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Synchrotron radiation photoelectron studies for primary radiation effects using a liquid water jet in vacuum: Total and partial photoelectron yields for liquid water near the oxygen K -edge
ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 1202–1206
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Synchrotron radiation photoelectron studies for primary radiation effects using a liquid water jet in vacuum: Total and partial photoelectron yields for liquid water near the oxygen K-edge Masatoshi Ukai a,, Akinari Yokoya b, Yusuke Nonaka a, Kentaro Fujii b, Yuji Saitoh c a
Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei-shi, Tokyo 184-8588, Japan Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan c Synchrotron Radiation Research Center, Japan Atomic Energy Agency (JAEA), Sayo-gun, Hyogo 679-5148, Japan b
a r t i c l e in f o
a b s t r a c t
Keywords: Soft X-ray synchrotron radiation Liquid water jet Auger electron spectroscopy Primary radiation effect
A new spectroscopy to identify the hydration structure playing important role in liquid-phase radiation damage is in progress using a laminar liquid water jet sample in vacuum in combination with soft X-ray synchrotron radiation. We present the total and partial electron yields for liquid water using a photoelectron spectroscopy. Partial electron yields for the K 1 1b1 1b1 Auger transition are obtained for the first time by measuring the electrostatically dispersed electron kinetic energy spectra as a function of photon energy of synchrotron radiation. & 2009 Published by Elsevier Ltd.
1. Introduction The geometrical structures of molecules in liquid and their electronic energies are of significant importance in the research of radiation interaction with matter to understand the mechanism of inducing damage as well as the mechanism of spacial- and timepropagation of unstable species in liquid. Primary processes of bio-chemically important molecules can provide proper prototypes of the above mentioned processes in a cell mimetic condition. It can be safely assumed that the conformational structure of, ca, DNA is determined by the hydrogen-bonded ‘‘hydration network’’ of water molecules surrounding it. The conformational structures further determine the electronic energies of hydrated complex and, presumably, the chemical reaction pathways in the physiological functions of the complex. This assumption should also allow us to have the following thermodynamical viewpoint on the induction of radiation damage and the electronic or chemical restoration. The intact bio-molecules in a cell are in their thermochemically and ‘‘biologically stable’’ states defined by the hydrationnetwork-aided natural conformation. The primary irradiation products, i.e., the ionic and excited sites of molecule induced by the primary radiation interaction, are regarded both as the cores of remaining radiation damage and as the localized excess energies. The excess energies should diffuse into the environment
Corresponding author.
E-mail address: [email protected] (M. Ukai). 0969-806X/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.radphyschem.2009.07.011
with time, which finally gives rise to a metastable thermal state involving a stable damage or restores the original state of natural molecular forms. The ‘‘hydration network’’ plays important roles both as the intermediary of ‘‘heat’’ transport to environment and as the structural memory of the original hydrogen-bonded conformation in restoration processes. We thus remark the necessity of experimental investigation of water as a bio-cell solvent. ESCA (electron spectroscopy for chemical analysis) study (Siegbahn, 1974) for water and water solution is expected to provide a certain evidence of chemical structures of molecules aided by hydrogen bonds. A new spectroscopic study using a soft X-ray synchrotron radiation for photoelectrons ejected from a liquid solution in the form of laminar jet in vacuum is in progress (Ukai et al., 2008). The technique of liquid beam in vacuum (Fuchs and Legge , 1979; Faubel et al., 1988; Faubel and Steiner, 1992; Faubel and Kisters, 1989) has been employed in several experiments of physicochemical spectroscopy (Faubel et al., 1997, 1999; Moberg et al., 1991; Morgner, 1998; Sobott et al., 1999; Horimoto et al., 2000). However, biological application is few. We present the results of total and partial photoelectron yields for liquid water. We remark that the environmental hydration gives rise to the influence on the eigen-state energy of the excited molecular orbital more largely than on the energies of the valence and inner shell orbitals (Ukai et al., 2008). Total photoelectron yields representing photoabsorption cross sections of water as a function of photon energy give a typical example of the environmental chemical shift of excited state. Another important aspect of the research for radiation interaction in liquid water is facilitating simulation of radiation
ARTICLE IN PRESS M. Ukai et al. / Radiation Physics and Chemistry 78 (2009) 1202–1206
risk. The extent of indirect radiation interaction due to secondary electrons and radicals produced in bulk cell liquid (Yokoya et al., 2006) can be evaluated with the use of the simulation parameters such as total cross sections for known primary radiation energies and partial cross sections for identified secondary electron energies in liquid water (Watanabe and Nikjoo, 2002; Watanabe et al., 2004). Bound electrons in a water molecule have large amplitudes of oscillator strength at the energies in the vicinity of oxygen K-shell edge to respond high-energy charged particles. As well as ESCA analysis to determine hydrated structures of molecules, photoelectron spectroscopy is a powerful tool to obtain yields of photoelectrons with identified energies.
2. Experiment In order to maintain a liquid water sample in vacuum we employ a liquid jet beam technique. The liquid sample maintained in the form of thin laminar-flow in vacuum is intersected by a mm-size synchrotron radiation beam. The photoelectrons ejected from the liquid beam are counted using a secondary electron multiplier. The technique of liquid jet is to delay the actual phase transition and to realize a practical size of liquid sample for experiments in vacuum vessel. The behavior of water sample has been described elsewhere (Ukai et al., 2008). Briefly, the liquid water introduced through mm-size orifice into vacuum with a certain stagnation pressure finds itself in the gas phase due to the abrupt decrease of atmospheric pressure. However, owing to the insufficient amount of thermal internal energy evaporation of water only occurs from the surface of the beam. Release of the
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heat of evaporation gradually decreases the temperature of the liquid as a function of the flight distance of the liquid beam in vacuum. Although the liquid beam will be finally caught into the solid phase, yet most of the water molecules are considered to remain in the liquid or supercooled liquid phase in a relatively long time compared with the time required for physical or spectroscopic processes. Hence, it is necessary to identify if the water sample is maintained in the liquid phase, which has been certified by measuring the temperature dependence of the photoabsorption spectra as a function of flight distance in vacuum (Ukai et al., 2008). Fig. 1 shows a schematic view of the present apparatus for photoelectron measurements. A water sample purified by Milli-Q system (Milli-pore, Tokyo) is admitted into vacuum through a 10 or 20 mm orifice on a platinum disk with the stagnation pressure of about 1.0–3.0 MPa using a liquid chromatograph sample injection pump. The vacuum vessel holding the liquid beam source is evacuated using a 2300 L/s turbo-molecular pump. To reduce the residual pressure in the vacuum vessel the waste sample after irradiation is collected by a liquid-nitrogen-cooling trap at the downstream of the jet beam. The vessel is also equipped with a liquid-nitrogen-cooling 20,000 L/s cryogenic evacuation panel. The pressure in the vacuum vessel is typically 1 10 2 Pa on the admission of the liquid sample. Another vacuum vessel holding a photoelectron spectrometer and secondary electron multiplier is connected to the vessel for the liquid beam source though an orifice of 1 mm in diameter and evacuated differentially using a 700 L/s turbo-molecular pump. The pressure of vacuum in the spectrometer vessel is typically 1 10 4 Pa on the admission of the liquid sample into the liquid beam source vessel.
Fig. 1. Schematic experimental apparatus of synchrotron radiation photoelectron spectroscopy for liquid water jet.
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The photoelectron measurements are carried out at SPring-8, Japan. The pseud-monochromatic undulator synchrotron radiation from the beam line BL-23SU is dispersed using a grazing incidence varied line spacing monochromator to provide the monochromatic synchrotron radiation (Saitoh et al., 2001). The monochromatic synchrotron radiation is refocused to be the spot of size slightly less than 0.04 mm at the irradiation point using two stages of refocusing mirror systems. The absolute photon energy is calibrated for the prominent resonant absorption peaks, such as the 1s-p vibrational peaks and the 1s-Rydberg progression of oxygen molecule in the gas phase (Saitoh et al., 2001; Coreno et al., 1999; Kosugi et al., 1992). The intensity of the refocused monochromatic synchrotron radiation is 1012 photons=s with the band pass of 0.5 eV (FWHM) at the photon energies in the region of 530–548 eV in the vicinity of oxygen K-edge. The photoelectrons ejected from the liquid water beam, entering into the spectrometer vessel though the above 1 mm orifice, and passing through electrostatic lenses are energydispersed with an hemispherical electrostatic analyzer and detected using a channel electron multiplier for measurements of photoelectron spectra, or are collected using a channel electron multiplier without energy dispersion for measurements of total photoelectron yields. The intensity of photoelectrons is normalized for the photon intensity monitored by the drain current from the post-focusing mirror at each photon energy. We refer these normalized photoelectron intensities to ‘‘photoelectron yields’’.
3. Results and discussion 3.1. Total photoelectron yields for liquid water near the O K-edge The total photoelectron yields from the liquid water jet are measured as a function of the incident photon energy of 530–547 eV. The results obtained with a stagnation pressure of 3 MPa at 300 K using the 10 mm nozzle are shown in Fig. 2. As described above, the phase state of water sample is identified to be in the liquid phase by the temperature dependent
measurements, so that the ‘‘contamination’’ of the electrons ejected from water vapor or ice is negligible (Ukai et al., 2008). The total photoelectron yields are enhanced to present a broad peak at photon energies of 536–540 eV which is understood to correspond to the photoionization of a K-shell electron of oxygen atom. A small enhancement at the photon energy around 534 eV should correspond to the excitation to an anti-bonding orbital. Since the K-edge energy (K-shell ionization potential) of liquid water is supposed to lie between these two peaks, we refer the former peak to ‘‘post-edge peak’’, and the latter to ‘‘pre-edge peak’’. The present result of total photoelectron yields is in good accordance with the previously measured total photoelectron yields (Wilson et al., 2004) and total X-ray fluorescence yields (Kashtanov et al., 2004). The total photoelectron yields for water vapor evaporated from the liquid water jet are also shown for reference at the bottom in Fig. 2. The ‘‘pre-edge peak’’ occurs at the photon energy of 534 eV and some other discrete peaks also appear at photon energies of 536 and 537 eV. The remarkable difference of the spectra between liquid and vapor is ascribed by a quantum chemical analysis to the delocalized outer electron behavior of hydrogen-bonded water clusters (Wernet et al., 2004).
3.2. Partial electron yields for liquid water near the O K-edge 3.2.1. Photoelectron spectra using an elecrostatic energy analyzer In order to obtain electron yields of identified kinetic energies, we employ photoelectron spectrometer using an electrostatic hemispherical analyzer. However, reliable electron energy analysis for a liquid water has been difficult. One of the reasons is an ambiguous electrostatic potential. This is due to the facts that the evaporated water produces thin coverage of insulator on the surface of electrodes and that photoelectron bombardment or attachment to the insulator surface causes an irregular electrostatic potential. To eliminate the effect of such an irregular electrification several important elements to determine the photoelectron kinetic energy are heated during the operation of the photoelectron spectrometer. Another reason of difficulty is an electrification of liquid water jet due to photoionization. Because of the high resistivity of neat water, positive charges on the surface of liquid jet induced by photoionization do not diffuse easily, so that the electrostatic
Main Auger Peak Energy (eV)
500
Fig. 2. Total photoelectron yields for water as a function of photon energy. The upper curve shows the yields for liquid water; the lower curve shows the yields for water vapor emerging from the liquid beam. The solid circles are the partial electron yields for the K 1 1b1 1b1 Auger transition. The partial yields are normalized to the height of the total yield at 537 eV for ease of comparison. Note: ‘‘pre-edge peak’’ region around 534 eV, ‘‘main edge’’ region at 535–536 eV, and ‘‘post-edge peak’’ region at 536–540 eV (see text). O K-shell ionization potential of liquid water is not correctly determined, but should lie in the main edge region of (b).
H2O h = 540 eV 490
480
470 0
100 200 300 400 Mirror Current (X-ray intensity) (nA)
500
Fig. 3. Effect of irregular electrification of liquid water jet by photoionization. Mean energy of the K 1 1b1 1b1 Auger peak moves to lower energy with increasing the intensity of photon beam (monitored by the refocusing mirror current).
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potential of water jet is not ground but positively electrified depending on the number of ejected electrons. Fig. 3 shows the mean kinetic energy of a prominent Auger electron where significant amount of shift to lower energy is observed by increasing the photon intensity as monitored by the drain current of post-focusing mirror. We estimate the correct electron energy by the extrapolation to the zero photon intensity with referring to the total photoelectron yields as described in Section 3.1. Electron kinetic energy spectra, thus obtained with the correction of electron energies, as a function of photon energy are shown in Fig. 4, which are obtained with the photon energy bandwidth of 0.1 eV and the nominal energy resolution of the spectrometer of 0.6 eV. Well-defined peaks in the electron kinetic energy spectra show that these electrons are ejected from the molecules lying near the surface of liquid water jet, since the electrons from the bulk domain are likely to lose their energies by inelastic collisions before they come out into vacuum. The present spectra are very similar to those for the thin ice condensed on Ru crystal (Coulman et al., 1990). Almost the same Auger electron spectra have been reported by Winter et al. (2007). In spite of a little broader bandwidth of the present electron energy analysis the absolute energies of the present spectra coincide with those by Winter et al. Features of spectra resemble the gas phase Auger electron spectra (Moddeman et al., 1971; Siegbahn, 1974). However, the fine structures observed in the gas phase Auger spectra (Hjelte et al., 2001) are not resolved in the present Auger spectra. In the Auger spectra by Winter et al. (2007) with a better resolution such fine structures have not been recognized. It can be safely interpreted that this is not due to the insufficient energy
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resolution of the electron spectrometer, but due to the nature of valence band of hydrogen-bonded water clusters. For convenience of discussion, electron orbitals of water are the inner valence 2a1 (O:2s), the outer valence 1b2 (OH bond) and the outer valence 3a1 and 1b2 (lone pairs). By comparing the present electron kinetic energy spectra with the gas phase Auger electron spectra, the prominent peak at the electron kinetic energy around 500 eV is ascribed to the K 1 1b1 1b1 Auger transition, and the peak around 480 eV, to the K 1 2a1 V transition, where V means one of the outer valence electrons. It is observed that electron kinetic energies for these prominent Auger transitions are not identical in the two regions of photon energy, i.e., in the region of ‘‘pre-edge peak’’ as mentioned in the previous section the K 1 1b1 1b1 Auger peak stays at 505 eV and the K 1 2a1 V Auger peak at 483 eV, whereas in the region of ‘‘postedge peak’’, the K 1 1b1 1b1 Auger peak moves to 501 eV and the K 1 2a1 V Auger peak to 479 eV. We remark that the difference in Auger peak energies is not due to the irregular electrification artifacts which are eliminated in the above mentioned procedure. Similar behavior was observed in the Auger electron spectra in the gas and solid phases which is called as the resonant Auger shift or spectator Auger shift and ascribed to the difference in the electric field within a molecular frame owing to the remaining or the ejected electron from the K-shell orbital of oxygen atom. Between the two regions of ‘‘pre-edge peak’’ and ‘‘post-edge peak’’ total photoelectron yields increase strongly, which can be specified as ‘‘main edge’’, where intermediate shifts of Auger peak energies are also presented. The oxygen K-shell ionization potential should lie in this area. Photoelectrons due probably to outer valence orbitals such as 1b1 , 3a1 , 1b2 , and 2a1 (Winter et al., 2004) are also obtained with smaller intensities in comparison with Auger electrons. 3.2.2. Partial photoelectron yields for K 1 1b1 1b1 Auger transition Shown in Fig. 2 are the partial electron yields for K 1 1b1 1b1 Auger peak as a function of photon energy, which are obtained as the intensity of Auger peak normalized for the photon intensity. The behavior of partial electron yields appears to be almost the same as that of total photoelectron yields. In spite of the more or less fluctuated data at larger photon energies, the partial electron yields in the region around the ‘‘pre-edge peak’’ trace the behavior of the total photoelectron yields. From the comparison of the partial yields with the total yields, we remark the followings:
Fig. 4. Electron kinetic energy spectra for liquid water at photon energies of 532.28 eV (below the ‘‘pre-edge peak’’) and 533.26–548.95 eV (covering both ‘‘preedge peak’’ and ‘‘post-edge peak’’ for O K-shell). Vertical lines indicate the shifts of mean energies of the prominent two peaks.
(1) Almost a constant fraction of the total electron yields is given to the cross section for the K 1 1b1 1b1 Auger final state in the whole range of the present photon energies. The nominal K 1 1b1 1b1 decay, which is the Auger transition of the 1b1 electron to the K 1 hole accompanied with the ejection of another 1b1 electron, seems to occur even in the ‘‘pre-edge peak’’ and ‘‘main edge’’ regions as well as in the ‘‘post-edge peak’’ region in spite of the above mentioned resonant Auger shift in the ‘‘pre-edge peak’’ region. It seems that the excited electron changes the molecular electric field but does not change the Auger transition probability, which should be examined for other Auger final states. (2) The behavior of ionization in the vicinity of liquid surface is very similar to that in the bulk water, which is concluded as follows. The electrons ejected from the water molecules in the vicinity of liquid surface can appear in the vacuum without inelastic scattering, so that they can be detected with the well-defined energy corresponding to the K 1 1b1 1b1 Auger transition and recorded in the partial electron yields. On the other hand, the electrons ejected from water molecules in the bulk readily lose certain amount of energies by a number of
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inelastic scattering before arriving in the vacuum, which are involved in the total electron yields together with the electrons ejected from the liquid surface. Thus the similarity between the total and the partial electron yields is ascribed to the similar ionization behavior at the liquid surface and in the bulk, which should also be examined by the further experiments and quantum chemical analysis. 4. Summary A new spectroscopy to identify the hydration structure playing important role in liquid-phase radiation damage is in progress using a laminar liquid jet sample in vacuum in combination with soft X-ray synchrotron radiation. We have presented the total and partial electron yields for liquid water with the use of a photoelectron spectroscopy. Extension to the partial photoelectron yields measurements for nucleotide/water solution will further provide insight of radiation damage to DNA. Acknowledgments The authors would like to thank the financial support of the Grant-in-Aid for Scientific Research from the Japan Society of Promotion of Science and the Cooperative Research Program of the Japan Atomic Energy Agency. They also thank Mr Y. Fukuda for his commitment for building and maintenance of the secondary refocusing mirror system. References Coreno, M., de Simone, M., Prince, K.C., Richter, R., Vondra cek, M., Avaldi, L., Camilloni, R., 1999. Chem. Phys. Lett. 306, 269.
¨ ¨ Coulman, D., Puschmann, A., Hofer, U., Steinruck, H.-P., Wurth, W., Feulner, P., Menzel, D., 1990. J. Chem. Phys. 93, 58. Faubel, M., Kisters, T., 1989. Nature 339, 527. Faubel, M., Shlemmer, S., Toennies, J.P., 1988. Z. Phys. D 10, 269. Faubel, M., Steiner, B., 1992. Ber. Bunsenges. Phys. Chem. 96, 1167. Faubel, M., Steiner, B., Toeneies, J.P., 1997. J. Chem. Phys. 106, 9013. Faubel, M., Steiner, B., Toeneies, J.P., 1999. J. Electron. Spectrosc. Rel. Phenom. 95, 159. Fuchs, H., Legge, H., 1979. Acta Astronautica 6, 1213. ¨ ¨ Hjelte, I., Piancastelli, M.N., Fink, R.F., Bjorneholm, O., Bassler, M., Feifel, R., Giertz, A., Wang, H., Wiesner, K., Ausmees, A., Miron, C., Sorensen, S.L., Svensson, S., 2001. Chem. Phys. Lett. 334, 151. Horimoto, N., Kohno, J., Mafune , F., Kondow, T., 2000. Chem. Phys. Lett. 318, 536. Kashtanov, S., Augustsson, A., Luo, Y., Guo, J.-H., Sˆathe, C., Rubensson, J.-E., ˚ Siegbahn, H., Nordgren, J., Agren, H., 2004. Phys. Rev. B 69, 024201. Kosugi, N., Shigemasa, E., Yagishita, A., 1992. Chem. Phys. Lett. 190, 481. ¨ Moberg, R., Bokman, F., Bohman, O., Siegbahn, H., 1991. J. Phys. Chem. 94, 5226. Moddeman, W.E., Carlson, T.A., Krause, M.O., Pullen, B.P., Bull, W.E., Schweitzer, G.K., 1971. J. Chem. Phys. 55, 2317. Morgner, H., 1998. Surf. Invest. 13, 463. Saitoh, Y., Nakatani, T., Matsushita, T., Agui, A., Teraoka, Y., Yokoya, A., 2001. Nucl. Instrum. Meth. A 474, 253. Siegbahn, K., 1974. J. Electron. Spectrosc. Relat. Phenom. 5, 3. Sobott, F., Wattenberg, A., Barth, H.-D., Brutschy, B., 1999. Int. J. Mass Spectrom. 185/186/187, 271. Ukai, M., Yokoya, A., Fujii, K., Saitoh, Y., 2008. Radiat. Phys. Chem. 77, 1265. Watanabe, R., Nikjoo, H., 2002. Int. J. Radiat. Biol. 78, 953. Watanabe, R., Yokoya, A., Fujii, K., Saito, K., 2004. Int. J. Radiat. Biol. 80, 11. Wernet, Ph., Nordlund, D., Bergmann, U., Cavalleri, M., Odelius, M., Ogasawara, H., ¨ ¨ I., Pettersson, I.G.M., Nissen, A., 2004. Naslund, I.øA., Hirsch, T.K., Ojamae, Science 304, 995. Wilson, K.R., Rude, B.S., Smith, J., Cappa, C., Co, D.T., Schaller, R.D., Larsson, M., 2004. Rev. Sci. Instrum. 75, 725. Winter, B., Weber, R., Widdra, W., Dittmar, M., Faubel, M., Hertel, I.V., 2004. J. Phys. Chem. A 108, 2625. ¨ Winter, B., Hergenhahn, U., Faubel, M., Bjornholm, O., Hertel, I.V., 2007. J. Chem. Phys. 127, 094501. Yokoya, A., Fujii, K., Ushigome, T., Shikazono, N., Urushibara, A., Watanabe, R., 2006. Radiat. Prot. Dosimetry 122, 86.
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Synchrotron radiation photoelectron studies for primary radiation effects using a liquid water jet in vacuum: Total and partial photoelectron yields for liquid water near the oxygen K-edge Masatoshi Ukai a,, Akinari Yokoya b, Yusuke Nonaka a, Kentaro Fujii b, Yuji Saitoh c a
Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei-shi, Tokyo 184-8588, Japan Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan c Synchrotron Radiation Research Center, Japan Atomic Energy Agency (JAEA), Sayo-gun, Hyogo 679-5148, Japan b
a r t i c l e in f o
a b s t r a c t
Keywords: Soft X-ray synchrotron radiation Liquid water jet Auger electron spectroscopy Primary radiation effect
A new spectroscopy to identify the hydration structure playing important role in liquid-phase radiation damage is in progress using a laminar liquid water jet sample in vacuum in combination with soft X-ray synchrotron radiation. We present the total and partial electron yields for liquid water using a photoelectron spectroscopy. Partial electron yields for the K 1 1b1 1b1 Auger transition are obtained for the first time by measuring the electrostatically dispersed electron kinetic energy spectra as a function of photon energy of synchrotron radiation. & 2009 Published by Elsevier Ltd.
1. Introduction The geometrical structures of molecules in liquid and their electronic energies are of significant importance in the research of radiation interaction with matter to understand the mechanism of inducing damage as well as the mechanism of spacial- and timepropagation of unstable species in liquid. Primary processes of bio-chemically important molecules can provide proper prototypes of the above mentioned processes in a cell mimetic condition. It can be safely assumed that the conformational structure of, ca, DNA is determined by the hydrogen-bonded ‘‘hydration network’’ of water molecules surrounding it. The conformational structures further determine the electronic energies of hydrated complex and, presumably, the chemical reaction pathways in the physiological functions of the complex. This assumption should also allow us to have the following thermodynamical viewpoint on the induction of radiation damage and the electronic or chemical restoration. The intact bio-molecules in a cell are in their thermochemically and ‘‘biologically stable’’ states defined by the hydrationnetwork-aided natural conformation. The primary irradiation products, i.e., the ionic and excited sites of molecule induced by the primary radiation interaction, are regarded both as the cores of remaining radiation damage and as the localized excess energies. The excess energies should diffuse into the environment
Corresponding author.
E-mail address: [email protected] (M. Ukai). 0969-806X/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.radphyschem.2009.07.011
with time, which finally gives rise to a metastable thermal state involving a stable damage or restores the original state of natural molecular forms. The ‘‘hydration network’’ plays important roles both as the intermediary of ‘‘heat’’ transport to environment and as the structural memory of the original hydrogen-bonded conformation in restoration processes. We thus remark the necessity of experimental investigation of water as a bio-cell solvent. ESCA (electron spectroscopy for chemical analysis) study (Siegbahn, 1974) for water and water solution is expected to provide a certain evidence of chemical structures of molecules aided by hydrogen bonds. A new spectroscopic study using a soft X-ray synchrotron radiation for photoelectrons ejected from a liquid solution in the form of laminar jet in vacuum is in progress (Ukai et al., 2008). The technique of liquid beam in vacuum (Fuchs and Legge , 1979; Faubel et al., 1988; Faubel and Steiner, 1992; Faubel and Kisters, 1989) has been employed in several experiments of physicochemical spectroscopy (Faubel et al., 1997, 1999; Moberg et al., 1991; Morgner, 1998; Sobott et al., 1999; Horimoto et al., 2000). However, biological application is few. We present the results of total and partial photoelectron yields for liquid water. We remark that the environmental hydration gives rise to the influence on the eigen-state energy of the excited molecular orbital more largely than on the energies of the valence and inner shell orbitals (Ukai et al., 2008). Total photoelectron yields representing photoabsorption cross sections of water as a function of photon energy give a typical example of the environmental chemical shift of excited state. Another important aspect of the research for radiation interaction in liquid water is facilitating simulation of radiation
ARTICLE IN PRESS M. Ukai et al. / Radiation Physics and Chemistry 78 (2009) 1202–1206
risk. The extent of indirect radiation interaction due to secondary electrons and radicals produced in bulk cell liquid (Yokoya et al., 2006) can be evaluated with the use of the simulation parameters such as total cross sections for known primary radiation energies and partial cross sections for identified secondary electron energies in liquid water (Watanabe and Nikjoo, 2002; Watanabe et al., 2004). Bound electrons in a water molecule have large amplitudes of oscillator strength at the energies in the vicinity of oxygen K-shell edge to respond high-energy charged particles. As well as ESCA analysis to determine hydrated structures of molecules, photoelectron spectroscopy is a powerful tool to obtain yields of photoelectrons with identified energies.
2. Experiment In order to maintain a liquid water sample in vacuum we employ a liquid jet beam technique. The liquid sample maintained in the form of thin laminar-flow in vacuum is intersected by a mm-size synchrotron radiation beam. The photoelectrons ejected from the liquid beam are counted using a secondary electron multiplier. The technique of liquid jet is to delay the actual phase transition and to realize a practical size of liquid sample for experiments in vacuum vessel. The behavior of water sample has been described elsewhere (Ukai et al., 2008). Briefly, the liquid water introduced through mm-size orifice into vacuum with a certain stagnation pressure finds itself in the gas phase due to the abrupt decrease of atmospheric pressure. However, owing to the insufficient amount of thermal internal energy evaporation of water only occurs from the surface of the beam. Release of the
1203
heat of evaporation gradually decreases the temperature of the liquid as a function of the flight distance of the liquid beam in vacuum. Although the liquid beam will be finally caught into the solid phase, yet most of the water molecules are considered to remain in the liquid or supercooled liquid phase in a relatively long time compared with the time required for physical or spectroscopic processes. Hence, it is necessary to identify if the water sample is maintained in the liquid phase, which has been certified by measuring the temperature dependence of the photoabsorption spectra as a function of flight distance in vacuum (Ukai et al., 2008). Fig. 1 shows a schematic view of the present apparatus for photoelectron measurements. A water sample purified by Milli-Q system (Milli-pore, Tokyo) is admitted into vacuum through a 10 or 20 mm orifice on a platinum disk with the stagnation pressure of about 1.0–3.0 MPa using a liquid chromatograph sample injection pump. The vacuum vessel holding the liquid beam source is evacuated using a 2300 L/s turbo-molecular pump. To reduce the residual pressure in the vacuum vessel the waste sample after irradiation is collected by a liquid-nitrogen-cooling trap at the downstream of the jet beam. The vessel is also equipped with a liquid-nitrogen-cooling 20,000 L/s cryogenic evacuation panel. The pressure in the vacuum vessel is typically 1 10 2 Pa on the admission of the liquid sample. Another vacuum vessel holding a photoelectron spectrometer and secondary electron multiplier is connected to the vessel for the liquid beam source though an orifice of 1 mm in diameter and evacuated differentially using a 700 L/s turbo-molecular pump. The pressure of vacuum in the spectrometer vessel is typically 1 10 4 Pa on the admission of the liquid sample into the liquid beam source vessel.
Fig. 1. Schematic experimental apparatus of synchrotron radiation photoelectron spectroscopy for liquid water jet.
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The photoelectron measurements are carried out at SPring-8, Japan. The pseud-monochromatic undulator synchrotron radiation from the beam line BL-23SU is dispersed using a grazing incidence varied line spacing monochromator to provide the monochromatic synchrotron radiation (Saitoh et al., 2001). The monochromatic synchrotron radiation is refocused to be the spot of size slightly less than 0.04 mm at the irradiation point using two stages of refocusing mirror systems. The absolute photon energy is calibrated for the prominent resonant absorption peaks, such as the 1s-p vibrational peaks and the 1s-Rydberg progression of oxygen molecule in the gas phase (Saitoh et al., 2001; Coreno et al., 1999; Kosugi et al., 1992). The intensity of the refocused monochromatic synchrotron radiation is 1012 photons=s with the band pass of 0.5 eV (FWHM) at the photon energies in the region of 530–548 eV in the vicinity of oxygen K-edge. The photoelectrons ejected from the liquid water beam, entering into the spectrometer vessel though the above 1 mm orifice, and passing through electrostatic lenses are energydispersed with an hemispherical electrostatic analyzer and detected using a channel electron multiplier for measurements of photoelectron spectra, or are collected using a channel electron multiplier without energy dispersion for measurements of total photoelectron yields. The intensity of photoelectrons is normalized for the photon intensity monitored by the drain current from the post-focusing mirror at each photon energy. We refer these normalized photoelectron intensities to ‘‘photoelectron yields’’.
3. Results and discussion 3.1. Total photoelectron yields for liquid water near the O K-edge The total photoelectron yields from the liquid water jet are measured as a function of the incident photon energy of 530–547 eV. The results obtained with a stagnation pressure of 3 MPa at 300 K using the 10 mm nozzle are shown in Fig. 2. As described above, the phase state of water sample is identified to be in the liquid phase by the temperature dependent
measurements, so that the ‘‘contamination’’ of the electrons ejected from water vapor or ice is negligible (Ukai et al., 2008). The total photoelectron yields are enhanced to present a broad peak at photon energies of 536–540 eV which is understood to correspond to the photoionization of a K-shell electron of oxygen atom. A small enhancement at the photon energy around 534 eV should correspond to the excitation to an anti-bonding orbital. Since the K-edge energy (K-shell ionization potential) of liquid water is supposed to lie between these two peaks, we refer the former peak to ‘‘post-edge peak’’, and the latter to ‘‘pre-edge peak’’. The present result of total photoelectron yields is in good accordance with the previously measured total photoelectron yields (Wilson et al., 2004) and total X-ray fluorescence yields (Kashtanov et al., 2004). The total photoelectron yields for water vapor evaporated from the liquid water jet are also shown for reference at the bottom in Fig. 2. The ‘‘pre-edge peak’’ occurs at the photon energy of 534 eV and some other discrete peaks also appear at photon energies of 536 and 537 eV. The remarkable difference of the spectra between liquid and vapor is ascribed by a quantum chemical analysis to the delocalized outer electron behavior of hydrogen-bonded water clusters (Wernet et al., 2004).
3.2. Partial electron yields for liquid water near the O K-edge 3.2.1. Photoelectron spectra using an elecrostatic energy analyzer In order to obtain electron yields of identified kinetic energies, we employ photoelectron spectrometer using an electrostatic hemispherical analyzer. However, reliable electron energy analysis for a liquid water has been difficult. One of the reasons is an ambiguous electrostatic potential. This is due to the facts that the evaporated water produces thin coverage of insulator on the surface of electrodes and that photoelectron bombardment or attachment to the insulator surface causes an irregular electrostatic potential. To eliminate the effect of such an irregular electrification several important elements to determine the photoelectron kinetic energy are heated during the operation of the photoelectron spectrometer. Another reason of difficulty is an electrification of liquid water jet due to photoionization. Because of the high resistivity of neat water, positive charges on the surface of liquid jet induced by photoionization do not diffuse easily, so that the electrostatic
Main Auger Peak Energy (eV)
500
Fig. 2. Total photoelectron yields for water as a function of photon energy. The upper curve shows the yields for liquid water; the lower curve shows the yields for water vapor emerging from the liquid beam. The solid circles are the partial electron yields for the K 1 1b1 1b1 Auger transition. The partial yields are normalized to the height of the total yield at 537 eV for ease of comparison. Note: ‘‘pre-edge peak’’ region around 534 eV, ‘‘main edge’’ region at 535–536 eV, and ‘‘post-edge peak’’ region at 536–540 eV (see text). O K-shell ionization potential of liquid water is not correctly determined, but should lie in the main edge region of (b).
H2O h = 540 eV 490
480
470 0
100 200 300 400 Mirror Current (X-ray intensity) (nA)
500
Fig. 3. Effect of irregular electrification of liquid water jet by photoionization. Mean energy of the K 1 1b1 1b1 Auger peak moves to lower energy with increasing the intensity of photon beam (monitored by the refocusing mirror current).
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potential of water jet is not ground but positively electrified depending on the number of ejected electrons. Fig. 3 shows the mean kinetic energy of a prominent Auger electron where significant amount of shift to lower energy is observed by increasing the photon intensity as monitored by the drain current of post-focusing mirror. We estimate the correct electron energy by the extrapolation to the zero photon intensity with referring to the total photoelectron yields as described in Section 3.1. Electron kinetic energy spectra, thus obtained with the correction of electron energies, as a function of photon energy are shown in Fig. 4, which are obtained with the photon energy bandwidth of 0.1 eV and the nominal energy resolution of the spectrometer of 0.6 eV. Well-defined peaks in the electron kinetic energy spectra show that these electrons are ejected from the molecules lying near the surface of liquid water jet, since the electrons from the bulk domain are likely to lose their energies by inelastic collisions before they come out into vacuum. The present spectra are very similar to those for the thin ice condensed on Ru crystal (Coulman et al., 1990). Almost the same Auger electron spectra have been reported by Winter et al. (2007). In spite of a little broader bandwidth of the present electron energy analysis the absolute energies of the present spectra coincide with those by Winter et al. Features of spectra resemble the gas phase Auger electron spectra (Moddeman et al., 1971; Siegbahn, 1974). However, the fine structures observed in the gas phase Auger spectra (Hjelte et al., 2001) are not resolved in the present Auger spectra. In the Auger spectra by Winter et al. (2007) with a better resolution such fine structures have not been recognized. It can be safely interpreted that this is not due to the insufficient energy
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resolution of the electron spectrometer, but due to the nature of valence band of hydrogen-bonded water clusters. For convenience of discussion, electron orbitals of water are the inner valence 2a1 (O:2s), the outer valence 1b2 (OH bond) and the outer valence 3a1 and 1b2 (lone pairs). By comparing the present electron kinetic energy spectra with the gas phase Auger electron spectra, the prominent peak at the electron kinetic energy around 500 eV is ascribed to the K 1 1b1 1b1 Auger transition, and the peak around 480 eV, to the K 1 2a1 V transition, where V means one of the outer valence electrons. It is observed that electron kinetic energies for these prominent Auger transitions are not identical in the two regions of photon energy, i.e., in the region of ‘‘pre-edge peak’’ as mentioned in the previous section the K 1 1b1 1b1 Auger peak stays at 505 eV and the K 1 2a1 V Auger peak at 483 eV, whereas in the region of ‘‘postedge peak’’, the K 1 1b1 1b1 Auger peak moves to 501 eV and the K 1 2a1 V Auger peak to 479 eV. We remark that the difference in Auger peak energies is not due to the irregular electrification artifacts which are eliminated in the above mentioned procedure. Similar behavior was observed in the Auger electron spectra in the gas and solid phases which is called as the resonant Auger shift or spectator Auger shift and ascribed to the difference in the electric field within a molecular frame owing to the remaining or the ejected electron from the K-shell orbital of oxygen atom. Between the two regions of ‘‘pre-edge peak’’ and ‘‘post-edge peak’’ total photoelectron yields increase strongly, which can be specified as ‘‘main edge’’, where intermediate shifts of Auger peak energies are also presented. The oxygen K-shell ionization potential should lie in this area. Photoelectrons due probably to outer valence orbitals such as 1b1 , 3a1 , 1b2 , and 2a1 (Winter et al., 2004) are also obtained with smaller intensities in comparison with Auger electrons. 3.2.2. Partial photoelectron yields for K 1 1b1 1b1 Auger transition Shown in Fig. 2 are the partial electron yields for K 1 1b1 1b1 Auger peak as a function of photon energy, which are obtained as the intensity of Auger peak normalized for the photon intensity. The behavior of partial electron yields appears to be almost the same as that of total photoelectron yields. In spite of the more or less fluctuated data at larger photon energies, the partial electron yields in the region around the ‘‘pre-edge peak’’ trace the behavior of the total photoelectron yields. From the comparison of the partial yields with the total yields, we remark the followings:
Fig. 4. Electron kinetic energy spectra for liquid water at photon energies of 532.28 eV (below the ‘‘pre-edge peak’’) and 533.26–548.95 eV (covering both ‘‘preedge peak’’ and ‘‘post-edge peak’’ for O K-shell). Vertical lines indicate the shifts of mean energies of the prominent two peaks.
(1) Almost a constant fraction of the total electron yields is given to the cross section for the K 1 1b1 1b1 Auger final state in the whole range of the present photon energies. The nominal K 1 1b1 1b1 decay, which is the Auger transition of the 1b1 electron to the K 1 hole accompanied with the ejection of another 1b1 electron, seems to occur even in the ‘‘pre-edge peak’’ and ‘‘main edge’’ regions as well as in the ‘‘post-edge peak’’ region in spite of the above mentioned resonant Auger shift in the ‘‘pre-edge peak’’ region. It seems that the excited electron changes the molecular electric field but does not change the Auger transition probability, which should be examined for other Auger final states. (2) The behavior of ionization in the vicinity of liquid surface is very similar to that in the bulk water, which is concluded as follows. The electrons ejected from the water molecules in the vicinity of liquid surface can appear in the vacuum without inelastic scattering, so that they can be detected with the well-defined energy corresponding to the K 1 1b1 1b1 Auger transition and recorded in the partial electron yields. On the other hand, the electrons ejected from water molecules in the bulk readily lose certain amount of energies by a number of
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inelastic scattering before arriving in the vacuum, which are involved in the total electron yields together with the electrons ejected from the liquid surface. Thus the similarity between the total and the partial electron yields is ascribed to the similar ionization behavior at the liquid surface and in the bulk, which should also be examined by the further experiments and quantum chemical analysis. 4. Summary A new spectroscopy to identify the hydration structure playing important role in liquid-phase radiation damage is in progress using a laminar liquid jet sample in vacuum in combination with soft X-ray synchrotron radiation. We have presented the total and partial electron yields for liquid water with the use of a photoelectron spectroscopy. Extension to the partial photoelectron yields measurements for nucleotide/water solution will further provide insight of radiation damage to DNA. Acknowledgments The authors would like to thank the financial support of the Grant-in-Aid for Scientific Research from the Japan Society of Promotion of Science and the Cooperative Research Program of the Japan Atomic Energy Agency. They also thank Mr Y. Fukuda for his commitment for building and maintenance of the secondary refocusing mirror system. References Coreno, M., de Simone, M., Prince, K.C., Richter, R., Vondra cek, M., Avaldi, L., Camilloni, R., 1999. Chem. Phys. Lett. 306, 269.
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