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

• Published in: Faraday discussions 2024-08, Vol.251, p.14-159
• Main Authors: Ritter, Michael E, DeSouza, Simone A, Ogden, Hannah M, Michael, Tara J, Mullin, Amy S
• Format: Article
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/d3fd00179b

Collisions of optically centrifuged CO 2 molecules with J = 244-282 ( E rot = 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 ν 3 fundamental IR probe transitions are measured for the J = 186-282 states of CO 2 , 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. Optically centrifuged CO 2 molecules with J = 244-282 are aligned with the excitation polarization while collision products with J = 76-100 have no polarization. Collisions relax 〈 m J 〉 at a rate of −2 per collision.