Joint AMO--Condensed Matter Physics Seminar

Matrix Isolation and Chemical Reactions in Impurity-Helium Solids

Vladimir Khmelenko
Laboratory of Atomic and Solid State Physics
Cornell University

Impurity-helium (Im-He) solids are porous solids created by injecting a mixed beam of helium gas and an impurity gas or impurity gases into superfluid 4He. The impurities or clusters of impurities from the beam are then surrounded by thin layers of solid helium. Van der Waals forces bind these components into a tenuous aerogel-like structure with nanoscale pores. We have used X-ray diffraction and ultrasound techniques to study the structure of Im-He solids. Clusters of impurities with sizes of order 50 Å and density ~1020 impurities per cm3 were observed by X-ray diffraction. The presence of a wide distribution of pore sizes in Im-He solids was revealed by ultrasound (from 80Å to 8600Å). Im-He solids are useful for matrix isolation of free radicals such as atomic hydrogen, deuterium and nitrogen. For example, local concentrations as high as ~1021 atoms/cm3 of stabilized isotopes of nitrogen atoms (14N and 15N) in nitrogen-helium solids were obtained. For the first time the N…N spin-pair radicals were observed in Im-He solids for both isotopes. High concentrations of the free radical impurities obtained in Im-He solids may possibly lead to energy storage applications. Another possible application of Im-He solids is the study of exotic tunneling chemical reactions. We use CW and pulse electron spin resonance (ESR) techniques to investigate Im-He solids containing atoms and molecules of hydrogen and/or deuterium. We have performed studies of low temperature (T~1.35K) tunneling chemical reactions in which deuterium atoms replace the hydrogen atoms bound in H2 or HD molecules to produce large and relatively stable concentrations of free hydrogen atoms. The kinetics of the reactions D+H2-->HD+H and D+HD-->D2+H have been investigated for Im-He solids with different initial ratios of hydrogen and deuterium ranging from 1% to 30%. Detailed studies of the structures of the H and D CW ESR lines have been performed. In addition to allowed ESR transitions of H and D atoms the satellite lines associated with the dipolar coupling of H and D atoms to the nuclear moments of hydrogen nuclei bound in neighboring molecules have been observed. From these observations we have determined the number of protons on neighboring molecules and their distances to the H and D atoms. The analysis of the modulation effects of the electron spin echo signals of H and D atoms allows us to estimate the number of deuterons on neighboring molecules and their distances to the atoms. The exchange tunneling reactions are capable of producing very high (up to 1018 cm-3) average concentrations of hydrogen atoms in Im-He solids. Quantum overlap phenomena involving quantum statistical effects or magnetic transitions might be observable at lower ( 30 mK) temperatures in these solids.

Thursday, September 22, 2005
2:00 PM
Gant Science Complex
Physics Department
Room P121

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