Joachim Heidberg

Fourier-transform infrared spectra of CO adsorbed on NaCl(100): structural changes at low temperatures

The monolayer of CO adsorbed on NaCl(100) forms a p(1×1) structure with CO aligned normal to the surface at temperatures above 35 K. This has been revealed from polarized infrared spectra, measured with a high-resolution Fourier-transform spectrometer. At temperatures above 35 K at monolayer coverage a single sharp absorption is observed with p-polarized light only, the wavenumber being 2155.21 cm−1 at 40 K. Below 35 K a second absorption at ∼2149 cm−1 is observed with p- and even stronger with s-polarized radiation. The p(1×1) structure described above is not in concert with these spectra. Monolayer spectra of isotopic mixtures of 12C16O/13C16O adsorbed on NaCl(100) at 19 K showed a decrease of the splitting with decreasing partial coverage. The data are most readily explained by the assumption of a correlation field splitting. The spectra of submonolayers show a temperature dependence similar to that of the full monolayer. The frequency shift and the observation of two bands suggest island formation.

Polarization-FTIR-spectroscopicstudy of the 2D-gas—solid transition, in CO2 on NaCl(100): reorientation, correlation field breakdown, heats of adsorption and 2D-condensation

Abstract Molecular reorientation and correlation field breakdown in the two-dimesional evaporation of CO 2 on NaCl(100) were measured by polarization Fourier-transform infrared spectroscopy. In the ordered 2D-condensed phase with p(2 × 1) structure a the correlation field exists, the CO 2 molecules being tilted 34° ± 5° from the surface. In the 2D-vapor the CO 2 molecules lie flat on surface, and no correlation field is detected. Both coexisting phases, clearly distinguished in the spectra, have constant density at constant temperature independent of coverage. From the variation of the 2D-vapor density with temperature the enthalpy of the 2D-evaporation was found to be Δ H v 2D = 3.0 ± 0.1kJ/mol. The isosteric heats of adsorption of the 2D-gas and the 2D-solid were determined to 32.4 ± 1.1kJ/mol and35.6 ± 1.3kJ/mol, respectively.

Structure and potential energy of the monolayer CO2 on NaCl(100)

Abstract In the polarized infrared spectra of the monolayer CO2 on NaCl(100), obtained in situ by cleaving under ultrahigh vacuum, a coupled site symmetry and correlation field splitting of the v2 bending vibration and the corresponding correlation field splitting of the v3 asymmetric stretching vibration are observed at 81 K. Derived from the spectra are the ordering of the monolayer at 81 K to a structure with two-dimensional space group pg and the tilt angle θ = 27° between the CO2 molecules and the surface. An adsorption potential for the CO2-NaCl and the lateral adsorbate interaction containing damping functions is introduced. The adsorbate structure with the global minimum of the potential energy is computed consistent with experimental results, including the heat of adsorption. In addition, the position of CO2 with respect to Na+ and Cl− is given, not directly available from the spectra. Considering the long range coupling of the dynamic dipoles, the s- and p-polarized absorption spectra v3 are calculated for a systematic variety of configurations. The best fit to the measured spectra was found for a configuration close to the adsorbate structure with the global minimum of the potential energy.

Site symmetry and correlation field splitting in the adsorbate CO2NaCl(100)

Abstract Site symmetry and correlation field splitting in the s- and p-polarized infrared spectra of the v2 bending vibration of monolayer CO2 adsorbed on NaCl(100) were detected. Also the asymmetric v3 stretching vibration of the adsorbate was observed and shows a splitting due to the correlation field. The monolayer coverage was established by measurement of adsorption isotherms. These demonstrate a two-dimensional first order phase transition in CO2NaCl(100).

Infrared-laser-induced desorption by resonant multiphoton excitation of adsorbate virbation in CH3FNaCl at different coverages

Abstract Desorption induced by resonant CO 2 laser pulses in CH 3 F-NaCl(film) on NaCl(lOO) single crystal surfaces at different coverages has been observed. The primary activation step is the multiphoton excitation of the most intense internal adsorbate vibration, the C-F stretching mode ν 3 . The variation of desorption yield with laser frequency was measured at constant fluence and coverage and was correlated with the corresponding linear absorption spectra. From desorption yield versus laser fluence plots, rate coefficients and characteristic minimum energy fluence values φ s for desorption at different surface coverages have been determined, showing increasing φ s with decreasing coverage at constant excitation frequency. By appropriate choice of laser frequency and fluence, it is possible to separate different adsorption phases by desorption.

Temperature dependence of the ν3 absorption of the monolayer CO2 adsorbed on NaCl(100): continuous change in adsorbate geometry and vibrational dephasing

Abstract CO2 on NaCl(100) at monolayer coverage forms an ordered overlayer with p(2×1) structure and pg symmetry, having two molecules per adsorbate unit cell. This results in a doublet ν3 absorption due to the correlation field. The splitting is not observed for diluted isotopes, e.g. 13C16O2 in the monolayer of 12C16O2 at natural abundance (∼1%). The polarized 12C16O2 doublet and the polarized 13C16O2 singlet have been recorded simultaneously in the temperature range between 5 and 80 K with a high-resolution FTIR spectrometer. The main and the diluted isotope show different behaviour with respect to the temperature dependence of the lineshapes. Because of the strong dynamic dipole-dipole coupling the doublet peak maxima are first of all determined by slight changes of the tilt angle ϑ (∼1.2°) and of the intermolecular angle Φ (∼0.6°) whereas the dephasing process dominates the temperature dependence of the singlet of the diluted 13C16O2. According to the exchange model for vibrational phase relaxation the experimental data have been analyzed, considering the thermal excitation of external modes at 29, 49 and 61 cm−1 (measured by inelastic He scattering [15]), and the damping according to the elastic continuum model with an optimized set of anharmonic coupling constants. Crude agreement between measured and calculated data is achieved only.

Infrared cryospectroscopic study of CONaCl adsorbates: Detection of surface diffusion at 25 and 40 K

The interaction of carbon monoxide with clean NaCl film surfaces at 12 to 77 K has been investigated by infrared spectroscopy. At low temperatures ≦ 15 K multilayer adsorption is preferred with formation of solid-like α-CO. On warming the sample to 25 K or exposing the surface at 25 K the sharp IR resonance of a new island adsorbate μ appears, which at temperatures ≳ 40 K converts irreversibly to the stable adsorbate. The substantial changes in the infrared spectra upon warming the adsorbate from 12 to 25 K and 40 K show that the CO diffusion on NaCl is hindered up to ∼ 40 K and that at ∼ 25 K the CO molecules diffuse on top of α-CO and of the μ-adsorbate with precipitation at the edge onto the virgin NaCl.

Infrared spectroscopy of carbon dioxide adsorbed on NaCl(100) at submonolayer coverage

Vibrational infrared spectroscopy has been used for the first time to study adsorption on insulator single crystal surfaces. Different carbon dioxide isotopes adsorbed on NaCl(100) have been investigated with FTIR-spectroscopy at low temperatures (≦80 K) under UHV. For submonolayer coverages the ν 3 absorption band of e.g. 12 C 18 O 2 is centered at 2304 cm −1 . This band saturates without a measurable frequency shift at monolayer coverage and a second band at 2314 cm −1 appears, assigned to the second layer, growing upon further gas admission. In a 12 C 16 O 2 / 13 C 16 O 2 coadsorbate at submonolayer and monolayer coverage the observed spectrum contains all bands at the same frequencies as found for the pure adsorbates.

FTIR-spectroscopy as a highly sensitive technique to study adsorption and desorption on ionic film and single crystal surfaces

Infrared absorption by CO and CO2 adsorbed on NaCl (film) and NaCl(100) single crystal surfaces has been measured under ultrahigh vacuum conditions as a function of temperature and pressure, using grating and FTIR transmission-absorption spectroscopy. The absorption line of a monolayer of CO on NaCl(100) due to the stretching vibration is extremely narrow (<2 cm−1) and onlyp-polarized light is absorbed at oblique incidence. Resonant desorption in CO-NaCl (film) has been detected. Thev3 absorption of a monolayer of CO2 on NaCl(100) appears as a doublet.

Infrarotabsorption durchp-Benzochinone, chemisorbiert an mit Tantal dotierten Natriumfluorid- und Barium fluorid-Aufdampfschichten

The IR absorption spectra of p-benzoquinone, p-benzoquinone-fií and p-benzoquinone-1802 adsorbed from the vapor on evaporated films of alkali and alkaline earth halides were taken. In addition the optical and ESR spectra of some systems were recorded. It has been found, that the films of NaF and BaF2 which were prepared as described in ref.1 and 6 contain small dotations of tantalum. Only upon adsorption on the Ta doped films NaF (Ta) and BaF2(Ta) do substantial changes in the spectra of the adsorbed species occur. These changes are attributed to chemisorption. On the other hand small changes in the IB spectra are measured upon adsorption e.g. on NaF(W), BaF2(W), NaOl(Ta), LiF(Ta), OaF2(Ta). These observations among others indicate physical adsorption. A complete assignment of the observed bands of chemisorbed p-benzoquinone and its above isotopie derivatives is presented. Upon chemisorption on NaF (Ta) for example the via (GO stretch) and vu (0—0 stretch) frequencies are lowered by 250 and 100 cm-1, respectively. Vibrational fundamentals become IR active. The optical spectrum of p-benzoquinone adsorbed on NaF ( ) shows