Examination and control of H$_3^+$ formation in ethane with intense laser pulses*

Guided by COLTRIMS identification of H$_3^+$ fragmentation channels, an adaptive learning algorithm supplied with 3D momentum based feedback is used to identify intense laser pulse shapes that control H$_3^+$ formation from ethane. Since a C$_2$D$_4^{2+}$-D$_2$ intermediate state is thought to lead to D$_3^+$ formation via roaming of the D$_2$, we use the D$_3^+$:C$_2$D$_4^{2+}$ ratio as the control objective. In a similar measurement, we control the ratio of D$_2$H$^+$ to D$_3^+$ produced from the D$_3$C-CH$_3$ isotopologue of ethane, which selects between trihydrogen cations formed from atoms on one or both sides of ethane. Both the D$_3^+$:C$_2$D$_4^{2+}$ and D$_2$H$^+$:D$_3^+$ ratios can be modified by a factor of two or more. In addition, 2D scans of linear chirp vs. third-order dispersion are conducted for a few fourth-order dispersion values while the D$_2$H$^+$ and D$_3^+$ production are monitored. These dispersion scans are not as successful at modifying the D$_2$H$^+$:D$_3^+$ ratio as the adaptive search.