Helium nanodroplets provide nanometer confinement and cooling to 0.4 K with least perturbations due to the quantum-liquid nature of superfluid He. While He droplets have been used for decades for the creation of tailor-made molecular complexes and their investigation with frequency-domain and mass spectrometric techniques, their potential has so far rarely been used for time-domain dynamical studies (Figure 1). As a future perspective, He droplets will allow us to study primary photochemical and photophysical processes, like energy and charge transfer, in molecular systems that are not accessible with other techniques.

Dynimics triggered by photoexcitation of atoms in He droplets

We set out to explore the opportunities and limitations of the He nanodroplet approach to femtosecond spectroscopy. As a first milestone, we were able to observe the quantum fluid response to photoexcitation of single chromophore atoms inside a He droplet (see Fig. 2 and Ref. [1]). We learned that a fast expansion of the solvation shell within 500 fs triggers the oscillation of the emerging He bubble with 25 ps period, and is followed by ejection of the excited chromophore from the droplet within ~50 ps.

Photodynamics of small molecules

In a second study we investigated vibrational wave packets in small dimer molecules (Fig. 3) and found that the influence of the quantum solvent He on the coherent nuclear motion is exceptionally low, much lower than any conventional solvent [3]. The demonstration that dynamics of fully solvated molecules are actually observable and only weakly influenced is a big step towards ultrafast photochemistry inside he droplets.

[3] Ultrafast photoinduced dynamics of single atoms solvated inside helium nanodroplets
B. Thaler, P. Heim, L. Treiber, M. Koch
J. Chem. Phys. 152, 014307 (2020)

[2] Conservation of Hot Thermal Spin-Orbit Population of ²P Atoms in a Cold Quantum Fluid Environment
B. Thaler, R. Meyer, P. Heim, S. Ranftl, J.V. Pototschnig, A. W. Hauser, M. Koch, W. E. Ernst,
J. Phys. Chem. A 123, 3977 (2019)