Formaldehyde (HCHO) plays a primary role in tropospheric chemistry. Its photochemical activity is an important source of radical species such as HCO, H, and subsequently HO2 as well as molecular hydrogen and carbon monoxide. As a source of hydrogen radicals (HOx = OH + HO2), HCHO plays a significant role in the oxidative capacity of the atmosphere, and an important part in the interrelated chemistries of ozone and the HOx and NOx (NO + NO2) cycles. Accurate atmospheric photolysis rates of HCHO are thus required in order to properly model tropospheric chemistry. Despite extensive studies HCHO’s spectroscopy and photochemistry remains to be well characterized. Absolute room temperature absorption cross sections for the A1A2 – X1A1 electronic transition of formaldehyde have been measured over the spectral range 30285 – 32890 cm-1 (304 – 330 nm) using ultraviolet (UV) laser absorption spectroscopy. Absorption cross sections were obtained at an instrumental resolution better than 0.09 cm-1 which is slightly broader than the Doppler width of a rotational line of HCHO at 300K (~0.07 cm-1) and so we were able to resolve all but the most closely spaced lines. Qualitative comparisons with spectral simulations show varying agreement depending on vibronic band. Refined state origins and transition dipole moments for each vibronic band have been reported. There is evidence of areas of perturbation and the need to optimize higher order spectral constants. Pressure broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. Comparisons between the available high-resolution studies and spectral simulations indicate that the HCHO absorption cross section is still not well characterized. The relative quantum yield for the production of radical products, H+HCO, from the UV photolysis of formaldehyde (HCHO) has been measured directly using a Pulsed Laser Photolysis – Pulsed Laser Induced Fluorescence (PLP – PLIF) technique across the same spectral region. Relative yields were normalized to a value of 0.69 at 31750 cm-1 based on the current NASA-JPL recommendation. The resulting absolute radical quantum yields agree well with previous experimental studies and show more wavelength dependent behavior than the recommendation. This provides support for the complicated competition among the various HCHO dissociation pathways.

Thesis (Ph.D.(Physics)) -- University of Limpopo, 2020 / The effects of O2, H2O and OH− and collectors are the major factors that determine the flotability behaviour of minerals. In particular, the influence of the chain length variation on xanthate collectors gives rise to increased recovery rates, and are still the most versatile collector for most minerals. This study explores the bonding behaviour, adsorption energies and electronic properties directly related to the reactivity of O2, H2O and OH−, ethyl xanthates (EX), normal propyl-xanthate (nPX), normal-butyl-xanthate (nBX–) and amyl-xanthate (AX) with the platinum antimony mineral surfaces: (100), (110) and (111) surfaces. We employed the ab-initio quantum mechanical density functional theory to investigate their adsorption and their electronic properties. In order to attain precise calculations, the cut-off energy of 500 eV was used for the bulk PtSb2, which was also transferred to the surfaces. To obtain accurate results the k point used for both the bulk and surfaces were 6x6x6 and 4x4x1, respectively. The bulk relaxation was found to give final lattice parameter of 6.531 Å. The DOS (Density Of States) indicated that both bulk and surfaces of PtSb2 had a metallic character, thereby indicating semiconducting behaviour. In cleaving the surfaces, all possible terminations were considered and the slab thickness was varied to obtain the desired stable surfaces. Their relaxed surface energies were 0.807 J.m-2, 1.077 J.m-2 and 1.074 J.m-2 for the (100), (111) and (110), respectively. These indicated that the (100) surface was the most stable and dominant plane for the platinum antimony. This fact is also observed in other minerals in general that low-index surfaces with lower surface energies indicates structural stability. The DOS showed stability with the EF (Fermi level/ Fermi energy) falling deep into the pseudo gap for all surface. The valence electrons on the surface were 5d96s1 for Pt and 5s25p3 for Sb as depicted from the Mulliken population charges and these electrons were actively involved in the hybridisation. The oxidation showed that the oxygen molecules preferred interacting with the Sb atoms than the Pt atoms for all surfaces. For the (100) surface we found that the Pt-O2peroxide adsorption site gave the strongest adsorption, while for the (110) surface we noted that the Sb2-O-O-Sb3 bridging gave the most exothermic adsorption. The case of the (111) surface showed the Sb2-O-O-Sb2 bridging to give the strongest exothermic adsorption, which dissociated and resulted in atomic bonding. Their atomic charges indicated that the oxygen molecules gain charges from the Pt and Sb atoms. In all cases, PtSb2 Bulk PtSb2 (100), (110) and (111) surfaces O2, H2O, OH-and Xanthates adsorptions the O2 interacting with Sb gained more charges, thus showing preferential adsorption to the Sb atoms. In addition, the Sb/Pt-bonded oxygens were more negative than the terminal or end-bonded oxygen atom for superoxide modes. These suggested that the 2p-orbital spin-down unoccupied orbital (LUMO) of O2 is fully occupied. The case of H2O molecules adsorptions on the three PtSb2 mineral surfaces indicated that the H2O adsorbed through van der Waals forces, in particular for multi adsorptions by physisorption process for the (100) and the (110) surfaces. However, on the (111) surface we observed chemisorption adsorption. For the (100) surface we found that the H2O-Pt was exothermic, while the H2O-Sb was endothermic and only showed exothermic from 5/8-8/8 H2O/Sb. The case of the (110) surface showed stronger adsorption of H2O on Pt than on Sb atoms, with a weaker adsorption on Sb2 atoms, while the adsorption on the (111) surface was stronger on Sb3 and weaker on Sb2 atoms. The full-coverage for the (110) surface gave –35.00 kJ/mol per H2O molecule, which is similar to the full coverage on the (100) surface (–38.19 kJ/mol per H2O molecule). Furthermore, the full monolayer adsorption on Sb2 and Sb3 for the (111) surface gave much stronger adsorption (–55.54 kJ/mol per H2O). In addition, the full-coverage on the (111) surface (i.e. on Pt1 and all Sb atoms) gave adsorption energy of –54.95 kJ/mol per H2O molecule. The adsorption of hydroxide on the surfaces showed stronger affinity than the water molecules. This suggested that they will bind preferentially over the water molecules. We also found that the OH–preferred the Sb atoms on the (100) surface, with a greater adsorption energy of –576.65 kJ/mol per OH– molecule for full-surface coverage. The (110) surface adsorption energy on full-surface coverage was –541.98 kJ/mol per OH molecule. The (111) surface full-coverage yielded adsorption energy of –579.53 kJ/mol per OH– molecule. The atomic charges related to both hydration and hydroxide adsorption showed charge depletion on both Pt/Sb and O atoms of the H2O and OH–. This suggested that there is a charge transfer into other regions within the orbitals. The adsorption of collectors on the PtSb2 surfaces to investigate their affinity with surfaces were performed considering different adsorption sites in order to find the most stable exothermic preferred site. In respect of the (100) surface, we noted that the bridging on Pt and Sb atoms by the collector involved the S atoms for all xanthates. Their adsorption energies showed that EX had strong affinity with the surface and the order was as: EX ≈ AX > nBX > nPX. In the case of the (110) surface the bridging on Pt atoms were PtSb2 Bulk PtSb2 (100), (110) and (111) surfaces O2, H2O, OH- and Xanthates adsorptions the most preferred sites for EX, nPX, nBX and AX. The order of adsorption energies was: nBX > nPX ≈ AX > EX. The (111) surface was observed to have the bridging on Sb2 and Sb3 atoms most exothermic for EX, nBX and AX, while the nPX showed the bridging on Pt1 and Sb3 atoms. The adsorption energies were found to have the nPX more stronger on the surface, with EX weaker and the order decreased as: nPX > nBX > AX > EX. This gave insights in the recovery of the minerals during flotation, that the use of EX or AX may float the platinum antimonide better based on the adsorption trends on the (100) surface, which is the most stable surface plane cleavage for platinum antimonide. The analysis of the electronic structures of the collector on the surface from density of states showed stability bonding of the collector on the surface, due to the EF falling deep into the pseudo gap for collector S atoms and surface Pt and Sb PDOS. The atomic charges computed indicated that the collectors behave as electron donors and acceptors to the Pt and Sb on the surface, respectively for the (100) surface. Interestingly for the (110) surface we observed that both surface Pt and Sb atoms lost charges, with a loss of charges on the collector S atoms. These observations suggested that the collectors S atoms offer their HOMO electrons to Pt and Sb atoms to form bond and simultaneously the Pt and Sb atoms donate their d-orbital and p-orbitals electrons to the LUMO of the collectors to form a back donation covalent bond, respectively. The (111) surface clearly showed that the surface Pt and Sb atoms lose charges to the collector S atoms, suggested a back donation covalent bonds. / National Research Foundation (NRF) and CSIR (Council for Scientific and Industrial Research) through Centre for High Performance Computing (CHPC)

The power of the McCumber theory [D. E. McCumber, Phys. Rev. 136, A954-957 (1964)] consists in its ability to accurately predict emission cross section spectra from measured absorption, and vice versa, including both absolute values and spectral shapes. While several other theories only allow the determination of integrated cross sections, the McCumber theory is unique in generating the spectral shape of a cross section without any direct measurements regarding that cross section. The present work is a detailed study of the range of validity of the McCumber theory, focussing particularly on those aspects that most critically affect its applicability to transitions of rare earth ions in glasses. To analyze the effect of the spectral broadening on the accuracy of the technique, experiments were performed at room and low temperature. The theory was tested by comparing the cross sections calculated using the McCumber relation with those obtained from measurements. At room temperature, a number of ground state transitions of three different rare earth ions (Nd, Er and Tm) in oxide and fluoride glass hosts have been studied. Special attention was paid to the consistency of the measurements, using the same experimental setup, same settings and same detection system for both absorption and fluorescence measurements. Other aspects of the experimental procedure that could generate systematic errors, like fluorescence reabsorption and baseline subtraction uncertainties in the absorption measurements, were carefully investigated. When all these aspects are properly accounted for, we find in all cases an excellent agreement between the calculated and the measured cross section spectra. This suggests that the McCumber theory is not limited to crystalinne hosts, but describes quite well the reciprocity between emission and absorption for the broader transitions of rare earths in glassy hosts. This good agreement does not hold, however, for the low temperature results. The distortion observed in this case follows the theoretically predicted behavior, and corresponds to the amplification of the gaussian wings that describes the inhomogeneous type of broadening. Our results suggest that the McCumber theory must be used with caution for temperatures below 200 K.

abstract: One strategic objective of the National Aeronautics and Space Administration (NASA) is to find life on distant worlds. Current and future missions either space telescopes or Earth-based observatories are frequently used to collect information through the detection of photons from exoplanet atmospheres. The primary challenge is to fully understand the nature of these exo-atmospheres. To this end, atmospheric modeling and sophisticated data analysis techniques are playing a key role in understanding the emission and transmission spectra of exoplanet atmospheres. Of critical importance to the interpretation of such data are the opacities (or absorption cross-sections) of key molecules and atoms. During my Doctor of Philosophy years, the central focus of my projects was assessing and leveraging these opacity data. I executed this task with three separate projects: 1) laboratory spectroscopic measurement of the infrared spectra of CH4 in H2 perturbing gas in order to extract pressure-broadening and pressure-shifts that are required to accurately model the chemical composition of exoplanet atmospheres; 2) computing the H2O opacity data using ab initio line list for pressure and temperature ranges of 10^-6–300 bar and 400–1500 K, and then utilizing these H2O data in radiative transfer models to generate transmission and emission exoplanetary spectra; and 3) assessing the impact of line positions in different H2O opacities on the interpretation of ground-based observational exoplanetary data through the cross-correlation technique. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2019

Oxidation flow reactors (OFRs) using low-pressure Hg lamp emission at 185 and 254 nm produce OH radicals efficiently and are widely used in atmospheric chemistry and other fields. However, knowledge of detailed OFR chemistry is limited, allowing speculation in the literature about whether some non-OH reactants, including several not relevant for tropospheric chemistry, may play an important role in these OFRs. These non-OH reactants are UV radiation, O(¹D), O(³P), and O₃. In this study, we investigate the relative importance of other reactants to OH for the fate of reactant species in OFR under a wide range of conditions via box modeling. The relative importance of non-OH species is less sensitive to UV light intensity than to water vapor mixing ratio (H₂O) and external OH reactivity (OHR_ext), as both non-OH reactants and OH scale roughly proportionally to UV intensity. We show that for field studies in forested regions and also the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. We find that O(¹D), O(³P), and O₃ have relative contributions to volatile organic compound (VOC) consumption that are similar or lower than in the troposphere. The impact of O atoms can be neglected under most conditions in both OFR and troposphere. We define “riskier OFR conditions” as those with either low H₂O (< 0.1 %) or high OHR_ext ( ≥  100 s⁻¹ in OFR185 and > 200 s⁻¹ in OFR254). We strongly suggest avoiding such conditions as the importance of non-OH reactants can be substantial for the most sensitive species, although OH may still dominate under some riskier conditions, depending on the species present. Photolysis at non-tropospheric wavelengths (185 and 254 nm) may play a significant (> 20 %) role in the degradation of some aromatics, as well as some oxidation intermediates, under riskier reactor conditions, if the quantum yields are high. Under riskier conditions, some biogenics can have substantial destructions by O₃, similarly to the troposphere. Working under low O₂ (volume mixing ratio of 0.002) with the OFR185 mode allows OH to completely dominate over O₃ reactions even for the biogenic species most reactive with O₃. Non-tropospheric VOC photolysis may have been a problem in some laboratory and source studies, but can be avoided or lessened in future studies by diluting source emissions and working at lower precursor concentrations in laboratory studies and by humidification. Photolysis of secondary organic aerosol (SOA) samples is estimated to be significant (> 20 %) under the upper limit assumption of unity quantum yield at medium (1 × 10¹³ and 1.5 × 10¹⁵ photons cm⁻² s⁻¹ at 185 and 254 nm, respectively) or higher UV flux settings. The need for quantum yield measurements of both VOC and SOA photolysis is highlighted in this study. The results of this study allow improved OFR operation and experimental design and also inform the design of future reactors.