Infrared laser-induced photodesorption of adsorbed and condensed phases

Journal of Electron Spectroscopy and Related Phenomena, 38 (1986) 65-74 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

INFRARED

LASER-INDUCED

CONDENSED

PHOTODESORPTION

65

OF ADSORBED

AND

PHASES

Ingo Hussla IBM Almaden

Research

Center,

K33/801,

650 Harry Road, San Jose, California

95120-6099.

U.S.A.

ABSTRACT Infrared laser-induced photodesorption occurs via pulsed excitation of internal vibrational modes of molecules adsorbed on surfaces. A review of these experiments is given. Excitation of the second overtone of the molecule-surface bond has been proposed theoretically as alternative photodesorption channel. Experimental results are reported for CO-Cu(100). No desorption was detected even though the overtone occurred within the CO2 laser range and incident intensities up to 100 MW/cm2 were used. PHENOMENON Infrared

AND SYSTEMS

laser-induced

desorption

of molecules

vibrational

adsorbate

studied crystals, detection

since

method.

been reported

adsorbed

and metal

Photodesorption

(ref.

(ref.15).

SF6-Si(ll1)

11,12)

CSHsN-Ag(film)

by Heidberg (ref.

(ref.

results

15).

16.17).

19) have also been presented.

work by Hess ef al., (ref. 20-24) CD,OH,

CD,OD,

phenomenon

phases

after

laser. The IR-LIPD

employing

has been

including

ionic

mass spectrometry

using tunable pulsed CO2 laser sources,

and condensates, (ref. 2-8),

including

SF,-NaCl(lOO)

C,H,/CH,F-NaCl(film)

et al., and C,H,N-KC1

(ref.

13-15).

13).

C,H,N-Ag(ll0)

(ref.

(ref.

CO/CH3F-Cu(polycrysta1)

15) (ref.

Other CO, laser photodesorption on condensed thick molecular Ccl,,

the

of internal

phenomenon

CSH5N-Ni(foi1)

CH,OD,

involving

excitation

of substrates

time-resolved

CSH5N/CSD5N-Ag(film)

for the systems

a

from a variety

experiments,

(ref. 2), CH,F-NaCl(film)

CO-NaCl

as CH,OH,

surfaces,

adsorbates

is

and condensed

modes with a pulsed infrared

for numerous

CH,F-NaCl(100)

(ref.

from

(IR-LIPD)

1978 and has now been observed

semi-conductors

Preliminary

VISITED

photodesorption

D,O,

CH,F

as have

(ref.

1).

(ref. 9-10).

C,H,N/C,D,N-KC1

by

(ref. Chuang

ef

15). al..

18) and NH3-Cu(100) experiments

included the

layers and bulk materials,

and C,H,CHO.

such

Using a HF/DF

laser, LIPD of H20 from optical surfaces like CaF2, NaCl, KCI, SiO, Cu has been claimed (ref. 25).

In a recent publication

0368-2048/86/$03.50

we reported

IR-LIPD

of NH,-Cu(100)

0 1986 Elsevier Science Publishers B.V.

and NH3-Ag(film)

after

exciting the N-H stretching 2.5-4.2 pm region. also investigated substrates

modes (ref. 26-30) with a tunable pulsed laser operating

Mixed ND,-NH,

molecular layers adsorbed on Cu(100) and Ag(film) were

(ref. 28-30), as well as Xe-NH3 co-condensates

;)f the mass of the desorbed -ignals monitor anslational

the

molecules

velocity

temperatures

and NaCl(film)

of

distribution

resonantly

the

absorption-desorption of the desorption

yield.

to an internal

Frequency

parameters

and intensity

about the rate of IR-LIPD,

systems

CH,F-NaCl(ref.

yield measured

modes

must

also be included

of the desorption temperature. infrared

an

yield depend The intensity

absorptivity

provides

with those obtained by

32) and

(ref. 7,43,38).

in order

Anharmonicity

to obtain

better

Time-of-flight

of the excited

agreement

spectra

between

of photodesorbing the desorption

(ref. 35). More recently, a lineshape function

desorption

IR-LIPD.

rate of both’single (ref. 30).

has been presented

mode

Comparison

(ref. 36).

dependence

Actually,

(ref. 30)

and theoretical

data

became available recently for

of the photodesorption

yield due to

line broadening has been recently calculated for CH,F-NaCl. mechanism

in LIPD

(ref.

was

56).

surface processes was presented

Results and conclusions

of experimental

and multi photon situations

Frequency

A phase-dissipative active

rate

systems known where the absorbed photon energy exceeds the

and inhomogeneous

surface processes

31), CO-NaCl(ref.

In this regard CO-NaCI (ref. 37) and (NHs) multilayer-Cu(lOO)

NHJND,-Cu(lO0)

laser-stimulated

(ref.

et al., for the

of proper spectral widths (for laser and system)

excited adsorbates

are model systems for one-photon on the desorption

CO-W(l10)

by Kreuzer

(ref. 34). Lucas and Ewing have calculated

applies to one-photon

energy.

38.39)

measured,

dependence

data (ref. 43,38,39).

there are not many adsorption

laser-driven

been

and substrate at maximum

has been emphasized

of vibrationally

which specifically

31),

The importance

and theoretical

homogeneous

not

rates have been published

CO-Cu(lOO)(ref.

species have been calculated

adsorption

In other cases, where the linear has

which can then be compared

approach,

32, 33,48).

input in these calculations

and lifetimes

and can be fit to

model calculations.

Using a master equation

experimental

mode.

system

like coverage

information

vibrational

species

can be obtained by measuring the laser frequency dependence

of the desorption

adsorption

These desorption

system are first recorded in order to optimize coupling

dependence

semi-empirical

of photodesorbing

adsorption-substrate

spectrum

on system

as the rise and decay

by using a Maxwell Boltzmann least square fit. In most cases linear

L the IR-laser frequency IR-spectrum

mass spectrometer

after laser adsorbate-interaction.

i-absorption spectra of the adsorption

(ref.

on Ag(film)

(ref. 29).

IR-LIPD data are measured with a time-resolved

strongly

in the

investigated Very

recently

and

found

an extended

the

theory

for

by Beri and George (ref. 50.51,52).

for the systems studied are reviewed below.

such as diffusion

to assist

(ref. SS), dissociation

and ionization

Other laser-induced can be induced by

67

IR-radiation. molecules

The intensities

observed for these surface reactions are smaller than for the same

in the gas phase (ref. 19). Laser-induced

the incident IR-LIPD,

laser beam heats

especially

the substrate.

when the IR-reflectivity

thermal desorption

This desorption of the substrate

(LITD) occurs when

mechanism

competes

with

is low and the absorptivity

is

high. Laser-induced particular,

surface reactions

in general, and resonant

vibrational

adsorbate

coupling in

have been reviewed recently in great detail by Chuang ef al, (ref. 28,29,44,49)

and

others (ref. 11.45,46,47,53).

EXPERIMENTAL

CONSIDERATIONS

Custom-built developed

IR cells and apparatus

(ref. 57.58). The instruments,

laser-induced

desorption

the molecularly 59).

experiment

desorbing

Otherwise,

desorption

by time-resolved

signals, a laser-in,

Fast ion current amplification

For this reason, slow electrometer

to be avoided, necessitating

laser-out

with cooled slits and discrimination

optical

set-up

the actual laser pulse event.

amplification

probe must be

operational

recorder or signal averager.

(ref. 59). The desorption

signal acquisition

device,

e.g., storage

Since the time scale of detection

backing pressure exposure

than by a single dosage, a different

chamber equipped

time-scale

output stability.

oscilloscope,

is in the microsecond

This procedure

adsorbate.

A difficult

of the substrate

to the gas molecules

of the laser, which is operating

(ref. 59) rather

The shutter

at a 1 Hz repetition

rate for

providing

a single

experimental

all measurements

onto

whose signal output is used to trigger the data acquisition. of the reflected beam

single

laser energy simultaneously

reflection-absorption

spectrum

LIPD problem is the spatial movement

spot on the crystal when the laser is tuned (grating, monochromator) range. Therefore.

range, laser

If the surface coverage is controlled

trigger method is used (ref. 59. 60): A shutter blocks the

also allows measurement

event,

transient

When the laser actually hits the surface, the light is reflected

IR detector,

desorption

is zeroed by laser pulse is

laser output for a certain time, chosen to obtain the desired Langmuir exposure. is timed by the trigger output

has

amplifier (ref. 59). Best

In most cases a signal from the thyratron-triggered

the desorption

a fast (photondrag)

has to be

of the mass spectrometer

pumped mass spectrometer

jitter in the ns range does not affect the measurements.

optimum

(ref.

devices to make sure that only molecules without any wall

collisions reach the electron impact ionizer.

by constant

mass spectrometry

of the mass spectrometer

the use of a large bandwidth

results have been obtained using a differentially

used to start

of the pulsed

laser light will scatter from the chamber walls, causing spurious effects

and signal broadening. accomplished.

under UHV have been difficulties

have been discussed in detail for the case of LITD, where

species are also detected

To obtain real-time

achieved.

for laser solid-interaction procedures and experimental

have to be reproduced

at different

to each of the

of the IR beam

through

its frequency

optical alignments.

68 REVIEW OF RECENT RESULTS ON IR-LIPD .

IR-LIPD can take place from both dielectric

as a monolayer.

Excitation

ring) leads to desorption frequency

of different

types of internal vibrational

(ref. 16, 19). The photoactive

than the modes directly associated

order is generally considered l

and metal surfaces at surface coverages as low

Resonant

photodesorption

of the incident principle,

bond and to first

as being decoupled.

laser radiation

is observed

dependence

spectrum of the adsorbate,

bend,

mode has a much higher vibrational

with the molecule-surface

with strong wavelength

the frequency

modes (stretch,

dependence

in a certain intensity

for both dielectric

on the desorption

range

and metal substrates.

yield matches

In

the IR-absorption

but examples of smaller linewidth (FWHM) due to multiphoton

absorption

have been reported.

absorption

spectrum

The importance

of measuring

and the IR-absorption-desorption

both the IR-(reflection)

spectrum

simultaneously

must be

emphasized. l

IR-LIPD

can be obtained with IR-laser pulses of 6 ns. Most experiments

are done with a

CO, laser having a pulse duration between 60 and 200 ns. No LIPD experiment radiation

has been performed

contributions l

and elevated

for IR-LIPD are in the order of OS-2 MW/cm2. substrate

temperatures,

washed

out by given or induced thermal

density,

such as alteration

expected in the intensity adsorbate,

to date, most likely because induced thermal

are overuling any resonant effect (ref. 72).

Incident laser intensities fluences

successfully

using CW

however,

the resonant

effects

of surface properties range mentioned

can induce

(ref. 29). or ablation

At higher laser

feature

of IR-LIPD

Effects

of high laser power

of subtrate

can be

material

are not

above (ref. 71). High vibronic excitation

IR-photoionization

and IR-photofragmentation

of the of the

adsorbed molecules (ref. 18). l

The desorption Typical However, different

yield appears to be higher on dielectric substrates

quantum

yields

of 2x10-l

since no systematic

and 5~10~

than on metal substrates.

are reported

study of yield vs. pulse duration

numbers might be also caused by the different

respectively

(ref.

4.27).

has been performed,

these

laser systems used in ref. 4 and ref.

27, respectively. l

Only a very few adsorption energy exceeds the adsorption systems

investigated

condensed photon.

systems are known where one-photon energy of an adsorbed

so far require multiphoton

layers, where the evaporation The intensity

dependence

molecule.

absorption.

vibrational Therefore

The situation

absorption most of the

is different

in

energy is in the order of the energy of a CO,

of the desorption

yield is of course different

for the two

cases (ref. 30). l

Fluence

or intensity

experiments

performed

“thresholds”

of LIPD have been reported.

so far have used quadrupole

mass spectrometry

However,

since all

in a time-of-flight

mode, which has poor sensitivity

mainly due to low electron impact ionization efficiencies

and a small solid angle, these “thresholds”

may be an artefact

due to lack of instrumental

sensitivity. l

The velocity distribution distribution.

of the desorbed particles appears “close” to a Maxwell-Boltzmann

but only very few

measured translational

temperatures

close to the substrate ionic

crystals

l

temperature

are found

spatial-resolved

or higher (ref. 30,18). than

with co-adsorbates

desorption

(multilayer)

from co-adsorbates

(ref. 9, 18). Conditions

such as low substrate contribution

(= low thermal

statistical

temperature

(ref.

from

3).

No

molecule.

Transition

probabilities

molecule,

tunneling

temperature,

via IR-LIPD

of the occupation

dipole transitions

surface potential

show no significant enhanced isotope uncertainty

(ref. 30). while

has been reported

low coverage and a non-metal

rate) will rise the probability

of

(ref. 30).

theory of IR-LIPD has been derived based on the master equation

the time evolution

inelastic

Molecules photodesorbing

subtrate

to the desorption

experiments

describing

laser-induced

The

from metal surfaces are

with one IR active component

subtrate

A quantum

the

yield within the 25% experimental

sucessful isotope separation l

for molecules photodesorbing

to be colder

in the desorption

preferential

signals have been reported.

IR-LIPD data have been published to date.

IR-LIPD experiments selectivity

real-rime desorption

processes

are calculated in the molecule,

for the resonant

of the vibrational

according to Fermi’s golden rule for the for the phonon-induced

heating mechanism,

into continuum

states of the adsorbed

cascades in the

and for the elastic and

states leading eventually

to desorption

(ref.

7,30-33). l

The numerical particular

results

of desorption

if one considers

rates are in agreement

spectral line widths and resonant

with the experiments,

heating mechanism

in

including

phonon and electron damping (ref. 7, 30). l

A phenomenological process

description

is the excitation

of thermally

of a localized

assisted IR-LIPD

adsorbate

is proposed:

vibration.

Bound

to bound

state

transitions

play essential roles in channeling the absorbed photon energy into the localized

vibrational

levels in the surface potential

the thermal desorption

excitation probability

via electron or phonon-mediated

of the surface potential.

Application:

IR-LIPD

might

IR-absorption-desorption co-adsorbates resonant

via IR-LIPD

heating

cannot

become

spectra,

a surface

e.g.

from

can be obtained cause desorption

cleaning” might become available.

aid resulting in

This thermal excitation

when the molecule is also internally

analytical

catalysts.

from dielectrics of the unexcited

can enhance the

excited or when it is coupled

with the elastic and inelastic tunneling processes, but the selectivity .

The primary

will suffer.

tool

in order

Successful

to

and in co-adsorbates species.

obtain

separation

“Selective

of

where surface

70

SECOND OVERTONE In this vibrational

section

EXCITATION

an alternative

excitation

IN CO-Cu(100)

IR photodesorption

of the surface-molecule

channel

low (300-500 cm-‘).

Only in a few adsorption

The frequency

of a second overtone

at 921 cm -l. fundamental

This frequency Cu-C vibration

excitation

treated

of this mode is

systems, such as H-W (ref. 67), can

a pulsed CO, laser be used for direct coupling into this mode. possibility

the direct

bond. This process has been theoretically

by George et al., (ref. 61-64) and Jedrzejek ef al., (ref. 65-66). normally

is considered:

Jedrzejek et al., suggested the

of the Cu-C bond using a high power pulse laser

value was calculated

based on the measured

of 339 cm (ref. 68) and the experimental

values of the

value of 69.7 kJ/mol

(ref. 69) for the isosteric heat of adsorption.

One-Dimensional

Microsconic

CO-Cu(lOO)(ref.

Thermal desorption

the transition

respectively.

Mechanical

Tm

g

Photcdesorption

ip

65.66).

of the chemisorptive Let

Quantum

is caused by phonon energy transfer

from the lattice to the vibration

bond and can be increased by laser vibrational rates

for phonon

be WR_,,, and Wi,,,,

Then the probability

P,(t)

coupling into this mode.

and laser-driven

transitions

that the atom is in a state at time t, is given by the

“master equation”:

@,W = 6t The total transition

rate W,_,

~W,__,P,W+ ~WmJmW.

is the sum of the phonon and laser contributions

W n-m = W:*,

(2)

+ Wl,,,.

The use of the master equation involves certain implicit assumptions: approximation”

is invoked by assuming that the rate of dissipation

heat bath is larger than the transition reduced

density

matrix

includes

multiphonon

of the adsorbed effects

oscillator-anharmonicity

(increase the effect

finally, transition

rates W,_,.

when

of Wg,,

I2

elements

The computation

continuum

of bound-to-continuum



of energy in the phonon

Em and E, > Debye energy),

rates between all levels. Wh_*

cm-2:1

(a) The “Markovian

(b) The off-diagonal

particle are neglected.

(spacing between

transitions

are possible),

(1)

m

m

close and

transitions

of the

of We_,,,

the effect

of

e.g. n -,

n+2

(m desorption)

and

are evaluated using the golden rule fomula

r nm/?a (3) [(Em - Ea)/fi -

aI2 + (+,m12

71 Here I is the absorption for the chemisorptive

line-width

bound to the surface

It is difficult incident

p is the dipole moment operator

field due to the laser.

The matrix

1n> and 1m> of the Morse oscillator representing

by the average lattice-atom

interaction.

the atom

Several of these quantities

with any precision.

to compute

the local field accurately

laser field is modified

molecules.

transition,

bond, and E is the local electric

element is taken between states

cannot be calculated

for the n-m

even for a perfectly

by local fields emanating

If phenomenological

Maxwell equations

described by the Fresnel formula.

flat surface.

from the metal and the surface

are used, the polarization

At infrared frequencies

The

of the metal is

the metal is close to being a perfect

conductor,

and the local field is roughly twice that of the incident laser. However, the Fresnel

equations

break down at points located too close to the surface and then the accuracy of these

equations

is unknown.

substantially. determine

The presence of any kind of roughness may also modify the local field

The effect of the polarization

in a satisfactory

Assuming

maximum

photon

to the local light intensity

molecule is equally difficult to

absorption

by a perpendicular

dipole,

I and is equal to 2I/eo*c*n

velocity of light, u = refractive

WA,,

of the neighboring

manner.

= 2:

(eu=vacuum

_Other

+I

chemisorbed achieved

are the dependence

c =

(4) -

aI2 + (rm,,12

(Large C = large local field.) of the dipole

bond, the role of anharmonicity

in the harmonic

constant,

I-nm/2a

I2



C is defined by C = I/(1&

uncertainties

dielectric

index):

[(Err, - E,)/g where the constant

1ti 12 is proportional

(n-n+1

moment

transition

on the length

of the

are off resonance

or are

case) and finally, the role of the width I,,

the line width of the

adsorbed species. The presence of I,m, in eq. (4) is required by the existence of dissipative processes coupled oscillator.

If Ir,,,, -, 0 a g-function

active only if its frequency

is equal to (Em - E,)/fi

to the anharmonic

absorption

bandwidth.

The presence

within

the smaller power broadened

allows all the n-+m transitions

of I,,

participation

even if they are not in resonance,

w-(Err,-E,)/R

is increased.

On the other

appears in eq. (4). and the photon is

though

hand if I,,

to have some

they are less and less effective has excessively

large values,

as then

wln-ml/r,, Numerical results have been reported using: Et-E,/R Kcal/mol. 200 ns. r,,, desorption

bond distance Q, = 2.27& potential = 30 cm-‘, (E3-EJ

= 339 cm-l, potential depth D = 16.6

width a = 2.464; laser energy lJ, pulse width

= 921 cm-’ and laser intensity

I = 50 GW/cm2(!).

rates have also been given as a function of local laser field parameter

C.

The

12 Experimental

Results

The described

photodesorption

CO, laser intensities was applied. successful

mechanism

up to 100 MW/cm2.

The experiments

was probed in the system CO-Cu(100)

The whole accessible

in CO-Cu(100)

(ref. 70). laser-induced

71) and very recently, for IR-LIPD in CH,F/CO temperature

CO, laser frequency

range

were carried out in an UHV system, which has been used for

LITD experiments

18). Substrate

using

ablation of copper (ref.

mixtures adsorbed on Cu (polycrystal)

of the clean (Ar+ sputtered)

(ref.

Cu(100) single crystal was 90K. To

assure that highest coverages are obtained, CO backing pressures up to lo-* mbar were applied. The CO surface p-polarized

coverage

was controlled

by LITD (ref. 70).

Angles of incidence

pulsed CO, laser (FWHM 60ns) were 45’, 67.5’ and 86”; the detection

the mass spectrometer

was O”, 22.5” and O”, respectively.

Up to 64 desorption

events were sampled.

found in ref. 18. Laser energy measurements before and after reflection

Time-of-flight

angle of

distance was 23 cm.

More details of the experimental taken on a pulse-to-pulse

were

of the

set-up can be basis (ref. 59)

from the single crystal.

No desorption of CO from Cu(lO0) single crystals could be obtained applying laser intensities of up to 100 MW/cm2 range from

910-1090

DISCUSSION

cm-l was used Also ion spultered rough cryslal surfaces

by direct excitation

on theoretical

grounds,

of the surface-molecule

intensities

efficient

channel

thermal desorption

system.

(surface

of CH,F has been obtained via CO, laser excitation

does

desorb

of IR-LIPD:

Finally,

A preliminary

experiment

order to obtain information states of the photodesorbing

On the other

to

hand, this

should

co-adsorbate

be drawn

to the microreversible excitation

one expects a release of IR radiation when adsorption

of

occurs.

system CO-Ni (film) (ref. 73),

has been observed after admission

of this early investigation

This where

of internal modes, while CO

If LIPD is induced by vibrational

has been reported for the adsorption

where IR adsorboluminescence 77K. The time resolution

attention

adrorbolummescence.

internal modes of the adsorbate,

laser

of copper for incident IR radiation.

desorption

phenomenon

of applying

makes it quite impossible

damage).

with our recent findings for CH3F/CO-Cu

18).

excitation

from clean Cu(lO0) surfaces is not achieved via laser

because of the high reflectivity

(ref.

excitation

of up to 100 MW/cm2.

The necessity

to cause photodesorption

experimentally

shows that CO desorption

latter result is in accordance

not

were used.

relaxation and damping of any vibrational

in this metal adsorbate

higher than 100 MW/cm2

handle this desorption experiment

laser frequency

via second overtone

does not occur with intensities

Clearly, this implies that phonon-assisted are extremely

The whole of the CO,

AND CONCLUSION

Photodesorption proposed

under the above described conditions.

of CO to a nickel film at

was 1s only and has to be improved

about the dynamics of LIPD. Also, the vibrational species should be probed in future experiments.

in

and rotational

73

ACKNOWLEDGEMENT I would like to thank Professor Beloit, Wisconsin, and Professor

R. Viswanathan,

Department

for his help with the experiments.

Peter Stair, Chemistry

Department,

Beloit College,

I am grateful to Professor

Northwestern

for providing laser and ultra high vacuum facilities.

of Chemistry,

University,

Eric Weitz

Evanston,

Thanks to Dr. M. R. Philpott

Illinois,

for critical

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