Emission cross sections and line shapes for photodissociating triiodide in ethanol: Experimental and computational studies

Susceptibility of two-dimensional resonance Raman spectroscopies to cascades involving solute and solvent molecules

Two-dimensional resonance Raman (2DRR) spectroscopies have been used to investigate the structural heterogeneity of ensembles and chemical reaction mechanisms in recent years. Our previous work suggests that the intensities of artifacts may be comparable to the desired 2DRR response for some chemical systems and experimental approaches. In a type of artifact known as a "cascade," the four-wave mixing signal field radiated by one molecule induces a four-wave mixing process in a second molecule. We consider the susceptibility of 2DRR spectroscopy to various types of signal cascades in the present work. Calculations are conducted using empirical parameters obtained for a molecule with an intramolecular charge-transfer transition in acetonitrile. For a fully impulsive pulse sequence, it is shown that "parallel" cascades involving two solute molecules are generally more intense than that of the desired 2DRR response when the solute's mode displacements are 1.0 or less. In addition, we find that the magnitudes of parallel cascades involving both solute and solvent molecules (i.e., a solute-solvent cascade) may exceed that of the 2DRR response when the solute possesses small mode displacements. It is tempting to assume that solute-solvent cascades possess negligible intensities because the off-resonant Raman cross sections of solvents are usually 4-6 orders of magnitude smaller than that of the electronically resonant solute; however, the present calculations show that the difference in solute and solvent concentrations can fully compensate for the difference in Raman cross sections under common experimental conditions. Implications for control experiments and alternate approaches for 2DRR spectroscopy are discussed.

Photoelectron-photofragment coincidence studies of I3- using an electrospray ionization source and a linear accelerator

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to examine the dissociative photodetachment (DPD) of I3-. The high beam energy PPC spectrometer for complex anions couples an electrospray ionization source, a hexapole accumulation ion trap and a linear accelerator to produce fast beams of I3- (M = 381 amu) anions, the heaviest system studied to date. Following photodetachment, the photoelectron and up to three photofragments are recorded in coincidence yielding a kinematically complete picture of the DPD dynamics at beam energies of 11 keV and 21 keV. Photodetachment leads to the production of stable I3, two-body DPD, as well as evidence for two- and three-body photodissociation. DPD is found to occur predominantly via the first excited A state, with some contributions from highly excited vibrational levels in the neutral ground state. With the ions thermalized to 298 K in the hexapole trap, there are significant contributions from vibrational hot bands. Three-body photodissociation at 4.66 eV is found to occur preferentially via a charge-symmetric process to form I + I- + I. In the future this method will be applied to other polyatomic systems with a large molecular mass, including multiply charged anions and complex clusters, in concert with a cryogenically cooled hexapole trap to reduce thermal effects.

Chemical Reaction Dynamics at Liquid Interfaces: A Computational Approach

Recent advances in experimental and theoretical studies of liquid interfaces provide remarkable evidence for the unique properties of these systems. In this review we examine how these properties affect the thermodynamics and kinetics of chemical reactions which take place at the liquid/vapor interface and at the liquid/liquid interface. We demonstrate how the rapidly varying density and viscosity, the marked changes in polarity and the surface roughness manifest themselves in isomerization, electron transfer and photodissociation reactions.

Direct Metal Writing and Precise Positioning of Gold Nanoparticles within Microfluidic Channels for SERS Sensing of Gaseous Analytes

We demonstrate a one-step precise direct metal writing of well-defined and densely packed gold nanoparticle (AuNP) patterns with tunable physical and optical properties. We achieve this by using two-photon lithography on a Au precursor comprising poly(vinylpyrrolidone) (PVP) and ethylene glycol (EG), where EG promotes higher reduction rates of Au(III) salt via polyol reduction. Hence, clusters of monodisperse AuNP are generated along raster scanning of the laser, forming high-particle-density, well-defined structures. By varying the PVP concentration, we tune the AuNP size from 27.3 to 65.0 nm and the density from 172 to 965 particles/μm², corresponding to a surface roughness of 12.9 to 67.1 nm, which is important for surface-based applications such as surface-enhanced Raman scattering (SERS). We find that the microstructures exhibit an SERS enhancement factor of >10⁵ and demonstrate remote writing of well-defined Au microstructures within a microfluidic channel for the SERS detection of gaseous molecules. We showcase in situ SERS monitoring of gaseous 4-methylbenzenethiol and real-time detection of multiple small gaseous species with no specific affinity to Au. This one-step, laser-induced fabrication of AuNP microstructures ignites a plethora of possibilities to position desired patterns directly onto or within most surfaces for the future creation of multifunctional lab-on-a-chip devices.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data

Photodissociation pathways of a trihalide series are systematically investigated by globally fitting vibrational wavepacket signals in broadband transient absorption spectra.

Photodissociation of gas-phase I3−: Comprehensive understanding of nonadiabatic dissociation dynamics

Photodissociation of the gas-phase tri-iodide anion, I3-, was investigated using photofragment time of flight (TOF) mass spectrometry combined with the core extraction method. An analysis of the TOF profiles provided the kinetic energy and angular distributions of photofragment ions and photoneutrals, from which the photoproduct branching fractions were determined in the excitation energy range of 3.26-4.27 eV. The measurement has revealed that (1) in the entire energy range investigated, three-body dissociation occurs preferentially as the "charge-asymmetric" process I-(1S)+I(2P3/2)+I(2P3/2) with the yield of approximately 30%-40%, where the excess charge is localized on the end atoms of the dissociating I3-, and that (2) two-body dissociation via the 3Piu(0u+)<--1Sigmag+(0g+) excitation proceeds as I-(1S)+I2(X 1Sigmag+)/I2(A 3Pi1u) or I(2P3/2)+I2-(X 2Sigmau+) with the yield of approximately 60%, while that via the 1Sigmau+(0u+)<--1Sigmag+(0g+) excitation alternatively as I*(2P1/2)+I2-(X 2Sigmau+) or I-(1S)+I2(B 3Piu) with the yield of approximately 60%. Ab initio calculations including spin-orbit configuration interactions were also performed to gain precise information on the potential energy surfaces relevant to the I3- photodissociation. The calculations have shown the presence of conical intersections and avoided crossings located along the symmetric stretch coordinate near the ground-state equilibrium geometry of I3-, which play key roles for the two-body and the three-body product branching. The nonadiabatic nature of the I3- photodissociation dynamics is discussed by combining the experimental findings and the ab initio results.

Photodissociation of ICN at the liquid/vapor interface of water

The photodissociation of ICN adsorbed at the liquid/vapor interface of water is studied using classical molecular dynamics with nonadiabatic surface hopping. The cage escape, geminate recombination to form ICN and INC and the subsequent vibrational relaxation of these two molecules (on their ground electronic states) is compared with the same process in bulk water and with previous photodissociation studies at liquid interfaces. We find that the reduced surface density and weaker solvent-solute interactions give rise to reduced rate of nonadiabatic transitions and that the probability for cage escape at the interface is significantly enhanced due to the possibility that one or both of the photodissociation fragments desorb into the gas phase. The overall desorption probability varies from 75% to 92% for ICN initially located just below the Gibbs surface (50% bulk density) to ICN located just above the Gibbs surface, respectively. The corresponding geminate recombination probabilities are 18% and 9%, respectively. The vibrational relaxation rate of the recombined ICN is slower than in the bulk by a factor of 2.3.

The dependence of thymine and thymidine Raman spectra on solvent

Recent work has focused on developing Raman spectroscopy as a noninvasive probe of DNA interactions with solvents, intercalants, proteins, and other ligands. Here, we report the Raman spectra of thymine in eight solvents and thymidine in nine solvents obtained with visible excitation. Raman spectra under acidic, neutral, and basic conditions were also obtained of both thymine and thymidine. Changes in both the frequencies and intensities of several of the vibrational bands in the 800–1800 cm–1 region are observed. No evidence of deprotonation in the different solvents is observed for either thymine or thymidine. Correlations of the observed frequency shifts of specific vibrational modes with characteristic properties of the solvent for both thymine and thymidine show a significant correlation with acceptor and donor numbers, measures of the hydrogen-bonding ability of the solvent, in both thymine and thymidine. These results are interpreted in terms of hydrogen-bonding interactions between the N-H protons of the thymine base and lone pairs of electrons on the solvent molecules and between the solvent hydrogens and lone pairs on C=O sites. The solvent-dependent intensity in vibrational bands of thymine between 1500 and 1800 cm–1 indicates a strong interaction between thymine and solvent at the C=O and N-H sites that leads to separation of the C=O stretches from the C=C stretch. The intensity variations with solvent were much smaller for thymidine than for thymine, perhaps as a result of replacing the N1 proton by the sugar. These results suggest that Raman spectroscopy is uniquely sensitive to specific interactions of thymine and thymidine with their environment.Key words: Raman spectroscopy, thymine, thymidine, solvent effects, hydrogen bonding.

Time-resolved electron detachment imaging of the I[sup −] channel in I[sub 2]Br[sup −] photodissociation

The evolution of the I(-) channel in I(2)Br(-) photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I(-) fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a approximately 300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragment's electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, approximately 1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I(-) fragment.