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Recently, a metastable state of solid $$^4$$ He has been produced by locally lowering the density of the solid below the melting density using a focused acoustic wave pulse. An unexpected instability of the solid has been found about 4 bar below the melting line. This paper reports on experiments precisely localizing in time the instability birth within the acoustic pulse. It is found that, as expected, the instability always appears during a depression swing of the wave. However a metastability pressure limit does not emerge clearly. Total stress duration seems also to play a role in the instability triggering, suggesting a fatigue effect. [ABSTRACT FROM AUTHOR]

Metal atoms in solid and liquid helium-4 have attracted some interest either as a way to keep the atoms in a weakly perturbing matrix, or using them as a probe for the helium host medium. Laser sputtering with nanosecond pulsed lasers is the most often used method for atom production, resulting however in a substantial perturbation of the matrix. We show that a much weaker perturbation can be obtained by using femtosecond laser pulses with energy as low as 10 μJ. As an unexpected benefit, the atomic density produced is much higher. [ABSTRACT FROM AUTHOR]

Laser cooling and trapping offers the possibility of confining a sample of radioactive atoms in free space. Here, we address the question of how best to take advantage of cold atom properties to perform the observation of as highly forbidden a line as the 6S–7S Cs transition for achieving, in the longer term, atomic parity violation (APV) measurements in radioactive alkali isotopes. Another point at issue is whether one might do better with stable, cold atoms than with thermal atoms. To compensate for the large drawback of the small number of atoms available in a trap, one must take advantage of their low velocity. To lengthen the time of interaction with the excitation laser, we suggest choosing a geometry where the laser beam exciting the transition is colinear to a slow, cold atomic beam, either extracted from a trap or prepared by Zeeman slowing. We also suggest a new observable physical quantity manifesting APV, which presents several advantages: specificity, efficiency of detection, possibility of direct calibration by a parity conserving quantity of a similar nature. It is well adapted to a configuration where the cold atomic beam passes through two regions of transverse, crossed electric fields, leading both to differential measurements and to strong reduction of the contributions from the M1-Stark interference signals, potential sources of systematics in APV measurements. Our evaluation of signal-to-noise ratios shows that with available techniques, measurements of transition amplitudes, important as required tests of atomic theory, should be possible in 133Cs with a statistical precision of 10-3 and probably also in Fr isotopes for production rates of &rsim;106 Fr atoms s-1. For APV measurements to become realistic, some practical realization of the collimation of the atomic beam as well as multiple passages of the excitation beam matching the atomic beam looks essential. [ABSTRACT FROM AUTHOR]

We demonstrate the production of an electric field inside a high temperature cesium vapor cell with external electrodes. This external control of the electric field, which is not possible with a glass cell in presence of a cesium vapor, is achieved using a cell made of sapphire, and is of particular interest for our ongoing Parity Violation experiment. We describe the main components and the implementation on the set-up, including the pulsed high voltage generator. With pulse duration not exceeding 200 ns the system provides a reversible longitudinal E-field of up to 2 kV/cm in the vapor at a density of ∼ 2×1014 at/cm3 without discharge. Atomic signals attest the application of the electric field in the cell, with the predicted value. Further improvements obtained with sapphire cells are also presented. [ABSTRACT FROM AUTHOR]

Abstract. We have developed a "temperature-tunable cell", for which the reflected intensity of a laser beam on a chosen window can be reduced below 0.5% of the incident intensity, if one can control the cell temperature with an accuracy better than 1 Celsius. This application relies on the use of windows made from highly parallel and smooth (0001) sapphire plates. More generally, such sapphire plates have proven to be extremely well-suited components in the field of atomic physics and laser applications. [ABSTRACT FROM AUTHOR]

Abstract. We demonstrate that the walls of glass cells become electrically conductive when exposed to a dry cesium vapor, glasses of different compositions leading to similar effects. We find dramatically different results for monocrystalline sapphire cells, which exhibit typical resistances of a few G OMEGA, nearly five orders of magnitude larger than glass cells in similar conditions. In spectroscopy experiments requiring the application of an electric field, low cell resistances imply leakage currants generating stray magnetic fields. The latter, at the origin of harmful effects in precise tests of the fundamental laws of physics performed in cesium cells, will thus be suppressed in sapphire cells. Moreover, with such cells it becomes possible to place the electrodes outside. A tentative interpretation of the surface electrical conductivity of glass cells is propounded by establishing a connection with the surface coverage by cesium atoms physically adsorbed on the glass surface. This results from the observed dependences of the cell conductance versus Cs vapor density in the mtorr range and versus the wall temperature up to 200 Celsius, which indicates an activation energy of 0.66 +/-0.05 eV. In the sapphire cell, where there is no hint of surface effects, the conductivity looks instead attributable to collisional processes occuring inside the vapor for Cs number densities *(This character cannot be converted in ASCII text) 10[sup 14] at/cm[sup 3]. [ABSTRACT FROM AUTHOR]

Two-beam, linear magneto-optical spectroscopy is a powerful tool for studying short-lived states. We present both measurements and a quantitative theoretical analysis of magneto-rotation observed in the forward scattering of a linearly polarised laser beam passing through an amplifying atomic medium placed in a longitudinal magnetic field. The probed transition connects two short-lived, excited atomic levels, the upper state (here the 7 S level of cesium) being prepared initially via another transition from ground state, excited by a linearly polarised pump beam. The probe polarisation undergoes three different magneto-optical processes: optical rotation, with separate contributions from the two transitions, and linear dichroism due to Hanle precession of the upper state alignment. Complete resolution of the hyperfine structures and ninety degree switching of the probe polarisation enable us to isolate all of these processes. To lowest order in optical thickness the relative intensities and lineshapes are well interpreted. [ABSTRACT FROM AUTHOR]

Saturation phenomena in two-photon step-by-step transitions, analyzed by monitoring the intermediate state population, provide a valuable tool to get absolute cross-sections, free of uncertainties associated with the densities of the molecular or atomic species. At first sight, however, there is an ambiguity concerning which cross-section must be ascribed to each one of the two steps. One way to resolve this ambiguity is to look at the dependence of the saturation of the intermediate state population upon the polarization of the incident beam. This is illustrated here in the case of the two-photon ionization of Cs molecules involving a bound-free transition with production of 5 D Cs atoms in the intermediate state. The dominant polarization-splitting mechanism proceeds from the strong dependence upon the 5 D magnetic quantum number of the photoionization cross-section, relative to circularly polarized photons, while a similar feature is much less striking with plane polarized light. For the time scale involved in the experiments analyzed in this paper, the polarization splitting is strongly affected by the hyperfine interaction which mixes the 5 D magnetic numbers. We have extended our previous simplified analysis to situations where the hf frequency is not assumed to be large compared to the inverse of the light pulse duration. Our theoretical treatment is based upon the solution of a quantum Liouville equation with a non-hermitian Hamiltonian and an external source term. In view of the rather large atomic state multiplicity, we have developed a method which bypasses a direct numerical solution of the Liouville equation: we solve first Schrödinger-like equations, the final answer is then obtained from a quadrature. We also discuss other mechanisms contributing to the polarization splitting. Although they are of small importance in the present experimental context, they could be of relevance for shorter pulses of light or when the transition probability of the second step process is no longer the dominant one. [ABSTRACT FROM AUTHOR]

Transition probability saturation has been used as a means of investigation of cesium photoionization via photodissociative states of Cs around 550 nm. We present here a theoretical interpretation of the data obtained with polarized light. It allows for dispersion of the cross-sections due to the multi-level structure of the dissociated molecules and of the 5 D product atoms. The hfs in 5 D is also taken into account. From best data fits we obtain determinations of the averaged cross-sections, free of uncertainty due to Cs or Cs densities. The 5 D ionization cross-section agrees with theoretical prediction. [ABSTRACT FROM AUTHOR]

The 5D level of cesium is populated by pulsed 540 nm photodissociation of Cs, and selectively detected via the 6P population that results by radiative decay. It is monitored by absorption of a delayed probe pulse. Its time evolution yields a value of 1260±80 ns for the radiative lifetime of the Cs 5D level, in good agreement with the calculation by Theodosiou [15]. [ABSTRACT FROM AUTHOR]