Neutral-fragmentation paths of methane induced by intense ultrashort IR laser pulses: Ab initio molecular orbital approach

Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 × 10¹³ to 1.2 × 10¹⁴ W/cm² (from 1.0 × 10¹⁰ to 3.0 × 10¹⁰ V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH₂ + 2H and CH₂ + H₂, in ¹T₂ superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited ¹A₁ states originated from the Jahn-Teller ¹T₂ states were substantially stabilized along the neutral-fragment path CH₄ → CH₂ + 2H and were located below the ionization threshold. On the other hand, the low-lying excited ¹B₁ states, which also originate from the Jahn-Teller ¹T₂ states, were embedded on the ionized state along the dissociation path to CH₂ + H₂. This indicates that ionic fragments, rather than neutral ones, are produced along the CH₂ + H₂ path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.