Optical Effects in the IR Reflection-Absorption Spectra ...

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J. CHEM. SOC. FARADAY TRANS., 1995, 91(21), 405-4008

Optical Effects in the IR Reflection-Absorption Spectra of Thin Water-ice Films on Metal Substrates

Published on 01 January 1995. Downloaded by Heriot Watt University on 28/04/2015 13:01:15.

Andrew B. Horn,*? Sally F. Banham and Martin R. S. McCoustra$ School of Chemical Sciences, University of East Anglia , Norwich, UK NR4 7TJ

The effect of film thickness upon the shape of the OH stretching band of thin films of water ice on a gold substrate has been analysed using classical optics. The structure observed in the OH stretching band of films with thicknesses comparable to the wavelength of the IR radiation arises because of a combination of an increased specularly reflected component, an s-polarised component and a relaxation of the metal surface selection rule.

Heterogeneous chemistry occurring on the surface of particles at low temperatures in the atmosphere has been effectively studied recently using extended thin films of ice, solid nitric acid hydrates and concentrated sulfuric acids.'-' Experimental techniques involving the use of flow-tube reactors' and Knudsen cells' have been used to obtain kinetic parameters such as reaction probability, whereas IR spectroscopic studies have given valuable information regarding surface reaction mechanism^.^-'^ Reflection-absorption IR spectroscopy (RAIRS), in which thin films of relevant material condensed upon highly reflecting substrates (e.g. gold foil) are subjected to a single external reflection of IR radiation at high angles of incidence, has a number of experimental advantages for the analysis of thin films5 To a first approximation, the spectra of these films obtained in a RAIRS configuration are qualitatively similar to transmission IR measurements. However, there are a number of small optical perturbations which may be observed upon closer inspection. For example, it has recently been observed that shifts in band position occur for ordered thin films of the nitrogen oxides N20, and N,O,, which are a direct consequence of the effect of the vicinity of a metal surface4 and coupling between the IR radiation and the longitudinal (LO) and transverse (TO) optical modes in the film. The T O mode gives rise to a vibrational transition dipole moment parallel to the surface and is effectively neutralised by the so-called metal surface selection rule (MSSR) whereas the LO mode, with a surface-normal transition dipole moment, is observed at a higher frequency. The degree of splitting between the modes (and hence the frequency shift) is referred to as Lydane-Sachs-Teller (LST) splitting.' The magnitude of the shifts is proportional to the oscillator strength of the individual IR absorptions; the effect has been observed previously in the spectra of thin films on transmitting substrates, where both the LO and T O modes are seen in the absence of an MSSR. For films of water ice, a complicated structure appears in the OH stretching region (2700-3700 cm- ') as the film thickness increases. This does not correspond to the changes caused by crystallite size or orientation effects" upon annealing, which are observed for a wide range of film thicknesses. In this communication, we report an analysis of the optics of the water/metal substrates used in such experiments and offer an alternative explanation of the effects.

'

The IR spectrum of a thin ice film at 80 K is shown in Fig. l(a). For very thin films (estimated thickness 50-100 nm), a broad band centred around 3220 cm-' is observed which shows structure in the form of two broad shoulders to low wavenumber. Upon annealing the sample, the band shape changes slightly as the ice film re-orders to produce what is thought to be a polycrystalline film [Fig. l(b)]. Fig. 2(a) shows spectra resulting from the deposition of amorphous films of H 2 0 at 80 K. As the thickness increases, the main peak of the absorption sharpens and shifts to higher wavenumber whilst a strong feature appears to grow out of the shoulder to lower wavenumber of the main absorption. This peak dominates the spectra of the thicker films. The spectrum obtained from D 2 0 films under identical conditions is shown in Fig. 2(a), for which similar behaviour is observed. The precise thickness of the thickest film has not been determined experimentally in either case, but is estimated to be of the order of 5-10 pm, i.e. comparable to the wavelength of the radiation. 0.220.20-

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t Present address : Department of Chemistry, University of York, Heslington, York, UK YO1 SDD. $. Present address: Department of Chemistry, University of Nottingham, Nottingham, UK NG2 2RD.

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Fig. 1 RAIR spectra of CQ. 50 nm ice films deposited on a gold substrate:(a)as deposited at 80 K ; (b)after annealing to 155 K

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 0.45

where Fo, y and So are the centre frequency, the half-width and the effective oscillator strength of the absorber. The complex refractive index as a function of frequency can be calculated directly from this equation. Refractive indices, derived from experimental data and recently re-evaluated by Toon et aZ.,14 are available in the literature and have been used here for full spectral simulation. The optical properties of the multiphase system can be described in terms of classical optics, using the well known Fresnel equations,' presented in an appropriate form by, for example, McIntyre and Aspnes.I6 For the interface between two phases j and k, the Fresnel coefficient r j k , which describes the relative electromagnetic field intensities on either side of the interface, can be written as

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where pk and &k are the magnetic permeability (ca. 1 in the mid-IR) and the complex relative permittivity (frequency dependent for the absorbing layer, effectively constant for the metal substrate) of the phase k. The superscripts s and p represent the polarisation state of the IR radiation, parallel and perpendicular to the surface, respectively. The function t k for each phase (effectively a complex cosine term) is related to the angle of incidence upon the first interface 8, and the complex refractive index of phase k ( n k ) according to the equation

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For a three-phase vacuum-absorber-metal system, the overall Fresnel coefficient can be written as

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where the term B describes the (wavelength dependent) change in phase of the radiation on travelling through the second phase,

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Fig. 2 RAIR spectra of varying thicknesses of ice deposited on a gold substrate at 80 K. (a) H,O ice; (b)D,O ice.

In order to explain these changes, the RAIR spectrum of the vacuum-absorber-metal system has been modelled more precisely using a classical optical approach. Initially, the optical properties of the absorbing layer have been simulated as a single absorption band using a complex dielectric function of the form13 E(?)

= E,

c2 + (G2 -so ?); - iy?

(5)

where d is the thickness of the film. In many treatments, linear approximations of the exponential function in eqn. (4), valid when d A, are used to simplify the computation; however, for thicker films it is necessary to use the full complex expression. Computation of the results presented in this paper was performed using version 5.0 of the MathCad mathematical package. Fig. 3 shows the predicted vibrational spectrum for an idealised absorbing layer, with optical parameters close to those typical of the OH stretching region of ice, as a function of thickness for s- and p-polarisation (the input parameters are described in the figure caption). The angle of incidence is taken as 75", since this angle is used in our experimental measurements. Higher angles of incidence will result in a slightly different band structure. In order to predict the combined, unpolarised spectrum recorded experimentally, the sum is shown in Fig. 4. It can clearly be seen from Fig. 3 that there are two effects contributing to the complicated band structure. First, the p-polarised response (which is the only one seen for very thin fdms becaue of the MSSR) starts to split as the film thickness increases. This can be explained by considering the effect

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

upon the optical properties of a material of a strong absorption band. The reflectivity is determined by the complex refractive index, n. This includes contributions from the effects of the absorption index k (related to the imaginary part of n ) and the real refractive index, n. These parameters are linked and large fluctuations in n are associated with large values of k. The complex refractive index (and hence the reflectivity) will vary across the spectrum and the thick-layer spectra will show the effects of a reflection maximum near the peak of absorption. Consequently, the absorption intensity will be distorted in the vicinity of its peak, since a reflected ray reaching the detector corresponds to a reduction in the amount of radiation able to enter the sample and be absorbed. This appears in the spectrum as a notch in the absorption contour, the relative depth of which increases with film thickness. The ultimate extent of this effect is observed in the pure reflectance spectra of strongly absorbing materials, e.g. polymers. The second effect is the appearance of a surface parallel (s-polarised) component. For ultrathin films, this absorption is forbidden by the MSSR: as the distance increases from the surface, the effect decreases and the s-polarised band is

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Fig. 4 Sum of s- and p-polarised absorbances from Fig. 3

observed. For normal incidence, this would have an antinodal maximum at A/4,where A is the wavelength of the radiation in the medium in question. The thicker film ( 5 pm) spectral simulation, in which both of these effects are combined, shows a remarkable similarity to the structure of the real RAIR spectrum. To verify the nature of this effect, simulations using experimental values of the optical properties of ice are shown in Fig. 5 and 6. The OH stretching absorption band of a thin layer of ice contains a number of individual, overlapping absorptions (due to the molecular symmetric and asymmetric stretches, overtones, combinations and solid-state effects) and cannot be truly represented by a single harmonic oscillator. The results show that the effect described above is noticeable only for the strongest bands in the spectrum whilst the weaker absorptions remain relatively unperturbed, in excellent agreement with the real spectra. This is commensurate with the known fact that strong selective reflection maxima are associated with sharp absorption bands of high intrinsic intensity. Overall, however, our calculated intensities are higher than in the measured RAIR spectra. This is most probably due to the lack of sophistication used in the model, which does not account for inhomogeneities such as roughness and uneven thicknesses in the film. For a real ice film, the roughness of the surface will result in a degree of diffuse reflectance from the top surface of the ice and a rather diffuse beam being reflection-absorbed in the film. Additionally, a real measure-

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Fig. 3 Predicted absorbance spectra for s- and p-polarisations for varying thicknesses of an absorbing material on a gold surface. Optical parameters are calculated from eqn. (l), using vo = 3220 cm-*, So = 0.1 and y = 150cm-'.

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

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wavenumher / cm-' Fig. 5 Predicted RAIR spectra for s- and p-polarisations for varying thicknesses of ice on a gold substrate, calculated using measured optical data from ref. 14

2 3

4 5 ment represents the average spectrum over a range of angles of incidence (ca. 12" in our system). Features observed in IR reflection spectra of icelmetal layers can be accurately predicted from classical Fresnel coefficients and the optical properties of ice. As the thickness of the ice film is increased, reflectance effects start to impinge upon the measured absorption spectrum, changing the overall shape of the band. The magnitude of the effect is correlated with the magnitude of the absorption coefficient and the film thickness. These effects must be taken into account when interpreting the IR spectra of species adsorbed on and absorbed in ice films of thicknesses comparable to the wavelength of light in the mid-IR spectral region. This work was performed with support from the SERC, the ACI, the CEC STEP programme (CT90-0071) and CEC Environment programme: We would also like t o thank Dr. John Sodeau for the use of equipment and Professor Norman Sheppard for illuminating discussions and useful comments during the preparation of the manuscript.

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M. Leu, S. B. Moore and L. F. Keyser, J. Phys. Chem., 1991,95, 7763. 0.W. Saastad, T. Ellerman and C. J. Nielsen, Geophys. Res. Lett., 1993,20, 1191. A. B. Horn, T. G. Koch, M. A. Chesters, M. R. S. McCoustra and J. R. Sodeau, J. Chem. Phys., 1994,98,946. T. G . Koch, A. B. Horn, M. A. Chesters, M. R. S. McCoustra and J. R. Sodeau, J. Chem. Phys., 1995,99,8362. M. R. S. McCoustra and A. B. Horn, Chem. SOC.Rev., 1994,23, 195. S. F. Banham, A. B. Horn, T. G. Koch and J. R. Sodeau, Faraday Discuss., 1995,100,in the press. M. A. Tolbert, B. G. Koehler and A. M. Middlebrook, Spectrochim. Acta, Part A, 1992,48,1303. B. G. Koehler, L. S. McNeill, A. M. Middlebrook and M. A. Tolbert, J. Geophys. Res., 1993,98,10563. H. Rieley, H. D. Aslin and S. Haq, J. Chem. SOC., Faraday Trans., 1995,91,2349. L. Delzeit, B. Rowland and J. P. Devlin, J . Phys. Chem., 1993, 97,10312. R. H. Lyddane, R. G. Sachs and E. Teller, Phys. Rev., 1941,59, 673. B. Schmitt, R. Grim and M. Greenberg, Proc. 22nd ESLAB Symposium on Infrared Spectroscopy in Astronomy, Eur. Space Agency, SP290,1989,213. H. W.Verleur, J. Opt. SOC.Am., 1968,58,1356. 0. B. Toon, M. A. Tolbert, B. G. Koehler, A. M. Middlebrook and J. Jordan, J. Geophys. Res., 1994,99,25631. 0. S . Heavens, Optical Properties of Thin Solid Films, Dover, New York, 1991. J. D. E. McIntyre and D. E. Aspnes, Surf Sci., 1971,24,417.

Communication 5/05492C; Received 17th August, 1995