Utilizing Quantum\nCascade Lasers for Ultranarrow Velocity\nResolution and Quantum-State Selectivity in Molecular Beam Scattering\nand Spectroscopy

We demonstrate the capability of a narrow linewidth quantum\ncascade\nlaser (QCL) to selectively excite a very narrow velocity range of\nnitric oxide (σ ≤ 7(3) m/s) with a pure ro-vibrational\nquantum state. By implementing a counter-propagating geometry, the\nmolecules are selectively excited according to the Doppler shift of\nthe ro-vibrational transition frequency such that the velocity width\nassociated with the excited molecules depends only on the QCL linewidth.\nWe demonstrate a velocity distribution limited by the effective linewidth\nof our free-running QCL (Γ = 3.2 MHz). Our development provides\na cost-effective, flexible approach to resolve quantum-state selective\nchemical dynamics with excellent velocity resolution in a wide variety\nof molecules with infrared-active transitions. This technique has\nbeen formulated to provide ultrahigh collisional energy resolution\nin molecular beams to delineate final quantum-state product pairs\nin studies of molecular collisions.