Scott G. Sayres

Photoelectron imaging of small silicon cluster anions, Sin− (n=2–7)

Photoelectron imaging experiments were conducted on small silicon cluster anions, Si(n) (-) (n=2-7), acquired at a photon energy of 3.49 eV (355 nm). Electronic transitions arising from the anion ground states are observed, and the evaluated vertical detachment energies agree well with previous measurements and theoretical calculations. The anisotropy beta parameters have also been determined for each unique feature appearing in the photoelectron angular distributions at the employed photon energy. Separate calculations using density functional theory are also undertaken to determine the relative atomic orbital contributions constructing the interrogated highest occupied and low-lying molecular orbitals of a specific cluster. A method to interpret the observed cluster angular distributions, term the beta-wave approach, is then implemented which provides quantitative predictions of the anisotropy beta parameter for partial wave emission from molecular orbitals partitioned by varying contributions of atomic orbital angular momenta. Highlighted in the beta-wave analysis is the ability of discriminating between disparate molecular orbitals from two nearly isoenergetic structural isomers of opposing point group symmetry for the Si(4) (-) and Si(6) (-) cluster ions, respectively.

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.

Strong-field ionization and dissociation studies on small early transition metal carbide clusters via time-of-flight mass spectrometry.

In this work, we report experimental results from the strong-field ionization and subsequent Coulomb explosion of narrow distributions of small (<40 atoms) heteronuclear clusters composed of transition metal (Ti, V, Cr, Nb, or Ta) and carbon atoms. Analysis of the resulting multiply charged ions was performed through time-of-flight mass spectrometry, and evidence regarding ionization dynamics was obtained. The data reveal the presence of enhanced ionization during exposure to the ultrashort (∼100 fs) pulse resulting in the formation of ions possessing significantly higher charge states than those predicted from atomic species. Regardless of the transition metal species, we observe the absorption of similar amounts of energy from the external field, as indicated by the maximum observed charge states in each experiment. These results are compared to our previously reported study on the strong-field ionization of transition metal oxide clusters. We observe identical maximum observable charge states for each of the transition metal species resulting from both metal oxide and metal carbide clusters.

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.

Exposing the Role of Electron Correlation in Strong-Field Double Ionization: X-ray Transient Absorption of Orbital Alignment in Xe+ and Xe2+

Orbital alignment measurements and theory are used to examine the role of electron correlation during atomic strong-field double ionization (795 nm, (1-5) × 10(14) W cm(-2)). High-order harmonic, transient absorption spectroscopy is used to measure the angular distributions of singly and doubly tunnel-ionized xenon atomic states via 4d core to 5p valence shell transitions between 55 and 60 eV. The experimental MJ alignment distributions are compared to results of a rate-equation model based on sequential ionization, previously developed for coherent electron motion, and now applied to account for the alignment prepared by tunneling ionization. The hole generated in the (2)P3/2 state of Xe(+) is measured to be entirely composed of |MJ| = 1/2, in agreement with theory. The result is a higher degree of alignment than previously reported. Because the model neglects effects of electron-ion recollision, the theory predicts a high degree of alignment in both spin-parallel (triplet) and antiparallel (singlet) terms of Xe(2+). However, the alignment generated with linearly polarized light is observed to be spin-state dependent. The measured alignments for triplet spin states ((3)P2 has |MJ| = [0 : 1 : 2] of [27±6 : 45±11 : 29±0] and (3)P1 has |MJ| = [0 : 1] of [56±2 : 44±2]) are in good agreement with the expectations of theory, which are [33 : 53 : 14] and [66 : 33], respectively. The results validate the rate equation model for sequential tunnel ionization. However, the alignment extracted for a singlet state is greatly diminished: (1)D2 is measured to be [18±1 : 39±2 : 43 ± 2] compared to theoretical expectation of [60 : 39 : 1] for |MJ| = [0 : 1 : 2]. The poor agreement with the sequential ionization model suggests that the alignment of (1)D2 is strongly influenced by the high propensity for the liberated first electron to return to and recollide with its parent atomic orbital. Therefore, although the influence of electron recollision appears minor in the triplet states and suggests sequential ionization, electron correlation between the ionic core and the first ionized electron cannot be ignored in the singlet state. Singlet states are likely to be generated through nonsequential double ionization over the intensity range where the experiments are performed.