Ultrafast Dynamics inside He Nanodroplets

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.

Fig. 1: A beam of superfluid He nanodroplets is generated by expansion of He gas through a cryogenic nozzle into vacuum. Loading of the droplets is very flexible: for example, a chromophore in microsolvation of water molecules can be produced by doping the chromophore first and subsequently adding a chosen number of water molecules. The system is then investigated with pump-probe photoionization and photoelectron and/or photoion detection.

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.

Fig. 2a: Fast response to photoexcitation. Time-resolved photoelectron spectrum of single In atoms solvated inside He droplets indicates an increase of the solvation shell within ~500 fs. The dashed line corresponds to TD-DFT simulations [1].
Fig. 2b: Slower response. The he bubble oscillates with 25 ps and the photoexcited In atom is ejected from the droplet after ~50 ps. The animation is obtained with time-dependent density functional theory (TD-DFT) [1].

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.

Fig. 3: Vibrational wave packet dynamics of In<sub>2</sub> molecules, observed as periodic modulation of the time-resolved photoelectron signal (left). Sliding-window Fourier analysis reveals that the oscillation amplitude decays with a half-life of 4 ps due to a combination pf dephasing and decoherence (center), whereas the oscillation frequency remains constant (right) [3].

[5] Observation of laser-assisted electron scattering in superfluid helium

L. Treiber, B. Thaler, P. Heim, M. Stadlhofer, R. Kanya, M. Kitzler-Zeiler, M. Koch

Nat Commun 12, 4204 (2021)


[4] Long-Lived Nuclear Coherences inside Helium Nanodroplets
B. Thaler, M. Meyer, P. Heim, M. Koch
Phys Rev Lett 124, 115301 (2020) | full text
Focus stories: APS Physics magazine | AIP Inside Science
Press coverage: | ChemEurope| Chemie.DE | APA | Der Standard | TU Graz

[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)

[1] Femtosecond photoexcitation dynamics inside a quantum solvent
B.Thaler, S. Ranftl, P. Heim, S. Cesnik, L. Treiber, R. Meyer, A. W. Hauser, W. E. Ernst, M. Koch
Nature Communications 9, 4006 (2018)
press coverage: Der Standard | ScienceDaily | EurekAlert! |Phys.Org


Markus Koch, Assoc.Prof.,
group leader
+43 316 873-8161

Leonhard Treiber
PhD student
+43 316 873-4531 - 8163

Michael Stadlhofer
PhD student
+43 316 873-4531 - 8163

Patricia Brugger
MSc student
+43 316 873-4531 - 8163 


active: P 33166

completed: P 29369