Photofragment Spectroscopy of FeCH₂ ⁺, CoCH₂ ⁺, and NiCH₂ ⁺ near the M⁺-CH₂ Dissociation Threshold

Photofragment spectra of jet-cooled MCH₂ ⁺ (M = Fe, Co, Ni) have been measured. Our investigation of NiCH₂ ⁺ represents the first reported spectroscopic study of this molecule. A spectroscopic threshold is observed for each of the three systems. In addition to imposing strict upper limits on the M⁺-CH₂ bond strengths, these results further the discussion concerning the interpretation of spectroscopic thresholds as thermodynamic limits. The measured upper limits are: D^○ ₀(Fe⁺-CH₂) ≤ 342 ± 2 kJ/mol, D^○ ₀(Co⁺-CH₂) ≤ 331 ± 2 kJ/mol, and D^○ ₀(Ni⁺-CH₂) ≤ 295 ± 5 kJ/mol. Three distinct, 2-nm-wide peaks are observed in the photofragment spectrum of CoCH₂ ⁺, but the spectra lack sharp structure above threshold.