Tetsuo Katayama

Real-time observation of surface bond breaking with an x-ray laser.

Surface Molecules Not Quite Desorbing The dynamics of molecules desorbing from or adsorbing on surfaces requires that molecules rapidly gain or lose a large amount or translational and rotational energy to enter or leave the gas phase. An intermediate precursor state has long been invoked in which molecules interact weakly with the surface but translate along it and exchange energy without forming localized surface bonds. DellAngela et al. (p. 1302) found evidence for such a state in changes in x-ray absorption and emission spectra of CO molecules adsorbed on a ruthenium surface after optical excitation rapidly heated the surface. The use of a free electron laser provided high time resolution for x-ray spectroscopy studies. Density function theory and modeling of high temperature states revealed a state that forms from molecules that have not overcome the desorption barrier during heating and that are bonded less strongly than the chemisorbed state. Changes in x-ray absorption and emission features reveal a weakly interacting precursor state to the chemisorbed state. We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells—chemisorbed and precursor state separated by an entropy barrier—explain the anomalously high prefactors often observed in desorption of molecules from metals.

Maxima in the thermodynamic response and correlation functions of deeply supercooled water

Pointing to a second critical point One explanation for the divergence of many of the thermodynamic properties of water is that there is a critical point in deeply supercooled water at some positive pressure. For bulk water samples, these conditions are described as “no mans land,” because ice nucleates before such temperatures can be reached. Kim et al. used femtosecond x-ray laser pulses to probe micrometer-sized water droplets cooled to 227 K (see the Perspective by Gallo and Stanley). The temperature dependence of the isothermal compressibility and correlation length extracted from x-ray scattering functions showed maxima at 229 K for H2O and 233 K for D2O, rather than diverging to infinity. These results point to the existence of the Widom line, a locus of maximum correlation lengths emanating from a critical point in the supercooled regime. Science, this issue p. 1589; see also p. 1543 Maxima in the isothermal compressibility and correlation length point to the existence of a second critical point in water. Femtosecond x-ray laser pulses were used to probe micrometer-sized water droplets that were cooled down to 227 kelvin in vacuum. Isothermal compressibility and correlation length were extracted from x-ray scattering at the low–momentum transfer region. The temperature dependence of these thermodynamic response and correlation functions shows maxima at 229 kelvin for water and 233 kelvin for heavy water. In addition, we observed that the liquids undergo the fastest growth of tetrahedral structures at similar temperatures. These observations point to the existence of a Widom line, defined as the locus of maximum correlation length emanating from a critical point at positive pressures in the deeply supercooled regime. The difference in the maximum value of the isothermal compressibility between the two isotopes shows the importance of nuclear quantum effects.

Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer

Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400u2009nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.

Response to Comment on “Maxima in the thermodynamic response and correlation functions of deeply supercooled water”

Caupin et al. have raised several issues regarding our recent paper on maxima in thermodynamic response and correlation functions in deeply supercooled water. We show that these issues can be addressed without affecting the conclusion of the paper.