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High resolution absorption spectra of the IF molecule in the X¹Σ⁺ ground state were observed with the Kitt Peak Fourier transform spectrometer in a F₂/I₂ flame. Iodine fluoride is the most unstable interhalogen compound. Accurate wavenumber measurements (±0.0002 cm⁻¹) were made for the 1-0, 2-1 and 2-0 bands and molecular constants were derived. The observation of a high resolution emission spectrum at 13 μm (750 cm⁻¹) is unusual. Seven bands (1-0 through 7-6) of the main isotopic form, ²⁸Si³²S, were observed along with three bands (1-0, 2-1 and 3-2) for each of the minor species, ²⁹Si³²S, ³⁰Si³²S and ²⁸Si³⁴S. More than 2450 lines were fitted for this important astrophysical molecule. Dunham coefficients were obtained for each isotopomer of SiS. Mass-reduced Dunham parameters, including Watson's correction due to the breakdown of the Born-Oppenheimer approximation, were also derived from the data. The first high resolution spectrum of a metal dihalides was recorded by infrared emission spectroscopy. The infrared emission spectrum of BeF₂ was observed in the region of the antisymmetric stretching mode, ν₃, and the combination band, ν₁ + ν₂. Eight vibration-rotation bands were rotationally analyzed and the spectroscopic constants are reported. The equilibrium beryllium fluoride bond distance (rₑ) was found to be 1.3729710(95)Å in BeF₂. Values of the vibrational frequencies of all the three normal modes were obtained from the spectra. The gas phase infrared spectrum of C₆₀ (buckminsterfullerine) has been observed in emission with the National Solar Observatory Fourier transform spectrometer at Kitt Peak. Bands attributable to the C₆₀ molecule are found at 527.1 cm⁻¹, 570.3 cm⁻¹, 1169.1 cm⁻¹ and 1406.9 cm⁻¹. Additional emission features are tentatively assigned to C₇₀ or combination bands of C₆₀. A new, strong emission is observed at 1010.2 cm⁻¹ belonging to an unknown molecule. A new electronic state of ³Π symmetry of PtO has been observed between 7100 cm⁻¹ and 8015 cm⁻¹ above the ground state. This new state was observed through an electronic transition to the ³Σ⁻ ground state of this free radical. Three independent electronic systems connecting the Ω = 0,1 and 2 spin components in the upper state to Ω = 1 component in the ground state have been recorded in emission with the Fourier transform spectrometer associated with the McMath Solar Telescope at Kitt Peak. (Abstract shortened with permission of author.)

This dissertation presents the results of several investigations into nonadiabatic reaction dynamics in the 0.5 to 10 K temperature regime. The free jet flow reactor technique for production of very low local temperatures and the method for extraction of reaction rate coefficients in this unique environment is reviewed. Ion-neutral reactions which exhibit nonadiabatic behavior are initiated by state selective resonantly enhanced multiphoton ionization and the reactions are subsequently studied using time of flight mass spectrometric detection. The importance of long lived collision complexes in nonadiabatic ion-neutral reactions is reemphasized. Collisional electronic spin orbit relaxation of Xe⁺(²P(½) is shown to be very inefficient for a wide variety of collision partners. A sequential two electron charge transfer mechanism is proposed to account for the high efficiency of the fine structure relaxation by methane and nitrous oxide both of which have open charge transfer channels. The results of fine structure state specific reactions of Ar⁺(²P(J)) with H₂, D₂, HD, CH₄ and CD₄ are presented as examples of nonadiabatic atom and electron transfer reactions. Preliminary results on vibrational-rotational relaxation of neutrals in the free jet at very low temperatures using a pump-probe technique are presented.