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February 18, 2006

 

 

Ultrafast Dynamics in Water

A thorough understanding of the chemical processes that are initiated when radiation interacts with aqueous systems is essential for many diverse fields, from condensed matter physics to medicine to environmental science. An incoming photon with enough energy to produce a core hole in a water molecule sets off motions that can affect bonding configurations, which in turn affect subsequent chemical-reaction pathways. However, it is a fundamental challenge for the radiation chemistry community to unravel the early time dynamics of electronically excited states in water because their short (femtosecond) time scales are difficult to access directly with pump–probe measurements. Using a combination of isotope substitution experiments and molecular dynamics simulations, researchers from Sweden, Germany, and the U.S. have shown that the ultrafast (0- to 10-fs) dissociation dynamics of liquid water can be successfully probed with x-ray emission spectroscopy.

Not-So-Still Waters

If you look deeply enough at even the stillest of waters, as deep as the molecular level, you will find a surprisingly turbulent, dynamic universe. The water molecules constantly form and break bonds under the influence of several weak interactions, balancing both cooperative and anticooperative effects. The situation gets further complicated when you add ionizing x-ray radiation to the mix: the energy from the radiation is enough to eject an electron from the core of the molecule, immediately resulting in the stretching and, sometimes, breaking of bonds. This process of radiolysis is commonly encountered in many fields and technologies, including biophysics, radiation physics, waste remediation, environmental cleanup, medical diagnosis, and radiation therapy.

The detailed study of substances in the condensed and dynamic liquid state poses unique challenges that aren't an issue with isolated gas molecules or solids with rigid structures. Only recently have experimental techniques capable of probing the dynamics of liquid water become available. Odelius et al. have applied a combination of approaches, both experimental and theoretical, to gain a better understanding of how the fate of a given water molecule can be decided in a split second by tiny radiation-induced jiggles.
X-ray emission spectroscopy (XES) is an element-specific tool capable of probing the local electronic structure of the occupied electronic states in complex systems. In the XES of water, the fluorescence from an electron reoccupying the oxygen 1s core level is measured. For hydrogen-bonded systems such as liquid water, ultrafast dynamics that occur during the lifetime of the core-excited state complicate the interpretation of the XES in terms of molecular structures but open a unique opportunity to study femtosecond reaction dynamics. This primary event of excitation or ionization of a water molecule in the radiolysis of aqueous systems governs all the subsequent steps in the radiolytic process.

core-hole dissociation

Simplified representation of the core-hole dissociation process. Excitation of a core electron in the blue oxygen atom leads to stretching of the O–H bonds and the ultimate dissociation of one of the OH groups, whose trajectories are represented by a "multiple-exposure" effect.

The molecular dynamics are strongly dependent on the nature of the electronic core-excited state, and by scanning through the x-ray absorption excitation energy, from preedge through postedge, different reaction dynamics can be probed. In addition, by performing the same measurements using molecules of different nuclear masses, any differences in the dynamics will become apparent, and an observed isotope effect in the XES would provide direct evidence of the importance of such processes.

High-brightness radiation from ALS Beamline 8.0.1 was used to excite both normal and deuterated liquid water in a custom-designed copper liquid cell with a 100-nm-thick silicon nitride membrane that enables the study of liquid water under ultrahigh vacuum conditions. The subsequent x-ray emission was recorded in the beamline's soft x-ray fluorescence endstation with a high-resolution Rowland-circle spectrograph. The temperature of the copper liquid cell was kept slightly above the freezing point of water to minimize temperature-induced effects.

disscociation of water spectrum

By controlling the excitation energy of the initial x-ray absorption, the ultrafast dissociation of hydrogen-bonded or uncoordinated OH groups can be selectively probed in liquid water in the x-ray emission spectrum.

The researchers found both an isotope effect and significant differences between the preedge and postedge XES. The preedge and postedge differences were explained in terms of the localization of the (lower-energy) preedge excitations, possible anisotropy in direction of the emitted x rays, and differing hydrogen-bond arrangements between the preedge and postedge cases. The researchers then used ab initio molecular dynamics simulations to model the effects of the finite lifetime of the core hole. In these simulations, the isotope effect due to the inertia difference between hydrogen and deuterium is apparent, with an especially striking difference in dynamics for the two different excited states: the postedge excitation results in dissociation of the hydrogen-bonded OH group(s), whereas for the preedge excitation the uncoordinated OH group dissociates.

In summary, experimental XES for normal and deuterated liquid water was analyzed against the background of ab initio molecular dynamics simulations. The excitation energy dependence in the XES data is explained in terms of differences in excited-state dynamics. Because of the inertia difference between hydrogen and deuterium, excited-state dynamics on the same time scale as the decay of the core-excited state will result in an isotope effect in the XES. The experimentally observed isotope effect is the key to unravelling the excited-state dynamics, but theoretical simulations were essential for proving the feasibility of the proposed mechanism.

Research conducted by M. Odelius, D. Nordlund, and L.G.M. Pettersson (Stockholm University); H. Ogasawara (Stanford Synchrotron Radiation Laboratory); O. Fuchs, L. Weinhardt, F. Maier, and E. Umbach (Würzburg University); C. Heske (University of Nevada, Las Vegas); Y. Zubavichus and M. Grunze (Heidelberg University); J.D. Denlinger (Advanced Light Source); and A. Nilsson (Stockholm University and Stanford Synchrotron Radiation Laboratory).

Research funding: Swedish Foundation for Strategic Research, Swedish Research Council, Swedish National Supercomputer Center, Center for Parallel Computers (Sweden), German Federal Ministry of Education and Research. Operation of the ALS and the Stanford Synchrotron Radiation Laboratory are supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: M. Odelius, H. Ogasawara, D. Nordlund, O. Fuchs, L. Weinhardt, F. Maier, E. Umbach, C. Heske, Y. Zubavichus, M. Grunze, J.D. Denlinger, L.G.M. Pettersson, and A. Nilsson, "Ultrafast core-hole-induced dynamics in water probed by x-ray emission spectroscopy," Phys. Rev. Lett. 94, 227401 (2005).

ALSNews Vol. 262, February 22, 2006

 

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