Journal of Electron Spectroscopy and Related Phenomena, 38 (1986) 153-158 Elsevler Science Publishers B.V.. Amsterdam - Printed in The Netherlands

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HIGH RESOLUTION INFRARED REFLECTION ABSORPTION SPECTROSCOPY WITH A CONTINUOUSLY TUNABLE INFRARED LASER

F. M. HOFFMANN, N. J. LEVINOS, B. N. PERRY AND P. RABINOWITZ Exxon Research and Engineering Company, Corporate Research Science Laboratories, Route 22 East, Annandale, New Jersey 08801, USA

ABSTRACT

A new, continuously tunable high resolution infrared laser has been applied to Infrared Reflection Absorption Spectroscopy (IRAS) of molecular adsorbates on well defined metal surfaces. The sensitivity, resolution and continuous tunability of the laser are demonstrated with vibrational spectra of gas-phase methane and of submonolayer CO adsorbed on Ru(001).

INTRODUCTION

Infrared Reflection Absorption Spectroscopy (IRAS) has become an important

method for performing vibrational spectroscopy on well defined surfaceslw3.

This technique is attractive because of its high resolution and its pressure

independence which allows operation both at UHV and higher pressures. The

high sensitivity of IRAS has been demonstrated in the spectral region above

1000 cm-I. However, expansion of the accessible spectral range to lower

frequencies depends critically on the availability of radiation sources with

higher intensity than is currently available from thermal emitters. At

present, synchrotron radiation4, free electron lasers5, and infrared lasers6

are being investigated as potential tunable sources of infrared radiation in

the 1000 to 10 cm-I range. Synchrotron radiation holds promise as an intense

broadband source in the spectral regime below 1000 cm-l (e.g. for FTIR

spectroscopy)4. Infrared lasers have potential as powerful narrowband sources

with extremely high resolution (0.20 to 0.01 cm-l) and intensity many orders

of magnitudes higher than thermal emitters. As a result, infrared lasers

offer an opportunity to perform IRAS at higher sample temperatures (without

interference from thermal background radiation), and to obtain peak shape

analysis and time resolution (lifetime measurements in the nano to picosecond

range) in the presently inaccessible spectral range below 1000 cm-l. In spite

of these apparent advantages infrared lasers have not been used previously for

IRAS except for photoaccoustic spectroscopy in the 9-11 pm region (900-1100

cm -l) accessible with CO2 lasers7s8. The reason for this is twofold: the

lack of laser tunability over a large wavelength range and the inability to

realize the singal-to-noise advantage of these sources due to amplitude and

frequency fluctuations.

0368-2048/86/$03.50 0 1986 Elsevier Science Publishers B.V.

154

We now report the first application to IRAS measurements of a newly

developed wideband tunable (10,000 to 900 cm-l), high resolution infrared

laser. The sensitivity and resolution of the technique are demonstrated with

vibrational spectra of submonolayer CO adsorbed on a Ru(001) surface.

EXPERIMENT

Stimulated Raman scattering in a gas phase medium can be used to extend

the tuning range of existing lasers from the visible into the infrared region

by generating and optimizing high order Stokes radiationg(a). The

continuously tunable infrared radiation used in our measurements was generated

in a recently developed hydrogen Raman laser g(b). The experimental system is

schematically depicted in Fig. 1. A frequency-doubled Nd-YAG laser is used to

pump a dye laser, producing pulses of 7 nsec width over the tuning range of

the dye. The dye radiation is directed into a hydrogen Raman cell (14 atm of

H2) and Raman converted third Stokes infrared radiation exits through a BaF2

window. A multiple pass arrangement in the Raman cell provides a substantial

reduction in the required pump power by increasing the net Raman gain through

the use of repeated focussing in the active medium l"all. The number of

passes can be adjusted from 3 to 21. In these experiments 21 passes were

used. Figure 2 shows the measured quantum efficiency for production of first

Stokes (0.97 urn), second Stokes (1.62um) and third Stokes (5.0um) radiation

with the pump radiation at 0.69 nm as a function of injected pump

Nd:YAG Pumped Dye-Laser

Fig. 1: Schematic diagram of the experimental apparatus.

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energy. By tuning over the range of a visible dye (600-800 cm-l) and

generating Stokes radiation, a corresponding range in the infrared can be

accessed for each Stokes order. By changing dyes, the full range of the

infrared between 10,000 and 900 cm-I is accessible.

Both continuous tunability and high resolution (0.2 cm-l) are demonstrated

in Figure 3 with a gas phase spectrum of methane. Figure 3a shows the R, Q

and P-branches of the C-H stretch in the range between 3180 and 2940 cm-l.

The inset (b) shows a high resolution spectrum (0.2 cm-l) of the Q-branch with

an expanded frequency scale over a range of 20 cm-I. Finally, curve (c) is

the baseline in the absence of methane.

The IRAS experiments were performed in a UHV chamber (10-l') Torr range)

equipped to perform IRAS, LEED (Low Energy Electron Diffraction), Auger

electron spectroscopy, thermal desorption, workfunction measurements and

EELS. Experimental procedures, preparation and cleaning of the ruthenium

crystal (8 mn diameter) were similar to those described earlier in detail12.

60 -

10 -

I 25.0

Laser Pump Energy(mJ)

Fig. 2: Measured quantum conversion efficiencies (photons out of MPG/Photons into MPC) for pump (0.69Pm) 1st Stokes (0.97um), 2nd Stokes (1.62vm) and 3rd Stokes (5,OPm) radiation vs. input pump energy.

156

CH4 (1 Atm N2)

w I I I I I

3150 3100 3050 3000 2950

Fig. 3: Vibrational-rotational spectrum of gas-phase methane (50 Torr CH4 in 1 atm NZ): (a) R, Q and P-branch (b) Q-branch dt an expanded scale (c) baseline in the absence of methane.

The infrared radiation from the Raman cell is passed through appropriate

filters to remove lower order Stokes radiation and is focussed onto the

Ru(001) sample at an angle of incidence of 80" from the surface normal. The

radiation consists of pulses of 1 mJ energy (at 2000 cm-l) at a repetition

rate of 10Hz. The incident light is linearly polarized and the polarization

plane is kept at 45" with respect to the plane of incidence. This gives

approximately equal components of light polarized parallel and perpendicular

to the plane of incidence. After reflection from the surface, the parallel

and perpendicular components of the radiation are analyzed using a ZnSe prism,

and are separately detected with a pair of pyroelectric detectors (Fig. 1).

Since the surface dipole selection rule on metals allows only vibrational

excitations with a dynamic dipole moment perpendicular to the surface3, only

the p-polarization is absorbed by the adsorbate. Thus one has a means to

ratio the signal. This method of polarization "modulation" reduces signal

fluctuations which result from laser pulse-to-pulse instabilities. It also

20,56 comu (001) 85K

C 5.5 cm-*

LU3U zu13

Wavenumber cm- 1

Fig. 4: Vibrational spectrum obtained with a single reflection from a CO layer (e = 0.65) adsorbed on Ru(001) at 85K. (Resolution 0.2 cm-l).

reduces the interference of atmospheric background absorption (mainly due to

water). This technique allows spectra to be collected at typical scan speeds

of 0.5 to 1 cm-l -1 set , at signal-to-noise ratios of 200 to 500, with an

inherent resolution of 0.2 cm-l.

Experimental results are shown in Fig. 4 with a vibrational spectrum of

the C-O stretch obtained for a layer of carbon monoxide adsorbed in molecular

form at 85K on a Ru(001) surface (e = 0.65). The C-O stretch frequencies at

2056 cm-l is in good agreement with previous measurements13. The high

resolution spectrum produced with the tunable laser shows a linewidth of 5.5

cm ml for the C-O stretch, which is considerably smaller than previously

measured14. However even at this small value it appears from temperature

dependent measurements15 that inhomogeneous broadening contributes to the

lineshape. Inhomogeneity is also suggested by a shoulder on the low frequency

side of the spectrum in Fig. 4. From these measurements we conclude that the

homogeneous linewidth is 5 cm-l or less. The measured linewidth gives a lower

limit of the estimated vibrational lifetime of T > 2 psec for chemisorbed CO

on Ru(001) (Ead-30kcal/mo117).

158

CONCLUSION

The data presented here clearly demonstrate that sufficient sensitivity

can be obtained with tunable infrared lasers to perform IRAS on submonolayers

of molecular adsorbates. To fully realize the potential of this technique,

will be necessary to extend the tunability of the laser to the region below

900 cm-l and to approach the fundamental sensitivity of the technique by

increasing the laser stability. We are presently working towards both of

those goals. The present restriction of the IR laser to the spectral range

above 900 cm-I is due to the cut-off of the BaF2 exit window on the Raman

cell. By employing windows with longer wavelength transmission, and using

more powerful pump radiation it should be possible to extend the spectral

range to lower frequencies.

it

ACKNOWLEDGEMENT

The authors are grateful to A. Kaldor for stimulating discussions and

enthusiastic support and to J. Hrbek and S. Dougal for technical support.

REFERENCES

(5)

(6)

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(9b)

I :!I (12)

(13)

(14)

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