Intense-Field Photoionization of Analogous Molecules by Focused Ultrafast Pulses*

The nature of a specific substituent in analogous molecules can dramatically affect their photodynamics. In previous work, we have shown that the dissociation rate for C-X bond breaking in the monohalobenzenes is strongly dependent of the value of X [\textit{Phys. Chem. Chem. Phys.}, \textbf{13}, 13783 (2011)]. We report on the ionization and fragmentation of analogous molecules in the focus of 50-fs, 800-nm laser pulses. Ion mass spectra are recorded as a function of intensity in the absence of the focal volume effect [\textit{Phys. Rev. Lett. }\textbf{100, }023002 (2008)]. Targets include carbon dioxide (CO$_{\mathrm{2}})$, carbon disulfide (CS$_{\mathrm{2}})$, and the substituted monoaromatics aniline (Ph-NH$_{\mathrm{2}})$ and nitrobenzene (Ph-NO$_{\mathrm{2}})$. We observe that the onset of ionization for CO$_{\mathrm{2}}$ and CS$_{\mathrm{2}}$ is dominated by parent ions. Ionization is insignificant for intensities that maximize alignment of CS$_{\mathrm{2}}$, which validates ultrafast electron diffraction experiments from aligned CS$_{\mathrm{2}}$ [\textit{Nature Comm. }\textbf{6, }8172 (2015)]. We observe REMPI in aniline including M$^{\mathrm{+}}$, M$^{\mathrm{2+}}$, \textellipsis with suppressed fragmentation. We observe minor amounts of parent ions and a propensity of fragmentation for nitrobenzene [T. Scarborough (in press) \textit{Phys. Chem. Chem. Phys.}]. Recent work at larger wavelengths will be discussed.