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Two-dimensional superstructures and softened C–H stretching vibrations of cyclohexane on Rh(111): Effects of preadsorbed hydrogen
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10.1063/1.3670014
/content/aip/journal/jcp/135/23/10.1063/1.3670014
http://aip.metastore.ingenta.com/content/aip/journal/jcp/135/23/10.1063/1.3670014

Figures

Image of FIG. 1.
FIG. 1.

A series of TPD spectra of (a) cyclohexane (mass 84) at a heating rate of β = 1.4 K/s and (b) produced hydrogen (mass 2) with β that continually decreases from 1.3 K/s at 250 K to 0.3 K/s at 400 K. All measurements were performed in the same heating condition. Cyclohexane molecules were dosed on the clean Rh(111) surface at 150 K. (c) Estimated fractional cyclohexane coverage on the clean Rh(111) surface. The amount of adsorbed cyclohexane was normalized by the saturated monolayer (0.17 molecules/surface Rh atom).

Image of FIG. 2.
FIG. 2.

(a) A phase diagram of superstructures observed by SPA-LEED as a function of cyclohexane exposure and coadsorbed hydrogen coverage. All measurements were carried out at 90 K, and data points are indicated by dots. (b)–(g) SPA-LEED patterns of overlayer structures 1–6 in (a) together with the notations of the commensurate and HOC structures. The incident electron energy is 75 eV. Reciprocal unit cells of cyclohexane domains are indicated by lines.

Image of FIG. 3.
FIG. 3.

(a) A STM image of submonolayer cyclohexane on the clean Rh(111) surface (region 1). The sample bias (Vs), tip current (It), and sample temperature (Ts) were 313 mV, 131 pA, and 86 K, respectively. The unit cell of the superstructure is indicated by a white rhombus. The white arrow in the figure shows one of the dislocation points at which misalignment of the molecular rows (indicated by white lines) is observed. (b) A Fourier transform (FT) image of the STM image in (a). Spots corresponding to the molecular and HOC periodicities are denoted by M and H, respectively. (c) A close-up STM image of the moiré structure in the same tunneling condition as the STM image in (a). (d) A line profile along the white line (A–B) in (d).

Image of FIG. 4.
FIG. 4.

(a) A STM image of the commensurate R19.1° structure (region 2) of cyclohexane adsorbed on the H/Rh(111) surface (Vs = 131 mV, It = 131 pA, Ts = 96 K). The unit cell of the commensurate structure is indicated by a white rhombus. The inset shows an image of the clean Rh(111) surface (Vs = 92 mV, It = 313 pA, room temperature). The scale and the sample orientation are the same in both images. (b) A line profile along the white line (A–B).

Image of FIG. 5.
FIG. 5.

(a) A STM image of the HOC R23.4° structure (region 3) on the H/Rh(111) surface (Vs = 884 mV, It = 156 pA, Ts = 150 K). (b) A FT image of the STM image in (a). Spots corresponding to the molecular and HOC periodicities are denoted by M and H, respectively. (c) A high-resolution STM image (Vs = 625 mV, It = 263 pA, Ts = 145 K). The unit cell of the HOC structure is indicated by a white rhombus. (d) A line profile along the white line (A–B) in (c).

Image of FIG. 6.
FIG. 6.

(a) A STM image of the HOC 5 × 5 structure (region 4) on the H/Rh(111) surface (Vs = 442 mV, It = 156 pA, Ts = 130 K). The inset is a FT image of the STM image. Spots corresponding to the molecular and HOC periodicities are denoted by M and H, respectively. (b) A high-resolution STM image (Vs = 743 mV, It = 131 pA, Ts = 91 K). The unit cell of the HOC structure is indicated by a white rhombus. (c) Line profiles along the white lines (A–B and C–D) in (b).

Image of FIG. 7.
FIG. 7.

STM images of (a) low-coverage (Vs = 884 mV, It = 131 pA, Ts = 117 K) and (b) high-coverage (Vs = 743 mV, It = 78 pA, Ts = 105 K) cyclohexane on the hydrogen saturated Rh(111) surface (region 6). The directions of the molecular rows and the steps are indicated by white lines and dashed lines, respectively. (c) A FT image of the STM image in (b). M1 and M2 spots correspond to molecular periodicities in the terraces 1 and 2, respectively. Streaks that derive from two steps in (b) are also indicated.

Image of FIG. 8.
FIG. 8.

(a) A series of TPD spectra of cyclohexane (mass 84) as a function of preadsorbed hydrogen coverage (β = 1.4 K/s). All measurements were performed at a constant cyclohexane exposure of seven shots. (b) Peak temperature as a function of hydrogen coverage.

Image of FIG. 9.
FIG. 9.

A series of IRAS spectra as a function of hydrogen coverage: (a) “normal” C–H stretching mode (2800–3000 cm−1) and (b) “softened” C–H stretching mode (2400–2800 cm−1). After preparation of hydrogen-adsorbed Rh(111) surfaces, a constant amount of cyclohexane (exposure of seven shots) was dosed at 150 K, followed by IRAS measurements at 20 K. (c) A schematic picture of adsorbed cyclohexane at a hollow site with chair conformation. Carbon and hydrogen atoms of cyclohexane are indicated by filled and open circles, respectively.

Image of FIG. 10.
FIG. 10.

Adsorption models for (a) a HOC R17.0° structure, (b) a commensurate R19.1° structure, (c) the HOC R23.4° structure, and (d) the HOC 5 × 5 structure. The positions of substrate Rh atoms and cyclohexane molecules are illustrated by gray circles and red dots. The unit cells of superstructures are indicated by blue lines. Adsorption geometries of molecules at the hollow sites are illustrated by black lines with the free CH2 groups (filled circles) and the softened C–H groups pointing to the surface (open circles).

Tables

Generic image for table
Table I.

Observed superstructures of submonolayer cyclohexane on the clean and hydrogen-preadsorbed Rh(111) surfaces.

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/content/aip/journal/jcp/135/23/10.1063/1.3670014
2011-12-20
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Two-dimensional superstructures and softened C–H stretching vibrations of cyclohexane on Rh(111): Effects of preadsorbed hydrogen
http://aip.metastore.ingenta.com/content/aip/journal/jcp/135/23/10.1063/1.3670014
10.1063/1.3670014
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