  In a recent paper, cond-mat/0405331 (Ref. 1), del Barco et al. reported
experimental studies on a Ni4 molecular system. They used an experimental
method (combining microwave spectroscopy with high sensitivity magnetic
measurements) that we have introduced before, cond-mat/0404410 (see also Ref. 2
and 3). Among other things, our technique allows us to monitor spin-state
populations in the presence of microwave magnetic fields. Absorption
line-widths give rough 'upper bounds' on the decoherence rate similar to
'standard' high frequency electron paramagnetic resonance (HF-EPR) techniques.
In the case of quasi continuous radiation our technique does NOT give directly
the spin-lattice relaxation time T1. For measurements like those of del Barco
et al., it simply gives access to the phonon-bottleneck time, a parameter that
is many orders of magnitude longer than the spin-lattice relaxation time. Any
conclusion concerning quantum coherence is preliminary.
