Nuclear resonant scattering of synchrotron radiation by physisorbed Kr on ${\mathrm{TiO}}_2$(110) surfaces in multilayer and monolayer regimes

Physisorbed Kr layers on ${\mathrm{TiO}}_2$(110) surfaces were investigated by means of nuclear resonant scattering (NRS) of synchrotron radiation at Kr thicknesses ranging from multilayer to monolayer. The NRS intensity was measured as a function of the Kr exposure, from which the NRS signal corresponding to monolayer was estimated as 0.23 cps. The time spectra measured at various thicknesses showed a monotonous decay without any quantum beat features. The recoilless fraction $f$ evaluated from the analysis of the time spectrum revealed a substantial reduction upon temperature rise from 19 to 25 K. As its origin, an order-disorder phase transition of the monolayer Kr is proposed.

Figure 1

Schematic of the experimental setup of NRS measurements from physisorbed layers in an ultrahigh vacuum chamber.

Figure 2

XR from physisorbed Kr layers as a function of $\theta$ at Kr exposure of 0 to 2000 L. Dashed curves are the calculated XR for the flat surfaces of rutil ${\mathrm{TiO}}_2$ and solid Kr.

Figure 3

(a) Nuclear resonant spectra at 9.4 keV by ${}^{83}\mathrm{Kr}$ nuclei in Kr physisorbed layers at Kr gas exposures of 6, 12, 18 L. The ${}^{83}\mathrm{Kr}$ isotope enriched (75%) Kr gas was used. Each point was obtained with the accumulation for 30 sec. (b) The deduced NRS intensity as a function of the exposure of the isotope enriched Kr gas.

Figure 4

Time spectra of NRS of a 9.4 keV x ray from ${}^{83}\mathrm{Kr}$ in physisorbed layers of Kr on a ${\mathrm{TiO}}_2$(110) surface (open circles). Solid curves are fits with Eq. (1). The spectrum (a) is multiplied by 20 for a clarity. (n) and (e) indicate that the natural or isotope enriched Kr gas was used, respectively.

Figure 5

Temperature dependence of the recoilless fraction $f$ for solid Kr (open squares) (Ref. [27]), monolayer Kr physisorbed on graphite (open circles) (Ref. [24]), and physisorbed Kr on ${\mathrm{TiO}}_2$(110) in the monolayer regime [present study].

Figure 6

Unit cell of the rutile ${\mathrm{TiO}}_2$(110) surface. Open circles and filled circles denote the ${\mathrm{O}}^{2-}$ ion and $\mathrm{T}{\mathrm{i}}^{4+}$ ion, respectively (Ref. [41]). A and B denote the adsorption sites of Kr which are energetically equivalent.