Phonon generation in condensed He by laser-excited atomic bubbles

We discuss the interaction between nanometer-sized defects (atomic bubbles) and elementary excitations (phonons) in quantum fluids and solids. We observe that optical excitations in embedded metal atoms induce bubble expansions/contractions that create strongly localized phonon wave packets in the quantum matrix. We derive the structure and dynamics of these vibronic excitations from the experimental laser-induced fluorescence spectra of Au and Cu atoms in liquid and solid He. The atomic vibrations are found to be strongly damped on the 50 Å and 5 ps scales in agreement with pure hydrodynamic estimations. The dynamics of quantum fluids and solids is described in terms of quantized elementary excitations (such as, e.g., phonons and rotons in HeII [1]) with energies E and momenta k of meV range (tens of K) and a few Å−1, respectively. The most widely used method for probing these excitations is the inelastic scattering of neutrons having comparable energies and momenta. Each scattering event creates one phonon (roton), whose parameters E and k can be determined from the energy and momentum of the scattered neutron. First neutron scattering experiments on superfluid helium were reported in the 1950s, and nowadays the technique is still widely used, in particular in connection with excitations in thin helium films [2], helium confined in nanoporous media [3], and possible manifestations of supersolidity [4]. Scattering experiments can also be performed with optical photons [5] (E 2.5 eV, k 10−4 Å−1). However, the smallness of the photon momentum implies that only k≈ 0 optical phonons (which exist only in solids) or pairs of counterpropagating excitations can be created. Impurity atoms and molecules represent another class of nanoscopic objects that may create elementary excitations in condensed helium [6–10]. However, their potential for probing the dynamics of quantum fluids and solids at the microscopic level has not been exploited in depth so far. The main advantage of atomic dopants over other (a)E-mail: [email protected] probes is that they can be interrogated by means of laser radiation. Therefore, a variety of laser-spectroscopic and time-resolved methods can be applied for obtaining information on the matrix excitations. The mechanism of impurity-phonon interactions at the microscopic scale is not quantitatively understood at present. In particular, it is not clear how the presence of dopants modifies the spectrum and dispersion relation of the excitations in the helium sample, e.g., via the appearance of localized modes which do not exist in pure condensed helium. Here we present a simple hydrodynamic model for describing the dynamics of a vibrating atomic bubble. Our analysis reveals two regimes of the impurity-phonon interaction which we shall refer to as quasiclassical and quantum. We analyze existing optical data of impurity atoms in condensed helium and present a new laser-spectroscopic study of condensed He doped with the transition-metal atoms Au and Cu. Our results demonstrate that in the quantum regime their laser-induced fluorescence spectra yield complete information on the structure and dynamics of the phonon wave packet generated by the atomic-bubble motion. The structure of the defects formed by metal atoms embedded in liquid and solid helium has been studied since the 1980s (reviewed in [11]). The valence electron of the metal atom strongly repels the closed-shell He atoms due to the Pauli principle. The dopant thus forms a nanometersized cavity, of radius Rb, called “atomic bubble”. Due to Published in " " which should be cited to refer to this work. ht tp :// do c. re ro .c h A to m + bu bb le en er gy (m eV ) Rb (Å) abrption linhape fluoscence linhape bubble relaxation bubble relaxation abs