Photoelectron spectroscopy studies of transient species by

Journal of Electron Spectroscopy 0 Elsevler Scientific Publuhng

Photoelectron

and Related Phenomena, 16 (1979) 46-59 Company, Amsterdam - Pnnted m The Netherlands

Spectroscopy

Studies

of Transient

45

Species

by John Dyke, Nevllle Department

Jonathan

of Chemistry,

and Alan Morns

The University,

Southampton,

SO9 5NH, Nampsh~re,

U K

Abstract The methods

used to generate

study by p e s

transient

are reviewed

from such studies

species

in sufficient

The type of information

is discussed

with

reference

concentrations

which

for

can be obtained

to actual investigations

that have

been made Introduction A new spectroscopic problems

technique

can usually

The range is normally

anticipated

but the level of success

estimates

Vacuum

ultraviolet

species

3s no exceptlon

capable

of making

structure purpose

of the species

of this article

which may be generated special

techniques

greater

photoelectron

spectroscopy

not without

contribution

of

to transient

overview

but it is

of the electronic

and of its molecular

a general

from such studies

as applied

its difficulties

to our understanding

under consideration is to provide

which

to a wide variety than was initially

often falls short of the most optimistic

It YS certainly

a valuable

be applied

considerably

ions

The

of the sort of information

and also to give some indication

have been adopted

to generate

transient

of the

species

Experimental The photoelectron species except

spectrometer

has to be purpose

built

that it must be capable

region of the spectrometer differentially transient

which

must also be pumped

in the photoionization

our experience,

the most convenient

on the buildlng

block principle

sources

special

inlet tubes,

can be introduced

a spectrometer

where

spectrometer

is that it can be easily analyser

caused by contamination. cannot be expected instrument measured

Hence, easy Resolution

to be maintaIned

over a long period

for argon

of delay dismantled

In

has been constructed

chambers

or photon

The other criterion Too frequently

minimises

the inevitable circumstances

Under operating

charrber "down-time" involved

from a "clean"

conditions,

IS normally

for such

resolution

in the photoionization

under the special

(FWHM) using He1 radiation

time of the

such as high temperature

at the level obtained

of trme.

and most probably

is one which

photoionization

cleaning

is not critical

1s kept as low as possible.

Then, modifications

with a minimum

of transient

The photoionization

the residence

chamber

replacement

analyser

efficiently

efficiently

is lost as a result of the build up of deposits and electron

for studies

The type of electron of being evacuated

in order to reach conditions

species

furnaces,

is most useful

25 meV as

the best whrch

can be

J DYKE etal

46 malntalned for an extended perfod.

The type of spectrometer used ln the studies which are discussed in this paper has been described previously (1).

A schematlc form is shown 1n Figure 1.

analyser 1s of the 150' hemispherical analyser type

dlffuslon pumps which allow rapld evacuation of tie apparatus. atmosphenc

pressures a vacuum of better than 10

The

P1 and P2 are 6" oil Starting from

mm 1s normally achieved in

less than 30 minutes. Various techniques are used to generate transient species.

The most important

of these so far have been discharge methods, pyrolysis and gas-phase atom-molecule reactions.

Since generation of high concentrations of the transient is a pre-

requlslte for a successful experiment, it is worth dlscusslng each of these methods in some detail.

c

3 PI

c

3 P2

Figure 1 Schematic diagram of a photoelectron spectrometer for studying transjent species, showing dlffuslon pumps (Pl, PZ), species production point (A), sample inlet tube (B) and photolonlration point (C). i) Discharge methods Although in principle any type of discharge may be employed, ln practice all studies have involved species generated by means of a microwave discharge through an appropriate cavity

The discharge method has a number of disadvantages.

High amongst these is the fact that the discharge itself normally extends over a range of perhaps 5 cm

It also generates high densities of electrons

Both

these factors prevent generation of the species as close to the photolonlzation region as one would ideally like

Additionally the micrcrwave discharge, although

UPS OF TRANSIENT

SPECIES

often an efficient no guarantee produced,

means of dissociating

that the desired

p e s

product

does not provide

of overlapping

molecules,

IS indiscriminate

species will be produced

that it will be a maJor

techniques problem

47 There

1s

or indeed if it IS

Unlike most other spectroscopic

a wide spectral

bands from different

region to explore

species

The

IS a very severe

restriction.

In spite of the problems the discharge method has been used to produce high concentrations

of CS (2,3,4), SO (5) and CF2 (6)

use of the technique generating hydrogen, produced

IS as a means

transients nitrogen

and oxygen

by dissociation

recorrt,ination reactions

species

In p e s.

(9,10), atomic

atomic owgen Almost hydrogen

for generatIng

with nitrogen

bromine

the reactions

atom abstraction

near colllslon

is an order of magnitude

frequency

of most general

by atomic fluorine efficiencies

that there are many reactions

which

are too slow

to yield

are those involving

However,

deteriorates

atomic fluorine

before

as a result of reactions

extensive

Fluorine

re-cleaning

IS required

atoms can be generated

or an inert gas/molecular the latter method with overlapping dissociation

involving

can be minimlsed

spectral

of molecular

because

bands. fluorine

used to study the SH radical

mixture

and surface

than 20 hours

of carbon tetrafluonde

In spite of the potential hazards,

the former

Under appropriate can be achieved

leads to too many problems conditions (13)

approximately

Thts technique

(13) (from the F/H2S reaction)

being used to study the formyl radical fluorine with formaldehyde)

discharge

rapidly

time for a spectrometer

IS rarely greater

by a microwave

fluonne

IS preferable

but the working

ensures

there are difficulties

of the spectrometer

This problem

occur with

of atomic fluorine

The most severe of these IS that the resolution materials

(11) and

(12)

Most such reactIons

can be used

which

hydroxyl

monoxide

monoxide

importance

and reactivity

and

litmted success

A few reactions

dioxide

to form bromine

transient

slower than the

chlorine with ozone to give chlorine

with molecular

certainly

pre-treated

resonance

it has been used only with

IS at the limit of acceptability

have been used are atomic hydrogen radicals

method

The main reason for this IS that many such reactlons frequency

Jn the presence

tube IS normally

such as laser magnetic

spectroscopy,

As a rule of thumb, a reaction which collision

usually

reactions

this has been a very profitable

spin resonance

molecule

(8) can all be

acid

for study by other techniques

electron

Atomic

In order to minlmise surface catalysed atom

the inside of the discharge

II) Gas phase atom-molecule Although

the biggest of atoms for

reactions

(7) as well as the atomic halogens

of the parent dlatomic

or bone

perhaps

high concentrations

by means of simple atom-molecule

of an excess of argon or hellurn. with phosphoric

of producing

However,

has been

and IS currently

(14) (produced by the reactlon

and the NH2 radical

80%

of atomic

(15) (by reaction of F with ammonia)

JDYKJXetal

48

111) Pyrolysis techniques Pyrolysis techniques have been widely used especially up to temperatures of 12 - 1300K.

Conventional electrically heated furnaces up to this temperature

11mit are readily constructed.

Often transportation of the gaseous species across

the photoionization zone is facilitated by passing an Inert gas or nitrogen through the furnace

Species such as NF2(16) and HBS(17-18) have been studied using this

method to produce the transient There are certain advantages to using this method of production. Not least of these 1s the fact that the spectra are often easy to asslgn since the primary dissoclatlon product is usually the transient species of interest. Even more directly, many gaseous species can be generated by heating of the solid material, e.g. studies of the metal halides have been made

(19-23)

Conventional current heating of furnaces becomes Increasingly dlfflcult at high temperatures

Apart from ensunng

that electrical interference does not occur,

one has the problems associated with the use of very high currents

Various

alternative heating methods have been employed Tncludlng laser heating However, the method which we have employed has involved

(24)

lnductlve heating

(25)

The temperature limit in radiofrequency heating is set by the type of susceptor used for the furnace material and temperatures of 2900K have been measured in recent work using carbon as the furnace material

A scale diagram of the type of

furnace which has been employed is shown in Figure 2

It was found in practice

that operation of the RF generator close to the spectrometer led to coaslderable Interference in the spectrometer detection circuitry

Effective screening of RF-

carrying or spectrometer components was not possible

The generator was therefore

used in a pulsed mode in conJunction with a linear gate in the detector circuit. Full details are given in reference 25.

This type of furnace has made possible

studies of a variety of transient species lncludlng the CH, radical(26) SiO molecule

(27)

and the

UPS OF TRANSIENT

SPECIES

49

Schematic of lnductlvely-heated furnace , showing spectrometer entrance Figure 2. slit (A), additlonal d>ffuslon pump facllitles for ionization chamber (B), lnductlon co11 (C), furnace (D), heat shields (E), outer shield (F), 0-ring vacuum seal (G), front shield (H), lonlzatlon chamber flange on which furnace assembly is mounted (I), ionization point (J), RF power feedthrough (K) and observation window for temperature measurement (L) Information

from

studies

Many spectroscopic high

resolution

of

transient

techniques

of

have been used to study

some methods

such

resonance,

microwave

and electronic

the

states

the

ground

present only

time

rarely

However, ions

p e s

It

is

from the

of not

the relate

As well it

still

of

of

course

in p.e

previously

neutral

this

one is

detected

molecule

as determining

that,

ordering

regarded

as isoelectronic

of

example

because

the

a great

relative ionic in

one sees that

positive

deal

ionic states

terms only

in

of in

state

energies

the

the

ions

least to

the

molecules,

neutral

molecule

can be applied

to

have been obtained

this

new data

valence

Since

selection

by one electron of

of

studies.

spin

at

Ions by optlcal

successes

The

electron usually

on small the

method which

governed

to make comparative

relative

concerning

have been observed the

that

characterized

most new data

of

not

One of

ve ease wl th which

has been possible

area

molecules

resonance, such

have concentrated

spectroscopic

energies

s

are

are well

transients

transient

magnetic

been obtained

relative

in

molecules

no general

IS therefore

1) Determination

states

studies

as laser spectroscopy

neutral

has new lnformatlon there

Since

of

species

type

isolated Fl gure

dlphosphorus

study

has been

has been accumulated in

I’$,

lonlc

ionization

of

Hence,

electrons.

rules,

molecules, 3 shows the

PN+ and PFI; which This

can be

1s an interesting

case does Koopmans’

J DYKEetal

50 theorem

Its breakdown

hold

correlation

and reorganlzatlon

considerable

theoretical

state

energies

these

effects

investigate observed

in the other energies

interest

are an essential A revlew

these

lonlc

problems

and accurate

prerequlslte

and more

has already

2

20

Schematic diagram Figure 3 Abscissa of N$, PN+ and P$

It is well Predlctlon these

energy

values

methods

which

to ldentjfy

of

for the ion?c

the magnitudes

are avallable

of

to

the expenmentally

(1)

I

’ .. *

16

._

14

72

10 eV

that the shape of a photoelectron of the molecule

the low-lying states (arbitrary units)

envelope

depends

on the potential

band

and appropriate

In the case of dlatomlc

obtalned

of

states

surfaces

(28,29)

can be

experimental

showing the correlation between I P.(eV) Ordinate Intensity

of the Franck-Condon

surfaces.

lnformatlon

18

of lonjc

understood

a problem

for lnvestlgatlng

generally

IL\

7s due to the effects

is in itself

been presented.

..I

I

PN+

the potential

This

of the theoretlcal

states,

11) Charactenzatlon

two molecules

1s determIned ionic state

on an accurate molecules,

curves

knowledge

a great

deal of

of the ions from the

of

by

UPS OF TRANSIENT observed

photoelectron

have usually neutral

molecule

of the ion measured

bands

provided

can be correctly

51

SPECIES

an adequate

For diatomic asslgned,

Hence,

assuming

often a reasonable

estimate

the electronic

transition

the band, Franck-Condon wave

using Morse

functions

In some cases where estimated within

we believe

to an accuracy

of the Franck-Condon observed

which

some value

potential

calculated

is known

(30)

There

by other

(5)

in the lonlc states

for bond lengths

evidence

agreement that the

1s not always

a good one

methods,

bond

4 shows a stick diagram

assumed

bond lengths

5 gives the potential

spectroscopic

If

the experlmental

can be used to estimate

using this information.

and

constant

ionic bond

spectroscopy,

Figure

at various

Figure

values

moment

the method

than ~0 011

calculated

with

is, however,

transition

that normally

of better

by optical

Normally,

the vibrational

of the bond lengths

can be made between

electronic

has been derived

envelopes

has been

over the width of

for assumed

to determine

bands

products

from the spectrum

to be lnvarlant

curves

good estimates

states of the SO+ Ion

SO+ have been observed

ionization

can be computed

of these

factors

the dissociation

of the ionic state

is assumed

energy

+O 005 a has been found

Nevertheless,

diagram

moment

comparisons

of a constant

curve for the

can be made for x, the anharmonicity

In this way and those obtained

assumption lengths

energy

techniques

that the photoelectron

can determlne

of the ion, can be obtained

potential

ones has given reasonably

spectroscopic

of the potential

that the adiabatic

envelopes

Matching

other

molecules,provlded

by p.e s , the dissociation frequency

earlier,

knowledqe

one normally

Zie the vibrational

lengths

As stated

for

energy

Since no ?on?c states of this information

is of

J DYKEietal

52

1540

1545

1517

Iso

exptl

Stick diagram showing calculated and observed intensities for the Figure 4 vlbrattonal fine structure in the ba ds assigned to the states of SO* shown Calculated bond lengths (re) are tn B . Perhaps one of the important features of this type of study is that one 1s able to predict reasonably precisely where optlcal transitIons shoutd occur.

Hence to

a large extent one has solved the search problem in optical spectroscopy which can be severe especially If one 1s attempting to make use of tunable lasers as probes.

It should therefore prove possible to locate and characterize some

of these states in more detail.

UPS OF TRANSIENT SPECIES

53

PO entlal energy diagram for the observed sta es of SO+ Figure 5 Ordinate Potential energy x10-4 (cm- t ) Bond length (Ft)

III)

Observation

of excited electronic

states of the neutral species

So far only one species, the metastable Attempts to observe the analogous

02(',9) state has been detected by p.e.s

state ln sulphur monoxide have not met with any

The importance of such studies would be that on

success

Abscissa

a new family of ionic states 1s generated. *e,, and *A9 states of 0; were observed

one-electron

ion,zation,

Hence in the case of 02('bg),

and characterized

the

for the first time

(32)

It 1s likely that progress In this particular area will be slow because of the formidable states.

difficulties

anslng

from the short lifetimes of most excited electronic

One area where progress may be made is ln attempting

excited atoms formed in simple atom-molecule

reactions

to detect electronically

There is a possibility

the reacttons of atomic fluorine with the hydrogen halldes for example, the formation of atomic halogen in the 'Pi state have Important consequences

(33,34)

that

lead to

Such processes would

in the studies of the dynamics of these simple

reactions and It would be of Interest to determine the branching ratios between the

?P

312

and 2P

3

states

In theory,

p e s

should be capable of achlevlng this

JDYKEetal

64 IV)

Observation of excited vibrational levels in neutral molecules

Only one such study involving vibratIonally excited nitrogen and hydrogen has been made (35), although "hot" bands are sometimes observed especially in high temperature experiments

In theory the p e s

technique should be quite a useful

one for studying vibrationally excited species since operating pressures ln the photolonlzation region are low compared with many other spectroscopic methods. Hence, highly exoerglc atom-molecule reactions such as those involving atomic fluorine might well be amenable to study

The aim of such work would be to

determine the lnitlal vibrational energy dlstnbutlon

in the product molecule

This would be a valuable complement to the Infrared chemiluminescence method and would have the advantage that the population of the ground vibrational level might also be measured

Additionally, the technique might be extended to

reactions which do not give products which emit in the near Infrared region and are hence difficult to study by the chemilumlnescence method v) Studies of neutral molecules It was stated earlier that the neutral species has usually been well characterized by other spectroscopic methods

Occasionally this is not the case.

of this are the methyl and t-butyl radicals

Two examples

Under these circumstances, some

information can be obtalned by the p e.s. method The p e

spectrum of the CH3 radical has been investigated on a number of

occasions (26, 36, 37) One of the studies (26)found well resolved vibrational structure associated with the band attributed to the first ionization potential. Analcgous fine structure was also observed for the deuterated molecule Application of the Teller-Redlich product rule assuming C3v symnetry led to predlcted vlbratlonal frequencies for the CD3 molecule, which were outside the experimental error limits of those observed. for the methyl radical could be excluded.

On this basis a pyramidal structure

Moreover, the frequency shifts were

compatible with those predicted assuming a planar model for the neutral radical. Hence one can say with some confidence that the methyl radical IS a planar molecule Ab lnltlo calculattons and.computed Franck-London envelopes support this conclusion The case of the t-butyl radical is more complex because of lack of knowledge of the many additional vibratIona

frequencies of this molecule.

However, a

photoelectron study using vibrational selection rule arguments as applied to p e s strongly support a pyramidal structure for the carbon skeleton in this molecule.(38) This conclusion 1s in accord with the interpretation which has been made of the large coupling constant for the tertiary 13C together with Its negative temperature dependence (39, 40, 41)but in disagreement with conclusions reached by other workers. (42,43) Recent Developments 1) High temperature studies The ability to obtain acceptable p.e

spectra of species generated at temperatures

UPSOFTRANSLENTSPECXES

55

in excess of 2000K could have important implications especially for studies of inorganic molecules.

One such species which has been studied is the SJO molecule

which was generated by heating polymeric SiO at % 2300K (27).

SiO (Xlc+) has the

electronic configuration ('1~)~ (2~)~ (3~)~ (lm)'+ (4~)~ (5~)~ (6~)~ (2m)'+ (7~)~. Bands anslng in the p e

from lonlzatlon involving the three outermost orbttals were observed spectrum as shown in Figure 6

On the basis of this spectrum it was

possible to construct a potential energy diagram for SiO+ and to compare experimental data with the results of multiple - scattering Xa calculations Work is presently underway to extend these studies to transItJon metal oxides

s,o+

IO<

xkz+)

-

sad

I

(dI+) +

-CO

Figure 6 Ordinate 11)

.

h

.

He1 photoelectron spectrum of SiO counts 5-l

Studies of fluonne

Sld

co+

Abscissa

--ia

(Aan) H

I P (eV).

atom abstraction reactions

This method is Important because of its wide spread appllcabillty and because of the small number of precursors which are present to complicate the spectrum. It has recently been

used to obtain the p.e

spectrum of the SH(X2n,) radical (13)

when other methods (discharge and pyrolysis) proved unsuccessful

The ground

state electronic configuration of the neutral SH radical can be written as (lu)2 (2u)2 (30)2 (lm)4 (4a)Z (5a)Z (2lT)3. The energies of the 2n and 5u levels are accessible with He1 radiation.

Hence five bands were observed in the photo-

electron spectrum as shown in Figure 7 lA,

lx;+,

These correspond to formation of 3~',

3n and ln.states of the positive ion.

J DYKE et al

66

SH*IX’>tcSHdrll n

lOOOr

SH*l’At+StilXbnt

0

18

17

t6

I5

He1 photoelectron Figure 7 atoms with hydrogen sulphide An Interesting (15)

This

Currently states

there

is particular

laser photoelectron resulting

states

considerably values

wqth

greater

the energy

assuming

The analogous

determined states

Very

recently

NH2 problem

the vertical

potentials

optimization

in order

ionization

A full

account

Future

developments

experience

certainly

were

A recent

present

to obtain

more

gave

and a

of the two states but we have triplet

and singlet

of NH2 and 0.67 eV for performed

information

with

the aid

on these states

the calculated

is 0 73 eV in good agreement be given

1s

has been

attention

are being

In progress,

(IAl)

This

in the spectrum,

potentials

CalculatTons

excited

work

separation

little

bands

(45-48) which

of the corresponding

separation

with

of

experiment.

in due course

the future with

has been that the developments

been the ones which Almost

potentials

to predict

of the ionic

(0 84~0 03) eV

the experimental

ionization

are still

of this work will

It 1s diffqcult

the NH2 radical

radical

calculations

for the energy

the separation

calculations

energies

(3B,) and first

has received

ionization

these

IO

from ammonia

the methylene

being

that "hot" bands were

experimentally

of geometry Although

with

theoretical

as 1 04 eV for the adiabatic

the vertical

In the relative

difference

value of 0 51 eV has been obtained (45)

involves

atom abstraction

of CH2 in Its ground

eV

II

study of the CH,- ion (44) ldentlfied

than the best

of (0.48+0.02)

reinterpreted

interest

detachment

in progress

by hydrogen

it is isoelectronic

from formation

electronic

is currently

1s generated

of NH2 because

12

spectrum of SH obtained from the reaction of fluorine Abscissa I P (eV) Ordinate counts s-l

study which

radical

I3

14

any confidence

which

have occurred

especially

as our

have not always

the most expected

however

there will

be a conslderable

increase

in interest

in

UPS OF TRANSIENT

57

SPECIES

molecular fragments generated at high temperatures.

The study of Ionic molecules

would be an obvious application of high temperature p e.s.

In favourable lomc

compounds, changes in chemical shift can be followed through the process of dlmerlzation, tnmenzatlon

and eventual polymerization

This InformatIon should

then be useful in understanding chemical shift data obtained from solid state studies.

There IS also the possibility that species generated by flash photolysis

methods will be studied

Both of these developments would be considerably

facilitated by a spectrometer of higher sensitivity. channel detector system developed by Siegbahn et al

In this respect the multi(49) which has an effective

count rate Improvement of approximately two orders of magnitude, offers exciting possibilities In both this and other fields. Undoubtedly there will also be continued development of the theoretical methods used for interpretation of spectra

Of particular interest IS the application of

the Green's function method (50) to the interpretat'lonof photoelectron spectra of transient species

Another useful development would be the improvement of

exrstlng methods for predict'lonof photoionization cross-sections of small molecules in order that relative experimental band intensities can be used in a routine way

as means of band assignment Acknowledgements We are extremely grateful for the considerable financial assistance which we have recieved from the Science Research Council

Above all we would like to

dedicate this article to the memory of our good friend and colleague Joe Hawkins who tragically died after a short Illness on August 4 1978 deeply aware that wIthout his unfailing good will in

and highly

skilled

We are

craftmanshlp

constructing the apparatus, much of the work described in this article would

not have been accomplished.

J DYKE et al

58

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27

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