Elsevier

Chemical Physics

Volume 377, Issues 1–3, 25 November 2010, Pages 1-5
Chemical Physics

High resolution X-ray emission spectroscopy of water and aqueous ions using the micro-jet technique

https://doi.org/10.1016/j.chemphys.2010.08.023Get rights and content

Abstract

Soft X-ray absorption (XA) and emission (XE) spectroscopy is a powerful method for probing the local electronic structure of light elements (e.g. C, O, N, S) and transition metals, which are all of importance for biochemical systems. Here, we report for the first time on the XE spectra of a liquid micro-jet sample in a vacuum environment. We developed a high resolution X-ray emission spectrometer and recorded the spectra of pure water in full agreement with those of the literature, as well as of an aqueous solution of NiCl2. For the latter system, ground state Hartree–Fock calculations using a self-consistent reaction field (SCRF) approach were carried out to specify the nature of the d-occupied orbital. Our results confirm the dark-channel-fluorescence-yield mechanism that we recently proposed for the case of metal ions in aqueous solutions. The ability to record absorption and emission spectra of an aqueous liquid-jet opens the way for the study of biochemical systems in physiological media.

Graphical abstract

Soft X-ray absorption (XA) and emission (XE) spectroscopy is a powerful method for probing the local electronic structure of light elements (e.g. C, O, N, S) and transition metals, which are all of importance for biochemical systems. Here, we report for the first time on the XE spectra of a liquid micro-jet sample in a vacuum environment. We developed a high resolution X-ray emission spectrometer and recorded the spectra of pure water in full agreement with those of the literature, as well as of an aqueous solution of NiCl2. For the latter system, ground state Hartree–Fock calculations using a self-consistent reaction field (SCRF) approach were carried out to specify the nature of the d-occupied orbital. Our results confirm the dark-channel-fluorescence-yield mechanism that we recently proposed for the case of metal ions in aqueous solutions. The ability to record absorption and emission spectra of an aqueous liquid-jet opens the way for the study of biochemical systems in physiological media.

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Introduction

Spectroscopic techniques based on photon-in/photon-out processes using synchrotron radiation have proven to be highly sensitive tools for investigating the local electronic structure of condensed matter and of chemical and biological systems. Soft X-ray absorption [1], [2], [3], [4], [5], [6], [7], [8], [9] and emission spectroscopy [10], [11], [12], [13], [14] of liquid systems can probe the elements of life as e.g. C, O and N through the K-edges [1], [8], [11], [15] and transition metals through the L-edges [5], [16], [17]. Recently, the XAS technique was further extended for investigating in situ the preparation process of solar cells [18], [19], as well as for probing the transition metal active centre of proteins and enzymes under physiological conditions [16], [17], [20]. In most of these recent XA studies on liquids, the samples were contained in a cell consisting of soft X-ray transparent thin (few hundreds of nm’s) membrane windows made of silicon nitride (Si3N4). Flowing the liquid behind the membrane, as can be done with flow-cells, additionally reduces the risks of sample damage with respect to static drop-behind-membrane cells. However, in using a membrane several issues have to be considered: (i) the membrane should be ideally transparent for the respective fluorescence light of interest. Therefore elements that are contained in the membrane material (Si and N) or are energetically close to them are not directly measurable. (ii) Since no membrane is ideally transparent for any energy range, measuring techniques which require a high incoming flux like, e.g. X-ray emission spectroscopy (XES), suffer from the loss of photons leading to long data acquisition times [3]. In the case of very low concentration samples, such as biochemical systems in physiological media, the loss of intensity is detrimental [16], [17], [20]. (iii) Interactions with the membrane, like hydrophobic or hydrophilic effects can induce artifacts in the spectra. As mentioned in the literature, the membrane can also react chemically upon X-ray radiation with the sample (e.g. oxidization of the inner surface of the Si3N4 membrane) and change its nature as a function of irradiation [14]. Such changes may affect the XA and XE spectra and cannot be neglected.

To overcome these issues, in this work fluorescence yield XAS and XES are combined for the first time with the micro-jet technique. We demonstrate this new approach by investigating the oxygen K-edge of liquid water and the Ni L-edge of an NiCl2 aqueous solution under resonant and non-resonant excitation. We present our preliminary results to demonstrate the capability of our new setup. A detailed discussion of our results, in particular for the case of aqueous Ni2+ ions will be given later. The series of XE spectra of water measured from the liquid-jet give qualitatively the same characteristics as what has been shown before for water behind an Si3N4 membrane and show a resolution comparable to the most recent publications on high resolution XES [10], [14]. For the aqueous NiCl2, concentrations down to 250 mM were measured, but according to the signal to noise ratio, even lower ion-concentrations can be measured with this setup. Moreover, the emission lines reflect what we recently proposed as the dark-channel-fluorescence-yield (DCFY) mechanism [21], which reveals electron delocalization in mixed orbitals between water and nickel. For the interpretation of the emission spectra of aqueous NiCl2, theoretical calculations using a Hartree–Fock (HF) approach combined with a self-consistent reaction field (SCRF) to include solvent effects were carried out.

Section snippets

Experimental setup (LiXEdrom)

The samples were measured with the LiXEdrom-setup at the U41 PGM beamline of BESSY. This spectrometer was developed for investigating liquid samples using the micro-jet technique. The XA spectra were recorded in the total fluorescence yield (TFY) mode with a GaAs diode (grounded and with shielded connections to avoid disturbing signals from electrons). For the XE measurements a Gamma Data MCP-CCD-detector combined with a self-developed grating holder was used in Rowland-geometry in a slit-less

Theoretical calculation

To reveal the nature of the molecular orbitals (MOs) of Ni2+ in water which are involved in the XE spectra we carried out ground state Hartree–Fock (HF) calculations using the Gaussian03 program package [23]. An often followed approach to include solvent effects into high level ab initio calculations are self-consistent reaction field (SCRF) methods. In these methods, the solvent is modeled as a continuum of uniform dielectric constant (called the reaction field) and the solute is placed into a

Results and discussion

In the following the results of the measurements obtained from liquid water will be discussed and compared to the recently published high resolution spectra of water [10], [14]. In the second section, we will present the XE spectra obtained from Ni2+ resonant excitation around the L3-edge of NiCl2 aqueous solution. We will compare these results with our results from drop-behind-membrane measurements at beamline 7, ALS-Berkeley lab [3] and verify the validity of the DCFY mechanism that we

Conclusion

Our new high resolution XE and XA spectrometer (LiXEdrom) setup for probing the local electronic structure of elements in aqueous solution has been used with a soft X-ray synchrotron light source and the micro-jet technique. With the advantages of the beamline U41-PGM at BESSY II, a small focus and a high flux, the spectrometer is able to probe freshly introduced liquid samples avoiding X-ray induced sample damage. As a demonstration of its capability, in this study we presented two examples of

Acknowledgments

The authors thank to Prof. Dr. Jan-Eric Rubbensson for his advices during the building of the XE spectrometer, Dr. Carlo Callegari, Agne Sulciute and Rafael Vescovi for their help during the beamtime and the entire workshop at BESSY/HZB for their support. We acknowledge support of the German-Russian Interdisciplinary Science Center (G-RISC) and the Helmholtz-Gemeinschaft through the young investigator fund VH-NG-635.

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    K.M.L. and R.K. contribute equally to this work.

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