【摘要】Understanding the details of the intensities and spectral shapes of x-ray absorption spectra is a long-standing problem in chemistry and physics. Here, I present detailed studies of x-ray absorption for prototypical liquids, solids and gases with the goal of enhancing our general understanding of core-level spectroscopy via comparisons of modern theory and experiment.;In Chapter 2, I investigate the importance of quantum motions in the x-ray absorption spectra of simple gases. It is found that rare fluctuations in atomic positions can be a cause of features in the spectra of gaseous molecules.;In Chapter 3, I explore a novel quantization scheme for the excited and ground state potential surfaces for an isolated nitrogen molecule. This allows for the explicit calculation of the \"correct\" transition energies and peak widths (i.e. without any adjustable parameters).;In Chapter 4, the importance of nuclear motion in molecular solids is investigated for glycine. We find that the inclusion of these motions permits the spectrum to be accurately calculated without any additional adjustable parameters.;In Chapter 5, I provide a detailed study of the hydroxide ion solvated in water. There has been recent controversy as to how hydroxide is solvated, with two principal models invoked. I show that some of the computational evidence favoring one model of solvation over the other has been either previously obtained with inadequate precision or via a method that is systematically biased.;In Chapter 6, the measured and computed x-ray absorption spectra of pyrrole in both the gas phase and when solvated by water are compared. We are able to accurately predict the spectra in both cases.;In Chapter 7, the measured x-ray absorption of a series of highly charged cationic salts (YBr3, CrCl3, SnCl4 , LaCl3 and InCl3) solvated in water are presented and explained.;In Chapter 8, the measured x-ray absorption spectrum at the nitrogen K-edge of aqueous triglycine is presented, including effects of various salts which can alter its solubility. This is used to show that while x-ray absorption is sensitive to salt interactions with small peptides, it is unlikely to be a sensitive probe for overall protein structures, i.e. to distinguish beta sheet from an alpha helix at the nitrogen K-edge.;Finally, in Chapter 9 future directions are discussed.