Impulsive Collision Dynamics of CO Super Rotors from an Optical Centrifuge

Generating Superrotors and Dynamics of Molecules in Extremely High Rotational States

The optical centrifuge was demonstrated in 2000 as a tool for preparing ensembles of molecules in extreme rotational states. Highly rotationally excited molecules, so-called superrotors, are observed as products of photodissociation and molecular collisions, in high-temperature environments in the atmospheres of Earth and exoplanets, and in the interstellar medium. Traditional optical excitation is limited to small changes in rotation, limiting experiments to relatively low rotational states. In this review, I discuss the use of a tunable optical centrifuge to prepare molecules in selected ranges of excited rotational states and investigations of their collisional relaxation using state-resolved polarization-sensitive transient IR probing. I examine the decay dynamics of population, alignment, and translational energy release, focusing on experimental results, and compare them with simulations that overestimate observed relaxation rates. A clear picture of near-resonant and nonresonant energy transfer pathways emerges and establishes the means to distinguish superrotor and bath collision products.

Spectroscopic study of energy transfer in collisions between vibrational excited H2 and CO2

The collisional energy transfer between vibrational excited H2(1, 7) and CO2 was investigated by exciting H2 to a vibrational excited state of v = 1, J = 7 by the stimulated Raman scattering technique. The coherent anti-Stokes Raman spectroscopy (CARS) technique determined that H2 was excited to the H2(1, 7) state. Varying the cuvette temperature, the number of H2(1, 7) particles was found to increase with the increase in H2 molar ratio α by scanning the intensity of the CARS spectrum, with peaks at different α at a temperature of 363 ± 15 K, but the peak temperature was not sensitive to α. Scanning CARS spectra after H2 collisions yielded that the energies of the excited states of H2 were mainly distributed in the vibrational and translational states, proving that the collisions between the excited states of H2 were linear collisions. The collisional transfer rate coefficients of H2(1, 7) and CO2 were obtained by fitting the Stern-Volmer equation as kv(H2) = (2.89 ± 0.30) × 10-13 cm3 s-1 and kv(CO2) = (8.23 ± 0.42) × 10-13 cm3 s-1. Exciting H2 to different states, it was found that the collisional transfer rate coefficient of CO2 was less affected by the energy of the vibrational excited H2. The rotational temperature was obtained from the Boltzmann distribution of the rotational dynamics, and it was found that the rotational temperature of CO2(0000, J) was about 3.4 times higher than that of CO2(0001, J), which proved that the energy of the vibrational excited H2 was mainly allocated to the higher rotational state of CO2(0000).

Transient IR spectroscopy of optically centrifuged CO2 (R186-R282) and collision dynamics for the J = 244-282 states

Collisions of optically centrifuged CO2 molecules with J = 244-282 (Erot = 22 800-30 300 cm-1) are investigated with high-resolution transient IR absorption spectroscopy to reveal collisional and orientational phenomena of molecules with hyper-thermal rotational energies. The optical centrifuge is a non-resonant optical excitation technique that uses ultrafast, 800 nm chirped pulses to drive molecules to extreme rotational states through sequential Raman transitions. The extent of rotational excitation is controlled by tuning the optical bandwidth of the excitation pulses. Frequencies of 30 R-branch ν3 fundamental IR probe transitions are measured for the J = 186-282 states of CO2, expanding beyond previously reported IR transitions up to J = 128. The optically centrifuged molecules have oriented angular momentum and unidirectional rotation. Polarization-sensitive transient IR absorption of individual rotational states of optically centrifuged molecules and their collision products reveals information about collisional energy transfer, relaxation kinetics, and dynamics of rotation-to-translation energy transfer. The transient IR probe also measures the extent of polarization anisotropy. Rotational energy transfer for lower energy molecules is discussed in terms of statistical models and a comparison highlights the role of increasing energy gap with J and angular momentum of the optically centrifuged molecules.

Enhancing reactivity of SiO+ ions by controlled excitation to extreme rotational states

Optical pumping of molecules provides unique opportunities for control of chemical reactions at a wide range of rotational energies. This work reports a chemical reaction with extreme rotational excitation of a reactant and its kinetic characterization. We investigate the chemical reactivity for the hydrogen abstraction reaction SiO⁺ + H₂ → SiOH⁺ + H in an ion trap. The SiO⁺ cations are prepared in a narrow rotational state distribution, including super-rotor states with rotational quantum number (j) as high as 170, using a broad-band optical pumping method. We show that the super-rotor states of SiO⁺ substantially enhance the reaction rate, a trend reproduced by complementary theoretical studies. We reveal the mechanism for the rotational enhancement of the reactivity to be a strong coupling of the SiO⁺ rotational mode with the reaction coordinate at the transition state on the dominant dynamical pathway.

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

The excess energy flow pathways during rotational and translational relaxation induced by rotational or translational excitation of a single molecule of and within each of four different neat liquids (H₂O, MeOH, CCl₄, and CH₄) are studied using classical molecular dynamics simulations and energy flux analysis. For all four liquids, the relaxation processes for both types of excitation are ultrafast, but the energy flow is significantly faster for the polar, hydrogen-bonded (H-bonded) liquids H₂O and MeOH. Whereas the majority of the initial excess energy is transferred into hindered rotations (librations) for rotational excitation in the H-bonded liquids, an almost equal efficiency for transfer to translational and rotational motions is observed in the nonpolar, non-H-bonded liquids CCl₄ and CH₄. For translational excitation, transfer to translational motions dominates for all liquids. In general, the energy flows are quite local; i.e., more than 70% of the energy flows directly to the first solvent shell molecules, reaching almost 100% for CCl₄ and CH₄. Finally, the determined validity of linear response theory for these nonequilibrium relaxation processes is quite solvent-dependent, with the deviation from linear response most marked for rotational excitation and for the nonpolar liquids.

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.

Optical pulses can be useful to create and control molecules in higher quantum states. Here the authors use optical pumping to create rotationally excited states of SiO⁺ molecular ion into super rotor ensemble.

State-resolved rotational distributions and collision dynamics of CO molecules made in a tunable optical centrifuge

State-resolved distributions and collision dynamics of optically centrifuged CO molecules with orientated angular momentum are investigated by probing the CO J = 29-80 rotational levels using high-resolution transient IR absorption spectroscopy. An optical centrifuge with tunable bandwidth is used to control the extent of rotational excitation in the sample. The rotational distributions are inverted with a maximum population in J = 62. Rotational levels with J > 62 have populations that correlate with the intensity profile of the optical trap. The full bandwidth trap excites CO up to the J = 80 level, while J = 67 is the highest level observed in the reduced bandwidth trap. Polarization-sensitive transient spectroscopy shows that the initial orientational anisotropy is r = 0.8 for levels with J ≥ 55, while anisotropy values are near r = 0.4 for levels with J < 50. The rotational distribution for J > 50 is broadened slightly by collisions, consistent with small |ΔJ| propensity rules for rotational energy transfer. Doppler-broadened line profiles show that the J = 60-80 levels have translational temperatures near T_trans = 300 K and that these temperatures remain constant for as much as 24 gas kinetic collisions. Doppler linewidths for levels with J < 60 are broadened by non-resonant rotation-to-translation energy transfer. Kinetic analysis of transient signals shows that collisions with thermal bath molecules are the predominant relaxation pathway.

Anomalous Intensities in the 2+1 REMPI Spectrum of the E 1Π-X 1Σ+ Transition of CO

We report on one-color experiments near 214 nm involving the photodissociation of jet-cooled OCS to produce high rotational states (40 < J < 80) of CO (X ¹Σ⁺, v = 0, 1) which were then ionized by 2+1 resonance-enhanced multiphoton ionization via the E ¹Π state. The nominally forbidden Q-branch of the two-photon E ¹Π-X ¹Σ⁺ transition is observed with intensity comparable to the allowed R-branch. The bright character of the high- J Q-branch lines can be described quantitatively as intensity borrowing due to mixing of the E ¹Π and C ¹Σ⁺ states, using J-dependent mixing coefficients extrapolated from the observed Λ-doubling in the lower rotational levels of the E state. In addition to the significant enhancement of Q-branch intensities above the values predicted by conventional two-photon line strengths for a ¹Π-¹Σ⁺ transition, the high- J lines of the R- and P-branches appear to be suppressed in intensity by approximately a factor of 3 compared to the unperturbed low- J line strengths, most likely due to perturbations associated with a ¹Σ^- state. The E-state rotational term values for J < 80, v = 0 derived from the present spectra agree within our measurement and calibration uncertainties with the extrapolations based on the molecular constants previously derived from rotational levels with J < 50. The E-X transition is attractive for future application to photodissociation dynamics and rotational polarization measurements of CO photofragments, with convenient access to state-selective probing on multiple rotational branches, which exhibit different sensitivity to fragment alignment.

The effect of CO rotation from shaped pulse polarization on reactions that form C2

The effect of CO rotational energy on bimolecular reactions to form electronically excited C₂ is reported here.