Photofragmentation and fragment analysis; Coriolis interactions in excited states of CH₃

Methyl radicals in their ground state (CH₃(X)) were created and excited by two- and one- color excitation schemes for CH₃Br and CH₃I, respectively, to record (2+1) REMPI spectra of CH₃ for resonant transitions to the Rydberg states CH₃**(np²A₂); n = 3, 4. Various new and previously observed vibrational bands were identified and analyzed to gain energetic information for the Rydberg states. Particular emphasis was placed on analysis of the rotational structured spectra centered at 70 648 and 60 700 cm^-1, due to transitions from to and for both Rydberg states, respectively. Vibrationally forbidden transitions between the and states, gain transition probabilities as a result of mixing of the ν₂ and ν₄ vibrational states due to Coriolis coupling between the two vibrational modes (intensity borrowing effect). As a result, the spectra are dramatically affected, both regarding line intensities and positions. This is the first direct evidence of a Coriolis interaction between two vibrational modes in Rydberg states of CH₃ (and in XH₃ molecules) based on simultaneous observation of spectra due to transitions to both interacting states. The analyses reveal close similarities between the Rydberg states and the ground state cation in terms of energy properties as well as the Coriolis interaction, as evident from comparison with recent work in relation to observation of CH₃⁺(X) in space (O. Berné et al., Nature, 2023, 621, 56-59). The effect of Coriolis interactions on predissociation of the Rydberg states/hence fragment products is discussed.