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The first part of this dissertation covers the microwave spectroscopy of the weakly bound complexes HI-HF, H₂S-SO₂, Ar-H₂S, and N₂O-HF. These molecules were investigated using a Flygare-Balle type pulsed-beam Fabry-Perot Fourier-transform microwave spectrometer. The spectrometer and its construction are described. The HI-HF complex was found to be hydrogen bonded through the hydrogen of the HF. The hydrogen bond length was found to be 2.720A with a structure such that the monomer bonds form an acute angle of 70°. For H₂S-SO₂ a structure with a S-S distance of 3.67A and H-O distances of 3.1A was obtained. It was established that the hydrogens of H₂S are equivalent is this species. A new set of transitions for Ar-HDS were observed indicating that the hydrogens in this molecule are not equivalent. Several new transitions were also observed for Ar-H₂³⁴S, Ar-H₂S and Ar-D₂S. New insight into the structure of the Ar-H₂S molecule was obtained. Several new transitions for the bent isomer of N₂O-HF were measured and a fit of the ground state constants for this species was performed including quartic distortion constants. The resulting fit improves calculated line centers by two orders of magnitude over previous results. The second part of this dissertation covers the infrared spectroscopy of the ν₆ and ν₈ bands of formic acid. Fourier transform data for these bands were obtained at 0.01 cm⁻¹ resolution using the spectrometer at the Kitt Peak National solar observatory. Diode laser data at 0.001 cm⁻¹ was obtained for the ν₈ band. A two state a-type Coriolis coupled Hamiltonian was used to perform a global fit on all the available data. A greatly improved set of spectroscopic parameters for these two bands were obtained. This new set of parameters has allowed several previously unassigned far I.R. laser and I.R. laser saturation lines to be assigned. Using these improved constants, it should be possible to predict the frequencies for several formic acid far I.R. laser transitions which are not yet accurately measured.

The UV-visible absorption spectrum of carbon vapor trapped in solid argon at low temperature contains many intense features, but only those of C₂ and C₃ have been identified. For this work, graphite vapor was produced under high vacuum conditions and condensed with argon onto a cold (∼10K) substrate. The resulting matrix-isolated carbon molecules were analyzed with both UV-visible and infrared absorption spectroscopy. Slight warming of the sample allowed formation of larger molecules and subsequent spectra traced the growth of their absorption features. The experiment associated infrared features to particular UV-visible features via their growth curves. The most reliable correlations are listed below. Theoretical calculations of equilibrium geometries and IR vibrational frequencies were performed on linear and nearly linear carbon chain molecules from C₃ to C₉ at the HF/SCF level of theory and from C₃ to C₇ at the MP2 level of theory, both using the 6-31G* basis set. Tentative assignment of the UV-visible features to molecules was made based on these considerations and on the following: the experimental IR/UV-visible correlations, previous experimental IR work, and modeling of the growth of the UV-visible features during matrix annealing. (UNFORMATTED TABLE FOLLOWS:) Molecule, UV-visible Feature (Å), IR Feature (cm⁻¹), Correlation Coefficient. C₃ (linear), 4100, 2040, .98. C₈ (linear), 3075, 1998, .97. C₈ (cyclic), 3900, 1804, .90. C₆ (linear), 2465, 1952, .86. C₇, 3480. C₉, 4480.

Recent improvements in infrared detector arrays make it possible for the first time to conduct detailed spectroscopic studies of a complete range of objects in the 0.9-1.35 μm region. In this dissertation, I examine the 0.9-1.35 μm spectra of planetary and proto-planetary nebulae, M dwarfs, young stellar objects, Seyfert galaxies, an H II region, and a Wolf-Rayet star. Line identifications are made for each of these objects, and extensive line lists are presented. I also investigate what the lines can tell us about each object. The 0.9-1.35 μm spectrum of the proto-planetary nebula AFGL 618 is dominated by recombination lines, low-ionization, shock-excited lines, and thermal and fluorescent H₂ lines. We use ratios of forbidden lines to show that there are two distinct physical regions in the lobes of AFGL 618, including one which must have been excited by shocks. We also show that the H₂ lines in the 0.9-1.35 μm region are ideal for detecting low levels of fluorescent H₂ emission, even when a strong thermal component is present. We present 0.6-1.5 μm spectra for M dwarfs ranging from M2 through M9. These spectra are compared with recent theoretical models, and a temperature scale is determined. In late-M dwarfs, the shape of the infrared spectrum and the depth of the 1.35 μm H₂O feature are good temperature indicators. The temperatures we derive for the M dwarfs are higher than the temperatures found in earlier studies and are in closer agreement with theoretical tracks of the lower main sequence. We present 0.9-1.35 μm spectra for 7 young stellar objects. These objects exhibit a wide variety of behavior, including strong fluorescent emission. We show that the infrared spectra can be used to study all of the regions that are detected with visible and red spectra. As a result, 0.9-1.35 μm spectroscopy should be quite useful for studying heavily embedded sources. The 0.9-1.35 μm spectra of high-excitation objects include a number of distinctive features including He II lines, several high ionization lines, and very strong (S III) lines. We find that the excitation level of a source can be estimated based on these features alone.

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