We have investigated the desorption of N2 from Ni(100) using thermal desorption spectroscopy (TDS). A modified Polanyi-Wigner equation has been used to obtain the desorption energy and the preexponential factor, both of which depend on the coverage of the adsorbate. We show that there is a large lateral interaction among the adlayer molecules when N2 goes down as ordered c (2 X 2) on Ni. In addition, the overlayer ordering, in the thermal desorption process, is observed to affect the thermal desorption spectra

Dowben, Peter A.

Varma, Shikha

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Peter Dowben Publications Research Papers in Physics and Astronomy May 1990 The effect of lateral interactions on the thermal desorption of N2 from Ni(100) Shikha Varma Syracuse University Peter A. Dowben University of Nebraska-Lincoln, pdowben@unl.edu Follow this and additional works at: https://digitalcommons.unl.edu/physicsdowben  Part of the Physics Commons Varma, Shikha and Dowben, Peter A., "The effect of lateral interactions on the thermal desorption of N2 from Ni(100)" (1990). Peter Dowben Publications. 137. https://digitalcommons.unl.edu/physicsdowben/137 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Peter Dowben Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. the effect of lateral interactions on the thermal desorption of N, from Ni(100) Shikha Varrna and P. A. Dowben Department ofPhysa'cs, Syracu~e vflf~er.~ib-~,  Syrsacusc, New York 63244-1130 (Received 24 August 1989; accepted 9 October 1989) We have investigated the desorption of W, from Ni( 108) using thermal desorption spectroscopy (TDS). A modified Polanyi-Wigner equation has been used to obtain the desorption energy and the preexponential factor, both of which depend on the coverage of the adsorbate. We show that there is a large lateral interaction among the adlayer molecules when N, goes down as ordered c ( 2  X 2) on Ni. In addition, the overlayer ordering, in the thermal desorption process, is observed to aEect the tkermal desorption spectra. The interaction of molecular nitrogen on Ni ( 180) has been the focus of several studies.'-"t has been observed that the adsorption of N2 on Ni ( 108) at 84 K results in the formation of either an ordered c(2 X 2) (with an average domain size of 34 A) or a disordered c(2 X 29 (with an average domain size of 13 A).' Since the disordered c(2 X 2)  N2 overlayer is a noneqnilibrium phase, probing the energetics of this phase cannot be accomplished using equilibrium techniques that have been applied to the study of ordered c(2 X 2 )  over- layers. We have, therefore studied the effects of the precur- sor skate and lateral interactiol:~ for N, on Ni(l90) using thermal desorption spectroscopy (TDS) . Ordering (as we%! as other phase changes) occurring dur- ing the thermal desorption process has a profound effect upon thermal desorption spectra, as we shall demonstrate for N2 on Ni( 100). The adlayer lateral interactions between adsorbed N2 molecules influence the apparent heat of ad- sorption of N2 on Ni ( 1 00). II. EXPERIMENTAL The experiments were carried out in a ultrahigh vacuum system equipped with low energy electron diffraction; (LEED) and Auger electron spectroscopy (AES). The thermal daorption experiments were undertaken at con- stant heating rates@ fallowing adsorption of ordered or dis- ordered overlayers of N, . By adsorbing at 85 K, N2 forms a disordered overlayer with an average domain size of 17 A as determined by LEED, as is described in detail e l se~here .~  Ordered overlayers were formed by annealing the Ni( 100) surface, repeatedly in an ambient N2 background, between f 20-130 K. These ordered overlayers have an average do- main size of 34 A or larger as determined by EEED.2 In this paper, we have assumed that the ordered c ( 2  X 2 )  structure @orresponds to a coverage of 8.05 x IQl4 molecules/cm2 at saturation (8 = 8.5). The Ni.6 188) crystal was cleaned by repeated Ar+ ion bornbarment and annealing to 708 R, and was ascertained to be clean by AES following each experiment. The tempera- ture was determined by a calibrated chromel-alumel ther- mocouple. 811. THEORY In thermal desorption spectroscopy an adsorbate is de- sorbed from the surface by heating the substrate. The gase- ous des~q t ion  products are detected by a mass spectrom- eter. The rate of daorption ( r )  can be described by the Bolanyi-Wigner (PW) equation"25 where 8 Is the adsorbate coverage, ~ ( 8 )  is the theta depen- dent preexponential factor, n is the order of desorption ki- netics, Ed ( 8 )  is the coverage dependent desorption energy, R is the gas constant, and Tis the temperature. Several ap- proaches have been taken to solve the rate parameters v and Ed. Simplified methods of analysis have been proposed by Redhead2' and Chan-Aris-Weinberg (glAWlZ7 fomuHa. For a first order reaction, the Redhead formula can be used to calculate Ed when a choice is made on the value of the preexponential. The usual choice is IOl3  s - I, although in the literature values between IIOL%and 10" s - have been report- ed.20 The disadvantage of CAW analysis is that the desorption parameters Ed and Y are obtained as a function of initial coverage 9, (at time t = 0 )  and not of the actual coverage 0. The dependence ofEd and v on r?, and not on 6 is possibly an unwarrrented assumption though when Ed and v are cover- age independent, the CAW formulas should give correct val- ues. Unfortunately for many overlayers, Ed and v do depend Temperature (Kelvin) PI<;. 1. Normalized thermal desorption spectra for A:: on Ni( 100). These spectracorrespor~d to the saturated overlayer ofthe disordered c ( 2  x 2 )  N2 for different heating rates, /3 = 0.9(A),  1.4(U), 3 ( 0 ) ,  5.3( Y f #/s. These have been normalized so that they correspond to the same coverage (inte- grated intensity under thertnal desorptiorl spectra) of -0.92 (see text). 28185 J. Vac. Sci. Techncsl. A 8 (3), May/Jun 1990 0734-2101/%0/8328Q5-Q5$0116Q @ 1990 American Vacuum Society 2645 2606 S. Varma and P. A. Dowben: Thermal desorption of N, from Ni(100) 2606 * u" n C .- - .- Ef + KL 5: 6 0 C 114 118 122 126 130 134 138 :m 115 I25 135 (b) Temperature (# I  (dl Temperature (K) FIG. 2. ( a )  Thermal desorption spectra for NL fronl an ordcred c(2 x 2) on Ni(100) and (b)  where N: forms a disordered ~ ( 2 x 2 )  on Ni(100). (c)  The experimental data (x) is compared with theoretical models for the saturated overlayer tllernial desorption spectra for ordered N2 adlayer and (d)  for the disordered adlayer. The heating rate is /?= 3 K/s. on coverage, 8. More complete analysis methods have been proposed by King,I2 B a ~ e r , ~ '  and Habenschaden and Kiip- per.2' We have used the modified PW thermal desorption model,28 in which lateral interactions between the adsorbed particles, manifest themselves by coverage-dependent de- sorption parameters: Ed ($1 = E ,  + hE'(@), ( 9 )  Coverage (8/8,,,) ---b 0.0 0.2 0.4 0.6 0.8 1.8 j I j  14.0 Coverage (molecules per surface unit cell) FIG. 3. Heat of desurption Ed ( 6 )  is plotted, as a fnlnction of coverage (0) for the ordered adlayer of N, on Ni! lW j (/? = 3 K/s). Also shown is the isosteric heat of adsorption, q., ( + ). On,,, -- 0.5, the saturation coverage for the ordered overlayer. J. Vac. Sci. Techno!. A, VoB. 8, No. 3, May/Jun 1990 0 0 .05  (2.10 0.15 0.20 Coverage (motecules per surface unit cell) FIG. 4. Desorptio~i energy Ed (0) plotted as a function of coverage for the NL in a disordered ~ ( 2 x 2 )  overlayer on Ni with /?= 0.9(A), 1.4(n), 3 ( X ), 5.3( + ) K/s. We have also plotted E, (8) for the ordered overlayer (0) and the isosteric heat of adsorption (B). Om,, = 0.5, the saturation coverage for the ordered overlayer. where E, is the desorption energy at zero coverage, v,, is the preexponential frequency factor at zero coverage, BE is the coverage dependent part of the desorption energy. and Tc is the compensation effect temperature. We will assume that the lateral interactions are pairwise and that the desorption occurs statistically. Then Ed (8)  is given Ed($) = E, - WH. (4) The interaction energy W is negative for attractive interac- tions and positive for repulsive interactions. Using Eqs. ( 1 ) , (3),  and (4) ,  we get the modified PW equation2" = v e p ( ) e x P [  H T -  -AE(Bi R T ~  1 .  Coverage (molecules per surface unit ce l l )  FIG. 5. Log v ( 8 )  plotted as afunction ofcoverage. Results areshown for N2 adsorbed in disordered ~ ( 2 x 2 )  overlayers on Ni( 100) with p= 0.9(A), 1.4(El), 3 (  X),  5.3( + ) K/s. The results for the ordered case (0) are also shown. &, = 0.5, the saturation coverage for the ordered overlayer. lag I/,- 2608 S. Varrna and P. A. Dowben: Thermal desorption of N, from Ni(100) 2608 TABLE 11. The values of E, (from PW and CAW ), W, v, (from PW and CAW) and Tc. These values are for ordered layers of N, on Xi( 100). The average er- ror bars have also been indicated. Wis negative for attractive interactions and positive for repulsive interactions. for comparison and give consistantly higher values for Eo than does the PW model. V. DISCUSSION As has been assumed for equilibrium studiesV2 our results indicate that N, on Ni( 108) adsorbs into a single site. This conclusion is based upon observing only the single feature in the thermal desorption spectra and the linear variation in In(R) versus 1/T plots.25 Angle-resolved photoemission studies3 are also consistent with a single molecular N, ad- sorption site. A comparison of thermal desorption results for the or- dered N2 overlayer with earlier equilibrium studies2 show that the desorption energy, for coverages > B = 0.1, is larger than the equilibrium heat of adsorption (Fig. 3 ) .  This may indicate an activation barrier to desorption. The activation barrier [Ea,, = Ed (0) - q,, 1, is - 6 kJ/mol at 8 = 0.25. Such an activation barrier is consistent with the existence of a precursor state observed earlier.2 The lower heat sf de- sorption as compared to q,, at low coverages, is a common artifact of thermal desorption spectroscopy and has been ob- served before" for N, on Ni f 1 10). We have noted that the saturation coverage of the disor- dered c(2 X2) overlayer is some 84% of the ordered over- layer, indicating that a large number of defects are frozen into this disordered overlayer, consistent with previous LEEB results.' A large drop in the equilibrium heat of ad- sorption can be associated2 with the completion of the over- layer and formation of antiphase domain boundaries. By freezing into the overlayer similar defects, the differential entropy of the adsorbed layer,2 Sad = A S  + S:, remains Barge for a wider coverage range since defects are not avoided or forced to growing domain edges. Efiectively, the pairwise interactions are lost, and Iateral interactions are substantiaf- By reduced, with large defect densities. This decrease in lateral attractive interaction energies is observed. For the ordered overlayers our results suggest that the overlayer has strongly attractive painvise interactions (Table 11) while the disordered overlayers have slightly re- pulsive interactions overall, in the overlayer, as indicated by the value of W (Table I).  The overall increase in Ed (8)  for the disordered overlayer with decreasing heating rate, ,8, supports tkis observation as well. Ordering does occur dur- ing the thermal desorption process and the smaller thefl, the more ordering will occur during thermal desorption and an- nealing of overlayer. Ordering effectively reduces the defect density resulting in a larger attractive lateral interaction term W and greater ($1, as is observed (Fig. 4). The CAW method of analysis results in Eo values that also reflect this increasing Ed (8)  f Tables 1 and 11). J. Vac. Sci. Technol. A, Vol. 8, No. 3, Yay/Jun I990 The hybridization of molecular N, orbitals to form bands has been observed for molecular N, adsorbed on Ni( 100) . 3  Such hybridization, as evidenced by strong dispersion of the bands, does indicate a strong lateral attraction between N, molecules on Ni( 100) as observed in tkis work. We have also used the PW model, taking in account the presence of lateral interaction, to calculate theoretical desorption spec- tra that fit our experimental curves quite well [Figs. 2(c) and 2 (d )  1. The same model with no mobile precursor did not give as good a fit for high coverages. In conclusion. we have determined that there are strong lateral interactions for the ordered N, overlayers on Ni( 100). This lateral interaction is strongly attractive and is about 55 kJ/mol(40 < / W / < 70 kJ/mol). A softening of the vibrational mode and an increase in the differential entropy associated with the antiphase domain boundaries2 is ob- served in general with an increasing defect density in a ms- lecular N, overlayer. We have observed that for the nonequilibrium phase of a disordered c(2 X 2) N, overlayer on Ni( 100), the heating rate for thermal desorption profoundly influences the results and apparent energetics of N2 interactions in the adsorbed overlayer. ACKNOWLEDGMENTS This work was funded by the NSF through Grant No. DMR-8820779. The authors would like to thank M. Cirunze and J. H. Block who assisted in the initial stages of this study. ' P. S. Bagus, C. R. Rrundle, M. 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The effect of lateral interactions on the thermal desorption of N\u3csub\u3e2\u3c/sub\u3e from Ni(100)

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The effect of lateral interactions on the thermal desorption of N\u3csub\u3e2\u3c/sub\u3e from Ni(100)

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