Ar(I) photoelectron spectra of unsaturated hydrocarbons

275

Journcll of Electron Spectroscopy and Related Phenomena, 15 (1979) 275-280 0 Elsevler Sclentlfic Pubbshmg Company, Amsterdam - Pnnted m The Netherlands

He (I) /Ne (I)/Ar (I) PHOTOELECTRON SPECTRA OF UNSATURATED

ANGULAR-DISTRIBUTION HYDROCARBONS

M H

KIBEL, M K

Physxal

LIVETT

Chemistry

and G L

Department,

NYBERG La Trobe Unlverslty,

Bundoora

3083, VW,

Australia

ABSTRACT

The energy dependence of ethylene, hexadiene

1,3-butadlene,

IS much

1s a symmetry-speclflc

Strong

contrlbutlon

among the ethylene,

parameter-based

assignment

cyclohexene,

all r-band

values.

for both 71 and 0 orbltals,

are observed

parameters

of the ?T- and selected

norbornadlene,

In the low energy region

less uniform,

slmllar threshold

observed

benzene,

has been determlned

the a-bands towards

of the anlsotropy

B-values

intra-band

while

whereas decrease

to the TI &values

of symmetric

character

T-orbltal

bands,

of

with decreasing

Marked

and benzene

1,4-cyclo-

the behavlour

energy dependencies

In the multiple

butadlene

1,3-and

U-bands

energy

have been

molecules

lntenslty

there

varlatlons

and the anlsotropy-

to the 1,4-cyclohexadlene

first

band 1s conflrmed

INTRODUCTION BesIdes

Its posltlon,

or energy,

lsed also by Its electron dlstrlbutlons

angular

a band In a photoelectron

dlstrlbutlon

(and their resulting

anlsotropy

In order to help wzth band assignment molecules

(benzene

Unlike

of electron undertaken

klnetlc because

energy

a yardstlck

compllcatlon

which

bands

however,

Investlgatlon

this behavlour

establishing

different

potential,

may therefore

[Z])considered the anlsotropy

be undertaken

In this paper. parameter

of this energy dependence

is of interest

for theoretical

In Its own right

calculations),

this energy-dependence

of a molecule.

parameters)

1s character-

of these angular

This has been the case with two of the

[l] and 1,4-cyclohexadrene

the lonlzatlon

spectrum

The determination

This paper

may present Includes

B IS a fun&Ion may then be

(and with an eye to

or In order

to elzmlnate

when comparing

data resulting

B-values

any

for the

from studies of

each type. In the fkrst, the unsaturated examined

In order to determlne

the region of greatest are utilized

hydrocarbons B(E)

varlatlon

For comparison

ethylene,

1,3-butadiene

for these archetypal

1s that near threshold,

with the a-orbltals,

and benzene

?T-orbltal compounds He(I),

the behavlour

are

Since

Ne(1) and Ar(I)source of the highest

276

MH

U-orbltal

rs also Included

For benzene,

been made based on prelrmlnary Prrmarlly

data, lower o-orbltals

In the second category,

cyclohexene,

1,3-cyclohexadiene

for which some postulates

have prevrously

are also studled

1,4-cyclohexadiene

and norbornadrene

KIBELetal

and the related compounds

have been examrned with He(I) and

NetI) sources In order that the asslgrunent, based on anlsotropy parameters energy

121,

former

(versus the opposite

of the symmetric above the antrsymmetrrc

though, the measurements

x-orbltal

In the other drenes) may be conflrmed

do provrde addItIona

at He(I)

comblnatlon

In the

In addltlon,

data on B(E) dependence

In a wrder

range of compounds

RESULTS The apparatus with Its rotating, detalled

[3], as has the procedure

component of lsotroplcally parameters

are displayed

retarding-field, for correctrng

scattered electron

In Figure l/2

6 from the angular-dlstrlbution

analyser has previously

been

the B-values to allow for a small

flux [4]

The resulting anlsotropy

The statrstlcal

error involved in obtalnlng

IS typically kQ.03, being less at the beglnnrngs

of

bands and of the spectrum, and greater at the ends, due to the Increase in noise accompanying

the increase rn Integral signal level

Nowever,

are not the mayor source of error, and when reproducibilrty previous

statlstlcal

fluctuations

IS taken into account the

figure needs to be increased to fU 10

DISCUSSION Several observations that all the n-orbital energy

follow from lnspectlon of these results. B-values decrease with decreasing

For those orbltals of Figure 2a (cyclohexene,

1,4-cyclohexadrene,

norbornadiene)

all posztive,

For the structurally

diverse compounds of Figure la, (ethylene, allene, 1,3-butadlene gradients vary over a considerable

range,

but they more

and benzene]the

however the average value This suggests that rn

both overall and for each compound

1s again about 0 06 eVeI

the energy dependence

B-values it may be feasrble to separate one

mayor) contrlbutlon present molecules, 1,3-cyclohexadiene 1s probably

of x-orbltal

which is not orbital specific this latter contribution Even here, however,

greater than the apparent

ad3acent U-band

The former

from another which IS

would be significant

Among the

(IP) x-orbital

slope observed, due to spectral overlap with the

atomrc orbital.

results for ethylene are replotted,

(the

only for benzene and

the actual slope of the second

(molecule non-specific)

to associate with the constituent

LS

1,3-cyclohexadlene,

not only are the gradients

are also all very slmllar in value, around 0 06 eV-'

corresponding

The most obvious

outgoing electron kinetic

contribution

it would be natural

In Figure Id the experlmental

together with orthogonalised

plane wave

[S] and

ANGULAR

DISTRIBUTION

FROM UNSATURATED

277

HYDROCARBONS

A_(2a) -

f-T- ORBITALS

I

(2 II

(ICI 0

I

Ii

I

’

1

”

”

1)

;z=c=c;

>C=F/

0

ETHYLENE

ALLENE

0 CYCLOHEXENE

Q1 3-

0 BENZENE

BUTADIENE

Q 1.3 - CYCLOHEXADIENE

I

1 0 l.L- CYCLOHEXADIENE

0 0 0 -0 -0 -1

I

I

2

I

I

4

I

I

6

I

11

6

b

JO

1

‘1)

12 KEieV

Figure l/2 Energy dependence of the anisotropy parameters for l'f-and U- orbltals of unsaturated hydrocarbons Respective key fqures (c) give symbols used on plots Figures in brackets correspond to the band number of each orbital type, startma fron low IP Short lmes m la and lb show intra-band varaatlon

278 the

MH atomx

carbon 2p (61 calculations.

a much better approxlmatlon

IUBELetal

It 1s seen that the latter curve 1s actually

to the experlmental

energy dependence of ethylene

B-values

than IS the former Amongst the U-orbitals

there 1s a wide variation

In the gradients,

this LS less for the structurally more similar compounds represented than for those of Figure lb

AgaIn the

both groups, at approximately

zero (ev-'1

energy dependence parallelling

of G-orbital

average slope 1s essentially

(73 that

results)

However such generallsatlons 1s as large as it 1s for

Two points relating to previous derived U-orbltal

lonlzatzon energy

(also apparent

B-values

from the present

have llmlted utlllty when the lndlvldual

experunents

emerge

The

fact that carbon 2p-

do not increase uniformly with decreasing

of a steady Increase In b-value with IncreasIng

[7] LS due not to a (negative slope) kAnetic energy dependence

but rather to the increase In the binding energy itself Secondly,

compact less nodal wavefunctlons)

[S] that all points

The difference

(91 lie on common curves

(or the associated

the present benzene results

c+ and i'r-bands) are at variance with the conclusion orbltal type

T-orbltal

the orbltals of Figure lb

anisotropy parameters

energy means that the observation

the same for

IS less than that of l'f-orbltals,

(for hydrocarbons)

are on average greater than those of b-orbltals

varlatron

In Figure 2b

Thus it can be said that on average the

anisotropy parameters

the previous observation

though again

more (both for

from each

lies both In the extended

lower energy range and In different values obtazned for some of the 74nm points Returning

to the a-orbital

anlsotropy parameters,

there are two such occupied orbltals In the molecule orbltal

that when

the @-values for the higher IP Now

(~2) lie below those of 711 In all cases except for 1,4-cyclohexadlene

calculations

[lo] predlcted

that while for norbornadiene

nodeless symmetric Tr-orbltal comblnatlon hexadlene

Recently It has been shown

l,+cyclohexadlene

complzcatron

[2] that whereas

for norbornadlene

and

evidence in favour of the orbltal Lnvetsion

arIses In that the anlsotropy

to different

energy, an undetermaned

Ne(1) source

which has the higher lonlzatlon

the situation is reversed

Although this seems clear-cut

dependence

the

the He(I) $-value of the first T-band 1s greater than that of the

second, for 1,4-cyclohexadiene

correspond

and 1,3-cyclohexadiene

would lie lower Ln energy, for 1,4-cyclo-

It would be the ant~synunetr~c combination

potential

possrble

it will be observed

electron kinetic energies

parameters

a

being compared

so that, since @ 1s a function of

fraction of the difference may actually be due to this

For this reason the anisotropy parameters

have been detennlned

It 1s clear from Figure 2a that because all the gradients

slmllar this in fact 1s a refinement which makes no essential difference

usrng a

are very to the

concluszon drawn Iust from the He(I) @values The posslbllity

of characterlzlng

anisotropy parameters

molecular

of the corxespondlng

orbltal synunetrles through the

photoelectron

bands was early appreciated

279

ANGULAR DISTRIBUTION FROM UNSATURATED HYDROCARBONS [11,12], and has remalned Were theoretical

a strong motlvatlon

calculations

ldentlflcatzon

avallable,

As It IS, however,

basis

from trends across generallsatlon,

l'r-orbitalhas a lower

the anlsotropy molecular

general

parameter

parameter

energy dependence

previously @ having

across

These

obtained

the latter,

lndlvidual

[7]

intensltles

analyser

of the

which

than 1s allowed The most

when possible,

band

This therefore between increase

does not

of lonlzed

(ormroxlmatel

of

this 1s not of

band lntensltles,

and SOI give

ObvLously

correction

orbltals

bands would corresponds

to be greater

them,

(for each photon

have relatzve

lntensltles

energy)

of one, two,

"rule of thumb"

for photo-

to note from the Table

Just how far are commonplace, 1/2 or 2

(benzene lel g with Ne(I))of

[133 that

grounds

the

low energy

factor

(relative)

lntenslty

of a factor of l/2 or 2 are unexceptionable, provide

a

(allowlng for degeneracy)

to the usual

factors of 2'3 or 3'2

an earlier claim

because against

1s to normallse

were equal

and It is interesting

(of Itself)

of the

than those

conclusion)

seems always

the intensltles

and there IS even one instance

wavelengths

speclflc

and that the occurrence

Furthermore,

analyser

matrix-elements

degenerate

supports

of

energy dependence

the relative

sections

l/E bandwldth

way of presenting

lntensltles,

varlatlons,

for a dlsperslve

This therefore

this does break down

that

no isolated phenomenon

total cross

to the total number

not Infrequent,

the photoelectron

f1el.d type there 1s no dlscrlmlnatlon

Then If all transition-Integral

electron

symmetric

the spectra are taken at 54 7O to the photon beam the

for by the theoretical

respectively

are linear

are stronger

of the electron(Ingulardlstrlbutlon,

In practice

all singly, doubly,

clearly

(contrary to the previous

(relative)

InformatIve

bands which

from this study comprises

is of the retarding

electrons,

one such

It will be noted

one(s)

energy dependencies

1s certainly

Because

are independent

direct measure

lndlcate

the nodeless

la are those energy dependencies

for these lntra-band

if not widespread,

In the Table

results

molecules

ldentlfled

An example of the long-awalted

It is also clear,

The final data avallable llsted

on Figure

Intra-band

to account

molecule

of @ seems to have emerged'

now been determlned,that

strong enough

rellabzllty)

on an lndivldual

must be emplrlcally

The present -TT -electron

studies

to this "rule", and that It 1s not Just a consequence

dependence

so) and non-zero

patterns

B-value than the antisymmetric

The other data represented anlsotropy

behavlour

that In multi

confIrm

symmetry

(of elgenvalue-comparable

would be possible

a range of compounds

namely,

all cases studled

of p-values

of orbitals

for angular-dlstrtiutlon

for invoking

l/3

changes

and that such an

autolonlzatlon

CONCLUSION The present photoelectron

data represent angular

the first general

dlstrlbutlons

zaturated hydrocarbons,and trends which have emexged

emphasis

study of the energy dependence

for polyatomlc has been placed

molecules

All compounds

on the n-bands

auger well for the contznuatlon

of

are un-

The several general

of such investlgatlons

280

MH

Figures In brackets of lonlzed oxbitals Ethylene

Relative Band Intensities are nonnallsed to the total number He(I)

Ref[l4]*

(allowing

for degeneracy

Ref[l4]*

NetI) (1 0)

0 24

lb3u[vl

0 17 (0 7)

0 22

0 24

lbsg[oll

0 21

(0 8)

0 28

0 36 (1 4)

0 36

(1 6)

0 31

0 27 (1 1)

0.34

3ag

[u2

1

0 41

0 22 (0 9) lbnu INS] * Dlfferential(90°)cross-sectIon 1,3-Butadlene lbg[rl (a) 1

He(I) 0 08 (0 6)

0 10

ArtI) 0 45 (0 9) 0 55

(0 7)

0 12

(0.8)

0 18

(1 3)

0 22 (1 6)

0 26

(1 8)

6ag*I aal

0 19 (1 3)

0 13

(0 9)

5butC u4 1

0 15

(1 0)

0 15

(1 1)

1

0 13

(0 9)

0 07

(0 5)

6bu*[

u2

5aUtCo5

1

Benzene lelgl~r(a)l 3e

2&[

01 (a

la

2u*[rn2(s)

I

He(I) 0 09 fl 1)

NetI) 0 08 (0 7)

0 13 (1 6)

0 24

I2 2)

1

0 12

(1.4)

0 22 (2.0)

(r)l

0 16

(1 9)

0 21

(1 9)

lb2utl~3(t)l

0.18

(2 2)

0.16

(1 4)

2b,ut[os(s)l

0 11 (1 3)

0 09

(0 8)

3alg

0

10

0

11 (1 3)

3elU+tlu2

[us(r)1

le2g[uswl

from dlsperslve

Net11 0 09 (0 6)

0 12 (0 9)

1

laubr2(s) 7ag[a13

0 06

0 13 (0 5)

0 19 lntensltles

KIBELetal

analyser,l/E

scaled

(1 1)

*,? overlapped

ArtI) 0 73 (2 9) 0 27

(1 1)

*,t overlapped

(1 2)

REFERENCES

6 7 8 9 10 11 12

13 14

T A Carlson and C-P Anderson, Chem Phys Letts , 10(1971)561 M.H Kxbel, M K Livett and G L Nyberg, J Elec.Spec ,(1978). F J Leng and G-L Nyberg, J Phys E, 10(1977)686 M H Klbel, FJ Leng and G.L Nyberg, J Elec Spec ,Precedlng paper L L Lohr, In Electron Spectroscopy, ed D A. Shirley, North-Holland, Amsterdam, 1972,p245 S T Manson, J Elec Spec , 2(1973)482 RM White, T A Carlson and D P Spears, J Elec Spec , 311974159 J.A Kxnslnger and J W Taylor, Int J Mass Spec and Ion Phys , 10(1972/73)445 B Jonsson and E Llndholm, Arklv Qslk, 39(1969)65 R Hoffmann, E Hellbronner and R Glelter, J Amer Chem Sot , 92(1970)706. D C Mason, A Kuppermann and D M Mlntz, In Electron Spectroscopy, ed D A. Shxley, North-Holland, Amsterdam, 1972,p269. T A Carlson, G E MaGulre, A E. Jonas, K L Cheng, C P. Anderson, C.C Lu and B P. Pullen, Ibxd, ~207, T.A Caxlson and C P Anderson, Chem Phys.Letts , 10(1971)561. F J Leng and G L. Nyberg, J Elec.Spec., 11(1977)293. J W Rabalals, T P. Debles, J L Berkosky, J.T Huang and FO Elllson, J Chem Phys., 61(1974)516