Phonon generation in condensed ⁴ He by laser-excited atomic bubbles

Abstract

-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 50Å 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 fewÅ −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 −4Å−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][7][8][9][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: peter.moroshkin@unifr.ch 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 4 He doped with the transition-metal atoms Au and Cu. Our results demon...

Laser ablation and spectroscopy of copper in liquid and solid4He

“…Therefore, it is identical for both transitions and strongly depends on the type of the matrix (liquid or solid). Recently, we have shown [23] that a simple hydrodynamic model may explain the PW shape and its spectral width.…”

Zero-phonon lines in the spectra of dysprosium atoms in superfluid helium

“…The hydrodynamic model of the atomic bubble leads to the eigenmodes of the bubble interface described by the spherical harmonics Y L,m (θ, ϕ). At the same time, these vibrations can be represented as localized phonon wavepackets or pseudolocal modes since their eigenfrequencies overlap with the phonon spectrum of liquid and solid He [5].…”

“…Besides the direct time-resolved measurements [4] and numeric calculations [6,7], the oscillations of the atomic bubbles can be investigated spectroscopically, by observing the phonon structure in the absorption and emission spectra of the impurities [5,8]. The electronic transitions of the impurity atom that are accompanied by the excitation of the bubble vibrations result in the formation of phonon wings (PW) in the impurity spectra.…”

Spectroscopy of gold atoms isolated in liquid and solid 4He matrices

“…The observed spectra reproduce the general trend reported earlier. 1,12 Valence electron transitions have very broad structureless lineshapes, whereas the inner-shell transitions possess a characteristic ZPL-PW substructure. The intensity ratio of the ZPL and PW features reflects the strength of the atom-phonon coupling.…”

“…The two types of excitation lead to distinctive spectral features in the laser-induced fluorescence emitted by the isolated species. 1 Outer shell transitions are strongly broadened and blueshifted with respect to the freeatom transition frequencies and have Gaussian-like lineshapes with a very small asymmetry. Under the same conditions the fluorescence spectrum from inner-shell excitations shows almost no shift and possesses a pronounced substructure, consisting of a very narrow zero-phonon line (ZPL) and a broader and redshifted phonon wing (PW).…”