Kinetics and Energetics of Oligomer Desorption from Surfaces

Abstract

The dynamics of oligomer desorption from surfaces has been studied by measuring the desorption kinetics of a set of straight chain alkanes [H͑CH 2 ͒ n H, with n 5 to 60] from the surface of single crystalline graphite. Desorption is observed to be a first-order process and the preexponent of the desorption rate constant has a value n 10 19.660.5 sec 21 and is independent of the oligomer chain length. More interestingly, we find that the barrier to desorption has a nonlinear dependence on chain length and takes the form DE This Letter reports the barriers to desorption ͑DE z des ͒ of straight chain alkanes [H͑CH 2 ͒ n H, with n 5 to 60] from the surface of graphite into vacuum and the finding of a nonlinear dependence of DE z des on chain length, n. This nonlinear behavior reveals something of the dynamics of the complex process or mechanism by which oligomeric species desorb from surfaces. Independent of its fundamental interest, this result has implications for a number of technologically important surface phenomena such as the evaporation of thin lubricant films from the surfaces of magnetic data storage disks and the desorption of alkanes from the surfaces of Fischer-Tropsch catalysts.The vast majority of measurements of molecular desorption kinetics from surfaces have used relatively small species for which the desorption process is considered simply as a displacement along the surface normal. This desorption mechanism can be adequately modeled using a single well-defined reaction coordinate through a fairly simple potential energy surface describing the interaction of the molecule with the surface. The desorption rate constant is usually considered to have the form given by the transition state theory [1]where h is Planck's constant and k B is Boltzmann's constant. The DE z des is the difference between the zero-point energies of the adsorbed state and the transition state for desorption. At best the multidimensional nature of the adsorbate-substrate potential energy surface influences the desorption kinetics through the partition functions for the adsorbed state ͑q͒ and the transition state for desorption ͑q z ͒.Consider the desorption of an oligomeric species such as H͑CH 2 ͒ 60 H from a surface. Many scanning tunneling micrographs (STM) of alkanes adsorbed on graphite at room temperature reveal an all-trans conformation stretched out and interacting with the surface along its length [2][3][4][5]. Since the desorbed state has no segments interacting with the surface one might predict that the DE z des should be linear in the chain length. There have been several prior sets of measurements which observe the effects of alkyl chain length on the desorption kinetics of species such as alkyl alcohols and simple alkanes adsorbed on metal surfaces [6][7][8][9]. In these cases the range of alkyl chain lengths has been limited to n # 12. Needless to say, over this limited range the measured values of DE

Desorption of n-alkanes from graphene: a van der Waals density functional study

“…As shown in Figure 2 the calculated adsorption energy values grow linearly with the size of the alkane molecule, N , with an off-set comparable to that from experiments. Although the coverage of adsorbed alkanes in our calculation is 0.2-0.3 ML (with full coverage defined as described further below) the subtraction procedure involving the two first terms in (4) ensures that all direct alkane-alkane interactions are eliminated. Thus our results should be compared with experimental results for single alkane molecules desorbed from otherwise clean and defectless graphene (coverage 0 ML), E 0 .…”

“…The n-alkanes measured were short, with the number of C atoms N ≤ 10. The desorption of n-alkanes from graphite surfaces was also measured by Paserba and Gellman [2][3][4], and from various other surfaces by a number of other groups [5], the surface materials including metals (Ag, Au, Cu, Pt, Ru), oxides (Al 2 O 3 , MgO), and semiconductors (Si).…”

Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

“…Commonly, the latter are assumed to be on the order of 10 13 -10 15 s Ϫ1 and small uncertainties of less than an order of magnitude will not give rise to any serious error of the activation energy. However, the assumption of constant preexponentials is sometimes problematic, 20 in particular if large adsorbates with many internal degrees of freedom are studied. 21 For thermal desorption of alkane chains from graphite, for example, preexponential factors have been calculated using transition state theory ͑TST͒ and were found to increase by over four orders of magnitude with chain length from 10 12 s Ϫ1 for methane up to 10 16 s Ϫ1 for C 12 H 26 .…”

Nucleation and growth of thin films of rod-like conjugated molecules

“…The molecule averages over many possible atomic adsorption sites to find the molecular adsorption site with the minimum energy. Directly related to this is a large number of internal vibrational degrees of freedom [47,48]. These have to be considered when discussing the interaction at interfaces.…”