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.

Direct observation of coherence transfer and rotational-to-vibrational energy exchange in optically centrifuged CO2 super-rotors

Optical centrifuges are laser-based molecular traps that can rotationally accelerate molecules to energies rivalling or exceeding molecular bond energies. Here we report time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO₂ at 380 Torr spun to energies beyond its bond dissociation energy of 5.5 eV (J_max = 364, E_rot = 6.14 eV, E_rot/k_B = 71, 200 K). The entire rotational ladder from J = 24 to J = 364 was resolved simultaneously which enabled a more accurate measurement of the centrifugal distortion constants for CO₂. Remarkably, coherence transfer was directly observed, and time-resolved, during the field-free relaxation of the trap as rotational energy flowed into bending-mode vibrational excitation. Vibrationally excited CO₂ (ν₂ > 3) was observed in the time-resolved spectra to populate after 3 mean collision times as a result of rotational-to-vibrational (R-V) energy transfer. Trajectory simulations show an optimal range of J for R-V energy transfer. Dephasing rates for molecules rotating up to 5.5 times during one collision were quantified. Very slow decays of the vibrational hot band rotational coherences suggest that they are sustained by coherence transfer and line mixing.

Selective rotational control in mixtures of molecular super-rotors

We demonstrate experimentally a method of all-optical selective rotational control in gas mixtures. Using an optical centrifuge-an intense laser pulse whose linear polarization rotates at an accelerated rate, we simultaneously excite two different molecular species to two different rotational frequencies of choice. The new level of control is achieved by shaping the centrifuge spectrum according to the rotational spectra of the centrifuged molecules. The shaped optical centrifuge releases one molecular species earlier than the other, therefore separating their target rotational frequencies and corresponding rotational states. The technique is applicable to molecules with non-overlapping rotational spectra in the frequency range of interest and will expand the utility of rotational control in the studies of the effects of molecular rotation on collisions and chemical reactions.

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.

Laser control of molecular rotation: Expanding the utility of an optical centrifuge

Since its invention in 1999, the optical centrifuge has become a powerful tool for controlling molecular rotation and studying molecular dynamics and molecular properties at extreme levels of rotational excitation. This technique has been applied to a variety of molecular species, from simple linear molecules to symmetric and asymmetric tops, to molecular ions and chiral enantiomers. Properties of isolated ultrafast rotating molecules, the so-called molecular superrotors, have been investigated, as well as their collisions with one another and the interaction with external fields. The ability of an optical centrifuge to spin a particular molecule of interest depends on both the molecular structure and the parameters of the centrifuge laser pulse. An interplay between these two factors dictates the utility of an optical centrifuge in any specific application. Here, we discuss the strategy of assessing and adjusting the properties of the centrifuge to those of the molecular rotors and describe two practical examples of optical centrifuges with very different characteristics, implemented experimentally in our laboratory.