Ultrafast Intramolecular Rearrangement Dynamics of Phenyl Triflate

Gas-phase time-resolved investigations are essential for elucidating the reaction dynamics of photoacid generators (PAGs) embedded within solid-state photoresists employed in extreme ultraviolet (EUV) photolithography. Ultrafast time-resolved measurements reveal that the photodissociation of phenyl triflate (C₆H₅OSO₂CF₃), a PAG utilized in modern computer chip fabrication, occurs within a ~3 picoseconds timescale. This dissociation is modulated by an intramolecular rearrangement that yields C₆H₅OCF₃ and releases SO₂. The rearrangement shows vibrational coherence in the ion yield of multiple fragments. These coherent oscillations correspond to an O-S-C bending mode, which brings the CF₃ group closer to the phenoxy group, with a frequency observed at 58 cm^-1. Therefore, this promoting mode modulates the yield of the molecular ion and the subsequent ion fragments of phenyl triflate. The reaction responsible for SO₂ release occurs at 12.4 eV (100 nm), thus secondary electrons generated by EUV excitation at 92 eV (13.5 nm) are critical in triggering these photochemical processes. Electronic structure calculations identify the promoting vibrational mode and its frequency (70 cm^-1) is in good agreement with experimentally observed vibrational coherence.