X-Ray Spectroscopy to Probe Water Structure

日時　2025年10月31日　（金）　15:00 – 16:30
場所　WPI-SKCM2研究棟（VBL）　2階　セミナー室（VBL204）
講師　Lars G.M. Pettersson
(Department of Physics, AlbaNova University Center, Stockholm University)

I will describe ongoing theoretical and experimental work using XAS, XES and EXAFS to elucidate the character of local structures in ambient liquid water. A consequence of a two-state picture of water¹ with a coexistence line between high- (HDL) and low-density liquid (LDL) that terminates at a critical point is that ambient water can be viewed as a supercritical liquid, i.e. the single-phase region beyond a critical point with fluctuations related to HDL and LDL. Here, I will describe our efforts to use X-ray techniques to determine the local H-bonded structures of these species at ambient conditions and their possible consequences to chemistry and biology.

XAS: We have performed PIMD simulations of ambient water using the MB-pol force-field and constructed the resulting distributions of structural descriptors (LSI, tetrahedrality, asymmetry, G₅, Tanaka ζ). We have built a genetic algorithm to select from the ~245,000 individual centroid environments in the simulation ~1,500 structures that reproduce the distributions from the full set. Computing XAS for these 1,500 (times 16 beads) 32-molecules clusters using TDDFT with our recently calibrated functional², we find that we still need to modify the PIMD MB-pol distributions using SpecSwap-RMC^{3, 4} in order to match XAS.

XES: We have performed full Kramers-Heisenberg calculations of XES on a 2D OH-stretch grid using water pentamers in tetrahedral (“LDL”) and asymmetric (“HDL”) geometry to analyze the importance of interference effects and various vibrational states in the intermediate core- and final valence-hole states. We find that the 3a₁ state becomes near-degenerate with the 1b₁ as the O-H is stretched, but its contribution in the 1b1 region disappears when vibrational interference is included⁵.

EXAFS: We have performed EXAFS measurements of Argon in water at four different temperatures (4, 15, 25, and 45℃). Combining EXAFS calculations using FEFF and SpecSwap-RMC to match experiment we determine the coordination shell around solvated Argon to ~20 molecules in a likely dodecahedral cage6. The assumption is that Argon acts as a neutral probe of existing LDL-like environments, but alternative hypotheses are also considered.

¹ A. Nilsson, and L. G. M. Pettersson, Nature Commun. 6 (2015) 8998.
² T. Fransson, and L. G. M. Pettersson, J. Chem. Phys. 160 (2024) 234105.
³ M. Leetmaa, K. T. Wikfeldt, and L. G. M. Pettersson, J. Phys.: Cond. Mat. 22 (2010) 135001.
⁴ L. G. M. Pettersson, and O. Takahashi, J. Non-Cryst. Solids 14 (2022) 100087.
⁵ F. Moncada, T. Fransson, and L. G. M. Pettersson, unpublished.
⁶ R. Candela, Y. Muñoz Muñoz, R. Ash, J. Öström, C. Kaminsky, D. Sokaras, U. Bergmann, and L. G. M. Pettersson, unpublished.

問合せ先　高橋修、出田真一郎（放射光科学研究所）