Study of the H-induced structural phase transitions on W(100) by surface infrared spectroscopy.

A vibrational study of the various H-induced phases on W(100) at room temperature has been carried out by surface infrared spectroscopy. A large shift (~35cm(-1) in the frequency of the W(2)H symmetric stretch mode, nu(1), is observed at low coverages (theta~0.3), indicating a local structural rearrangement (~~0.03Angstrom ) of the W atoms as the commensurate (square root 2x square root 2)R45 degrees phase is established at theta = 0.3. The incommensurate phase, I-(square root 2x square root 2), is made up of reconstructed domains (nu(1)~~1300cm (-1)) separated by domain walls of finite site and with unreconstructed W atoms as evidenced by a new H vibration at ~~1100cm(-1). As the coverage is increased (theta>0.5), the reconstructed domains decrease in size with a substantial expansion of the W dimer and finally disappear (theta~~1). The disordered surface that ensues is brought back into registry with the bulk at saturation (theta=2).