Infrared emission spectroscopy of CO on Ni

JournaIofElectron Spectroscopy andRehtedPhenomena, 29(1983)113-118 ElsevlerScientlficPub~mgCompany.Amsterdam-_PtrntedmTheNetherlands

INFRARED

EMISSION

SPECTROSCOPY

S. CHIANG,

R. G. TOBIN,

Department

of Physics,

and Materials Berkeley,

OF CO ON Ni*

and P. L. RICHARDS University

and Molecular

California

113

of California

Research

Division,

at Berkeley, Lawrence

Berkeley

Laboratory,

94720

ABSTRACT We report the first observation of thermally emitted infrared radiation from vibrational modes of molecules adsorbed on clean, single crystal metal surfaces. The observation of emission from CO adsorbed on Ni demonstrates the surface sensitivity of a novel ap aratus for infrared vibrational spectroscopy, with a A resolution of 1 to 15 cm- P over the frequency range from 330 to 3000 cm-l. liquid helium cooled grating spectrometer measures the thermal radiation from a room temperature, single crystal sample, which is mounted in an ultrahigh vacuum system. Measurements of frequencies and linewidths of CO on a single crystal Ni sample, as a function of coverage, are discussed.

INTRODUCTION Vibrational

spectroscopy

cules on surfaces Many techniques

can be used to obtain

limitations.

Electron

high surface

sensitivity

energy

both good resolution

and high surface

Photon spectroscopies

molecular

infrared

specific

infrared absorption

ground which

spectra,

to the surface.

but each has its own of inherently

range, but resolution

Inelastic

and small frequency

tunneling

sensitivity,

spectroscopy

but because

the interpretation

better

yields

the sample must

of the results

is not

(ref.2).

not as surface absorption

atoms and mole-

has the advantage

so that linewidths

(ref.1).

be in the form of a tunnel junction, always clear

vibrational

loss spectroscopy

to achieve,

are often unobservable

adsorbed

of their bonding

over a large spectral

than 30 cm-' is difficult shifts

can be used to identify

and to study the chemistry

easily

obtain

as electron

spectroscopy signal

suffers

directly

of 1 to 10 cm-l, but they are Conventional

from the difficulty

is very small compared

arises from reflection

absorption

resolution

spectroscopies.

to the experimental

from the bulk metal

by placing

reflection-

that the surface

a thermometer

back-

(ref.3). Measuring

on the sample

the

allows the

*Work supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U. S. Department of Energy under contract No. DE-AC03-76SF00098. 036%2048/83/0000+000/$03.00

0 1983 ElsevlerSaentific

PubhshmgCompeny

114 observation

of the same small surface

arising

from the bulk absorption

thermal

sources,

however,

room temperature infrared

of background,

experiment

source and apparatus vantages

ratus to measure equilibrium

measurement,

the infrared

Historically,

background

crystal samples

experiment.

EMISSION

infrared

astronomy,

however,

In order to exploit the adand constructed

from a single crystal

sample

that no radlatlon

tor. In practice

APPARATUS have often been limited by intrinsic

Modern photoconductors

developed

are limited by statistical

eter

from sources

we approximate

mately

at liquid nitrogen

infrared system,

emission

grating

spectrometer

1s equipped

consists

sample can also be heated Because the parallel surface,

face interacts near grazing the sample

for maximum

orders of diffraction,

by an off-axis

parabololdal

approxi-

amounts

in the UHV diffraction The

of gas.

normal to the sur-

the sample

is placed at

(ref.6). Radiation

by a low-emissivity polarizer

removes

through

to eliminate

by a plane mirror,

from

KRS-5

the compo-

After passing

the light is filtered

mirror.

(UHV)

field must be zero at a con-

to the surface.

reflected

vacuum

and mass spectrometry.

surface sensitivity

parallel

ultrahigh

field component

light, a cold infrared

polarized

spectrom-

the sample with

on a manipulator

there. Therefore,

slit of the spectrameter,

ond and higher collimated

of the electric

into the cooled spectrometer

lens. To reduce background nent of the radiation

the entire

for low energy electron

and dosed with controlled

with the molecules

incidence

is coupled

the entrance

is mounted

only light with an electric

it is

comes from the sample.

argon ion sputtering,

component

are used,

reach the detec-

of a liquid helium temperature

coupled to a conventional

with facilities

(LEED), Auger spectroscopy,

in the

(77K). Under these conditions,

reaching the detector

apparatus

as shown in Fig. 1. The sample

system, which

ducting

(%5K) and surrounding

temperature

80% of the radiation

Our infrared

by cooling

for low-

fluctuations

other than the sample

this condition

to liquid helium temperatures

baffles

an appa-

in thermal

modes.

photon stream, even at very low flux levels. When such detectors desirable

an

of the

A room temperature source emits significantly -1 , a range which includes the frequencies of

experiments

by the detector.

at

reduction,

by instability

modes and also of most adsorbate-substrate

DESIGN OF THE INFRARED

is thennodynam-

of this background

we have designed

emission

at room temperature.

noise produced

single

should be less influenced

ln the range from 100 to 3000 cm most vibrational

Since emission

and conventional

than a reflection

of an emission

with

low temperatures

, an infrared emission experiment benefits from

can easily be used. Because

emission

background

sensitivity

at extremely

of low thermal mass.

to absorption

the same reduction

(refs. 4 and 5). Adequate

can only be obtained

(~1.5K) and with substrates lcally equivalent

signal on top of the smaller

sec-

and then

The beam is diffracted

by the

115

CrossSecttonof UHV System

Off-axlsParaboloIdal

RodlotIon Shield

Vlewport

Fig. 1. Optical layout of infrared emission apparatus, with liquid helium cooled spectrometer on the left and ultrahigh vacuum system on the right. See text for details.

grating

and refocused

Our detector sensitive

to radiation

IS ampllfled

indicate

would

efficiency

permit the measurement

International,

frequencies

by long term instabilities

measure

(ref.7).

above 1 Hz, the instru-

noise are negligible. limited sensitivity

from a CO coverage

the sensitivity

in the optics

we can routinely

is

in the photon stream, most of which and amplifier

of s7%, the photon-noise

of emission

which

with a cold JFET input stage

to noise ratio of unity in a one Hz bandwidth.

has not yet been implemented,

development,

amplifier

Detector

onto the detector.

higher than 330 cm-'. The photocurrent

limited by the fluctuations

With our spectrometer

paraboloid

from Rockwell

that, at modulation

comes from the sample emission.

with a signal

off-axis

with frequencies

by a translmpedance

Our measurements ment is noise

by a second

is a Si:Sb photoconductor

of 0.005 monolayer, Because modulation

of the instrument

and electronics.

emission

is now limited

Even without

further

from 0.05L CO on Ni, with a line

center signal to noise ratio of 4 in one second of-integration. Our instrument three different resolution

is able to measure

signals

from 330 cm-l to 3000 cm-l using

diffraction

gratings. With slit widths of 1 mn, the instrumental -1 -1 ranges from about 1 cm at 400 cm -' to about 15 cm-l at 3000 cm ,

which should be sufficient adsorbate-substrate

to measure

and molecular

frequencies

vibrations.

and linewidths

of both

116 EXPERIMENTAL

DETAILS

We have observed stretching

vibration

single crystal,

thermally

emitted

infrared

radiation

of CO on a clean single crystal

obtained

from the Materials

Research

versity, was cut at about 8" from a (100) surface. UHV system with base pressure bardment oxygen

of 2 x lo-"

(1uA at 1000 eV), heating

from the carbon-oxygen

of nickel. The nickel of Cornell

Uni-

The crystal was mounted

Laboratory

in a

torr and cleaned by argon ion bomin 2 x 10e8 torr

up to 65O"C, and oxidation

at 600°C to remove residual

carbon. The surface

cleanliness

was monitored

by LEED and Auger spectroscopy. Infrared

data were obtained

clean Ni surface, letting

by first measuring

then exposing

a reference

10m8 torr CO gas into the UHV system through

sec. The spectra nate variations

in blackbody

experiments, together

with spectral

to yield

of the

a leak valve for 5 or 10

of Ni with CO were divided by the reference

ground was then subtracted.

spectrum

the sample to doses of 0.05L or O.lOL CO by

intensity The results

and optical efficiency. from 3 or 4 completely

scans of only one minute

the series of spectra

spectrum

to elimi-

A constant

back-

independent

each, were then averaged

shown in Fig. 2. The instrumental

reso-

lution was 15 cm-'.

Fig. 2. Observed

emission

spectra

Cr...” NCI tnn\ao _,^....^A _.._C..^^ .._ 3LqJpz” 3ur-I ace a> I,“III

I.l\l”“,O

a function of frequency for CO exposures from 0.05L to 1.2L.

E iii .:

‘...(

w

-.

‘*

---*

-......::

._--...:.--

.* ..**

:’ .*

..“‘-.

**

-.

“2’ . _:-.:*** *.-z*....-..-

:

*.

..

-.....-0 6 L

* .

\‘.. **.*....a0.4 L _...* *‘.. _ *-.-w.- +%.*-- .* *.-.*:...... 0 2 L &..4:~ -.

d-J*:

...*.. ---: ..*._.** I

I800

1900

Frequency

2000

(cm”)

2100

117 DISCUSSION We observe The strongest

two prominent

emission

band falls in the range 2010-2035

35 cm-'. A weaker

terminally

bonded

in these frequency

and bridge bonded molecules,

Figures 3 and 4 show the peak positions bands as a function each spectrum

of this baseline quencies

of coverage.

is the primary

shift continuously

of about

in the range

ranges have been attributed

respectively

and integrated

A quadratic

before the calculation

vibration.

cm-' and has a width

band, with a width of 60-80 cm-', is observed

cm-'. Peaks falling

1850-1930

bands due to the C-O stretching

baseline

(ref.8). intensities

from

in the determination

source of error in these values.

upward with increasing

of the two

has been subtracted

of areas; uncertainty

to

exposure,

The peak fre-

reaching

a total

shift of 25 cm -' for the on-top site and 35 cm-' for the bridge site at saturation. These shifts are comparable

to those previously

9 and lo), but they are much smaller 11, and 12) and on evaporated

observed

than those observed

Nl films

on Nl(100)

on Ni(ll1)

(refs.

(refs. 9,

(ref.5).

I

101 x On-top

0

0.4 0.8 1.2 CO Exposure (L)

3. Frequencies of observed infrared lines on Ni(100)8" stepped surface as a function of CO exposure. x's mark the peak position for the on-top site. o's mark the centroid of the line for the bridge site.

OO-

. CO

. Exposure

. (L)

Fig. 4. Areas of the observed infrared peaks on Ni(l00)8" stepped surface as a function of CO exposure. x's mark the on-top site; e's mark the bridge site. The large errors are due primarily to difficulty in determining the baseline.

118 All infrared measurements have observed

band. If these observations they suggest

of CO on Ni, both on single

that the bridge site band is much broader are representative

that the coupling

to the metal

crystals

and on films,

than the terminal

of natural

vibrational

is considerably

stronger

site

lifetimes, for mole-

cules bonded to two Ni atoms than for those bonded to only one.

CONCLUSION We have shown that infrared resolution

emission

probe of the vibrational

finements

of the apparatus

frequency

range of our Instrument

substrate

modes,

spectroscopy

spectra

hold the promise includes

and we will soon attempt

is a sensitive,

of adsorbed

molecules.

of still greater the frequencies

to observe

high-

Further

sensitivity.

reThe

of many adsorbate-

such modes on a Ni(100)

sample.

REFERENCES

6 7 8

H. Ibach, H. Hopster, and B. Sexton, Applic. of Surf. Sci. 1, 1 (1977). P. K. Hansma, Phys. Rep. m, 145 (1977). Robert G. Greenler, J. Chem. Phys. 44, 310 (1966). R. B. Bailey, T. Iri, and P. L. Richards, Surf. Sci. lOJ, 626 (1980). H. J. Levinson, R. 6. Tobin, and P. L. Richards, these proceedings. Robert G. Greenler, Surf. Scl. 69, 647 (1977). E. L. Dereniak, R. R. Joyce, and R. W. Cappas, Rev. Scl. Inst. 48, 392 (1977). R. P. Eischens, S. A. Francis, and W. A. Pliskin, J. Phys. Chem. 60, 194 (1956). J. C. Bertollni and B. Tardy, Surf. Sci. 102, 131 (1981). M. J. Dignam, in Vibrations at Surfaces: Proceeding of an International Conference at Namur, Belgium, R. Caudano, J. M. Gllles, and A. A. Lucas (Eds.), Plenum, New York (1982). W. Erley, H. Wagner, and H. Ibach, Surf. Sci. 8& 612 (1979). J. C. Campuzano and R. G. Greenler, Surf. Sci. !3J, 301 (1979).

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