Transient IR (0001-0000) Absorption Spectroscopy of Optically Centrifuged N2O with Extreme Rotation up to J=205

Abstract High-resolution transient IR absorption spectroscopy was used to measure (0001− 0000) R-branch transition frequencies for N2O molecules in extreme rotational states, with quantum number up to J = 205 and energies as high as Erot=17000 cm−1. A population inversion of rotationally excited N2O states was prepared with an optical centrifuge and probed in a multi-pass IR cell using a quantum cascade laser. The optical centrifuge is based on 800-nm ultrafast, chirped laser pulses that optically trap molecules and accelerate them angularly to extreme rotational states. This work substantially increases the range of observed transitions for this band beyond J = 100 transitions previously reported and provides benchmark measurements for theoretical predictions based on a polyad model of effective Hamiltonian. Transient Doppler-broadened IR line profiles of N2O show that optical excitation of the sample is selectively partitioned into rotation, prior to Doppler broadening from rotation-to-translation collisional energy transfer. These results demonstrate how high-J transitions can be measured without thermal heating by coupling optical centrifuge excitation with high-resolution transient IR absorption probing.