with J ≤ 280*

We report new IR spectra of N₂O and CO₂ in extreme rotational states made with an optical centrifuge. Spectroscopic signatures of high-J rotational states may be important in the characterization of exoplanet atmospheres, for which only a fraction of observed spectral lines have been identified. In our experiments, an optical centrifuge based on shaped ultrafast laser pulses is used to trap and spin molecules into high-J states with oriented angular momentum. High-resolution transient IR absorption spectroscopy of N₂O with J ≤ 205 (E_rot = 17700 cm^-1) and CO₂ with J ≤ 280 (E_rot = 30700 cm^-1) was measured near λ=4.3 μm using the output of a frequency-stabilized quantum cascade laser. The observed transition frequencies for N₂O and CO₂ agree well with predictions based on an extrapolation from low-J transitions, with the exception of two CO₂ spectral perturbations that result from mixing with (03³0) and (11¹0) vibrationally excited states. Transient Doppler-broadened line profile measurements confirm that rotation is excited selectively in the optical centrifuge, without broadening in the spectral lines. These experiments demonstrate how new IR transitions of high-J states can be measured with the challenges associated with thermal heating in a gas sample.