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That an inelastic electron tunneling experiment can be a powerful tool to investigate molecular vibrational spectra was first shown by JAc~VVlC and LAMBE (1). Since that t ime much progress has been accomplished on this field: a fairly up-to-date review can be found in WOLF (2). Tunnel junctions where the insulating barrier is a thin (50--100)• film of Polyvinyl Formate have been extensively investigated by our group in the last few years (3.5). For the reader's opportunity let us briefly summarize the main results that have been obtained. First ly it was shown in (3) that, when the insulating barrier is a Formvar film, proper vibrational modes can be transferred from the metallic electrodes to the