A novel photoacoustic spectrophotometer (PAS) for the measurement of gas-phase and aerosol absorption over the UV-visible region of the spectrum is described. Light from a broadband Hg arc lamp is filtered in eight separate bands from 300 to 700 nm using bandpass interference filters (centered at 301 nm, 314 nm, 364 nm, 405 nm, 436 nm, 546 nm, 578 and 687 nm) and modulated with an optical chopper before entering the photoacoustic cell. All wavelength bands feature a 20-s detection limit of better than 3.0 Mm(-1) with the exception of the lower-intensity 687 nm band for which it is 10.2 Mm(-1). Validation measurements of gas-phase acetone and nigrosin aerosol absorption cross sections at several wavelengths demonstrate agreement to within 10% with those measured previously (for acetone) and those predicted by Mie theory (for nigrosin). The PAS instrument is used to measure the UV-visible absorption spectrum of ambient aerosol demonstrating a dramatic increase in the UV region with absorption increasing by 300% from 405 to 301 nm. This type of measurement throughout the UV-visible region and free from artifacts associated with filter-based methods has not been possible previously, and we demonstrate its promise for classifying and quantifying different types of light-absorbing ambient particles.
Laboratory data have a dual and critical role in interpreting information obtained from the Cassini spacecraft in its passes through the Titan ionosphere. Firstly, in situ mass spectra are obtained by Cassini and their conversion into atmospheric molecular composition requires chemical modeling to create agreement between the observed mass spectra and those determined from the models. Secondly, once agreement is obtained, then the chemical model can be considered to represent the evolution of the Titan atmosphere. As a contribution to these endeavors in the past, laboratory measurements have been made in the Selected Ion Flow Tube (SIFT) of the reactions of a series of ring molecules with the important ionospheric ion CH3+. These reactions showed that a dominant reaction channel is association. In the present study, this work has been extended to reactions of another important Titan ion C3H3+. These ion-molecule reactions have also been studied at room temperature using a SIFT. Reactions have been studied in detail with benzene, toluene and pyridine and show again that association is very important. The loss of ionization in the ionosphere is then controlled by electron-ion dissociative recombination of the association ions and their progeny. The recombination reactions have been studied as a function of temperature (300 to 550 K) using a flowing afterglow. These combined data have been used to develop a subset of the chemistry and test its viability. They have indicated that association of the important Titan ions with the abundant nitrogen, followed by switching of the nitrogen for the ring compounds, can build up larger species, perhaps resulting in multi-rings. Recombination of such species can affect the ionization balance and provide species which can contribute to the parallel neutral chemistry. Species are suggested that should be looked for in the in situ mass spectra.
Abstract:The copper complex of 5,10,15,20-tetrakis-(3,4-dibenzyloxyphenyl)porphyrin (CuTDBOPP) was synthesized and characterized with electronic absorption spectroscopy and ESI high-resolution spectrometry. In the electronic spectrum, there was a shift in the Soret band from 424 nm to 420 nm that indicated that the insertion of the metal ion was successful. Additionally, the number of Q bands decreased from four peaks to a single peak at 541 nm. The molar absorptivity of the Soret and Q band are 3.4 × 10 5 cm −1 ·M −1 and 1.8 × 10 5 cm −1 ·M −1 , respectively. The ESI HRMS was in excellent agreement with simulated isotopic distribution spectra. CuTDBOPP was incorporated into a series of Gratzel cells where the open current voltage was recorded.
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