The effect of CO rotational energy on bimolecular reactions to form electronically excited C 2 is reported here. The reactions are initiated by CO multiphoton absorption of 800 nm light in strong optical fields using two different polarization configurations based on shaped chirped pulses. The observation of Swan band emission indicates that C 2 (d 3 Π g ) is a reaction product. The optical polarization is in the form of either an optical centrifuge or a dynamic polarization grating. In each case, the strong field aligns CO molecules and induces multiphoton absorption. Power-dependent measurements indicate at least seven photons are absorbed by CO; CO(a 3 Π) is a likely reactant candidate based on kinetic modeling. Relative reaction efficiencies are determined by measuring Swan band emission intensities. For a CO pressure of 100 Torr and an optical intensity of I = 2.0 × 10 13 W cm −2 , the relative C 2 (d 3 Π g ) yield with the dynamic polarization grating is twice that with the optical centrifuge. The extent of CO rotational energy was determined for both optical polarizations using high-resolution transient IR absorption for a number of CO states with J = 62–73 and E rot up to 10 400 cm −1 . Optical centrifuge excitation generates at least 2.5 times more rotationally excited CO molecules per quantum state than the dynamic polarization grating. The results indicate that the effect of large amounts of CO rotational energy is to reduce the yield of the C 2 products.
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Rotational energy transfer kinetics of optically centrifuged CO molecules investigated through transient IR spectroscopy and master equation simulations
A combined experimental and theoretical study of quantum state-resolved rotational energy transfer kinetics of optically centrifuged CO molecules is presented. In the experiments, inverted rotational distributions of CO in rotational states up to J=80 were prepared using two different optical centrifuge traps, one with the full spectral bandwidth of the optical centrifuge pulses, and one with reduced bandwidth. The relaxation kinetics of the high-J tail of the inverted distribution from each optical trap was determined based on high-resolution transient IR absorption measurements. In parallel studies, master equation simulations were performed using state-to-state rate constants for CO-CO collisions in states up to J=90, based on data from double-resonance experiments for CO with J=0-29 and a fit to a statistical power exponential gap model. The model is in qualitative agreement with the observed relaxation profiles, but the observed decay rate constants are smaller than the simulated values by as much as a factor of 10. The observed decay rate constants also have a stronger J-dependence than predicted by the model. The results are discussed in terms of angular momentum and energy conservation, and compared to the observed orientational anisotropy decay kinetics of optically centrifuged CO molecules. Models for rotational energy transfer could be improved by including angular momentum effects.
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- Award ID(s):
- 1800531
- PAR ID:
- 10338096
- Date Published:
- Journal Name:
- Faraday Discussions
- ISSN:
- 1359-6640
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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State-to-state rotational energy transfer in collisions of ground ro-vibrational state 13 CO molecules with N 2 molecules has been studied using the crossed molecular beam method under kinematically equivalent conditions used for 13 CO + CO rotationally inelastic scattering described in a previously published report (Sun et al. , Science , 2020, 369 , 307–309). The collisionally excited 13 CO molecule products are detected by the same (1 + 1′ + 1′′) VUV (Vacuum Ultra-Violet) resonance enhanced multiphoton ionization scheme coupled with velocity map ion imaging. We present differential cross sections and scattering angle resolved rotational angular momentum alignment moments extracted from experimentally measured 13 CO + N 2 scattering images and compare them with theoretical predictions from quasi-classical trajectories (QCT) on a newly calculated 13 CO–N 2 potential energy surface (PES). Good agreement between experiment and theory is found, which confirms the accuracy of the 13 CO–N 2 potential energy surface for the 1460 cm −1 collision energy studied by experiment. Experimental results for 13 CO + N 2 are compared with those for 13 CO + CO collisions. The angle-resolved product rotational angular momentum alignment moments for the two scattering systems are very similar, which indicates that the collision induced alignment dynamics observed for both systems are dominated by a hard-shell nature. However, compared to the 13 CO + CO measurements, the primary rainbow maximum in the DCSs for 13 CO + N 2 is peaked consistently at more backward scattering angles and the secondary maximum becomes much less obvious, implying that the 13 CO–N 2 PES is less anisotropic. In addition, a forward scattering component with high rotational excitation seen for 13 CO + CO does not appear for 13 CO–N 2 in the experiment and is not predicted by QCT theory. Some of these differences in collision dynamics behaviour can be predicted by a comparison between the properties of the PESs for the two systems. More specific behaviour is also predicted from analysis of the dependence on the relative collision geometry of 13 CO + N 2 trajectories compared to 13 CO + CO trajectories, which shows the special ‘do-si-do’ pathway invoked for 13 CO + CO is not effective for 13 CO + N 2 collisions.more » « less
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Quantum state-resolved spectroscopy was recently achieved for C60 molecules when cooled by buffer gas collisions and probed with a midinfrared frequency comb. This rovibrational quantum state resolution for the largest molecule on record is facilitated by the remarkable symmetry and rigidity of C60, which also present new opportunities and challenges to explore energy transfer between quantum states in this many-atom system. Here we combine state-specific optical pumping, buffer gas collisions, and ultrasensitive intracavity nonlinear spectroscopy to initiate and probe the rotation-vibration energy transfer and relaxation. This approach provides the first detailed characterization of C60 collisional energy transfer for a variety of collision partners, and determines the rotational and vibrational inelastic collision cross sections. These results compare well with our theoretical modeling of the collisions, and establish a route towards quantum state control of a new class of unprecedentedly large molecules.more » « less
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Abstract 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.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d2fd00068g
