Reactive collisions in helium nanodroplets studied with high resolution infrared spectroscopy
Morrison, Alexander Michael
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The work presented in this thesis is centered around the use of infrared lasers to characterize molecular clusters, hydrocarbon radicals and products of combustion reactions embedded in liquid helium droplets. Much of the work involves the development and automation of a high power, infrared optical parametric oscillator (OPO). The OPO is capable of producing >1 Watt of continuously tunable idler output between 2.2 and 4.6 micrometers. The developed hardware and software allows for several hundred wavenumbers of efficient, automatic, continuous tuning of the idler wave with an absolute frequency resolution of about 20 MHz. The rotational and vibrational relaxation dynamics of the methyl radical are studied. The linewidths of these transitions in the ro-vibrational spectrum vary considerably and are rationalized in terms of the anisotropy in the He-methyl potential energy surface. This anisotropy couples the molecular rotation to the collective modes of the droplet with some rotational states being coupled more efficiently, leading to reduced lifetimes and broadened linewidths. Highly exothermic reactions have been observed to take place in the cold, dissipative environment of the helium droplet. However, it was not entirely clear whether the reaction products remain in the droplets or are expelled. In this thesis, the exothermic reaction, methyl radical plus molecular oxygen, was studied. This exothermic reaction is known to lead barrierlessly to the methyl peroxy radical, releasing about 30 kcal/mol of bond energy that is dissipated by the evaporation of helium atoms. In this study, the infrared spectroscopy shows that the reaction products are still present in the droplet and are cooled to the droplet temperature of 0.37 K.