Polarization infrared spectroscopy study of quasi-orthorhombic acetylene thin films on KCl (100)

The growth of ultrathin films of acetylene on KCl (100) single-crystal surfaces has been studied by means of low-energy electron diffraction (LEED) and polarization infrared spectroscopy (PIRS) in transmission geometry at $40\mathrm{K}$. IR spectra in the region of the asymmetric stretch vibration ${\nu }_3$ and the asymmetric bending mode ${\nu }_5$ were recorded at different coverages. The PIRS spectra as well as the observed $(\sqrt{2}\times \sqrt{2})R45°$ diffraction pattern with two glide planes are consistent with a parallel orientation of the molecules with respect to the surface as expected for the formation of the low-temperature orthorhombic phase of ${\mathrm{C}}_2{\mathrm{H}}_2$. A refined analysis of the infrared spectra within the dynamic dipole-dipole coupling approach confirms that the lateral orientation of the molecules within one layer is close to the T-shaped geometry favored by the intermolecular quadrupole-quadrupole interaction. Deviating from what was assumed in a previous study [J. P. Toennies et al., Phys. Rev. B 65, 165427 (2002)], the lateral orientation of the molecules in subsequent layers is not characteristic for the orthorhombic phase: essential features in the IR spectra point towards a statistical stacking arrangement of two inequivalent layer types within the films. A structural model is proposed, which is consistent with all available experimental results.

Figure 1

Experimental PIRS spectra in the region of the ${\nu }_3$ mode recorded during exposure of a bare KCl crystal to $2.5\times {10}^{-9}\mathrm{mbar}$ acetylene. Each spectrum is labeled with the respective accumulated exposure in units of langmuir $(1\mathrm{L}={10}^{-6}\text{Torr}\mathrm{s})$. The sample temperature was $40\mathrm{K}$.

Figure 2

Experimental PIRS spectra of the ${\nu }_3$ mode recorded at $40\mathrm{K}$ during exposure of a monolayer ${\mathrm{C}}_2{\mathrm{H}}_2$ on KCl (100) to $2.5\times {10}^{-9}\mathrm{mbar}$ acetylene. Each spectrum is labeled with the respective accumulated exposure in addition to the exposure needed to prepare the monolayer at $75\mathrm{K}$.

Figure 3

Comparison between experimental PIRS spectra ($p$ polarization) of the ${\nu }_3$ mode in different film preparations after an exposure of $20\pm 2\mathrm{L}$. (A) Film grown on a bare KCl (100) surface at $40\mathrm{K}$. (B) Film grown at $40\mathrm{K}$ on a monolayer ${\mathrm{C}}_2{\mathrm{H}}_2∕\mathrm{K}\mathrm{Cl}$ (100), which was prepared at $70\mathrm{K}$. (C) Same as (B) but different KCl (100) single crystal.

Figure 4

Experimental PIRS spectra of the ${\nu }_5$ mode recorded at $40\mathrm{K}$ during exposure of a monolayer ${\mathrm{C}}_2{\mathrm{H}}_2$ on KCl (100) to $5\times {10}^{-9}\mathrm{mbar}$ acetylene. Each spectrum is labeled with the respective accumulated exposure in addition to the exposure needed to prepare the monolayer at $75\mathrm{K}$.

Figure 5

LEED $(\sqrt{2}\times \sqrt{2})R45°$ diffraction pattern of a multilayer film of acetylene on KCl (100). The primitive reciprocal lattice vectors of the substrate are labeled (10) and (01), respectively. The only visible type of superstructure peaks are {$\{\frac{3}{2},\frac{1}{2}\}$; the beam orders $\{\frac{1}{2},\frac{1}{2}\}$ and $\{\frac{3}{2},\frac{3}{2}\}$ appear extinguished, consistent with two glide planes along the directions [100] and [010].

Figure 6

Structure models of acetylene films on KCl (100) in perspective sideview (top) and topview (bottom). (A) One monolayer with adsorption site over the cations (smaller white balls) exhibiting $(\sqrt{2}\times \sqrt{2})R45°$ symmetry and two glide planes. (B) Three layers with an equivalent orientation of the superimposed molecules in the first and third layers. (C) Three layers with a crossed orientation of the superimposed molecules in the first and third layers. This structure is possible within growth model II (see text), but not within growth model I.

Figure 7

Calculated PIRS transmission spectra of acetylene multilayers in the region of the ${\nu }_3$ mode using the parameter set given in Table . (A) Growth model I and (B) growth model II. As explained in the text, the spectra of model II are the average of spectra from $2^N$ different film structures of $N$ layers assumed to coexist on the surface with equal abundance. In addition, an experimental PIRS spectrum (bold solid line) of a film containing $\sim 6$ layers is shown in each diagram.

Figure 8

Comparison between experimental (bold solid line) and calculated PIRS transmission spectra of two selected structural configurations AAABAA (dashed line) and BAABAA (solid line) of an acetylene film containing six layers.

Figure 9

Calculated PIRS transmission spectra of acetylene multilayers in the region of the ${\nu }_5$ mode using the parameter set given in Table . (A) Growth model I and (B) growth model II. As explained in the text, the spectra of model II are the average of spectra from $2^N$ different film structures of $N$ layers assumed to coexist on the surface with equal abundance. In addition, an experimental PIRS spectrum (bold solid line) of a film containing $\sim 6$ layers is shown in each diagram.