Session V32: Chemical Physics of Extreme Environments II

Kinetics, mechanisms and products of reactions of Criegee intermediates.

The atmospheric ozonolysis of alkenes such as isoprene produces Criegee intermediates which are increasingly recognized as important contributors to oxidation chemistry in the Earth's troposphere. Stabilized Criegee intermediates are conveniently produced in the laboratory by ultraviolet photolysis of diiodoalkanes in the presence of O$_{\mathrm{2}}$, and can be detected by absorption spectroscopy using their strong electronic bands in the near ultraviolet region. We have used these techniques to study a wide range of reactions of Criegee intermediates, including their self-reactions, and reactions with carboxylic acids and various other trace atmospheric constituents. In collaboration with the Sandia National Laboratory group led by Drs C.A. Taatjes and D.L. Osborn, we have used photoionization and mass spectrometry methods, combined with electronic structure calculations, to characterize the products of several of these reactions. Our laboratory studies determine rate coefficients for the Criegee intermediate reactions, many of which prove to be fast. In the case of reactions with carboxylic acids, a correlation between the dipole moments of the reactants and the reaction rate coefficients suggests a dipole-capture controlled reaction and allows us to propose a structure-activity relationship to predict the rates of related processes. The contributions of these various Criegee intermediate reactions to the chemistry of the troposphere have been assessed using the STOCHEM-CRI global atmospheric chemistry model.   

Direct Measurement of the Unimolecular Decay Rate of Criegee Intermediates to OH Products.

Ozonolysis of alkenes is an important non-photolytic source of OH radicals in the troposphere. The production of OH radicals proceeds though formation and unimolecular decay of Criegee intermediates such as syn-CH3CHOO and (CH3)2COO. These alkyl-substituted Criegee intermediates can undergo a 1,4-H transfer reaction to form an energized vinyl hydroperoxide species, which breaks apart to OH and vinoxy products. Recently, this laboratory used IR excitation in the C-H stretch overtone region to initiate the unimolecular decay of syn-CH3CHOO and (CH3)2COO Criegee intermediates, leading to OH formation. Here, direct time-domain measurements are performed to observe the rate of appearance of OH products under collision-free conditions utilizing UV laser-induced fluorescence for detection. The experimental rates are in excellent agreement with statistical RRKM calculations using barrier heights predicted from high-level electronic structure calculations. Accurate determination of the rates and barrier heights for unimolecular decay of Criegee intermediates is essential for modeling the kinetics of alkene ozonolysis reactions, a significant OH radical source in atmospheric chemistry, as well as the steady-state concentration of Criegee intermediates in the atmosphere.   

Probing neutral atmospheric collision complexes with anion photoelectron imaging.

Photodetachment of anionic precursors of neutral collision complexes offers a way to probe the effects of symmetry-breaking collision events on the electronic structure of normally transparent molecules. We have measured the anion photoelectron imaging (PEI) spectra of a series of O$_{\mathrm{2}}^{\mathrm{-}}\cdot X$ complexes, where $X$ is a volatile organic molecule with atmospheric relevance, to determine how the electronic properties of various $X$ molecules affect the low-lying electronic structure of neutral O$_{\mathrm{2}}$ undergoing O$_{\mathrm{2}}-X$ collisons. The study was motivated by the catalog of vibrational and electronic absorption lines induced by O$_{\mathrm{2}}-$O$_{\mathrm{2}}$, O$_{\mathrm{2}}-$N$_{\mathrm{2}}$, and other collisions. The energies of electronic features observed in the anion PEI spectra of O$_{\mathrm{2}}^{\mathrm{-}}\cdot X$ ($X \quad =$ hexane, hexene, isoprene and benzene) relative to O$_{\mathrm{2}}^{\mathrm{-}}$ PEI spectroscopic features indicate that photodetachment of the anion does indeed access a repulsive part of the O$_{\mathrm{2}}$ -- $X$ potential. In addition, the spectra of the various complexes show an interesting variation in the intensities of transitions to the excited O$_{\mathrm{2}}(^{\mathrm{1}}\Delta_{\mathrm{g}})\cdot X$ and O$_{\mathrm{2}}(^{\mathrm{1}}\Sigma _{\mathrm{g}}^{\mathrm{+}})\cdot X$ states relative to the ground O$_{\mathrm{2}}(^{\mathrm{3}}\Sigma _{\mathrm{g}}^{\mathrm{-}})\cdot X$ state. With $X \quad =$ non-polar species such as hexane, the relative intensities of transitions to the triplet and singlet states of O$_{\mathrm{2}}\cdot X$ are very similar to those of isolated O$_{\mathrm{2}}$, while the relative intensity of the singlet band decreases and becomes lower in energy relative to the triplet band for $X \quad =$ polar molecules. A significant enhancement in the intensities of the singlet bands is observed for complexes with $X \quad =$ isoprene and benzene, both of which have low-lying triplet states. The role of the triplet states in isoprene and benzene, and the implications for induced electronic absorption in O$_{\mathrm{2}}$ undergoing collisions with these molecules, are explored.   

Photoelectron Spectroscopy of Transition Metal Hydride Cluster Anions and Their Roles in Hydrogenation Reactions

The interaction between transition metals and hydrogen has been an intriguing research topic for such applications as hydrogen storage and catalysis of hydrogenation and dehydrogenation. Special bonding features between TM and hydrogen are interesting not only because they are scarcely reported but also because they could help to discover and understand the nature of chemical bonding. Very recently, we discovered a PtZnH$_{\mathrm{5}}^{\mathrm{-}}$ cluster which possessed an unprecedented planar pentagonal coordination between the H$_{\mathrm{5}}^{\mathrm{-}}$ moiety and Pt, and exhibited special $\sigma $-aromaticity. The H$_{\mathrm{5}}^{\mathrm{-}}$ kernel as a whole can be viewed as a $\eta ^{\mathrm{5}}$-H$_{\mathrm{5}}$ ligand for Pt. As the second example, an H$_{\mathrm{2}}$ molecule was found to act as a ligand in the PdH$_{\mathrm{3}}^{\mathrm{-}}$ cluster, in which two H atoms form a $\eta ^{\mathrm{2}}$-H$_{\mathrm{2}}$ type of ligation to Pd. These transition metal hydride clusters were considered to be good hydrogen sources for hydrogenation. The reactions between PtH$_{\mathrm{n}}^{\mathrm{-}}$ and CO$_{\mathrm{2}}$ were investigated. We observed formate in the final product H$_{\mathrm{2}}$Pt(HCO$_{\mathrm{2}})^{\mathrm{-}}$.   

Total Cross Section Measurements and Velocity Distributions of Hyperthermal Charge Transfer in Xe$^{\mathrm{2+}}+$ N$_{\mathrm{2}}$

Guided-ion beam measurements of the charge exchange (CEX) cross section for Xe$^{\mathrm{2+}}+$ N$_{\mathrm{2}}$ are reported for collision energies ranging from 0.3 to 100 eV in the center-of-mass frame. Measured total XS decrease from 69.5$\pm $0.3 Angstroms$^{\mathrm{2\thinspace }}$(Angs.) at the lowest collision energies to 40 Angs.$^{\mathrm{2}} $at 100 eV. The product N$_{\mathrm{2}}^{\mathrm{+}}$ CEX cross section is similar to the total CEX cross section while those of the dissociative product, N$^{\mathrm{+}}$, are less than 1Angs.$^{\mathrm{2}}$ for collision energies above 9 eV. The product N$_{\mathrm{2}}^{\mathrm{+}}$CEX cross section measured here are much larger than the total optical emission-excitation cross sections for the N$_{\mathrm{2}}^{\mathrm{+}}$ ($A)$ and ($B)$ state products determined previously in the chemiluminescence study of Prince and Chiu suggesting that most of the N$_{\mathrm{2}}^{\mathrm{+}}$ products are in the $X$ state. Time-of-flight (TOF) spectra of both the Xe$^{\mathrm{+}}$ and N$_{\mathrm{2}}^{\mathrm{+}}$ products suggest two different CEX product channels. The first leaves highly-vibrationally excited N$_{\mathrm{2}}^{\mathrm{+}}$ products with forward scattered Xe$^{\mathrm{+}}$ (LAB frame) and releases between 0.35 to 0.6 eV translational energy for collisions below 17.6 eV. The second component decreases with collisional energy and leaves backscattered Xe$^{\mathrm{+}}$ and low-vibrational states of N$_{\mathrm{2}}^{\mathrm{+}}$. At collision energies above 17.6 eV, only charge exchange involving minimal momentum exchange remains in the TOF spectra.   

Aerosol droplets: Nucleation dynamics and photokinetics

This talk addresses two fundamental aerosol processes that play a pivotal role in atmospheric processes: The formation dynamics of aerosol particles from neutral gas phase precursors and photochemical reactions in small aerosol droplets induced by ultraviolet and visible light. Nucleation is the rate determining step of aerosol particle formation. The idea behind nucleation is that supersaturation of a gas leads to the formation of a critical cluster, which quickly grows into larger aerosol particles. We discuss an experiment for studying the size and chemical composition of critical clusters at the molecular level. Much of the chemistry happening in planetary atmospheres is driven by sunlight. Photochemical reactions in small aerosol particles play a peculiar role in this context. Sunlight is strongly focused inside these particles which leads to a natural increase in the rates of photochemical reactions in small particles compared with the bulk. This ubiquitous phenomenon has been recognised but so far escaped direct observation and quantification. The development of a new experimental setup has finally made it possible to directly observe this nanofocusing effect in droplet photokinetics.   

\textbf{Single Scattering Albedo of fresh~biomass burning aerosols measured using cavity ring down spectroscopy and nephelometry}

An accurate measurement of optical properties of aerosols is critical for quantifying the effect of aerosols on climate. Uncertainties still persist and measurement results vary significantly. The factors that affect measurement accuracy and the resulting uncertainties of the extinction-minus-scattering method are evaluated using a combination of cavity ring-down spectroscopy (CRDS) and integrating nephelometry and applied to measure the optical properties of fresh soot (size 300 and 400 nm) produced from burning of pine, red oak and cedar. We have demonstrated a system that allows measurement of optical properties at a wide range of wavelengths, which can be extended over most of the solar spectrum to determine ``featured'' absorption cross sections as a function of wavelength. SSA values measured were nearly flat ranging from 0.45 to 0.6. The result also demonstrates that SSA of fresh soot is nearly independent of wavelength of light in the 500-680 wavelength range with a slight increase at longer wavelength. The values are within the range of measured values both in the laboratory and in field studies for fresh soot   

Catching Conical Intersections in the Act; Monitoring Transient Electronic Coherences by Attosecond Stimulated X-Ray Raman Signals

Conical intersections (CIs) dominate the pathways and outcomes of virtually all photochemical molecular processes. Despite extensive experimental and theoretical effort, CIs have not been directly observed yet and the experimental evidence is inferred from fast reaction rates and vibrational signatures. We show that short X-ray pulses can directly detect the passage through a CI with the adequate temporal and spectral sensitivity. The non-adiabatic coupling that exists in the region of a CI redistributes electronic population but also generates electronic coherence. This coherent oscillation can then be detected via a coherent Raman process that employs a composite femtosecond/attosecond X-ray pulse. This technique, dubbed Transient Redistribution of Ultrafast Electronic Coherences (TRUECARS) is reminiscent of Coherent Anti-Stokes Raman Spectroscopy (CARS) in that a coherent oscillation is set in motion and then monitored, but differs in that the dynamics is electronic (CARS generally observes nuclear dynamics) and the coherence is generated internally by passage through a region of non-adiabatic coupling rather than by an externally applied laser.