STRUCTURE AND DYNAMICS OF CO/MGO(001) - A HELIUM ATOM SCATTERING STUDY

The structure and dynamics of CO adsorbed on MgO(001) surfaces cleaved in situ under UHV has been studied in the temperature range from 36 to 59 K by means of helium atom scattering. The monolayer was prepared at T-sample = 36 K under a CO pressure of 1 x 10(-8) mbar. Two stable adsorbate phases were observed. For the lower temperature phase < 40 K a c(4 X 2) pattern was found. Increasing the temperature to 51 K leads to a (1 X 1) symmetry. The change in the diffraction pattern is accompanied by desorption of CO from the MgO surface. The observed phase transition is reversible only if gaseous CO is offered during cooling down to < 40 K again. Thus, the measured (1 X 1) symmetry corresponds to a coverage of theta < 1. While in time-of-flight experiments only one dispersion-free mode at approximate to 9 meV was found for the '(1 X 1)' phase, an additional mode at approximate to 10.5 meV was measured for the low-temperature c(4 X 2) structure. From the intensity ratio of both Einstein modes a c(4 X 2) unit cell containing six partially tilted molecules is suggested, while the '(1 x 1)' is attributed to a CO lattice gas.