The photodissociation of HF in solid argon is studied by nonadiabatic molecular dynamics simulations, using Tully's 'surface hopping' method. The model includes 12 electronic states, described by DIM method. An extremely fast 'spin-flip' transition is found corresponding to a population buildup in the (3)Pi state, already within about 1 fs after a vertical Frank-Condon excitation into the (1)Pi state. The effect is particularly striking in view of the relatively weak spin-orbit coupling in HE. The mechanism of this exceptionally fast spin-flip is discussed and a physical interpretation is provided. Comparison is made to the slower spin-flip computed previously for F-2/Ar, and found both theoretically and experimentally for ClF/Ar. (C) 2007 Elsevier B.V. All rights reserved.