^{1}, John R. Morris

^{1}and Diego Troya

^{1,a)}

### Abstract

We present an experimental and theoretical study of the dynamics of collisions of the CO molecule with organic surfaces. Experimentally, we scatter CO at and 30° incident angle from regular (-terminated) and -fluorinated (-terminated) alkanethiol self-assembled monolayers (SAMs) and measure the time-of-flight distributions at the specular angle after collision. At a theoretical level, we carry out classical-trajectory simulations of the same scattering process using CO/SAM potential-energysurfaces derived from *ab initio* calculations. Agreement between measured and calculated final translational energy distributions justifies use of the calculations to examine dynamical behavior of the gas/surface system not available directly from the experiment. Calculated state-to-state energy-transfer properties indicate that the collisions are notably vibrationally adiabatic. Similarly, translational energy transfer from and to CO rotation is relatively weak. These trends are examined as a function of collisionenergy and incident angle to provide a deeper understanding of the factors governing state-to-state energy transfer in gas/organic-surface collisions.

This work has been supported by the NSF (Grant Nos. CHE-0547543 and CHE-0549647), and the AFOSR under Grant No. FA9550-06-1-0165. Diego Troya is a Cottrell Scholar of Research Corporation.

I. INTRODUCTION

II. COMPUTATIONAL DETAILS

A. Potential-energysurfaces

1. Ab initio calculations

2. Analytic CO/SAM potentials

B. Classical-trajectory calculations

III. EXPERIMENTAL DETAILS

IV. RESULTS AND DISCUSSION

A. Comparison between theory and experiment

B. Comparison with rare gas scattering

C. The effect of initial CO rovibrational excitation

D. The effect of collisionenergy

E. The effect of the angle of incidence

V. CONCLUDING REMARKS

### Key Topics

- Energy transfer
- 76.0
- Self assembly
- 61.0
- Surface scattering
- 56.0
- Surface dynamics
- 51.0
- Surface collisions
- 44.0

## Figures

The orientations investigated with *ab initio* calculations in this work. Carbon atoms are brown and oxygen atoms are red.

The orientations investigated with *ab initio* calculations in this work. Carbon atoms are brown and oxygen atoms are red.

Calculated intermolecular potential energy for the system as a function of CO center-of-mass, C, distance. The displayed curves are for the approach that yields the deepest van der Waals well [approach (i) in Fig. 1]. MP2/CBS corresponds to a complete basis set limit estimate based on MP2 energies. Note that the CCSD(T)/aug-cc-pVDZ and MP2/aug-cc-pVDZ data overlap within the thickness of the plotted curves.

Calculated intermolecular potential energy for the system as a function of CO center-of-mass, C, distance. The displayed curves are for the approach that yields the deepest van der Waals well [approach (i) in Fig. 1]. MP2/CBS corresponds to a complete basis set limit estimate based on MP2 energies. Note that the CCSD(T)/aug-cc-pVDZ and MP2/aug-cc-pVDZ data overlap within the thickness of the plotted curves.

Calculated intermolecular potential energy for the system as a function of CO center-of-mass–C distance. The displayed curves are for the approach that yields the deepest van der Waals well [approach (c) in Fig. 1]. MP2/CBS corresponds to a complete basis set limit estimate based on MP2 energies.

Calculated intermolecular potential energy for the system as a function of CO center-of-mass–C distance. The displayed curves are for the approach that yields the deepest van der Waals well [approach (c) in Fig. 1]. MP2/CBS corresponds to a complete basis set limit estimate based on MP2 energies.

Comparison of fp-CCSD(T)/CBS and analytic intermolecular potential-energy surfaces of CO approaches to the and molecules. (a)–(i) correspond to the approach geometries shown in Fig. 1. ○ , *ab initio* data;—, fit; ◻ , *ab initio* data; , fit

Comparison of fp-CCSD(T)/CBS and analytic intermolecular potential-energy surfaces of CO approaches to the and molecules. (a)–(i) correspond to the approach geometries shown in Fig. 1. ○ , *ab initio* data;—, fit; ◻ , *ab initio* data; , fit

Measured (symbols) and calculated (lines) product translational energy distributions in collisions of CO with the indicated SAMs at and . Note that the experimental distributions correspond to those molecules which scatter from the SAM in a ±0.5° acceptance window in the in-plane, forward-scattering specular direction , while the calculated distributions include the scattering flux integrated over all final angles. The distributions are normalized to unit area.

Measured (symbols) and calculated (lines) product translational energy distributions in collisions of CO with the indicated SAMs at and . Note that the experimental distributions correspond to those molecules which scatter from the SAM in a ±0.5° acceptance window in the in-plane, forward-scattering specular direction , while the calculated distributions include the scattering flux integrated over all final angles. The distributions are normalized to unit area.

Calculated average final translational energies as a function of the final scattering angle in collisions of CO with the indicated SAMs . The arrow corresponds to the scattering angle at which the experimental results were obtained. The symbols correspond to the average final translational energies determined from experiment (circle, ; triangle, ).

Calculated average final translational energies as a function of the final scattering angle in collisions of CO with the indicated SAMs . The arrow corresponds to the scattering angle at which the experimental results were obtained. The symbols correspond to the average final translational energies determined from experiment (circle, ; triangle, ).

Calculated final translational energy distributions for collisions of Ne, CO, and Ar with a (a) and a (b) at and . The distributions are normalized to unit area. Data for Ne and Ar are from Ref. 18

Calculated final translational energy distributions for collisions of Ne, CO, and Ar with a (a) and a (b) at and . The distributions are normalized to unit area. Data for Ne and Ar are from Ref. 18

Final CO rotational distributions for various initial rotational states when scattering from (a) - and (b) at and .

Final CO rotational distributions for various initial rotational states when scattering from (a) - and (b) at and .

Total energy deposited into the indicated SAM surfaces as a function of CO initial rotational energy. and .

Total energy deposited into the indicated SAM surfaces as a function of CO initial rotational energy. and .

Calculated CO final translational energy distributions as a function of collision energy scattering from (a) and (b) with a angle of incidence.

Calculated CO final translational energy distributions as a function of collision energy scattering from (a) and (b) with a angle of incidence.

## Tables

Parameters of the Buckingham potential describing the CO/SAM interactions. (Units are such that if internuclear distances are given in Å, then the potential energy is in kcal/mol.)

Parameters of the Buckingham potential describing the CO/SAM interactions. (Units are such that if internuclear distances are given in Å, then the potential energy is in kcal/mol.)

Comparison of *ab initio* and analytic energy and geometry of the van der Waals mimima for , systems. (Energies below the asymptote in . Values between parentheses correspond to the distance between the centers of mass of the pairs in angstroms. The *ab initio* data correspond to fp-CCSD(T)/CBS values.)

Comparison of *ab initio* and analytic energy and geometry of the van der Waals mimima for , systems. (Energies below the asymptote in . Values between parentheses correspond to the distance between the centers of mass of the pairs in angstroms. The *ab initio* data correspond to fp-CCSD(T)/CBS values.)

Comparison of intermolecular interaction parameters for CO– and rare-gas and systems.

Comparison of intermolecular interaction parameters for CO– and rare-gas and systems.

Scattering properties as a function of initial CO rovibrational state in collisions of CO with - and . (30° incident angle; collision energy. Values within parentheses correspond to averages considering only those molecules undergoing a direct mechanism.)

Scattering properties as a function of initial CO rovibrational state in collisions of CO with - and . (30° incident angle; collision energy. Values within parentheses correspond to averages considering only those molecules undergoing a direct mechanism.)

Scattering properties as a function of collision energy in collisions of CO with - and . (30° incident angle; CO states: , ; values within parentheses correspond to averages considering only those molecules undergoing a direct mechanism.)

Scattering properties as a function of collision energy in collisions of CO with - and . (30° incident angle; CO states: , ; values within parentheses correspond to averages considering only those molecules undergoing a direct mechanism.)

Final CO properties as a function of the incident angle in collisions with - and . ( collision energy and CO vibrational state. Values within parentheses correspond to averages considering only those molecules undergoing a direct mechanism.)

Final CO properties as a function of the incident angle in collisions with - and . ( collision energy and CO vibrational state. Values within parentheses correspond to averages considering only those molecules undergoing a direct mechanism.)

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