Interaction of acetylene molecules with the MgO(100) surface: LEED experiments and potential-energy calculations

N

ELSEVIER

surface science Surface Science 375 (1997) 315 330

Interaction of acetylene molecules with the MgO(100) surface: LEED experiments and potential-energy calculations D. Ferry a, j. Suzanne a,,, P.N.M. Hoang b, C. Girardet b a CRMC2_CNRS 1, Dbpartement de Physique, Facult~ des Sciences de Luminy, Case 901, 13288 Marseille Cedex 9, France b Laboratoire de Physique Mol~culaire (URA-CNRS 772), FacultO des Sciences La Bouloie, Universit~ de Franehe-Comt~, 25030 Besangon Cedex, France Received 27 July 1996; accepted for publication 25 October 1996

Abstract

Structural and thermodynamical low-energy electron diffraction experiments were performed on MgO(100) single crystals cleaved in-situ under ultra-high vacuum conditions. Acetylene adsorption experiments were conducted within the temperature range 40 94 K and the Call2 pressure range 10-9-10 6 Torr. We have determined experimentally a commensurate (2 × 2) herringbone structure containing two molecules and having two glide planes. Bulk condensation of acetylene has been studied and the latent heat of sublimation AHsub.=5.08_+0.3 kcal tool -1 determined. The isosteric heat of adsorption Qst = 6.9 _+0.4 kcal mol I and lateral interaction energy Q Iq= 3.1+_0.4 kcal mol-1 have been measured and compared with semi-empirical potential calculations which detail the nature of the various contributions to these physical quantities. The stable adsorption sites are the Mg atoms, and the calculated geometry is in good agreement with our LEED observations. Moreover, the influence of the molecular motions on the isosteric heat of adsorption is evaluated to take temperature effects into account. © 1997 Elsevier Science B.V. All rights reserved. Keywords: Acetylene; Low energy electron diffraction (LEED); Magnesium oxides: Semi-empirical models and model calculations: Single crystal surfaces

1. Introduction

Considerable experimental and theoretical emphasis has been given to the adsorption of molecules on metallic and dielectric substrates. Most of the earliest works on two-dimensional (2D) physisorbed monolayers have been conducted on rare-gas atoms on noble or transition metals, graphite and MgO substrates, both experimentally [1-7] and theoretically [5,8-10]. In the case of *Corresponding author. Fax: + 33 91 269305; e-mail: [email protected] 1 Also associated with the Universities of Aix-Marseille 2 and 3.

molecular adsorbates, the physical properties of physisorbed films depend upon numerous factors as the shape and electrostatic properties of the admolecule, the corrugation of the substrate surface, etc. In this context, the study of the interaction of molecules bearing strong multipole moments with ionic surfaces is particularly interesting because they are adequate probes of the surface electric field. Acetylene (C2H2) is an example of a polar molecule which presents no dipole but a strong quadrupole moment 0 = 7 . 2 + 0 . 5 D A -~ [-11 ]. One expects an important interaction of this molecule with the surface electric-field gradient as well as a strong molecule-molecule lateral interaction. This molecule has been the subject of

0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0039-6028 ( 9 6 ) 0 1 2 9 6 - 4

316

D. Ferry et al. / Surface Science 375 (1997) 315-330

experimental and theoretical investigations on various substrates, including metals [ 12-15 ], graphite [16-19], and ionic substrates such as NaC1 [20-22] and MgO [23,24]. In the case of MgO(100), the value of the surface charge q*= Iq/el is not clearly known, and calculations based on the semi-empirical, self-consistent, tight binding approach have been done [25] leading to q*= 1.14. Furthermore, semi-empirical potential calculations have investigated the influence of this parameter on the interaction energy and the geometry of polar molecules adsorbed onto MgO(100) [26,27]. It has been shown that the adsorption energy of a single adsorbed NH3 molecule varies almost linearly with the effective surface charge, and a comparison with experimental results leads to a value of q* = 1.2_+0.1 [28]. The present paper deals with both LEED measurements and theoretical interaction-potential calculations of the acetylene monolayer adsorbed on MgO(100). Section 2 is devoted to the experimental part, including the LEED structure determination of the C2H/ monolayer, the concomitant thermodynamics of the layer, and the bulk condensation studied by equilibrium adsorption isotherms and isobars. Calculations concerning a single C2H2 admolecule and the adsorbed monolayer structure are developed in Section 3. They are followed by a general discussion in Section 4.

2. Experiments

2.1. Experimental set-up and sample preparation Our experiments have been conducted in an ultrahigh vacuum (UHV) chamber where the background pressure reaches P < 10-lo Torr after baking the system at T=473 K for 48 h. Our UHV chamber is equipped with a four-grid LEED apparatus (Fig. 1) featuring a low electron-beam intensity I < 10 - 9 A, which reduces possible perturbation of the adsorbed molecules. A channel plate intensifier and a transparent fluorescent screen give an LEED pattern viewed from the rear of the LEED optics. A mirror at 45 ° from the LEED optics axis allows us to observe the diffraction pattern in a direction perpendicular to this axis through a window. The

Sample

Direction of observation

I

I I

[i i i

s

I

;

l

:l !

i |

i

,

Four-grid optics Channel plate// intensifier

Mirror Electron gun ~ Transparent fluorescent screen

Fig. 1. Schematic representation of the low-energy electron diffraction system.

MgO(100) surface is exposed to the electron beam during the time t < 10 s of an LEED pattern acquisition in a Macintosh IIcx using a video camera (Panasonic Wu BE 600/G). We use MgO single crystals of purity 99.99% and dimensions 4 m m x 2 m m x l 0 m m from Pikem. They are cleaved in situ, with a residual gas pressure of P < 1 0 -l°Torr, using a special cleaver [29] built in our laboratory and mounted on the sample manipulator. The temperature of the sample is lowered using a low-temperature copper sample holder thermally link to a closedcycle cryocooler (CTI-cryogenics), which allows us to reach T = 30 K. The thermal link between the cold head and the sample holder is realized by a copper rod followed by two copper braids allowing all motions of the sample manipulator: translations along x, y and z, rotations around the z axis and around an axis perpendicular to the sample surface plane. We measure the sample temperature with a platinum resistor (R = 100 f~ at T = 273 K) which is connected to a temperature controller providing a temperature stability ATe0.05 K. We estimate the absolute temperature accuracy to be + 1 K. We measure the pressure with a standard Bayard Alpert-type ionization gauge (Varian model

D. Ferry et al./Surface Science 375 (1997) 315-330

UHV-24) connected to a vacuum gauge controller (Granville-Phillips model 307). Both ionization gauge and controller have been calibrated with nitrogen as a reference. Because of different ionization coefficients, corrections must be made for different gases. In the case of acetylene, the pressure indicated by the gauge has to be divided by 1.95 [30] to obtain the true pressure. A cylindrical mirror analyser (CMA) Auger spectrometer (RIBER OPC 105) is used in the 1001500eV energy range in order to monitor the cleanliness of the MgO(100) surface. It does not show visible surface contamination (within the Auger spectrometer sensitivity) before and after adsorption and desorption of C z H 2 molecules (Fig. 2). A mass spectrometer (Balzers Q M G 311) is used to analyze the background impurities, which consist mainly of molecular hydrogen, before and after introducing the acetylene gas. It should be mentioned that molecular hydrogen does not physisorb onto MgO at temperatures above T = 30 K. The C z H 2 gas, from Air Liquide, is 99.60% pure. A supplementary gas-cleaning procedure is applied in order to reach a higher degree of purity. We first introduce the gas into a 1 1 gas tank at a pressure of about 6 x 10 -1Torr, then it is con5.0 10 4 ~ L I" "q'~*~ I 1. [ I I..h q I I. L .I I

4.0 10 4

~

~ ] l

,I I -I

(b)

3.0 10 4 2.0 10 4

0.0 10° ~, ~' 250 450

~; 650 850 E (eV)

" " 1050

1250

Fig. 2. ~,uger electron spectra performed (a) on a freshly cleaved crystal and (b) after five acetylene adsorption experiments. The primary energy of the electron beam is Ep = 3000 eV and the pic-to-pic modulation is Vpp=1 V. In both spectra, oxygen (O K L L ) and magnesium (Mg K L L ) transitions are clearly visible at E = 5 1 0 e V and E = l 1 8 6 e V , respectively, whereas the carbon (C K L L ) transition at E = 2 7 2 eV is not visible. This indicates that, within Auger spectrometer sensitivity, the surface is not contaminated by carbon during our experiments.

317

densed in a glass cold finger at liquid-nitrogen temperature, where it solidifies. The pressure above the C z H 2 solid is then around 10 -1 Torr. At this stage, the remaining gas is pumped out using a turbomolecular pump (Balzers TPU 110) until the background pressure reaches 10-VTorr. Solid acetylene is then warmed to room temperature and evaporated into the tank. This process is repeated until the equilibrium pressure above the solid acetylene is that of pure C z H 2 at T = 77 K, i.e. P = 5 X 10 - 7 Tort. No impurity is then detectable in the gas phase with the mass spectrometer at this level of purification, except for the UHV chamber background impurities. Before an experiment, the MgO crystal is cooled down to the desired temperature and cleaved in a background pressure lower than 10 l°Torr. Hence, the first experiment is performed on a perfectly clean surface. 2.2. LEED data collection and analysis We have automated our LEED experimental set-up in order to improve the data acquisition process and their subsequent treatments. Various video systems have been described in the literature [31,32]. Let us briefly describe our system: a video camera (Panasonic Wu BL 600/G) takes the LEED pattern through a window (Fig. 1) and the analogical video signal is sent to a Macintosh IIcx (Apple) computer via an acquisition card (Neotech) which converts the incoming signal to a PAL picture (768 x 512 pixels with 256 grey levels). This data acquisition process is monitored by a computer program written in our laboratory. Besides the data acquisition system, a diffractedspot intensity-analysis computer program has also been made in our laboratory in order to perform a fast automatic analysis and to reach a better accuracy in the data treatments. 2.3. Experimental results Besides surface structure determination, LEED is used to determine the amount of adsorbed molecules onto the substrate by measuring the intensity of four MgO diffraction spots (1,0, 0,1, 1,0 and 0,1) versus pressure at constant temper-

318

D. Ferry et al. / Surface Science 375 (1997) 315-330

ature (equilibrium isotherms) or versus temperature at constant pressure (equilibrium isobars). At a constant electron energy, the intensity of the diffracted spots decreases as the number of adsorbed molecules increases. All LEED equilibrium adsorption isotherms and isobars are normalized by assuming that the intensity decrease of a diffraction spot of the substrate is proportional to the number of adsorbed molecules [33,34], taking the coverage 0 = 0 before acetylene gas introduction and 0= 1 at the beginning of the plateau following monolayer condensation. This method is commonly used when stepwise adsorption isotherms (isobars) are observed for physisorbed systems [5]. When 3D crystallites start to grow at supersaturation, we observe that the intensity decreases continuously with time until the spots are no longer visible.

(a)

2.3.1. Structure of the adsorbed acetylene monolayer In the entire temperature range studied here (40 K < T,~f,~, < 94 K), we find that acetylene forms an ordered two-dimensional solid phase. As an illustration, Fig. 3a shows the observed LEED pattern at T = 80 K after about 5 rain of exposure to acetylene gas at a pressure of 2 × 10 - 9 Tort. Extra spots due to acetylene appear very dim at first, but their intensity increases until it reaches a constant value at monolayer saturation. At the end of each experiment, the MgO sample is warmed up to room temperature and then cooled again the next day for a new experiment. Successive experiments following the one carried out directly after cleavage show LEED patterns of lower quality. A maximum of five experiments are performed on any one crystal. The cleanliness of the MgO(100) surface is checked using Auger electron spectroscopy between successive experiments. It does not show any surface contamination by carbon species (Fig. 2), as was the case for some other molecules [35]. The LEED pattern shown in Fig. 3a and its schematical representation in Fig. 3b can be interpreted in terms of the following symmetries: (V~ x V~)R45 ° or (2 x 2) with two glide planes along the (110) directions. However, steric and theoretical considerations favor the (2 × 2) symmetry (see Section 3.3.2). Furthermore, previous

(b) Fig. 3. (a) LEED pattern of an acetylene monolayer adsorbed on MgO(100) at T = 8 0 K and a C2H2 gas pressure of 2 x 10 .9 Torr. The electron energy is E = 113 eV. The four spots at the corners are from the MgO(100) substrate, and the four other spots are due to acetylene (see Fig. 3b). This pattern can be explained by a commensurate (2 x 2) structure where {1,0} spots are clearly missing, suggesting two glide planes perpendicular to the cell axis in real space. (b) Schematic representation of the LEED pattern (see Fig. 3a) of C2H2/MgO(100). The unit cell (A*, B*) of the (2 x 2) commensurate overlayer in reciprocal space is shown. In this figure, large squares represent MgO diffracted beams, full circles are visible diffracted beams due to acetylene, and empty circles are missing spots suggesting glide planes. The MgO(100) surface unit cell is indicated by a* and b*.

D. Ferry et al./Surface Science375 (1997) 315 330

neutron diffraction experiments with deuterated a c e t y l e n e C z D 2 have shown that this is indeed the case [23]. The absence of (h,0) and (0,k) spots with h and k odd is consistent with glide planes perpendicular to the unit-cell axes [36]. Besides these systematic extinctions due to the pgg symmetry, we do not observe in our LEED patterns the {1,2} reflections, whereas they have been observed in neutron experiments. These different behaviors between LEED and neutron diffraction can be explained by the weak coherent scattering of electrons by hydrogen compared to that of neutrons by deuterium. Taking this consideration into account, a classical kinematical calculation can show that the LEED {1,2} reflection intensities are indeed negligible. In real space, the (2 x 2) unit cell has a parameter A = 5.96 A and can contain two molecules (Fig. 12), considering the size of C z H 2. The accuracy of our measurements is estimated to be 1%. It is reasonable to think that the strong quadrupole moment 0 = 7 . 2 + 0 . 5 D ~-1 of C z H 2 probably leads to a herringbone packing, similar to that found experimentally for C z D 2 o n MgO(100) by neutron diffraction experiments [23]. A commensurate (2 x 2) herringbone structure containing two molecules which lie parallel to the surface plane satisfies the pgg symmetry. It is worth mentioning that our proposed structure is close to the arrangement of C2D2 molecules in the (001) face of the lowtemperature orthorhombic bulk crystal phase of acetylene, which is stable below T = 133 K [37]. This face presents a rectangular unit cell containing two molecules arranged in a herringbone structure. Furthermore, the dimensions of this latter cell (A = 6.193 A and B=6.005 A) are comparable to the dimensions of our proposed unit cell ( A = B = 5.96 A). We have observed the commensurate (2 x 2) structure between T = 4 0 K and T = 9 4 K, which are, respectively, the lowest temperature that we have explored and the highest temperature allowed for this study due to the large equilibrium pressure value of the acetylene monolayer.

2.3.2. Thermodynamics of acetylene on MgO( lO0) 2.3.2.1. Monolayer condensation. Equilibrium adsorption isotherms are measured within the tern-

319

1.00 t

i

0.95 0.90 !

o.85 T I/I o 0.80 i 0.75

•

0.70 [

0.60

lO-tl

.....

lO-tO

t ....... I .....

10-9

lO-S

1 ,

10"~

.... ~-

10 .6

Pressure (Torr) Fig. 4. LEED equilibrium adsorption isotherm of acetylene on MgO(100) at T = 8 9 K. The electron energy is E = 112.58eV. I/I o represents the averaged value of intensity I of the four MgO {1,0} equivalent spots versus CzHz pressure normalized to the averaged value I 0 at coverage 0=0. The plateau features monolayer condensation.

perature range 88-94 K and the equilibrium pressure of the acetylene monolayer within the range 10 - 9 10 - 6 Torr. Fig. 4 shows an LEED equilibrium adsorption isotherm performed at T = 89 K. This curve features a very sharp decrease of the MgO {1,0} spots, corresponding to condensation of the first layer of acetylene molecules. It is followed by a plateau along which the intensity remains approximately constant within an order of magnitude in pressure. Hence, the acetylene coverage stays at a constant value along the plateau, and we take this coverage 0 equal to 1 monolayer (ML) and normalize the amount adsorbed to its intensity value as explained in Section 2.3. Two reproducible normalized equilibrium adsorption isotherms in the monolayer regime are exhibited in Fig. 5. Those stepwise isotherms are characteristic of a first-order 2D g a s ~ 2 D solid phase transition. Their reversibility confirms that C z H 2 molecules do not dissociate during experiments, and indicates that the rnonolayer is physisorbed on MgO(100). Eight adsorption isotherms and three adsorption isobars have been measured. An adsorption isobar is shown in Fig. 6. A Clausius-Clapeyron straight line log P = f ( 1 / T ) at a constant coverage 0=0.5 has been determined from our experiments, as shown in

D. Ferry et al./Surface Science 375 (1997) 315-330

320 1°1

........

1

0~ O o

g g

I

........

-6.8

I

•T=89K

0.9 0.8 0.7 0.6 0.5 0.4 0,3

° ° o go

I

p. . . . i . -

t

I . . . . q. . . . I . . . .

• T=93K

•

,t

k

oO

Oo°•

0.2 0.1

o

-7.4

~

-7.6

~

-7.8

•

AA

"8"0

10-11

10 -10

10 .9

10 "s

10 .6

10 .7

2Dgas

-8.2

"1 10

[ ....

t ....

* P=2.6

P ....

[ ....

:

'. . . . . ," 13

........ 14

15

1/T (103. K "1)

Fig. 5. Normalized L E E D equilibrium adsorption isotherms of C2H2/MgO(100). Q, T = 8 9 K; A, T = 9 3 K with primary energy of E = 112.58 eV and E = 112.45 eV, respectively.

....

~

",' . . . . . . . . . . . . . . 11 12

Pressure (Torr)

15

J.

OAIA

I ....

I ....

~ ....

Fig. 7. Loglo[P(Torr)] versus 1/T(K) phase diagram for acetylene on MgO(100), extracted from equilibrium LEED adsorption isotherms and isobars. We have indicated three different phase existence domains. The slopes of these straight lines give the isosteric heat of adsorption Qst = 6.9 + 0.4 kcal m o l - i and the latent heat of sublimation AHsub. = 5.08 +0.3 kcal mol 1

10 S T o r r

25 e"

35

.d

45

line following the relation

55-

.=

Monolayer

~.Oo

°

°

°

°

I 8 In P 1 Qst---R

65

"

(2)

75• 8 5

. . . .

105

I

Q . . . .

100

•

O ~

~ . . . .

95

~ . . . .

90

~ . . . .

85

I

. . . .

80

I

75

. . . .

~ . . .

70

65

Temperature (K) Fig. 6. L E E D equilibrium adsorption isobar of C2H2/MgO(100 ) up to bulk condensation. The 2D g a s ~ 2 D solid transition is clearly visible at T = 8 9 . 3 K, where the curve shows a sharp step. After monolayer condensation, the decrease of the temperature leads to a large plateau due to monolayer completion within the temperature range 88-75 K before the bulk condensation at T=71 K. We do not observe secondlayer condensation for C2H2/MgO(100) within the temperature domain used.

Fig. 7. The equation of this line is log [P(Torr)] =

- 1497.62 + 8.98. T(K)

(1)

The isosteric heat of adsorption Qst is obtained from the slope of this Clausius-Clapeyron straight

We find an isosteric heat of adsorption of 6.9_ 0.4 kcal mol-l. Note that this value is close to that found for acetylene on NaCI(100), which is Qst = 7.2_+0.5 kcal mo1-1 [20]. In addition, we can evaluate the lateral interaction energy. Indeed, during the C2H2 adsorption, the 2D gas and the 2D solid phases are in thermodynamical equilibrium on the surface. At low coverage (0<0.2 ML), C2H2 first adsorbs into a 2D gas phase. At about 0.2 ML a critical coverage 0c is reached, above which the additional adsorbing molecules condense into a 2D solid phase. For a first-order phase transition, equilibrium adsorption isotherm (isobar) curves exhibit a characteristic vertical step along which the 2D gas phase and 2D solid phase densities remain constant. We determine 0c graphically at the pronounced kink before the step. Therefore, the lateral interaction energy Qfl can be obtained by applying the Clausius-Clapeyron

321

D. Ferry et al./Surfitce Science 375 (1997) 315 330

equation to the 2D phases

10

---"

9 8

7 -6

(3) E

5 4

where q~= kT(O/ao) is the spreading pressure of the 2D gas in equilibrium with the dense 2D solid, a0 being the molecular area at full monolayer condensation. The lateral interaction energy QII is then obtained from the Ahrrenius plot of ~b shown in Fig. 8. As a result, we find Qll = 3.1 +0.4 kcal m o l - ~. Studying the variation of the isosteric heat of adsorption with coverage (Fig. 9) is an other way to estimate this quantity. We assume that the difference AQst between the value of Qst at monolayer coverage and its value at very low coverage ( 0 ~ 0 ) corresponds to the lateral interaction energy. We find here AQ~t ~ 3 kcal t o o l - l , which is in good agreement with the value of QII previously determined. Note that the large value of Qll characterizes the lateral cooperative effects between CzH 2 molecules. It may be correlated with the appearance of superstructure spots on the L E E D pattern when C2Hg coverage reaches 0 ~ 0.4, which clearly indicates the formation of 2D islands during the growth of the (2 x 2) solid phase.

-6.3

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Q

-6.4 -6.5 -6.6

-6.8 - ~ -6.7 -6.9

-i

-7 -7.1 ~. . . . 10.6

-+ ' ' ' ~-~ 10.7 10.8

I0.9 103/T(K)

11

11.1

11.2

Fig. 8. Arrhenius plot of the spreading pressure q~. The slope of the straight line corresponds to the lateral interaction energy between C2H2 molecules. Here we obtain an experimental value of QII= 3.1 +_0.4 kcal mol 1.

J

3

'i

2

0

'

I

.

.

.

.

.

.

.

.

0.1 0.2 073 014 015 0.6'- 0 . 7 ' Coverage

.

018 0~.9

I--

1

1.1

Fig. 9. Evolution of the isosteric heat of adsorption Qst versus acetylene coverage 0. The full line represents the mean value of Qst at monolayer-coverage saturation.

2.3.2.2. Bulk condensation. The condensation of C z H 2 up to the bulk has been followed by measuring a set of seven LEED equilibrium adsorption isobars in the pressure range 2 x 10-8-2 x 10 -~ Torr and the temperature range 65-105 K. These curves are normalized as explained in Section 2.3.2.1. A complete equilibrium adsorption isobar starting at zero coverage and continuing up to bulk condensation is shown in Fig. 6. It first exhibits a vertical step which is the signature of the monolayer condensation. This step is followed by a plateau where the intensity remains approximately constant, which is attributed to the completion and to the compression of the monolayer. Finally, a sharp drop of the intensity occurs when bulk condensation begins. We do not observe second-layer condensation in our experiments, whereas previous neutron experiments have shown it at T = 131 K [23]. We have plotted the Clausius Clapeyron line corresponding to the acetylene bulk condensation in the phase diagram shown in Fig. 7. The equation of this line is

log [P(Torr)] -

- 1109.12 T(K)

+ 8.11.

(4)

The heat of sublimation determined from this equation (AHsub.=5.08+0.30 kcal tool -1) compared very well to the available published value (AHsub.(C2H2) = 5.029 kcal m o l - 1 [38]).

D. Ferry et al./Surface Science 375 (1997) 315-330

322

Table 2 Lennard-Jones potential parameters

3. Calculations

3.1. Interaction potential Within the approximation of a rigid MgO substrate and an undeformed molecule, we describe the interaction energy V as a sum of a moleculesubstrate potential VMsand of a molecule-molecule potential VMM. The interaction potential VMs between a C2H2 molecule and the MgO substrate is a sum of electrostatic, induction and dispersion-repulsion contributions |/1

-I- l/DR

a (A)

3.69 2.13 0.52 15.09 5.67 1.05 1.38 7.46 2.81

3.21 2.53 2.76 2.89 2.7l 2.64 2.98 3.05 2.87

asee Ref. [41]. bsee Ref. [26].

(5)

VMs= V~s + -Ms ~ -Ms.

The electrostatic contribution V~s describes the interaction between the charges of Mg and O ions and the distributed charges q, dipoles #, quadrupoles O, octupoles £2 and hexadecapoles • located on the atomic C and H sites [39] (see Table 1). The distributed multipole analysis (DMA) which has been extensively discussed elsewhere [40,41], has the following advantages: (i) it describes the non-local character of the electronic distribution in the molecule, and (ii) it converges rapidly in the multipolar expansion, being more accurate than the usual point multipolar distribution. The induction contribution Vhs comes from the mutual adsorbate-substrate polarization and remains weak. The last contribution VDs R takes into account the dispersion-repulsion between atoms of C, H, Mg and O by assuming a pairwise Lennard-Jones form with parameters e and a (see Table 2) [26,42]. The interaction potential VMM between two adsorbate molecules is written as a sum of three

Table 1 Distributed multipole analysis of the acetylene molecule (in atomic units)

Charge q Dipole # Quadrupole O Octupole .O Hexadecapole ~

C C" H Ha Mg-Mg b O-O b H-O H-Mg C-Mg C-O C-H

• (meV)

H

C

C

H

0.108 0 0 0 0

-0.108 - 1.012 -0.501 -- 3.345 1.329

-0.108 1.012 -0.501 3.345 1.329

0.108 0 0 0 0

contributions, as VMS

VMM= V~,M+ VhM+ VMM D..

(6)

V~M describes interactions between the distributed charges, dipole, quadrupole, octupole and hexadecapole moments of two acetylene molecules. VhM characterizes the mutual polarization of the electronic cloud of each molecule by the electrostatic field of the others, and V~RMis the dispersionrepulsion interaction term described by a pairwise Lennard-Jones form between C and H atoms. Finally, the total interaction potential between a C 2 H 2 adlayer and the MgO substrate is then a sum of pairwise interactions

•

j

(7) where r i and Qi define the position of the center of mass and the orientation of the ith C2H2 molecule with respect to an absolute frame tied to the substrate (Fig. 10). The origin of the absolute frame is chosen at the position of a Mg atom. Note the value of the nearest-neighbor distance a=2.98 A between Mg (or O) atoms in the substrate.

3.2. Equilibrium structure The calculations of the minimum energy configuration are performed at T = 0 K for various situations. We first consider a single C z H 2 molecule adsorbed on MgO(100) and determine the mini-

D. Ferry et al./Surjace Science 375 (1997) 315 330

323

Z

~ ~ . f

•

Y

Mg

@o

"10

Z

"30 -

O

H

/_/< (a) lo

,

o.8!

Fig. 10. Geometry of the MgO(100) surface. The (x, y) 2D frame corresponds to axes along M g rows, two consecutive Mg atoms being at a distance a = 2.98 A. The usual (0, ~b) angles and the r vector locate the C2H2 molecule with respect to the fixed 3D frame (x, y, z).

7 -~

,/

: ..~;,o.~

l,,~~

,'

3.3. 3.3.1.

Results Isolated

acetylene

admolecule

Figs. 1 la and 1 lb exhibit, respectively, the potential energy surface and the equipotential map expe-

:

]

' ,' %

'

-.oo

,

'

0.6:

~ ' t

i

, '

i 051

I

~: I '

i

i

.

04,

.

.

.

o .

~"

' ~ ;

'

' ,

02

i' ° !

:7''

~

o3!

mum potential energy surface experienced by this molecule when it moves above the MgO surface. At each position (x, y) of the center of mass, the potential energy V is minimized with respect to the distance z between the molecular center of mass and the surface plane and to the orientations (0, q~). The next step concerns the monolayer structure determination. We consider only commensurate structures of the type (n x rn)R~ which can be rotated or non-rotated (q5 =0) with respect to the substrate frame (x, y). The x axis is chosen along a Mg row. The minimization procedure [43] consists of a numerical search for the potential minin mum ~ i ×,,~R.(r, Q) connected with the (n x m) unit cell containing s molecules, with respect to the 5s degrees of freedom (three for the position (x, y, z) and two for the orientation (0, ~b) of each molecule). The cyclic conditions are applied to the other cells by assuming that equivalently adsorbed molecules move similarly in every cell. We have investigated the influence of the effective surface ionic charge o n ~n(ninrn)Rga(t', ~r~) by using different values of q*.

. . . .

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.

, ~ ' '~ ' ~ - -1400

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~ ,

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_~.~,(~~ . , ,,

LO:4.

0:6

0:6 ~ (377' 018

09'

1:0

(b)

Fig. l 1. (a) Potential surface V m i n ( X , y) experienced by the C2H 2 admolecule. The origin x = 0 , y = 0 corresponds to a Mg site, whereas x/a= 1 (a=2.98 A) defines the distance between two consecutive Mg atoms, which are the most stable adsorption sites. The molecule lies flat above the surface with its center of mass located at z = 2.47 A, above a Mg atom, its symmetry axis being collinear to a Mg row. (b) Equipotential m a p experienced by the admolecule as it moves on the MgO{ 100) surface. Mg atoms are located at each corner of the figure.

rienced by a C 2 H 2 molecule as it moves along the MgO(100) surface. The effective ionic charge is taken as q* = 1. When the molecular center of mass lies above the Mg and O atoms, the corresponding energies are - 3 2 8 and - 9 7 meV, respectively. The absolute minimum is found above Mg atoms, which are the most stable adsorption sites. Equilibrium valleys for C 2 H 2 molecules are along Mg rows with an energy variation of 98 meV between two nearest Mg neighbors. In the Mg-O directions, the energy variations are equal to 244.5 meV. This clearly means that acetylene is trapped above Mg atoms, since high potential

D. Ferry et al. / Surface Science 375 (1997) 315-330

324

barriers prevent it migrating towards the next Mg or O atoms. The linear shape and the electric characteristics of CzH2 lead to an orientational configuration where this molecule lies flat (0 = 90 °) above the surface in the most stable adsorption site. The molecular center of mass lies at z = 2.47 * above the surface, and the CzHz symmetry axis is collinear to Mg rows (~b=O° or q~=90°).

3.3.2. Acetylene monolayer The minimization procedure of V has been applied to a set of various primitive cells with increasing area. Note that the molecular area in the (100) lattice plane of the bulk low-temperature acetylene crystal form is equal to 18.69 A2. We have first considered (n × 1) cells. A (1 × 1) unit cell is prohibited because of its small area of 8.88 A 2. Therefore, a (2 × 1) cell with an area of 17.76 A 2 seems to be more suitable as far as steric consideration have to be taken into account. However, we have to consider an important point, which is the large quadrupole moment 0=7.2_+0.5 D,~ -1 of the acetylene molecule. Indeed, it is well known that the interaction between two quadrupoles is minimized for a perpendicular configuration, a parallel configuration being unfavourable from an energetical point of view. This is supported by the T-shaped structure exhibited by C2H2 molecules in the (100) plane of the bulk. The steric and the T-shaped considerations allow us to exclude the possibility of (2 × 1), (3 × 1) or (V~ x V~)R45 ° unit cells which would contain only one molecule, implying that C z H 2 molecules would be parallel to one another in an ordered monolayer structure. This now leads us to consider ( 2 × 2 ) and ( V ~ × 2

(~Mg

0

c

QH

Fig. 12. Real-space unit cell (A, B) of the calculated (2 x 2) commensurate structure of C2H2 adsorbed on MgO(100). The MgO(100) surface unit cell (a, b) is also shown. Coordinates and orientations of molecules are given in Table 4.

V~)R45 ° unit cells which have an identical area of 35.52 A2 and can reasonably contain two molecules. Calculations performed on the (V~ x 2V~)R45 ° phase with two molecules per unit cell do not lead to a stable geometry. The most stable calculated geometry corresponds to the non-rotated (2 x 2) phase containing two molecules, as shown in Fig. 12. Table 3 gives the characteristics of this phase versus the effective ionic charge q*. The lateral position of the molecular centers of mass and the orientations do not depend on this charge value. The molecular centers of mass are located above Mg atoms. The first molecule lies at the

Table 3 Characteristics of the geometry of the most stable structure for a C2H 2 monolayer adsorbed on MgO(100) q*'~

Molecule b

x (~,)

y (A)

z (A,)

0 (°)

~b (°)

2

i 2 1 2 1 2

0 2.98 0 2.98 0 2.98

0 2.98 0 2.98 0 2.98

2.39 2.39 2.49 2.49 2.53 2.53

90 90 90 90 90 90

60 120 60 120 60 120

1.2 1

~q* is the effective ionic surface charge. bMolecule 1 is located at the origin of the unit cell, whereas molecule 2 lies at the center of the cell.

D. Ferry et al./Surface Science 375 (1997) 315-330

origin of the unit cell with its molecular axis parallel to the surface plane (0=90 °) and rotated by about ~b= 60 ° with respect to the x axis. The second molecule is located at the center of the unit cell with its molecular axis parallel to the surface plane (0=90 °) and rotated by about ~ = 120 ° with respect to the x axis. The z distances between the center of mass of each molecule and the MgO(100) surface are identical for a given value of q*. Results of a study concerning the relative contributions to the potential are presented in Table 4. It shows that the molecule-substrate interactions are dominant whatever the q* value is (larger than or equal to 1). In the same way, the competition between the dispersion-repulsion and the electrostatic contributions is always dominated by the electrostatic terms in the molecule-molecule and the molecule-substrate interactions. It is the consequence of the large quadrupole moment value of acetylene. Calculations were also performed using a point multipole analysis (PMA) and a distributed multipole analysis (DMA). Using PMA leads to results that are 10% larger than using DMA. The results presented in Table 4 were obtained using DMA. Table 4 does not report the value of the induction contribution, which is found to be very small (V~=- 13.2 meV, using polarizabilities ~lt= 35.4× 1025 cm 3 and a ± = 12.7 × 1025 cm 3 from Ref. [44]). This represents only about 1.3% of the total energy V.

4. Discussion and comparison with experiments 4.1. Monolayer structure

In the case of physisorption, relatively weak electrostatic and van der Waals forces responsible Table 4 Contributions to the total energy versus effective surface ionic charge q* (energy per molecule in kcal tool - l ) q*

V~s

VMM

2 1.2 l

0.91 --15.25 - 1.35 --7.32 -1.93 --5.59

--1.30 - 1.30 --1.30

--2.60 -2.60 --2.60

V

VMs/ VMM

--18.24 - 12.57 --11.42

3.68 2.22 1.93

325

for the adsorption leave the electronic structure of the adsorbate basically unperturbed. Acetylene bears a large molecular quadrupole moment and the dominant adsorbate-surface interaction for C z H 2 o n MgO(100) is expected to result from electric-field gradient-quadrupole interactions, the adsorbate-adsorbate interactions probably being quadrupole quadrupole. In this context, the calculated monolayer equilibrium structure, i.e. the herringbone geometry (see Table 3) with t w o C 2 H 2 molecules in the (2 x 2) unit cell, agrees fairly well with the LEED results and with the existence of two glide planes perpendicular to the real-space unit-cell vectors. Moreover, it confirms results of previous neutron diffraction experiments [23] performed at 131 and 150 K on uniform MgO powders presenting only (100) faces. The four-fold symmetry of the MgO(001) substrate and the pgg symmetry of the adsorbate lead to four equivalent domains on the surface. The good agreement between the experimental structure obtained by various techniques and the theoretical results allows us to conclude that the structure of acetylene adsorbed onto MgO(100) is now well established. It is interesting to compare our present results with previous experimental and theoretical data available on the one hand for acetylene adsorbed on NaCI [20,45,46] and graphite [11,16 18] substrates, and on the other hand for ethane adsorbed on MgO(100) [43]. A structure presenting a square symmetry and a T-shaped geometry between neighboring molecules has been determined for acetylene adsorbed on NaCI(100) using polarized infrared spectroscopy [20]. A bilayer structure was initially proposed [20], but recent helium-atom diffraction experiments have shown two monolayer structures having much larger unit cells with T-shaped arrangements of the molecules [45]. Periodic Hartree-Fock calculations performed on CzH2/NaCI(100) have considered a (~/2 x ~v/2)R45° structure containing two molecules arranged in a T-shaped geometry [46]. Acetylene molecules are located above Na ions with their molecular axes parallel to the surface (0 = 90 °) and collinear to Na rows (~b=0 ° or ~b=90°). Both experimental and theoretical results [20-24,45,46] show that the adsorption of acetylene on NaCI(100) and MgO(100) leads to commensurate

326

D. Ferry et al./Surface Science 375 (1997) 315 330

solid phases exhibiting the square symmetry and the T-shaped arrangement. Such a geometry obtained for square ionic substrates still appears to be the most stable on a covalent hexagonal substrate (graphite), as shown both experimentally [11,16] and theoretically [18]. These results are not surprising if we consider the T-shaped structure exhibited by Cell2 molecules in the (100) plane of the bulk crystal at low temperature. However, note that for bulk C2Hz the stability of such a crystal structure is generally explained by the donoracceptor-type bonding between the T-shaped molecules involving the proton and the triple-bond n electrons. Our calculations based on classical electrostatic interactions together with dispersionrepulsion contributions seem to give a similar geometry for acetylene on the NaC1 substrate. In the case of ethane adsorbed onto MgO(100), a rectangular (2V~ x V~)R45 ° structure containing two molecules has been observed by LEED and neutron diffraction experiments [43]. The lengthened shape of C 2 H 6 and its concomitant small multipolar nature due to high point symmetry make this molecular species behavior an interesting basis of comparison since the lateral interactions as well as the interaction with the substrate are dominated by dispersion-repulsion contributions [43]. A comparison of these experimental results with semi-empirical potential calculations has shown that ethane molecules lie fiat above the surface and are ordered in a T-shaped geometry, like CzH2.

4.2. Adsorption site and motions Our LEED experiments as well as neutron diffraction experiments are unable to identify the adsorption site of the adsorbed C2H 2 molecules. We have tried to solve this problem by studying the adsorption of an isolated molecule onto the surface. The potential-energy surface map (Fig. 1la) experienced by the molecule clearly indicates that its center of mass is located above a magnesium atom where the energy V= -328 meV (7.54 kcal mol -l) is a minimum. Note that the C z H 2 molecule lies parallel to the surface by minimizing its strong electrostatic interaction with the substrate due to the high n-electron density around

the triple C------C bond. Another local minimum, above oxygen atoms, is found at a much higher energy of V = - 9 7 meV (2.23 kcal mol- 1). Relative positions of those minima mean that the energetic corrugation of the surface is large. Moreover, the energy variation along Mg rows, which correspond to equilibrium valleys, is equal to 98 meV (2.25 kcal mo1-1) indicating that molecules cannot migrate along the equilibrium valleys. The various motions of the admolecule have been calculated at the equilibrium site. We find that the frequency connected to the center of mass motion perpendicular to the surface is relatively high, about 145 cm -1 (0.414 kcal mol-1). This is also the case for the 0 motion of the molecular axis with a frequency equal to 193.4 cm -~ (0.552 kcal mol-a). The molecular motions parallel to the surface are much lower and significantly anisotropic along the directions x and y since their frequencies are, respectively, 100.7 cm- ~(0.287 kcal mol -~) and 74.9cm -1 (0.214 kcal mol-~). By contrast, the ~bmotion of the molecule axis appears to be nearly free, with a frequency equal to 23.4 cm -1 (0.067 kcal mol-1).

4.3. Isosteric heat of adsorption and latent heat of sublimation In addition to determining the monolayer structure, our experimental set-up also allows us to measure thermodynamical quantities such as the isosteric heat of adsorption Q~t and the lateral interaction energy Qll- These quantities can be calculated with our model and compared with the experimental values (Table 5). Table 4 shows the various contributions to the total energy versus the effective substrate charge q*. We have investigated the influence of q* on the energy because it was shown in a previous theoretical work [25] that q*= 1.14, and experimental results of ammonia adsorbed onto MgO(100) and semi-empirical potential calculations led to a value of q* = 1.2 _+0.1 [28]. These values are appreciably different from the ionic value of 2. Here our aim is not to establish a precise value of q*, but to observe a tendency. It is clear that the difference between the experimental isosteric heat of adsorption Qst --- 6-9 _+ 0.4 kcal mol-1 and the calculated total energy V for

327

D. Ferry et al./Surface Science 375 (1997) 315-330

Table 5 lsosteric heat of adsorption Qst, lateral interaction energy Qii and interatomic distances of acetylene molecules adsorbed onto various substrates Bulk C2H2(100) plane

CzHz adsorbed onto

Qst (kcal mol - 1 ) QLI(kcal mo1-1) Molecular area Nearest distance H-..H Nearest distance C---C Nearest distance C-..H

Exp. Calc. Exp. Calc. (,~z) (A) (A) (A)

MgO (100)

NaC1 (100)

6.9 + 0.4 11.4° 3.1 +0.4 3.9 17.76 2.98 3.56 2.65

7.2 + 0.5a 8.0b 0.96a 3.1b 15.84b 2.85b 3.43b 2.39b

Graphite (0001) 6.40 +_0.04~¢1 6.45d 1.3+0.1 d 18.76d 3.14d 3.77d 2.74d

--

18.59f 2.95f 3.64f 2.73f

~From Rel~[20], bfrom Ref. [46], ffrom Ref. [-17], d from Ref. [18], ffor q*=l, rfrom Ref. [37]. various values of q* (Table 4) is minimum for q* = 1, even if the calculated value V = 11.4 kcal m o l is still larger than the experimental one. This difference can be partially explained by the fact that our calculations are performed at T - - 0 K, whereas the thermodynamical experiments were conducted at about 90 K. When corrections due to temperature effects are taken into account in our calculations, through the frequencies of oscillations of adsorbate determined above, the calculated value decreases to V = 10.5 kcal mol -x. However, even if the agreement appears better, this correction cannot explain the remaining difference between the experimental and calculated values. It should be pointed out that there are significant differences between calculated and experimental values of the isosteric heat of adsorption for some molecules such as methane [26], ethane [43] and hydrocarbons more generally. These disagreements suggest that we should improve our interaction potentials for these particular molecules. Table 5 summarizes experimental and theoretical data of bulk acetylene on the one hand, and acetylene adsorbed onto MgO(100), NaCI(100) and graphite (0001) on the other hand. It is interesting to note that our measured value Qst for C2H2/MgO(100) is close to that determined experimentally for C2Hz/NaCI(100): Q~t = 7.2___0.5 kcal tool -1 [20]. For this latter system, periodic H a r t r e e - F o c k calculations lead to Qst=8.0 kcal mol - I [46], in good agreement with the experimental value. Adsorption is clearly governed by

electrostatic forces because of the large quadrupole m o m e n t of the molecule and the electric surface field gradient for both systems, C2H2/MgO(I00) and C2H2/NaCI(100). As a consequence, if we assume that the main contribution to the isosteric heat of adsorption Qst is due to the electrostatic interactions, the effective ionic surface charge q* of MgO(100) should be close to that of NaCI(100). The adsorption of acetylene onto graphite (0001) faces has been studied both experimentally [-11,16-18 ] and theoretically [ 18 ]. The experimental isosteric heat of adsorption Qst=6.40+0.04 kcal m o l - a and the calculated value V = 6.45 kcal mol-1 are still in good agreement. Graphite is not ionic, and interactions between the molecules and the surface are mainly due to dispersion-repulsion terms. It indicates that the dispersion-repulsion contribution is probably well described, and we conclude that in the case of CzH2/MgO(100) the difference between the experimental and the calculated value of Qst could be due to an overestimation of the electrostatic contribution. In addition to the isosteric heat of adsorption Qst, we have measured the latent bulk heat of sublimation A H ~ b . = 5 . 0 8 + 0 . 3 kcal tool -1. This value agrees with the available published value which is AH~ub.(C2H2)=5.029 kcal mo1-1 [38]. Moreover, a comparison between this value of AH~ub. and the significantly larger value (by about 40%) of the isosteric heat of adsorption Qst for the monolayer indicates a strong interaction of mole-

328

D. Ferry et al./Surface Science 375 (1997) 315-330 -20

cules with the MgO surface. This feature is corroborated by the calculations presented in Table 4.

-40

4.4. Lateral interaction energy We observe that there is a good agreement between the calculated lateral interaction energy VMM= 3.9 kcal mol- 1 and the experimentally determined value Qit=3.1___0.4 kcal mo1-1. This agreement is still better if we take into account temperature effects which lower the calculated value, leading to VMM=3.6 kcal mol-1. Moreover, this result is consistent with the variation of the isosteric heat of adsorption with coverage (Fig. 9) since Qst ~ 4 kcal mol-1 when the coverage is close to zero, indicating a lateral interaction energy of about 3 kcal mol-X. Table 4 shows that moleculemolecule interactions are governed by electrostatic forces due to the large value of the C2H 2 quadrupole moment. As a consequence, the pgg square structure is the most favourable. This is corroborated by the close analogy with the (100) lattice plane of the bulk CzH 2 crystal [37] or the lowest density monolayer adsorbed on graphite [11,18] as shown in Table 5. For the latter case, the square symmetry does not result from the small corrugation of the graphite substrate, which furthermore has a hexagonal symmetry, but is a direct consequence of the lateral interactions. Experimental values of the lateral interaction energy QId between acetylene molecules adsorbed onto MgO(100), graphite (0001) and NaCI(100) are, respectively, 3.1 _+0.4, 1.3_+0.1 [18] and 0.96 kcal tool-1 [20]. Table 5 shows that the values of the molecular area of acetylene adsorbed onto MgO(100) and graphite (0001) are, respectively, 17.76 and 18.76 £ 2 [ 1 8 ] . They are close to the molecular area value in the (100) plane of the C2H 2 bulk low-temperature crystal phase, which is 18.59 ~2 [37]. In the case of C2H2/NaCI(100), this value is lower, and equal to 15.84A 2 [46]. The interatomic distances between the molecules are thus smaller in this latter case than in the other systems (see Table 5), and repulsive lateral interactions can occur between the adsorbed molecules. Fig. 13 shows the calculated lateral interaction energy as a function of the distance between the centers of mass of neighboring CzH 2 molecules. To

-60 ~"

-80 -100

c

Solid

-120

~raphite

_#

-140 -160 -180 ~

3.8

'

'

3.9

4.0

'

I

4.1

. . . .

I

4.2

. . . .

i

4.3

~ ,

,

i

4.4

. . . .

45

Center of mass distance (A)

Fig. 13. Lateral interaction energy per molecule versus centers of mass distance between acetylene neighboring molecules. We have indicated particular points corresponding to the (100) plane of the bulk low-temperature solid phase and to C2H 2 adsorbed onto various substrates.

perform this calculation, we have considered a floating monolayer having the most stable (2 × 2) configuration found for CaH2/MgO(100), and we have varied the unit-cell parameter. Distances between the centers of mass corresponding to the bulk and to the observed two-dimensional solid phases of C2H2 adsorbed onto MgO(100), graphite (0001) and NaCI(100) are indicated by arrows on the curve. They show attractive lateral interaction energies between acetylene molecules adsorbed onto MgO(100) and graphite (0001), while the case of NaCI(100) is clearly repulsive. QII thus contains a repulsive contribution in this latter situation (Fig. 13), whereas the lateral interactions between C2H2 molecules are minimized on MgO(100).

4.5. Monolayer stability Additional information concerning the monolayer stability can be extracted from our thermodynamical study. As the free-energy difference between the monolayer and the bulk phase is written #2D-ff30--RT In (P2D/P3D), the measurement of the ratio P2D/P30 of monolayer to bulk

D. Ferryet al./Surface Science375 (1997) 315-330

329

condensation pressures at a given temperature leads to an estimate of the stability of the twodimensional film in comparison with that of the bulk phase: the lower PzD/P3t), the better the stability of the monolayer. At T = 9 4 K (the highest temperature explored in our work because of the high value of the monolayer equilibrium pressure) we obtain PzD/P3D=3 × 10 -4. An extrapolation of our data at T = 1 5 0 K would give P z D / P 3 D = I . 8 x l O -2, which is consistent with the ratio measured by Coulomb et al. [23]. At T = 150 K, this ratio is about 2 x 10 - 2 for acetylene adsorbed on graphite (0001) [ 17]. In addition, such a stability suggests that the melting temperature of the monolayer may be relatively high. This idea is supported by neutron diffraction experiments showing that the (2×2) solid phase exists at T=210 K [47], i.e. 17.4 K above the 3D triple point of acetylene (T~ = 192.60 K) [48]. A measure of the melting temperature of the acetylene monolayer was not possible with our experimental technique, because at 210 K the equilibrium pressure of the acetylene monolayer is much larger than the maximum pressure of about 10-6Torr allowed in the chamber to perform LEED experiments.

a second layer before bulk condensation, as was observed by neutron diffraction experiments at T-- 131 K [23]. These experiments were performed by introducing a constant C2H2 pressure at a temperature of around 100 K and cooling down our MgO sample until the {1,0} substrate diffracted beams were no longer visible in the LEED pattern (bulk condensation). It is expected that second-layer condensation appears at a temperature close to that of bulk condensation for a given pressure, but we have never observed it. It is possible that the second layer appears above a characteristic critical temperature, as was shown for ethylene adsorbed on graphite (0001) [-49]. In this latter system, the number of adsorbed layers changes with temperature, and the second layer appears only above a critical temperature T = 79.9 K. In our case, the existence of a characteristic temperature associated with second-layer formation within the temperature range 65-130 K could explain why we have not observed its condensation in our temperature domain (65 90 K).

4.6. Growth mode

We have combined LEED experiments and semiempirical potential calculations to study the adsorption of acetylene onto MgO(100). LEED measurements of the structure and interactionpotential calculations allow us to propose a commensurate (2 × 2) herringbone structure containing two molecules which are located above Mg atoms. Acetylene is physisorbed upon the MgO(100) surface with an experimental value of the isosteric heat of adsorption of Qst --- 6.9 +_0.4 kcal mol- 1 and a lateral interaction energy Qll=3.1+0.4 kcal mo1-1. The adsorption process is completely reversible. Calculations show that these measured values are mainly due to the electrostatic adsorbate-substrate and adsorbate-adsorbate interactions. Bearing in mind that crucial problems such as the accuracy of potential parameters and the existence of surface defects on the surface are far from being solved, it would be interesting to improve the potential between hydrocarbons, to study the melting of the monolayer, and to under-

During our LEED experiments, superstructure spots due to acetylene appear when coverage reaches 0~0.4, which indicates the formation of 2D islands during the growth of the monolayer on the MgO(100) surface, as in the case of C2H2 on NaCI(100) [22]. It is consistent with strong attractive lateral interactions between adsorbed molecules, as has been shown previously in this work, since Qll represents about 45% of the measured isosteric heat of adsorption Qst- Furthermore, acetylene molecules do not dissociate onto the surface, except at defect sites, as shown by the reversibility of equilibrium isotherms and Auger electron spectroscopy spectra, which do not show the presence of carbon species on the surface after desorption (Fig. 2). Our LEED equilibrium adsorption isobars conducted up to the bulk (Fig. 6) do not show the presence of a step characterizing the formation of

5. Conclusion

330

D. Ferry et al./Surface Science 375 (1997) 315-330

s t a n d w h y s e c o n d - l a y e r f o r m a t i o n is n o t o b s e r v e d b e l o w T = 131 K.

References [1] E.R. Moog and M.B. Webb, Surf. Sci. 148 (1984) 338. 1-2] K. Kern, R. David, R.L. Palmer and G. Comsa, Phys. Rev. Lett. 56 (1986) 2823. [3] K. Kern, Phys. Rev. B 35 (1987) 8265. [4] E.D. Specht, A. Mak, C. Peters, M. Sutton, R.J. Birgeneau, K.L. D'Amico, D.E. Moncton, S.E. Nogler and P.M. Horn, Z. Phys. 69 (1987) 347. [5] T. Meichel, J. Suzanne, C. Girard and C. Girardet, Phys. Rev. B 38 (1988) 3781. [6] M. Hamichi, A.Q.D. Faisal, J.A. Venables and R. Kariotis, Phys. Rev. B 39 (1989) 415. [7] J. Cui and S.C. Fain, Phys. Rev. B 39 (1989) 8628. [8] G. Vidali, G. Ihm, H.Y. Kim and M.W. Cole, Surf. Sci. Rep. 12 (1991) 133. [9] L.W. Bruch, Surf. Sci. 125 (1983) 194. [10] J. Vilain and M.B. Gordon, Surf. Sci. 1 (1983) 125. [11] A. Terlain, Ph.D. Thesis, Universit6 de Nancy I, 1984. [12] P.C. Stair and G.A. Somorjai, Chem. Phys. Lett. 41 (1976) 391. [13] J.E. Demuth, Chem. Phys. Lett. 45 (1977) 12. [14] L.L. Kesmoldel, R.C. Baetzold and G.A. Somorjai, Surf. Sci. 66 (1977) 299. [15] S. Bao, K.M. Schindler, P. Hofmann, V. Fritzsche and A.M. Bradshaw, Surf. Sci. 291 (1993) 295. [16] P. Thorel, C. Marti, G. Bomchil and J.M. Alloneau, Suppl. Le Vide, les Couches Minces 201 (1980) 119. [17] J. Menaucourt, A. Thomy and X. Duval, J. Chim. Phys. 77 (1980) 959. [18] C. Peters and J.A. Morrison, Surf. Sci. 165 (1986) 355. [19] M. Trabelsi and Y. Larher, Surf. Sci. 275 (1992) L631. [20] S.K. Dunn and G.E. Ewing, J. Phys. Chem. 96 (1992) 5284. [21] S.K. Dunn and G.E. Ewing, Faraday Discuss. 96 (1993) 95. [22] S.K. Dunn and G.E. Ewing, J. Vac. Sci. Technol. A 11 (1993) 2078.

[23] J.P. Coulomb, Y. Lahrer, M. Trabelsi and I. Mirebeau, Mol. Phys. 81 (1994) 1259. [24] D. Ferry and J. Suzanne, Surf. Sci. 293 (1996) L19. [25] S. Russo and C. Noguera, Surf. Sci. 262 (1992) 245. [26] A. Lakhlifi and C. Girardet, Surf. Sci. 241 (1991) 400. [27] S. Picaud, A. Lakhlifi and C. Girardet, J. Chem. Phys. 98 (1993) 3488. [28] M. Sidoumou, V. Panella and J. Suzanne, J. Chem. Phys. 101 (1994) 6338. [29] T. Angot, J. Suzanne and J.Y. Hoarau, Rev. Sci. Instrum. 62 (1991) 1865. [ 30] R.L. Summers, Ionization Gauge Sensitivities as Reported in the Literature, National Aeronautics and Space Administration Technical Note TND 5285. [31] K. Heinz, K. M011er, T. Engel and K.H. Rieder, Structural Studies of Surfaces (Springer, Berlin, 1982). [32] M.A. Van Hove, W.H. Weinberg and C.M. Chan, LowEnergy Electron Diffraction (Springer, Berlin, 1986). [33] J. Unguris, L.W. Bruch, E.R. Moog and M.B. Webb, Surf. Sci. 87 (1979) 415. [34] J. Unguris, L.W. Bruch, E.R. Moog and M.B. Webb, Surf. Sci. 109 (1981) 522. [35] V. Panella, J. Suzanne, P.N.M. Hoang and C. Girardet, J. Phys. I France 4 (1994) 905. [36] International Tables for X-Ray Crystallography (Kynoch, Birmingham, 1965). [37] H.K. Koski and E. S~indor, Acta Crystallogr. B 31 (1975) 350. [38] W. Braker and A.L. Mossman, Matheson Gas Data Book, 5th ed. (Matheson Gas, East Rutherford, 1971). [39] A.J. Stone, private communication. [40] A.J. Stone, Chem. Phys. Lett. 83 (1981) 233. [41] A.J. Stone and M. Alderton, Mol. Phys. 56 (1985) 1047. [42] J.L. de Coen, G. Elefante, A.M. Liquori and A. Damiani, Nature 216 (1967) 910. [43] P.N.M. Hoang, C. Girardet, M. Sidoumou and J. Suzanne, Phys. Rev. B 48 (1993) 12183. [44] J.O. Hirschfelder, C.F. Curtiss and R. Bird, Molecular Theory of Gases and Liquids (Wiley, New York, 1954) p. 949. [45] A. Glebov, R.E. Miller and J.P. Toennies, to be published. [46] A. Allouche, Surf. Sci., to be published. [47] J.P. Coulomb, private communication. [48] Handbook of Chemistry and Physics, 48th ed. (The Chemical Rubber Co., 1968). [49] J. Menaucourt, A. Thorny and X. Duval, J. Phys. 38 (1977) C4-195.