Title :
Soft X-ray absorption spectroscopy in liquid environments
Objective:
1. To determine is the disturbance is addition of energy from the x-rays.
2. Collecting high-resolution soft x-ray emission spectra,
3. To study the interactions between soft x-rays and liquid water.
4. To find out more about the interactions among water molecules themselves and the influence of temperature and isotope substitution.
Research Methodology:
Instrument:
UHV chamber
Rowland soft X-ray emission spectrometer
Data Analysis:
The FY-SXA spectra were recorded at the BESSY-II UE56/1-SGM beamline using a 5mm x 5mm GaAsP photodiode to detect the total fluorescence yield. Using a grating with 1200 lines/mm the beamline resolution was set to 100 meV by suitable choice of entrance and exit slit settings. The liquid sample cell was introduced into a Rowland soft X-ray emission spectrometer on a XYZ manipulator as. The soft X-ray beam is coupled in and out of the liquid volume through one SiNx membrane of 150 nm thickness and 250µm × 500µm windowarea, resulting in a X-ray transmission ranging from 0.1 at 200 eV to 0.96 at 1600 eV photon energy for normal incidence or detection. Liquid can be circulated through sample volume in the measuring position allowing for in situ changes of e.g. electrolyte concentrations, pH values, etc. as well as for easy temperature control in an external cooler or heater within the fluid loop. As the pressure in the UHV chamber is not affected by the liquid sample cell, efficient photon detectors requiring high vacuum conditions such as CsI covered microchannel plates can be employed. We have chosen a normal incidence, grazing detection geometry in order to minimize saturation effects, which can distort FY-SXA spectra for concentrated species.
Theory:
The geometric structure of liquid water has been investigated in detail by many techniques, but many details are still under debate, such as the actual number of hydrogen bonds between the various water molecules. Even less is known about the electronic structure. Since it is the intermittent bonding between water molecules that gives liquid water its peculiar characteristics, the electronic structure plays a crucial role in understanding the properties of the liquid state. Consequently, information essential for insight into chemical and biological processes in aqueous environments is lacking. To address this need, researchers from Germany and the U.S. have used soft x-ray spectroscopy at the ALS to gain detailed insight into the electronic structure of liquid water. Their spectra show a strong isotope and a weak temperature effect, and, for the first time, a splitting of the primary emission line in x-ray emission spectra. By making use of the internal "femtosecond clock" of the core-hole lifetime, a detailed picture of the electronic structure can be painted that involves fast dissociation processes of the probed water molecules. On the theoretical side, ab initio molecular dynamics calculation for liquids have only recently become feasible. Experimentally, standard techniques of soft X-ray spectroscopy are difficult to apply due to high vacuum requirements. We have designed a sample cell through which a liquid can be circulated in a UHV chamber. Soft X-rays are coupled in and out of the sample volume by a SiNx membrane of 150 nm thickness, allowing to perform photon-in photon-out spectroscopy and in particular to record fluorescence yield (FY) soft X-ray absorption (SXA) and soft X-ray emission (SXE) spectra. Here we report on FY-SXA experiments on various electrolytes, complexes and bio-molecules, illustrating the capabilities of the instrument and the technique.
Result:
A classical application of SXA spectroscopy in the near edge region (NEXAFS, XANES) is the analysis of the local electronic structure and in particular the oxidation state within a sample. The different oxida-tion state of the central iron atom in the cyano complexes is apparent in the different fine structure of the spectra. Due to the high stability of the cyano complexes and the transition times for the X-ray absorption process in the femtosecond range, we expect that the spectra can be modeled by static electronic structure calculations with averages over only a few configurations. Nevertheless, the spectra of the salts in aqueous solution differ clearly from the solid state spectra, possibly due to the presence of different local geometries, the presence of higher solvation shells and a presumably increased K–Fe distance in solution. Effects of photoreduction by the X-ray beam can be seen in a small Fe2+ contribution in the solid K3Fe(CN)6 spectrum. NaCl/H2O is an omnipresent electrolyte on earth. Earth’s oceans consist on average out of a 0.6M NaCl solution. Bodily fluids and cells of, e.g. mammals contain many ions, with Na+ and Cl− being the ions with by far the highest concentrations in plasma and interstitial liquid. A 0.15M solution of NaCl is isotonic to human blood. Na+ ions in aqueous solution are surrounded by water molecules with the O atoms directed towards the Na+ ion. Calculations indicate that the first solvation shell is made up out of six H2O molecules, with an Na–O distance of 2.3Å for certain concentrations. Electrostatic effects in electrolyte solutions are described under certain simplifying approximations by Debye–Hückel theory, which predicts the existence of a cloud of oppositely charged counter ions around any given ion. Mathematically, the counter ion charge can be envisioned as being concentrated in a shell of radius rD, the Debye length. For 0.1Mand 5.0MNaCl/H2O, rD = 9.6 and 1.4 Å, respectively. Due to its approximations in particular treating ions as point charges Debye–Hückel theory cannot be expected to be quantitatively valid for concentrations above 0.1 M. The Na–Cl nearest neighbor distance in solid NaCl is 2.82 Å. The numbers calculated above nevertheless suggest that the counter ions and ions in concentrated solutions will come in into close proximity. At room temperature, 6.14M NaCl is a saturated solution. We present Na 1 s FY-SXA spectra for 0.1 and 5.0M aqueous NaCl solutions. Clear changes as a function of concentration can be observed, in particular increased spectral weight in he low energy part of the spectrum around 1075 eV. Even for 5M NaCl, saturation effects in FY-SXA are below 1% in our experimental geometry (glancing angles: in = 90◦,out < 10◦), as calculated by the formulas in ref. We assume that the observed spectral changes are due to Cl− ions entering the first H2O coordination sphere around the Na+ ions. We are currently carrying out electronic structure calculations in order to explain the experimental observations. The effects of solvent exchange on NaI solutions, where we compare Na 1 s FY-SXA spectra for 1M NaI solutions in water and ethanol. Clear changes in the electronic structure at the sodium site are visible. Somewhat similar to the case of increased concentration of NaCl dissolved in water , spectral weight is redistributed to lower energies in the ethanol complex as compared to the solution in water. While there is no experimental data on the structure of the Na complex in ethanol, a five-fold coordination of Ni with methanol and the presence of one Cl atom in the same coordination shell has been observed for solutions of NiCl2 in methanol. If a similar geometry, including an Iodine atom in the first coordination shell, is present in our situation, the redistribution of spectral weight may be attributed to the proximity of the counter ion, analogous to the aqueous solutions of NaCl. In order to clarify the reasons underlying the observed changes in the spectra, electronic structure calculations based on molecular dynamics calculations have to be carried out. X-ray absorption studies at the Fe 2p absorption edges in biologically relevant molecules are presented in
Fig. 1. Fe 2p FY-SXA spectra for solid Fe-Phthalocyanin and haem-chloride (red circles) compared to solutions of the respective compounds in ethanol (blue circles).
The haem complex is crucial in O2 transport in blood for respiration in humans, consisting of Fe2+ in a porphyrin ring molecule. In chordate animals four haem complexes are coupled to four peptide chains, forming hemoglobin. In other phyla of animals such as, e.g. spiders, haem like molecules are colloidally solved in blood. The electronic structure of haem and Fe-Phthalocyanin as a model substance has been investigated in solutions in ethanol. In Fig. 1, Fe 2p absorption spectra are compared for solid samples and room temperature saturated solutions. The observed differences between solid state and solution are more pronounced in the case of Fe-Phthalocyanin, presumably due to stronger intermolecular coupling in closely stacked solid Phthalocyanins. In the solutions, the difference in the ring structure clearly produces a different electronic structure locally at the Fe sites.
Conclusion:
We have demonstrated the possibility to perform photon-in photon-out spectroscopy in the soft X-ray range on liquid samples, employing a simple, UHV compatible liquid sample cell. Depending on electrolyte concentration and the solvent environment, we observe changes in the local electronic structure in sodium electrolyte systems. The most pronounced changes are interpreted as being caused by the presence or absence of counter ions in the first coordination shell of sodium. Electronic structure calculations in order to check this hypothesis and to disentangle the influence of different structural configurations in solution are currently in progress. Depending on the fluorescence yield of the atomic species under investigation, concentrations down to 100mM can currently be easily investigated using a photodiode as detector. For sodium electrolytes, it is thus possible to investigate the concentration interval from concentrations encountered in human blood to saturated solutions. For many biological studies, however, spectroscopy of species concentrated in the 1mM range and below is required. In this low concentration regime, saturation effects in fluorescence yield detection become negligible and a large solid angle can be used for detection. In this way, an increase of the detectable signal by a factor of 100 is easily achievable. If stray light due to reflected X-rays and parasitic UV-radiation can be suppressed, this gain in signal will translate into increased sensitivity. Furthermore, single photon counting detection with high quantum efficiency can easily be implemented in the vacuum environment, leading to a further increase in sensitivity. As a result, we expect to be able to study electrolytes with concentrations below 1mM in the near future, i.e. in a concentration range relevant to many biological questions.
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