Photodesorption and photoreactions at surfaces

Nuclear Instrument\

and Methods

in Physics Research

I365 (1902)

Nuclear Instruments 8 Methods in Physics Research Sf?iiC~l1 B

lXILlX6

North-Holland

Photodesorption

and photoreactions

at surfaces

H.J. Krcuzer

will survey the main ideas of photodesorption

We

ultraviolet infrared

LO the infrared. laser-molecular

As

examples

and photoreactions

we discuss photodissociation

photophysics

and photochemistry

at surfaces

we

are

dealing with a solid surface on which molecules arc adsorbed. Elcctromagnctic radiation from the infrared to the X-ray regime is impinging on this system interacting in a variety of ways with the solid, the ;Idsorbatc or both, as schematically depicted in fig. I. If the photons arc absorbed by the solid, they can gcncrate phonons, clcctron-hole pairs, plasmons, cxcitons or any other type of collective excitation in the solid, depending on the energy of the incoming radiation. By further decay into phonons these primary excitations can heat up the solid in the surface region with heat conduction taking the energy into the bulk. In addition, if the mean free path of the primary excitations is long enough so that they can propagate back to the surface from their point of generation inside the solid, they can couple into the adsorbed molecules (i) by vibrationally exciting the surface bond or some internal mode of the

Fig.

I. Schematic

surface coupling

with

by electronic

transitions

and pl~(~todes(~rpt~(~l~hy

vjbr~ti(~n~i coupling.

1. Introduction In

at surfaces induced by photons with energies from the

induced

view of a molecule

iln~inging

photons

into the substrate

adsorbed

from

or adsorbate

cxcitations.

Olh8-.SS3X/SZ/%OS.OO,c: 1992 - Elsevier

the

on a bolicl

UV to the IR

producing

various

molcculc, or (ii) by exciting clcctronic states of the molecule, or (iii) by transferring charge onto the molcculc. With visible and UV radiation direct clcctronic excitation of the adsorbed molecules is also possible, whcrcas with IR radiation internal vibrational modes of the molecule (typically with wavcnumbcrs of a few thousand cm ‘) can be excited as well as the vibrations of the surface bond (i.e. vibrations of the molcculc as a whole with rcspcct to the surface, typically with w~~v~nunlb~rs of a few hundred cm ‘1. Elcctronic excitations usually show a thr~sh(~ld cffcct as a functi(~n of the cncrgy of the incoming photons, whcrcas IR excitations of the adsorbed molecule are resonant in nature. The resulting effects can be classified as (i) absorhatc-substrate bond cleavage leading to photodcsorption, and (ii) intra-adsorbatc bond clcavagc Icading to surface photochemistry. i.e. photorcactions, photoctching etc. The efficiency of photodcsorption and photorcactions at surfaces is controled by 6) the cross sections of the initial excitations. (ii) the time scales of the rcactions as contparcd to those of quenching processes, and (iii) the length scales, i.c. mean free paths of the primary excitations in the substrate. To discuss time scales. wc recall that in gas phase phot~)r~actions WC obscrvc tluorcsccncc on times of the order of IO -” s (e.g. in henzcnc) whcrcas photodissociation (e.g. 01 CH,Br into CtI,: and Br-) proceeds within IO ” s. On a surface any excitation of the adsorhcd molcculc will hc yucnchcd by coupling to the vibrational and electronic dcgrccs of freedom of the suhstratc. The time scale of vibrational quenching is given by the inverse vibrational frcqucncy of the surface bond, i.e. typically 10- ” s. If phonon-mediated cncrgy transfer is not very efficient as it is for weakly physisorbcd spccics, one might gain another order of magnitu~ic. Electronic quenching can be faster than IO- ” s, but can bc as slow as IO- ” s, in particular if it proceeds via clcc-

Science Publishers B.V. All rights resewed

IV. SPUTTERING/DESORPTION

tronPholc that

pair coupling.

one

except

in very special

at surfrtccs The

questions

surface

region

radiation:

or the

How

to

the

dots

adsorbatc;

proceed

&sorption

cst;lblishcd

molecules

field

incoming

the field the

or reaction

then

channel.

i.e. from

(left

panel

vibration

to

many

rcspccts

vey WC will

and

direct third

photochemistry

in its infancy.

look

to just

duccd

by

will

infrared

sur-

basic thcorctical ;I few.

clcctronic

bc dcvotcd

in

a num-

In this short In

at photodissoci~itioti

or suhstratc-mcdiatcd section

is still

arc already

[Id].

on outlining

cxamplcs

WC will

thcrc

rcvicws

conccntratc

confining

section

along

the

next

caused

by

excitation.

The

to photodusorptiori

in-

Iascr-molecul~ir

vibrational

cou-

cscapc

along

the particle

up

will

3. If the cncrgy.

curve

with

remain

return

state;

trapped

excited

state

(ii) If quenching the

ground

escape If

sccnar-

state

minimum,

is such that it winds

of the

(iii)

of

the particle

from

ET. gained

E, > 0, it will gl-ound

Scvcl-al

to the ground

tl away

state

place

the possibilities

ion formation);

;I di5tancc

in a continuum

electronic

pling.

electronically

curve

takes

tl away from

is inefficient, this includes

or positive

arrow

ET.

transistate

dcgrcc\

a distance

that

is cfficicnt.

curve

clcctronic

initial

2 (recall

of ncgativc energy

excited

cncrgy

pho-

the excited

de-excitation

will travel kinetic

its

onto

I in a Franck-Condon

If quenching in

arrow

is short

an incoming

;I repulsive

by subdratc

gaining

ios C~SLIC: (i)

of the

of excitation

motion.

of fig. 7). Before the particle

of mass

The mini-

is the position

time

the molcculc

arrow

quenching

freedom,

will

lift

fir41 consider

the \urfacc photophysics

ber of ocmprchcnsivc ideas.

through

If the

will

of the center the surface.

curve

to that of nuclear

ton or electron

the cncrgq the

state

compared

Wc

into

from

molcculc.

state curve

translation? While

in the ground

tion.

excitation

of the clistancc molecule

adsorbed

the sur-

into

how< does

initial

;I>

in the

clectromagnctic

co~lplc

and (iii)

from

arc

distribution

tion as ;I function of the adsorbed mum

phenomenon.

by theory

is the electric due

(ii)

it is very unlikely

dxorhed

cases. wshcrcas photodissocintit,n

to be anawcrcd

(i) what

transfer

from

is by now a well

follows:

fact

In summary.

sees fluorescence

state

cncrgy

as ;I particle

in its

E‘, < 0. the particle

at the surface,

i.e. quenching

will

will

he

pcrfcct. 2. Electronic

In

excitations

whcrc For our purposes

bc: (i) direct (ii)

on the adsorbed

molcculc.

The origin

get ionized

photocxcitation

by the incoming

by photoclcctrons in pairs

the or

and to a Icsscr dcgrcc induced

substrate;

ground

system

excitation the

of incoming

wc have

photons: by the

plotted

state of the coupled

and of the rclcvant

resulting

image

the cxcitcd

adparticlc

at the

[5,6]. as illusthe energy dparticlecxcitz

01

towards

electron

a dccpcr

of the substrate.

will

Icave

as a neutral

the surface

that

[7-IO].

such circumstance+

energy

ctatc.

or photon

must get dc-cxcitcd

cloud

surface.

spccics

Under

” s. If the kinetic ground

the

creates

IO

tronic

a situation

also shows an attractive in

force

by

into the clcctron

7 WC depict

to physisorbcd

by an incoming to the surface.

photons.

clcctronic

The

further

pertains

well closer

arc best discussed picture

a situation

of fig.

state curve

in-

substrate:

local hating

of photocxcit~itioli

in fig. 2, where

substrate

from

(iv) through

in the McnLcl-CJomer-Redhead tratcd

(iii)

plasmons

by absorption

The encrgetics

gcneratcd

can

panel

assumcd

minimum, Such

photons

the electronic

01’

of and CIcctron

electronPholc surfucc

excitation

ionization

excitation

coming

“clcctronic

right

the excited

also includes

the adsorbatc” attachment

the term

the

gained

Othenvisc.

potential as it moves

typically

within

is large enough, particle

it

in its elec-

clucnching

is again per-

fect. To

make the connection fig.

3 that.

with

in

reactions.

one cwpccts the intramolecular

Distance

yuitc

dissociative

WC note

from

similar

surface

to

reactions. gas

potential

phase cn-

in the gas phase. yield beam.

it could

sorption

>.

and tables

at surfaces

cross

sections

and

at

Zhou.

A

and White

Photodesorption tional coupling

curve

different shifted in

the

clcctronically

that

ol’ the

in the position

its

will

for from

width.

of its minimum

Thus.

the

channels

WC discuss faces.

Cowin

on Nit

1I I ).

that with

two examples

From

photons

cmcrging

is not possible

Below

energy

cV.

Reducing

results

the

nism

gas-phasc-likc

for the second

involving

mation

ii

brink

ct al.

a Pd(l and

Their NO

with at low

component

of

rotational

with 193 nm

nm the

of 0.0

only

the

first

peak

suh-

NO

tcmpcratures of 3 X IO

of NO

around of

‘* cm’.

NO1

for

with

i.e. about

initial

from

For

the

the

vibration.

(,-states

the

dcgrccs

I‘ = 2.

is

3,

of

to the phonons

m;lkes

laser

;I

intcn-

(ignoring

an

faster)

as indicated

to the

of frce-

Bec~u~scthe

typically (or

(i)

AB-

a photon

_.

now).

to the surface

transfer

the

molecule

in fig. 4. cncrgy

vibration

namely (ii)

and for high for

solid

niolcculc~substratc

oscillators.

to

cncrgctic

wells

further

bond vibration

latter

vibration

more

internal

i.e.

I

partic-

of the

fig. 4). Absorbing

anhurmonicitics

molecular

magnitude

at this

vibration,

beam

A

B = F. We

order than

01 the

by the widths

transfer

from

the

bond vibration.

i-states

in fig.

energy

from

of the solid

4, is 410~. the

surface

two sccnar-

on

3.5

heating.

two components or less

of

inde-

and rota-

115-130 K spacer.

can bc A

translational

800-YOO with

internal

the

of parts

3 and

the phonon

laser

;m

treat

at 105 K

substrate

the

(iii) (see

into

WC will

heating

the

of coupled

and

up

of

view

surface

tuned

consisting

direct

vihra-

on ;I solid

laser

is transparent

I’ = 0 to I‘ =

further

by

further

bctwccn

more

from

molecules

pho-

was kept

of

form

photochcmpublished

simplicity

molecular

incoming

molcculc-surface

to the l’or-

translational

of the order

the

[3] listing

by Hasscl-

cncrgics

showed

from

transition

of the potential

i.c.

a monolayer

with

For

can

of the substrate

;I mccha-

substrate-mcdi~ited

component.

cncrgy.

of is

adsorbed

itself

WC

vibration,

photochcm-

by Ho

review

infrared

so that

internal

of CH ;C1.

invoke Icading

to avoid

surface dom

initial

[-I].

an

as a system

A-B

questions

excitations.

with

complex the

the

photodissociation

as ;I quasi-diatomic

can bc ignored

sitics

photolysis

system

desorption

photodissociation

section

tlucnccs

undergoing

interfaces

vibration.

frequency

cncrgy

an experiment

The

with

abfor a

hy infrared laser-molecular

that the solid

IR

peak appears

acted as a spacer

spectra

of photon with

that

photons

A sIow

temperatures

associated

to 248

of

which

time-of-night

pendcnt

energy suggests

adsorbed

of NO,.

molecules.

tional

of

example

11) surface

and 0.3 cV

with

at a kinetic

attachment

WC describe

irradiated

arc

measurcmcnt

substrate

[ 171.They

physisorption

than 20

radicals.

i

second

todissociation

with

radicals

irradiation

direct

clcctron

of CH

ular

of 1.7 cV.

more

CH,

peak we must

initial

strntc-mcdiatcd As

photon

assume

i4 possible

a second

spectrum

is sc’cn. This

from

whereas

after

15 monolaycrs,

in the time-of-flight

at surCl1,CI

it is known

energy

1I I)

Ni(

in ;I time-of-flight

kinetic

bccond peak

have \tudicd

;I kinetic

on

that

and B, c.g. for CH ;F wc have A = CH

at 24X nm. Adsorbing

CH,CI

observed

photons.

the molcculc

layer,

further

a molecule

ii-r-adiatcd

(even after’ clucnching)

[I I]

with

WC consider and

intramolecular

of 193 nm photolysis

This

indeed

also

particlc

excited

gas phase photochemistry

radicals

of

htatc.

of photochemistry

and coworkers

monolaycrs

to hc

for dissociation.

CH.,

the correct

excited

state ground

and possibly

clcctronically

also bc vihrationally

opening

excited

electronic

with

formulae

in the metal.

photodesorption,

adsorbate-metal

Zhu

Frcsnel

of systems

3.

crgy

from

have been compiled fol-

of the photon

by comparison

occurred

photoreactions.

istry

the photodissociation

a dielectric

of photons

Extcnsivc istry

calculated

surface

absorption

measuring

of angle of incidence

be cstablishcd

curves

metal

P :

w

By

as a function

fast and

K is the result

a measured 10 timcs

cross

the value

Fig. 4. The mechanism of photo&sorption by infrared molecular vibrational coupling.

laser-

IV. SPUTTERING/DESORPTION

Hcrc Q is the cffeotivc charge characterizing the dynamic dipolc of the rrdsorbcd muleculc. The rclcvant

molecular vibrational frequency is Sf with ii homogeneous l&width 1’1. In cq. 17) we have !‘=I;. -i- I; whcrc Y1 is the laser ~~n~~v~dt~, f is the Iascr irrtcnsity. The iransition rate dcpcnds on the anpic of incidence of the laser through F( 0) = 4s: sin’@( II, -t ff +(t

-,z;Z)

r;rn%~“q

‘, (8)

whcrc ~2, is the index of r&action of the suhsfratc, FVom the master cyuntion (4) one can calculate the rate constant of phutrldesnrption, rd( I. T), as a func-

log,OI (W/cm’) Fig. 5. Photodesorption

yield (or rate)

T = 100 K for CH,F/NaCI

for different

V,). Dashed line is without coherent

tion of laser substrate. To connect photodcsorption

YaN,,[l

-cxp(

intensity,

I,

adsorption

two-quantum

and tcmpcraturc,

it with experiment, yield is given by pr,,t,)].

vs laher intensity

WC note

at

energies

processw.

T of the that

the

(4)

where N,, is the number of initially adsorbed molcculcs, and t, is the length of the laser pulse. The constant ot proportionality involves characteristics of the detector system. If r,t, e I. the yield is proportional to the rate constant. As input into the theory WC need (i) the vibrational frequency. f2. of the adsorbed molcculc, (ii) its dynamic dipole moment and (iii) the depth and range of the surface potential. All this informaticm can be obtained from independent measurcmcnts. Parameters arc laser intensity and temperature. The output is the yield. WC briefly mention that the dcpendcncc on the angle of incidence of the laser beam, calculated according to cq. (8) from Frcsncl formulae, suggests that for a perpendicularly adsorbed molecule the best angle of incidence on an insulating surface is about 60 O. In fig. 5 we show the dependence of the photodcsorption yield, Y. on the laser intensity, I, for scvcral values of possible potential well depths chosen so that 2-, 3- and 4-photon processes are necdcd to lift the adsorbed molecule into the desorption continuum. We note that the calculated yield curves arc very nonlinear, quite similarly to experimental findings. The straight line portion on the log-log plot can be fitted as Y a I” with powers N = I .4, 2.6 and 3.7, respectively. In the gas phase one would expect powers 2, 3 and 4, respectively. The differences arc due to cncrgy loss from the laser into the heat bath of the solid. For surface reactions simple counting ideas about the order of a process do not hold; indeed, the whole concept of reaction order becomes dubious as it changes so dra-

matically with intensity and tcmperaturc: this is an important lesson to remember. We said earlier that resonant heating in photodesorption is expected on metal surfaces whcrc thcrc is efficient coupling of the molecule to the phonon degrccs of freedom of the substrate. This idea has been checked in co-adsorption cxpcrimcnts [ 171: equal amounts of NH, and ND, were adsorbed on a Cu surface at 90 K and the laser was tuned into a NH, vibration at 3370 cm ’ with the corresponding ND, vibration off by about 1000 cm ‘. If direct resonant photodesorption were dominant, only NH 1 should bc dctcctcd. What was found were equal amounts of NH 3 and ND,, suggesting that the final dcsorption process was indcpendcnt of the initial excitation. i.e. thermal in nature. One point about this experiment is still unrcsolved. i.c. the fact that the translational tempcraturc of the dcsorbing molcculcs was mcasurcd to bc lower than the initial surface tempcraturc. Obviously the laser cannot cool the surface A likely explanation arises from the fact that the energy of the dcsorbing particles is stored partly in translation and partly in rotation. The fact that the translational energy is so low would then suggest that the particles dcsorb rotationally hot. We briefly summarize the work on the frcqucncy dependcncc of the photodesorption yield [ l8,lY]. The theory has been worked out to account for homogencous and inhomogcncous broadening of the molecular vibrational lcvcl [2()], for the anharmonicity of the molecular vibration and for I‘-/‘ coupling accounting for cncrgy transfer bctwccn adaorbcd molcculcs [2l]. If the laser line width is larger than the homogcncous width of the molecule, then, in the abscncc of inhomogcneous broadening, the desorption yield has a width equal to that of the laser. Under the opposite situation, the desorption yield width is that of the homogeneous IR lincwidth of the molcculc. The latter prediction has been vcrificd for CO on NaCI, a simple system in which a single photon is sufficient to cause dcsorption. If we work in the rcgimc whcrc the yield vs intensity curve obeys a power law with cxponcnt N, Y a I”, one expects a narrowing of the dcsorption yield curve for cu> I. If inhomogcneous broadening is dominant, one expects the dcsorption yield to be proportional to the frequency distribution in the molcculcs, except at the highest intensities where intcrcsting double peaks are predicted. The anharmonicity of the molecular vibration posts an interesting problem. Because the I’ = I + 2 and higher transitions in a molecule usually differ by a few a laser of ten cm -’ from the (3= 0 + I transition, width less than I cm ’ cannot excite more than the first vibrational level. Yet this is not enough to cause photodcsorption. Fortunately the intimate coupling of IV. SPUTTERING/DES(>RPTION

the

molecule

to

mechanism

phonons

the

to supply

the

of the

energy

ncous absorption of a phonon phonon process [I Y,?2].

solid

diffcrcncc

provides

DIET

a

[Al X.-L.

I photon-l

in a cohcrcnt

IV,

4s.

G. Betz

and P. Varga

(Springer.

Berlin.

I YYO).

by simulta-

Zhou.

X.-Y.

Zhu

and J.M.

White.

Surf.

Sci. Rep..

in press.

[51 D. Menzel and [hl P.A. Redhead.

R. Gomcr,

[71 P.R. Antonicwicz,

4. Outlook

[Xl P.

Fculner.

Phy’.

[‘)I Z.W.

Ciortel,

[I II

Krewcr.

and

E.P. Marsh.

T.L.

J.P.

I’hys.

C‘owin,

Phys. 01 (19x4)

II?1

E.

Cassuto

1131J. ll-rJ

Gortel.

P. Feulner

and

Il.

Rev.

B13 (IYY

Menxl.

XYSI.

A.

Wlrr.dGcki.

Phys.

Gilton.

W. Mrirr.

Rev.

Lctt.

S. Jakuhith.

and G. Ertl.

Hcidherg.

hl

M.R.

Schneider

(IYXX)

2725:

H.

S. Ncuttesheim,

J. (‘hem.

Stein.

Phq’s. (‘hem.

(Neuc

H.J. Krcurcr

and D.N.

Lawr-Induced

and

J. C‘hem.

and

I21 (IYXO)

Lowy.

M. Wall’. A.

Phys. 03 (IYYO) 1X7.

C. Riehl

Folge)

Z.W.

[I71

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Molwular

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( 19x7)

Gortel.

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(‘hem.

A.

Nestmann.

Z.

115. I’hys.

Lrtt.

7X (IYXI)

H.J.

Piercy

( IYXS) 34X’). [IHI I I.J. Krcwer,

Z.W.

Am.

Krruer.

11. Srki. I’.

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Z.W.

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at

Surfaces,

J. Opt.

P. Piercy

and

R. Teahima.

50.13. T.J.

C‘huang.

and

R.

Gortel

Teshima. and

Z.W.

Gortel.

Phya.

P. Piercq.

Rev.

J. Opt.

I I.J. 1~37 Sot.

24X. P. Piercy.

Phys. Rec. B3h (IYX7)

[?()I

Physics

no. ‘7.

B 72 (10x3)

~lu\da.

Kreuer.

[I’)1 Z.W.

R. Trahima

and

1I.J.

Kreuxr,

R. Teshima

and

H.J.

Krewrr.

R. Teahima

and

ll.J.

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R. Teshima

and

H.J.

Krenxr.

305’).

P. Piercy.

Surf. Sci. 156 (1YX6) 1.12.

[I]

T.J. C‘huang.

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Surf.

Z.W.

Sci. Rep. 3 (lYX3)

Gortel Procesws

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al.

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Lin

[Xl

Z.W. Surf.

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Z.W. Surf.

in

in: Advances

and Spzctrowq~y.

Singapol-c.

[3] W. Ho. in: Desorption

PI1

1.

and H.J. Kreuzer,

Transition\,

I)

50.

[IhI

References

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and Z.W.

1Y37.

I Inasrlhrink.

Sot. Am.

This review wax made poxsiblc by ;I grant fun&xi by the Network of Ccntrcs of Exccllcnce programmc in association with the Natural Scicnccs and Engineering Council of Canada.

Kreuzer

53 (10X-l) 671.

H.J.

Gortel

8X6.

73x7.

[I51 Acknowledgement

H.J.

Phys, Rev. B3.i (IYX7)

IlO1 Z.W.

Phys. 41 (IYC4) 331 1.

Rev. B? I (IYXO) 3X I

D. Mewel.

Phys. Rev. Lctt.

After many years of rather haphazard work. photophysics and photochemistry at surfaces is cmcrging as a fascinating new field of surface science. A lot of good work still remains to be done. What is nccdcd in particular arc complctc cxpcriments. i.e. whcrc all adsorption-dcsorption parameters of the system arc known before one starts photocxpcrimcnts. As for clcctronic effects, theory has not advanced much bcyond model calculations. The situation is much better with the theory of photodcsorption by IR lascr-molccular vibrational coupling where many detailed prcdictions remain to be tcstcd cxpcrimentally.

J. Chem.

C‘an. J. Phys. 32 (lY6-l)

Gortcl.

P. Pirrcy.

Sci. l7Y (IYX7) Gortrl.

176.

P. Picrcy.

Sci. I66 (IYXh)

LI IO.