Studies by UV and IR Absorption of Particular Motions of NO 2 − in Single Crystals of NaNO 2

The polarization states of the ν 2 IR absorption band of NO   2 −(ν 2 ≈ 828 cm −1 at 20 °C) and of the zero‐phonon line ν 00 of the UV electronic transition of NO   2 −(ν 00 ≈ 385 nm at 10 K), are studied between 10 and 300 K. When the temperature increases, the ν 2 (‖ b ) and ν 00 (‖ a ) absorption lines are no longer perfectly polarized along the b ‐and a ‐axes, respectively, and this depolarization is strongly dependent on the orientation of the wave vector q of the incident light ( q ‖ a , b or c ). This result may be explained by the NO   2 −interaction with low‐frequency waves.