Ionization and fragmentation of methane with intense mid-infrared fields

We investigated the ionization and fragmentation rate of methane (CH4) at several wavelengths between 3 and 4 um. We found that the amount of fragmenting ions relative to the intact molecular ions exhibit a pronounced wavelength dependence and is peaked at around 3.3 to 3.6 $\mu $m. In contrast, the feature is absent in the same measurements with deuterated methane (CD4). The results suggested that the resonance of C-H bond stretching mode is playing a significant role in the dissociation processes. Moreover, by comparing the total ion yields of CH4 with that of CD4, we found that the overall ionization rate of CH4 is also enhanced at around 3.3 to 3.6 $\mu $m. This result has important implications in understanding tunnel ionization in the presence of vibrational resonance.