Super Rotors Research Articles

Precisely spun super rotors

Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. Here, we demonstrate a broadband optical approach to pump trapped SiO+ molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state N = 67, corresponding to the peak of a 9400 K distribution, had a narrow N spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C2Π state. Significant centrifugal distortion of super rotors pumped up to N = 230 allowed probing electronic structure of SiO+ stretched far from its equilibrium bond length.

Hydroxyl super rotors from vacuum ultraviolet photodissociation of water

Hydroxyl radicals (OH) play a central role in the interstellar medium. Here, we observe highly rotationally excited OH radicals with energies above the bond dissociation energy, termed OH “super rotors”, from the vacuum ultraviolet photodissociation of water. The most highly excited OH(X) super rotors identified at 115.2 nm photolysis have an internal energy of 4.86 eV. A striking enhancement in the yield of vibrationally-excited OH super rotors is detected when exciting the bending vibration of the water molecule. Theoretical analysis shows that bending excitation enhances the probability of non-adiabatic coupling between the tilde B and tilde X states of water at collinear O–H–H geometries following fast internal conversion from the initially excited tilde D state. The present study illustrates a route to produce extremely rotationally excited OH(X) radicals from vacuum ultraviolet water photolysis, which may be related to the production of the highly rotationally excited OH(X) radicals observed in the interstellar medium.

Control of molecular rotation with an optical centrifuge

The author of this tutorial was shortlisted as one of the four dissertation prize finalists at DAMOP 2017 when presenting this work. The tutorial is based on the author's PhD thesis which studied the applicability of an optical centrifuge, the novel tool that utilizes non-resonant broadband laser to excite molecular rotation. It allows one to produce and control molecules in extremely high rotational states, so-called molecular 'super rotors'. The thesis included the study of certain physical properties of these super rotors, such as optical, magnetic, acoustic, hydrodynamic, and quantum mechanical properties.

Importance of rotational adiabaticity in collisions of CO2 super rotors with Ar and He.

The collision dynamics of optically centrifuged CO2 with Ar and He are reported here. The optical centrifuge produces an ensemble of CO2 molecules in high rotational states (with J ∼ 220) with oriented angular momentum. Polarization-dependent high-resolution transient IR absorption spectroscopy was used to measure the relaxation dynamics in the presence of Ar or He by probing the CO2 J = 76 and 100 states with Erot=2306 and 3979 cm-1, respectively. The data show that He relaxes the CO2 super rotors more quickly than Ar. Doppler-broadened line profiles show that He collisions induce substantially larger rotation-to-translation energy transfer. CO2 super rotors have greater orientational anisotropy with He collisions and the anisotropy from the He collisions persists longer than with Ar. Super rotor relaxation dynamics are discussed in terms of mass effects related to classical gyroscope physics and collisional rotational adiabaticity.

Anisotropic kinetic energy release and gyroscopic behavior of CO2 super rotors from an optical centrifuge.

An optical centrifuge is used to generate an ensemble of CO2 super rotors with oriented angular momentum. The collision dynamics and energy transfer behavior of the super rotor molecules are investigated using high-resolution transient IR absorption spectroscopy. New multipass IR detection provides improved sensitivity to perform polarization-dependent transient studies for rotational states with 76 ≤ J ≤ 100. Polarization-dependent measurements show that the collision-induced kinetic energy release is spatially anisotropic and results from both near-resonant energy transfer between super rotor molecules and non-resonant energy transfer between super rotors and thermal molecules. J-dependent studies show that the extent and duration of the orientational anisotropy increase with rotational angular momentum. The super rotors exhibit behavior akin to molecular gyroscopes, wherein molecules with larger amounts of angular momentum are less likely to change their angular momentum orientation through collisions.

Impulsive Collision Dynamics of CO Super Rotors from an Optical Centrifuge.

We report state-resolved collision dynamics for CO molecules prepared in an optical centrifuge and measured with high-resolution transient IR absorption spectroscopy. Time-resolved polarization-sensitive measurements of excited CO molecules in the J=29 rotational state reveal that the oriented angular momentum of CO rotors is relaxed by impulsive collisions. The translational energy gains for molecules in the initial plane of rotation are threefold larger than for randomized angular momentum orientations, indicating the presence of anisotropic kinetic energy. The transient data show enhanced population for CO molecules in the initial plane of rotation immediately following the optical centrifuge pulse. A comparison with previous CO2 super rotor studies illustrates the behavior of molecular gyroscopes; spatial reorientation of CO2 J=76 rotors takes substantially longer than that for CO J=29 rotors, despite similarities in classical rotational period and rotational energy gap. High-resolution transient IR absorption measurements of the CO J=29-39 rotational states show that the collisional depopulation rates increase with J quantum number.

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

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.

Rotational relaxation in ultracoldCO+Hecollisions

Cold and ultracold collisions involving rotationally hot CO molecules are investigated using quantum me- chanical coupled channel, coupled states, and effective potential scattering formulations. Quenching rate co- efficients are given for initial rotational levels near the dissociation threshold. The stability of the CO super rotors against collisional decay is compared to previous investigations involving homonuclear molecules. It is found that quasiresonant transitions provide a significantlystronger contribution to the total relaxation rate than in the comparable case of O2. As in the case of H2, sharp structures in the distribution of total quenching rate coefficients are found at rotational levels where quasiresonant scattering is not allowed. Experimental schemes to cool and trap neutral polar mol- ecules using a time-varying electric fieldhave been proposed 1,2 and recently realized 3 . Meijer and co-workers have applied the so-called Stark decelerator to slow down meta- stable CO molecules 1 . Schemes to produce diatomic mol- ecules in highly excited rotational states have also been pro- posed 4,5 and recently realized 6 . Theoretical studies have suggested that the collisional dynamics of such rota- tionally hot molecules would be particularly interesting at low temperatures 7-1 1 . These studies concentrated on homonuclear diatomic molecules where it was found that quasiresonant vibration-rotation QR VR transitions contrib- uted significantly in quenching the highly rotationally ex- cited molecules. Another recent investigation showed that QR VR transitions have a neglible effect on the relaxation of rotationally hot oxygen molecules and that the quenching is very efficient for all rotational levels and dominated by pure rotational deexcitation 12 . Each of the theoretical investi- gations 7-12 assumed a helium atom collision partner. He- lium buffer gas cooling 13 has proven to be an effective technique for loading molecules into a magnetic trap 14 and it has recently been shown that the cooling technique may be applied to a beam of molecules 15 . Therefore, col- lisions involving helium atoms are of considerable interest in ultracold molecular physics. In this work, we perform relax- ation studies for helium collisions with CO. We compute an extensive amount of collisional data that may be used as a point of comparison to the molecular hydrogen and oxygen systems studied previously and investigate whether hetero-

Emission spectrum for polar super rotors in a cold buffer gas

Rate coefficients for cold and ultracold He + CO collisions are used together with radiative lifetimes to investigate the relaxation of rotationally hot polar molecules in the presence of a cold or ultracold helium buffer gas. Starting with an initial Gaussian distribution of highly excited rotational states, such as might be produced by an optical centrifuge, the time-dependent relaxation of a collection of super rotors is studied and in the case of CO an emission spectrum which contains the collisional information is predicted as a function of buffer gas density. Implications for the experimental detection of cold and ultracold quasiresonant atom-diatom scattering is discussed.

Cold collisions involving rotationally hot oxygen molecules

Cold and ultracold collisions involving rotationally hot oxygen molecules are investigated using quantum-mechanical, coupled-channel, coupled-states, and effective-potential scattering formulations. Quenching rate coefficients are given for initial rotational levels near the dissociation threshold. The stability of the oxygen ``super rotors'' against collisional decay is compared to previous investigations involving hydrogen molecules where the rotational inertia was significantly smaller. In contrast to hydrogen, all possible states of rotationally hot oxygen are quenched very rapidly during a collision with a buffer gas helium atom, and the quenching efficiency is always dominated by pure rotational transitions.

Prospects for cooling and trapping rotationally hot molecules

Cold collisions involving highly rotationally excited diatomic molecules are investigated. As the translational energy is lowered, the total inelastic cross section decreases sharply for specific channels where quasiresonant transitions are no longer energetically allowed. Rate coefficients are given for collisional quenching of rotationally excited ${\mathrm{H}}_{2},{\mathrm{D}}_{2},{\mathrm{T}}_{2},$ and ${\mathrm{O}}_{2}$ at zero temperature. The specific rotational states that are stable against collisional relaxation would be interesting prospects for cooling and trapping. The application of collisional cooling methods together with recently developed schemes for producing rotationally hot molecules may allow high densities of ultracold ``super rotors'' to be achieved.

Cooling and trapping of molecules in highly excited rotational states

Collisional relaxation of highly rotating hydrogen molecules is investigated as a function of energy. Calculations demonstrate that inelastic collisions are dramatically suppressed for specific rotational levels of the molecule as the energy is lowered due to the closing of quasiresonant rotation-vibration channels. It is predicted that a ${}^{3}\mathrm{He}$ buffer gas may be used to load these highly excited molecules into a trap without a significant loss of population. It is further predicted that evaporative cooling may be used to cool the ``super rotors'' to even lower temperatures.

Vibration–rotation transfer in molecular super rotors

The collisional behavior of (X)6Li2 molecules in very high rotational levels of v=0 is considered. Highly efficient vibration–rotation transfer is predicted in these “super rotors” particularly when the conditions for quasiresonant transfer are fulfilled. This requires simultaneous near-resonance in energy and in angular momentum. Values of Δj for which quasiresonant vibration–rotation transfer (QRT) occurs become smaller as initial rotor state increases and transfer is likely to become particularly fast for Δj=2, predicted to occur when ji=130. This behavior is contrasted with the inefficiency of pure rotational transfer within the v=0 level for fast-rotating molecules. QRT will take place for quite cold collisions and thus will provide competition for the spinning-up process used to create the super rotors.

Scheme for state-selective formation of highly rotationally excited diatomic molecules

A scheme is proposed for making highly rotationally excited diatomic molecules (“super rotors”) in their ground vibrational and electronic state, e.g., 6Li2X 1Σg+ (v=0,J⩾115) where the rotational energy exceeds the bond strength (E(0,J)−E(0,0)⩾D00). Such levels, while strictly speaking quasibound, have very long tunneling lifetimes (>1011 s for J⩽130), and should have very interesting and unique collisional properties, especially at low temperature. The rotation of the molecules is “spun up” by sequential irradiation by R branch photons in the A 1Σu+–X 1Σg+ bands starting with cold molecules at low J. Spontaneous emission to other vibrational levels is overcome by using a pump laser and its multiple Raman sidebands as in previous work on “spinning down” diatomics.