Strong-field photofragmentation of methanol leading to H₃⁺ formation

H₃⁺ formation in strong-field photofragmentation of methanol (CH₃OH) is known to originate predominantly from the dication (CH3OH2+), and prior work has established H₂ roaming as a key mechanism that enables H₃⁺ production [1-3]. Here, we use coincidence momentum imaging of a CH₃OH⁺ ion beam following 790 nm strong-field interaction to directly quantify the dication versus monocation contributions and to search for complementary signatures of the roaming pathway. We measure the [H₃⁺ + COH⁺] channel and find that the neutral-cofragment channel [H₃⁺ + COH] is below our detection limit (<0.1% of the corresponding ionic-cofragment yield), providing direct evidence that H₃⁺ formation from the monocation (CH₃OH⁺) is negligible under our conditions. Building on the established roaming picture, we then test a key mechanistic consequence: if a neutral H₂ roams on the dication surface but fails to abstract a proton, it should be detectable as H₂ in coincidence with dication fragment(s), for example H₂ + CHOH²⁺ or three-body breakup into H₂ + Ion₁ + Ion₂. We discuss the extent to which our measured coincidence channels support these expected “failed abstraction” signatures of H₂ roaming in CH₃OH²⁺.