State-Specific Collision Dynamics of Molecular Super Rotors with Oriented Angular Momentum

Abstract

An optical centrifuge pulse drives carbon dioxide molecules into ultrahigh rotational states with rotational frequencies of ω ≈ 32 THz based on the centrifuge frequency at the full width at half-maximum of the spectral chirp. High-resolution transient IR absorption spectroscopy is used to measure the time-evolution of translational and rotational energy for a number of states in the range of J = 0-100 at a sample pressure of 5-10 Torr. Transient Doppler profiles show that the products of super rotor collisions contain substantial amounts of translational energy, with J-dependent energies correlating to a range of ΔJ propensities. The transient population in J = 100 is short-lived, indicating rapid relaxation of high J states; populations in J = 36, 54, and 76 increase overall as the super rotor energy is redistributed. Transient line profiles for J = 0 and 36 are consistently narrower than the initial ambient sample temperature, showing that collision cross sections for super rotors increase with decreasing collision energy. Quantum scattering calculations on Ar-CO2(j) collisions are used to interpret the qualitative features of the experimental results. The results of this study provide the groundwork for developing a more complete understanding of super rotor dynamics.

Dynamics and Hydrodynamics of Molecular Superrotors

“…However, rotational timescales are typicallym uch longer than vibrational ones. One is the "optical centrifuge" [12][13][14][15][16][17] that uses al aser that changes the orientationo fi ts linear polarization vector at an accelerating rate, which engages the molecules at the focusi nto fast, concerted rotation. This notion was used to obtain ag eneral description of the RT relaxation dynamics of fast-spinning molecules, [8] corroborated by the observed distribution of rotational states in ag as.…”

“…Several experimental methodsn ow exist for acceleration of the rotation of molecules and possibly turning them into SRs under ambient conditions. One is the "optical centrifuge" [12][13][14][15][16][17] that uses al aser that changes the orientationo fi ts linear polarization vector at an accelerating rate, which engages the molecules at the focusi nto fast, concerted rotation. The "molecularp ropeller" method [18][19][20][21] uses two short pulses with special timing and polarizations to align the molecules and then kick them into unidirectional rotation.…”

Impulsive Collision Dynamics of CO Super Rotors from an Optical Centrifuge

“…This result is also consistent with previous double resonances tudies on Jspecific rotational relaxation in CO-COc ollisions that find thermalization is complete for CO J = 14 in seven hard-sphere collisions. [30] The lifetimes presentedi nF igure 8s how that the decay of population in the CO J = 29 state is much slower than that for J = 35-39. [9] Initially, ar elatively narrow asymmetricd istributiono fs uper rotor states is prepared with the opticalf ield on.…”

“…Large translational energy releases are seen in CO 2 J = 76 following collisions of CO 2 super rotors, with the J = 76 state having atranslational temperature as high as 4000 K. [7] Higher J-states of CO 2 (such as J = 100) probedwith perpendicular IR light have smaller translational energy releases, with initial temperatures near 400 Kt hat increaset hrough collisions to 800 K. [30] These observations show that more translational energy is found for states furtherd own the cascade. The J = 76 state of CO 2 is further down in the energy cascade from the initial J = 220 rotors than is the J = 29 state of CO from the initial J = 50 state.…”

“…[31] Recent molecular dynamics simulations by Steinitz et al provide an estimate for what the initial distribution might look like for N 2 and CO 2 in an opticalc entrifuge and how that distri-bution evolves. [17,30] This asymmetry increases for the higher J states,w ith high J molecules being more likely to lose rotational energy than gain it. As the system relaxes back to equilibrium, the distribution of rotational states broadens substantially,w ith am aximum intensity at mid-J states that are lower than the initially excited molecules, but highert han the states with the greatest population at 300 K. [9] J-dependent opticalc entrifuge studies from our lab are consistentw ith the simulationr esults.…”