The effect of molecular vibrations and surface structure on the chemisorption of methane on platinum

In this thesis, I report state-resolved measurements of the chemisorption probability of CH4 on Pt(111) and Pt(110)-(1×2) for several rovibrationally excited states (2ν3, ν1+ν4, and 2ν2+ν4) in addition to the ground state. Measurements of the state resolved reactivity as function of the incident translational energy lead to state-resolved reactivity curves for each of the states under study. The relative efficacy of activating the dissociation reaction is obtained for each excited state by comparing the increase in reactivity observed upon excitation of a particular state to the effect of increasing the translational energy of CH4 in the ground state. The results provide clear evidence for mode specific reactivity with the highest efficacy for the stretch-bend combination (ν1+ν4), followed by the stretch overtone (2ν3) and the bend overtone state (2ν2+ν4). The results demonstrate that vibrational activation of CH4/Pt chemisorption process does not simply scale with the total internal energy of the incident CH4 molecule, which is a central assumption of the PC-MURT statistical model for dissociative chemisorption reactions developed by the group of Harrison [Ukraintsev et al., Chem. Phys., 1994. 101(2): p. 1564]. On the contrary, the qualitative predictions of the vibrationally adiabatic model proposed by Halonen et al. [J. Chem. Phys., 2001. 115(12): p. 5611] are in good agreement with our results. The higher efficacy of the ν1+ν4 state can also be rationalized by observing that, at the transition state, the breaking C-H bond is both stretched and bent from its equilibrium geometry, therefore I suggest that this state might have a significant projection on the reaction coordinate [Psofogiannakis et al., J. Phys. Chem. B, 2006. 110 : p. 24593 ; Anghel et al., Phys. Rev. B, 2005. 71 : p. 4]. Comparison between the state-resolved reactivity for CH4(2ν3) on Pt(111) and Ni(111) is used to obtain information about differences in barrier height and transition state location for the dissociation on the two different metals [Bisson et al., J. Phys. Chem., 2007. 111: p. 12679]. Finally, for the more corrugated Pt(110)-(1×2) surface, I determined the state-resolved sticking coefficients for different polar and azimuthal angles of incidence. Comparison between the reaction probability for incidence parallel and perpendicular to the missing rows of this surface shows shadowing effects that are consistent with predominant reactivity of the top layer Pt atoms.

State-Resolved Reactivity of CH4(2ν3) on Pt(111) and Ni(111): Effects of Barrier Height and Transition State Location

Rainer Beck, Régis Bisson, Thanh Tung Dang, Plinio Maroni, Marco Sacchi, Bruce Yoder

Quantum state-resolved sticking coefficients on Pt(111) and Ni(111) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2ν3) at well defined kinetic energies in the range of 10-90 kJ/mol. The ground state reactivity of CH4 is approximately 3 orders of magnitude lower on Ni(111) than on Pt(111) for kinetic energies in the range of 10-64 kJ/mol, reflecting a difference in barrier height of 28 ± 6 kJ/mol. 2ν3 excitation of CH4 increases its reactivity by more than four orders of magnitude on Ni(111), whereas on Pt(111) the reactivity increase is lower by two orders of magnitude. We discuss the observed differences in the state-resolved reactivity for the ground state and 2ν3 excited state of methane in terms of a difference in barrier height and transition state location for the dissociation reaction on the two metal surfaces.

2007

Quantum state-resolved studies of direct and precursor-mediated dissociative chemisorption of silane on Si(100)

Thanh Tung Dang

In this work, I present the results of my studies on the state-resolved reactivity of silane (SiH4) on the Si(100)-(2x1) surface. The results demonstrate a co-existence of both a direct and a precursor mediated mechanisms for the dissociative chemisorptions of SiH4 on Si(100)-(2x1). Above an incident kinetic energy of 34 kJ/mol, where SiH4 chemisorption occurs via the direct mechanism, I find that both translation and vibrational energy activate the chemisorptions. Vibrational energy is slightly less effective than translation energy in promoting the reaction since adding 51.6 kJ/mol of vibrational energy leads to the same reactivity increases as adding 47 kJ/mol of translational energy. Vibrational excitation leads to at most a factor of 4 increase in reactivity of SiH4 on Si(100)-(2x1). While translational activation was reported before [Jones et. al, Chem. Phys. Lett., 1994, 229, 401], this is the first time vibrational activation is observed for SiH4 chemisorption on Si(100)-(2x1). I also investigate mode-specific reactivity by comparing the reactivity of SiH4 excited to two different nearly isoenergetic vibrational states, |2000> and |1100>. The results show that the reactivity of SiH4 excited to the |2000> state is consistently higher than that of the |1100> state over the range of incident kinetic energy studied. This mode-specific reactivity of SiH4 on Si(100)-(2x1) is rationalized in terms of different vibrational amplitude of the |2000> and the |1100> states: the former contains two quanta of Si-H stretch in a single Si-H bond, whereas the latter contains one quantum of Si-H stretch in each of two Si-H bonds. Below 34 kJ/mol, where SiH4 chemisorption on Si(100)x(2x1) occurs via the precursor-mediated mechanism, the effect of internal vibrational energy on the reactivity is found for the first time. While increasing translational energy decreases SiH4 reactivity, vibrational energy increases its reactivity. After the incident SiH4 molecule is trapped in the weakly bound precursor state, its vibrational energy appears to be conserved long enough that it can still promotes the chemisorption reaction once the trapped SiH4 encounters a reactive site. The chemisorption of SiH4 is, however, also weakly activated with vibrational energy in the precursor-mediated mechanism, in which a reactivity enhancement of less than a factor of 2 is observed for vibrationally excited SiH4. The use of isotopic selective excitation in deposition experiments to control the isotopic abundance of Si is not successful due to the fact that SiH4 chemisorption on Si(100)-(2x1) is weakly activated with vibrational energy. With a reactivity enhancement of at most a factor of 4, the relative isotope abundance of 28Si is expected to increase by only 0.6% in the laser excitation experiment. Such a small change in the natural isotopic abundance could not be detected reliably in the limited analysis time by our SIMS setup. However, this experiment can be applied to other system, where vibration excitation is more efficient to promote the reaction. Furthermore, vibrational activation of precursor-mediated gas-surface reaction discovered in this work will open up a new possibility to control gas-surface reaction of physisorbed species on surfaces.

EPFL2007

Photoexcitation and photoionization dynamics of doped liquid helium-4 nanodroplets

Evgeniy Loginov

The photoexcitation and photoionization dynamics of sodium atoms deposited on the surface of helium nanodroplets and aromatic molecules (aniline, phenol and toluene) embedded in the interior of droplets have been investigated by a variety of spectroscopic techniques. The mean droplet sizes varied in the range of ≈ 2 000 - 20 000 atoms. For the first time, the excitation spectra of Na-doped helium droplets corresponding to Rydberg states of Na atoms have been measured from the lowest excited 3p state up to the ionization threshold. All lines in the excitation spectra are shifted and broadened with respect to atomic lines. The desorption of bare excited Na atoms and NaHen, n=1-4, exciplexes was observed upon excitation. The experiments revealed that the relative abundance of desorbed species, their internal energy states, their speed and angular distributions are specific to the state of sodium atom to which it was excited to on the droplet's surface. In the lowest excited states (3p, 4s, 3d and 4p), we observed rather regular dynamics. The photoelectron spectroscopy of products revealed the desorption of excited sodium atoms in the initially excited state and in lower lying states which were populated by radiative decay of the higher ones. The velocity distributions showed interesting characteristics: the mean kinetic energy of desorbed sodium increased linearly with excitation frequency, while angular anisotropy varied monotonically. In contrast, the recorded velocity distributions of exciplexes did not manifest systematically such regular properties. It was found that the excitation spectra and relative abundances of exciplexes depended on the mean droplet size, while the velocity distributions of desorbed species were, in general, not dependent on size. The tentative explanation of the observed features is based on the approximation that "Na-helium nanodroplet" interaction potentials can be described by a sum of Na-He pair potentials over the helium atoms constituting the nanodroplet. The shapes and shifts of the transitions to low excited states of Na obtained within this model are in a good agreement with experimental observations. The velocity distributions of desorbed Na atoms can be qualitatively interpreted by the overall interaction potentials of sodium with a helium nanodroplet where they act as counterparts in a diatomic system. In contrast, the NaHen exciplex formation on the surface of nanodroplets appears to be mainly governed by the Na-He pair potentials. The excitation to higher than 4p states of Na on the droplet's surface revealed complex situations. The experimental spectra correlated poorly to those calculated within the pair-wise interaction model. The velocity distributions of desorbed sodium and exciplexes often consisted of two components compared to one in the lower states. The photoelectron spectra disclosed the presence of species in the states that could not be populated by radiative decay of the higher ones, indicating that relaxation plays an important role during the detachment. The experiments on the photoionization of Na-doped helium droplets revealed that their ionization threshold is red-shifted with respect to a free sodium atom. The time-of-flight mass spectra showed that the directly created Na+ ion was solvated in the droplet. The indirect evidence of creating special Na-droplets in high Rydberg states, when the ionic core remains inside the cluster and the weakly bound electron orbits outside, was obtained. The shape of ionization transition and the droplet size dependence of the ionization threshold are in line with the model of a pair-wise additive interaction of the created sodium ion with helium atoms in the nanocluster. The photoionization of aromatic molecules (aniline, phenol, and toluene) in helium droplets was studied with photoelectron spectroscopy. The photoelectron spectra resemble closely those of gas phase molecules except for the droplet size dependent shift. This shift is caused by the lowering of the ionization threshold upon solvation and can be readily estimated. The individual peaks in photoelectron spectra are broadened, which is thought to partially reflect the rearrangement of helium upon ion solvation. The droplet size and kinetic energy dependences of the peak broadening towards lower energy may be attributed to the relaxation of the photoelectrons as they pass through a helium droplet.