SAND IN THE LABORATORY. PRODUCTION AND INTERROGATION OF GAS PHASE SILICATES.

Given its technological importance, the literature abounds with models for plasma enhanced chemical vapor deposition of the SiH$_4$/O$_2$/Ar system. In a continuing effort to identify and characterize the optical spectra of Si$_3$ generated in a SiH$_4$/Ar pulsed discharge sourcefootnote{The electronic spectrum of Si$_3$ I: the triplet D$_{3h}$ system� Reilly, N. J.; Kokkin, D. L.; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C., J. Chem. Phys. 136(19), 194307, 2012.}, we detected, via two dimensional (2D) LIF, a relatively strong electronic transition in the 570-600,nm region that is strongly enhanced by the addition of a small amount of O$_2$. The excitation spectrum shows resolved band structure at the pulsed laser resolution of 0.5,cm$^{-1}$ and exhibits a radiative lifetime of 1.97,$mu$s. The dispersed fluorescence exhibits three vibrational progressions and an unusually small splitting of approximately 50,cm$^{-1}$. Here we report on efforts to identify the molecular carrier of these bands, with particular interest paid to species resulting from oxygen impurities in the silane discharge

Effect of Facial Encumbrance on Excimer Formation and Charge Resonance Stabilization in Model Bichromophoric Assemblies

Excimer formation and charge resonance stabilization in covalently linked bichromophoric systems with flexible spacers are important processes relevant to biochemistry and functional materials. Requiring a π-stacked cofacial arrangement of a pair of aromatic molecules at a van der Waals contact, the underlying geometrical reorganization that accompanies these events continues to be debated. Here we use a variety of methods including two-color resonant two-photon ionization spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy (HB), and laser-induced fluorescence (LIF) excitation and emission spectroscopy to compare the gas-phase spectroscopy and dynamics of the van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1). The goal of this work is to probe the influence of methyl substitution on the dynamics of excimer formation and charge resonance (CR) stabilization. The fluorene dimer, (F)2, displays lifetime broadened electronic spectra and the dominance of excimer emission, consistent with a rapid (picoseconds) formation of a π-stacked excimer upon electronic excitation. Ion yield measurements of (F)2 reveal a lowering of the ionization potential (IP) by some 0.38 eV relative to the monomer, reflecting significant CR stabilization. These trends are mirrored in the 9-methylfluorene dimer, (MF)2, as one face of the π-system remains open. In contrast, the electronic spectrum of the dimethyl-substituted dimer, (F1)2, shows narrow features representing a single band system, and analysis of the torsional structure in dispersed fluorescence spectra identifies this as emission from the locally excited state of a tilted (non-π-stacked) dimer, with no evidence of excimeric emission. The structure of this dimer reflects the increased importance of C–H/π interactions in the dimethyl-substituted system, as increased steric constraints block a cofacial approach. The IP of (F1)2 shows CR stabilization which is roughly 1/2 of that in π-stacked (F)2 dimer. Extensive theoretical calculations support these findings and show the importance of sandwich-type configurations for excitonic delocalization and CR stabilization

THE QUINTESSENTIAL BOND OF MODERN SCIENCE. THE DETECTION AND CHARACTERIZATION OF DIATOMIC GOLD SULFIDE, AuS.

The gold sulfur bond is becoming ever more important to a vast range of scientific endeavors. We have recorded the electronic spectrum of gas-phase AuS, at vibrational resolution, over the 440-740,nm wavelength range. By application of a synergy of production techniques, hot hollow-cathode sputtering source and cold laser ablation molecular beam source, excitation from both spin components of the inverted $^2Pi$ ground state is possible. Excitation into four different excited electronic states involving approximately 100 red-degraded bands has been observed. The four excited states have been characterized as $a^4Sigma_{1/2}$, $A^2Sigma^+_{1/2}$, $B^2Sigma^-_{1/2}$ and $C^2Delta_i$. The observed red-degraded vibronic bands where then globally analyzed to determine an accurate set of term energies and vibrational constants for the excited and ground electronic states. The electronic configurations from which these states arise will be discussed

Cofacially Arrayed Polyfluorenes: Spontaneous Formation of π-Stacked Assemblies in the Gas Phase

Understanding geometrical and size dependencies of through-space charge delocalization in multichromophoric systems is critical to model electron transfer and transport in materials and biomolecules. In this work, we examine the size evolution of hole delocalization in van der Waals clusters of fluorene (i.e., (F)n), where a range of geometries are possible, reflecting both π-stacking and C–H/π interactions. Using mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), we measure electronic spectra and vertical ionization potentials (IPs) in the gas phase. Results are compared with model covalently linked assemblies (denoted Fn), exhibiting a sterically enforced cofacial (i.e., π-stacked) orientation of chromophores. For both systems, an inverse size dependence (i.e., 1/n) of IP vs cluster size is found. Surprisingly, the values for the two sets fall on the same line! This trend is examined via theory, which emphasizes the important role of π-stacking, and its geometrical dependencies, in the process of hole delocalization in multichromophoric assemblies

Strength of π-Stacking, from Neutral to Cation: Precision Measurement of Binding Energies in an Isolated π-Stacked Dimer

π-Stacking interactions are ubiquitious across chemistry and biochemistry, impacting areas from organic materials and photovoltaics to biochemistry and DNA. However, experimental data is lacking regarding the strength of π-stacking forces—an issue not settled even for the simplest model system, the isolated benzene dimer. Here, we use two-color appearance potential measurements to determine the binding energies of the isolated, π-stacked dimer of fluorene (C13H10) in ground, excited, and ionic states. Our measurements provide the first precise values for π-stacking interaction energies in these states, which are key benchmarks for theory. Indeed, theoretical predictions using ab initio and carefully benchmarked DFT methods are in excellent agreement with experiment

C–H/π and C–H–O Interactions in Concert: A Study of the Anisole–Methane Complex using Resonant Ionization and Velocity Mapped Ion Imaging

Noncovalent forces such as hydrogen bonding, halogen bonding, π–π stacking, and C–H/π and C–H/O interactions hold the key to such chemical processes as protein folding, molecular self-assembly, and drug–substrate interactions. Invaluable insight into the nature and strength of these forces continues to come from the study of isolated molecular clusters. In this work, we report on a study of the isolated anisole–methane complex, where both C–H/π and C–H/O interactions are possible, using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI). Using 2CAP and VMI, we derive the binding energies of the complex in ground, excited, and cation radical states. The experimental values from the two methods are in excellent agreement, and they are compared with selected theoretical values calculated using density functional theory and ab initio methods. The optimized ground-state cluster geometry, which is consistent with the experimental observations, shows methane sitting above the ring, interacting with anisole via both C–H/π and C–H/O interactions, and this dual mode of interaction is reflected in a larger ground-state binding energy as compared with the prototypical benzene–methane system

The Role of Torsional Dynamics on Hole and Exciton Stabilization in π‐Stacked Assemblies: Design of Rigid Torsionomers of a Cofacial Bifluorene

Exciton and charge delocalization across π‐stacked assemblies is of importance in biological systems and functional polymeric materials. To examine the requirements for exciton and hole stabilization, cofacial bifluorene (F2) torsionomers were designed, synthesized, and characterized: unhindered (model) MeF2, sterically hindered tBuF2, and cyclophane‐like CF2, where fluorenes are locked in a perfect sandwich orientation via two methylene linkers. This set of bichromophores with varied torsional rigidity and orbital overlap shows that exciton stabilization requires a perfect sandwich‐like arrangement, as seen by strong excimeric‐like emission only in CF2 and MeF2. In contrast, hole delocalization is less geometrically restrictive and occurs even in sterically hindered tBuF2, as judged by 160 mV hole stabilization and a near‐IR band in the spectrum of its cation radical. These findings underscore the diverse requirements for charge and energy delocalization across π‐stacked assemblies

π-π stacking vs. C–H/π interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9′-dimethylfluorene

Studies of exciton and hole stabilization in multichromophoric systems underpin our understanding of electron transfer and transport in materials and biomolecules. The simplest model systems are dimeric, and recently we compared the gas-phase spectroscopy and dynamics of van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1) to assess how sterically controlled facial encumbrance modulates the dynamics of excimer formation and charge resonance stabilization (CRS). Dimers of fluorene and MF show only excimer emission upon electronic excitation, and significant CRS as evidenced in a reduced ionization potential for the dimer relative the monomer. By contrast, the dimer of F1 shows no excimeric emission, rather structured emission from the locally excited state of a tilted (non π-stacked) dimer, evidencing the importance of C–H/π interactions and increased steric constraints that restrict a cofacial approach. In this work, we report our full results on van der Waals clusters of F1, using a combination of theory and experiments that include laser-induced fluorescence, mass-selected two-color resonant two-photon ionization spectroscopy, and two-color appearance potential measurements. We use the latter to derive the binding energies of the F1 dimer in ground, excited, and cation radical states. Our results are compared with van der Waals and covalently linked clusters of fluorene to assess both the relative strength of π-stacking and C–H/π interactions in polyaromatic assemblies and the role of π-stacking in excimer formation and CRS

The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri-hexabenzocoronene, C42H18

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to have an oscillator strength of f=1.4x10^-3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12 cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.Comment: 9 pages, 1 figure. Fixed a typo on the frequency of the 'b' ban

Application of two dimensional flourescence spectroscopy to transition metal clusters

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.9]{NiONiO2.eps} \end{wrapfigure} Determining the physical properties (bond lengths, angles, dipole moments, etc) of transition metal oxides and dioxides is relevant to catalysis, high temperature chemistry, materials science and astrophysics. Analysis of optical spectra is a convenient method for extraction of physical properties, but can be difficult because of the density of electronic states and in the case of the dioxides, presence of both the oxide and superoxide forms. Here we demonstrate the application of two dimensional fluorescence spectroscopy\footnote{N.J. Reilly, T.W. Schmidt, S.H. Kable, \textit{J. Phys. Chem. A.}, 110(45), 12355-12359, 2006} for aiding in the assignment and analysis. Particular attention will be paid to the spectroscopy of first row transition metal monoxides and dioxides of Nickel, NiO and NiO$_2$, and Manganese, MnO. Furthermore, the application of this technique to discovering the spectrum of other transition metal systems such as Metal-dicarbides will be outlined