Time evolution of a pumped molecular magnet-A time-resolved inelastic neutron scattering study

Introducing an experimental technique of time-resolved inelastic neutron scattering (TRINS), we explore the time-dependent effects of resonant pulsed microwaves on the molecular magnet Cr₈F₈Piv₁₆. The octagonal rings of magnetic Cr³⁺ atoms with antiferromagnetic interactions form a singlet ground state with a weakly split triplet of excitations at 0.8 meV. A 4.6 tesla field was applied to tune the splitting between two members of the triplet excited level [Formula: see text] to resonance with 105 GHz (0.434 meV) microwaves. The time-dependent occupations of the ground state [Formula: see text], lower lying levels [Formula: see text] and [Formula: see text], and higher energy states [Formula: see text] were extracted during and after 20 s long microwave pulses incident along the (101) direction of a Cr₈F₈Piv₁₆ crystal held at 1.9 K. At significantly elevated spin temperatures, we found underpopulation relative to thermal equilibrium of [Formula: see text] and spin-lattice thermalization time scales ranging from 1.6(2) s to 5.7(2) s depending on the power level. This contrasts with the relaxation time [Formula: see text]s inferred for [Formula: see text] from in situ Electron Spin Resonance measurements. By probing a broad range of excited states during intense microwave pumping, TRINS thus provides a first view of long lived excited states in a molecular antiferromagnet.