Adsorption of PF3 on Cu(001): Ordered overlayer structures and frustrated translational modes

The adsorption of PF3 on Cu(001) was studied by means of high resolution he lium atom scattering (HAS) and SPALEED. PF3 adsorbs at surface temperatures below 210 K and forms a lattice gas at exposures less than 0.8 L. Saturati on is reached for exposures greater than 3 L. At these coverages PF 3 forms an ordered c(4 x 2) layer for surface temperatures above 145 K. For temper atures below 145 K an incommensurate, hexagonal PF 3 layer was observed tha t necessarily excludes uniquely on-top site PF3 molecule to surface coordin ation. A new vibrational mode in the thermal energy regime corresponding to the frustrated translation parallel to the surface was identified; the ene rgies of excitation were 3.3 meV and 3.5 meV, respectively, for the isolate d molecules and the c(4 x 2) structure. No dispersion of the frustrated tra nslation in the ordered c(4 x 2) phase was found. The results are discussed in terms of adsorbate-adsorbate and adsorbate-substrate interactions. (C) 1999 American Institute of Physics. [S0021-9606(99)70211-4].