We use x-ray emission spectroscopy to elucidate the molecular structure of liquid methanol, water, and methanol-water solutions. We find that molecules in the pure liquid methanol predominantly persist as hydrogen-bonded chains and rings with six and/or eight molecules of equal abundance. For water-methanol solutions we find evidence of incomplete mixing at the microscopic level. Our results provide a new explanation for a smaller entropy increase in the solution due to water molecules bridging methanol chains to form rings.

We use x-ray emission spectroscopy to examine the influence of the intermolecular interaction on the local electronic structure of liquid water. By comparing x-ray emission spectra of the water molecule and liquid water, we find a strong involvement of the a(1)-symmetry valence-orbital in the hydrogen bonding. The local electronic structure of water molecules, where one hydrogen bond is broken at the hydrogen site, is separately determined. Our results provide an illustration of the important potential of x-ray emission spectroscopy for elucidating basic features of liquids.

Soft x-ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close-lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x-ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×1012 photons per second on the sample with a 1% bandpass) is enough to allow soft x-ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two-dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three-axis coordinate system in order to cover the different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two-dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2-in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.

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We present carbon K emission spectra of diamond excited with high-resolution undulator radiation. The valence-band emission spectra are shown to be strongly dependent on the excitation energy, up to 20-30 eV above the C K edge. It is proposed that the dependence is indicative of the resonant inelastic scattering description of these emission spectra, i.e. , the absorption-emission process should be described as a single scattering event where the momenta of the photoelectron and the valence hole in the final state are related by momentum conservation.PACS numbers: 78.70.g, 71.25.Rk X-ray absorption and the consequent decay through xray emission near an absorption edge are generally assumed to be two independent one-photon processes. With the exception of the x-ray resonant Raman scattering [1-3] and emission from molecules [4,5], there has been no clear experimental evidence questioning the validity of this assumption and thus no need to interpret this type of photon-in-photon-out experiment by the general x-ray inelastic scattering treatment [3]. Recent synchrotron radiation studies showing strong excitation energy dependencies in the x-ray emission spectra [5][6][7][8][9][10] were mostly attributed to multielectron processes such as shakeup satellites within the "absorption followed by emission" picture [11]; i.e. , the absorption and emission processes are independent.In this Letter we report carbon K emission spectra of diamond excited with high-resolution synchrotron radiation. The intensities of valence-band features are shown to be strongly dependent on the energy of the incident photons which excite the C 1s electrons into successive symmetry points in the conduction band, suggesting the presence of correlation between the absorption and emission processes. Furthermore it appears that this correlation can be related to the momentum of the photoelectron and that of the valence hole in the final state. We propose that the excitation energy dependence can be explained most straightforwardly by treating the absorption-emission process as a single inelastic scattering process with well-defined momentum conservation.In this interpretation, as a result of the negligible momentum transfer from the soft-x-ray photon, the final state of the solid prefers to have equal momentum for the photoelectron and the valence hole. This enhances emission from symmetry points where the photoelectron has been excited to. Since, with the availability of the next generation synchrotron radiation sources, emission experiments excited with high-resolution photons will be performed with increasing frequency, our results have important implica-tions for interpreting these experiments. Finally, the use of the momentum-resolved inelastic scattering as a new band mapping technique will be discussed briefly.Diamond was chosen for this study because it has a very simple and broad band structure that has been well studied.For this system of delocalized sp electrons, correlation eA'ects, which have been found to be very important in the prev...