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.