Photoinduced Molecule Formation of Spatially Separated Atoms on Helium Nanodroplets

Real-time tracking of energy flow in cluster formation

Femtosecond time-resolved spectroscopy has shaped our understanding of light-matter interaction at the atomic level. However, the photoinduced formation of chemical bonds, especially for larger aggregates, has evaded observation due to difficulties to prepare reactants at well-defined initial conditions. Here, we overcome this hurdle by taking advantage of the exceptional solvation properties of superfluid helium, which allow us to stabilize atoms in a metastable, foam-like configuration with 10 Å interatomic distance. We apply photoexcitation with a femtosecond laser pulse to collapse such a dilute metastable aggregate of Mg atoms formed inside a nanometer-sized He droplet, and track cluster formation at a characteristic time of (450 ± 180) fs through photoionization with a time-delayed second pulse. We find that energy pooling collisions of electronically excited Mg atoms occur during cluster formation, leading to transient population of highly-excited Mg atoms, up to 3 eV above the excitation photon energy. Relaxation and conversion to nuclear kinetic energy drives cluster fragmentation and ejection of ionic fragments from the droplet. Our results demonstrate the potential of He droplets for bond formation studies, and for revealing involved energy- and charge transfer dynamics, like photon energy upconversion.

Conservation of Hot Thermal Spin-Orbit Population of 2P Atoms in a Cold Quantum Fluid Environment

The 0.4 K internal temperature of superfluid helium nanodroplets is believed to guarantee a corresponding ground-state population of dopant atoms and molecules inside this cryogenic matrix. We have recorded 6s ← 5p excitation spectra of indium atoms in helium droplets and found two absorption bands separated by about 2000 cm^-1, a value close to the spin-orbit (SO) splitting of the In ²P ground state. The intensities of the bands agree with a thermal population of the ²P_1/2 and ²P_3/2 states at 870 K, the temperature of the In pick-up cell. Applying femtosecond pump-probe spectroscopy, we found the same dynamical response of the helium solvation shell after the photoexcitation of the two bands. He-density functional theory simulations of the excitation spectra are in agreement with the bimodal structure. Our findings show that the population of SO levels of hot dopants is conserved after pick-up inside the superfluid droplet. Implications for the interpretation of experiments on molecular aggregates are discussed.