Dissociative ionization of methanol in medium intense femtosecond laser field using time-of-flight mass spectrometry

• Dissociation follows step-wise dissociation with no sign for Coulomb explosion.
• CH3OH+, CH3O+, CH3+ form at 1.4×1013, 1.7×1013, 2.0×1013 W/cm2, respectively.
• C+, CH+, CH2+, OH+, and O+ and the H+ is detected at 2.8⁎1013 W/cm2.
• H2+ is detected at 3.0×1013 W/cm2.
• Among all ions, only H+ was found to have angular anisotropic distribution.

800 nm 100 fs laser combined with TOF mass spectrometer was used to investigate the dissociative ionization of methanol, CH3OH in the laser intensity less than 4×1013 W/cm2. The results showed that the dissociation follows a sequential step-wise dissociation pattern, with no sign for Coulomb explosion. At very low intensity 1.4×1013 W/cm2, only the parent ion CH3OH+ appears. At 1.7×1013 W/cm2, the dissociation products are mainly due to hydrogen elimination from the O–H or C–H bonds forming CH3O+ and CH2OH+ primary ions. At 2.0×1013 W/cm2, the C–O bond starts to break forming CH3+, which simultaneously dissociates to CH2+. The primary fragment ions CH3O+ and CH2OH+ start to dissociate into lower secondary fragments as CH2O+ and CHO+ for the former and CHOH+ and COH+ for the latter. At 2.8×1013 W/cm2, secondary dissociation fragments dominate the spectrum including C+, CH+, CH2+, OH+, and O+ and the H+ is detected. At 3.0×1013 W/cm2, H2+ is detected. For all ions formed, the ion yield increases with laser intensity till it reaches maximum, after which it drops down manifesting the dissociation of these ions into smaller fragments. Among all ions, only H+ was found to have angular anisotropic distribution.