Fast nuclear-spin conversion ofH2trapped and polarized in aCO2matrix

Fast ortho-to-para conversion of molecular hydrogen in chemisorption and matrix-isolation systems

“…They also found the ortho -to- para ratio of H 2 trapped in solid Ar to be decreased by ∼25% after 2–3 days and attributed this o-p conversion to the accidental contamination of magnetic impurities, though the accurate conversion rate was not determined. In a recent study ( Yamakawa et al, 2020 ), H 2 was trapped and polarized in solid CO 2 , so that the conversion rate of H 2 was derived from the time evolution of its infrared absorption band, as is expounded below.…”

“… Nuclear spin conversion of H 2 trapped in solid CO 2 at 5.4 K ( Yamakawa et al, 2020 ) (Copyright 2020; American Physical Society). (A) Induced infrared-absorption bands of H 2 (a) 240 s and (b) 3,360 s after the sample deposition.…”

“…In most of the condensed systems, the vibrational-frequency shift of H 2 with respect to the gas phase, , was negative: ( ) cm -1 at site A (B) of solid CO 2 ( Yamakawa et al, 2020 ), cm -1 in solid N 2 and CO ( Warren et al, 1980 ), cm -1 on amorphous D 2 O ice ( Hixson et al, 1992 ). Despite the relatively small red-shift, the conversion rate of H 2 in solid CO 2 at 5.4 K was even higher than that on amorphous H 2 O ice at 9.2 K, , measured by Ueta et al, 2016 , who also reported the monotonical increase of the rate with temperature below 14 K. This result is not explained by the electric-field-induced conversion mechanism ( Sugimoto and Fukutani, 2011 ); instead, probable is the three-step conversion model ( Ilisca and Ghiglieno, 2016 ; Ilisca, 2018 ), which consists of electron exchange between H 2 and a solvent, hyperfine contact interaction in H 2 , and spin-orbit interaction inside the solvent.…”

“…Interestingly enough, the conversion time-constants (the inverse of the conversion rate) of H 2 in non-magnetic systems accompanied by energy gaps are distributed quite widely: a few or tens of minutes in MOFs ( FitzGerald et al, 2010 ) and solid CO 2 ( Yamakawa et al, 2020 ), and tens or hundreds of hours in crystalline Si ( Lavrov and Weber, 2002 ; Hiller et al, 2007 ; Peng et al, 2009 ) and the cage of C 60 ( Chen et al, 2013 ). Note that the CO 2 film deposited below ∼9 K has a porous structure with the enhanced surface area and exhibits a unique feature of “thermal spikes”, which are abrupt temperature rises due to the structural rearrangement during the film deposition ( Arakawa et al, 1979 ).…”