A femtosecond-scale laser pulse makes molecular bonds

For centuries, scientists have steered the outcome of chemical reactions using macroscopic variables such as temperature, pressure, concentration, or pH. Since the advent of lasers, they have also exploited a microscopic variable—the internal structure of the reactants themselves. By shaping an ultrashort light pulse, researchers can tune the time-dependent amplitude and phase of its optical field so that the pulse’s multiple Fourier components induce multiple transitions among the electronic and vibrational degrees of freedom of atoms or molecules. The transitions then change their wavefunction and its time evolution. (See the article by Ian Walmsley and Herschel Rabitz, Physics Today, August 2003, page 43 .) So far, such coherent control of atoms and molecules has been limited to unimolecular processes such as excitation, ionization, and dissociation. Now a collaboration led by experimentalist Zohar Amitay (Technion–Israel Institute of Technology) and theorist Christiane Koch (University of Kassel in Germany) has demonstrated coherent control over bond formation in a two-particle reaction. The researchers fired 70-fs pulses at 1000-K magnesium atoms, which can absorb multiple IR photons and form Mg₂ dimers. As shown in the potential energy diagram, during each pulse, two IR photons excite a pair of colliding atoms in their ground state into a vibrational dimer state (shaded pink band) within the lowest excited electronic state (red curve). A subsequent excitation populates a higher excited state (blue band) whose symmetry, unlike that of the first excited state, allows UV fluorescence to the ground state. The team found that positively chirped pulses—those whose instantaneous frequency increases with time—enhanced the yield of detected dimers by up to a factor of five compared with unshaped pulses. (L. Levin et al., Phys. Rev. Lett. 114, 233003, 2015 .)