N. Katz

Molecular hydrogen formation on astrophysically relevant surfaces

Recent experimental results about the formation of molecular hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic hydrogen diffusion and desorption, the barrier for molecular hydrogen desorption, and the probability of spontaneous desorption of a hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first- and second-order kinetic processes within a single model. Using the values of the parameters that best fit the experimental results, the efficiency of hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.

H2 formation on interstellar grains in different physical regimes

An analysis of the kinetics of H2 formation on interstellar dust grains is presented using rate equations. It is shown that semi-empirical expressions that appeared in the literature represent two different physical regimes. In particular, it is shown that the expression given by Hollenbach, Werner & Salpeter [ApJ, 163, 165 (1971)] applies when high flux, or high mobility, of H atoms on the surface of a grain, makes it very unlikely that H atoms evaporate before they meet each other and recombine. The expression of Pirronello et al. [ApJ, 483, L131 (1997)] – deduced on the basis of accurate measurements on realistic dust analogue – applies to the opposite regime (low coverage and low mobility). The implications of this analysis for the understanding of the processes dominating in the Interstellar Medium are discussed.

Bragg spectroscopy of the multi-branch Bogoliubov spectrum of elongated Bose-Einstein condensates

We measure the response of an elongated Bose-Einstein condensate to a two-photon Bragg pulse. If the duration of the pulse is long, the total momentum transferred to the condensate exhibits a nontrivial behavior which reflects the structure of the underlying Bogoliubov spectrum. It is thus possible to perform a spectroscopic analysis in which axial phonons with a different number of radial nodes are resolved. The local density approximation is shown to fail in this regime, while the observed data agree well with the results of simulations based on the numerical solution of the Gross-Pitaevskii equation.

Beliaev damping of quasiparticles in a Bose-Einstein condensate.

We report a measurement of the suppression of collisions of quasiparticles with ground state atoms within a Bose-Einstein condensate at low momentum. These collisions correspond to Beliaev damping of the excitations, in the previously unexplored regime of the continuous quasiparticle energy spectrum. We use a hydrodynamic simulation of the expansion dynamics, with the Beliaev damping cross section, in order to confirm the assumptions of our analysis.

Direct Observation of the Phonon Energy in a Bose-Einstein Condensate by Tomographic Imaging

The momentum and energy of phonons in a Bose-Einstein condensate are measured directly from a time-of-flight image by computerized tomography. We find that the same atoms that carry the momentum of the excitation also carry the excitation energy. The measured energy is in agreement with the Bogoliubov spectrum. Hydrodynamic simulations are performed which confirm our observation.

Optically Induced Polarons in Bose-Einstein Condensates: Monitoring Composite Quasiparticle Decay

Nonresonant light scattering off atomic Bose-Einstein condensates is predicted to give rise to hitherto unexplored composite quasiparticles: unstable polarons, i.e., local impurities dressed by virtual phonons. Optical monitoring of their spontaneous decay can display either Zeno or anti-Zeno deviations from the golden rule, and thereby probe the temporal correlations of elementary excitations in the condensates.

High sensitivity phonon spectroscopy of bose-einstein condensates using matter-wave interference

We observe in time-of-flight images a matter-wave fringe pattern for low momentum excitations. The contrast of these fringes is a sensitive spectroscopic probe explained by a Bogoliubov excitation projection and approaches the quantized excitation regime

Decoherence and dephasing in strongly driven colliding Bose-Einstein condensates

We report on a series of measurements of decoherence and wave-packet dephasing between two colliding, strongly coupled, identical Bose-Einstein condensates. We measure, in the strong-excitation regime, a suppression of the mean-field shift, compared to the shift which is observed for a weak excitation. This suppression is explained by applying the Gross-Pitaevskii energy functional. By selectively counting only the nondecohered fraction in a time-of-flight image we observe oscillations for which both inhomogeneous and Doppler broadening are strongly suppressed. If no post-selection is used, the decoherence rate due to collisions can be extracted and is in agreement with the local density average calculated rate.

Three-wave mixing of bogoliubov quasiparticles in a Bose-Einstein condensate

A dressed basis is used to calculate the dynamics of three-wave mixing between Bogoliubov quasiparticles in a Bose condensate. Because of the observed oscillations between different momentum modes, an energy splitting, analogous to the optical Mollow triplet, appears in the Beliaev damping spectrum of the excitations from the oscillating modes.

Spectroscopic and collisional splitting in the spectrum of a strongly driven Bose condensate

The paper studies the spectrum of a /sup 87/Rb BEC in the presence of a dressing traveling optical lattice. The dressing Rabi frequency /spl Omega//sub d/ is determined by performing Rabi oscillations in the time domain. The spectrum of the dressed condensate is measured with a perturbative Bragg probe and presented without and with the dressing lattice beams, where splitting in frequency with magnitude similar to /spl Omega//sub d/ clearly seen for the latter.