10�3 [19]) suggests some systematic error in thequantum yield determination. Scaling our resultsaccordingly though, still sets our maximum yield ofSOþ

2 fluorescence at 0.55 of the value quoted by Dujar-din and Leach [19]. To our knowledge, no other confir-mation for the SOþ

2 emission exists, but it seems unlikelythat it could have been an artificial result, because it wasonly seen in a particular spectral region [19].

The known SO+ emission [19,20] involves a transitionfrom the A2P state to the X2P ground state. Due to thelong lifetime of about 2.4 ls [19], the SO+ peak in Fig. 6,illustrates the discrimination effect seen for �slow� ionicemissions.

The photon–ion–ion spectrum in Fig. 4(b) shows�slow� photons to be favoured by a long SO+ residencetime, thus indicating an emission from the molecularion. For the S+ and O+ peaks, short (<10 ns) and longneutral lifetimes are observed. In contrast to the S+

peak, Field and Eland did not observe the O+ peak, soagain we suggest that k < 250 nm for the short lifetimeemission.

4.6. Nitrous oxide (N2O)

The electron–ion–photon spectrum of N2O (Fig. 7),shows a strong Nþ

2 –photon coincidence. The A2P–X2Remission of Nþ

2 is ruled out by its long wavelength.Detection of the B2R–X2R emission is likely, but the

Fig. 7. Electron–ion time difference (+hm) spectrum of N2O at25.6 nm.

electron–photon decay is not described well by a singlelifetime of 60 ns [21] (Fig. 8). The logarithmic plot showsat least one additional, considerably longer lifetime. Wehypothesize that the observation is a predominantly Nþ

2

�B� state emission, but with significant contributionsfrom vibrational levels outside the Franck–Condon zoneof the N2 molecule.

Coincidences with NO+ are also seen at this wave-length, but are attributed to a double ionization processexplained later. Similarly, N+ and O+ coincidences alsorequire consideration of ion pair emissions for theirexplanation.

A new emission in coincidence with Nþ2 þOþ is ob-

served at 25.6 nm (Fig. 9). The decay curve is consistentwith that of Nþ

2 in the singles spectrum, suggesting emis-sion from the ionic fragment. This is supported by thephoton–ion–ion decay (Fig. 4(c)). The measuring andstatistical errors are large enough to include the possibil-ity that all O+ ions in the single ion–photon spectrum at25.6 nm, are due to this process. However, a long life-time, not consistent with ionic emission, is alsoobserved.

The discovery of an emission in coincidence withNO+ + N+ is more surprising, bearing in mind the ab-sence of NO+ + N-photon coincidences (the number ofNO+–photon coincidences that are seen in the single-ion spectrum, are no more than would be expected fromion pair contamination). The short wavelength of theNO+ emission is at the limit of our photomultiplier sen-sitive range, and is thus detected only weakly, as seenfrom NO photoionization (/f = 0.13%). Emission fromthe N+ fragment can be ruled out by a lack of availableenergy.

Fig. 9 shows the ion pair peak, with (a) and without(b) photons. It is clear that the dissociation of metasta-ble N2O

2+ ions to N+ + NO+ contributes a dispropor-tionate amount to the fluorescent process. Whenviewing the ion pairs with respect to photons (Fig.

Fig. 8. (a) A logarithmic plot of the Nþ2 electron–photon coincidences

taken at 30.4 nm. (b) The same decay curve with a superimposedsimulated decay of a 60 ns lifetime.

Fig. 9. (a) Quadruple coincidences of E–I–I + photons. Crossesillustrate average arrival times for ionic fragments in mass spectra.(b) Electron–ion–ion coincidences of N2O at ionized at 25.6 nm.

Fig. 10. Electron–ion time difference (+hm) spectrum of CS2 at 30.4nm.

S. Taylor, J.H.D. Eland / Chemical Physics 315 (2005) 8–16 15

4(d)), an unusual shape is seen that we interpret as a cor-relation between the time of dissociation and photonemission. Our conclusion is that the emission is froman excited state of N2O

2+ to a dissociative state. Theaverage photon emission time of 123 ns is consideredan underestimate for the lifetime because of the appara-tus discrimination. This process was observed weakly,and possibly ambiguously, at 30.4 nm excitation.

The N+ peak in the single ion spectrum includes avery long (neutral) lifetime and at 25.6 nm an additionalshort lifetime.

4.7. Carbon disulphide (CS2)

Known emissions coincident with S+ and CS+ frag-ments [18] are observed (Fig. 10). For CS+, the emissionlifetime is consistent with that of the CS+ emission fromOCS. In a new ion pair emission involving CS+ and S+,the same lifetime is found, so CS+ is assumed to be theemitter. For S+, a short and long lifetime component areobserved; the overall appearance is very similar to theO+ peak from OCS, suggesting a dominant contributionfrom the partner fragment in both cases. For C+, onelifetime of below 10 ns and another between 50 and100 ns are observed.

The most interesting result from CS2 is the observa-tion of a new emission from CS2þ

2 . At 30.4 nm, the life-time was measured as 400�200

100 ns.

4.8. Carbon tetrafluoride (CF4)

Known CFþ3 –photon coincidences are observed and

are due to fluorescence from CFþ4 followed by dissocia-

tion [22]. Much weaker coincidences are found forCFþ

2 and Fþ. The former has a very short lifetime whilethe F+ peak has a lifetime of about 20 ns.

5. Conclusion

The efficient photon detection of our apparatus hasenabled a very sensitive PIFCO experiment to be devel-oped. The large angular acceptance for photons also al-lowed the introduction of an electron detector, enablingtriple or quadruple coincidence data to be obtained.This had the advantage of allowing all photons to beattributed to the appropriate ion, and allowed photonarrival times to be measured with respect to the nar-rowly spread electron times. A further advantage wasin sensitivity, due to the elimination of broad fluores-cence curves and a reduction in background.

We have observed photons in coincidence with newfragments from most molecules tested. We also see forthe first time, examples of fragment emissions fromion pairs and see evidence for emissions from two newdoubly charged ions.

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