Time-resolved surface kinetics by IR diode laser reflection-absorption spectroscopy

Journal of Electron Spectroscopy and Reluted Phenomena, 54155 (1990) 573-580

573

Elsevier Science Publishers B.V., Amsterdam

Time-Resolved Surface Kinetics by IR Diode Laser Reflection-Absorption Spectroscopy Eric Borguet and Hai-Lung Department of Chemistry University of Pennsylvania Philadelphia, Pennsylvania

Dai

19104-6323

ABSTRACT An IR reflection-absorption technique using diode lasers as light sources was developed with a capability of /.tsec time resolution for studying surface kinetic A polarization subtraction scheme was used to eliminate the absorption by processes. species other than the adsorbate studied. The technique was first applied to the study of CO adsorption on, and thermal desorption from, a Cu(100) surface. On a timescale of one second, a 0.002 monolayer sensitivity was demonstrated. Absorption traces directly monitoring the CO concentration on the surface with real-time resolution at various frequencies near the CO stretching mode allow the in situ observation of adsorption and The observations suggest that there is more than one adsorption desorption. configuration for CO on Cu(100).

1.

INTRODUCTION

Vibrational spectroscopy has been an important tool for studying the structure and The most commonly used vibrational reactions of molecules adsorbed on surfaces [l]. spectroscopic techniques are electron energy loss spectroscopy (EELS) [2], infrared reflection absorption spectroscopy (IRAS) [3], Raman spectroscopy [4], helium atom scattering [S], and sum-frequency generation [6], to name a few. Each has its advantages and disadvantages [7], and hence its particular domain of applicability. Using primarily dispersive and Fourier Transform spectrometers, IRAS has achieved a level of sensitivity comparable to that of EELS [8], and has exitended its wavelength range down to the molecule-substrate stretching region (~500 cm ) [9]. As an optical technique, it is inherently capable of time resolution down to lo-t4 seconds, though this has to be achieved using lasers and has only begun to be exploited recently [lo]. The use of lasers in IRAS has the potential to overcome many of the problems associated with using black body sources, but so far they have only been used to monitor The principal advantages of lasers are their high phenomena at constant coverage. magnitude greater than blackbody sources), orders intensity (several spectral directionality (enabling optimal grazing incidence geometries to be achieved), high degree of polarization (allowing the surface IR selection rule to be used to its greatest advantage) and coherence [ 111. In principle, lasers can be employed in IRAS for surface In practice, however, intensity [12] studies to achieve shot noise limited sensitivity. and polarization fluctuations [13], rather than detector noise, have been the limiting factors. 0368-2048/90/$03.50

0 1990 Elsevier Science Publishers B.V.

574

Diode lasers have been employed in several surface studies to date [14]. These However, the spectral brightness of investigations have concentrated on spectroscopy. diode lasers and the fast time response of contemporary IR detectors can enable these CW Applications involving gas phase species devices to be used in fast kinetic studies. have achieved microsecond time resolution and sensitivities of better than 1W absorbance [15]. In the solution phase sensitivity of 1O-3 absorbance at sub-picosecond time resolution has been demonstrated [16] using up-conversion detection of IR laser light. In this paper we describe a novel application of diode lasers to the study of surface kinetic phenomena. The CO/Cu( 100) system was chosen as it has been studied by a number of techniques [17], and thus provides a benchmark system against which to test our approach.

2.

TIME-RESOLVED SPECTROSCOPY

DIODE

LASER

DIFFERENCE

REFLECTANCE

Most surface infrared spectroscopies make use of well known surface selection rules first developed by Greenler [ 181. In essence, because of the screening properties of metals, infrared absorption will only occur for species that have a component of their dynamic dipole moment perpendicular to the surface. S-polarized light will not interact P-polarized light alone carries with the adsorbed species and thus acts as a reference. the absorption signal. The subtraction of the reference from the signal should therefore eliminate any fluctuation common to both and yield a null signal in the absence of absorption. In this time-resolved IRAS, the polarized monochromatic output of the diode laser is directed onto the crystal at grazing incidence. The polarization is adjusted to yield equal intensity s and p laser beams after reflection, which are monitored by two detectors separately. A time dependent perturbation (e.g., exposure of clean surface to CO) will cause a change only in the intensity of the p-polarized reflected light and a deviation from zero of the difference signal. The signal (absorption trace) can be obtained at different fixed wavelengths to obtain a time-dependent spectral profile. This methodology overcomes many of the problems associated with conventional IRAS namely absorption by atmospheric species and etalon effects which have limited the sensitivity of previous studies. These artifacts arise only when scanning in wavelength and are not present in our experiment which is sensitive only to the anisotropy of light absorption by the adsorbates. The principal sources of noise in this diode laser difference reflectance spectroscopy are the mechanical refrigerators in which the lasers and detectors are housed. In particular, the 3.5 Hz jitter of the cold head which cools the laser causes the diode laser itself to move in and out of the focus of the collimation optics with consequent amplitude modulation [19a] of the intensity at the detector of l-5%. In order to approach the shot noise limit, modulation frequencies of >lMHz must be employed [19b]. Amplitude modulation limits detection to absorptions >O.l% [19c]. Remedies have been proposed in the literature [19] to circumvent this problem caused by refrigerator vibration. The best solution would be the use of a static cooling dewar to replace the compressor type refrigerator. Fluctuations in polarization and intensity as a function of wavelength [14c] did not affect our experiment as the signal was nulled initially at each different wavelength. The matching of detectors in a dual beam apparatus is important. The responsivities of our detectors [21] were observed to vary due to fluctuations in the cold finger temperature. To improve the stability of the detectors an additional temperature

575

stabilizer [21] was needed to maintain the temperature at a level where fluctuations were minimal and where the responsivities of the detectors were as similar as possible. There is, in principle, no limit to the timescale accessible in this methodology. The current setup, which uses IR detectors to directly monitor absorption, should enable sub-microsecond time resolution. Indeed, on short timescales experimental schemes ,involving repetitive perturbations on systems which regenerate themselves are more readily employed. It will thus be possible to take advantage of multiple shot signal averaging. 3.

EXPERIMENTAL A. Ultra High Vacuum Apparatus

The UHV chamber is equipped with a 300 a.m.u. quadrupole mass spectrometer, an ion gun, a Bayard-Alpert type UHV ion gauge and a leak valve. The chamber is pumped by a 170 l/s turbo molecular pump (which was also used to pump the bakeable gas admission manifold), a 120 l/s ion pump and a 35 l/s noble ion pump. Pressures lower than 2x10-10 Torr were routinely achieved after a 36 hour bakeout at 15O’C. The crystal (12 mm x 8 mm x 1.5 mm) was prepared by standard metallographic techniques to a mirror finish, and its orientation verified to within 2’ by Laue back-diffraction. Two spark eroded grooves on the top and bottom edges of the crystal enabled it to be firmly attached to the sample heater by means of two small stainless steel clamps. The sample heater is a self contained package which can provide up to 40 W of heating power at 1200°C (maximum recommended temperature) [20]. The molybdenum casing provides for excellent thermal conductivity. The heater and crystal assembly is clamped into the copper base of a liquid nitrogen dewar. This arrangement ensures the mechanical stability of the crystal, while allowing cooling to 90 K and heating to temperatures greater than 850 K. The cooling time constant for the system was 100 seconds, and heating rates of up to 5 K/s were also possible, the linearity of the ramp being maintained over 150 K. Because of the difficulty of spot welding to copper the Chromel-Alumel thermocouple wires (0.005 inch) were spot welded instead to one of the clamps, within 2 mm of the crystal edge. B.

Optical Arrangement

The IR source consisted of a lead-salt tunable diode laser mounted on the vibrationally isolated cold finger of a cryogenic refrigerator [21]. The diode lasers are composition tunabte, commercially available devices capable of covermg_,a range of 300 cm to 3300 cm . Each particular laser diode can be tuned 100-300 cm above the composition determined threshold frequency by varying both laser temperature and injection current. Above threshold the laser generally emits in several modes, spaced =2 cm-’ apart. Each individual mode, which is about 0.0003 cm-’ wide, can itself be tuned about 0.5 cm-‘. These are thus stepwise continuously tunable devices, with numerous short (l-2 cm-‘) gaps in the tuning range. The linearly polarized, divergent (~30 ), output of the laser is collimated by an off-axis parabola (OAP) and then passes through a monochromator, as shown in Figure 1, which serves as both a tunable grating filter and a wavelength calibration. The monochromator was independently calibrated against the absorption spectrum of a liquid standard. The emergent single mode radiation is collimated to a diameter of 7 mm by a refractive Galilean telescope. A 500 Hz mechanical chopper was placed at the telescope

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W : WAVEPLATE PB:

POLARIZER

RD: BEAMSPLITTER

M : MONOCHROMATOR

Figure 1: Schematic illustration

REFERENCE

SD : SIGNAL DA:

BEAM

BEAM

DIFFERENYIAL

of experimental

DETECTOR

DETECTOR AMPLIFIER

apparatus.

A ?J2 waveplate [22] serves to rotate the plane focus to modulate the laser intensity. of polarization of the incident radiation which is directed onto the crystal at grazing The reflected incidence (87’) through a differentially pumped BaF, window [23]. radiation exits through a second BaFa window and is separated into its s and p components by means of a wire grid polarizer 1243. The polarizer, oriented 45* with respect to the incident beam, transmits p-polarized light and reflects s-polarized light. The s and p components are each focused onto two separate copper-doped germanium photoconductive detectors housed in a cryogenic refrigerator [21]. The output of each detector was amplified with a 200 kHz bandwidth preamplifier [25] and fed into a homemade differential amplifier. The difference signal output was sent to a lock-in amplifier [26] operating with a time constant of one second for the experiments described here. The lock-in output, as well the chamber pressure, partial pressure and thermocouple voltage were continuously recorded and stored on a computer for subsequent analysis. All gases used were 99.999% purity and used as received.

C. Experimental Procedure The sample was prepared by sputtering with Ne*(SOO eV, 5 @/cm*) at room temperature, and annealing to 850 K for a few minutes before each experiment. The IR absorption, as a function of time, of CO on Cu(100) during an absorption or desorption process was recorded in the following manner. The laser was tuned by varying both the temperature and current to the desired wavelength region. A single mode was selected using the monochromator and the waveplate adjusted to yield a null signal from the differential amplifier. The crystal was initially flashed to 230 K to remove any species that could have adsorbed from the background, and then allowed to cool. When the crystal temperature had fallen below 95 K the copper surface was exposed to CO by backfilling the chamber to -5x10-9 Torr while the reflectance difference signal was monitored. The presence of a species absorbing at that wavelength was indicated by a decrease in the reflected p-polarized light and a consequent deviation from zero of the difference signal. After

3-5 L exposure the leak valve was closed and pressure fell to <5x1W” Tot-r in <3 minutes. The sample was then heated at a constant rate (1.5 K/s) to drive off adsorbed CO while the partial pressure at m=28 a.m.u. and the reflectance difference signal were monitored. This procedure was repeated 3-4 times before tuning the laser to a new wavelength. Before and after each run a BaF, slide was inserted in one of the beam paths after the polarization beamsplitter. The resultant signal simulated absorption and provided a calibration for the experiment.

4.

RESULTS

AND DISCUSSION

IR reflectance difference traces were recorded, for both adsorption and desorption, at a number of wavelengths from 2070 cm-’ to 2110 cm-‘. The IR absorption peak for the CO/Cu(lOO) is known to shift from 2077 cm-’ at low coverage to 2088 cm-’ at saturation coverage (0=0.57). After exposure of 1.8 L, corresponding to 8=0.5, a sharp ~(2x2) LEED pattern was reported [ 17b]. Saturation is only achieved after 5-6 L exposure, and is associated with a new phase indicated by a ~(74 2x-12)R45’ LEED pattern [17g]. In the case of adsorption traces the IR signal was first plotted as a function of Figure 2 illustrates an adsorption trace recorded at exposure for each wavelength. 2085 cm-‘. The nonlinear behavior is due to the shift in IR absorption band shape and

16-

161 4*12-

86-

0 0.0

0.4

0.8

1.2

EXPWRE(lANGMUIRS)

IR difference reflectance Figure 2: recorded at 2085 cm-’ during adsorption of CO (left).

,

, 120

,

, 160

,

,

,

200

TEMPERATURE(K)

Figure 3: IR difference reflectance at 2085 cm-’ while heating sample at 1.5 R/s (right).

578

position [17b], as well as the change in the perpendicular component of the dynamic The noise level is equivalent to dipole moment [17g], as a function of coverage. absorption by 0.002 of a full monolayer. Desorption traces require a correction before the IR difference reflectance as a function of temperature, at a given wavelength, can be extracted from the data. The bare metal s and p reflectivities do not vary in the same manner as a function of temperature. Therefore even in the absence of IR absorbing species an IR difference reflectance signal is observed. A desorption difference reflectance trace is therefore recorded at each wavelength with no prior CO exposure. This trace is subtracted from the desorption trace for a CO covered surface. These clean surface traces also serve to correct for any changes in sensitivity upon going from one wavelength to another. As the IR difference reflectance signal is a strong function of the angle of incidence at which measurements are made, slight changes in this angle are of consequence [13]. A desorption trace taken at 2085 cm-’ is shown in Figure 3. To facilitate the interpretation of the experimental data, plots of intensity as a function of wavenumber are generated. Figure 4 is an example of such a spectrum. It represents the spectral profile at T=120 K during a temperature ramp. The raw data are best fit to a sum of two Lorentzians. The evolution of the spectral profile as a function of temperature during a temperature ramp is illustrated by creating a three dimensional plot of intensitywavenumber-temperature as shown in Figure 5. 1 20

10 15

8

8

10

5

2080

2100

0 10

I 2090

, 2090

I 2100

/

I 2110

I

2120

WAVENUMBEA (CM-‘)

Figure 4: Intensity as a function of wavenumber at 120 K during a temperature ramp. The solid triangles are the experimental points. The solid lines are the Lorentzian fits (see text).

Figure 5: Fitted IR difference reflectance spectra for 1.5 K/s temperature ramp. The spectra shown are sections of the intensity-temperature-wavenumber hypersurface taken at 92 K, 100 K, 110 K, 120 K, 130 K, 140 K, 145 K, 150 K, 160 K, 165 K, 170 K, and 175 K.

579

Upon heating, the intensity of the low frequency peak is observed to decrease up to about 125 K. This is accompanied by a peak in the thermal desorption spectrum. Subsequently, the IR spectrum resolves into two peaks. The low frequency peak initially narrows, with an accomp~ying increase in intensity, before broadening out and decreasing in intensity. This regime is associated with the principal TDS peak. The high frequency feature persists until 180 K. This peak is attributed to the presence of defects or steps. At very low coverages a high temperature feature is present in TDS. It coincides with a feature in the TDS of a sputter damaged surface. The reported line shape for CO/Cu(lOO) is a Lorentzian type function with a high frequency tail [17b]. Our line shapes indicate the presence of inhomogeneous broadening, probably due to a small misalignment (~2’) of the crystal [17fl. The apparently anomalous peak height associated with the defect sites can be at~but~ to intensity borrowing. The reported LEED patterns suggest two species [17g] at high coverage and only one at coverages below 8=0.5. The behavior of the lower frequency peak, which initially broadens with temperature and then narrows at -140 K, is indicative of the passage from one configuration to another. Narrow peaks generally indicate ordered structures. The spectral behavior as a function of temperature thus suggests the existence of two configurations for CO on Cu(lO0). A quantitative analysis of this system is currently underway and will be published elsewhere.

5.

CONCLUDING

REMARKS

A novel application of diode lasers to IRAS has enabled us to follow adsorption and desorption in situ and in real time. The method is sensitive to 0.002 of a full monolayer of CO on a subsecond timescale. In principle, the experimental approach should enable extension to faster timescales (10-’ seconds initially) as has occurred for gas and liquid phase with even higher sensitivity.

ACKNOWLEDGEMENTS This work is supported in part by the NSF (MRL 85190959). Ac~owledgement is also made to the Donors of the Petroleum Research Fund administered by the American Chemical Society for additional support. HLD acknowledges the receipt of a Camille and Henry Dreyfus Foundation New Faculty Grant and a Teacher-Scholar Award, and an Alfred P. Sloan Fellowship. The authors are grateful to Professor E. Ward Plummer for equipment support as well as helpful discussions.

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