Infra-red reflection absorption spectroscopic study on adsorption structures of acrolein on polycrystalline gold and Au(111) surfaces under ultra-high vacuum conditions

Infra-red reflection absorption (IRA) spectra were measured at 80 K under ultra-high vacuum conditions for acrolein adsorbed on two kinds of gold films: Au(111) and polycrystalline gold surfaces. Upon increasing the amount of exposure from 0.02 to 200 L (1 L=l x 10(-6) Torr.s), the adsorbate at Au( 111) gave rise to a series of sharp IRA bands due to a CH2 out-of-plane wagging vibration [omega(CH2)] successively. indicating discrete adsorption states, i.e. 964 (type 1)-->978(type 1')-->992(type)-->1003 cm(-1)(type 3). All these states have the molecular plane parallel to the surface: type I is in an isolated state, and type 3 is in an associated state with a tn two-dimensional arrangement, whereas type 3 forms an ordered multilayered structure. Type 1' was tentatively assigned either to a trapped state at step sites or to all associated state forming small oligomers at the surface. Only type 3 gives rise to IRA bands due to nu(C=O). which appears at 1677 cm(-1) as a singlet at relatively small exposure levels and splits into doublets, giving the 1656 and 1672 cm(-1) components at 2.0 L. The doublers were explained as being due to a crystal field splitting, which conforms to the fact that the adsorbate forms an ordered three-dimensional arrangement. The IRA spectrum of type 3 is readily converted to that of a more stable polycrystalline state upon increasing the temperature from 80 to 100 K, Thus. type 3 is a thermodynamically metastable state. Acrolein adsorbed on a polycrystalline gold film assumes an amorphous state in the exposure level of 0.06-4.8 L, giving broad IRA bands due to nu(C= O) and omega(CH2) in the 1686-1699 and 974-991 cm(-1) regions, respectively. The IRA spectra of acrolein adsorbed on Ag(111) were also measured, which indicated that the adsorbates exist in a less ordered state than those on Au(111). although a multilayered structure gives IRA features that are almost identical with those of type 3.