Broadband optical cavity absorption spectroscopy in the near-ultraviolet: applications in atmospheric chemistry

A novel spectroscopic method, incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS), has been modified and extended to measure absorption spectra in the near-ultraviolet with high sensitivity. The near-ultraviolet region extends from 300 to 400 nm and is particularly important in tropospheric photochemistry; absorption of near-UV light can also be exploited for sensitive trace gas measurements of several key atmospheric constituents. In this work, several IBBCEAS instruments were developed to record reference spectra and to measure trace gas concentrations in the laboratory and field. An IBBCEAS instrument was coupled to a flow cell for measuring very weak absorption spectra between 335 and 375 nm. The instrument was validated against the literature absorption spectrum of SO2. Using the instrument, we report new absorption cross-sections of O3, acetone, 2-butanone, and 2-pentanone in this spectral region, where literature data diverge considerably owing to the extremely weak absorption. The instrument was also applied to quantifying low concentrations of the short-lived radical, BrO, in the presence of strong absorption by Br2 and O3. A different IBBCEAS system was adapted to a 4 m3 atmosphere simulation chamber to record the absorption cross-sections of several low vapour pressure compounds, which are otherwise difficult to measure. Absorption cross-sections of benzaldehyde and the more volatile alkyl nitrites agree well with previous spectra; on this basis, the cross-sections of several nitrophenols are reported for the first time. In addition, the instrument was also used to study the optical properties of secondary organic aerosol formed following the photooxidation of isoprene. An extractive IBBCEAS instrument was developed for detecting HONO and NO2 and had a sensitivity of about 10-9 cm-1. This instrument participated in a major international intercomparison of HONO and NO2 measurements held in the EUPHORE simulation chamber in Valencia, Spain, and results from that campaign are also reported here.

X-ray and optical studies of soft x-ray transients in M31

Soft X-ray transients (SXTs) are a subgroup of low-mass X-ray binaries consisting of a neutron star or a black hole and a companion low-mass star. SXTs exhibit a sudden outburst by increasing the luminosity from ∼ 1033 to ∼ 1036−38ergs1. After spending a few months in outburst, SXTs switch back to quiescence. Optical study of the binary system during the quiescence state of SXTs provides an opportunity to discriminate between BH binaries and neutron star binaries. The first part ot this research is composed of result of 10 years joint project between Hubble space telescope and Chandra, to study SXTs in M31. The other part of this thesis focused on the light curve of bright SXTs in M31. Disc irradiation is thought to be capable of explaining the global behaviour of the light curves of SXTs. Depending on the strength of the central X-ray emission in irradiating the disc, the light curve may exhibit an exponential or a linear decay. The model predicts that in brighter transients a transition from exponential decline to a linear one may be detectable. In this study, having excluded super-soft sources and hard X-ray transients, a sample of bright SXTs in M31 (Lpeak > 1038ergs1) has been studied. The expected change in the shape of the decay function is only observed in two of the light curves from the six light curves in the sample. Also, a systematic correlation between the shape of the light curve and the X-ray luminosity has not been seen.

Broadband cavity-enhanced absorption spectroscopy with incoherent light

Although broadband incoherent light does not efficiently couple into a high-finesse optical cavity, its transmission is readily detectable and enables applications in cavity-enhanced absorption spectroscopy in the gas phase, liquid phase and on surfaces. This chapter gives an overview of measurement principles and experimental approaches implementing incoherent light sources in cavity-enhanced spectroscopic applications. The general principles of broadband CEAS are outlined and general “pros and cons” discussed, detailing aspects like cavity mirror reflectivity calibration or the establishment of detection limits. Different approaches concerning light sources, cavity design and detection schemes are discussed and a comprehensive overview of the current literature based on a methodological classification scheme is also presented.