Transient Infrared Absorption Studies of Molecular Super Rotors Prepared in a Tunable Optical Centrifuge

Abstract

In the work presented here, an ultrafast, angularly accelerating optical trap, called an optical centrifuge, rotationally excites gas-phase molecules such as N2O, CO, and CO2 to extremely high-J rotational levels with oriented angular momentum. Molecules in highly energetic rotational levels are known as super rotors and relax to thermal equilibrium. The state-resolved collision dynamics are investigated using polarization-sensitive high-resolution transient infrared absorption spectroscopy. Three different optical centrifuge traps are used to impart angular momentum to the gas-phase molecules: a full optical bandwidth, a reduced optical bandwidth, and a tunable optical bandwidth. New IR spectral lines of N2O with J=140-205 (E_rot=8,200-17,400 cm^(-1)) are reported. Polarization-dependent transient measurements of N2O in J=195 reveal high orientational anisotropy of r=0.85 produced by the centrifuge. The nearly-nascent rotational distributions of CO are investigated using two pressures and two optical centrifuge bandwidths. The shapes of the distributions beyond the peak at J=62 mimic the intensity profile in the fall-off region of the shaped optical pulses. The capture and acceleration efficiencies of CO and CO2 at comparable angular frequencies using three clipped chirps are also investigated. Nascent rotational distributions show that CO2 and CO exhibit narrow and broad distributions, respectively, due to differences in molecular polarizability anisotropy. Surprisingly, the ratio of [CO2]:[CO] trapped by the centrifuge is nearly 1.5, despite CO2 having twice as many states as CO and about 3-fold larger polarizability anisotropy. The relaxation dynamics of CO and N2O with He and Ar buffer gases indicate that He is more efficient at rotational quenching than is Ar, and leads to products with larger recoil.