Core-to-valence spectroscopic detection of the CH2Br radical and element-specific femtosecond photodissociation dynamics of CH2IBr.

Abstract

Element-specific single photon photodissociation dynamics of CH2IBr and core-to-valence absorption spectroscopy of CH2Br radicals are investigated using femtosecond high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy. Photodissociation of CH2IBr along both the C-I or C-Br reaction coordinates is observed in real-time following excitation at 266 nm. At this wavelength, C-I dissociation is the dominant reaction channel and C-Br dissociation is observed as a minor pathway. Both photodissociation pathways are probed simultaneously through individual 4d(I) N(4/5) and 3d(Br) M(4/5) core-to-valence transitions. The 3d(Br) M(4/5) pre-edge absorption spectrum of the CH2Br radical photoproduct corresponding to the C-I dissociation channel is characterized for the first time. Although the radical's singly occupied molecular orbital (SOMO) is mostly localized on the central carbon atom, the 3d(Br) → π*(SOMO) resonances at 68.5 eV and 69.5 eV are detected 2 eV below the parent molecule 3d(Br) → σ*(LUMO) transitions. Core-to-valence XUV absorption spectroscopy provides a unique probe of the local electronic structure of the radical species in reference to the Br reporter atom. The measured times for C-I dissociation leading to I and I* atomic products are 48 ± 12 fs and 44 ± 4 fs, respectively, while the measured C-Br dissociation time leading to atomic Br is 114 ± 17 fs. The investigation performed here demonstrates the capability of femtosecond time-resolved core-level spectroscopy utilizing multiple reporter atoms simultaneously.