Measurement of the infrared photodetachment cross section ofNO−

We have measured the infrared photodetachment cross section of the negative molecular ion ${\mathrm{NO}}^{\mathrm{-}}$ for photons in the range 3000–4150 ${\mathrm{cm}}^{\mathrm{-}1}$ (0.372–0.514 eV), using a coaxial ion-beam–laser-beam apparatus. The cross section declined by 56% as the photon energy increased. There was no evidence for an expected threshold in the cross section near 4000 ${\mathrm{cm}}^{\mathrm{-}1}$ from the channel NO(v’=2)←${\mathrm{NO}}^{\mathrm{-}}$(v’ ’=0). This surprising result awaits theoretical interpretation.