Laser-induced Desorption Of NO Research Articles

Dissipative quantum dynamics using the stochastic surrogate Hamiltonian approach.

Based on the original Stochastic Surrogate Hamiltonian approach for the simulation of open quantum system dynamics, we present a modified algorithm for the swap procedure. The implementation is tested with nuclear relaxation dynamics of model systems known from literature, i.e., the harmonic oscillator and the Morse oscillator. Finally, the stochastic surrogate Hamiltonian is applied to simulate the dynamical electronic excitation and relaxation of a photodesorption process. This is the first application of the stochastic surrogate Hamiltonian in an ab initio context. A comparison to a surrogate Hamiltonian benchmark allows us to evaluate the results obtained. For this purpose, the well-studied laser-induced desorption of NO from NiO(100) is chosen.

Highly Efficient Ultrafast Energy Transfer into Molecules at Surface Step Sites

Ultrafast laser-induced desorption of NO and CO molecules on a platinum surface reveals that the rate of energy transfer between the laser-heated electrons and the adsorbate depends critically on the precise adsorbate location. An analysis based on a simple electronic friction model suggests that this femtosecond energy transfer occurs ∼3 times faster to adsorbates at step sites compared to adsorbates located on atomically flat surface regions. Density functional theory calculations of vibrational damping rates by electron−hole pair excitations corroborate the increased rate of energy transfer at step sites and show that it is caused primarily by an increased local density of adsorbate-induced states around the Fermi level.

Two-dimensional surrogate Hamiltonian investigation of laser-induced desorption of NO∕NiO(100)

The photodesorption of NO from NiO(100) is studied from first principles, with electronic relaxation treated by the use of the surrogate Hamiltonian approach. Two nuclear degrees of freedom of the adsorbate-substrate system are taken into account. To perform the quantum dynamical wave-packet calculations, a massively parallel implementation with a one-dimensional data decomposition had to be introduced. The calculated desorption probabilities and velocity distributions are in qualitative agreement with experimental data. The results are compared to those of stochastic wave-packet calculations where a sufficiently large number of quantum trajectories is propagated within a jumping wave-packet scenario.

Three-dimensional ab initio simulation of laser-induced desorption of NO from NiO(1 0 0)

Laser-induced desorption of NO molecules from a NiO(1 0 0) surface is studied on an ab initio level. Based on ab initio NiO-cluster calculations a three-dimensional potential energy surface was constructed for the electronic ground and a representative excited state. Quantum wave packet calculations on these surfaces allow the simulation of experimental velocity distributions of the desorbed NO molecules. Analysis of the wave packet dynamics demonstrates that the experimentally observed bimodality of the velocity distributions is caused by a bifurcation of the wave packet on the excited state potential, where the molecular motion parallel to the surface plays a decisive role.

Multi-dimensional mixed quantum?classical description of the laser-induced desorption of molecules

A mixed quantum–classical method for the simulation of laser-induced desorption processes at surfaces is implemented. In this method, the nuclear motion is described classically, while the electrons are treated quantum mechanically. The feedback between nuclei and electrons is taken into account self-consistently. The computational efficiency of this method allows a more realistic multi-dimensional treatment of desorption processes. We apply this method to the laser-induced desorption of NO from NiO(100) using a two-state two-dimensional potential energy surface derived from ab initio quantum chemical calculations; we extend this potential energy surface to seven dimensions employing a physically reasonable model potential. By comparing our method to jumping wave-packet calculations on exactly the same potential energy surface we verify the validity of our method. We focus on the velocity, rotational, and vibrational distributions of the desorbing NO molecules. Furthermore, we model the energy transfer to the substrate by a surface oscillator. Including recoil processes in the simulation has a decisive influence on the desorption dynamics, as far as the velocity and rotational distribution is concerned. In particular, the bimodality in the velocity distribution observed in low dimensions and in the experiment disappears in a high-dimensional treatment.

Simulation of laser-induced desorption of NO from NiO(1 0 0)

We have implemented a mixed quantum-classical scheme for the simulation of the laser-induced desorption from surfaces. This method allows a multidimensional simulation of the desorption process on realistic time scales. Using ab initio derived potential energy surfaces, we compare the mixed quantum-classical scheme to jumping wave-packet calculations for the laser-induced desorption of NO from NiO(1 0 0). Our results demonstrate the significant role of the multidimensionality in the desorption process, in particular the importance of surface recoil processes.

Femtosecond ultraviolet laser-induced desorption of NO from NiO(100)/Ni(100)

We have studied the ultraviolet laser-induced desorption of NO molecules from an epitaxial film of NiO(100) on Ni(100) for pulse durations of 550 fs and at a photon energy of 3.95 eV. For applied laser intensities up to 450 MW/cm2, the highest intensities employed in this work, a linear dependence of the desorption yield on the laser intensity is observed. A desorption cross section of (1.9±0.3)⋅10−17 cm2 is then derived. The molecules are detected with rovibrational state selectivity by (1+1)-REMPI via the A 2Σ+(v′=0,1,2,3)←X 2Π(v″=0,1,2,3) γ-band transitions. The rotational population distributions are nonthermal, with rotational temperatures of about 290 K at low and of about 770 K at high rotational energies. The relative populations in the 2Π1/2 and 2Π3/2 fine-structure states and of the Λ-doublet states are examined for the given set of (v″,J″) quantum numbers. The vibrational states v″>0 are significantly populated approaching vibrational temperatures of 3800 K. Velocity distributions of the desorbed molecules are measured for individual rovibrational states. Bimodal distributions with a distinct rotational-translational coupling are observed. Further, molecules desorbing in the excited fine-structure state 2Π3/2 show a higher velocity than those desorbing in 2Π1/2 The results are compared with earlier ones obtained for the same system with nanosecond desorption pulses at hν=6.4 eV.

Selectivity on Laser-Induced Desorption of NO and CO from Transition Metal Surfaces

Selectivity on Laser-Induced Desorption of NO and CO from Transition Metal Surfaces

Bending vibration of NO on Pt(111) at the intermediate-excited state in photostimulated desorption

Laser-induced desorption of NO from Pt(111) at 80 K is studied using a nanosecond-pulsed laser at 193 nm. The desorbed NO molecules are state-selectively detected by the resonance-enhanced multiphoton ionization method. The tilt angle at the time of NO desorption is estimated from the rotational quantum number dependence of the time-of-flight spectra for the desorbed molecules using a simple impulse model. The potential surface for the frustrated rotation (bending vibration) of NO chemisorbed on the Pt(111) surface is found to be nearly flat at the intermediate-excited state from an analysis of the mean vibrational amplitude.

Computer Simulations of State-Specific Laser-Induced Desorption of NO from NiO

Computer Simulations of State-Specific Laser-Induced Desorption of NO from NiO

State-resolved studies of the laser-induced desorption of NO from Si(111) 7×7: Low coverage results

The results of a quantum-state-resolved study of the laser-induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption-laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the rest-atom localized, intrinsic surface state of the 7×7 reconstructed surface. It is suggested that the surface state hole drives the desorption by neutralization of a NO−δ adsorbate.

Surface-state-mediated photochemistry: Laser-induced desorption of NO from Si(111).

Laser-induced desorption of NO from Si(111) has been investigated using state-specific detection techniques. Characterization of the internal-state population distributions with desorption-laser wavelengths in the range 1907--335 nm indicate that, at low initial NO coverages, desorption is driven by a common, nonthermal mechanism. The wavelength dependence of the yield and the effects of coadsorbates on the yield establish that photogenerated holes in a Si intrinsic surface state mediate the laser-adsorbate interaction.

Laser-excited hot-electron induced desorption: A theoretical model applied to NO/Pt(111)

We present a theoretical model for stimulated desorption due to the interaction of energetic substrate carriers with molecular adsorbates. The model is based on the premise that optically excited hot electrons scatter into an unoccupied valence electron resonance of the adsorbate, thus forming a temporary negative molecular ion which then experiences an enhanced attraction towards the substrate. Neutralization of the ion returns the adsorbed molecule to one of the continuum states of the molecule/substrate potential energy surface, possibly in an internally excited state. The consequences of such a model are worked out using semiclassical wave packet dynamics which, in the short time limit relevant to the present situation, can be brought to an analytic realization. The model provides considerable insight into recent experiments on the laser-induced desorption of NO from Pt(111).

Laser-induced desorption of NO from Si(111): effects of coverage on NO vibrational populations

Laser-induced desorption of NO from Si(111) has been investigated using state-specific detection techniques. The observed energy partitioning in the desorbed NO varies significantly with the NO coverage. Characterization of the energy partitioning with desorption-laser wavelengths in the range 355–1907 nm indicates that different substrate electronic excitations are responsible for the desorption at low NO coverage vs saturation coverage. The relationship between the substrate electronic excitation and observed vibrational-state-population distributions is explored.

State-resolved evidence for hot carrier driven surface reactions: Laser-induced desorption of NO from Pt(111)

State-specific diagnostics are used to characterize the laser-induced desorption of NO from Pt(111). Two desorption channels are observed; one is consistent with thermal activation, while the other is driven by adsorbate interactions with hot carriers. For this latter channel, the observed dependence of the desorption yield on the wavelength of the incident laser pulse (1907, 1064, 532, and 355 nm) and the wavelength dependence of the kinetic energy distributions establish the nonthermal nature of the excitation process. The inverted spin–orbit population, the non-Boltzmann rotational state distributions, and the vibrational state population are interpreted in terms of a desorption mechanism involving a temporary ion resonance.

Non-Boltzmann rotational and inverted spin–orbit state distributions for laser-induced desorption of NO from Pt(111)

The internal state distributions of NO desorbed from a Pt(111) surface by visible and near-visible laser radiation (355, 532, and 1064 nm) were measured by laser-induced fluorescence. Non-Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption-laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short-lived negative-ion resonance intermediate.

Optically driven surface reactions: Evidence for the role of hot electrons.

Evidence is presented for the role of excited conduction electrons in the laser-induced desorption of NO from Pt(111). State-specific detection of the desorbed NO establishes that the rotational distributions are non-Boltzmann, that the spin-orbit population is inverted, and that both the translational and vibrational distributions are uncorrelated with the laser-induced surface-temperature jump. The role of optically excited substrate electrons in driving the desorption process is evidenced by a dramatic dependence of the vibrational and translation energy distributions on the desorption-laser wavelength.

Laser-induced desorption: Thermal and nonthermal pathways

Distributions of internal states and kinetic energies are reported for the laser-induced desorption of NO from a platinum foil. Two desorption channels are observed, one with internal-state populations which are well fit by Boltzmann distribution functions that correspond to the peak surface temperature. The second desorption channel displays population distributions which are non-Boltzmann: kinetic energies exceed the peak surface temperature by a factor of ≈5; kinetic energies depend on J; the spin–orbit states are inverted; and the vibrational population exceeds that associated with the maximum surface temperature by a factor of 50. The wavelength dependence of the non-Boltzmann desorption signal suggests that a substrate mediated desorption process is operative.