Astrochemistry and combustion studies with shock waves

Abstract

Shockwaves and polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM) and on Earth. Combustion chemistry provides the fundamental reaction routes and mechanisms for PAH formation. These fundamental chemical principles governing hydrocarbon combustion can be utilized to describe the formation and destruction of PAHs in astrophysical environments under similar shocked environments. The primary focus of my thesis is PAHs, specifically examining the thermal processing of PAHs through shock waves in astrochemistry and the formation of aromatics, which eventually form PAHs during the pyrolysis of hydrocarbons in combustion experiments. The formation and breakdown of PAHs are assisted by shockwaves under well-defined reaction times, high temperatures, and highpressure conditions generated via shock tube facilities. Home-built shock tube facilities in the Laboratory for Hypersonic and Shockwave Research (LHSR) lab were used to investigate high-temperature chemical kinetics and spectroscopy experiments relevant to astrochemistry and combustion. The ’single pulse nature’ in the shock tube causes the reflected shock to increase the temperature up to ∼6000 K for ∼2-3 ms of dwell time. The molecules are heated behind the reflected shock wave through collisions that facilitate energy transfer between the driver gas, argon, and the sample molecule, while the wall of the shock tube remains at an ambient temperature. Chapter 01 explores the occurrence of PAHs both on Earth and in space, highlighting the significance and justification for investigating PAHs in the pyrolysis of both hydrindane and methylcyclopentane. Chapter 02 examines the specifics of the shock tube facilities, emission spectroscopy tools, gas chrovi matography instruments, and the computational methodologies used in this thesis. For the astrochemistry experiments discussed in Chapter 03, a new Material Shock Tube (MST) facility was established with an optical access chamber focusing on time-resolved and time-integrated emission spectroscopy in the UV-Vis-NIR region. The processing of interstellar dust analogues, including PAHs and fullerenes, via UV irradiation or ion/electron bombardment has been extensively studied; however, investigations through shock processing remain limited. The prototypical PAH coronene (C24H12) was selected for the shock processing study with a focus on the C2 (Swan band) time-resolved emissions in the UV-vis region. Key findings from the research show that the shock waves enable the conversion of polycyclic aromatic PAHs into smaller hydrocarbons CnHx, a potential DIB carrier, and carbon clusters that subsequently recombine during the cooling phase into graphene-like structures, affecting the carbon lifecycle of the interstellar medium. The identification of CnHx (where n = 3–9 with a predominance of odd-numbered n, and x = 1–3) species as potential carriers of a broad green emission seen in the laboratory suggests a possible link to the 5450 ˚A diffuse interstellar band (DIB). The results were supported by the collection and analysis of solid residues post-shock, as well as molecular dynamics (MD) simulations conducted by various collaborators. This study establishes a foundation for future research focused on understanding the complex interactions among PAH and shocks, as well as the evolution of carbonaceous materials in the interstellar medium. The second section of my thesis is related to combustion experiments conducted in the shock tube, as discussed in Chapters 04 and 05. For combustion experiments, the existing Chemical Shock Tube (CST) facility was modified to study the thermal decomposition of hydrocarbon fuel surrogates. Transhydrindane (C9H16), comparable to trans-decalin ((C9H18), was selected as a bicyclic alkane substitute for diesel or jet fuel, while methyl cyclopentane ((C6H12)), a branched monocyclic alkane, was chosen for naphtha or gasoline in the pyrolysis experiments. A wide variety of hydrocarbons, such as alkanes, alkenes, cycloalkanes, and aromatics, were identified through GC (gas chromatography) analysis. The experimental results were validated against a detailed kinetic model developed from high-level quantum chemistry and rate calculations, which provides fundamental insights into the elementary steps of the thermal decomposition. These studies will be further used within the laboratory to develop surrogates for ongoing RP-1 (rocket propellant) fuel experiments in the shock tube and the new flow reactor facility. In conclusion, Chapter 06 summarizes the findings from the shock tube experiments used to develop and validate theoretical kinetic models, which in turn facilitate the interpretation of experimental observations, resulting in an in-depth understanding of PAH chemistry in both Earth and astrophysical settings. In addition to these chapters, the appendix contains the detailed reaction mechanisms for the pyrolysis of both hydrindane and methylcyclopentane