Exploring molecular interactions in the condensed phase with full rotational resolution Klaus von Haeften1, Luis Guillermo Mendoza Luna2, Nagham Shiltagh1 , Mark J. Watkins1, Nelly Bonifaci3, Frédéric Aitken3 1 Kanano GmbH, Caecilie-Auer-Weg 7, 89075 Ulm, Germany 2Department of Physics & Astronomy, University of Leicester, UK 3 G2ELab-CNRS, Equipe MDE, 25 Av. des Martyrs, BP 166, 38042 Grenoble Cedex 9, France

Probing superfluidity using molecules Klaus von Haeften XXXVI RSEF meeting, molecular physics at the edge Probing superfluidity using molecules Sharp rotational lines in superfluid 4He droplets, indicating free rotation By replacing superfluid 4He with non-superfluid 3He the sharp lines broaden into a single, broad feature When 60 4He is added, the sharp lines reappear Infrared spectrum of OCS in seeded beams, pure 4He, mixed 3He-4He droplets and pure 3He, Sartakov, Toennies, Vilesov, J. Chem. Phys. 128, 134316 (2012) Infrared spectrum of OCS in pure 3He, pure 4He and mixed 3He-4He droplets, Grebenev, Toennies, Vilesov, Science 279, 2089 (1998)

Non-superfluid density follows molecular rotation Angular momentum coupling increases moments of inertia and line-widths Early interpretation: binding sites of helium atoms (rigid). Advanced models: 'binding sites' of non-superfluid density (fluid). Figure: Whaley group

Working Hypothesis Normal liquid interface depends on - molecule-helium interaction (type of molecule, electronic/vibrational/rotational excitation) - pressure P - temperature T …need experiment in bulk helium He2* is a promising candidate for molecular spectroscopy in bulk helium Droplet helium work suggests existence of He2* in equilibrium with bulk, but no evidence has been presented yet.

Fluorescence spectroscopy of He2* in the Vis/Nir spectral ranges Klaus von Haeften molecular spectroscopy in bulk helium Fluorescence spectroscopy of He2* in the Vis/Nir spectral ranges Fluorescence from transitions between electronically excited states => Possibility to do spectroscopy in the visible range Collisions and solvation will broaden and shift lines e- Rydberg State of He2* 7 Å He2+

Previous experiments in superfluid helium show unsolvated He2* Klaus von Haeften molecular spectroscopy in bulk helium Previous experiments in superfluid helium show unsolvated He2* Population of high J means molecules are not in equilibrium Rotational resolution 160 keV electron beam excitation Corona discharge No rotational resolution Hill, Heybey, Walters, Phys. Rev. Lett. 26, 1213 (1971) Dennis et al., Phys. Rev. Lett. 23, 1083 (1969) Li et al., Eur. Phys. J. D 47,22821 (2009)

Experiment Klaus von Haeften molecular spectroscopy in bulk helium Cryogenic micro cell Excite He in large pressure range (1 – 100 bar) …vary interaction strength and kinetic energy via P and T… Mobility measurements Fluorescence spectroscopy First version High pressure cell mounted on cold finger (HELIOX cryogen-free fridge, Courtesy SRC Uni-Leicester) Second version Corona discharge running at room temperature

Measurement of ion mobility Determination of hydrodynamic radius He+ ions in supercritical helium Mobility µ from I(V) curves Relation between mobility, µ, and radius, r, via Stokes equation.  is viscosity. By knowing how size changes with pressure we can now investigate compressibility

Condensation, boiling and solidification of He+ clusters compressibility liquid solid H. Gharbi Tarchouna, J. Chem. Phys. Lett. 6, 3036 (2015)

Data analysis Selection of data Points Fitting of spectral lines with Lorentzian functions. Determination of Lineshifts Line intensities Linewidths

Analysis of line shifts Klaus von Haeften molecular spectroscopy in bulk helium Analysis of line shifts Different lineshift coefficients: Two different molecules contribute to spectra

Lineshift coefficients Luis G. Mendoza Luna et al., J. Phys. Chem. Lett. 7, 4666 (2016)

Pressure dependence of line intensity Intensity ratio between solvated (Q) and un-solvated excimers (P>2) 1 2 3 4 Q1 P2 Pressure induced rotational cooling in the liquid phase (but not in the gas phase) liquid phase gas phase

Model of solvation of He2* Low pressure High pressure Liquid helium Liquid helium Luis G. Mendoza Luna et al., J. Phys. Chem. Lett. 7, 4666 (2016)

Determination of solvated contribution of He2* fluorescence Klaus von Haeften molecular spectroscopy in bulk helium Determination of solvated contribution of He2* fluorescence Black: measured spectrum Blue: solvated He2* contribution T < 20 K Red: simulated spectrum Black: measured spectrum Simulation: Trot = 750 K

Atomic spectra Klaus von Haeften molecular spectroscopy in bulk helium singlet triplet

Singlet-triplet conversion Intensity ratio between triplet and singlet 3s -> 2p atomic lines Dissociative recombination gas phase liquid phase He2+ + e- -> He + He* 3s1S 3s3S He2+ He + He Triplet has far higher intensity than 3:1 Pressure dependence observed

Observations and Conclusions Klaus von Haeften Observations and Conclusions Mobility measurement shows growth and compression of He ion clusters. R reaches maximum, depending on temperature, T, and pressure, P. The higher T, the smaller is the maximum cluster size. Further increase of P compresses (and solidifies) the clusters. Regions of higher compressibility than bulk liquid helium are attributed to a liquid state of the clusters. Fluorescence spectra show contributions from solvated and un-solvated (He2* in gas bubbles) He2* in liquid helium. Pressure induces rotational cooling at a rate of at least 1010-1011K/s in the liquid phase, but not in the gas phase. Lowest rotational quantum state populated within 1 ns at pressures > 3bar. Q(1) and P(2) lines of He2* solvated in liquid helium are not resolved as expected for a hindered rotor. Very efficient pressure-induced transfer from singlet to triplet levels of atomic helium H. Gharbi Tarchouna et al., J. Chem. Phys. Lett. 6, 3036 (2015); Luis G. Mendoza Luna et al., J. Phys. Chem. Lett. 7, 4666 (2016)

Acknowledgements Funding: The Leverhulme Trust Royal Society Klaus von Haeften molecular spectroscopy in bulk helium Acknowledgements Funding: The Leverhulme Trust Royal Society British Council CONACYT Erasmus Technical Help: CMP group, Stuart Thornton Space Research Centre, Department of Physics & Astronomy, Duncan Ross, Jamie Williams, Jon Lapington Andy Underhill and Mechanical Workshop staff, Dept. Physics & Astronomy Frédéric Aitken Luis G. Mendoza Luna Nagham Shiltagh Mark J. Watkins Nelly Bonifaci

Formation of He2*-Hen clusters in the gas phase Klaus von Haeften free to hindered rotation in liquid helium Formation of He2*-Hen clusters in the gas phase Eloranta, Apkarian, J.Chem.Phys. 115, 752 (2001) Cluster formation? He2*+He must have long range minimum! LHe and He2*-Hen have different boiling temperatures