  The electronic structure of a quantum spin liquid compound,
EtMe3Sb[Pd(dmit)2]2, has been studied with angle-resolved photoemission
spectroscopy, together with two other Pd(dmit)2 salts in the valence bond solid
or antiferromagnetic state. We have resolved several bands that have negligible
dispersions and fit well to the calculated energy levels of an isolated
[Pd(dmit)2]2 dimer. EtMe3Sb[Pd(dmit)2]2 being a Mott insulator, its lower
Hubbard band is identified, and there is a small gap of ~ 50 meV between this
band and the chemical potential. Moreover, the spectral features exhibit
polaronic behavior with anomalously broad linewidth. Compared with existing
theories, our results suggest that strong electron-boson interactions, together
with smaller hopping and on-site Coulomb interaction terms have to be
considered for a realistic modeling of the organic quantum spin liquid systems
like the Pd(dmit)2 salt.
