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

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Abstract
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 2P1/2 and 2P3/2 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 16 000 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.