New Method to Study Spin Conversion of a Nuclear-Spin Rotor with Low Tunnel Splitting

A new method is developed to measure the spin-conversion time ${\tau }_{\mathrm{con}}$ in low-tunnel-frequency systems. The experiment uses field cycling and consists of three steps: saturation of the tunnel reservoir by Zeeman-tunnel resonance; waiting, away from resonance, during a variable time interval $t_W$; and measurement of the decreased tunnel temperature $T_T$, by use of a saturated Zeeman reservoir. A plot of $T_T$ vs $t_W$ yields ${\tau }_{\mathrm{con}}$. ${\tau }_{\mathrm{con}}$ of C${\mathrm{H}}_3$ rotors in copper acetate is measured at 15, 12.5, and 6 K and turns out to be extremely long, at least ${20T}_1$. This discrepancy is discussed and compared with the one found in lithium acetate.