Optical and transport studies of Ni(dmit<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>-based organic conductors

Single-crystal Ni(dmit${)}_{2}$ salts, (${\mathrm{Ph}}_{4}$P)[Ni(dmit${)}_{2}$${]}_{3}$, (${\mathrm{Bu}}_{4}$N${)}_{2}$[Ni(dmit${)}_{2}$${]}_{7}$\ensuremath{\cdot}${2\mathrm{C}\mathrm{H}}_{3}$CN, and (${\mathrm{Me}}_{3}$S)[Ni(dmit${)}_{2}$${]}_{2}$, have been synthesized. All show semiconducting behavior in their temperature-dependent dc conductivities. Room-temperature polarized reflectance measurements have been made over the range between 100 and 32 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (12 meV--4 eV). For light polarized along the Ni(dmit${)}_{2}$ stacking axis, all spectra show an energy gap, with superimposed vibrational fine structure at low frequencies and charge-transfer bands at high frequencies. Band gaps determined from the optical conductivities are consistent with thermal activation energies from dc transport measurements. The stacking-axis conductivity shows the effect of electron-molecular vibration interaction; analysis for (${\mathrm{Me}}_{3}$S)[Ni(dmit${)}_{2}$${]}_{2}$ yields a dimensionless electron-phonon coupling constant \ensuremath{\lambda}\ensuremath{\sim}0.27. For light polarized perpendicular to the stacking axis, only weak vibrational features are observed. \textcopyright{} 1996 The American Physical Society.