Dynamics in Solid Hydrogen below 4 K David T. Anderson Department of Chemistry, University of Wyoming Laramie, WY th International Symposium on Molecular Spectroscopy

Outline Growing H 2 clusters in the condensed phase Impurity induced H 2 nuclear spin conversion Chemical reaction dynamics at low temperature

Preferential formation of ortho-H 2 -HF in slit jet spectra j=0 H2H2 H F H F H H j=1 D 0 = cm -1 D 0 = cm -1 Expt: C. M. Lovejoy, D. D. Nelson, Jr., and D. J. Nesbitt, JCP 87, 5621 (1987). Theory: D. C. Clary and P. J. Knowles, JCP 93, 6334 (1990). para-H 2 -HFortho-H 2 -HF A. R. W. McKellar is studying X(H 2 ) n clusters in the gas-phase

13.8 K K Grow H 2 crystals via rapid vapor deposition S. Tam and M. E. Fajardo, Rev. Sci. Instrumen. 70, 1926 (1999). 15 K2 K

Growing solid parahydrogen crystals S. Tam and M. E. Fajardo, Rev. Sci. Instrumen. 70, 1926 (1999). room temperature dopant (e.g., CH 3 F) vacuum shroud radiation shield optical substrate T = 2.4 K IR beam atmosphere vacuum cryostat cold tip catalytic converter Fe(OH) 3

Experimental set-up IR ortho/para converter p-H 2 gas turbo pump dopant Specifications sample-in-a-vacuum liquid helium cryostat (1.7 – 5.0 K) variable temperature ortho/para converter ( K) 180 l s -1 turbo pump (<10 -4 torr during deposition) IR diagnostics – Bruker IFS120 (0.008 cm -1 )

Studying CH 3 F(ortho-H 2 ) n clusters in solid parahydrogen para-H 2 London dispersion weaker ortho-H 2 dipole-quadrupole stronger binding energies that differ by 5 cm -1 (60 J mol -1 ) are significant at T ≤ 4 K

CH 3 F(ortho-H 2 ) n clusters in solid hydrogen [ortho] /ppm K. Yoshioka and D.T. Anderson, JCP 119, 4731(2003). [CH 3 F] = ppm n =

ortho-H 2 clusters around CH 3 F H 3 CF para-H 2 ortho-H 2 n = 1 n = 2n = 3 n = 0

as-deposited 1 st annealing 1 st annealing increases cluster size 2.5 K 4.4 K 7 ppm CH 3 F 5.7% ortho-H 2

Takes time to approach low temperature cluster equilibrium 4.4 K → 1.8 K 0 hr 1 hr 2 hr 3 hr difference at 1.8 K spectrum is still changing after 3 hours!

ortho-H 2 can move by a process called “rotational diffusion” after before this process can be facile even at T = 2 K!!

4.3 K 1.8 K ? =9 ≥12 Reversibly grow larger clusters at low temperature 4.3 K 1.8 K

Reversibly “boil-off” outer ortho-H 2 molecules T = 1.8 KT = 4.4 K

Low temperature irreversible process – CH 3 F induced H 2 nuclear spin conversion 1 H½ F½ atomspinnuclear magnetic moment

CH 3 F catalyzed ortho-H 2 → para-H 2 conversion minutes

Summary ortho-H 2 molecules preferentially cluster around polar impurities even in solid molecular hydrogen solid molecular hydrogen is dynamic at liquid helium temperatures CH 3 X catalyzes H 2 nuclear spin conversion while OCS or N 2 O do not

The usual suspects (2004) Kazuki Yoshioka Subrahmanyam Garimella Paul Raston Adam Oman Britney Lorenz Becker Funding The American Chemical Society Research Corporation Department of Defense (AFOSR) The National Science Foundation Collaborators Mario E. Fajardo, Eglin AFB, Florida R. J. Hinde, University of Tennessee

annealed, 1.83 K annealed, 1.67 K N 2 O(ortho-H 2 ) n N 2 O(ortho-D 2 ) n Assigning X(H 2 ) n cluster spectra in SMH