Strong field-induced dissociation by intense mid-infrared pulses was investigated in bromofluoroform monocation (CF3Br+) and iodobenzene dication (C6H5I2+) using ab initio molecular dynamics calculations. In both systems, bond -selective dissociation was achieved using appropriate laser polarizations and wavelengths. For CF3Br+, energetically disfavored fluorine elimination was strongly enhanced at wavelengths of 7 to 8 μm with polarization along a C–F bond. This is the result of two effects: the deposition of high enough kinetic energy into the molecule by the laser field and the near-resonant excitation of the C–F stretching mode. At shorter and off-resonant wavelengths, bromine elimination becomes significant due to rapid intramolecular vibrational energy redistribution (IVR). For C6H5I2+, the branching ratios for the dissociation of the ortho-, meta-, and para-hydrogens can be controlled simply by changing the laser polarization. These results show the general applicability of bond selective dissociation of cations by intense mid-infrared laser fields.

Mode-selective chemistry has been a dream of chemists since the advent of the laser in the 1970s. Despite intense effort, this goal has remained elusive due to efficient energy randomization in polyatomic molecules. Using ab initio molecular dynamics calculations, we show that the interaction of molecules with intense, ultrashort mid-infrared laser pulses can accelerate and promote reactions that are energetically and entropically disfavored, owing to efficient kinetic energy pumping into the corresponding vibrational mode(s) by the laser field. In a test case of formyl chloride ion photodissociation, the reactions are ultimately complete under field-free conditions within 500 fs after the laser pulse, which effectively overcomes competition from intramolecular vibrational energy redistribution (IVR). The approach is quite general and experimentally accessible using currently available technology.Keywords: intramolecular vibrational redistribution; mode-selective chemistry; strong field dissociation;

We measured the photoelectron spectra and angular distributions of partially aligned N2, O2, and CO2 in the rescattering plateau of above threshold ionization (ATI). The measured ATI electrons have relatively low collision energies (<15 eV). The photoelectron angular distributions (PAD) show clearly species and energy dependence. A simple two-center interference model was not able to consistently retrieve structural properties. We conclude that due to the interplay between the electrons and rescattering potential, the molecular structural information is obscured and cannot be extracted conveniently. However, the sensitivity of the PAD to the scattering potential in laser-induced electron diffraction promises a practical tool for studying electron–ion scattering dynamics.

Imaging and controlling reactions in molecules and materials at the level of electrons is a grand challenge in science, relevant to our understanding of charge transfer processes in chemistry, physics, and biology, as well as material dynamics. Direct access to the dynamic electron density as electrons are shared or transferred between atoms in a chemical bond would greatly improve our understanding of molecular bonding and structure. Using reaction microscope techniques, we show that we can capture how the entire valence shell electron density in a molecule rearranges, from molecular-like to atomic-like, as a bond breaks. An intense ultrashort laser pulse is used to ionize a bromine molecule at different times during dissociation, and we measure the total ionization signal and the angular distribution of the ionization yield. Using this technique, we can observe density changes over a surprisingly long time and distance, allowing us to see that the electrons do not localize onto the individual Br atoms until the fragments are far apart (∼5.5 Å), in a region where the potential energy curves for the dissociation are nearly degenerate. Our observations agree well with calculations of the strong-field ionization rates of the bromine molecule.