Interatomic Processes in Helium Nanodroplets
Ionization of matter by energetic radiation generally causes complex secondary reactions that are hard to decipher. Helium nanodroplets are ideally suited to unravel these secondary processes thanks to their unique properties -- the simple electronic structure of the He constituent atoms, the extremely weak binding of He atoms to one another and the quantum-fluid properties of He nanodroplets [1].
The simplest interatomic decay process occurs when several He atoms in a He nanodroplet are resonantly excited by an intense VUV pulse. Then, pairs of He* excited atoms can exchange energy and decay by the reaction He* + He* → He + He+ + e [2]. This process, traditionally termed Penning ionization, can be clearly identified by through the emission of a characteristic electron.
More complex secondary ionization processes at higher photon energies involve elastic and inelastic scattering of the electron thereby creating multiple He* excitations in a droplet, see the figure above [3], the simultaneous photoionization and excitation of a He atoms followed by energy-transfer ionization of a neighboring He atom in the reaction He+* + He + e → He+ + He+ + 2e [4], and photo double-excitation of a He atoms followed by energy transfer ionization by the reaction He** + He → He* + He+ + e [5].
Helium nanodroplets are mostly used as ultracold and inert substrates for spectroscopic studied of embedded molecules [6]. When the He in droplets is excited or ionized, the transfer of energy or charge can also lead to ionization of the embedded molecules [7,8]. For atoms and molecules bound to the surface of the droplets, these processes are surprisingly efficient [9,10,11].
References
[1] A. C. LaForge, L. Ben Ltaief, S. R. Krishnan, N. Sisourat and M. Mudrich, Rep. Prog. Phys. 87, 126402 (2024)
[2] L. Ben Ltaief, K. Sishodia, R. Richter, B. Bastian, J. D. Asmussen, S. Mandal, N. Pal, C. Medina, S. R. Krishnan, K. von Haeften, and M. Mudrich, Phys. Rev. Research 6, 013019 (2024)
[3] L. Ben Ltaief, K. Sishodia, S. Mandal, S. De, S. R. Krishnan, C. Medina, N. Pal, R. Richter, T. Fennel, and M. Mudrich, Phys. Rev. Lett. 131, 023001 (2023)
[4] M. Shcherbinin, A. C. LaForge, V. Sharma, M. Devetta, R. Richter, R. Moshammer, T. Pfeifer, M. Mudrich, Phys. Rev. A 96, 013407 (2017)
[5] B. Bastian, J. D. Asmussen, L. Ben Ltaief, H. B. Pedersen, K. Sishodia, S. De, S. R. Krishnan, C. Medina, N. Pal, R. Richter, N. Sisourat, M. Mudrich, Phys. Rev. Lett. 132, 233001 (2024)
[6] Molecules in Superfluid Helium Nanodroplets
[7] M. Shcherbinin, A. C. LaForge, M. Hanif, R. Richter, M. Mudrich, J. Phys. Chem. A 122, 1855-1860 (2018)
[8] A. C. LaForge, V. Stumpf, K. Gokhberg, J. von Vangerow, F. Stienkemeier, N. V. Kryzhevoi, P. O’Keeffe, A. Ciavardini, S. R. Krishnan, M. Coreno, K. C. Prince, R. Richter, R. Moshammer, T. Pfeifer, L. S. Cederbaum, and M. Mudrich, Phys. Rev. Lett. 116, 203001 (2016)
[9] D. Buchta, S. R. Krishnan, N. B. Brauer, M. Drabbels, P. O’Keeffe, M. Devetta, M. Di Fraia, C. Callegari, R. Richter, M. Coreno, K. Prince, F. Stienkemeier, R. Moshammer, and M. Mudrich, J. Phys. Chem. A 117, 4394 (2013)
[10] A. C. LaForge, M. Shcherbinin, F. Stienkemeier, R. Richter, R. Moshammer, T. Pfeifer, and M. Mudrich,Nature Physics 15, 247–250 (2019)
[11] L. Ben Ltaief, K. Sishodia, J. D. Asmussen, A. R. Abid, S. R. Krishnan, H. B. Pedersen, N. Sisourat and M. Mudrich, Rep. Prog. Phys. 88, 037901 (2025)