  We report on an infrared spectroscopy study of mobile holes in the
accumulation layer of organic field-effect transistors based on rubrene single
crystals. Our data indicate that both transport and infrared properties of
these transistors at room temperature are governed by light quasiparticles in
molecular orbital bands with the effective masses m* comparable to free
electron mass. Furthermore, the m* values inferred from our experiments are in
agreement with those determined from band structure calculations. These
findings reveal no evidence for prominent polaronic effects, which is at
variance with the common beliefs of polaron formation in molecular solids.
