Time-resolved ultrabroadband far-to-mid-infrared spectroscopy directly reveals doorway-mediated vibrational energy flow in an energetic crystal (β-HMX).

Understanding how vibrational energy flows from molecular vibrations to lattice phonons is critical for controlling the sensitivity of energetic molecular crystals. The intervening low-frequency doorway modes play a key role in this process, yet their ultrafast dynamics remain largely unexplored due to the lack of experimental access. Here, we overcome this limitation using time-resolved ultrabroadband far-to-mid-infrared spectroscopy, which allows us to directly monitor vibrational energy transfer (VET) involving doorway and lattice modes across the full 100-1700 cm-1 range in crystalline β-HMX-a spectral window inaccessible to conventional mid-infrared techniques. This broad range reveals a complete three-stage energy flow pathway: sub-picosecond vibrational energy redistribution among high-frequency modes, a 2-25 ps multi-stage VET network (vibrational → doorway → lattice), and final ∼140 ps thermalization. Doorway-mode-mediated pathways govern VET, with an efficient route from NO2 symmetric stretch (ν20) to short-lived doorway modes (ν8-ν10, N-NO2 in-plane bend) and then to the lattice. These results provide the first direct experimental validation of the doorway mechanism in an energetic molecular crystal and demonstrate the unique power of ultrabroadband infrared spectroscopy for revealing previously hidden vibrational channels.