Probing the ultrafast relaxation of superfluid helium

Helium nanodroplets are intriguing quantum systems featuring extraordinary properties such as an extremely low temperature (0.37 K) and frictionless motion (superfluidity). Because of these properties, and because helium droplets are chemically inert and completely transparent to infrared and visible light, they are often used as the “ideal test-tube” for probing embedded molecules by spectroscopy. But helium nanodroplets are also ideal model systems for studying the photodynamics in weakly-bound nanostructures, both experimentally and theoretically: He atoms have a simple electronic structure, interatomic binding is extremely weak, and the structure of helium nanodroplets is homogeneous and nearly size-independent due to their superfluid nature. So how does a superfluid droplet itself react when it is directly excited by a laser pulse? This question was answered by an international research team lead by M. Mudrich (Uni Aarhus, Denmark) and F. Stienkemeier (Uni Freiburg, Germany) in an experiment carried out at the LDM beamline of the free-electron laser FERMI. In this experiment, helium nanodroplets of variable size were resonantly excited into their lowest excited states by wavelength-tunable extreme-ultraviolet (XUV) laser pulses (pump) generated by FERMI. The subsequent relaxation dynamics was probed on the femtosecond time scale by photoionizing the droplets using an ultraviolet laser pulse (probe), and by measuring the energy of the emitted photoelectron as a function of the pump-probe delay (see Figure 1). 

Figure 1.    Time-resolved electron spectra showing the electronic relaxation into the metastable 1s2s state of a single helium atom. The upper and lower horizontal dashed lines indicate the photoelectron energy corresponding respectively to direct ionization of He by absorption of one pump and one probe photon, and to ionization of atoms that have decayed into the metastable 1s2s state; the dashed curve indicates the most-probable decay path.

The surprising finding is that despite the extremely weak binding of the He atoms in the droplets and the superfluid nature thereof, energy dissipation is very efficient even for the lowest excited states; more than 1 eV of electron energy is dissipated in less than 1 ps due to the coupling of the excited electron to the nanofluidic motion of the helium atoms in the droplet. Supported by high-level model calculations carried out by the group of M. Barranco (Uni Barcelona, Spain and Uni Paul Sabatier-Toulouse, France), the researchers identified three elementary relaxation steps: Ultrafast electron localization on a single excited helium atom, electronic relaxation of that atom into metastable states, and the formation of a void bubble around the atom as shown in Figure 2, which eventually bursts at the droplet surface, thereby ejecting the excited atom into vacuum. These results show that it is now possible to follow the relaxation dynamics of electronically excited free nanodroplets in great detail using ultrashort tunable XUV pulses. These results help understanding how nanoparticles interact with energetic radiation. This is important for various research fields, including atmospheric science, radiation damage in biological matter, light-harvesting mechanisms in natural and artificial complexes, and photocatalysis. Furthermore, knowledge about the ultrafast response of nanoparticles to intense XUV and x-ray laser pulses is instrumental for devising and interpreting experiments that probe the structure and dynamics of unsupported nanoparticles by direct diffraction imaging, which is now becoming possible using novel light sources such as laser-based high-harmonic radiation sources and x-ray free-electron lasers.

Figure 2.    Snapshot of the density distribution of a helium nanodroplet consisting of 1000 atoms, 4 ps after its resonant excitation. The density was simulated using time-dependent density functional theory (TD-DFT); the yellow region indicates the excitation localized on a single He atom, which is surrounded by an empty bubble. At later times, this bubble bursts at the droplet surface and ejects the excited atom into the vacuum.



This research was conducted by the following research team:

M. Mudrich1, A. C. LaForge2, A. Ciavardini3, P. O’Keeffe3, C. Callegari4, M. Coreno3, A. Demidovich4, M. Devetta5, M. Di Fraia4, M. Drabbels6, P. Finetti4, O. Gessner7, C. Grazioli8, A. Hernando9, D. M. Neumark10, Y. Ovcharenko11, P. Piseri5, O. Plekan4, K. C. Prince4, R. Richter4, M. P. Ziemkiewicz7,10, T. Möller12, J. Eloranta13, M. Pi14, M. Barranco15, F. Stienkemeier16


Department of Physics and Astronomy, Aarhus University, Denmark
Institute of Physics, University of Freiburg, Freiburg im Breisgau, Germany and Department of Physics, University of Connecticut, USA
CNR-ISM, Italy; Present address: CERIC-ERIC, Trieste, Italy
Elettra - Sincrotrone Trieste ScpA, Trieste, Italy
Dipartimento di Fisica, Università degli Studi di Milano, Italy; Present address: CNR-IFN, Milano, Italy
Laboratory of Molecular Nanodynamics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Chemical Sciences Division, Lawrence Berkeley National Laboratory, USA
CNR-IOM, Istituto Officina dei Materiali, Trieste, Italy
Kido Dynamics, EPFL Innovation Park Bat. C, Lausanne, Switzerland, IFISC (CSICUIB) and Instituto de Fisica Interdisciplinar y Sistemas Complejos, Palma de Mallorca, Spain
10 Department of Chemistry, University of California, Berkeley, USA
1 1Institut für Optik und Atomare Physik, TU-Berlin, Germany; Present address: European XFEL, Germany
12 Institut für Optik und Atomare Physik, TU-Berlin, Germany
1 3Department of Chemistry and Biochemistry, California State University at Northridge, USA
14 Departament FQA, Facultat de Física, Universitat de Barcelona, Spain and Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Spain
15 Departament FQA, Facultat de Física, Universitat de Barcelona, Spain, and Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Spain, and Laboratoire des Collisions, Agrégats, Réactivité, IRSAMC, CNRS et Université Paul Sabatier-Toulouse 3, France
16 Institute of Physics, University of Freiburg, Freiburg im Breisgau, Germany

Contact persons:

Marcel Mudrich, email:



M. Mudrich, A. C. LaForge, A. Ciavardini, P. O’Keeffe, C. Callegari, M. Coreno, A. Demidovich, M. Devetta, M. Di Fraia, M. Drabbels, P. Finetti, O. Gessner, C. Grazioli, A. Hernando, D.M. Neumark, Y. Ovcharenko, P. Piseri, O. Plekan, K.C. Prince, R. Richter, M.P. Ziemkiewicz, T. Möller, J. Eloranta, M. Pi, M. Barranco, and F. Stienkemeier. “Ultrafast relaxation of photoexcited superfluid He nanodroplets”, Nature Communications 11, 112 (2020) DOI:10.1038/s41467-019-13681-6

Last Updated on Tuesday, 28 January 2020 16:24