Showing papers by "Richard J. Saykally published in 2010"

The structure of ambient water

Abstract: We review the spectroscopic techniques and scattering experiments used to probe the structure of water, and their interpretation using empirical and ab initio models, over the last 5 years. We show that all available scientific evidence overwhelmingly favors the view of classifying water near ambient conditions as a uniform, continuous tetrahedral liquid. While there are controversial issues in our understanding of water in the supercooled state, in confinement, at interfaces, or in solution, there is no real controversy in what is understood as regards bulk liquid water under ambient conditions. © 2010 Taylor & Francis.

Exciton Dynamics in CdS−Ag2S Nanorods with Tunable Composition Probed by Ultrafast Transient Absorption Spectroscopy

Abstract: Electron relaxation dynamics in CdS−Ag2S nanorods have been measured as a function of the relative fraction of the two semiconductors, which can be tuned via cation exchange between Cd2+ and Ag+. The transient bleach of the first excitonic state of the CdS nanorods is characterized by a biexponential decay corresponding to fast relaxation of the excited electrons into trap states. This signal completely disappears when the nanorods are converted to Ag2S but is fully recovered after a second exchange to convert them back to CdS, demonstrating annealing of the nonradiative trap centers probed and the robustness of the cation exchange reaction. Partial cation exchange produces heterostructures with embedded regions of Ag2S within the CdS nanorods. Transient bleaching of the CdS first excitonic state shows that increasing the fraction of Ag2S produces a greater contribution from the fast component of the biexponential bleach recovery, indicating that new midgap relaxation pathways are created by the Ag2S mate...

Investigation of protein conformation and interactions with salts via X-ray absorption spectroscopy

Abstract: Nitrogen K-edge spectra of aqueous triglycine were measured using liquid microjets, and the effects of Hofmeister-active salts on the spectra were observed. Spectra simulated using density functional theory, sampled from room temperature classical molecular dynamics trajectories, capture all major features in the measured spectra. The spectrum of triglycine in water is quite similar to that in the presence of chaotropic sodium bromide (and other halides), which raises the solubility of proteins. However, a new feature is found when kosmotropic Na 2 SO 3 , which lowers solubility, is present; this feature results from excitations of the nitrogen atom in the terminal amino group of triglycine. Both direct interactions between this salt and the protonated amino terminus, as well as corresponding changes in the conformational dynamics of the system, contribute to this new feature. These molecular measurements support a different mechanism for the Hofmeister effect than has previously been suggested based on thermodynamic measurements. It is also shown that near edge X-ray absorption fine structure (NEXAFS) is sensitive to strong direct interaction between certain salts and charged peptides. However, by investigating the sensitivity of NEXAFS to the extreme structural differences between model β-sheets and α-helices, we conclude that this technique is relatively insensitive to secondary structure of peptides and proteins.

Measurement of Bromide Ion Affinities for the Air/Water and Dodecanol/Water Interfaces at Molar Concentrations by UV Second Harmonic Generation Spectroscopy

Abstract: Recent experimental and theoretical work has demonstrated that certain anions can exhibit enhanced concentrations at aqueous interfaces and that the adsorption of bromide is particularly important for chemical reactions on atmospheric aerosols, including the depletion of ozone. UV second harmonic generation resonant with the bromide charge-transfer-to-solvent band and a Langmuir adsorption model are used to determine the affinity of bromide for both the air/water and dodecanol/water interfaces. The Gibbs free energy of adsorption for the former is determined to be −1.4 kJ/mol with a lower 90% confidence limit of −4.1 kJ/mol. For the dodecanol/water interface the data are best fit with a Gibbs free energy of +8 kJ/mol with an estimated lower limit of −4 kJ/mol.

Communication: Near edge x-ray absorption fine structure spectroscopy of aqueous adenosine triphosphate at the carbon and nitrogen K-edges

Abstract: Near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen and carbon K-edges was used to study the hydration of adenosine triphosphate in liquid microjets. The total electron yield spectra were recorded as a function of concentration, pH, and the presence of sodium, magnesium, and copper ions (Na+/Mg2+/Cu2+). Significant spectral changes were observed upon protonation of the adenine ring, but not under conditions that promote π-stacking, such as high concentration or presence of Mg2+, indicating that NEXAFS is insensitive to the phenomenon. Intramolecular inner-sphere association of Cu2+ did create observable broadening of the nitrogen spectrum, whereas outer-sphere association with Mg2+ did not.

Effect of Surface Active Ions on the Rate of Water Evaporation

Abstract: Current understanding of the vapor−liquid exchange kinetics of liquid water is incomplete, leading to uncertainties in modeling the climatic effects of clouds and aerosol. Initial studies of atmospherically relevant solutes (ammonium sulfate, sodium chloride) indicate that their effect on the evaporation kinetics of water is minimal, but all those constituent ions are also expected to be depleted in concentration at the air−water interface. We present measurements of the evaporation kinetics of water from 4 M sodium perchlorate solution, which is expected to have an enhanced concentration of perchlorate in the surface layer, using Raman thermometry of liquid microdroplets in a free evaporation regime. We determine the evaporation coefficient γe to be 0.47 ± 0.02, ca. 25% smaller than our measured value for pure water (0.62 ± 0.09). This change, while small, indicates that direct interactions between perchlorate ions and evaporating water molecules are affecting the evaporation mechanism and kinetics and s...

Importance of electronic relaxation for inter-coulombic decay in aqueous systems.

Abstract: Inspired by recent photoelectron spectroscopy (PES) experiments on hydroxide solutions, we have examined the conditions necessary for enhanced (and, in the case of solutions, detectable) intercoulombic decay (ICD)--Auger emission from an atomic site other than that originally excited. We present general guidelines, based on energetic and spatial overlap of molecular orbitals, for this enhancement of ICDbased energy transfer in solutions. These guidelines indicate that this decay process should be exhibited by broad classes of biomolecules and suggest a design criterion for targeted radiooncology protocols. Our findings show that PES cannot resolve the current hydroxide coordination controversy.

An analysis of the NEXAFS spectra of a molecular crystal: α-glycine

Abstract: The nitrogen K-edge near edge x-ray absorption fine structure spectrum of α-crystalline glycine has been calculated for temperatures ranging from 0 to 450 K. Significant temperature dependent spectral changes are predicted. The calculated room temperature spectrum is in good agreement with the experiment. At high temperatures, molecular motions strongly influence the spectrum, as any unique spectrum from an individual instantaneous configuration does not resemble the experimental result or the average calculated spectrum; complex coupled motions in this prototypical molecular crystal underlie the observed spectral changes.

Monopeptide versus Monopeptoid: Insights on Structure and Hydration of Aqueous Alanine and Sarcosine via X-ray Absorption Spectroscopy

Abstract: Despite the obvious significance, the aqueous interactions of peptides remain incompletely understood. Their synthetic analogues called peptoids (poly-N-substituted glycines) have recently emerged as a promising biomimetic material, particularly due to their robust secondary structure and resistance to denaturation. We describe comparative near-edge X-ray absorption fine structure spectroscopy studies of aqueous sarcosine, the simplest peptoid, and alanine, its peptide isomer, interpreted by density functional theory calculations. The sarcosine nitrogen K-edge spectrum is blue shifted with respect to that of alanine, in agreement with our calculations; we conclude that this shift results primarily from the methyl group substitution on the nitrogen of sarcosine. Our calculations indicate that the nitrogen K-edge spectrum of alanine differs significantly between dehydrated and hydrated scenarios, while that of the sarcosine zwitterion is less affected by hydration. In contrast, the computed sarcosine spectr...

Nuclear quantum effects in the structure and lineshapes of the N2 near-edge x-ray absorption fine structure spectrum

Abstract: We study the relative ability of several models of x-ray absorption spectra to capture the Franck-Condon structure apparent from an experiment on gaseous nitrogen. In doing so, we adopt the Born-Oppenheimer approximation and a constrained density functional theory method for computing the energies of the x-ray-excited molecule. Starting from an otherwise classical model for the spectrum, we systematically introduce more realistic physics, first by substituting the quantum mechanical nuclear radial density in the bond separation R for the classical radial density, then by adding the effect of zero-point energy and other level shifts, and finally by including explicit rovibrational quantization of both the ground and excited states. The quantization is determined exactly, using a discrete variable representation (DVR). We show that the near-edge x-ray absorption fine structure (NEXAFS) spectrum can be predicted semiquantitatively within this framework. We also address the possibility of non-trivial temperature dependence in the spectrum. By using constrained density functional theory in combination with more accurate potentials, we demonstrate that it is possible to improve the predicted spectrum. Ultimately, we establish the predictive limits of our method with respect to vibrational fine structure in NEXAFS spectra.

Recombination-pumped triatomic hydrogen infrared lasers

Abstract: Mid-infrared laser lines observed in hydrogen/rare gas discharges are assigned to three-body recombination processes involving an electron, a rare gas (He or Ne) atom, and the triatomic hydrogen ion (H3+). Calculations of radiative transitions between neutral H3 Rydberg states support this interpretation, and link it to recent results for hydrogenic/rare gas afterglow plasmas. A mechanism for the population inversion is proposed, and the potential generality and astrophysical implications of such molecular recombination laser systems are briefly discussed.

Special issue devoted to molecular complexes in our atmosphere and beyond

Abstract: This special issue is a follow up of the workshop organized by the Solvay Institutes on April 20–23, 2010, in Brussels, devoted to ‘Molecular complexes in our atmosphere and beyond’. Molecular complexes, viz. free, isolated, van der Waals and hydrogen-bonded molecules, both charged and neutral, are more often considered as potential contributors to several important physical and chemical processes in such environments as planetary atmospheres and interstellar clouds. There is an essential and growing need for more systematic experimental and theoretical investigations in the laboratory, which, in turn can stimulate dedicated remote sensing missions to explore these frontier molecular systems. The aim of the workshop was to gather experts from different scientific areas to design a viable plan for addressing these issues. One of the major features at the workshop was that our atmosphere invited itself via the now famous Eyjafjallajökull volcano eruption, which started the weekend before the workshop. The organization was maintained, but about one third of the 90 participants had to cancel their travel to Belgium. A special talk was actually devoted to the remote sensing of ashes and gases from the eruption and kindly delivered by Dr. Cathy Clerbaux (CQP, ULB and LATMOS, Paris VI), leading the atmospheric chemistry activities around the IASI/Metop monitoring satellite mission. Despite the eruption and disruption, the meeting produced a very interesting synopsis of the field. The sessions focused on ‘Experimental investigations in the laboratory’; ‘Molecular pairs in our atmosphere’; ‘Aerosols’; ‘Atmospheric water’; ‘Atmospheric chemistry’ and ‘Astro complexes’, successively. Among other topics, the role of the water dimer in our atmosphere, the importance of line profile modelling, the known but still overwhelming difficulty in accounting for non-rigidity and large amplitude motions in spectral analysis, the role of aerosols and clathrates, the present and future status on the investigation of Titan’s atmosphere and the increasing role of ever more focused satellite missions were all stressed. Theoretical and instrumental developments are progressing rapidly, opening new laboratory capabilities in this field. Experimentally, sophisticated new approaches, e.g. nonlinear optical and photofragment imaging, are regularly performed, the search for complexes in the overtone range, far above the dissociation energy, is successful, and synchrotron radiation promises to further stimulate the study of complexes in the range of the large amplitude vibrations. Aside from the water dimer, remote sensing of molecular complexes seems still much of a challenge, however, particularly in the interstellar medium. Recent and forthcoming missions might, however help in this respect, despite not being dedicated to the observation of weakly bound molecules. More specifically, spectral continua in various wavelength ranges may carry relevant information on dimers and provide unexpected access to their remote sensing. Finally, the role of molecular complexes was consistently addressed in various fundamental scientific phenomena such as nucleation, crystallization and evaporation, catalysis in atmospheric reactions, symmetry issues and statistical thermodynamics, usually from the powerful microscopic point of view. Important, focused messages were delivered at the workshop including the following: Yes, the water vapor dimer is very likely to be present in our atmosphere and yes it is expected to contribute a profound fraction of the radiation budget; beware of fakes when searching for evidence of complexes and make sure that monomer contributions are accounted for up to the latest details, including all refinement in the line profile investigation; H3 and H þ 5 are fundamental astrophysical species receiving lots of interest and deserving even more; aerosols and clathrates are most interesting and important objects, on which ‘not so low’ resolution spectroscopy may yield relevant information in the astro context. A small part of the excitement of this meeting has thus been transferred into this special issue and the guest editor warmly thanks all contributors for their special effort towards this limited, nevertheless quite rich output from the workshop. It is hoped that it provides some landmarks in the field.

Cavity Ringdown Laser Absorption Spectroscopy: History, Development, and Application to Pulsed Molecular Beams

Abstract: The measurement of electronic spectra of supersonically cooled molecules and clusters is a widely used approach for addressing many problems in chemistry. The most established techniques for making such measurements are laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI), and both have been employed very successfully in a large number of studies. However, both methods often fail for systems containing more than a few atoms, due to rapid internal conversion, predissociation, or other dynamical processes. Even for small systems, the vibronic band intensities are often contaminated by intramolecular relaxation dynamics; in such cases, these techniques cannot be used for reliable intensity measurements. For clusters that exhibit rapid photofragmentation, depletion spectroscopy can be employed quite effectively to measure their vibronic structure, but again, dynamic effects complicate the interpretation of spectra. The same considerations apply to other types of “action” spectroscopy. It would often be preferable to measure the electronic spectra of molecules and clusters in direct absorption, as this approach is the most straightforward and accurate means of determining absolute vibronic band intensities and for accessing states that are invisible to LIF or REMPI. The problem, of course, is that direct absorption methods are generally orders of magnitude less sensitive than the “action” techniques and are, therefore, difficult to apply to transient species, such as clusters or radicals. In this review, we describe a relatively new direct absorption technique that we have developed for measuring the electronic spectra of jet-cooled molecules and clusters with both high sensitivity and high spectral resolution. The method is based on measurement of the time rate of decay of a pulse of light trapped in a high reflectance optical cavity; we call it cavity ringdown laser absorption spectroscopy (CRLAS). In practice, pulsed laser light is injected into an optical cavity that is formed by a pair of highly reflective (R > 99.9%) mirrors. The small amount of light that is now trapped inside the cavity reflects back and forth between the two mirrors, with a small fraction (∼1 R) transmitting through each mirror with each pass. The resultant transmission of the circulating light is monitored at the output mirror as a function of time and allows the decay time of the cavity to be determined. A simple picture of the cavity decay event for the case where the laser pulse is temporally shorter than the cavity round trip transit time is presented in Figure 1. In this case, the intensity envelope of these discrete transmitted pulses exhibits a simple exponential decay. The time required for the cavity to decay to 1/e of the initial output pulse is called the “cavity ringdown” time. Determination of the ringdown time allows the absolute single pass transmission coefficient of the cavity to be determined with high accuracy, given the mirror spacing. The apparatus is converted to a sensitive absorption spectrometer simply by placing an absorbing medium between the two mirrors and recording the frequency dependent ringdown time of the cavity. Ideally, the ringdown time is a function of only the mirror reflectivities, cavity dimensions, and sample absorption. Absolute absorption intensities are obtained by subtracting the base-line transmission of the cavity, which is determined when the laser wavelength is off-resonance with all molecular transitions. † IBM Predoctoral Fellow. Current address: Sandia National Laboratories, M/S 9055, Livermore, CA 94551-0969. ‡ Los Gatos Research. 25 Chem. Rev. 1997, 97, 25−51

INVITED TOPICAL REVIEW The structure of ambient water

Abstract: We review the spectroscopic techniques and scattering experiments used to probe the structure of water, and their interpretation using empirical and ab initio models, over the last 5 years. We show that all available scientific evidence overwhelmingly favors the view of classifying water near ambient conditions as a uniform, continuous tetrahedral liquid. While there are controversial issues in our understanding of water in the supercooled state, in confinement, at interfaces, or in solution, there is no real controversy in what is understood as regards bulk liquid water under ambient conditions.