Surface Science 272 (1992) 347-351 North-Holland surface science Ethane adsorbed on MgO(100) single crystal surfaces: a high resolution LEED study M. Sidoumou, T. Angot and J. Suzanne CRMC2-CNRS, Ddpartement de Physique, Facult~ des Sciences de Luminy, Case 901, 132,~8 Marseilh" Codex 9, France Received 14 October 1991; accepted for publication 27 January 1992 Ethane adsorbed on MgO(l(10) single crystal surfaces has been studied in the monolayer range using high resolution LEED. Two 2D ordered solid phases S t and S 2 have been observed with coexistence of S t + S 2 with increasing coverage. S 2 is a V~- × 2~2 R45 ° commensurate solid phase close to the (1111) plane of bulk C2H 6. For S t, we propose an oblique cell with a = 4.65 ,~, b = 4.53 ,~ and 1, = 71 °. lt. Introduction 2. Experimental set-up Molecular adsorbates on a well characterized, homogeneous uniform surface of ionic substrates is of current interest both theorctica!ly il-3] and experimentally [4-14]. Indeed, the combination of simple van der Waals adsorbate-substrate in- ~eractions due to dispersion forces and clectro- static interaction between the molecular multi- poles and the ionic substrate electric field leads to new kinds of surface phase transitions and structures. In particular, the molecule-molecule interaction can be strongly perturbed or frus- trated by the molecule-substrate counterpart. As far as MgO substrates are concerned, methane [5,7,8,11,12], nitrogen [7-9] and ethane [10] monolayer structures have been investigated by neutron diffraction [5,8-10], helium atom scat- tering [11,12] and LEED [6,7,13-15]. We present here a LEED study of an ethane monolayer ad- sorbed on single crystal MgO(100) .~urfaces which agrees qualitatively with neutron diffraction ex- periments on IVigO uniform powders [1/3], show- ing two solid phases S t and S 2. We will show that the structure proposed from the neutron diffrac- tion data for the S~ solid phase cannot explain our LEED data. We will propose a new structure for S t . The experimental set-up h;~s hcc~ described elsewhere [13,14] Shortly, it fc;~i ~ +~i~ra-high vacuum (UHV) chamber with -,. turb,.)molecular pump achieving a base pressure of about 1() -'~ Torr after baking the system for 24 h. A ion punlp allows us to get a residual gas background of P< 1 x 10--~t' Torr. A mass spectrometer (Varian QMG 311) gives *,he partial pressures of each remaining gas. It also allows us to check the purity of the ethane gas used in this experiment. A CMA Auger spectrometer is used to monitor the cleanliness of the MgO surface. The MgO crystal (Spicer Co, purity 3N and 4N) is cleaned under vacuum with a special cleaver built in our laboratory [15]. The temperature of the sample is lowered using a low temperature sample holder thermally linked to a closed-cycle cryocooler al- lowing us to reach 35 K. The ter,~perature is measured by a platinum resistor (1~!(. ~ ~} at 0°C) ,.onnected to a temperature controller providing a temperature stab!!!ty AT = 0.05 K. We estimate the absolute temperature calibration at + 1 K. The sample manipulator provides translation in the x, y and z directions, rotation around a vertical axis, plus azimuthal rotation around an axis perpendicular to the sample surface plane. ()039-6028/92/$05.(10 e; 1992 - Elsevier Science Publishers B.V. All rights reserved