State-Specific Collision Dynamics of Molecular Super Rotors with

Article pubs.acs.org/JPCA

State-Specific Collision Dynamics of Molecular Super Rotors with Oriented Angular Momentum Matthew J. Murray, Hannah M. Ogden, Carlos Toro, Qingnan Liu,† David A. Burns, Millard H. Alexander, and Amy S. Mullin* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States 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. Highresolution transient IR absorption spectroscopy is used to measure the timeevolution 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.

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rotational states, termed super rotors,31 were prepared by an angularly accelerating optical field.14,26 They used the optical centrifuge to adiabatically excite Cl2 molecules to states near J ≈ 420, where dissociation was detected. A key feature of both of these approaches is that the molecules have a unidirectional sense of rotation so that the ensemble of rotors has oriented angular momenta. Molecular super rotors exhibit a number of interesting collisional properties. There is evidence that super rotor angular momentum orientation is metastable with respect to collisions.32,33 Also, it has been suggested that macroscopic gas vortices may result from the super rotors’ extreme angular momenta.34 In the work reported here, we have used a high power optical centrifuge coupled to a high-resolution transient IR absorption spectrometer to investigate the time evolution of the translational and rotational energy profiles of optically centrifuged CO2 super rotors. Previous studies from our group have studied CO2 molecules in the range from J = 60−80 and have shown that transient absorption signals are a result of collision-induced relaxation of the centrifuge-excited molecules.28 Recent studies have shown that the highly oriented nature of the molecules prepared by the optical centrifuge is preserved through many Jchanging collisions.35 We present a set of experiments studying

INTRODUCTION Preparing all initial conditions of a molecular collision, including the internal energy and orientation of both collision partners, could lead to reaction efficiencies that approach unity.1 Significant enhancements in chemical reaction rates are obtained if molecules have added energy in motions that overlap with the reactive coordinate.2−5 A number of schemes have been envisioned to control as many conditions as possible in molecular collisions, including molecular orientation.6−14 The dynamics of nonreactive collisions are affected by the translational, vibrational, and rotational degrees of freedom, leading to a rich variety of observed behaviors.15−21 Because rotational energy states are relatively closely spaced (compared with vibrational and electronic energies), J-changing collisions are common, with many collision systems exhibiting a propensity for small changes in rotational quantum numbers.22−25 Most studies have focused on J-changing collisions within a thermal distribution of states. Here we investigate the rotationally inelastic collisions of molecules in ultrahigh rotational states that are initially prepared with a well-defined plane of rotation and a uniform direction of rotation. With the advancement of nonresonant laser schemes, new tools are available for controlling the orientation of rotational motion in the lab frame.14,26−30 Kitano et al. used a pair of crosspolarized pulses of light to nonadiabatically achieve angular momentum orientation of benzene molecules in the JK = 00, 10, 11, 22, and 33 states.27 In another approach, oppositely chirped pulses of light were used by Ivanov, Corkum, and coworkers to create an optical centrifuge, in which molecules in extreme © 2015 American Chemical Society

Special Issue: Dynamics of Molecular Collisions XXV: Fifty Years of Chemical Reaction Dynamics Received: August 14, 2015 Revised: October 13, 2015 Published: October 15, 2015 12471

DOI: 10.1021/acs.jpca.5b07941 J. Phys. Chem. A 2015, 119, 12471−12479

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Figure 1. (a) Spectra of oppositely chirped pulses, which combine to form the optical centrifuge. (b) Experimental geometry of IR probe and optical centrifuge (OC) beams.

the IR probe power was reduced significantly to avoid saturating the probe transitions. The experiments described here were performed with the IR and optical centrifuge (OC) beams crossing at 90°, as shown in Figure 1b. The optical centrifuge beam propagates along the z axis and prepares molecules with an angular momentum vector ⎯→ ⎯ JOC that is nearly parallel to the optical centrifuge propagation ⎯⎯⎯→ vector k OC and the z axis. Individual CO2 molecules that are optically centrifuged map out a rotating disk that spins with the ⎯⎯⎯→ electric field of the centrifuge pulse EOC. The ensemble of molecules has a unidirectional sense of rotation, corresponding to oriented angular momentum vectors. The propagation vector ⎯→ ⎯ kIR of the linearly polarized IR probe intersects the optical ⎯⎯⎯→ centrifuge propagation vector k OC at 90°. For the measurements reported here, the IR polarization was perpendicular to the x−y plane defined by the rotating disk of centrifuged molecules. Five CO2 states were investigated in this study: J = 0, 36, 54, 76, and 100. The optical centrifuge was focused down to a beam waist ω0= 25 μm in all sets of experiments, except the J = 100 state where the beam waist was ω0= 51 μm. Measurements of the J = 0, 36, and 54 states used the OPO and R branch transitions (ΔJ = +1) of the combination band (00°0 → 10°1). The J = 76 and 100 states were measured using the QCL with P branch transitions (ΔJ = −1) of the fundamental (00°0 → 00°1) band. For the J = 0, 54, and 76 states, the pressure was maintained at 10 Torr. For J = 100, the pressure was 7.5 Torr. For J = 36, the pressure was lowered to 5 Torr to reduce background absorption. The average time between collisions for these studies is 10 ns for 10 Torr, 15 ns for 7.5 Torr, and 20 ns for 5 Torr based on the gaskinetic collision frequency. To compare transient signals for different measurements on a common scale, time t is converted to average number of collisions N using the relationship N = t·z, where z is the gas-kinetic collision frequency. For CO2 at 300 K: z = 1.0 × 108 s−1 at 10 Torr; z = 6.7 × 107 s−1 at 7.5 Torr; and z = 5.0 × 107 s−1 at 5 Torr. Transient absorption changes were detected with a liquid-nitrogen-cooled InSb photodiode. Wavelength modulation was used to lock the IR frequency to the fringe of a scanning confocal etalon, thereby stabilizing the IR frequency during transient measurements. Transient line profiles were measured by collecting transient absorption at a series of closely spaced, discrete IR frequency steps (∼0.001 cm−1) across individual ro-vibrational transitions. Most of the line broadening is Doppler broadening; contributions from pressure broadening are assessed in the data fitting.

the transient populations and translational energies of a number of rotational states ranging from J = 0−100 to provide a broader picture of the collisional energy transfer dynamics of super rotors.

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EXPERIMENTAL METHODS The setup of the optical centrifuge high-resolution transient IR absorption spectrometer has been previously described.28,29 A Ti:sapphire laser system (Coherent) generates an ultrafast (40 fs) pulse centered at λ0 = 807 nm. The pulse is amplified, stretched to 50 ps, and given a positive chirp. The beam is split in half spectrally with two pairs of gratings and the spectral chirp of one beam is reversed. Figure 1a shows the spectrum of the oppositely chirped pulses. The output power of the pair of oppositely chirped pulses is increased with a 10 Hz Nd:YAGpumped multipass amplifier to a combined power of ∼50 mJ per pulse. The pulses are circularly polarized in opposite directions, recombined, and focused inside a Pyrex sample cell containing 5−10 Torr of CO2 gas (99.99%, Matheson Trigas). The recombined pulses generate a linearly polarized electric field that angularly accelerates over the duration of the pulse. The angular acceleration is a direct result of the timedependent frequency difference between the two chirped pulses. The spectral chirp is shown in Figure 1a; the full width at halfmaximum (fwhm) corresponds to a rotational frequency of ω ≈ 32 THz. A molecule with an anisotropic polarizability aligns with this rotating electric field and therefore gains rotational energy. A CO2 molecule rotating at ω ≈ 32 THz has a classical rotational energy of ∼18 200 cm−1, corresponding to a rotational quantum number near J = 220. Population changes for individual CO2 rotational states following the centrifuge pulse were measured using highresolution transient IR absorption. Two IR probe sources were used depending on the rotational state under investigation. A quantum cascade laser (QCL) from Daylight Solutions operating between λ = 4.2 and 4.5 μm was used to probe states J = 76 and 100 with the fundamental (00°0 → 00°1) transition. The QCL provides linearly polarized, high-resolution light with ΔνIR < 3 × 10−4 cm−1 and a power of ∼60 mW. The second IR source is a mid-IR optical parametric oscillator (OPO) by Lockheed Martin Aculight with output between λ = 2.5 and 3.2 μm. The J = 0, 36, and 54 states of CO2 were probed using combination band (00°0 → 10°1) transitions. The absorption strength of the combination band is ∼60 times weaker than the fundamental, thereby minimizing interference from background CO2 absorption.36 The IR light from the OPO is linearly polarized with a resolution of ΔνIR < 3 × 10−5 cm−1 and ∼1 W output power. In each case, 12472

DOI: 10.1021/acs.jpca.5b07941 J. Phys. Chem. A 2015, 119, 12471−12479

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Figure 2. Line center transient absorption measurements of CO2 J = 100 (a), 76 (b), 54 (c), 36 (d), and 0 (e) after excitation by the optical centrifuge. The detector response is shown as a dashed green line in panels a and e. Note that the signals are slower than the detector response in all cases.

Figure 3. Time-dependent Doppler-broadened line profiles for J = 100 (a), 76 (b), 54 (c), and 0 (d) are shown at 25, 50, 75, and 100 gas-kinetic collisions after optical centrifuge excitation.

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RESULTS AND DISCUSSION

results from quantum scattering calculations on the Ar−CO2(J) collision system. The trapping efficiency of the optical centrifuge depends on the intensity of the optical pulse and the chirp rate. From Figure 1a, the rotational frequency of the optical centrifuge at fwhm of the spectral chirps is near ω = 32 THz, corresponding to CO2 rotational states near J = 220. If the intensity profile of the pulse was uniform, one would expect the rotors to have a narrow

Here we report the evolution of translational energy and rotational state populations for 5 states of CO2 with J between J = 0 and 100 following optical centrifuge excitation. We first report the transient signals at line center, corresponding to a single Doppler slice. We then describe the time-dependent Doppler profiles of these states. Experimental results are compared with 12473

DOI: 10.1021/acs.jpca.5b07941 J. Phys. Chem. A 2015, 119, 12471−12479

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DOI: 10.1021/acs.jpca.5b07941 J. Phys. Chem. A 2015, 119, 12471−12479