Juddha Thapa

Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences



Committee Chair

Fabien Goulay

Committee Co-Chair

Brian Anderson

Committee Member

Michelle Richards-Babb

Committee Member

Kenneth Showalter

Committee Member

Kung Wang


The incomplete combustion of fuels inside of an internal combustion engines generates unwanted byproducts such as soot. Because of health and environmental concerns, soot formation has been a very active area of research in combustion chemistry. However, the mechanism of formation of soot is still not well understood. It has been proposed that the soot formation is initiated by the reaction of small free radicals with abundant hydrocarbon fuel molecules producing aromatic ring structures at high temperatures. These aromatic ring structures further react to form polycyclic aromatic hydrocarbons (PAHs) that are stable at the high temperatures of combustion environments. These PAHs collide and stick with each other forming dimers, trimers, tetramers, etc. Eventually such stable PAHs-stabilomers condense and transform into solid particles (soot). To minimize pollutants and increase the efficiency of engines, it is very important to understand the chemistry of the elementary reactions at the molecular level.;The reactions of hydroxyl free radicals with polycyclic aromatic hydrocarbon precursor molecules are studied experimentally in a quasi-static gas cell using laser pump-probe spectroscopy. Hydroxyl free radicals are generated by pulsed laser photolysis (PLP) using the third (355 nm) or fourth (266 nm) harmonic of Nd:YAG laser and their concentration is monitored as a function of laser delay-time using a frequency-doubled tunable dye laser perpendicular to the photolysis laser. The off-resonance fluorescence from the hydroxyl free radicals at 310 nm is collected by a photomultiplier tube (PMT) placing it orthogonal to the photolysis and probe laser beams. The reactions of hydroxyl radicals (OH) with phenylacetylene and fulvenallene have been investigated from 298 K to 450 K. The concentrations of the hydrocarbon reactants are measured using FTIR spectroscopy and UV absorption. The room temperature reaction rate of the OH + phenylacetylene reaction is measured to be 8.75(+/-0.73)x10-11 cm3s-1. The reaction rate coefficient is pressure and temperature independent over the 1-7.5 Torr and 298-423 K pressure and temperature ranges. The rate coefficient is larger than that expected based solely on association with the aromatic ring, which suggests reaction with the triple bond. For the OH + fulvenallene reaction, the room temperature rate coefficient is found to be 8.8(+/-1.7)x10-12 cm3s-1 with the negative temperature dependence. The comparison of the experimental rate coefficients with the calculated abstraction rate coefficients suggests that over the experimental range, association of hydroxyl radical (OH) to fulvenallene plays the significant role toward the formation of PAH precursors.