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 study 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 adsorbates on both substrates take on discrete adsorption states, as clearly probed by IR bands associated with CH2 twisting (tau(CH2)) and CH2 wagging (omega(CH2)) modes. The frequencies of these bands indicated that the adsorbates are weakly pi-bonded to the substrates. At low exposures (0.04-0.2 langmuir) 1,3-butadiene on both substrates takes an s-trans form and occupies two kinds of adsorption sites; the adsorbate at one 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) and at 905 cm(-1) on Ag(111). The adsorbate in the alpha-state, which lies parallel to the substrate surfaces, is predominant at a lower temperature (25-30 K), and upon raising the temperature up to 90 K, the alpha-state is partially converted to the beta-state. The conversion accompanies a tilting of the molecular plane, keeping the C=C bonds parallel to the surfaces. The enthalpy difference between the alpha- and beta-states on Ag(111) was estimated to be about 170 J/mol. At 0.3-0.5 langmuir the adsorbates saturate on the surfaces, giving the tau(CH2) and 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 langmuirs), the adsorbates on both substrates form ordered multilayers, giving 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 monolayer coverage and the multilayered states mainly consist of the s-trans form. 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.