Infrared absorption by a tunneling proton in crystalline 5-bromo-9-hydroxyphenalenone

5-bromo-9-hydroxyphenalenone absorbs infrared radiation strongly at 83 cm(- 1) when cooled to 5 K. The absorption is attributed to a transition from th e ground state to the first excited state of a tunneling proton in a double -well potential of the intramolecular hydrogen bond. The temperature depend ence of the spectrum was studied in detail. The linewidth was exceptionally large. Two mechanisms of the broadening are discussed. The atomic motion i n a deuterated crystal corresponding to the tunneling freezes at 34 K in a deuteration-induced phase transition. This is the first spectroscopic evide nce for the absorption of radiation by a tunneling proton in a neat molecul ar crystal.