Identifying isotopic effects in intense ultrafast laser-driven D$_2$H$^+$ fragmentation*

The triatomic hydrogen molecular ion is instrumental as a benchmark toward understanding the strong-field dynamics of polyatomic molecules. Using a crossed-beams coincidence three-dimensional momentum imaging method, we demonstrate clear isotopic effects in the fragmentation of D$_2$H$^+$ induced by 7 fs (40 fs), 790 nm laser pulses at an intensity of 10$^16$ W/cm$^2$ (5$\times$10$^{15}$ W/cm$^2$). Our experiment uniquely separates all fragmentation channels and provides kinematically complete information for the nuclear fragments. We show that for dissociative ionization of D$_2$H$^+$ there is a large difference in branching ratios of the two-body channels, where H$^+$+D$_2^+$ dominates D$^+$+HD$^+$, and the three-body channels, where H$^+$+D$^+$+D dominates D$^+$+D$^+$+H. In contrast, the dissociation channels display minimal differences.