VIBRATIONAL FREQUENCIES OF C2H4 AND C2H6 ADSORBED ON POTASSIUM, INDIUM, AND NOBLE-METAL FILMS

Surface enhanced Raman spectroscopy (SERS) was applied to determine th e vibrational frequencies of C2H4 and C2H6 adsorbed to cryocondensed m etal films of potassium, indium, copper, silver, and gold at 40 K unde r ultrahigh vacuum conditions. In the case of indium and noble metals, downshifted (E) and unshifted (N) frequency bands with respect to the band position in condensed C2H4 Or C2H6 films are resolved. The nu(E) C-H stretch frequencies are downshifted by 10-30 cm(-1) for C2H4 and C2H6 commonly. C2H4 exhibits additional mode softening of the nu(E) C- C stretch and the delta(E) CH2 scissor vibrations on noble metal surfa ces, but not on indium, yielding pi sigma-parameters for the E-species of 0.1 (Ag), 0.2 (Cu), and 0.25 (Au). On potassium C2H4 reacts and is at least partially cleaved, whereas C2H6 is stable and forms a physis orbed layer featuring N-frequencies exclusively.