The Dissociation Of Polyatomic Molecules In Intense Femtosecond Laser Fields

The dissociation of polyatomic molecules in the intense femtosecond laser field (10¹³-1014W/cm²) was investigated both experimentally and theoretically.Methane and acetone molecules were irradiated by 800 nm, ¹00 fs pulsed laser with different intensities. A linear time-of-flight (TOF) mass spectrometer was coupled to the laser system and used to detect the ions produced. Experimental observations show that methane and acetone molecules can dissociate in the intense laser field and the molecular dissociation proceeds through the stepwise mechanism, not Coulomb explosion. We applied the field-assisted dissociation (FAD) model and the quasi-classical trajectory (QCT) calculations to predict the fragmentation patterns of methane and acetone molecules, which are consistent with the experimental observations.We have also investigated the pulse width effect on the dissociation of methane ions in the intense femtosecond laser field using 800 nm, 8.0×10¹³W/cm² laser pulses with a width ranging from 60 fs to 180 fs, The observed relative yield of the primary fragment CH₃⁺ increases with increasing pulse width and tends to saturate when the pulse width is longer than 120 fs. The FAD model and the quasi-classical trajectory (QCT) calculations were applied to predict the dissociation probability of CH4+. The calculated probability is corrected with the molecular orientation effect and the spatial distribution of laser intensity. The modified results show that the dissociation requires at least 23 fs and saturates with long pulse widths (≥100 fs). The result is consistent...