About The Speaker

Vladimir Khmelenko is a Research Professor in Physics at Texas A&M University and is known for the first experimental observation of the exchange tunneling reactions of hydrogen isotopes in the solid phase and for discovery and characterization of impurity-helium solids. He received his PhD from Institute of Chemical Physics Russian Academy of Science in 1984 for his work on free radicals at low temperatures. He also received his Dr. of Sciences degree in Physics and Mathematics from Institute for Energy Problems of Chemical Physics Russian Academy of Sciences in 2002 for his work on matrix isolation and chemical reactions in condensed helium. In 1998 he continued his work at Cornell University in David Lee’s lab as Research Associate on studying free radicals at low and ultralow temperatures. In 2010 he moved to Texas A&M University where he continues research on atoms and molecules in quantum solids at low and ultralow temperatures.

Event Details

We present an experimental study of quantum diffusion of atomic hydrogen in solid H₂ films at temperatures below 1 K. The atoms are generated via electron impact dissociation by running a radio frequency discharge above the film at T = 0.7 K. We are able to distinguish between the diffusion of the atoms moving towards each other followed by recombination from the pure spatial diffusion driven by the density gradient [1]. The rate obtained for pure spatial diffusion of H atoms in the H₂ films was two orders of magnitude larger than that calculated from H atom recombination, the quantity used in all previous work to characterize the mobility of H atoms in solid H₂. We studied the influence of the film’s ortho-para composition on recombination and also on pure spatial diffusion [2,3]. We found that the H-atom diffusion was significantly enhanced by injection of phonons into the film. Our results provide the first measurement of the pure spatial diffusion rate for H atoms in solid H₂, the only solid-state system beside ³He − ⁴He mixtures, where atomic diffusion does not vanish even at temperatures below 1 K.  

1. S. Sheludiakov, D. M. Lee, V.V. Khmelenko, J. Järvinen, J. Ahokas, and S. Vasiliev, Purely spatial quantum diffusion of H atoms in solid H₂ at temperatures below 1K, Phys. Rev. Lett. 126, 195301 (2021).
2. S. Sheludiakov, D. M. Lee, V.V. Khmelenko, Yu. A. Dmitriev, J. Järvinen, J. Ahokas, and S. Vasiliev, Purely spatial diffusion of H atoms in solid normal– and para-hydrogen films, Phys. Rev. B 105, 144102 (2022).
3. S. Sheludiakov, C.K. Wetzel, D. M. Lee, V.V. Khmelenko, J. Järvinen, J. Ahokas, and S. Vasiliev, Studies of accumulation rate of H atoms in solid H2 films exposed to 0.1 and 5.7 keV electrons, Phys. Rev. B 107, 134110 (2023).