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

Abstract

Charge inversion mass spectrometry was used to produce the electronically excited species CHn (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 D3h symmetry, leading to its further subsequent dissociation.