CORE HOLE DECAY AND FRAGMENTATION OF MOLECULES ; ELECTRON-ION COINCIDENCE STUDIES OF N2

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CORE HOLE DECAY AND FRAGMENTATION OF MOLECULES ; ELECTRON-ION COINCIDENCE

STUDIES OF N2

W. Eberhardt, E. Plummer, I.-W. Lyo, R. Murphy, R. Carr, W. Ford

To cite this version:

W. Eberhardt, E. Plummer, I.-W. Lyo, R. Murphy, R. Carr, et al.. CORE HOLE DECAY AND FRAGMENTATION OF MOLECULES ; ELECTRON-ION COINCIDENCE STUDIES OF N2. Journal de Physique Colloques, 1987, 48 (C9), pp.C8-679-C8-692. �10.1051/jphyscol:19879118�.

�jpa-00227226�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n012, Tome 48, decembre 1987

CORE HOLE DECAY AND FRAGMENTATION OF MOLECULES ; ELECTRON-ION COINCIDENCE STUDIES OF N,

W. E B E R H A R D T , E.W. PLUMMER' , I.-W. LYO* , R. MURPHY*, R. C A R R * * and W. K. FORD* *

Exxon Research and Engineering Company, Route 22E, Annandale, NJ 08801, U.S.A.

*university of Pennsylvania, Department of Physics, Philadelphia, PA 19104, U.S.A.

**SSRL, PO Box 4349, Bin 69, Stanford, CA 94305, U.S.A.

* * * Montana State University, Physics Department, Bozeman, MT 59717. U.S.A.

ABSTRACT

Coincidence studies between energy selected electrons and the ions produced in the decay of a core hole excited state in free molecules reveal the individual pathways of soft x-ray induced fragmentation processes. In this type of

measurement it is possible to correlate the ion fragmentation products, their kinetic energy and electronic configuration with the hole configuration in the singly or doubly charged molecular electronic states populated in the electronic decay of the initial core hole excited state. This data produces a new insight into the potential energy curves of the singly and doubly charged molecular ions and into energy dissipation in highly excited molecular systems.

INTRODUCTION

A detailed knowledge and understanding of the interaction between radiation and matter on the molecular level is needed in many areas of science. These areas span from astrophysics, where these processes are relevant in understanding the formation and existence of molecules in outer space and in high a1 titude atmospheres of planets, to radiation damage in biological cell s. In chemistry radiation, and specifically soft x-rays, could potentially be used to produce radicals or to selectively activate bonds within a molecule. Of course this information also serves as a testing ground for theory, where very complex calculations are needed to get the details of the potential energy curves of the highly excited states we are dealing with here. The fragmentation of molecules also involves similar processes as the desorption from solids or solid surfaces induced by electronic excitations. A comparison between desorpt ion from surfaces and fragmentation of the same molecule in gas phase will give insight into charge reneutral i zati on mechanisms, which suppress 1 argel y the desorption on ions.

Absorption of a soft x-ray photon creating a core hole in an isolated molecule leads to a chain of events, the end result of which is usually the production of ionic fragments of the parent molecule [I-41. The initial photoabsorption process leaves the molecule (or molecular ion) in a highly excited state, which decays filling the core hole either by an Auger decay or by fluorescent decay. For a low Z atom like Carbon, Nitrogen, or Oxygen the Auger decay dominates, increasing the charge state of the molecule by at least one.

The Auger decay exchanges the core hole for two holes in the valence levels.

This molecular ion formed by the electronic decay of the core hole does fragment into various products depending upon the specific valence hole configuration.

Previous studies have shown that the ion fragmentation pattern depends upon the site of the core hole in the molecule and the excited electronic configuration of the molecule before the core hole decays [I-41. What has been missing in most of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879118

C9-680 JOURNAL DE PHYSIQUE

these investigations is the measurement of the fragmentation products for each individual hole configuration populated in the electronic decay of the core hole.

These measurements require that the ions be recorded in coincidence with the energy resolved Auger electrons. Coincidence spectra of this kind for gas phase CO have successfully been measured previously [3]. Here we present a complete analysis of the coincidence data for N2, dealing with both the events triggered by ionization of a 1s electron and the subsequent Auger decay as well as the events following N ls+x* excitation and deexcitation.

In some of the previous studies, it was observed that the fragmentation process occurs site specific [1,2,4], i .e. that the molecules fall apart around the nuclear center where the initial core electron excitation occurred. This site specificity was later also renamed "memory effect" [.5]. With one exception noted so far [6], the initial core hole excited state however is a stable

configuration. The origin of the site specific fragmentation is therefore not in the local character of the excitation process, but due to the fact that in the electronic decay the valence wavefunctions are favored which have a large overlap with the core hole excited state. Thus locally around the site where the core hole was initially created bonding electrons are removed from the system and it becomes unstable and falls apart.

We will be dealing with two different types of core electron excitation:

complete ionization and alternately the creation of a neutral core hole excited state by promoting the N 1s electron into the first unoccupied molecular orbital, the lx orbital. Both of these states decay rapidly via a two electron

relaxafion process. In order to clarify the language, we will use the term Auger decay only in connection with the processes following the complete ionization of a core electron in the initial absorption of a soft x-ray photon. The electronic decay of a neutral core hole excited state will be described as deexcitation.

Technically this is an autoionization process, which is described by an Auger type matrix element. The difference to the normal Auger decay is that the initial state is a neutral configuration and the final state a singly ionized configuration of the molecule.

The electronic configurations populated in these decays are substanti a1 ly lower in energy than the core hole excited states. Most of the energy of the core hole excited state is carried away by the emitted electron. Therefore these states also can be excited by lower energy photons or in electron scattering experiments. However, in general, the singly ionized outer valence hole

configurations of the diatomic and triatomic molecules are stable configurations.

Only two hole-one electron and higher excited electronic states are dissociative.

The cross section for direct excitation of these states by photons or electron scattering is rather small and is superimposed on the background of the continuum of the lower enersy sinqle electron ionization events. Therefore it is

advantageous to siudy these fragmentation channels by populating and

characterizin~ the electronic confiquration via Auqer decav or deexcitation of a core hole excited state. In the discussion of our-results-we will obviously include the previous findings connected with a direct excitation into these electronic configurations [7-91.

EXPERIMENTAL DETAILS

The general experimental arrangement, including the differential pumping stages, has been described previously [lo]. The electrons are detected by a commercial cylindrical mirror analyzer (CMA). Opposite the CMA and in line with its axis a TOF spectrometer detects the produced ions. This detector and the coincidence electronics setup are shown schematically in Fig. 1. The photon beam, the gas beam, and the axis of the CMA-TOF spectrometer combination are all at right angles with respect to each other. The intersection of the photon and gas beams is located in the center of the extraction region of the TOF mass spectrometer. The ions produced have large kinetic energies (up to more than 20 eV per particle) such that only ions traveling along trajectories of type d will be detected. All others will be discriminated against by the various apertures.

Thus for each ionic species we expect to observe two peaks centered around the arrival time of an ion of that specific mass but with no, or only thermal, initial kinetic energy. The separation, AT, of the two peaks is proportional to the kinetic energy released in the fragmentation process.

The coincidence experiments are conducted by starting a Time to Digital Converter (TDC) with the electron spectrometer set to detect electrons of a specific core hole decay channel. The time difference between the (fast) electron pulse and the ion pulse yields, as explained, both the mass to charge ratio and kinetic energy of the ions, which are produced in this specific event.

The major experimental difficulty in any coincidence experiment is to

differentiate between true and accidental coincidences. This is made more severe by a synchrotron source which is pulsed, since the accidental coincidences carry the time structure of the source. In these experiments the gas pressure, light intensity and the electronic timing were adjusted to give the best signal to noise. At best the signal to background was better than 20 to 1. Some of the curves shown later have had the machine time structure removed by using a portion of the spectrum containing only accidental coincidences. The di scriminat.ion between true and accidental coincidences is discussed in more detail in Ref. 11.

The quasimonochromatic radiation of energy 1200 eV produced by the 36 pole undulator at SSRL beam 1 ine V was used to ionize a beam of N2 molecules.

Additionally we used the mini undulator at X17T at NSLS, where the fundamental could be tuned to coincidence with the wavelength of the N Is** transition.

Even though we had to reduce the undulator intensity in order to lower the background signal due to accidental coincidences, we have found that an undulator is very helpful if not essential to perform this type of experiment in the soft x-ray region. In our experience, counting rates in the electron channel on bending magnet lines are too low to tune the detection system for optimum performance in the coincidence mode. Typically we ran at a counting rate of several hundred c/sec in the electron channel and about 3*104 c/sec in the ion channel. Under these conditions the typical coincidence rate was between 10-2 and 0.5 (c/sec)

.

REGION

DRIFT REGION

TIME

-

Fig. 1 Schematic view of the coincidence setup. The interaction region is given by the intersection of the photon beam and the gas beam. The electrons are detected on one side of the interaction region by a commercial CMA, whereas the ion mass-to-charge ratio is determined by the time of flight spectrometer.

JOURNAL DE PHYSIQUE

RESULTS AND DISCUSSION

I. The electronic decay of the core hole excited state

The core hole excited state, created by the absorption of a soft x-ray photon, has a lifetime of about 10-14 sec. The decay following N 1s electron ionizaton produces the well known Auger spectrum of N2 [12-131. The assignment of the peaks observed in this spectrum to various valence hole configuration has been discussed extensively in the literature [14]. Here we will just adopt this assignment.

The electron emission spectrum produced in the decay of the Nls core to bound state resonance on the other hand is quite different from the Auger spectrum as shown earlier [15]. The final states populated in this deexcitation spectrum are singly charged states of N2+, which could be single hole states or two hole one electron states compared to the molecular ground state. This deexcitation spectrum is shown in the top part of Fig. 2 on a binding energy scale, where the binding energy EB is defined as EB = hv-Ekin. Ekin is the kinetic energy of the emitted electron and hv is the photon energy. As shown earlier [16] we can get an empirical assignment at least for the single hole final states of the DES spectrum by comparing it to a normal photoemission spectrum on this binding energy scale as done in Fig. 2. From this comparison we can immediately assign the peak at EB 16.7 eV to the lxU-1 (A~II,) single hole state of N2+ and the shoulder at EB

-

15.6 eV to the 3ag-1 (X2xg+) ground state

Fig. 2 Deexcitation spectrum (top) of Np produced in the electronic decay of the N

ls+l?r

(n*) core electron excitation compared to the standard photoemi ssion spectrum on a binding energy scale as explained in the text.

of the molecular ion. The other strong peaks in the DES spectrum at EB = 26 eV, 32 eV and 50 eV have to be assigned to two hole-one electron final states. In making the assignment of these so called spectator lines in the DES spectrum, we use the same principles, which have guided the interpretation of the DES spectrum of N20 [17]. The deexcitation process is governed by the same matrix element as the Auger process, the difference being only that the DES transition is N2* to N2+, whereas the Auger transition is between N2+ and N$+. Therefore we assume that strong Auger lines will also show up as strong lines in the spectator part of the DES spectrum. Table I lists the strongest Auger lines of N2 and their calculated energies and intensities [14]. The corresponding DES 1 ines and the total symmetry derived by coupling the additional lxg electron to the Auger final state is listed also. In this coupling scheme it is taken into account that in DES we should only observe doublet final states for the case of N2.

In order to predict the energy of the spectator peaks, we use a calculation for atomic nitrogen [17], in order to estimate the screening energy due to the presence of the extra lxg electron in the final state. Thus we estimate this contribution to be about 18 eV. Taking the experimental Auger energies and adding the screening energy of the lxg electron, we get an empirical energy estimate for the DES spectator final states. This does not take into account any specific spin and angular momentum coup1 ing of the lxg spectator electron. The values derived from this empirical scheme are listed in Table I also.

Some of the DES final states are identical to X-ray emission final states populated in the DES decay of shake up initial states. Therefore we can also use the energies of some selected XES final states, calculated [18] at the

internuclear distance of core excited N2 as guidance for the DES assignment.

Altogether the assignment as given by the listing in Table I gives quite good agreement with the observed experimental spectrum shown in Fig. 3. We clearly can identify three groups of lines and thus give an assignment for the experimentally observed DES-spectator final states.

Table I

AES States DES States

EB(a) Config. Int.(b) Config. EB (emp) (c) ~ ~ ( c a l c ) (dl

(a) Ref. 13 fb) Ref. 14

(cj Semi -empirically determined binding energy; for explanation see text.

(d) Ref. 18

C9-684 JOURNAL DE PHYSIQUE

I I. Total Energy Conservation

Having established the assignment for the electronic final states populated in the decay of the various core hole excited states, we can now think about the assignment of the fragmentation channels by performing the Auger-electron ion coincidence studies. In this context it is very helpful to consider the whole process in a total energy diagram and to apply the energy conservation principle first. The energy deposited into the molecule by the absorption of the

soft-x-ray photon corresponds to the N(ls)+x* excitation energy (401 eV) in one case and to the binding energy of a N(ls) hole (409.9 eV) in the case of complete core electron ionization. In the latter case any excess energy is carried away by the ejected photoelectron, whereas the first event requires the photon energy to match the excitation energy exactly.

The core hole excited state again decays by the ejection of an electron, which carries away most of the energy. The energy remaining in the system is the binding energy of the Auger or DES final state as defined above. This energy can be determined by measuring the kinetic energy of the emitted electron and

subtracting it from the energy deposited in the molecule in the core hole excitation step. The DES spectrum was plotted on this binding energy in Fig. 2 and Fig. 3 displays our N2 Auger spectrum plotted vs. this binding energy scale.

1 1 1 1 1 ) 1 1 1 , 1 1 1 1 1

-

In

.- - ^{( 0 )} 24,a, -

C -

3

e

_{0 -}

-

_Y ⁵

1

³

^I

> -

V)

-

1

1%

-

1 . , , 1 , , , 1 , , , 1 ,

100 80 60 40

BINDING ENERGY (eV)

( b ) N;+

f--

3

+d;~+j

22'+~1

^N++N+I

>-

C3 a: ^{l q l ' l -}

-

-

- -1

I- 100 80 60 40

BINDING ENERGY (eV)

Fig. 3 (a) The Nls Auger spectrum of N2, plotted as a function of Binding Energy.

The region marked CY can fragment into N+

+

N+, whi 1 e the NZ+

+

N channel up in opens the region. ~ 2 2 + can fragment into NZ+

+

N+ in the 7 region of the Auger spectrum and at 6 it is possible to form ~ 3 + . (b) A plot of the energy thresholds for formation of various ion fragments or mu1 tiply charged states of N2. (c) A plot of the total kinetic energies of the fragmentation products as a function of the binding energy of the molecu- lar ion state. Each diagonal line is for a different set of products.

I n o r d e r t o i l l u s t r a t e t h e meaning o f t h i s reference scale l e t us consider peak

#1 i n t h e Auger spectrum, which occurs a t a b i n d i n g energy o f 43.4 eV. This peak corresponds t o a f i n a l s t a t e c o n f i g u r a t i o n w i t h two holes i n t h e outermost 3ag o r b i t a l and t h e b i n d i n g energy means t h a t i t r e q u i r e s 43.4 eV o f energy t o reach t h i s s t a t e from t h e ground s t a t e o f t h e N2 molecule.

The energy thresholds f o r any s p e c i f i c fragment can be p r e d i c t e d using an a d i a b a t i c energy cycle. The f i r s t step i s t o add t h e energy necessary t o d i s s o c i a t e t h e molecule, then each atom i s i o n i z e d t o a s p e c i f i c charge s t a t e by t h e a d d i t i o n o f t h e a p p r o p r i a t e energy [19]. The energy c a l c u l a t e d using t h i s procedure i s t h e minimum energy t h a t must be added t o t h e molecule t o produce t h e designated i o n i c products. These t h r e s h o l d energies a r e p l o t t e d i n Figure 3b, below t h e Auger spectrum o f N2 shown i n Fig. 3a. F i g u r e 3b a l s o i n c l u d e s t h e t h r e s h o l d energies f o r formation o f t h e doubly and t r i p l y i o n i z e d molecule, f o r example t h e t h r e s h o l d f o r forming ~ 2 2 + i s 42.7 eV [14]. The t r i p l e i o n i z a t i o n p o t e n t i a l was estimated from t h e f i r s t and second i o n i z a t i o n p o t e n t i a l s , by t a k i n g t h e h o l e - h o l e i n t e r a c t i o n i n t o account. N o t i c e t h a t t h e t h r e s h o l d energy f o r t h e p r o d u c t i o n o f N+ t N+ i s 38.8 eV i n d i c a t i n g t h a t t h e lowest energy s t a t e o f N22+ i s u n s t a b l e and p o t e n t i a l l y can fragment i n t o N + [ ~ P ]

+

N + [ ~ P ] w i t h -2 eV k i n e t i c energy p e r N i o n .

The r e l a t i o n s h i p between t h e b i n d i n g energy o f t h e i n i t i a l 2 h o l e s t a t e o f N22+, t h e fragmentation products and t h e i r k i n e t i c energy can be d i s p l a y e d using Fig. 3c. I n t h i s f i g u r e t h e t o t a l k i n e t i c energy o f t h e fragmentation products i s p l o t t e d a g a i n s t t h e Auger e l e c t r o n b i n d i n g energy. Each one o f t h e diagonal l i n e s represents a s p e c i f i c fragmentation channel, i .e. l i n e 1 i s f o r t h e N+

+

^N+

channel w i t h b o t h ions i n t h e 3~ ground s t a t e . Consider as an example t h e s t a t e o f ~ 2 2 + marked as peak #4 i n t h e Auger spectrum o f F i g . 3a. T h i s s t a t e has a b i n d i n g energy o f 49.7 eV and consequently t h e o n l y a v a i l a b l e channel f o r fragmentation i s N+ t N+. The t h r e s h o l d f o r f o r m a t i o n o f NZ+ t N i s 54 eV. I f t h i s molecular i o n s t a t e fragments i n t o two N i o n s i n t h e 3~ ground s t a t e , l i n e

#I o f Fig. 3c shows t h a t each i o n w i l l have a k i n e t i c energy o f -5.5 eV (10.9 eV t o t a l k i n e t i c energy). I n c o n t r a s t i t i s p o s s i b l e t h a t t h e s p e c i f i c s t a t e o f

~ ~ marked as peak #4 does n o t o r can n o t (s i n o r symmetry r e s t r i c t i o n s see 2 + r e f . [20]) decay i n t o t h e ground s t a t e o f N+( P).

S

A new l i n e must be drawn on Fig. 3c s h i f t e d t o t h e r i g h t i n b i n d i n g energy by t h e energy r e q u i r e d t o e x c i t e t h e N ions.

.

For example i f t h e N i o n s were b o t h i n t h e 1S s t a t e then 8.1 eV o f e x c i t a t i o n energy i s r e q u i r e d and t h e fragmentation products f o l l o w i n g t h e decay o f t h e molecular i o n s t a t e #4 would have o n l y -2 eV t o t a l k i n e t i c energy.

The experimental p o i n t s included i n Fig. 3c a c t u a l l y represent some o f t h e fragment channels i d e n t i f i e d by t h e coincidence s t u d i e s as discussed i n t h e f o l l o w i n g . T h i s type o f a n a l y s i s r e v e a l s any i n t e r n a l energy o f t h e fragments, which would otherwise escape d e t e c t i o n i n our experiments.

I 1 I. E l e c t r o n I o n Coincidence Studies; Auger F i n a l States

Fig. 4 shows a s e r i e s o f Auger e l e c t r o n - i o n coincidence s e c t r a f o r Np.

The h i g h r e s o l u t i o n TOF mass/charge spectra f o r t h e r e g i o n o f N!+ [m/e=7] i s shown on t h e l e f t and t h e m/e=14 r e g i o n i s presented on t h e r i g h t . These a r e t h e o n l y two r e g i o n s o f t h e TOF spectra where s i g n a l was observed. When a fragment 1 ik e N+ has k i n e t i c energy t h e TOF spectra w i l l e x h i b i t two peaks as explained above, t h e s h o r t e r t i m e peak i s from t h e ions d i r e c t e d towards t h e d e t e c t o r and t h e l o n g e r time peak i s produced by those ions t h a t were o r i g i n a l l y headed away from t h e d e t e c t o r b u t a r e turned around by t h e e x t r a c t i o n f i e l d . The k i n e t i c energy o f t h e fragmentation products can be determined from t h e t i m e o f f l i g h t spectra u s i n g computer s i m u l a t i o n o f t h e i o n t r a j e c t o r i e s i n t h e TOF

spectrometer. The l e t t e r [A->F] associated w i t h each s e t o f TOF spectra i s used t o l a b e l t h e energy p o s i t i o n i n t h e Auger spectrum a t t h e t o p l e f t o f Fig. 4.

The Auger spectrum a t t h e t o p r i g h t i s l i k e t h e one shown i n Fig. 3a, taken w i t h no e x t r a c t i o n v o l t a g e i n t h e TOF. I n o r d e r t o increase t h e i o n c o l l e c t i o n e f f i c i e n c y i n t h e TOF an e x t r a c t i o n v o l t a g e must be used which degrades t h e e l e c t r o n r e s o l u t i o n , producing t h e t y p e o f Auger spectrum a t t h e t o p l e f t .

JOURNAL DE PHYSIQUE

TIME OF FLlG HT ( p sec)

Fl'g. 4 TOF ion mass spectra of N p taken in coincidence with the Auger electrons at the various kinetic energies indicated by the arrows A through F.

The m/e=7 portion of the TOF spectrum is shown on the left and the m/e =

14 portion on the right. The kinetic energy (in eV) of the fragments is indicated above the strongest peaks. The extraction voltage was +50 eV.

Now let's examine the details of the TOF mass spectra as a function of the binding energy of the Auger electron [or hole configuration]. The TOF spectra marked A were accumulated for a binding energy of -43 eV, corresponding to the highest energy peak in the Auger spectrum [marked 1 in Fig. 3a]. There is some disagreement in the 1 iterature concerning the assignment of this peak 12,14,22],

J

with the two possibilities being the 31&,[3ag-l,lxu-11 or the 1Cg+[3ug- ] states of N$+. Our data shows that the primary product at point A is either N22+ or N+

with no kinetic energy. It is impossible to have N22+ decay into N+

+

N+ with no kinetic energy because of the Coulomb explosion between these two positively charged ions. Therefore our ion-coincidence data in conjunction with the high resolution Auger data proves that the majority of the intensity in this peak comes from a state of N22+ that is trapped in a virtual bound state with its minimum at approximately the same interatomic spacing as Np. This is the

1xg+[3ug-21 state which according to theory has a 2.4 eV potential barrier for dissociation. Apart from having only a very small Auger transition probability, the minimum of the 311U state is displaced such that the transitions occur onto the repulsive part of the potential curve and dissociation occurs 1211. The weak peaks in the TOF spectra A on either side of the main ~ ~ peak are produced by f 2 N+ ions with -4.5 eV kinetic energy. They are most likely a consequence of the poor electron resolution picking up contributions from other hole configurations.

All of the N2+2 species observed are metastable and will eventually decay into two N ions. The metastable N2+2 ions must live long enough to escape from the extraction region into the drift tube (-1 p sec). If the molecular ion fragments in the drift tube the TOF spectrometer will not measure the kinetic energy because the m/e ratio stays the same and the kinetic energy is small compared to the acceleration voltage.

Point B [EB-47 eV] in the Auger spectrum is the region of the most intense emission, and according to the calculation by Agren [14], the majority of the intensity comes from the following confi urations; 46% from 1~ [lxU-21, 27% from

9

lxg+[lnu-21, and 27% from llIu[30g:l, lnu:

1.

The calculated ofential energy

P

curve for the 1~ state is repulsive while both the llIu and

Cg+

states exhibit shallow potentiaq minima for N ~ Z + [21] However again the transitions are located so high on the repulsive branch of these potential energy curves that dissociation has to occur. This is in qualitative agreement with the

experimental observation displayed in panel B of Fig. 2. The central peak in the TOF spectrum is from N22+ due to the energy smearing of the Auger electrons in the ion extraction field while the-two broad peaks on either side are caused by N+ with 4.5

+

0.6 eV kinetic energy. This kinetic energy is plotted on Fig. 3c showing that it is consistent with fragmentation into N+

+

N+ both in 3~ state.

Panel C is the coincidence spectrum taken at a binding energy of -50 eV which averages over the peaks labeled 4 and 5 in Fig. 3a. The data shows that at this point in the Auger spectrum the only fragmentation products are ~ 2 2 + and N+

[with 5.5

+

0.8 eV kinetic energy]. There is a small contribution of N2+ signal seen in the left hand panel of C which again is a consequence of our poor electron energy resolution picking up signal from dee enough in the Auger

E

spectrum to be above the threshold for formation of N

+

N (54 eV). Peak #4 in the Auger spectrum is assigned to the lllg[2uU-1, lxu-11 state of ~ ~ and accord- 2 + ing to theoretical calculations [21] the potential energy curve for this state is repulsive. In contrast peak #5 has been assigned to the 1Cu[2au-1, 3ugyl] state of N22+ [21]. The potential energy curve for this state exhibits a minimum and rather pronounced dissociation barrier. The ion coincidence data at point C is consistent with this picture. The llI state has a binding energy of 49.7 eV so that fragmentation into 2N+ ( 3 ~ ) woul! produce ions with 5.4 eV kinetic energy while we measure 5.5

+

0.8 eV. The observation of a ~ ~ signal in panel C 2 + indicates that the lCu state of N22+ is indeed temporarily trapped in a virtual bound state.

When the energy of the electron analyzer is moved to point D[EB-60 eV] in the Auger spectrum a significant contribution from ~ 2 + is present in the TOF spectra, because the binding energy is above the threshold for formation of N2+

+

N. The low intensity of the peaks in this region of the spectrum ( f l region of

C9-688 JOURNAL DE PHYSIQUE

Fig. 3a) coupled with our poor energy resolution prohibits any quantitative assignment of the configuration of the fragmentation products from our data.

At point E in the Auger spectrum the decay of the hole states marked by the 7 in Fig. 3a are measured. These states correspond to one hole in the 2ag and a second hole in an outer valence state i .e. 2au, InU, or 3ag. It is amazing that the only observed decay channel is N22+ -> N2+

+

^N Conservation of momentum allows the energy of the neutral N to be determined and therefore the total kinetic energy (13.4

+

2.0 eV). Agren [14] has shown theoretically that the -y

region of the N2 Auger spectrum is composed of three main peaks associated with the 1Cg[2a -l,3ag-11 state at 6 7 eV, the 1Cu[2a -l,2au-11 state at 71 eV and the llIu[20 -1,?nU-l] state at 73 eV. The measured iinetic ener y is sufficient to allow ?he lxg state to decay into the ground sate of N+~(?P

8

)

+

N(~sO), but the kinetic energy is too small, even takin the appreciable uncertainty of our

9

measurements into account, for lCU and lIU states. The explanation is quite obvious. These singlet states of N22+ can not fragment into a doublet plus a quartet [20]. There are three decay channels in approximately the correct energy range with the correct electronic and multiplet structure; ~ 2 + [ 2 ~ 0 ]

+

N[~DO] with a threshold energy of 56.3 eV, N2+[2~0]

+

N[~PO] with a threshold energy of 57.4 eV, and ~+2[4P]

+

N[~sO] at 61 eV. The last two channels are slightly more favored by the experimental data than the first possibility.

Region 6 of the Auger spectrum in Fig. 3a and marked by F in Fig. 4 corresponds to 2 holes in the 2ag orbital. It is now energetically possible to form N23+, but this triply charged species can not be formed by a second Auger decay of the states at 6. There is too much energy stored in the remaining 2a hole, so only shake-off in the first Auger process can form N$+. The sum of !he kinetic energies for N2+ and N+ observed at F is plotted on Fig. 3c, which shows that it is consistent with the fragmentation of N23+->N2++N+.

r I t I I I I I I I I I ~

I

-

- -

20 - I

'>*-

I N + ~ ( ~ P ~ ) + N ( ~ P O ) -

L

-

10 - -

5 - -

N'(aP)+N+(3P) I

I

Fig. 5 Potential energy curves for selected states of N22+ created by combining the experimental data presented in this paper with the theoretical calculations of Thul strup and Andersen [21].

Our data for the fragmentation of ~ ~ can be combined with the theoretical 2 + calculations of Thulstrup and Andersen [21] to create a partial set of potential energy curves for different molecular ion states of N$+. Figure 5 displa s

I

these potential energy curves. The potential energy curves shown for the

1

and lCu states [at top of Fig 51 are based totally upon experimental data using ?he facts that the 2og is a strongly bonding orbital, that one of the dissociation products is neutral, and that the total ion kinetic ener y is only consistent

9

with a fragmentation process ending with ~ + 2 ( 2 ~ 0 ) and N( PO). At present we know the excitation energy at the spacing of N2, the energy and configuration of the fragmentation products at large separation, the space of the coulomb repulsive curve at large separation for two charged particles, and which states have a virtual bound state. Combining high resolution Auger spectra with our data will enable the experimentalist to determine the shape and relative position of the virtual bound potenti a1 we1 1 s.

IV. Electron Ion Coincidence Studies; DES Final States

Tuning the fundamental of the NSLS X 17T undulator to coincide with the excitation energy of the N 1s+llrg ( x * ) transition at 401 eV, we create a large number of neutral core electron excited N2 molecules. The width of the undulator fundamental is very large since it has only 10 periods, such that the core ionization is still the predominant event. However, the electron emission spectrum generated in the decay of these core hole excited states as shown in Fig. 6, exhibits several lines (shaded) which originate from the deexcitation of the x* transition. The individual decay spectra for both the x* deexcitation and the Auger decay following N Is ionization have been shown in Figs. 2 and 3.

The ion spectra taken in coincidence with the electron emission at a kinetic energy around 384 eV (A), 375 eV (B), and 363 eV (C) are shown in Fig. 7.

Curve A only shows the presence of N2+, whereas in Curve B N+ and N2+ are observed. Curve C is shown for comparison only. The electronic configurations curve C is taken in coincidence which are a result of the Auger decay and have been discussed in the previous section.

We have to add here that the coincidence measurements are taken with a static ion extraction field applied across the ionization region. Due to the vertical width of the photon beam, which. was restricted by apertures in the coincidence mode, this will cause an energy smearing of the energy distribution curve of the detected electrons. This uncertainty is estimated to be on the order of +3 eV under the present conditions.

I \ c ^I

2000. - ^N2 -

-

%

.

0 B A

- _*

-

'1 1000. - -

C C 0

-

0.0 I

300. 350. 400.

Kinetic Energy (eV)

Fig. 6 Electron kinetic energy distribution for N2 produced in the decay of a core ionized or core excited state (shaded region).

C9-690 JOURNAL DE PHYSIQUE

The e l e c t r o n s e m i t t e d i n r e g i o n A o f t h e spectrum (Fig. 6) correspond t o t h e x2Cg+ and A ~ I I ~ s t a t e s o f N2+. I n a s i n g l e p a r t i c l e n o t a t i o n these s t a t e s are described as a 30 - 1 o r lnu-1 h o l e s t a t e r e s p e c t i v e l y . These are s t a b l e i o n i c c o n f i g u r a t i o n s an8 consequently we o n l y observe N2+ i n t h e coincidence spectrum.

More i n t e r e s t i n g i s r e g i o n B o f t h e e l e c t r o n emission spectrum. Now we are observing both N+ and N2+ i n t h e i o n spectrum. The energy remaining i n t h e molecule a f t e r emission o f an e l e c t r o n w i t h 375 eV k i n e t i c energy i s 26 eV. This i s w e l l below t h e double i o n i z a t i o n p o t e n t i a l o f N2 (42.7 eV). Therefore, even though t h e r e i s a t e c h n i c a l ambiguity i n t h e degeneracy o f N2++ and N+ i n any TOF measurement, t h e peak a t a f l i g h t t i m e o f 5 psec i n spectrum B has t o be

unambiguously assigned as N+. We a l s o want t o add here t h a t t h e N2+ peak observed i n t h e same spectrum i s n o t caused by t h e energy smearing i n o u r e l e c t r o n d e t e c t i o n channel due t o t h e i o n e x t r a c t i o n f i e l d . Otherwise we would have t o observe a peak around 5 psec f l i g h t t i m e i n spectrum A.

0.0 2.5 5.0 7.5

Flight Time (psec)

Fig. 7 TOF i o n s p e c t r a measured i n coincidence w i t h t h e o b s e r v a t i o n o f an e l e c t r o n having a k i n e t i c energy o f 384 eV (A), 375 eV (B), and 363 eV (C).

From the width of N+ peak in Fig. 78 we conclude that the kinetic energy of the N+ fragments is about or less than 1 eV. For comparison, the N+ fragments observed under the conditions of Fig. 7C have a kinetic energy of 4.5 eV as discussed above.

At a binding energy of about 26 eV we are triggering on the first main peak of the spectator DES states. The final states contributing to this peak are the five states 1 isted on top of Table I. The potential energy curves [21,23,24 of these states all exhibit more or less pronounced minima. The lowest energy lIg ?!

state converges to the ~ ( 4 ~ 0 )

+

^N+(~P

4

dissociation channel at 24.29 eV, whereas the other states dissociate into N(~D )

+

^N+(~P) at 26.67 eV. The common feature of these states is that the minima are located at large intermolecular distances compared to the core hole excited state. Therefore the transition occurs high onto the repulsive branch of the curve. Only the c2Cu+ state has a substantial potential energy barrier and a minimum near the distance of the core excited state. Transitions into this state probably account for the observation of a N2+

signal in the coincidence spectra. However vibrational levels v'

>

4 of the

~2

,+

state can dissociate via a potential curve crossing into a 411U state into

C

N( SO) and N+(~P). This transition is induced through the spin-orbit interaction only and therefore has a very small transition rate. However since the c2CU+

state is a long lived configuration the total transition probability is not negligible [7-9,241.

The main peak of the DES spectrum is centered at a binding energy of 26 eV.

This experimental value is about 1 eV lower than the X-ray emission final state calculations [18]. The measured ion energy of 1 eV is consistent with a dissociation channel into ~ ( 4 ~ 0 )

+

N+(~P). On the other hand the ion coincidence peak exhibits a pronounced enhancement for ions with very low kinetic energies.

This has to come, either from a transition into the higher (v' ² 4) vibrational levels of the c2CU+ state or, from direct channels into ~ ( 2 ~ 0 )

+

N+(~P) at 26.67 eV. Due to the smearing of the electron distribution in the ion extraction field we presently cannot distinguish between these two channels.

In coincidence with the second largest peak of the DES spectrum at EB = 32 eV, we only see N+ ions (not shown in Fig. 7). The kinetic energy of these ions has increased to about 3.5 eV, showing that these potential energy curves also connect to one of the two lowest dissociation pathways. From the symmetry of these DES states either dissociation channel is allowed [20].

CONCLUSIONS

Studying the production of ionic fragments in coincidence with the energy selected electrons emitted in the decay of the primary core electron excitation reveals the individual pathways of soft x-ray induced fragmentation in free molecules. We can experimentally determine quite a few details of the potential energy curves of the singly and doubly charged molecular ion and thus gain insight into the involvement of individual valence electrons into the molecular bond.

The next generation experiment will have better energy resolution in the electron channel so that specific electronic and even vibronic states can be resolved. At the same time we will study also ion-ion coincidences in order to verify some of the assignments.

ACKNOWLEDGMENTS

We would like to thank the staff of SSRL and NSLS for their support and encouragement. This experiment is partially supported by NSF under grant No.

NSF-DMR-851919. The SSRL beam line V is funded by NSF grant No. NSF-DMR-8108343 and by ONR under N00014-82-C-0722. Research at SSRL and NSLS is supported by DOE.

C9-692 JOURNAL DE PHYSIQUE

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