Molecular desorption induced by heavy particle bombardment of solids

243

International Journal of Mass Spectrometry and ToonPhysics, 53 (1983) 243-254 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

MOLECULAR

BARBARA

DESORPTION

INDUCED

BY

HEAVY PARTICLE BOMBARDMENT

OF SOLIDS*

J. GARRISON?

Department PA 16802

of Chemistry, (U.S.A.)

The

Pennsylvania

State

University,

University

Park,

ABSTRACT A classical dynamics model is used to investigate nuclear motion in solids Of interest are the mechanisms due to bombardment by energetic atoms and ions. of ejection and cluster formation both of elemental species such as Ni, and Arn and molecular species where we have predicted intact ejection of benzene-C6H6, The pyridine-CBHBN, napthalene-CIOHB. biphenyl-CI2HID and coronene-C24HI2. results presented here show that the energy distributions of the parent molecular species, e.g. benzene, are narrower than those of atomic species, even though the ejection processes in both cases arise from energetic nuclear collisions. The bonding geometry also influences the ejection yield and angular distribution. The specific case of n-bonded and o-bonded pyridine on a metal surface is discussed with comparisons between the calculated results and experimental data. These calculations provide a means of interpreting SIMS, FABMS and possibly even PDMS experimental data. INTRODUCTION The

bombardment

interest

due to the

molecules (ref.1)

that

are

ejected

in these

Our

goal

Many

mechanisms

to

and presented May

tAlfred 0020-7381/

by energetic

of large in the

solid

novel

original

are formed

from

and

the

oxide

beams

species.

sample

during

nitrous

particle

such

has attracted These

species

can

be

as a dodecanucleotide

bombardment (ref.2).

event,

Numerous

for example other

examples

proceedings.

of the clusters

*.Dedicated

that

has been

ship

15-18

ejection present

or clusters

CNO(N203)3]+ appear

of solids

to understand

to the

involving

Professor

original

both

the

ejection

mechanisms

configuration

of atoms

the motion

R. M. Macfarlane

at a symposium

held

in his

of the atomic

occassion

honor

at College

P. Sloan

Research

SS/$OS.OO

0

Fellow

1983 Elsevier Science Publishers B.V.

relation-

in the sample.

nuclei

on the

1983.

and the

of

and/or

his

50th

Station,

of

birthdav

TX USA

244 electrons

can

However,

if

energy

in

atomic

nuclei

process

be

the

.

Thus

The

(SIMS), mass

to

fast

spect

favorable

the

classical

are

not

the

ejection

ry

procedure

classical

model

secondary

.

the

results

from

can

that

be used

to

experiments

in

the to for

ion

mass

the

plasma

at

the

order

to

give

of

approximating

ry

desorption is

and

are

microscopic us

of

can

there

model

cascades

look

motion

s pect romet

where

classical

collision

the

bombardment

observables

and

circumstances the

with between

model

the

(FABMS),

the

In

particle

collisions

understanding

spectrometry

be concluded

then

for

molecules.

a heavy

energetic

dynamics

sputtering, mass

be

the

by

important.

processes

further

which

insight

into

mechanisms. the

theoretical

adsorbed interaction integrated

are of

time

used

to

particles,

cluster

formation.

monitor

the

by

the

the

experimental

and

atoms,

The

the

analyze

3-12).

of

final

as

of

particles

total and

procedure

microscopic

Assuming

positions

such

and

equations

all

distributions

classical

solid

Hamilton’s

momenta

angular of

the

(ref.

observables

distributions,

events

atoms

and

cascade.

One advantage

collision

all

of

positions

collision

the

energy

array

among

yield the

during

consists

a finite

potential to

determine

ejected

model

molecules

a pairwise

be

the

experiments

can

from

motion

can

of

between it

model

Briefly,

function

point

ejecting

bombarded

must

a classical

from

for

is

there

a starting

(PDMS)

accessible

possible

as

data

data

The

eV range

bombardment

agreement

experimental

responsible sample

predictions

atom

romet

be

liquid)

developed

nuclei.

be compared

to

(or

100-10000

we have

atomic

proposed

a solid

is

mechanisms

as

a

and

momenta

yield

of

possible

that of

one

can

various

processes.

MECHANISMS From

the

classical

formation

dynamical

mechanism

cluster basic

OF CLUSTER FORMATION

yields,

in In

mechanisms

treatment,

detail

and

general,

of cluster

identity

such

as metals,

or

atomic

with

each

in

the

cluster apply In

by to

this

sites

clusters case

on

the

calculations radius

a recombination

-

the

the

type in

although

indicate

that

5 angstroms.

clusters.

This

deduction

in

of

the

surface

observed

that

there

are

systems

with

for

on a solid,

(ref.

above

many

need

absence

of

originate however, structure

an At-25

.

3-5)

in

do not

them

rearrangement,

We have

First,

region

process

the

of

examine

cluster

about three atomic

the

ejected

atoms

crystal

to

of

arise

from to

contiguous

ionic

the eject

would

experiments.

forces

a circular-region

complicates

can

form

description SIMS

from

long-range from

cluster

the This

types

to

the

information

suggest

adsorbates

cluster

to

semiquantitative

MnOm observed

most

possible

(ref.12).

of

the

surface,

straightforward observed

of

provide

near-surface

type

atoms

is

calculations

formation

interact

other

to

these

it

the of.

any composition from

sol id

of

the

argon

a

245 via this

mechanism

(ref.

clusters

(CsI)nCs+

with

We would

13).

n as large

as

also

speculate

70 (ref.14)

also

that

the alkali

form

by this

halide

basic

mechanism. A second

type

as simple

as carbon

mentioned

above.

interaction that

this

although energetic the

with energy

of cluster monoxide In the

difference

metal

original

atoms

atomic

chemical

state.

example,

the

If

case,

is only

(ref.6).

can

about

CO were

1 eV. to allow

ejection

by the

molecular

that

into

which

can be

11 eV, but

the

indicate

of CO molecules,

ion beam

systems

or by

it is easy

and to determine

oxygen

the amount

is -

Calculations

of the molecule

dissociated

suggest

species

as the dodecanucleotide

be dissociated

For such

configurations

molecular

the CO bond strength

is sufficient

of them

calculations

invotves

or as complicated

first

the surface

- 15 percent

emission

and carbon

the surface

atoms,

of CO observed

to infer

for

should

drop

dramatically.

Fig, 1. Placement of molecules on Ni(001). For graphical efficiency alI molecules are shown on the same Ni(001) crystal face. However, the ejection dynamics of each molecular type is determined in a separate calculation. The outline of each molecule is the approximate van der Waals size. The nearest neighbor nickel distance is 2.49A.

Although

the

illustrated

with

molecules

is

series

of

more

carbon

monoxide,

obvious.

molecules

adsorbed

molecular

just

are

naphthalene,

The

geometry

From

a few

cases

a large from

in

of

compared

to

several

parts

the

moves

i n one energy

of

energies

the

are

factors

2-3

metal process.

particle

has

ejection

mechanism

progress

on the

eject

intact It

because

the

cases

for

sample

of

molecules

the

is that

the

organic

that

in

to

be small to

strike

entire

molecule

is

that

the

kinetic

of

eVs.

molecule

after

three

benzene that

during

the

the

primary

This

molecular

struck,

These

monoxide, we found

the

water

more

molecule

femtoseconds.

the

will

the

carbon

of

absorb the

possible

so

eject

be applied

shows

can

molecules,

to

200

and

thousands

parts

than

ice

it so

1.

(ref.8-9).

Second, atoms

thus,

larger

molecules

can

of

freedom

either

Fig. intact.

process

manner

or of

different

less

of

dissipated

hundreds

the

is

been

ejection

strike

time

for

to

that

a severing

make

that

is

Most

.

parent

the

the

bombardment

forces

there

are

collision

the

difficult

observed the

the

time

in

unknown.

intact Work

solids.

molecules

can

in

also

(ref.15).

is

However,

Thus

hit

than

for

would

The

the

has

rather

in

atoms

ejection

particle

eVs

However,

by

this

individual I);

shown

be ejected

dissociating.

Fig.

surface.

are

is

a

on a metal

structures

may

be

have

molecules,

favor

degrees

a concerted

Finally,

valid

factors

which

bioorganic

molecules

molecule,

(see

in

of

species

without

atom

molecule

of

these

internal

organic

a metal

equally

coronene.

often

a large

of

for the

on a metal

adsorption

molecular

performed understand

r-bonds

diameter

whose

to

molecules

a-bond

can

bioorganic

been

first

which

or

the

three

many

have

benzene

mechanism

large

surface

The

n-bond

each the

collision

primary

tens

are

for

has

di rection.

of

are

calculations,

molecule

framework

Calculations

coronene

that

an energetic

massive size

and

ejection to

on a Ni (001)

approach

assumed

intact

extrapolation

either

sizes

we find

theoretical

energy

or

examined can

biphenyl

First,

the

atoms

whose

this

(ref.8-10).

which

we have

all

our

ejection

pyridins

structures,

In

the

necessarily

and

Larger

behind

not

of

than

principles

organic

mechanisms

surface

basic

one

of

govern

the

rips

part

off the of

two the

determinations

the

an

related

formed

from

H, CH,

or

of

the

rings

is

molecule

of

rearrangement

feature

fragments minus

between

all

interesting

molecule

structure

quantitative

channels to

molecule. observed influences

the

occur

to

are

not

within

arise

from

case

of

with

the

nature

is

essentially

knowfl. we have -

the

0.2

pS

an energetic

biphenyl

some

of

yields

that

collisions These

In

fragment

fragmentation

direct C2H2.

the

however,

frequency. fragmentation

process. The involving we find

final both that

mechanism intact the

for

cluster

ejection

and

observed

NiCO,

ejection recombination.

Ni2CO

and

NiFeCO

In

the

clusters

a hydrid

mechanism

case

of

CO on

NigFe,

form

by

a

247 recombination apparently

of ejected

no direct

surface

states.

propose

a reaction

Ni+ + C6H6

In the

with

these

of cationized

ejected

species

There

CO molecules.

moieties

and such

linear

and

ions,

as NiC6H6+

is

bridge-bond

we

NiC6H6+

that

the

(I)

Ni supplies

(Fef.16)

the

and that

the

charge

is based

ionization

on the

potential

that no

fact

of Ni is less

than

of benzene. This

dimer

final

hybrid

ion of the

molecular

unit

surface

from

the original

The turns

region

fact that

out,

is four

times

25 percent

The

for

of the ejected

clusters

may

be different

discouraging.

situations

where

the precise

nature

One example

involves

theoretically.

atoms

of oxygen oxygen

coverage

coverage

effect

~(2x2)

of this

them

This

is also

is that

surface,

sort

the

on Ni(001).

that

may from

be useful those

ratio

than

for

calculated

there

and the

As it

of the

[c(ZxZ)coverage]

a change

for this

on the

Concepts

Each

in the

is somewhat

and distinguishing

distinct

(ref.17).

molecules

atoms

The reason

mechanisms

several

clusters

other

of the

entities.

[p(2x2)surface],

lower.

with

formation

of surface

50 percent

oxygen

for the

1) or of water

interacts

cluster

as a function

(ref.18).

are

no

02 formation in testing

that

for

proceed

phases.

DISTRIBUTIONS energy

distribution

are characterized approximately

molecular

ejected

crystal collision

collision

species

in Fig. face

from

(ref.19).

the distribution

n=2.

Since

as the Ag+ of collision

and the

C&H6

The energy

narrower, with

ejected

and a high

however,

energy

the molecular

distributions and often

of benzene species

and on the same

energies

molecules.

that

cause

However,

tail

the

that

experiments goes

is characteristic

distributions

a monolayer

ions

in bombardment

energy

This distribution

experimental

a system

species

at l-5eV

cascade.

are much

2a are

cascade

the Ag atoms

of atomic

by a peak

as E'" where

non-equilibrium Shown

are

coverage

is much

island-growth through

the

be predicted

neighboring

(ref.

composition

there

ratio

oxygen

probability

ENERGY

the

higher

the model

closely

may be responsible

and then

arrangement

can

02-/O-

intact

to form

however,

rearrangement measured

mechanism

dodecanucleotide

ejects

near

ions

case

>

C6H6+ is observed

with

Fe atoms

between

of the type

surface

The presumption

that

Ni and

relation

of the

terminate

of a parent

at -1OeV.

for Ag+,

C6H5+

and

adsorbed

on a

Ag(lll)

is ejecting

timescale ejection energetic

during

one would to be the

C2H2+

the same expect

same

collisions

for

with

248

the molecular benzene

at higher shown

than

correlated C2H2+

are

energies

in Fig.

energy

bond

species

molecules

Note that

of the

distribution

should

the same

by the

peak

of the

of the

be higher

since

Thus should

Since

the

to the

its binding from

the have

the

energetic a distribution

energy

C2H2 + distribution

species

distributions

physical

then

C2H2 + fragment

distribution.

energy

The energy

fragmentation,

The fragments

the

C6H5+

to the binding

energies.

illustrate

and do cause

depleted.

as is illustrated

2a..

that

can

peak

distribution

is at a higher

position

surface, energy

can

the

peak

includes

calculations

be

(Fig.

of the

two

C-C

2b)

phenomena.

b

Bios voltage

20

IO

0

Energy (e!‘)

Wl

Energy Distributions. a) Experimental Ag+, C6H5+ and C2H2+ ion Fig. 2. plotted versus intensities from one monolayer of benzene adsorbed on Ag(ll1) the voltage on the sample. The primary particle,is Ar+ with energy 1 keV incident on the sample at a polar angle of 45O. The secondary particles are The raw data has been smoothed. This data collected at a polar angle of 60". has been graciously provided by D. W, Moon, R. J. Bleiler and N. Winograd b) Calculated C6H6 and C2H2 energy distributions from prior to publication. one monolayer of benzene on Ni(OO1). All ejected particles have been counted. The calculations are described in ref. 9. The energy resolution is 5eV.

It

is tempting

to use

a key to understanding must

be taken,

distinctive calculations tail.) not

also

In fact

approximated present

however,

shapes

the

as collision

of energy predict

the

energy

the mechanisms

the

cascades

can

of the for the give

ejected

in Fig.

of metal

atoms

during

the

distribution form

even

ejection

of Fig.

though event.

2.

2b can

three

(The

to have

a thermal

as

Care

rise to at least

as shown

C6H6

particles

desorption.

distribution

calculated solid

responsible

distributions

by a Maxwell-Boltzmann in the

distributions

a high

energy

be reasonably equilibrium

The calculations

is

indicate

that

energy

charged)

distributions

species

could

of elemental

possibly

(and preferably

be the most

useful

both

experimental

mechanisms

as these

particles

energetic

collisions.

Even

however,

one can obtain

from

experiments

SIMS

(~250

eV} primary

MATRIX

that

large

composition

influence

of the solid

on the types

only

for the

Fig.

3 are SIMS spectra

data

in Fig.

ionization

'rapidly than

to the

be fragmented

energy

E-2

in

distributions

if low energy

(ref.20).

3b was

(ref.

monolayer

coverage.

This

ions.

on Ni(OO1)

where

16).

SIMS spectrum

This

benzene

they

attenuate

however,

does

dosed

the

contain

is shown CnHm+

where

reason

have

3d.

n < 30.

with

They

due to the

low density

It is obvious parent

from

spectrum.

species

favorable

studies

are

However, to extract

parent

species

molecule,

are observed.

Here

on the

then,

C6H6, data,

at a et al. to

We believe

strongly

in

the is

as there

of large 3d.

the

is an

compound,

solid

The

clusters

are

it would

The calculated

3b, however,

the electronic

affects

one where

cationization,

to be the most

Fig.

spectrum,

and C3 species,

3~).

to be the

e.g.,

Fig.

This

of

Ni surface.

as long

a variety

layers

Lancaster

preparation

appears

from

[ref.

n > 6 in the calculations

were of an unknown

species

experimental

NiC6ti6+ peak.

(Fig.

with

identified

of benzene

corresponding

the Cl, C2,

the sample

in that

the parent

the

are

ionization,

if the sample

The comparable

but a large

of

from

peaks

study

adsorbed

benzene

at all masses

that

easily

means

spectrum

NiC6H6+

masses.

on solid

species

and

one

for C6H6

peaks.

peaks

molecules

Fig.3b-d

interesting

observed.

3a) predicts

CnHm+

Ni2+

The

to 3

Here the multiple NiC6H6+

not in

substrates.

langmuirs

and Winograd

The low coverage

difficult species.

The mass

Ni+,

of lower

performed

observe

of benzene

can be most

energetically benzene

been

these

2100

Shown

to approximately

the

3c.

fragments

of Karwacki

we do not observe

the mass

in Figure

The predominant

the work

with

has a

is true

exposed

Ni(OO1)

SIMS experiments

and cationized

of 77 K (ref.17,21).

in Figure

agreement

Ni2+,

hydrocarbon

Two SIMS experiments temperature

the surface

is shown

Ni+,

only

performed

This

motion.

different

corresponds

contains

also

nuclear

for three

dose

bombarded

to eject.

ion bombarded

This

spectrum

and Winograd

is being

for the

taken

for Ar+ 16).

which

observed

but also

of benzene

obtained

of benzene

Karwacki

or matrix

of species

process

langmuirs

which

off more

are used

cannot

and

EFFECTS

The

(Fig.

fall

ion beams

neutral

for comparing

different

here,

the

abundant

be

spectrum organic

has no C6H6+

environment

influences

peak

250

100% -

C6H6

Ni xv2

H

CH NIGH, Ni,

CzHz C,H, C,H, C, H,

I

I

1

0

I

NiC,H$

100% -

Ni+

x1/5 K+

Ni2+ b

100%

Ni+ x l/2

I I Fig. 3. Benzene mass spectra. identified. a) Calculated, langmuirs of C6H6 on Ni(OOl), of C6H6 on Ni(OOl), (ref.16).

I

.I. Ia MOSS

I

I

Niz

NiC,Hz

I

, c

I

I

--*I

d

The most intense peak in each grouping has been (ref.9). b) Experimental SIMS, 3 (ref.16). c) Experimental SIMS, 2100 langmuirs d) Experimental SIMS, sol id benzene [ref. 17).

251 MOLECULAR

ORIENTATION

It is of

interest

to the surface the surface

with

pyridine,

however, remainder

most

striking

molecular from

for the

pyridine.

Very

bond

molecule

be no efficient into

ordered

pyridine

a single

ejection

collision, surface

modes

of transferring away

totally

from

molecules,

for these lead

low

(ref.

then,

the

The of

cleavage

of a

the

There

appears

with

to

the

the original

ejection

expect

on a metal,

of

is clear

is struck,

Obviously

not necessarily

9).

ejection

of collisions

atom The

yield

two structures

the specific

affects

In

nitrogen

computed

the surface.

molecules

the

to dissociate.

energy

cloud.

the

atom

with

the surface.

to molecular

a carbon

or tends

One would of organic

is that

requires

the

from

bonded

interaction

through

away

is extremely

When

on the

processes.

and

similar

spectra

a liquid,

from

or an

solid. orientational

measurements system

the

of pyridine benzene

but

rearranges

intensity

of the A9C6H5+

on the silver

surface

ion intensities, rearranges

on

while

coverages

ion monotonically

Ag(lll]

increases

and finally

and

increase

In this

(ref.22).

to o-bond

increase

in SIMS

the pyridine

n-bonds

to the

surface.

decrease

again

as the

The

coverage

The AgC5H5N+

then

at low

the benzene

as

to one monolayer.

initially

of the

and

flat

Ag(lll),

(monolayer)

the

are broad

normal. angle

At

polar with

species

ions

on

pyridine angle

a peak

have

where

distribution

0.15

been

illustrated

and C5H5NH+

as the molecule pyridine

of the C5H5NH+

The polar

coverage

measured.

(M-H)+

in bonding ion sharpens

angle

The

(n-bonded), results

4.

For monolayer

are

believed

(benzene)

with

the

L benzene

L pyridine

the molecules of

influences

-2.5

in Figure

at e = 20" measured

the onset of the change

distribution

systems

u-bonded)

Ag(ll1)

are

also

(ref.22).

for four

(monolayer,

measurements

for low coverage

on the surface

ejected

ejected

4.5 L pyridine

benzene

the polar

at higher

is increased

of various

distribution

surface

adsorbed

of the molecules

distributions

12 L thiophene

on

benzene

been confirmed

recently

to one monolayer.

distributions (monolayer),

have

to the surface

however,

The arrangement

(pyridine)

and

on the surface,

is increased angular

effects

n-bonds

coverages

and

that

stays

of a monolayer

These

these

system in yields

of the trajectories

translation

fragmentation a sample

almost

is pointing

pyFidine

for molecules the

via the r-electron

the two cases

difference

simply,

mechanisms In benzene,

atoms

occurs

between

for the

ejection

six carbon

bonding

of the organic

structure

the

vs. PYRIDINE

orientations.

of the molecule

major

during either

molecule

among

difference

an analysis

N-N1

to compare

the

species

reasons

BENZENE

different

is shared

while

the

EFFECTS:

respect

and to

of

to lie (M+H)+

the

configuration,

however,

dramatically

and the

262 peak

moves

to

with

a peak

The polar

13 = 10”.

at e = lO"C,

angle

indicating

distribution

that

it also

of thiophene,

is a-bonded

is narrow

to the

surface.

8 (degrees] Fig. 4. Normalized polar angle distribution of molecular ion yields for 4.5 L pyridine (-, (M+H}+), 0.15 L pyridine (----, (M+H)+), 2.5 L benzene (==-=, (M-H)+), and 12 L thiophene (--9-g -=-, M+) on Ag(ll1) at 153K. The pyridine and benzene data is from ref. 22 and thiophene data has been supplied by the same authors.

FRAGMENTATION There fragments -0.2

has been form

x 10-12s

larger

considerable

primarily after

species

the

during

The calculations

show

from

speculation direct

primary

the that

collisions

estimated

that

approximately

than

5 eV of internal

have

less the

individual

these

molecules

other

hand,

will

to

to the

at the,surface

bond reach

probably

three

has struck)

(Fig. quarters

energy

strengths

dissociate.

or from

of the

(ref.9). intact.

the

Since

(i.e.

within

dissociation

this

numerous

benzene value

expect

The energetic

of

as 10-4s).

calculations

ejected

we would

observed

as long

to form

From

3a).

the

surface

(often

possible

in benzene

the detector

at the

detector

it is definitely

in direct

with

collisions

particle

flight

as to whether

fragments we

have

molecules is comparable

the majority molecules,

of

on the

253 At this approaches Recently

stage

it is necessary

to help Moon

elucidate

(ref.23)

has proposed

distributions

as a means

He finds

that

for chlorobenzene

probably

form

by direct

well

as benzene

molecular

to design

the different

a method

adsorbed

collisions

ions

between

adsorbed

on silver,

the polar

that

surface.

by gas phase

or theoretical

of fragmentation. angle

the fragmentation

on Ag(lll)

on the

formed

experiments modes

of examining

of differentiating

and pyridine

or fragment

clever

possible

the CgH5+

schemes.

and Cl-

For the chlorobenzene he found

that

decomposition

ions as

neither

of a cationized

species.

In that

the case

the

dissociation Spectra

with

species

during

cluster

yields.

clusters

are

CLOSING

in the

cluster

size.

to the

initially

has

though

formed

the surface.

near

after

then,

make

has shown (ref.24).

in ion

deCFeaSe

7Ous,

however,

in the

decomposition

a noticeable

experiments

detector

and a decrease

Here

detector

to the

taken

work

are due to

a monotonic

Spectra

the

recent

size

flight

ion intensity

(CsI)13Cs+

flight

the

exhibit

ion intensities.

These

(ref.141,

cluster

effect

no statement

of larger

on the

as to how the

STATEMENTS

importance

dynamics

procedure

is very

hypothesis

numerous

experimental

data

results

with

experiments

collision processes.

to see

that

powerful

examples

cascade

from

very well. from final

is set

developed

in heavy

SIMS

experiments

collisional

femtoseconds

up and

and

and laser

data where

is responsible

events

the

and

calculations

its predi_ctions

of the

experiments. provides

can be compared.

desorption

cascades

the

bombardment

collisional

experimental

This model

to investigate

particle

for describing

which

plasma

if indeed

in the

has been

processes

against

compare feasible

model

of collisional

a working shown

with

(CsI)15Cst

A classical This

clusters

during

bombardment

increasing

and

halide

species

0.211~ after

an increase

(CsI)14Cs+

alkali

in ion intensity

of metastable

taken

intensity show

of the

oscillations

are

mass

can

We have have

be used

fit to

spectrometry

important.

PDMS experiments for the actual

It is quite that

a

desorption

264 ACKNOWLEDGMENT The Moon,

interaction

R. J. Bleiler,

solidifying use their support

many data

of the

A. P. Sloan gratefully

with E. J.

of the

as well National

Foundation

those

who

Karwacki

ideas

presented

as for many Science and the

have and

supplied

here.

stimulating

Foundation, Camille

the

N. Winograd,

and

experimental has

I thank

greatly

them

for

conversations.

the Henry

Office

of Naval

Dreyfus

data, helped

allowing

D. W. in me to

The financial Research,

Foundation

the

is

acknowledged.

REFERENCES

: 3 4 5 6

; 9 :: 12 13 14 15 :; 18 19 20 21 22 23 24

C.J. McNeal and R.D. Macfarlane, J. Am. Chem. Sot., 103 (1981) 1609. R.G. Orth, H.T. Jonkman and J. Michl, J. Am. Chem. Sot., 103 (1981) 1564. B.J. Garrison, N. Winograd and D.E. Harrison Jr., J, Chem. Phys, 69 (1978) 1440. and B.J. Garrison, Surface Science, 78 N. Winograd, D.E. Harrison Jr. (1978) 467. B-J. Garrison, N. Winograd and D.E. Harrison Jr., Phys. Rev. B, 18 (1978) 6000. N. Winograd, B.J. Garrison and D.E. Harrison Jr., J. Chem. Phys., 73 (1980) 3473. K-E. Foley and B. J. Garrison, J. Chem. Phys., 72 (1980) 1018. 6-J. Garrison, J. Am. Chem. Sot., 102 (1980) 6553. B-J. Garrison, J. Am. Chem. Sot., 104 (1982) 6211. B.J, Garrison, J. Am. Chem. Sot., 105 (1983) 373. N, Winograd and B.J. Garrison, Accts. of Chem. Res., 13 (1980) 406. l3.J. GaFrison and N. Winograd, Science, 216 (1982) 805. B.J. Garrison and N. Winograd, Chem. Phys. Lett., in press. T.M. Barlak, J-R. Wyatt, R.J. Colton, J.J. Decorpo and J.E. Campana, J. Am. Chem. sot., 104 (1982) 1212. D-W. Brenner and B.J. Garrison, work in progress. E.J. Karwacki and N. Winograd, Anal. Chem., 55 ('1983) 79. G.M, Lancaster, F. Honda, Y. Fukuda and J.W. Rabalais, J. Am. Chem. SOC., 101 (1979) 1951. T. Fleisch, W.N. Delgass and D.E. Harrison Jr., N. Winograd, B.J. Garrison, J. Vat. Sci. Tech., 16 (1979) 629. D.W. Moon, R.J. Bleiler and N. Winograd, unpublished results. Ph.D. R.G. Hart and C.B. Cooper, Surf. Sci. 82 (1979) 549; E.J. Karwacki, thesis, The Pennsylvania State University, (1982). Andrade, Anal. Chem., 50 (1978) H.T. Jonkman, J. Michl, R.N. King and J-D. 2078. D.W. Moon, R.J. Bleiler, E.J. Karwacki and N. Winograd, J. Am. Chem. Sot., in press. in press. D.W. Moon and N. Winograd, J. Mass Spec. and Ion Phys.. W. Ens, R. Beavis and K.G. Standing, Phys. Rev. Lett., 50 (1983) 27.