Dawoud, Jamal N (2006) Structures and stabilities of H₂ adlayers on ionic crystal surfaces. PhD thesis, Concordia University.

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Abstract
Our Monte Carlo (MC) simulations investigate the possible formation of a series of structures, p (2{604}2)[arrow right] p (8{604}2)[arrow right] p (4{604}2)[arrow right] p (6{604}2) with coverages =0.5, 0.625, 0.75 and 0.83 respectively, by H 2 molecules on the MgO(001) and LiF(001) surfaces. On H 2 /MgO(001) the sequence p (2{604}2)[arrow right] p (4{604}2)[arrow right] p (6{604}2) is possible and is stable up to 9 K, while on H 2 /LiF(001) the progression p (2{604}2)[arrow right] p (8{604}2)[arrow right] p (4{604}2) is found. These structures are consistent with recent Helium Atom Scattering (HAS) results in terms of coverage and thermal stability, but disagree in terms of symmetry. The HAS work found " c " type structures whereas the MC simulations (without quantum considerations) yield a " p " type structures. The p (2{604}2) structures contains two H 2 molecules per unit cell, with each molecule lying parallel to the plane of the surface ([straight theta] = 90{493}) directly above every other cationic site. The molecules adopt a "T" configuration with respect to their nearest neighbours. For the p (8{604}2), p (4{604}2) and p (6{604}2) structures, there are two kinds of adsorption sites: a parallel site, as in the case of p (2{604}2) structure, and a tilted site, where the H 2 molecules sit between cationic and anionic sites with the molecular axis directed towards the anionic site, with [straight theta] {598} 60{493}. To reconcile the results of the simulations and experiments, the quantum mechanical rotational motion of the adsorbed H 2 molecules was studied using perturbation theory. These calculations show that the adsorbed hydrogen molecules are azimuthally delocalized and hence these structures are indeed c type. For the H 2 /NaCI(001) system, the MC simulations of the bilayer system show that the formation of a second layer is possible, in which the molecules lie tilted ([straight theta] = 45{493}) at the top of the Cl  sites in a unit cell of p (2{604}1) symmetry. It also found the molecules in the bottom layer lie flat over the Na + sites and arranged in a unit cell of p (2{604}1) symmetry, where the molecules adopt a tee configuration along diagonals, and have the same orientation along the rows. In terms of thermal stability, the bottom layer is stable up to 12 K whereas the upper layer is orientationally disordered at T > 5 K and molecules desorb at T > 8 K. These predicted structures are in agreement with the experimental findings in terms of coverage and thermal stability but disagree in terms of symmetry since the PIRSATR and HAS experimental findings show a (1{604}1) structure. To solve this problem, the rotational motion of H 2 molecules has been studied using perturbation theory and it is found that quantum effects will azimuthally delocalize the orientation of the molecular axis of H 2 . Thus, those predicted structures become a (1{604}1) structure.
Divisions:  Concordia University > Faculty of Arts and Science > Chemistry and Biochemistry 

Item Type:  Thesis (PhD) 
Authors:  Dawoud, Jamal N 
Pagination:  xxvi, 255 leaves : ill. ; 29 cm. 
Institution:  Concordia University 
Degree Name:  Ph. D. 
Program:  Chemistry and Biochemistry 
Date:  2006 
Thesis Supervisor(s):  Jack, David B 
ID Code:  8897 
Deposited By:  Concordia University Libraries 
Deposited On:  18 Aug 2011 18:38 
Last Modified:  05 Nov 2016 01:11 
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