The methyl iodide multiphoton ionization spectrum with intermediate resonance in the A-band region

The 5pπ→6p Rydberg excitations of methyl iodide-h₃ and -d₃ are observed using multiphoton ionization spectroscopy as two-photon resonances when excited in the 28 000-32 000 cm^-1 region. The two-photon methyl iodide resonances abruptly cease at 32 000 cm ^-1, and at higher frequencies are replaced by resonances originating at the ²P_1/2 and ²P_3/2 levels of the iodine atom. A tentative assignment is also made for a two-photon resonance in the methyl radical in this region. The replacement of methyl iodide resonances with those of iodine and methyl fragments beginning at 32 000 cm^-1 correlates directly with the onset of a one-photon intermediate resonance in the A band of methyl iodide. When pumping with light of frequency less than 16000 cm^-1, the 5pπ→6p excitations of methyl iodide are observed as four-photon resonances. However, on entering the A-band region via two-photon resonance beyond 32 000 cm^-1, no fragment transitions appear. Instead, we observe the three-photon resonance to the (5pπ, 6s) state of the parent molecule. The different behavior is thought to be due to excitation to a directly dissociative component state (A₁ or E) in the case of one-photon A-band resonance, and to a bound A₂ state in the case of two-photon A-band resonance.