Initial investigation of the wavelength dependence of optical properties measured with a new multi-pass Aerosol Extinction Differential Optical Absorption Spectrometer (AE-DOAS)

Atmospheric aerosols directly affect climate by scattering and absorbing radiation. The magnitude of the impact is dependent upon the wavelength of light, but is often estimated near 550 nm. When light scattering and absorption by aerosols is approximated, the wavelength dependence of the refractive index for specific components is lost. As a result, climate models would have inherent uncertainties for aerosol contributions to radiative forcing when considering the entire solar spectrum. An aerosol extinction differential optical absorption spectrometer has been developed to directly measure aerosol extinction at mid-ultraviolet to near infrared wavelengths. The instrument consists of a spectrometer coupled to a closed White-type multi-pass gas cell with an adjustable path length of up to approximately 20 m. Laboratory measurements of various gases are compared with known absorption cross sections. Additionally, the extinction of monodisperse samples of polystyrene latex spheres are measured and compared to Mie theory generated with refractive index values from the literature to validate the new instrument. The polystyrene experiments also emphasize the ability of the new instrument to retrieve the wavelength dependent refractive index, especially in the ultraviolet wavelength regions where variability is expected. The spectrometer will be a significant advancement for determining wavelength dependent complex refractive indices in future laboratory studies as well as provide the ability to monitor ambient aerosol light extinction

Evidence for partial quenching of orbital angular momentum upon complex formation in the infrared spectrum of OH-acetylene

The entrance channel leading to the addition reaction between the hydroxyl radical and acetylene has been examined by spectroscopic characterization of the asymmetric CH stretching band of the π-hydrogen bonded OH-acetylene reactant complex. The infrared action spectrum observed at 3278.6 cm−1 (origin) consists of seven peaks of various intensities and widths, and is very different from those previously reported for closed-shell HF/HCl-acetylene complexes. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum in the complex, which in turn is caused by a significant splitting of the OH monomer orbital degeneracy into 2A′ and 2A″ electronic states. The magnitude of the 2A′−2A″ splitting as well as the A rotational constant for the OH-acetylene complex are determined from the analysis of this b-type infrared band. The most populated OH product rotational state, jOH = 9/2, is consistent with intramolecular vibrational energy transfer to the ν2 C≡C stretching mode of the departing acetylene fragment. The lifting of the OH orbital degeneracy and partial quenching of its electronic orbital angular momentum indicate that the electronic changes accompanying the evolution of reactants into products have begun to occur in the reactant complex

Infrared spectrum and stability of a π-type hydrogen-bonded complex between the OH and C2H2 reactants

A hydrogen-bonded complex between the hydroxyl radical and acetylene has been stabilized in the reactant channel well leading to the addition reaction and characterized by infrared action spectroscopy in the OH overtone region. Analysis of the rotational band structure associated with the a-type transition observed at 6885.53(1) cm−1 (origin) reveals a T-shaped structure with a 3.327(5) Å separation between the centers of mass of the monomer constituents. The OH (v = 1) product states populated following vibrational predissociation show that dissociation proceeds by two mechanisms: intramolecular vibrational to rotational energy transfer and intermolecular vibrational energy transfer. The highest observed OH product state establishes an upper limit of 956 cm−1 for the stability of the π-type hydrogen-bonded complex. The experimental results are in good accord with the intermolecular distance and well depth at the T-shaped minimum energy configuration obtained from complementary ab initio calculations, which were carried out at the restricted coupled cluster singles, doubles, noniterative triples level of theory with extrapolation to the complete basis set limit

(2+1) Resonance Enhanced Ionization Spectroscopy of a State Selected Beam of OH Radicals

A state-selected beam of hydroxyl radicals is generated using a pulsed discharge source and hexapole field. The OH radicals are characterized by resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the nested D  and 3  Rydberg states. Simplified spectra are observed from the selected ∣MJ∣ = 3/2 component of the upper Λ-doublet level of the lowest rotational state (J = 3/2) in ground (v″ = 0) and excited(v″ = 1–3) vibrational levels of the OH X  state. Two-photon transitions are observed to the D (v′ = 0–3) and 3 (v′ = 0,1) vibronic levels, extending previous studies to higher vibrational levels of the Rydberg states. Spectroscopic constants are derived for the Rydberg states and compared with prior experimental studies. Complementary first-principle theoretical studies of the D  and 3  Rydberg states [see M. P. J. van der Loo and G. C. Groenenboom, J. Chem. Phys. 123, 074310 (2005), following paper ] are used to interpret the experimental findings and examine the utility of the (2+1) REMPI scheme for sensitive detection of OH radicals

Spectral aerosol extinction (SpEx): a new instrument for in situ ambient aerosol extinction measurements across the UV/visible wavelength range

We introduce a new instrument for the measurement of in situ ambient aerosol extinction over the 300– 700 nm wavelength range, the spectral aerosol extinction (SpEx) instrument. This measurement capability is envisioned to complement existing in situ instrumentation, allowing for simultaneous measurement of the evolution of aerosol optical, chemical, and physical characteristics in the ambient environment. In this work, a detailed description of the instrument is provided along with characterization tests performed in the laboratory. Measured spectra of NO2 and polystyrene latex spheres (PSLs) agreed well with theoretical calculations. Good agreement was also found with simultaneous aerosol extinction measurements at 450, 530, and 630 nm using CAPS PMex instruments in a series of 22 tests including nonabsorbing compounds, dusts, soot, and black and brown carbon analogs. SpEx measurements are expected to help identify the presence of ambient brown carbon due to its 300 nm lower wavelength limit compared to measurements limited to longer UV and visible wavelengths. Extinction spectra obtained with SpEx contain more information than can be conveyed by a simple power law fit (typically represented by Ångström exponents). Planned future improvements aim to lower detection limits and ruggedize the instrument for mobile operation

Photodissociation of the OD radical at 226 and 243 nm

The photodissociation dynamics of state selected OD radicals has been examined at 243 and 226 nm using velocity map imaging to probe the angle–speed distributions of theD(2S) and O(3P2) products. Both experiment and complementary first principle calculations demonstrate that photodissociation occurs by promotion of OD from high vibrational levels of the ground X 2Π state to the repulsive 1 2Σ− state

Spectroscopic detection and decay dynamics of the hydroxyl radical and its reactant complexes with carbon monoxide and acetylene

Excited electronic states of the hydroxyl radical are investigated using two types of measurements. First, laser-induced excitation originating from high vibrational levels (primarily v = 3, 4, and 5) of the X 2Π ground state to the repulsive 1 2Σ − potential results in D(2S) and O(3P 2) photodissociation products. The angle-speed distribution of the products, and thus their total kinetic energy release, are ascertained using velocity map imaging. Experimental results and complementary theory provide a consistent description of the photodissociation dynamics. The second experiment is focused on the spectroscopic characterization and dynamics of the D 2Σ− and 3 2Σ − Rydberg states of OH through resonance enhanced multiphoton ionization (REMPI). A jet-cooled and hexapole state selected beam of OH radicals yields simplified two-photon spectra of the D 2Σ − (v′ = 0–3) and 3 2Σ− (v′ = 0,1) vibronic levels, extending previous studies. First-principle theoretical studies inspired by this experiment are used to interpret its findings and to examine the utility of this REMPI scheme for sensitive detection of OH radicals. The structure, stability and/or dynamics of the OH-acetylene and OH-CO reactant complexes are studied in their ground and excited electronic states. Laser-induced fluorescence and fluorescence depletion methods provide a broad and relatively unstructured electronic excitation spectrum for the OH-CO reactant complex in the OH A-X (1,0) region. The electronic spectrum is consistent with a Franck-Condon mapping of the ground state radial distribution on a strongly attractive excited state potential that undergoes rapid decay dynamics. OH-acetylene is characterized by infrared action spectroscopy utilizing both the OH overtone and asymmetric CH stretching excitations of the complex. Analysis of the rotational band structure (2νOH) reveals a T-shaped π-hydrogen bonded structure. Investigation of the predissociation dynamics upon 2ν OH excitation establishes an upper limit of 956 cm−1 for the complex stability. The spectrum observed upon excitation of the asymmetric CH stretching mode displays unexpected rotational band structure. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum upon complex formation. The partial quenching of the electronic orbital angular momentum of OH indicates that the electronic changes accompanying the evolution of reactants into products have begun to occur in the OH-acetylene complex

Optical Properties and Associated Hygroscopicity of Clay Aerosols

Airborne mineral dust particles contribute a significant fraction to the total aerosol mass, thus they make a substantial contribution to the Earth\u27s radiative budget by direct scattering and absorption of radiation. Quantifying their contribution is complicated by the variability of optical properties as a function of water uptake. To improve understanding, we directly measured the relative humidity (RH) dependence of extinction [fRHext(RH, Dry)] for three key silicate clay components (illite, kaolinite, and montmorillonite) of mineral dust aerosols through cavity ring-down spectroscopy at 532 nm. The three clays studied show significant differences in fRHext(RH, Dry) at three RH values, and reasons for this are explored. With 68% RH as an example, we used the fRHext(RH, Dry) and Mie theory to calculate a growth factor for comparison with other measurement techniques. Humidified tandem differential mobility analyzer and quartz crystal microbalance growth factors from the literature are larger than our optical measurements indicate. An apparent decrease in particle size calculated from optical measurements for illite and kaolinite was further investigated by determining the aerosol electrical mobility size distribution of 68% RH and dry clay particles at that indicated shrinkage of approximately 10% at elevated humidity. Direct optical measurement has advantages because the effects of irregular shape and internal voids are observed. Our calculated growth factors provide a lower limit and can be incorporated into climate models in conjunction with other results to reduce the uncertainty associated with the optical response to water uptake on clay aerosols

ELECTRONIC EXCITATION SPECTROSCOPY OF THE OH-CO REACTANT COMPLEX

Author Institution: University of Pennsylvania; Department of Chemistry, University of PennsylvaniaThe electronic excitation spectrum of the OH-CO reactant complex in the OH A-X (1,0) spectral region has been investigated using a fluorescence depletion technique. This technique combines infrared overtone excitation of OH-CO at $1.4 \mu m$ with ultraviolet excitation of OH-CO between 275 and 300 nm, which induces a fluorescence signal. When the IR and UV transitions originate from a common ground state, the IR excitation reduces the ground state population and causes a depletion in the UV laser induced fluorescence signal. Scanning only the IR laser yields a rotationally resolved infrared spectrum of the pure OH overtone band of OH-CO at $6941.8 cm^{-1}$, while tuning only the UV laser results in a broad and relatively unstructured electronic spectrum for OH-CO spanning from 33500 to $36000 cm^{-1}$. The electronic spectrum peaks around $34600 cm^{-1}$ and exhibits a secondary shoulder near $35300 cm^{-1}$. The breadth and position of the electronic spectrum reflect the Franck-Condon window on the $OH A^{2}\Sigma^{+} (v^{\prime} = 1) + CO$ excited state potential. The lack of observable structure in the electronic spectrum is attributed to extensive homogeneous line broadening, most likely arising from rapid electronic quenching of OH $A^{2}\Sigma^{+}$ by the CO partner. The electronic excitation spectrum observed for the OH-CO will also be compared with analogous spectra obtained for complexes of OH with other partners

HYDROGEN BONDED OH−C2H2OH-C_{2}H_{2} REACTANT COMPLEX CHARACTERIZED BY INFARED ACTION SPECTROSCOPY

Author Institution: Department of Chemistry, University of PennsylvaniaThe entrance channel to the $OH + C_{2}H_{2} \rightarrow HOCHCH$ reaction has been characterized by infrared spectroscopy of a binary hydrogen-bonded complex between the chemically reactive partners. Infrared action spectra of the $OH-C_{2}H_{2}$ reactant complex have been recorded using an optical parametric oscillator operating in the OH overtone region near $1.4 {\mu}m$ and the asymmetric acetylenic fundamental region near $3.0 {\mu}m$. The $OH (v = 1$ or 0) fragments from vibrational predissociation are detected by laser-induced fluorescence. The pure OH overtone band of $OH-C_{2}H_{2}$ is observed at $6885.6 cm^{-1}$ (band origin), shifted $85.7 cm^{-1}$ to lower energy of the OH monomer transition. The pure OH overtone band exhibits rotationally resolved structure that is characteristic of an A-type transition of a near-prolate asymmetric top. The spectrum also shows interesting gaps between the P, Q and R branches, indicating that the orbital and spin angular momentum of the unpaired electron of OH is unquenched. The P and R line positions have been used to determine values of $\frac{1}{2}(B + C)$ for the upper and lower vibrational states, and yield a center of mass separation between the two subunits of $3.34(3) {\AA}in$ both vibrational states. The spectroscopic data, taken together with the results of ab initio calculations and previous work on the HF-acetylene and HCl-acetylene complexes, show that the OH-acetylene complex is T-shaped, with a hydrogen bond formed between the H atom of OH and the $\pi$ system of the C-C bond. The infrared spectrum in the asymmetric stretch region of acetylene is centered at $3281 cm^{-1}$, with a much smaller spectral red shift of $14 cm^{-1}$, but exhibits more complicated band structure with multiple Q-branches arising from a B-type transition