Dissociation mechanism of electronically excited CHn (n = 3–5) neutrals formed by near-resonant neutralization using charge inversion mass spectrometry

Charge inversion mass spectrometry was used to produce the electronically excited species CH_n (n=3–5) from their corresponding positive ions by neutralization with an alkali metal target, and then to subsequently detect and mass-analyze the negative ions formed from the neutral fragments produced from the dissociation of the excited neutrals. The trapezoidal shape and the intensity of the peak associated with CH₂^- ions in the charge inversion spectrum of CH₃⁺ ions indicated that the CH₃ neutrals dissociated mainly into CH₂ + H without a large activation barrier. The most intense peak in the spectrum of CH₄⁺ ions was that associated with CH₂^- ions, and this peak comprised a combination of both trapezoidal and triangular shaped peaks. The trapezoidal shaped peak was attributed to CH₂^- ions resulting from direct dissociation of CH₄ into CH₂ + H₂. The concurrent dissociation of CH₄ into CH₃ + H was followed by the further subsequent dissociation of the deformed CH₃ fragments into CH₂ + H, and this was proposed to be the origin of the triangular shaped component of the CH₂^- peak. In the spectrum of CH₅⁺ ions, the CH₃^- peak was much less intense than the CH₂^- peak, which was proposed to be the result of the geometry of the CH₃, formed from the dissociation of CH₅ into CH₃ + H₂,being substantially distorted from the D_3h symmetry, leading to its further subsequent dissociation.