INFRARED REFLECTION-ABSORPTION SPECTROSCOPIC STUDY ON THE ADSORPTION STRUCTURES OF 1,3-BUTADIENE AT AU(111) AND AG(111) SURFACES

Infrared reflection-absorption spectroscopy (IRAS) was applied to stud y the adsorption structures of 1,3-butadiene on Au(111) and Ag(111) in the temperature range from 25 to 95 K. On increasing exposures from 0 .01 to a few langmuirs (langmuir = 10(-6) Torr.s) at 40 K, the adsorba tes on both substrates take on discrete adsorption states, as clearly probed by IR bands associated with CH2 twisting (tau(CH2)) and CH2 wag ging (omega(CH2)) modes. The frequencies of these bands indicated that the adsorbates are weakly pi-bonded to the substrates. At low exposur es (0.04-0.2 langmuir) 1,3-butadiene on both substrates takes an s-tra ns form and occupies two kinds of adsorption sites; the adsorbate at o ne site (alpha-state) gives rise to the omega(CH2) band at 916 cm(-1) on Au(111) and at 908 cm(-1) on Ag(111), and the adsorbate at another site (beta-state) gives the omega(CH2) band at 908 cm(-1) on Au(111) a nd at 905 cm(-1) on Ag(111). The adsorbate in the alpha-state, which l ies parallel to the substrate surfaces, is predominant at a lower temp erature (25-30 K), and upon raising the temperature up to 90 K, the al pha-state is partially converted to the beta-state. The conversion acc ompanies a tilting of the molecular plane, keeping the C=C bonds paral lel to the surfaces. The enthalpy difference between the alpha- and be ta-states on Ag(111) was estimated to be about 170 J/mol. At 0.3-0.5 l angmuir the adsorbates saturate on the surfaces, giving the tau(CH2) a nd omega(CH2) bands at 1005 and 914 cm(-1) on Au(111) and at 1002 and 906 cm(-1) on Ag(111). When the exposure is increased further (1-4 lan gmuirs), the adsorbates on both substrates form ordered multilayers, g iving doulets bands at 1022 and 1016 cm(-1) for the tau(CH2) band and those at 920 and 916 cm(-1) for the omega(CH2) band. Both the monolaye r coverage and the multilayered states mainly consist of the s-trans f orm. A second conformer (s-cis or gauche form) was found to exist only in a few monolayers which exist between the monolayer coverage state and the multilayered state.