Precision Vibrotational Spectroscopy of a General Trapped Molecular Ion

Inelastic Recoil Spectroscopy (IRS) is a single-molecule action spectroscopy method that enables mid-infrared vibrational–rotational spectra of individual trapped molecular ions. By monitoring recoil events that occur when an excited molecule undergoes inelastic collisions with a cold buffer gas, IRS translates photon absorption into a measurable motional signature without destroying the ion. This approach overcomes the averaging intrinsic to ensemble spectroscopy, revealing vibrational and rotational structure one molecule at a time. We demonstrate a unique strength of single-molecule spectroscopy by measuring the distinct spectra of ortho and para nuclear spin isomers in the propargyl cation, C₄H₃⁺.