Laboratory Studies of Atmospheric Reactions Using Infrared Cavity Ringdown Spectroscopy

Abstract

Infrared cavity ringdown spectroscopy provides for sensitive spectroscopic detection with a time scale of microseconds. This thesis discusses the implementation of infrared cavity ringdown spectroscopy for studies of atmospheric reactions. Chapter 1 details the infrared cavity ringdown spectrometer assembled in our laboratory. Pulsed infrared radiation was generated by pumping an optical parametric amplifier (OPA) with 532 nm light from a doubled Nd:YAG laser, and a tunable dye laser provided the signal frequency. The ringdown decay was fit to an exponential curve to extract a ringdown time from which concentrations of absorbing species could be determined. Initial measurements indicated that our apparatus is a highly sensitive spectroscopic probe of infrared transitions of molecules with absorption linewidths greater than 1 cm^(-1). Our detection limit for the j = 7 line of the v_3 band of methane in a 43 inch cell was 10^(13) molec/cc of methane. Chapter 2 describes initial kinetics experiments performed with our apparatus for the reactions of HO_2 + HO_2 and HO_2 + ClO. Our results for the rates of the HO_2 + HO_2 reaction and the HO_2 + ClO reaction agree well with literature values, demonstrating the capabilities of our system for use in kinetic studies. We were unable to observe the formation of the postulated HOOOCI intermediate from the reaction of HO_2 + ClO due to spectral interference from H_2O_2 in the region where HOOOCl is predicted to absorb. Chapter 3 discusses studies of the reaction pathways undergone by alkoxy radicals. We found that photolysis of the alkyl nitrite precursor at 248 nm resulted in an unexpectedly high rate of decomposition of the alkoxy radicals, and we attributed this result to excess energy in the alkoxy radicals following photolysis at this wavelength. With photolysis at 351 nm, no decomposition products were detected. At this wavelength, we observed an -OH peak at 3675 cm^(-1) for the n-butoxy and n-pentoxy radicals, which is evidence of the isomerization channel. For the n-butoxy radical, we determined that the ratio of the rate for its reaction with O_2 versus its isomerization (k_(O2)/k_(isom)) was (4.0±0.1)*10^(-20) cm^3 molec^(-1) when probing the isomerization product milliseconds after the generation of the n-butoxy molecule and (3.6±1.4)* 10^(-20) cm^3 molec^(-1) when probing the isomerization product 10-20 µs after the generation of the nbutoxy radical. For the n-pentoxy radical, we placed an upper limit on the ratio of k_ (O2)/k_(isom) of 5.6*10^(-21) cm^3 molec^(-1).