Robert G. Greenler

Infrared Study of Adsorbed Molecules on Metal Surfaces by Reflection Techniques

The problem of obtaining the infrared spectrum of a molecular monolayer adsorbed on a bulk metal is discussed. The intensity of an infrared absorption band in radiation reflected from the surface is calculated for (a) various optical constants of the adsorbed layer and the metal, (b) various thicknesses of the adsorbed layer, (c) various angles of incidence, and (d) both states of polarization of the incident radiation. The absorption factor for infrared radiation polarized parallel to the plane of incidence typically has a peak at an incident angle of about 88°, where the absorption is 5000 times greater than at normal incidence. The absorption of a thin layer by the reflection technique, at optimum conditions, is calculated to be about 25 times greater than by transmission through the unsupported film at normal incidence.

Reflection Method for Obtaining the Infrared Spectrum of a Thin Layer on a Metal Surface

Previous calculations have shown that a thin, moderately absorbing layer on a metal surface has an absorption factor in the infrared which varies greatly with the angle of incidence and the state of polarization of the incident radiation. The requirements on a spectroscopic system, which uses the insights of these calculations, are discussed under the realistic conditions of multiple reflections with a low f/number beam. A method is described for obtaining many reflections between closely spaced mirrors. Radiation emerges from a narrow aperture between the mirrors, the aperture serving as an effective source for the infrared monochromator. A system has been constructed and its use is illustrated by the spectrum of a cellulose acetate layer on a silver mirror.

An infrared study of the adsorption of CO on a stepped platinum surface

Abstract The adsorption of CO at 85 K on the Pt-s[4(111)×(100)] surface has been investigated using infrared reflection-absorption spectroscopy (IRAS). TPD and LEED. Both IRAS and TPD measurements indicate that adsorption of CO takes place initially in a linearly bound configuration at sites associated with the steps (ν(CO) = 2065–2078 cm −1 ). Subsequently CO adsorbs with a reduced sticking coefficient on the (111) terraces, giving rise to a second band in the linear region (ν(CO) = 2086–2097 cm −1 ). The observed vibrational coupling is indicative of one-dimensional domains of step CO at low coverage and of two-dimensional domains, containing both step and terrace CO, at high coverages. The frequency of the step CO singleton is surprisingly close to that of the (hypothetical) terrace CO singleton. The IR extinction coefficient for CO adsorbed at the step site is a factor 2.7 greater than for CO in the two-dimensional domains at high coverage. Unlike Pt(111), adsorption on the stepped surface does not give rise to a bridging species.

The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

Abstract The adsorption of CO on Ni(111) has been studied using infrared reflection-absorption spectroscopy combined with LEED, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. At low CO coverage (θ = 0.05) CO adsorbs on threefold sites with a strecthing frequency given by ω CO = 1817 cm −1 . At θ = 0.30 all molecules have shifted to two-fold sites, and θ = 0.50, where a c(4 × 2) structure is observed, ω CO = 1910 cm −1 . At θ = 0.57, with a (√7/2) × √7/2)R19.1° structure, one quarter of the molecules are adsorbed on top of the nickel atoms with the others in two-fold sites. Molecules bonded on the top sites give rise to a band at 2045 cm −1 . The frequency shift due to dipole-dipole interactions is small compared with the shift resulting from bonding to different crystallographic sites.

The effect of index of refraction on the position, shape, and intensity of infrared bands in reflection-absorption spectra

Abstract The effect of the index of refraction on the appearance of infrared bands is discussed for a newly developing method of obtaining the infrared spectrum of a thin layer on a metal surface. The term, reflection-absorption spectroscopy, is proposed as a designation for this technique. Band shapes and shifts are calculated for reflection-absorption spectra, using previously published values for the optical constants. Examples are shown which indicate that the variation of index of refraction for an extremely strong band causes considerable changes from the usual transmission spectrum, but that the reflection-absorption spectrum of a weak band is much like the transmission spectrum. The calculations done with published data on the 609 cm−1 band of Cu2O are compared with experimental results obtained with an oxide layer formed on a copper surface.

The metal-surface selection rule for infrared spectra of molecules adsorbed on small metal particles

Abstract The metal-surface selection rule predicts that some infrared absorption bands should be absent from the spectra of molecules adsorbed on metal particles. It predicts that those bands corresponding to molecular vibrations with an oscillating dipole moment parallel to the surface should be suppressed. We calculate that this selection rule should apply to adsorption on metal particles larger than about 20 A in diameter; for smaller particles the rule is weakened.

The interpretation of CO adsorbed on Pt/SiO2 of two different particle-size distributions

Abstract The infrared transmission spectra have been obtained for CO adsorbed on silica-supported platinum particles with two different particle-size distributions. The smaller has a mean measured diameter of 11 A; the larger, 39 A. The spectra have been interpreted by comparison with calculated spectra from CO adsorption on model particles. The differences in the infrared spectra for the different particle sizes are attributed to the relative number of surface atoms in the Pt particles with different coordination numbers.

An infrared spectroscopic study of carbon monoxide adsorbed on polycrystalline gold using the reflection-absorption technique☆

Abstract The Cz.sbnd;O stretching band is recorded for CO adsorbed on an evaporated gold film. This band appears at 2120 cm−1 for low coverage and shifts to 2115 cm−1 for maximum coverage. The reflection-absorption system produces a measurable band from as little as 1 20 of monolayer coverage of CO, using a standard spectrometer with no additional signal processing. From the band intensities adsorption isobars and isosteres are plotted. From the isosteres, heats of adsorption and entropy changes on adsorption are obtained for coverages ranging from 0.1 to 0.6 monolayer. The isosteric heat of adsorption for coverages from 0.1 to 0.6 averages 13.1 kcal mole . The difference between the gas phase entropy at 1 atm and the differential molar entropy of the adsorbed layer averages 26.3 cal mole ° C .

Stepped single-crystal surfaces as models for small catalyst particles

Abstract Infrared spectroscopy is used to study the vibration of CO adsorbed on small, silica-supported platinum particles. Different samples have average Pt-particle sizes varying from 11 to 105 A in diameter. The infrared spectrum shows that all CO molecules are linearly bound to single Pt atoms; however, three bands can be detected on each of the different samples. The bands appear at 2081, 2070 and 2063 cm−1 and shift to higher wavenumbers (by about 6 cm −1) as the coverage increases. The intensities of the three bands vary with particle size in the same manner as the relative number of face, corner and edge atoms in our samples vary with particle size. This correlation leads to the assignment of the three bands to CO, linearly bound to face, corner and edge atoms in order of decreasing wavenumber. Some confirmation of these assignments are obtained from infrared reflection-absorption measurements of CO adsorbed on three extended-crystal surfaces of Pt: a (111) crystal, a crystal with (111) terraces and (100) steps, and a crystal with (111) terraces and kinked steps. These surfaces are described as (111), (533) and (432). The stepped crystals give bands for CO molecules associated with terrace sites appearing at about 2085 cm−1 and with step and kink sites appearing at about 2065 cm −1. Within ± 2 cm −1 there is no difference between the frequencies of CO adsorbed on step and on kink sites.

Experimental development of reflection-absorption spectroscopy: Infrared spectrum of carbon monoxide adsorbed on copper and copper oxide ☆

Abstract The reflection-adsorption technique is used to obtain the infrared spectrum of a monolayer or less of carbon monoxide adsorbed on an evaporated copper film. The band is located at 2105 cm −1 and is obtained with a signal-to-noise ratio in the range 5 to 15. In this technique, the infrared beam is multiply reflected between two closely-spaced parallel metal surfaces covered with the adsorbed layer. The CO band is used to investigate the dependence of the signal-to-noise ratio on the spacing of the metal surfaces. The existence of an optimum value of the spacing is demonstrated. The contribution to the absorption band of infrared rays with different angles of incidence is investigated and explained in terms of an optimum number of reflections and its variations with angle of incidence. After the copper surface is progressively exposed to oxygen, a slight shifting of the CO band to 2113 cm −1 is observed. Further exposure gives rise to a new band of adsorbed CO at 2135 cm −1 , interpreted as CO adsorbed on copper oxide.