Vibrational Frequencies of C2H4 and C2H6 Adsorbed on Potassium, Indium, and Noble Metal Films

Abstract

Surface enhanced Raman spectroscopy (SERS) was applied to determine the vibrational frequencies of C 2 H 4 and C 2 H 6 adsorbed to cryocondensed metal films of potassium, indium, copper, silver, and gold at 40 K under 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 C 2 H 4 or C 2 H 6 films are resolved. The ν E C-H stretch frequencies are downshifted by 10-30 cm -1 for C 2 H 4 and C 2 H 6 commonly. C 2 H 4 exhibits additional mode softening of the ν E C-C stretch and the δ E CH 2 scissor vibrations on noble metal surfaces, but not on indium, yielding π-parameters for the E-species of 0.1 (Ag), 0.2 (Cu), and 0.25 (Au). On potassium C 2 H 4 reacts and is at least partially cleaved, whereas C 2 H 6 is stable and forms a physisorbed layer featuring N-frequencies exclusively