Temporary Negative Ion States in Hydrocarbons and Their Derivatives

9 Temporary Negative I o n States i n H y d r o c a r b o n s a n d T h e i r Derivatives 1

2

K. D. JORDAN and P. D. BURROW 1

Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260 Department of Physics and Astronomy, University of Nebraska, Lincoln, NB 68588

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2

Electron scattering experiments, in particular elec tron transmission spectroscopy (ETS), have provided a wealth of data on the temporary negative ion states of polyatomic molecules. Following brief introduc tions to the transmission technique and to the char acteristics of resonances, we examine the spectra of several "simple" unsaturated hydrocarbons and discuss shape resonances arising from the temporary occupation of π* orbitals and, in the case of ethylene, the Feshbach resonances associated with Rydberg orbitals. The importance of long-range interactions in anion states is illustrated with data in several non -conjugated dienes. The classification of higher lying resonances in benzene is discussed with regard to their "shape" and "core-excited" characteristics. Finally we examine resonances associated with σ* orbitals and the decay of such states in the disso ciative attachment channel. Evidence for resonances in the cross sections for electron scattering from polyatomic molecules, including hydrocarbons, can be found in the literature as far back as the late 1920's (1,2). The authors of these papers, however, were unaware that the pronounced low energy peaks in the cross sections of molecules such as ethylene and acety lene were due to temporary negative ion formation. Haas (3), in 1957, was apparently the first to observe that strong vibrational excitation accompanied such a peak, and to invoke an unstable nega tive ion complex as the means through which the excitation takes place. Since the renaissance of electron scattering studies beginning in the late 1950 s, temporary anion states have been observed in all atoms and virtually all small molecules which have been examined. For the most part, such studies were carried out by physicists who focused their attention on atoms, diatomics and a few selected t r i atomic molecules. Much of this work concentrated on achieving a detailed understanding of the decay modes of temporary anions into f

0097-6156/ 84/0263-0165$06.00/ 0 © 1984 American Chemical Society

Truhlar; Resonances ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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the available channels, i n particular the angular scattering d i s t r i bution, excitation of v i b r a t i o n a l l e v e l s and electronic states, and fragmentation by d i s s o c i a t i v e attachment (4). Beginning i n 1965, a number of groups i n i t i a t e d studies of tem porary anion formation i n hydrocarbons (5-8), with extensive c o n t r i butions from the groups of Compton (9) and Christophorou (10) at Oak Ridge. Except for the work of Hasted and coworkers (7), these stud ies r e l i e d primarily on the trapped-electron (11) or SFg scavenger methods (12) for the detection of temporary negative ion states. Both of these methods are sensitive to the production of slow elec trons, t y p i c a l l y those with energies less than 100 meV, which are produced just above the threshold for an i n e l a s t i c process. For the observation of temporary anions, these data present some problems of interpretation. To y i e l d a s i g n a l , resonances l y i n g below the elec t r o n i c a l l y excited states of a molecule must decay strongly into highly excited v i b r a t i o n a l levels of the ground state of the neutral molecule which are nearly coincident with the resonance. The tech nique i s therefore l e s s sensitive to resonances of short l i f e t i m e s since these decay with smaller probability into the required high v i b r a t i o n a l l e v e l s . Higher l y i n g resonances, furthermore, may be confused with or overlap peaks a r i s i n g from the excitation of e l e c tronic states of the neutral molecule. F i n a l l y , l i t t l e information about the p r o f i l e s of the resonances i s derivable with these methods, and the energy resolution i s not generally s u f f i c i e n t to provide information on the v i b r a t i o n a l structure of the resonances. Despite reservations concerning the use of these methods to locate and characterize resonances i n complex molecules, they pro vided ample evidence for temporary negative ions i n a great variety of hydrocarbons. It must be said, though, that i n comparison to the widespread adoption of photoelectron spectroscopy (PES) by chemists during t h i s same period of time, the work on resonances appears to have made rather l i t t l e impact on the chemical community as a whole, and surprisingly l i t t l e work was i n i t i a t e d by chemists i n this area. In our view, t h i s was i n part a consequence of the methods u t i l i z e d , which did not permit a "global" picture of the temporary negative ion states of molecules. This i n turn made i t d i f f i c u l t to establish the connection between anion energies and the electronic structure, as described by molecular o r b i t a l s . Indeed, the complementary r e l a t i o n ship between the cation energies determined by photoelectron spec troscopy and anion energies deduced from electron scattering measurements was not f u l l y appreciated. Electron Transmission Spectroscopy

(ETS)

In 1971 a new v a r i a t i o n on the transmission method was introduced by Sanche and Schulz (13). The technique incorporated the trochoidal monochromator of Stamatovic and Schulz (14) and a modulation scheme to obtain the derivative of the electron current transmitted through a gas c e l l . This combination provides a r e l a t i v e l y simple and very sensitive means of locating resonances as they appear i n the t o t a l scattering cross section. In p a r t i c u l a r the energy resolution (20-50 meV), which i s substantially better than that found i n most trapped electron and SFg scavenger studies, i s s u f f i c i e n t to observe the v i b r a t i o n a l structure possessed by anions long l i v e d enough to


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Temporary Negative Ion States

undergo appreciable nuclear d i s t o r t i o n . Furthermore the apparatus i s simpler than e l e c t r o s t a t i c a l l y focused instruments and much l e s s sen s i t i v e to the d i f f i c u l t i e s encountered i n studies of reactive compounds. B r i e f l y , the trochoidal monochromator, which incorporates an a x i a l magnetic f i e l d and a crossed e l e c t r i c f i e l d for energy disper sion, produces a beam of current, t y p i c a l l y 10~ to 10~ A, which i s directed into a gas c e l l . The gas density i n the c e l l i s adjusted so that the current a r r i v i n g at the electron beam c o l l e c t o r i s reduced to approximately e of i t s i n i t i a l value at energies where the scat tering i s large. The r e j e c t i o n of the scattered electrons takes place primarily at a retarding electrode following the gas c e l l , where a potential b a r r i e r r e f l e c t s electrons whose a x i a l v e l o c i t i e s have been reduced below a selected value. In the absence of the potential b a r r i e r at the c e l l e x i t , some scattered electrons w i l l be rejected by other means such as multiple scattering and c o l l i s i o n s with the aperture edges. At low energies an additional r e j e c t i o n mechanism becomes s i g n i f i c a n t : Electrons which are e l a s t i c a l l y scat tered into a cone around 180° can re-enter the monochromator where they disperse and are l o s t (15). The transmitted beam thus r e f l e c t s the loss due to d i f f e r e n t i a l e l a s t i c scattering rather than the t o t a l cross section. As we i l l u s t r a t e l a t e r , the v i s i b i l i t y of v i b r a t i o n a l structure i n a resonance i s often enhanced i n the "backscattering" mode. F i n a l l y , i n the innovation introduced by Sanche and Schulz (13), a small ac voltage i s applied to a cylinder within the gas c e l l thus modulating the energy of the electrons, and the derivative with respect to energy of the unscattered or "transmitted" beam i s detected. Following studies i n hydrocarbons by Sanche and Schulz (16) and Nenner and Schulz (17), we began applying ETS to chemical problems (18) i n 1975, being p a r t i c u l a r l y motivated to correlate the anion states of hydrocarbons with t h e i r electronic structure. Electron transmission studies are now being c a r r i e d out by a number of other groups i n the U.S. and i n Europe. A bibliography of published work i s available from the authors. In t h i s paper we f i r s t review the c h a r a c t e r i s t i c s of resonances i n molecules, and then discuss recent experimental r e s u l t s i n poly atomic systems, with examples chosen to i l l u s t r a t e the types of i n formation that can be provided by ETS. We discuss some of the problems uncovered, and the r o l e that other electron scattering methods and theory could play i n solving these problems. 8

9


- 1

Resonance Characteristics Temporary anion states may be broadly c l a s s i f i e d either as shape resonances or core-excited resonances (4). The former are well described by a configuration i n which the impacting electron attaches to an atom or molecule i n one of the o r i g i n a l l y unoccupied o r b i t a l s . In the l a t t e r , electron capture i s accompanied by electronic e x c i t a t i o n , giving r i s e to a temporary anion with a two-particle-one-hole (2p-lh) configuration. One can further d i s t i n g u i s h core-excited resonances into those i n which the resonance l i e s energetically below i t s parent state and those i n which i t l i e s above. The former are referred to as Feshbach resonances and the l a t t e r as core-excited


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shape resonances since they can also be viewed as resulting from electron attachment to an excited state of the neutral molecule. Although a l l three types of resonances appear i n ET spectra, shape resonances are dominant at low energies i n unsaturated molecules, and we w i l l devote most of our discussion to these. Next, we review b r i e f l y the connection between the l i f e t i m e s of temporary anions and o r b i t a l symmetry. In the case of an atom, the p o l a r i z a t i o n and centrifugal terms combine to give an electron-atom interaction of the form: v ( r )

=

« M^Li +


2r

2r

at s u f f i c i e n t l y large distances from the atom. Here a denotes the p o l a r i z a b i l i t y and £ the angular momentum associated with the o r b i t a l into which the electron i s captured. When £ ^ 0, the resonance has a f i n i t e l i f e t i m e due to tunneling of the electron through the angular momentum b a r r i e r . ETS measurements i n the group I l a and l i b atoms provide examples of pure p-wave and d-wave shape resonances (19). The usual view i s that i n the absence of an angular momentum b a r r i e r , there i s no temporary electron capture or time delay of the scattered electron. The extension of this picture to shape resonances i n molecules requires that the single value of I be replaced by an i n f i n i t e sum over angular momentum components. However, i f the molecular symmetry i s s u f f i c i e n t l y high, resonances may be well characterized by only one or two I values. For example, the IIg shape resonance of N2 i s well described by a d-wave angular scattering d i s t r i b u t i o n (4). CO, on the other hand, has important £=1 and 1=2 components i n i t s angular d i s t r i b u t i o n , the former a r i s i n g from the d i f f e r e n t s i z e of the oxygen and carbon 2p o r b i t a l s . One important consequence of the lower symmetry of CO i s that i t sIT shape resonance has a shorter l i f e t i m e than that of N , as r e f l e c t e d , for example, by the much weaker v i b r a t i o n a l structure i n the ET spectrum (20). Molecules such as benzene and ethylene have s u f f i c i e n t l y high symmetry that their shape resonances should be dominated by a single angular momentum component. Indeed, from the limited angular scattering studies that have been carried out (21,22), t h i s appears to be the case. On the other hand, many polyatomic molecules with no o v e r a l l symmetry have been found by ETS to have shape resonances (20). This implies that the wavefunctions of the "extra" electrons i n these systems have a sizeable mixture of components with £>0, and i t indicates that the unsaturated portion of the molecule possesses l o c a l symmetry. For most of the polyatomic molecules we have studied, the shape resonances have l i f e t i m e s i n the range of 1 0 ~ to 1 0 ~ seconds, although resonances at energies close to 0 eV may have much longer l i f e t i m e s (4). The longer-lived shape resonances t y p i c a l l y display well defined structure due to nuclear motion, whereas those i n which the electron detaches i n a time short compared to that required for appreciable motion of the nuclei are broad and featureless, with widths of the order of 1 eV or more. Theoretical calculations (23) have shown that structure due to nuclear motion may s t i l l be weakly v i s i b l e i n cases i n which the electron detaches i n a time as short as one tenth of the period of the p a r t i c u l a r v i b r a t i o n . 2

2

2

13

15


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Core-excited resonances, which are the counterpart to shake-up processes i n PES, usually occur at higher energies and are generally less prominent than shape resonances. The two types of core-excited resonances have very d i f f e r e n t l i f e t i m e s ; the core-excited shape resonances have short l i f e t i m e s because they decay rapidly into their "parent states, and do not t y p i c a l l y display v i b r a t i o n a l structure. On the other hand, Feshbach resonances usually have long l i f e t i m e s and sharp v i b r a t i o n a l structure, since their decay must be accompa nied by electron rearrangement (4). In general, 2p-lh core-excited resonances i n which the p a r t i c l e o r b i t a l s derive from valence o r b i t als l i e above one or more of their parent states, while those derived from Rydberg o r b i t a l s l i e below (4). This c h a r a c t e r i s t i c ordering follows from the r e l a t i v e amounts of Coulomb repulsion i n the two cases. Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: September 28, 1984 | doi: 10.1021/bk-1984-0263.ch009

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Ethylene, Butadiene, and Hexatriene Electron transmission data i n the sequence of molecules — ethylene, butadiene, and hexatriene — i l l u s t r a t e many of the features of tem porary anion states i n unsaturated compounds. Our most recent meas urements (24) i n these compounds are shown i n Figure 1, which d i s plays the derivative of transmitted current as a function of electron energy. Ethylene, the prototypical alkene, has been studied f r e quently using transmission methods (7,16,18). The shape resonance near 1.7 eV was assigned to electron capture into the b2g(7T*) o r b i t a l by Bardsley and Mandl (25). In contrast to e a r l i e r studies, our work (18) showed weak undulations due to nuclear motion which we attributed to excitation of the C-C stretch mode. The weak struc ture i s consistent with an anion l i f e t i m e shorter than a period of this v i b r a t i o n . Walker et a l . (22) have studied the excitation of the v i b r a t i o n a l l e v e l s of the electronic ground state proceeding through t h i s resonance and found that the C-C stretch i s the domi nant mode which i s excited. Theoretical calculations (26) have shown that the equilibrium structure of the anion i s highly nonplanar. I t thus appears that electron detachment occurs s u f f i c i e n t l y rapidly that the molecule has l i t t l e opportunity to undergo outof-plane d i s t o r t i o n . The ET spectrum of trans-butadiene shows two well-defined reso nances which we attributed (18) to occupation of the two empty TT* o r b i t a l s . The lower resonance l i e s below that of ethylene and exhibits sharper structure. Figure 2 shows these data on an expanded energy scale for both "high r e j e c t i o n " conditions i n which the signal derives from the t o t a l scattering cross section, and "low r e j e c t i o n " which r e f l e c t s the d i f f e r e n t i a l e l a s t i c scattering near 180° (15). The symmetric C-C vibrations of the anion are the most pronounced, but there i s evidence for low frequency out-of-plane modes as w e l l . The upper resonance l i e s above that of ethylene and i s featureless. These r e s u l t s provide a nice confirmation of expectations based on TT MO theory and show also the interplay between symmetry, reso nance energy, and anion l i f e t i m e . Although the f i r s t o r b i t a l of butadiene has a leading p a r t i a l wave of £=1, and that of ethylene has £=2, the substantially lower energy of the former r e s u l t s i n a slower rate of tunneling. The increased l i f e t i m e makes possible the


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I 0

1

1

1

I

I

i

1

2 3 4 ELECTRON ENERGY (eV)

I

I

1

i

I

I

1

i

I

1

I

i

r

L

5

Figure 1. The derivative of transmitted current as a function of electron energy i n ethylene, butadiene, c i s - and trans-hexatriene (from Ref. 24).



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appearance of the low-frequency modes i n the spectrum. The ethylene anion, with i t s shorter l i f e t i m e , simply does not l i v e long enough to permit i t s low-frequency out-of-plane modes to appear i n the ET spectrum. In the lower portion of Figure 1, the ET spectra (27) of the c i s and trans isomers of hexatriene, separated chromatographically, are displayed. Two pronounced shape resonances are seen i n the spectra of each isomer. Hexatriene might be expected to have three shape resonances, one associated with each of i t s empty TT* o r b i t a l s . The ground state anion, however, i s stable, and the two features observed i n the ET spectra therefore correspond to the excited states. The energy of the f i r s t resonance of the c i s isomer i s 0.55 eV lower than that of the trans isomer. This was unexpected since the TT I P ' S of the c i s and trans isomers agree to better than 0.1 eV (28). We have attributed the "extra" s t a b i l i t y of t h i s anion of cis-hexatriene to a strong C 2 - C 5 bonding i n t e r a c t i o n i n the TTJ o r b i t a l . This i n t e r action i s much more important i n the f i r s t excited anion state than i n the ground or second excited anion states because of the larger charge densities at the C and C5 atoms. Since the f i l l e d TT o r b i t a l also has large densities at these s i t e s , we concluded that longrange through-space interactions are more important i n the anion state due to the more extended nature of the anion wavefunction. This interpretation was supported by c a l c u l a t i o n s . The importance of such interactions has been noted for several other anions (29-33). At higher energies, other types of resonances are found i n these molecules. In ethylene, narrow Feshbach resonances above 6.6 eV were f i r s t observed by Sanche and Schulz (16). In Figure 3 we show the spectrum i n t h i s region measured at somewhat higher resolution. The f i r s t Rydberg state of the neutral molecule i s characterized by the C2RV*" ground state core plus a 3s(aig) e l e c tron. Sanche and Schulz (16) suggested that the f i r s t Feshbach resonance i s formed by adding a second 3s electron to t h i s Rydberg state. The spectrum serves to illuminate several of the characteris t i c s of core-excited resonances. The "doublet" structure, repeated at i n t e r v a l s of approximately 170 meV, i s c h a r a c t e r i s t i c of the V 2 C-C symmetric stretch. The spacing between the f i r s t and second features i n each pair i s 60 meV. We a t t r i b u t e these to two quanta of the CH t o r s i o n a l mode, i . e . 2Vi+. These c h a r a c t e r i s t i c energies i n the anion are quite close to those of the Rydberg "parent" state, as expected. The existence of the low frequency modes i s a clear i n d i c a t i o n of the long l i f e t i m e of the Feshbach resonance r e l a t i v e to that of the B g shape resonance discussed previously. At energies above that of the singlet Rydberg state at 7.11 eV, indicated by S i n Figure 3, the v i b r a t i o n a l l e v e l s of the i f " (3s) resonance may decay into the TT~ 3s parent state thus shortening the l i f e t i m e and broadening the structure. At the time these data were taken, however, i t was not clear why the t h i r d doublet, which l i e s below the s i n g l e t Rydberg state, was also broadened. This puzzle was resolved by data of Wilden and Comer (34) who located the com panion t r i p l e t Rydberg state at 6.98 eV (with 2Vj+ at 7.03 eV), indicated by T on Figure 3, into which the resonances may also decay. 2

2

2

2

2

1

1


2


172

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ELECTRON ENERGY (eV)

Figure 2. The d e r i v a t i v e of transmitted current i n 1,3-butadiene. The curve marked "low r e j e c t i o n " i s obtained by rejecting only those e l a s t i c a l l y scattered electrons which return to the mono chromator. The "high r e j e c t i o n " data i s derived by rejection of a l l scattered electrons (from Ref. 24).

2I

I 6.6

,

I , I , I • I 6.8 7.0 7.2 7.4 ELECTRON ENERGY (eV)

,

l_ 7.6

Figure 3. The derivative of transmitted current i n ethylene showing the resonances associated with the lowest singlet (S) and t r i p l e t (T) Rydberg states.


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We have not detected Feshbach resonances i n butadiene or hexatriene, and i n general, i t appears that such resonances are much weaker i n larger hydrocarbons than i n d i - and triatomic molecules (16). Most hydrocarbons, including butadiene and hexatriene, how ever, display broad resonances at high energy which could be due either to shape resonances with attachment into a* o r b i t a l s or to core-excited resonances i n which electron capture into a TT* o r b i t a l i s accompanied by TT TT* excitation. Examples of such resonances w i l l be discussed l a t e r . A d e f i n i t i v e assignment of these broad states awaits a study of t h e i r decay channels by electron energy loss methods.


Nonconjugated

Dienes

The problem of long range interactions between widely separated subunits i n molecules i s of importance i n many areas of chemistry and biology. Although many studies have appeared characterizing these interactions i n excited states and cationic species, very l i t t l e has been done i n gas-phase anions. For these reasons we have undertaken a study of such interactions i n nonconjugated dienes and diones. Several of the dienes we have examined (35) are shown below:

The excellent agreement between the measured s p l i t t i n g s of the anion states and the values obtained from Koopmans' theorem ( i . e . , derived from the energies of the appropriate v i r t u a l o r b i t a l s of SCF calculations on the neutral molecules) supports the use of an o r b i t a l picture for interpreting these experiments. In compounds (1) and (4) , the two double bonds are s u f f i c i e n t l y close that much of the s p l i t t i n g between the TT* o r b i t a l s arises from d i r e c t , or throughspace, interactions. On the other hand, the IT* o r b i t a l s of (2) and (5) are s p l i t by 0.8 eV and 0.6 eV, respectively. These s p l i t t i n g s are over an order of magnitude greater than those which could a r i s e from through-space interactions i n these compounds. Rather, they a r i s e from the through-bond interaction due to the O - TT* and a* - TT* mixing made possible by the nonplanarity of the molecules. To understand better the nature of these interactions, we con sider the 7r =(7r -Hr ) d n"~ »(Tr -Tri ) delocalized o r b i t a l s formed from the TT and TT l o c a l i z e d o r b i t a l s . In the absence of through-bond interaction, the TT and TT"* o r b i t a l s would be nearly degenerate. The 7T+ o r b i t a l i s symmetric with respect to the plane bisecting the molecule " p a r a l l e l " to the double bonds, while the TT" o r b i t a l i s antisymmetric. Consequently the i r o r b i t a l can mix only with sym metric a o r b i t a l s and the TT"" o r b i t a l with antisymmetric 0* o r b i t a l s . Since the symmetric and antisymmetric o r b i t a l s are at appreciably d i f f e r e n t energies, t h i s mixing causes a s p l i t t i n g between the TT and IT" o r b i t a l s . A detailed analysis of the r e s u l t s f o r (2) shows that the TT o r b i t a l i s s t a b i l i z e d because i t mixes more strongly +

a n

a

A

,

a

a

D

)

d

+

+

+


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with Q o r b i t a l s than with O o r b i t a l s , while the TT" o r b i t a l i s desta b i l i z e d due to a greater mixing with O o r b i t a l s than with 0 * orbitals. Theoretical work suggests that through-bond s p l i t t i n g s of a few tenths of an eV are possible even when the double bonds are sepa rated by 9 - 10 1 as i n (3). Unfortunately, we have not been able to confirm this by ETS since the widths of the resonances are much greater than the expected s p l i t t i n g s . It i s possible that measure ments of the e x c i t a t i o n functions of the v i b r a t i o n a l l e v e l s or the angular scattering d i s t r i b u t i o n could provide information on the s p l i t t i n g s i n these cases.


Shape and Core-excited Resonances i n Benzene The benzene molecule and i t s derivatives have been extensively stud ied using a v a r i e t y of spectroscopic techniques. Not only does benzene serve as a prototype for aromatic systems, but the degener acy of i t s highest occupied and lowest unoccupied molecular o r b i t a l s makes i t an important system for the study of substituent and JahnTeller effects. The shape resonances of benzene and some of i t s derivatives have been explored by a number of investigators using ETS (16,17,20, 36). With the exception of questions regarding pseudo-Jahn-Teller problems (37), for example i n the alkyl-benzenes, substituent e f f e c t s on the doubly-degenerate E 2 ground state anion near 1.1 eV can be generally understood i n terms of inductive and mesomeric effects. In the remainder of t h i s section we consider the higher l y i n g resonances i n benzene. In general, the simple c l a s s i f i c a t i o n of resonances involving valence o r b i t a l s as either "shape" or "coreexcited" appears to break down as the anion energy increases (38). For example, the parentage of the l B 2 g "shape" resonance at 4.8 eV i s not clear-cut, since i t i s known to decay into e l e c t r o n i c a l l y excited states of benzene as well as the ground state (39,40). It has been suggested previously that t h i s anion must be described by an admixture of both shape and core-excited configurations (17). In Figure 4 we show the transmission spectrum of benzene at higher energies. The lowest feature i s the second "shape" resonance ( B 2 g ) . Above t h i s l i e several smaller features which must corre spond primarily to core-excited states. Detailed studies of the decay channels of these resonances have not yet been c a r r i e d out, although some information i s a v a i l a b l e . The v e r t i c a l arrows i n Figure 4 indicate the energies at which maxima occur i n the e x c i t a t i o n functions for \)\ of the X*Ax ground e l e c t r o n i c state of ben zene, as observed by A z r i a and Schulz (50), and for the lowest v i b r a t i o n a l l e v e l of the f i r s t t r i p l e t excited state ( B i ) as measured recently by A l l a n (41). The decay of the resonances into a number of other excited states has also been observed (41). The shading i s a very crude i n d i c a t i o n of the half-width of the peaks occurring i n these cross sections. Note that the peaks i n the e x c i t a t i o n func tions w i l l generally correlate with the midpoints of the structures in the transmission spectrum. This comparison shows that a given high-lying resonance may decay into either the ground state or an e l e c t r o n i c a l l y excited 2

U

2

2

g

3

u




JORDAN A N D BURROW

7 9 11 ELECTRON ENERGY (eV) Figure 4. The d e r i v a t i v e of transmitted current i n benzene. The largest feature i s the B g resonance at 4.8 eV. The v e r t i c a l arrows locate the maxima for decay into the X * A i ground state and the B i excited state of benzene. The shaded regions show roughly the half-widths of these peaks (from Ref. 24). 2

2

g

3

u


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176

state, or both. In more complex molecules such as naphthalene or styrene, the TT + TT* transitions occur at s t i l l lower energies, and the number of core-excited resonances i s even greater. Thus, accounting for the structure i n the t o t a l cross sections i s a con siderable challenge for theorists since a number of configurations are required for an adequate description of such resonances. A d i s cussion of these points and conjectures concerning their r e l a t i o n ship to the l i f e t i m e of the anions have been presented elsewhere (38).


Resonances Associated with O* Orbitals The shape resonances described i n the previous sections have a l l been associated with TT* o r b i t a l s . Resonances formed by attachment into the O* o r b i t a l s of unsubstituted hydrocarbons t y p i c a l l y occur at high energy (E > 5 eV) and are usually broader than TT* resonances. In compounds i n which they are hard to discern i n the t o t a l cross section, they may appear more readily i n the cross sections for v i b r a t i o n a l excitation, since direct excitation of v i b r a t i o n i s weak, at least for l e v e l s which are not allowed o p t i c a l l y , and hence the i n t e r f e r i n g background i s small. Such broad a* resonances, for example, have been observed i n methane (42) and ethylene (22) using such measurements. We have examined the r o l e of substituent groups on hydrocarbons to learn which systems introduce low-lying anion states associated with a* o r b i t a l s . ETS studies have shown that groups containing f i r s t row heteroatoms such as F, N, or 0 f a i l to s t a b i l i z e the 0 * o r b i t a l s s u f f i c i e n t l y to produce prominent resonances at low energy in the ET spectra. The s i t u a t i o n i s quite d i f f e r e n t for heterocompounds substituted with second, t h i r d , and fourth row atoms; a l l of these compounds are found to have well defined O* resonances below 4 eV. In Figure 5, the ET spectra of C H C 1 , C H C 1 , C H C I 3 , and C C I 4 are reproduced (43). Each of these molecules shows one or two reso nances which we a t t r i b u t e to C-Cl 0 * o r b i t a l s . Support for t h i s assignment i s provided by the number of resonances which appear and the trends i n their r e l a t i v e energies as the number of CI atoms increases, as well as by theoretical calculations. C H 3 C I has a single broad resonance, centered at 3.5 eV. As expected, C H C 1 has two resonances corresponding to the bonding and antibonding combina tions of the two l o c a l i z e d C-Cl O* o r b i t a l s . Both C H C 1 and CCli+ should each have only two low-lying anion states, doubly degenerate in the former and t r i p l y degenerate i n the l a t t e r . The ET spectrum of C H C I 3 shows two resonances as expected, but that of C C I 4 displays only one low-lying resonance. In this case the increasing s t a b i l i t y brought about by CI substitution causes the A j anion to be bound, and thus only the T anion i s seen i n the spectrum. There i s no sign of v i b r a t i o n a l structure i n any of the resonances. In the absence of other data, i t i s not clear whether the widths derive p r i marily from the short l i f e t i m e s or from the repulsiveness of the anion curves i n the Franck-Condon region. The anion states appearing i n the ET spectra may also give r i s e to stable anion fragments v i a the d i s s o c i a t i v e attachment process. Because of the large electron a f f i n i t y of the chlorine atom, the O 3

2

2

2

3

2

2

2


2

9.

JORDAN AND BURROW


177

CH3CI


CHgClg

CHCI3

CHCIgF

CCIgFg

0

1

2 3 4 ELECTRON ENERGY (ft)

5

6

Figure 5. The four upper curves show the derivative of transmit ted current i n the chloro-substituted methanes. The v e r t i c a l arrows indicate the experimental midpoints of the resonances. The v e r t i c a l l i n e s locate the t h e o r e t i c a l anion energies and show the o r b i t a l symmetries. The theoretical energies are normalized to the experimental data only at the A\ resonance i n CHC&3. Repro duced with permission from Ref. 43. Copyright 1982, American Institute of Physics. 2



178

RESONANCES

resonances i n the chloromethanes described above may decay readily i n t h i s channel. In the simplest picture, the y i e l d of stable anions i s viewed as a r i s i n g from a cross section for attachment modulated by an escape probability which w i l l be influenced strongly by the l i f e t i m e of the resonance and the rate of motion on the anion surface. The escape p r o b a b i l i t y may modify considerably both the shape and posi tion of a resonance as viewed by ETS. As the resonance l i f e t i m e shortens, the y i e l d of stable anion fragments i s reduced, and the maximum i n the y i e l d i s shifted to lower energies. One should not therefore expect the energies of the peaks determined i n the ETS and DA measurements to agree. Indeed, i n the chloromethanes the f i r s t peak for C l ~ production occurs at much lower energy than that for electron capture into the resonance. In substituted unsaturated compounds both 0" and TT resonances may appear at low energy i n the ET spectra. Our data i n the chloroethylenes (44) reveal that the O* resonances l i e t y p i c a l l y 1 - 2 eV above the ground state TT* resonance. Dissociative attachment i n the unsaturated compounds i s more interesting than i n the chloromethanes described above because the energy of the E anion, which correlates with the R( E) + C 1 ~ ( S ) asymptote, rapidly decreases as the C-Cl bond i s stretched, r e s u l t i n g i n a crossing of the E and II surfaces. Out-of-plane motion of the chlorine atom would thus cause the sur faces to undergo an avoided crossing. The available d i s s o c i a t i v e attachment data for the chloroethylenes show a peak i n the Cl"~ pro duction close to that for forming the IT anion (45) which we believe can be understood from the coupling of the two anion surfaces i n this energy region. Over the past few years, considerable information (29,36,46,48) has been acquired on

Temporary Negative Ion States in Hydrocarbons and Their Derivatives

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