Vacuum ultraviolet photoelectron spectroscopy of atoms and molecules

257

Journal of Electron Spectroscopy and Related Phenomena, 16 (1979)257-267 0 ElsevlerSc~en~f~cPubllshlngCompany,AmateKlam-~~dmTheNetherlands

VACUUM ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF ATOMS AND MOLECIJ’LES

JAMES A.R. SAMSON Behlcn Laboratory of Physics, University of Nebraska, Lincoln, NE

68588 U.S.A.

ABSTRACT For a complete study of the photoionization of atoms and molecules it is essential to make use of the technique of Photoelectron Spectroscopy and the continuum characteristics of synchrotron radiation. A brief review is given of the application of the above techniques in measuring partial photoionization cross sections and the angular distribution assymetry parameter 8.

Selected results are given,

which are compared to theoretical values.

INTRODUCTION Photoelectron spectroscopy is now a well established branch of photon collision physics and photochemistry. been able to obtain a large

In past studies of photoionization of gases we have amount of information from the conventional techniques

of absorption spectroscopy, ionization yields, and mass analysis of the photoionized products.

However, a blank area has existed.

its state of excitation or internal energy? ejected'

When an ion was produced what was

From what orbital was the photoelectron

In a few cases these questions could be answered by the indirect approach

of studying any fluorescent radiation that was produced.

However, the direct ap-

proach of studying the kinetic energy of the photoelectrons completely opened up this blank area.

Now there Is virtually no area in the field of photoionization

that cannot be studied, given a sufficient number of atoms. The technique of photoelectron spectroscopy is currently being used in two modes. In one case, undispersed radiation from resonance lamps provide electron energy spectra from which ionization potentials and vibrational spacings can be determined at two or three fixed photon energies.

The other approach has been to

use a many line spectral source or a continuum and disperse the radiation with a vacuum uv monochromator. of photon energy.

The motive here is to obtain information as a function

Although much valuable information can be obtained by the use

of undispersed resonance lamps the reason for their continued use is because of their simplicity and convenience, low cost, and in some cases because of the greater flux available within the resonance line.

However, more information can be obtained

JAR

258

SMSON

by the use of many discrete lines or especially from a continuum source of vacuum uv radiation. The most intense continuum and the one with the widest spectral range is, of course, the continuum obtained from synchrotron radiation The continuum allows measurements to be made within discrete structure and to study threshold phenomena as a function of the photoelectron energy

It is the purpose of this

review to show, briefly, how photoelectron spectroscopy has contributed to our understanding of the process of photoionization and of atomic and molecular structure.

DISCUSSION In the photoionization of an atom or molecule the photon energy can be absorbed by any of the various channels that are available. Theoretical analysis of the photoionization process must consider absorption by each of these channels then sum them to produce a total photoionization cross section. Clearly, it is desirable to measure these specific or partial cross sections to provide the most sensitive test of a particular theoretical model, and to do so as a function of photon energy. The partial cross sections are obtained by measuring the number of electrons within each energy group and comparing this to the total number of electrons produced.

This ratio is called the branching ratio.

That is, it is the fraction of

the total cross section that is involved in a specific photoionization process. Thus, the partial cross section o for ejecting an electron from the jth orbital j is given by,

?

= (N~/EN ) a(tota1) , Aj

where a(tota1) is the total photoionization cross section and Nj represents a fixed fraction of the electrons produced from the jth orbital. As long as this fraction remains constant as the photoelectron energy varies then the measured branchfng ratio is correct. This requires the collection efficiency of an electron energy analyzer to be calibrated as a function of electron energy. Figures 1 and 2 show two types of collecting efficiency curves, which depend upon whether the electron energies are analyzed at a fixed energy (by use of a retarding/accelerating lens system) or at variable energies by changing the voltage on the analyzer electrodes As can be seen from the Figures there is a large discrimination between low and high energy electrons in either case. Owing to the variation in the angular distribution of photoelectrons as a function of their energy the measured branching ratio will not, in general, be correct because electron energy analyzers sample only a certain small solid angle of the ejected electrons. In the vacuum uv region where dipole transitions dominate, the angular distribution of the photoelectrons ejected from atoms or molecules is given,

259

UPS OF ATOMS AND MOLECULES

ELECTRON

Fig. 1. Collecting efficiency curve of a cylindrical mirror electron energy analyzer with the use of a retarding/accelerating lens (pass energy = 3V).

for plane polarized

Nj =

9/47G

where

11 +

radiation,

and the direction P2(cos8)

Fig. 2. Collecting efficiency curve of a cylindrical mirror analyzer wlthout the use of a lens

by the relation,

(21

Bp2hJse)1,

0 is an asymmetry

can take on values

parameter

defining

to apply

the electron

angular distribution

ranging from -1 to +2, EJ is the angle between of the ejected

electron,

= 312 cos26 - l/2 (refs. l-2).

uv monochromators

WI

ENERGY

is partially

and the Legendre

Normally,

plane polarized.

to the case of partially

plane polarized

the electric vector

polynomial

the radiation

Thus, Eq.

and

emitted by vacuum

(2) has been generalized

or elliptically

polarized

radla-

tion, namely,

a

N

~~~14~)

~1 + 3148

f (l+~) co~2ex

+

M~COS~~~

-

2131 I,

(3)

3 when

the photon

direction

is along

Ox and By refer to the direction axis, respectively Either

of the photoelectron

parallel

P is the degree of polarization

= 54' 44' the expression Y

as shown in Fig. 3, and the angles with respect

to the x and y

(refs. 3-4).

the x or y axis must be oriented

polarization. ex = 6

the z-axis,

containing

to the direction

of the incident

8 and P vanish.

of maximum

radiation.

When electrons

When are

JAR

260

SAMSON

Fig. 3. Coordinate system showing the direction of ejection of a photoelectron The photon beam is in the direction of the with respect to the x, y, and z-axis z*-axis.

observed

in this direction

lax distribution

accepting

ratios.

effects become important,

range, then Eq. (2) is not strictly

(Uj/4")

with BC = 1.

T BnPn n=O

etc., are usually 1' B3, B4, the dipole approximation.

energies

continuum

in the keV angle.

(4)

are less than one or two hundred

small and Eq.

energies.

Figures

(refs. 6, 7).

as a function of photon

state into the ionization

the ion that are not accessible

However,

structure

of 02.

continuum

by

reveals the discrete

4(a,b) illustrate

the electronic

spectroscopy

For direct photoionization

energy.

electron

(4) tends to that obtained

there should be little difference

coincide with a discxete autoionizing tion of the excited

out that

radiation),

energy levels of 02 as revealed by photoelectron

584 and 736 i, respectively

distribution

as they do for photon

spectrum of a molecule most dramatically

nature of the electron binding

ionization

Tseng -et al. (ref. 5) have pointed

(for unpolarized

When the photon energies

and vibrational

This Thus,

at this "magic angle" in order to measure

(~0~8s) ,

volts B

The photoelectron

radiation.

on 54' 44'.

correct and there would be no magic

They give the more general expression

Nj =

of the incident

of thefr angu-

If angles other than 54' 44' are used then we must have a

of 8 and P and use Eq. (3).

if multipole

are independent

a conical shell centered

should observe electrons

true branching knowledge

measured

and the degree of polarization

is also true for analyzers all analyzers

the quantities

in the vibrational

intensity

the 736 i line happens When this happens populates

from the direct photoionization

at

into the

vibration

process

to

the relaxalevels of

Thus,

UPS OF ATOMS

J

AND

I

I

I

I

I

IS

I6

I7

I

13

12

II

261

MOLECULES

14

IONIZATION Fig.

4a.

584

photoelectron

i

spectrum

POTENTIAL of

I

16

I

I

20

21

WV)

02

73.6lnn

02’

I

I9

0 VT”

x %* “‘OrRi4rr$ 0

4 6 lIIIIIIIIlIIIIIII,ee

2 I

8

10

I2

H

l6la

4

ELEC

Fig.

736 i

4b.

measurements numbers of

by

of the

photoelectron

spectrum

vibrational use

of

suitable

autoionizing

When the

vibrational

obtain

the

photon

energy.

branching Then,

can be

obtained

as

spacing

the

tunable

of

can

02.

be

character

extended of

to

higher

synchrotron

vibrational

quantum

radiation

and

the

selection

state

integrated

states. intensities ratios by

use

a function

of

within

the

of of

various

Eq. the

(1)

each

electronic

states the

photon

partial energy.

that

are

are

accessible

photoionization Results

of

to

the

cross this

nature

we given

eection have

262

JAR

SAMSON

been obtained for at least the first three ionic states of 02, N2, and CO2 (refs. 8-10).

Figure 5 illustrates

theoretical

the partial

results of Davenport

(ref

cross sections

11).

been made of the partial photoionization most of the calculations

a broad resonance

show two overlapping

theoretical

agreement with experiment.

peaking at 31 eV in the X*Z

resonances.

Davenport

suggests

the

calculations

cross section of N2 (ref. 12-14).

show poor agreement with experiment.

for the B State are in excellent predict

Several

for N2, including

Davenport's

have

However, results

His calculations

also

state. The experimental data g that the higher energy resonance

(about 28 eV) is related to the theoretical "final-state" resonance, whereas the lower energy state, at approximately 23.6 eV, is caused by two-electron excitations, which were not included in his calculations. section

The peak observed

in the total cross

curve (refs. 15-16) at 23 6 eV is clearly caused by the resonances + 2'

in both

the X and A states of N

14-l

’ 4

‘1.’

’

’

’

’

’

’

’

’

’

’ -

Illlll”‘ll”~“l 014

18

22

26

30

34

38

42

46

PHOTON ENERGY(&) Fig. 5. Partial photoionization cross sections for the X, A, and B states of N+ (uncorrected for electron angular distribution). The solfd curves are the cal-2 culated values from Davenport (ref. 11). (from Plummer et al. ref. 9).

In the case of atoms, the experimental of the asymmetry in illustrating

parameter

values of the partial

$ have been extremely

the importance

of including

correlation

of cross sections.

This effect is particularly

of the 5s-electrons

in Xe.

correlations

correlations between

tance of interchannel

being considered.

the s-electrons coupling

and

theory, especially

effects in the calculation

noticeable

Figure 6 shows various

imental data (ref. 17), of the 5s Xe photoionization and without

cross sections

useful in gufding

in the photoionization

calculations,

with some exper-

cross sections

(ref. 18) with

The results clearly show the need for

and both the 5p and 4d-electrons.

is also illustrated

in measurements

The impor-

of 6, the photo-

263

UPS OF ATOMS AND MOLECULES

1.2 -(mb)

photon

energy

(Ry)

Partial photoionization cross section for the 5s electrons in Xe as a funcFig. 6. tion of photon energy. The dashed curve represents the RPAE calculation taking into account correlations between the 5s and 4d shells, whereas, the solid curve represents correlations between the 5s and (4d -t 5p) shells. The dotted curve does not take correlations into account. The experimental points are from Samson and Gardner, ref. 17 (from Amusia, ref. 18).

electron

angular distribution

theory and experiment the theoretical and 4d electrons correlations

parameter.

Figure 7 shows the comparison

for fl for the 5p-electrons

in Xe.

results obtained when the interchannel are neglected,

are included

whereas,

(ref. 19).

between

The dashed curve represents correlations

between

the 5p

the solid line curve is obtained when these

The experimental

and Torop -et al. (ref. 21) are in excellent

agreement

data of Dehmer -et al (ref. 20 with the calculations that

include full correlations.

Fig. 7. Angular distrfbutfon pararaeter f3 for the 5p electrons in Xe as a function of photon energy (1Ry = 13.6 eV). Theoretic81 data: --I---results obtained when interchannel correlations between 5p and 4d electrons are neglected. results obtained when these correlations are Included (from Amusia and Ivanov, ref. 19). Experimental data:l , D&met et al., ref. 20; 0, Torop et al., ref. 21.

264

JAR

SAMSON

The expected B-value (in the dipole approximation) for electrons with zero angular momentum, that is, s-shell electrons, is g = 2.

Measurements at 584 i of the

f3-parameterfor the 6s-electrons in Hg by Niehaus and Ruf (ref. 22) gave the result g = 1.68. 304 ;.

Dehmer and Dill (ref. 23) found B = 1.4 for the 5s-electrons in Xe at

The deviation from f3= 2 can be explained when spin-orbit interactions are

considered that allow s + up

and s -t ~~~~~ transitions to take place (ref 24). l/2 In fact, Starace et al. (ref. 25) have predicted that even in the nonrelativistic case the photoelectron angular distribution of s-electrons in open-shell atoms (other than those with R = 0 for the outer shell) will fluctuate with the energy of the incident radiation because the various possible couplings of the continuum p-waves to the ion core give rise to several distinct final states. An example of the calculated variation in the value of B for the 6s-electrons in Cs is shown in Fig. 8 (ref. 26).

The vertical arrows indicate the photoelectron energies where the radial

matrix elements R1,2,3,2 for the a + ~112,312 transitions go to zero. Also shown in the Figure (dashed curve) are the results calculated by Marr (ref. 27) using the experimental results of the degree of spin polarization of electrons produced in the photoionization process (ref. 28)

There is excellent agreement in the shape

of the two curves. However, there is considerable displacement in their relative positions.

PHOTOELECTRON

ENERGY CV)

Angular distribution parameter 6 for the 6s electrons in Cs as a function Fig. 8. of photon energy. The solid line represents the theoretical results of Ong and Manson, ref. 26. The dashed curve represents the semi-emperical results of Marr, ref. 27.

Some of the earliest studies that utilized photoelectron spectroscopy were concerned with measuring the ratio for producing the rare gas ions in their 2P l/2,3/2 states From their statistical weights a ratio of 2 was expected. However, most measurements have shown the ratio to be nonstatistical (refs. 29-32). This effect has also been observed in Cd, Hg, and Cs (refs. 33-35).

Although these ratios

UPSOFATOMSANDMOLECULES appeared

266

to be nearly

constant

aa a function

of photon energy it has recently been

shown for Cs and Xe that if the ratios are studied over a sufficiently

extended

photon

(ref. 32, 34).

energy

range the ratios will tend toward the statistical

The experimental

values

to 110 eV, are shown in Fig. 9 (refs. 30, 32, 36). in the data between the presence

autoionizing

a pressure

effect occurs

40.81 eV.

Their data, plotted

pressure

measurements.

energies

to indicate

("10%) at photon

and 60 eV

energies

solid line curve in Fig

et al

energies

(shown by the

(ref. 32) have shown that

at these two photon energies,

represent

their lowest

circle data at the above

These error bars are typical of all

The rise in the ratio to the statistical

They used a Dirac-Fock

9

coupling between

scatter

of 21 2 and 30 55 eV but not at

results of Ong and Manson

coupling

range 13.4

caused by

occurs just in the region of the Cooper minimum,

The theoretical

interchannel

in this region

the spread to be expected.

effect of spin-orbit

This is possibly

Error bars are placed on the closed

the closed circle data points. higher photon

resonances

In fact, Wuilleumier

ref. 37).

energy

There is considerable

(13.436 eV) and 33 eV.

threshold

of numerous

shaded rectangle,

values

of the ratios found in Xe, for the photon

and treats exchange

(ref

value for between

40

38) are shown by the

formulation

exactly.

that includes

However,

the

the effects

the 5p and 4d channels were not included

of

in the cal-

culation.

SENERGY 0

I

l20-

20

I

I

1

60

T’

80

I

T’

I

p’ f f{

__-________

-----

too ‘T

- -

-I

Ii

$J

bX

(eV)

40

15

+ Lm-

T

--i-7-i

Xe

I

20

I 40

I

I

I

I 80

60 PHOTON

ENERGY

I

I

I

100

(eV)

2 2 Pl,2 branching ratios for Xe as a function of photon energy The '312 ----- statistical weight value; theoretical values, Ong and Manson, ref 38 Experimental pointso, Wuilleumier et al., ref. 32, l , Samson et al., refo. 29, 30, A , Dehmer, ref. 36. Shaded area represents region of numerous autoionizing line* Fig. 9.

ratio is found to vary dramatically within autoionizing resonances The 2p1,2,3,2 (ref. 30, 39). For example, in the vicinity of the Xe 5~5p~6p(~l?~) window resonance at 20.95 eV the ratio is observed

to change from 1.54 to 3.0.

The calculations

of

266

J A R SAMSON

Starace predict that the ratio will increase to about 9.0 at the minimum of the resonance and, in fact, variations in the ratio within resonances are sxpected as This may explain the very low ratio for the a general phenomenon (ref. 40) 2 cross section ratio for Hg at 584 i as observed by Hotop (ref. 41) 2P He P3/2 l/2 observed a ratio of 0.28 (see Fig. 10) rather than the statistical weight value of 2.0.

From Mansfield's absorption spectrum of Hg (ref. 42) the 584 i line appears

to coincide with a broad absorption feature and this may perturb the ratio. More data in the vicinity of the 584 i line will be necessary to clarify this result.

584A - Hg -_-_-_-____

-mr,s

I’, 40

40

‘P%

1

I1I JS

Electron Enerov IeV) Fig.

10.

A portion of the 584 i photoelectric spectrum of Hg as a function of elec-

tron energy illustrating the relative strengths of the 2P and 2P3,2 lines. l/2 data are not corrected for the analyzer transmission. (from Hotop, ref 41).

The

There are many interesting effects in photon-atom interactions and it is clear that such effects as spin-orbit coupling and correlation effects between electrons within the same orbit and between neighboring orbits are very important in explaining these interactions

It is also clear that the use of synchrotron radiation and the

technique of photoelectron spectroscopy will be required to study many of the details of photoionization in atoms and molecules

ACKNOWLEDGEMENT It is a pleasure to acknowledge the US Department of Energy, the National Aeronautics and Space Administration (under Grant #NGR28-004-021), and the Atmospheric Sciences Section of the National Science Foundation for their support of our program on Photoionization of atoms and molecules.

I would also like to express my appre-

ciation to Professor Hotop for making available his unpublished results on Hg and to Professor Manson for his revised figure showing the Cs asymmetry parameter.

UPSOFATOMSANDMOLECULES

267

REFERENCES

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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