Gas–surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers

Abstract

Molecular beams of CO(2) and Ar were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro ((CF(2))(7)CF(3)-) functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation on model organic surfaces. Ar collisions, for incident energies ranging from 25 to 150 kJ mol(-1), exhibit final energy distributions that depend significantly on the terminal functional group of the SAM. The long-chain CH(3)-terminated monolayers serve as an excellent energy sink for dissipating the incident translational energy. For example, at 150 kJ mol(-1), greater than 90% of the collision energy is transferred to the CH(3)-SAM surface for specularly-scattered atoms (θ(i) = θ(f) = 30° from normal). However, the OH-SAM is a more rigid collision partner due to the formation of an intra-monolayer hydrogen bonding network and the (CF(2))(7)CF(3)-SAM (F-SAM) provides a high degree of rigidity due to the massive CF(3) groups. The final energies for the triatomic, CO(2), scattering from the three surfaces are remarkably similar to the results for Ar scattering. The only significant difference in the translational energy transfer dynamics for these two gases appears in collisions with the OH-SAM. Strong gas-surface attractive forces between CO(2) and the OH-SAM surface appear to counter the rigidity of the hydrogen-bonding network to help bring the majority of the molecules to thermal equilibrium at all incident energies up to 150 kJ mol(-1), resulting in increased energy transfer in comparison to Ar. The similarities in energy transfer for Ar and CO(2) final energy distributions in scattering from the CH(3)- and F-SAMs suggest that the internal degrees of freedom in the triatomic play only a small role in determining the outcome of the gas-surface collision under the scattering conditions employed in this work.

Kinematics and dynamics of atomic-beam scattering on liquid and self-assembled monolayer surfaces

“…For a given collision energy, C 2 F 6 molecules absorb significantly more energy than C 2 H 6 molecules in collisions with Ar, 14 whereas a fluorocarbon surface absorbs much less energy than a hydrocarbon surface in collisions with Ar and other atoms or molecules. 2,9,10,[15][16][17][18] Enhanced internal excitation in C 2 F 6 vs. C 2 H 6 is attributed to the more efficient energy transfer to the lower frequency modes in the heavier species. 19 When fluorocarbon and hydrocarbon chains condense and organize to form surfaces, the inter-chain motions on the hydrocarbon surface have lower frequencies than those on the fluorocarbon surface.…”

On probing ions at the gas–liquid interface by quantum state-resolved molecular beam scattering: the curious incident of the cation in the night time

“…20 Of particular interest to this work, Morris and co-workers have observed CO 2 collisions with OH terminated SAMS and see increased IS scattering compared to CH 3 terminated SAMS, which is ascribed to increased attraction of the quadrupolar CO 2 to the dipole of the OH group overcoming the rigidity of the surface. 21 Contributions from our group have exploited infrared laser absorption methods to monitor the quantum state-resolved rovibrational distribution of scattered molecules as well as high resolution Dopplerimetry to probe final quantum state-resolved translational distributions 22,23 In the present study, we extend these quantum state-resolved laser spectroscopy tools to explore scattering of CO 2 at hyperthermal kinetic energy (15 kcal mol À1 ) from a series of gas-alkali halide/glycerol solutions. One main goal of this work is to exploit the quantum state-resolved scattered flux distributions as a ''messenger'' for the presence/absence of cations or anions in the surface layer.…”

Visualization of Thermal Transport Properties of Self-Assembled Monolayers on Au(111) by Contact and Noncontact Scanning Thermal Microscopy

“…Further experimental evidence to support this notion is that the Δ T distribution observed for C 16 /C 4 SAM (Figure d) remains almost identical when the domain size is reduced (Figure S5). Another factor that results in the Δ T contrasts is that accommodation coefficients , of the SAMs for air around the surface might be assumed. According to reported accommodation coefficients of n -alkanethiol SAMs for argon, SAMs with long alkyl chains show a larger accommodation coefficient than those with short alkyl chains, indicating that the former accepts heat from gas more effectively than the latter.…”