Matt W. Ross

Delocalized electronic behavior observed in transition metal oxide clusters under strong-field excitation

Heterogeneously composed clusters are exposed to intensity resolved, 100 fs laser pulses to reveal the energy requirements for the production of the high charge states of both metal and nonmetal ions. The ionization and fragmentation of group V transition metal oxide clusters are here examined with laser intensities ranging nearly four orders in magnitude (∼3 × 10(11) W/cm(2) to ∼2 × 10(15) W/cm(2)) at 624 nm. The ionization potentials of the metal atoms are measured using both multiphoton ionization and tunneling ionization models. We demonstrate that the intensity selective scanning method can be utilized to measure the low ionization potentials of transition metals (∼7 eV). The high charge states demonstrate an enhancement in ionization that is three orders of magnitude lower in laser intensity than predicted for the atomic counterparts. Finally, the response from the various metals and the oxygen is compared to elucidate the mechanism of enhanced ionization that is observed. Specifically, the sequence of ion appearances demonstrates delocalized electron behavior over the entire cluster.

Influence of clustering and molecular orbital shapes on the ionization enhancement in ammonia

The Coulomb explosion of clusters is known to be an efficient source for producing multiply charged ions through an enhanced ionization process. However, the factors responsible for obtaining these high charge states have not been previously explored in detail and remain poorly understood. By comparing intensity-resolved visible laser excitation experiments with semi-classical theory over a range spanning both multiphoton and tunneling ionization regimes, we reveal the mechanism in which extreme ionization proceeds. Under laser conditions that can only singly ionize individual molecules, ammonia clusters generate ions depleted of all valence electrons. The geometries of the molecular orbitals are revealed to be important in driving the ionization, and can be entirely emptied at the energy requirement for removal of the first electron in the orbital. The results are in accord with non-sequential ionization arising from electrons tunneling from three separate molecular orbitals aided through the ionization ignition mechanism.

Onset of Coulomb explosion in small silicon clusters exposed to strong-field laser pulses

It is now well established that, under intense laser illumination, clusters undergo enhanced ionization compared to their isolated atomic and molecular counterparts being subjected to the same pulses. This leads to extremely high charge states and concomitant Coulomb explosion. Until now, the cluster size necessary for ionization enhancement has not been quantified. Here, we demonstrate that through the comparison of ion signal from small covalently bound silicon clusters exposed to low intensity laser pulses with semi-classical theory, their ionization potentials (IPs) can be determined. At moderate laser intensities the clusters are not only atomized, but all valence electrons are removed from the cluster, thereby producing up to Si4+. The effective IPs for the production of the high charge states are shown to be ?40% lower than the expected values for atomic silicon. Finally, the minimum cluster size responsible for the onset of the enhanced ionization is determined utilizing the magnitude of the kinetic energy released from the Coulomb explosion.

Ionization and Coulomb explosion of small group 10 transition metal oxide clusters in strong light fields

The ionization properties of small group 10 metal oxide clusters are explored using ultrafast pulses centered at 624 nm. Maximum atomic charge states resulting from Coulomb explosion were observed to be Ni(3+), Pd(3+), Pt(5+), and O(2+) species with similar ionization potentials ~30-35 eV. Ion signal as a function of laser intensity of each charge state of Ni, Pd, Pt, and O resulting from Coulomb explosion was mapped and compared to that predicted from semi-classical tunneling theory using sequential ionization potentials to quantify observed enhancements in ionization. The saturation intensity (I(sat)) of each charge state is measured and compared to previous studies on group 5 transition metal oxides. The atomic charge states of nickel showed a large enhancement in ionization compared to palladium and platinum, reflective of the differing bonding properties of each metal with oxygen. Results indicate that nickel oxide clusters undergo a greater extent of ionization enhancement as a result of multiple ionization mechanisms. The ionization enhancement behavior of each metal oxide species is explored herein.

Ultrafast ionization and subsequent coulomb explosion of zirconium oxide and tungsten carbide “superatomic” cluster species and comparison to group 10 metals

Ultrafast pulses centered at 624 nm are used to explore the ionization behavior of small zirconium oxide, palladium oxide, tungsten carbide, and platinum clusters. ZrxOy and WxCy have been identified as “superatom” species as they are isovalent with PdxOy and PtxCy clusters, respectively. Intensity selective scanning (ISS) technique is used to study the high charge states of each metal as a function of laser intensity. The ion signals of each atomic charge state are compared to those predicted from semi-classical atomic tunneling theory using sequential ionization potentials. A clear difference in ion signal appearance potentials is observed in early-group transition metal clusters vs. late transition metal clusters, indicating the significance of the d-orbitals in the overall ionization behavior of these clusters. The ionization behavior of each cluster species is explored in the context of transition metal clusters.

Structure, Reactivity, and Fragmentation of Small Multi-Charged Methane Clusters

Small methane clusters (CH4)n are irradiated using intense femtosecond laser excitation at 624 nm. The ionized species and those resulting from their fragmentation are detected via time-of-flight mass spectrometry (TOF MS). We find evidence of bound, multiply charged methane molecules and clusters resulting from Coulomb explosion upon exposure to highly energetic, ultrafast radiation. The assignment of the mass spectra is done after first-principles calculations (at the (R)MP2/aug-cc-pVXZ (X = D,T) level) on the charged (CH4)n(q+) clusters (n = 1-4, q = 1-4). We also considered the cluster stabilities and fragments that may result from intracluster molecular reactivity. Complex intracluster ion-molecule reactions induced by photoionization are expected to occur. Interestingly, we show that multi charged small methane clusters undergo intracluster reactions and fragmentations which are different from those observed for isolated methane ions or for large ionized methane clusters.

Extreme Ionization Leading to Coulomb Explosion of Small Palladium and Zirconium Oxide Clusters and Reactivity with Carbon Monoxide

Reported herein are strong-field ionization studies of small, neutral Pd(x)O(y) and Zr(x)O(y) clusters made using ultrafast laser pulses (~100 fs) centered at 624 nm. An enhancement in ionization of nearly 1.5 orders of magnitude lower in laser intensity than predicted from literature values is observed for both systems due to clustering. The change in enhancement upon addition of carbon monoxide at different pressures was also studied. Enhancement of high charge states of palladium was found to decrease upon CO addition, whereas in the case of the zirconium system, high charge states of zirconium were observed to increase. Pd and ZrO showed similar reactivity trends with CO and were found to have similar reactivity ratios in accord with their isovalent nature.

Ionization and Coulomb explosion of small uranium oxide clusters

Femtosecond pulses are used to study the strong-field ionization and subsequent Coulomb explosion of small uranium oxide clusters. The resulting high atomic charge states are explored as a function of laser intensity and compared to ionization rates calculated using semi-classical tunneling theory with sequential ionization potential values. The gap in laser intensity between saturation intensity values for the 7s, 6d, and 5f orbitals are identified and quantified. Extreme charge states of oxygen up to O4+ are observed indicating multiple ionization enhancement processes occurring within the clusters. The peak splittings of the atomic charge states are explored and compared to previous results on transition metal oxide species. Participation of the 5f orbitals in bonding is clearly identified based on the saturation intensity dependence of oxygen to uranium metal.

Strong-field ionization and coulomb explosion of chlorine weakly bound to small water clusters.

Clusters exhibit an enhancement in ionization rates under intense, ultrafast laser pulses compared to their molecular/atomic counterparts. Studies of ionization enhancement of weakly bound molecules to clusters have not been previously characterized and quantified. We demonstrate that weakly bound ClO to (H(2)O)(n) (n = 1-12) clusters and weakly bound HCl to (H(2)O)(n) (n = 1-12) clusters produce high atomic charge states of chlorine via Coulomb explosion. Density functional theory (DFT) was used to qualitatively compare the interaction energy of ClO with respect to the number of water molecules as well as HCl with respect to the number of water molecules. The chlorine ion signal intensity for each atomic charge state was observed to be dependent on the molecule-cluster bond strength. The observed ionization enhancement was quantified using semiclassical tunneling theory, and it was found that the Cl(3+-5+) and O(2+) charge states are enhanced in ionization. Possible mechanisms of ionization enhancement are explored for weakly bound chlorine species.

Ionization enhancement in silicon clusters and germanium atoms in the presence of zirconium

Molecules/clusters have been shown to undergo an enhancement in ionization under ultrafast laser pulses. This enhancement results in the lowering of the laser intensity required to observe ion signal from higher atomic charge states resulting from Coulomb explosion of clusters. Here, we explore the effect of using an early-group transition metal as an electron source in the formation of small silicon clusters on the observed enhancement in ionization. Intensity selective scanning is used to measure the onset of ion signal for the atomic charge states of silicon, germanium, zirconium, and oxygen. Additionally, the kinetic energy released values for the resulting high charge states of silicon are measured and compared to those previously observed using a copper electron source. A significant increase in ionization enhancement is observed upon using zirconium metal, despite a decrease in cluster size. Germanium metal with zirconium is studied for comparison and shows a larger enhancement in ion signal than silicon, indicating that atomic mass may be significant.