New Mexico State LANSCE Professor
Dept. of Physics, MSC 3D
Las Cruces, New Mexico
phone: (505) 646-5199
fax: (505) 646-1934
Los Alamos Neutron Scattering Center (LANSCE) Select cancel if prompted for username and password.
Engineering Physics II
Dr. Urquidi’s research involves the study of molecular liquids (e.g. water, HF, and acetic acid) and amorphous materials (e.g. high and low density amorphous ice and optically relevant glasses) through the use of neutron and high energy X-ray diffraction techniques combined with molecular dynamics (MD) and reverse Monte Carlo (RMC) simulations as aids in data interpretation. The benefits of using thermal neutron scattering in conjunction with high energy X-rays for condensed matter research are considerable. They both serve as bulk probes due to their high penetrating power and yield complimentary structural information. For example, Fig. 1 shows the pair distribution functions for 64:36 CaO:Al2O3 (calcium aluminate oxide) using both neutrons (shown in red) and high energy X-rays (shown in blue) with a beam energy of about 100keV . The advantage of using both techniques in conjunction becomes quite evident. Because neutrons interact with the nucleus they are sensitive to light atoms. On the other hand, X-rays interact with the electron distribution and so are more sensitive to atoms of higher Z. In this example the O-O correlations are better seen in with the neutron probe while the Ca/Al-X correlations are better detected with the X-rays. By combining the techniques in this manner, first order differences may be taken to obtain difference correlation functions that help elucidate the local bonding environment .
In other work, the measurement of structural differences between the hydrogenated and deuterated forms of LDA ice at 120K, have been carried out using 98keV X-ray diffraction techniques . The maximum observed isotope effect in LDA ice is ~3.4% at 40K (black curve, Fig. 2) compared to a maximum effect of ~1.6% previously measured in liquid water at room temperature (red curve). An understanding of this isotope effect is important to neutron scattering because of the use of isotopic substitution for contrast matching.
Other research topics include the behavior of liquid water at interfaces, with particular interest in biologically relevant surfaces and interactions (e.g. the hydrophobic effect associated with Hoffmeister salts). Also of interest is the relationship between supercooled water and the purported high to low density transition [4,5], which may have implications as to the existence of a second critical point for the liquid.