We have used photon correlation spectroscopy and quartz crystal microbalance techniques to examine the relaxation dynamics of ultrathin (hϽ400 Å) polystyrene films in both supported and freely standing geometries. These studies probe relaxation dynamics of polymer films in which the glass transition temperature (T g ) is reduced below the bulk value. Both the shape of the relaxation function and the dependence of relaxation time on temperature above the glass transition are remarkably similar to that of the bulk polymer, though the range of relaxation times is shifted according to the shift in T g . The results indicate that the microscopic relaxation dynamics of thin films remain similar to that of the bulk polymer even, in the extreme case in which the T g value is shifted more than 70 K below the bulk value. ͓S1063-651X͑98͒50508-9͔PACS number͑s͒: 68.60. Ϫp, 61.20.Lc, 62.80.ϩf, 64.70.Pf The increasing number of applications for polymer thin films has spurred a surge of activity aimed at increasing our understanding of the properties of these materials. Polymers in a thin film configuration may have physical properties different from those of the bulk material due to interfacial interactions and effects of molecular confinement. Of particular interest are anomalies in the glass transition temperature T g , which have been recently reported for thin polymer films. The T g values have been measured for polymer films supported by substrates ͓1-5͔ as well as for freely standing films ͓4,6͔. These experiments reveal that the T g values decrease for decreasing film thickness unless there is a strongly attractive polymer-substrate interaction ͓2͔. Films of polystyrene ͑PS͒ have been extensively studied on a number of different substrate materials encompassing both wetting ͓1,3,5͔ and nonwetting ͓4͔ systems. The measured T g values show only weak substrate dependent behavior. While the strength of the polymer substrate interaction does not strongly influence the measured T g for supported PS films, the simple presence of a substrate has been shown to alter dramatically the T g value in recent studies involving freely standing films ͓4,6͔.The glass transition temperature is intrinsically related to structural relaxation and thus in polymers to the segmental mobility. Structural relaxation dynamics of glass forming materials are generally well described by the stretched exponential functionwhere the stretching parameter ␤ describes the shape of the relaxation time distribution. The variation of the average relaxation time ͗͘ϭ͐(t)dt with temperature T generally obeys the empirical Vogel-Tammann-Fulcher ͑VTF͒ equationwhere the parameter 0 is a microscopic relaxation time, B describes the fragility of the glass former, and T 0 for polymers is generally T 0 ϳT g Ϫ50 K. For bulk polymers the relaxation behavior is well characterized by Eqs. ͑1͒ and ͑2͒. In order to help elucidate the origin of the large T g reductions observed for thin polymer films, a systematic study of the relaxation dynamics near T g must be performed. ...

Adsorption of CO, has been studied on clean and potassium-dosed Rh( 11 1) surface\ bv mean\ of UPS and XPS. CO1 adsorb.\ molecularly at 90 K on clean Rh without a xtrang influence on the electronic structure of the adsorbed layer. Adsorptton of C'02 on potassium-dosed Rh(l II) (6, = 0.33) at 100 K and annealing the adsorbed iayr zt 131 K producrd thrw pwks st 5.2. ti 7 and 10.9 CV in the Hell spectrum and 532.X for O(lh) and 290.5 cV for C( Is) in XPS. rhcw emissions were attributed to the formation of CO; radical antons. At higher temperaturrz the CO, radical is transformed into carbonate and CO. Carbonate species i\ charxterired hq the 3.5. X.4 and 10.2 eV peaks in UPS. and by C(ls) at 2X9.0 eV and O(ls) at 531.X eV levels III XPS. At lower potassium coverage ( 8 K = 0.1). carhonatr formatwn was not ohxrved, but the C'O1 anion radical dissociated to CO and 0 at I31 179 K. This pnuxs was accompanied hy the ;tppearanc~! t)f photoemission peaks at X.2. 11.2 and 6.0 eV.

The segregation of boron and its reactivity towards oxygen has been investigated by means of AES, XPS, UPS and ELS (in the electronic range) in the temperature range 11)0-1300 K. The segregation of boron in a Rh foil started from 700 K. The ~egregated boron produced a peak in XPS for the B(ls) level a~ 187.8 eV and emissions in UPS at 4,0 and 8,6-9.0 eV for B(2p) and B(2sp2), respectively, Analysis of the results suggested that the segregated boron on Rat foil mainly forms dimers or islands, instead of isolated monomers, without any significant charge transfer between rhodium and boron, Upon oxygen adsorption the B(ls) and O(ls) levels shifted to higher binding euergy (to 191,5 ~lnd 532.6 eV, respectively) and a new loss in the EELS was produced at 9,4 eV, demonstrating a strong chemical interaction between oxygen and boron. The interaction occurs at as low as 159 K, as indicated by tbe development of the 9.4 eV loss feature, It is assumed that boron suboxides are formed in which the B-B bond is retained, The cleavage of the B-B bond starts above 400 K attd is completed at 750 K, when the 2sp: hybrid state at 8.6-9.0 eV in the UPS, due to the B-B bond, is no longer detected. Formation of a polymer-like B:O3 species is proposed which reacts with elemental boron above 900 K to give B:O~. I. fail'eductionThe presence of adatoms (either as impurities or promot.rs) on metal surfaces can drastically influence the surface reactivity; this is in most cases attributed to an electronic interaction between the adatoms and the metals. However, there is increasing evidence that surface adatoms can interact directly with gaseous molecules, thereby strongly influencing the reactivity [1].We recently demonstrated that boron impurity segregating to a Rh surface dramatically alters the reactivity of the Rh surface towards N-and O-contain. ing moieties, such as CN [2], NO [3], CO 2 [41 and H~O 151, A possible reason for this phenomenon is that boron forms very strong bonds with N and O, which can promote the processes of surface dissociation of the adsorbed molecules. Following surface dissociation, we detected the formation of dissociation products via thermal desorption, and a new feature at 9,4 and 7.2 eV 0169-4332/89/$03.50