Infrared Reflection−Absorption Spectra of C₂H₄ and C₂H₆ on Cu:  Effect of Surface Roughness

Abstract

Infrared reflection absorption spectroscopy (IRRAS) of the highly symmetric molecules C 2 H 4 and C 2 H 6 adsorbed as mono-and multilayers onto copper films is studied in relation to the type of metal-film roughness. Spectra of C 2 H 4 show Raman lines on cold-deposited Cu films but not on Cu deposited at room temperature. For C 2 H 6 , the IR spectra from both types of metal films are similar; the surface infrared selection rule holds and no Raman bands are observed. The Raman lines that appear in the IR spectra already at low exposures are attributed to species adsorbed at special defect sites, identical to the so-called active sites in surface enhanced Raman scattering (SERS). The IR excitation mechanism by transient electron transfer to the adsorbate π* state can deliver a discrete vibrational band of a Raman-active vibration only under certain circumstances, for example, for adsorbates at the "SERS-active sites". C 2 H 6 at these sites cannot deliver Raman bands in IRRAS, because it has no π* state. We also discuss IRRAS measurements on Cu(111) and Cu(110) single crystals, where Raman bands of C 2 H 4 have been observed.

IR spectroscopy of adsorbates on ultrathin metal films

“…We observed Raman lines in IR spectra only for metal surfaces with roughness on the atomic scale and only for adsorbate molecules with a non-vanishing density of states at the Fermi level of the metal, i.e. not for C 2 H 6 [13].…”

“…(By the way, to detect three "normal" Raman lines in SEIRA [11] a bigger field enhancement than achieved in this work is necessary.) The suspicion corroborated by IRRAS studies [13] is this: The normal Raman lines ("R") in the IR spectra are due to molecules on metal sites where they are physisorbed. Therefore, they are observed only below 100 K. This is in agreement with SERS results which prove a first layer mechanism also for species physisorbed to small (111) facets [39].…”

Electronic Mechanisms of SERS

“…As expected, for the 300 K film, only the IR modes of ethylene are observed. Closer inspection of the spectra, also in the C-H stretch region, shows that only IR-active modes with the IR dipole moment normal to the local surface are observed, the so-called surface-selection rules hold [54]. This is very different for the cold-deposited silver film.…”

“…The best indication have been achieved with ethylene C 2 H 4 and parallel measurements of SERS and infrared reflection absorption spectroscopy (IRRAS) of the same copper samples [54]. Ethylene has an inversion centre and therefore there is a mutual exclusion rule between infrared and Raman activity of the vibrational modes, see the top fig.…”

“…5 (the 4 C-H stretch vibrations are not displayed). Figure 8 shows the IRRAS spectra of two Cu films [54], one prepared at room temperature (300 K), the other at 50 K. Both films have been cooled to 50 K and then exposed to a low background pressure of C 2 H 4 . The IRRAS spectra have been taken continuously during this time, the average achieved exposure during a run is given in the figures.…”