Thermal Desorption of Large Molecules from Solid Surfaces Kristen A. Fichthorn and Radu A. Miron Departments of Chemical Engineering and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 5 June 2002; published 21 October 2002) We use molecular-dynamics simulations and importance sampling to obtain transition-state-theory rate constants for thermal desorption of an n-alkane series from Au(111). We find that the binding of a large molecule to a solid surface involves different types of local minima. The preexponential factors increase with increasing chain length and can be substantially larger than typical estimates for small molecules. Our results match recent experimental studies and indicate that a proper treatment of conformational isomerism and entropy, heretofore not found in coarse-grained models, is essential to quantitatively describe the thermal desorption of large molecules from solid surfaces. DOI: 10.1103/PhysRevLett.89.196103 PACS numbers: 68.43.De, 68.43.Fg, 68.43.Mn, 68.43.Vx The equilibrium and dynamics of large molecules at surfaces are important in assembly, catalysis, thin films, lubrication, molecular electronics, and microfluidics. With the current emphasis toward miniaturization of technology, tighter control is needed of molecular pro- cesses at surfaces. Increased understanding of the equili- brium structures and dynamics of large molecular adsorbates is important toward this end. n-Alkanes, with chemical formula C N H 2N2 , are pro- totypical large molecules that exhibit many of the rich features associated with molecular adsorption. These nonpolar, chainlike molecules are relatively inert and they physically adsorb to a variety of solid surfaces, including metals [1–7], metal oxides [8], and graphite [9 –11]. n-Alkane desorption energies are experimentally observed to increase with increasing chain length, a trend that has been loosely associated with n-alkanes aligning their C-C bonds parallel to the surface, in the all-trans conformation. To support this idea, ultrahigh vacuum studies with various techniques [2–4,7,10,11] provide evidence that the ‘‘flat’’ and all-trans conformation oc- curs almost exclusively at low temperatures. Since the alkane-surface interaction is most likely dominated by van der Waals dispersion forces, which are pairwise, it is expected that the desorption energy should increase lin- early with increasing chain length. In contrast, n-alkane binding energies have been experimentally observed to increase in a less-than-linear way with increasing chain length [1,2,9]. In perhaps the best example of this trend, Paserba and Gellman used temperature-programed de- sorption (TPD) to measure the desorption energies for a series of 21 n-alkanes, with 5  N  60 carbons, ad- sorbed to graphite [9]. They showed that the desorption energy for this series increases as N 1=2 , and they attrib- uted this trend to the existence of partially bound alkane conformers near the desorption temperature. In this Letter, we consider the microscopic basis for such a model. The desorption dynamics of large molecules can be characterized by both an activation energy E d and a prefactor  0 , so that the rate constant k is given by k   0 expE d =k B T. The activation energy is emphasized in most studies and  0 is assumed to take on the ‘‘typical’’ value of 10 13 s 1 . In this Letter, we show that the pre- exponential factor for large molecules can be substan- tially larger than the typical value. We use molecular dynamics (MD) to simulate the thermal desorption of a series of n-alkanes, ranging from methane (CH 4 ) to n-dodecane (C 12 H 24 ) from Au(111), focusing on the low-coverage limit of a single alkane molecule. To describe these molecules, we adopt the united-atom (UA) model [12], in which CH N (N  2–4) groups are modeled as single interaction centers. We describe the UA-Au interaction using a Lennard-Jones (12-6) potential, truncated at a distance of 8:2  A, with parameters adjusted to match the desorption energy of n-hexane from Au(111) [1]. We assume that each UA interacts equally with the surface, neglecting possible variations in the polarizability for C and H atoms in different local environments. From studies of n-alkanes in the fluid phase, we expect these variations to be small [13]. Finally, we model the Au(111) surface as a five-layer slab with 64 atoms per layer. Atoms in the bottom two layers are fixed to their bulk, equilibrium positions. Atoms in the middle layer are maintained at a constant temperature to provide a heat bath for atoms in the top two layers. We represent Au-Au interactions using a Lennard-Jones (12-6) potential with parameters chosen to yield the lattice constant and the bulk cohesive energy of Au. It should be noted that the main role of the surface is to mimic the fcc(111) structure of Au(111) and to provide a heat bath for the adsorbate. We obtain n-alkane desorption rates using transition- state theory (TST), in which the desorption rate is given as a canonical average of the flux of adsorbed molecules passing through the transition state to the vacuum above. This average can be written as [14] k TST  1 2  2k B T m  1=2 R R FRe VR=k B T dR R R e VR=k B T dR ; (1) VOLUME 89, NUMBER 19 PHYSICAL REVIEW LETTERS 4NOVEMBER 2002 196103-1 0031-9007= 02 =89(19)=196103(4)$20.00  2002 The American Physical Society 196103-1