Laser-induced Desorption Of CO Research Articles

Multicoverage Study of Femtosecond Laser-Induced Desorption of CO from Pd(111).

We study the strong coverage dependence of the femtosecond laser-induced desorption of CO from Pd(111) using molecular dynamics simulations that consistently include the effect of the laser-induced hot electrons on both the adsorbates and surface atoms. Adiabatic forces are obtained from a multicoverage neural network potential energy surface that we construct using data from density functional theory calculations for 0.33 and 0.75 monolayer (ML). Our molecular dynamics simulations performed for these two trained coverages and an additional intermediate coverage of 0.60 ML reproduce well the peculiarities of the experimental findings. The performed simulations also permit us to disentangle the relative role played by the excited electrons and phonons on the desorption process and discover interesting properties of the reaction dynamics as the relevance that the precursor physisorption well acquires during the dynamics as coverage increases.

Electrons and Phonons Cooperate in the Laser-Induced Desorption of CO from Pd(111)

Femtosecond laser induced desorption of CO from a CO-covered Pd(111) surface is investigated with abinitio molecular dynamics with electronic friction that incorporates effects due to the excited electronic and phononic systems, as well as out-of-phase coadsorbate interactions. Our simulations show evidence of an important electron-phonon synergy in promoting CO desorption that has largely been neglected in other similar systems. At the saturated coverage of 0.75ML, effects due to CO-CO interadsorbate energy exchange are also important. Our dynamics simulations, in concert with site-specific desorption energy calculations, allow us to understand the large coverage dependence of the desorption yields observed in experiments.

Ultrafast Vibrational Dynamics of CO Ligands on RuTPP/Cu(110) under Photodesorption Conditions

We have studied CO coordinated to ruthenium tetraphenylporphyrin (RuTPP)/Cu(110) and directly adsorbed to Cu(110), using femtosecond pump-sum frequency probe spectroscopy, to alter the degree of electron-vibration coupling between the metal substrate and CO. We observe the facile femtosecond laser-induced desorption of CO from RuTPP/Cu(110), but not from Cu(110). A change in the vibrational transients, in the first few picoseconds, from a red- to blue-shift of the C–O stretching vibration under photodesorption conditions, was also observed. This drastic change can be explained, if the cause of the C–O frequency redshift of Cu(110) is not the usually-assumed anharmonic coupling to low frequency vibrational modes, but a charge transfer from hot electrons to the CO 2π* state. This antibonding state shifts to higher energies on RuTPP, removing the C–O redshift and, instead, reveals a blueshift, predicted to arise from electron-mediated coupling between the coherently excited internal stretch and low frequency modes in the system.

Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)

We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3–4 higher desorption yield at comparable fluence when desorption is induced via 400nm light, compared to 800nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.

Velocity distribution of CO desorbing from NiO(1 0 0)/Ni(1 0 0) after picosecond UV laser irradiation

The UV laser-induced desorption of CO from an epitaxial film of NiO(1 0 0) on Ni(1 0 0) yields a linear dependence of the desorption signal on the laser intensity with a cross-section of (8.3 ± 0.9) × 10 −18 cm 2. Rotational and vibrational temperatures of 870 K and 1340 K, respectively, are observed. State-resolved velocity distributions are observed, where one part can be described by a Maxwellian flux distribution with 〈 E kin〉/2 k ranging from 1000 to 1800 K. In addition a faster component is present which gains in importance for increasing J″. A distinct rotation–translational coupling is found.

Lateral velocity distributions in laser-induced desorption of CO from Cr 2 O 3 (0001): experiment and theory

Quantum state resolved determination of lateral velocity distributions in laser-induced desorption of CO from a Cr2O3(0001) surface is investigated. Experimentally, Doppler profiles are obtained via a (1+1′)-REMPI detection scheme for various rovibrational states and desorption velocities. The experimental findings are directly compared to theoretical results from high-dimensional stochastic quantum dynamical wave packet calculations based on complete first-principles potential energy surfaces. Reasonable quantitative agreement between experiment and theory concerning the total width as well as the full width at half maximum of the lateral velocity distributions is found. Depending on the rotational state a single-peaked or a double-peaked shape of the Doppler profiles is predicted theoretically. In order to support our theoretical predictions we suggest refined experimental investigations of the state-resolved Doppler profiles with an enhanced signal to noise ratio and further theoretical studies concerning the nature of the electronically excited intermediate.

Laser-Induced Desorption of CO from Cr2O3 (0001): Ab Initio Calculation of the Four-Dimensional Potential Energy Surface for an Intermediate Excited State

Ab initio cluster calculations for an excited state of the system CO/Cr2O3 (0001) are presented. A CO(a 3Π)-like state generated by an internal electronic excitation of the CO molecule (5σ → 2π*) is considered to be a possible intermediate for the laser-induced desorption of CO from Cr2O3 (0001). A four-dimensional potential energy surface (PES) for this state is calculated at the CASSCF level of theory. A few possible alternatives for the desorption intermediate are also discussed. The interaction mechanism between the excited CO molecule and the surface is analyzed, and it is shown that the change of symmetry of the quadrupole moment of the CO molecule is responsible for the topology of the calculated PES.

Optical sum frequency studies of adsorption and laser-induced desorption of CO on W(110)

The adsorption and laser-induced desorption behavior of CO on W(110) has been studied using sum frequency generation (SFG) and thermal desorption spectroscopy (TDS). The temperature and coverage dependence of the CO-stretch SFG-signal was measured. Laser irradiation of CO on the surface showed CO desorption and almost no dissociation of CO by visible laser pulses of 15–45 mJ/cm 2 without damaging the surface. Simulations of the surface reactions are in agreement with these measurements. Furthermore, it was observed that the non-resonant SFG-intensity change caused by CO or O adsorption is correlated to the work function change.

Femtosecond Laser-Induced Desorption of CO from Cu(100): Comparison of Theory and Experiment.

Desorption of CO from Cu(100) by femtosecond laser pulses was investigated using state and time-resolved techniques. The results are well described by two models based on frictional coupling between the CO and the substrate degrees of freedom: an empirical, 1D model, and recent, full-dimensionality, molecular dynamics calculations.

Simulations of the femtosecond laser-induced desorption of CO from Cu(100) at 0.5 ML coverage

Simulations of the femtosecond laser-induced desorption of CO from Cu(100) at an initial temperature of 95K are reported, employing molecular dynamics with electronic frictions. Results are obtained for saturation (0.5 ML) CO coverage, which are compared with additional simulations in the zero coverage limit. In both cases, the desorption yield depends nonlinearly on laser fluence, with initial nonadiabatic energy flow occurring selectively from hot substrate electrons to the molecule-surface frustrated rotation. There is significant enhancement of the desorption yield at saturation relative to the zero coverage limit. This is due to conversion of momentum in the plane of the surface to normal momentum, by collisions between an energetic adsorbate and its neighbors, and energy pooling between molecules in the overlayer.

Photodesorption dynamics of CO from Si (111): the role of surface defects

Laser-induced desorption of CO from an ion beam etched and annealed Si (111) surface is reported. Stateresolved measurements of the desorbed CO reveal very high translational and vibrational energy contents, with the rotational excitation being quite low. The results suggest that the CO photodesorption is derived from CO bound to ion beam etch-induced active “defect” site(s) on the Si(111)7 × 7 surface that are only minimally influenced by either the ion beam etching conditions or the anneal. The very high translational energy in the CO implies desorption from active “defect” site(s) either in the very-near-surface region or on the surface. Comparison of the photoyields at 266 and 355 nm suggests that the desorption mechanism cannot be described using existing models for thermal- or carrier-mediated processes.

UV laser-induced desorption of CO from Cr 2O 3(111). A fully quantum state resolved mapping of the desorbing species

The laser-induced desorption of CO adsorbed on an epitaxially grown film of Cr 2O 3(111) has been studied using 193 nm (6.4 eV) laser light. The velocity flux distributions of the desorbing neutral CO in its ground electronic state were recorded quantum state specifically by making use of a (1 + 1′) REMPI (resonance enhanced multiphoton ionization) technique. Single rovibrational states of the desorbing CO could be selected by using the (B 1Σ + ← X 1Σ +) transition in the VUV at ≈ 115 nm for the first excitation step of the REMPI process. Excellent resolution was obtained in contrast to the most commonly used (2 + 1) REMPI process for the same intermediate state. The velocity flux distributions were bimodal for ν″ = 0 and monomodal for ν″ = 1. The high desorption cross section of (3.5 ± 1) × 10 −17 cm 2 is typical for the desorption from oxide surfaces.

Simulations of femtosecond laser-induced desorption of CO from Cu(100)

Trajectory calculations of laser-induced desorption of CO from Cu(100) were performed employing molecular dynamics with electronic frictions and fluctuating forces to account for nonadiabatic energy exchange between nuclear and electronic degrees of freedom. The simulations were performed with an initial surface temperature of 95 K in the low coverage limit, with rapid electronic heating and cooling modeled via the electronic fluctuating forces. The calculated desorption yields are in rough agreement with available experiments, supporting the underlying mechanism of direct nonadiabatic energy transfer from hot electrons to adsorbate motion. Preferential nonadiabatic energy flow into frustrated rotational modes of CO on Cu is identified as a significant contributor to desorption.