Infra-Red Cryospectroscopic Study on the Interaction of CO with a (111)Ni- and NaCl-Film at Low Temperature under Ultrahigh Vacuum

Abstract

The interaction of carbon monoxide with clean nickel and sodium chloride surfaces has been investigated for the first time by infrared cryospectroscopy under ultra-high vacuum conditions at low adsorbent temperatures. The adsorbents were NaCl films, deposited onto an air cleaved (100)NaCl face used as such or covered by a (111)Ni film.

When nickel and NaCl surfaces at T ≤ 25 K are exposed simultaneously to CO, no infra-red absorption due to Co-Ni adsorption is observable but extraordinarily sharp infra-red bands of a new metastable phase, μ-CO-NaCl and of a metastable solid-like phase $\overline{\alpha }$-CO are detected at 2153 and 2138 cm⁻¹, respectively.

Infra-red absorption due to Co-Ni chemisorption first appears at 2094 cm⁻¹ on warming the nickel adsorbent to ~ 35 K.

At very low temperature, T ≤ 15 K, no infra-red adsorption of μ and α-CO-NaCl adsorption systems is detectable. Only the IR band of $\overline{\alpha }$-CO is observed. Evidence is presented that surface migration of CO on NaCl is strongly hindered up to temperatures T ≤ 40 K. An ultrahigh vacuum helium cryostat was developed allowing salt and metal evaporation in the 10⁻⁸ Pa (10⁻¹⁰mbar) range as well as optical measurements in the spectral region between 220 nm and 50 μm. Cold alkali halide adsorbents turned out to be useful in trapping and analyzing desorbates from nickel surfaces acting as catalyst in CO conversion. Thus UHV cryospectroscopy may be applied to the analysis of desorbates from surfaces of any optional temperature under the clean and defined conditions of ultrahigh vacuum.