An instrument to measure the rotational and vibrational energies of heterogeneous reaction products has been constructed. Reactant atoms pass through a membrane made of the material being studied. After recombination and desorption occurs, the product gas is analyzed via either electron beam or laser‐induced fluorescence. Surface composition is monitored by soft x‐ray appearance potential spectroscopy. Results for nitrogen molecule desorption from clean, polycrystalline iron are presented in order to illustrate the capabilities of the apparatus.