Fundamental symmetries studies with cold trapped francium atoms at ISAC

Francium combines a heavy nucleus (Z = 87) with the simple atomic structure of alkalis and is a very promising candidate for precision tests of fundamental symmetries such as atomic parity non-conservation measurements. Fr has no stable isotopes, and the ISAC radioactive beam facility at TRIUMF, equipped with an actinide target, promises to provide record quantities of Fr atoms, up to 10¹⁰/s for some isotopes. We discuss our plans for a Fr on-line laser trapping facility at ISAC and experiments with samples of cold Fr atoms. We outline our plans for a measurement of the nuclear anapole moment – a parity non-conserving, time-reversal conserving moment that arises from weak interactions between nucleons – in a chain of Fr isotopes. Its measurement is a unique probe for neutral weak interactions inside the nucleus.      

Francium sources and traps for fundamental interaction studies

Francium is one of the best candidates for atomic parity nonconservation (APNC) and for the search of permanent electric dipole moments (EDMs). APNC measurements test the weak force between electrons and nucleons at very low momentum transfers. They also represent a unique way to detect weak nucleon-nucleon interactions. EDMs are instead related to the time-reversal symmetry. Preliminary to these fundamental measurements are precision studies in atomic spectroscopy and the development of magneto-optical traps (MOT), which partially compensate for the lack of stable Fr isotopes. At LNL Legnaro, francium is produced by fusion of 100-MeV ¹⁸O with ¹⁹⁷Au in a thick target, followed by evaporation of neutrons from the compound nucleus. Francium diffuses inside the hot target (1200 K) and is surface ionized for injection at 3 keV in an electrostatic beamline. Typically, we produce 1×10⁶ (²¹⁰Fr ions)/s for a primary flux of 1.5×10¹² particles/s. We have studied Fr yields as a function of primary beam energy, intensity, and target temperature. Information on the efficiency of bulk diffusion, surface desorption and ionization is deduced. The beam then enters a Dryfilm-coated cell, where it is neutralized on a heated yttrium plate. The escape time of neutral Fr (diffusion + desorption) is approximately 20 s at 950 K, as measured with a dedicated setup. In the MOT, we use 6 orthogonal Ti:sapphire laser beams for the main pumping transition and 6 beams from a stabilized diode repumper. Fluorescence from trapped atoms is observed with a cooled CCD camera, in order to reach noise levels from stray light equivalent to approximately 50 atoms. Systematic tests are being done to improve the trapping efficiency. We plan to further develop Fr traps at LNL; in parallel, we will study APNC and EDM techniques and systematics with stable alkalis at Pisa, Siena, and Ferrara.     

A high-resolution time-of-flight mass spectrometer for the detection of ultracold molecules

We have realized a high-resolution time-of-flight mass spectrometer combined with a magneto-optical trap. The spectrometer enables excellent optical access to the trapped atomic cloud using specifically devised acceleration and deflection electrodes. The ions are extracted along a laser beam axis and deflected onto an off-axis detector. The setup is applied to detect atoms and molecules photoassociated from ultracold atoms. The detection is based on resonance-enhanced multi-photon ionization. Mass resolution up to m/Δm_rms=1000 at the mass of ¹³³Cs is achieved. The performance of this spectrometer is demonstrated in the detection of photoassociated ultracold ⁷Li¹³³Cs molecules near a large signal of ¹³³Cs ions. PACS 07.75.+h; 32.80.Rm; 37.10.Gh     

Transmission of ultracold atoms through a micromaser: detuning effects

The transmission probability of ultracold atoms through a micromaser is studied in the general case where a detuning between the cavity mode and the atomic transition frequencies is present. We generalize previous results established in the resonant case (zero detuning) for the mesa mode function. In particular, it is shown that the velocity selection of cold atoms passing through the micromaser can be very easily tuned and enhanced using a non-resonant field inside the cavity. Also, the transmission probability exhibits with respect to the detuning very sharp resonances that could define single cavity devices for high accuracy metrology purposes (atomic clocks).Received: 25 March 2003, Published online: 17 February 2004PACS: 42.50.-p Quantum optics - 32.80.-t Photon interactions with atoms - 32.80.Lg Mechanical effects of light on atoms, molecules, and ions     

An intense cold beam of metastable helium

We have produced and characterised a slow, bright and intense atomic beam of metastable helium atoms, suitable for atomic physics experiments. The maximum continuous flux attained was 2×10¹⁰ atoms/s, while a typical longitudinal peak velocity of the beam was ∼26 m/s with a divergence in the range of 15 mrad to 30 mrad. PACS 32.80.Pj; 32.80.Lg; 39.10.+j     

Atomic parity violation in a single trapped radium ion

Atomic parity violation (APV) experiments are sensitive probes of the electroweak interaction at low energy. These experiments are competitive with and complementary to high-energy collider experiments. The APV signal is strongly enhanced in heavy atoms and it is measurable by exciting suppressed (M1, E2) transitions. The status of APV experiments and theory are reviewed as well as the prospects of an APV experiment using one single trapped Ra^?+? ion. The predicted enhancement factor of the APV effect in Ra^?+? is about 50 times larger than in Cs atoms. However, certain spectroscopic information on Ra^?+? needed to constrain the required atomic many-body theory, was lacking. Using the AGOR cyclotron and the TRI??P facility at KVI in Groningen, short-lived ^212???214Ra^?+? ions were produced and trapped. First ever excited-state laser spectroscopy was performed on the trapped ions. These measurements provide a benchmark for the atomic theory required to extract the electroweak mixing angle to sub-1% accuracy and are an important step towards an APV experiment in a single trapped Ra^?+? ion.     

Photoionization of helium with ultrashort XUV laser pulses

Coupled-channel calculations for multiphoton ionization probabilities of helium through interaction with intensive short laser pulses are presented. Besides Slater-like orbitals we use regular Coulomb wavepackets in our configurational interaction basis to describe the continuum. Linearly polarized laser pulses of 3.8 fs duration and 2.96 x 10¹⁴ Wcm^-2 peak intensity have been used for frequencies between 0.2-1.2 a.u. The results are compared with other ab initio calculations.Received: 8 April 2003, Published online: 9 September 2003PACS: 32.80.Fb Photoionization of atoms and ions - 32.80.Wr Other multiphoton processes     

Nonlinear beam shaping by a cloud of cold Rb atoms

First experimental investigations are reported on nonlinear beam shaping due to the interaction between an intense laser beam and a cloud of laser cooled rubidium atoms. Resonant excitation of the F = 3 ↔ F ^′ = 4 hyperfine transition is considered. The single-pass interaction through the cold vapor causes an increase in the laser beam intensity in the forward direction (zero transverse wavevector component) when observed in Fourier space, for sufficiently high values of saturation. A qualitative explanation of the observations based on a two-level model for a resonantly excited transition proves acceptable. The observations are compatible with an interpretation based on nonlinear index-induced focusing of an incident beam with curved wavefront, as is used in z-scan measurements. Simple physical considerations allow us to deduce the conditions for the observability of optical patterns in the beam transmitted by a cold atomic cloud.     

Comparative short-term stability for a Cs beam and an expanding cold atomic cloud of Cs used as atomic frequency standards

We have compared the stability of two distinct atomic frequency standards. One is based on an atomic cold cloud of ¹³³Cs atoms in free expansion, interrogated by a Rabi resonance. The other standard is based on a thermal Cs beam interrogated by the separated oscillatory field method (Ramsey’s method). The atomic expanding cloud standard shows better stability and technical advances based on its simplicity.     

Sub-Doppler fluorescence on the atomic <Emphasis Type="Italic">D</Emphasis><Subscript>2</Subscript> line of a submicron rubidium-vapor layer

An extremely thin cell (ETC) with the thickness of a Rb atomic vapor layer in the range of 100–300 nm was fabricated. It is demonstrated that a simple laser-diode technique with a single resonant light beam is sufficient to observe separately all of the atomic hyperfine transitions of the D ₂ line of Rb (780 nm) and also allows us to measure the relative transition probabilities of the hyperfine transitions. The onset of collisional self-broadening of the hyperfine transitions as the number density of atoms increases was studied. The detrimental role of the atoms with slow longitudinal velocity in the sub-Doppler response of the Rb ETC is demonstrated by studies in which the cell is tilted from normal incidence of the laser beam. It is also shown that using an ETC allows us to resolve in a moderate external magnetic field the Zeeman splitting of the hyperfine transitions of the ⁸⁷Rb D ₁ transition F _g=1F _e=1,2. Received: 19 February 2003 / Revised version: 4 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +374/32-31172, E-mail: david@ipr.sci.am     

Investigation of highly excited states of calcium by three-photon ionization

The three-photon ionization in Ca from S₀ ground state is studied. The two-photon process is a near -- resonance process with one of the following bound states: 4s4d ¹D₂, 4p P₂, 4s6s ¹S₀, 4 D₂ and 4 S₀ while the third photon reach either directly the continuum or one of the autoionizing states. The succession of bound states as well as the transitions above the ionization limit are discussed. The dynamics of the multiphoton excitation processes is also discussed and radiative decay of 4 S₀ Ca state with two-photon excitation as well as (the measured) decay times of the Ca autoionizing states using the proper line profiles for different quantum numbers has been determined.Received: 29 September 2003, Published online: 18 May 2004PACS: 32.70.-n Intensities and shapes of atomic spectral lines - 32.70.Cs Oscillator strengths, lifetimes, transition moments - 32.80.Fb Photoionization of atoms and ions - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)     

Photoassociation experiments with ultracold metastable helium

The present article gives a review of various photoassociation (PA) experiments performed at ENS with a gas of ultracold atoms of metastable helium in the 2³S₁ state, using a PA laser beam red-detuned from the 2³S₁-2³P transitions. Molecular spectra close to the D₂ atomic line (2³S P₂) are presented. All the measured lines are identified as a signature of molecular bound states having a strong (if not pure) quintet spin character at short interatomic distance. Close to the D₀ atomic line (2³S P₀), giant helium dimers can be produced [see Phys. Rev. Lett. 91, 073203 (2003)]. A laser set-up improved recently allows us to measure very accurately the binding energy of the ro-vibrational ground state of the 0 _u ⁺ purely long-range potential and the agreement with the theory published previously is excellent. Finally, preliminary results on 2 photon PA spectroscopy are given.Received: 1 July 2004, Published online: 26 October 2004PACS: 32.80.Pj Optical cooling of atoms trapping - 33.20.Kf Visible molecular spectra - 34.50.Gb Electronic excitation and ionization of molecules - 34.20.Cf Interatomic potentials and forcesJ. Léonard: Present address: Van der Waals-Zeeman Institute, Universiteit van Amsterdam, The Netherlands.M. Walhout: Permanent address: Calvin College, Grand Rapids, MI, USA.     

Pump-probe measurement of atomic parity violation in cesium with a precision of 2.6%

We present the atomic parity violation measurements made in Cs vapour using a pump-probe scheme. After pulsed excitation of the 6S -7S forbidden transition in the presence of a longitudinal electric field, a laser beam resonant with one of the 7S -6P transitions stimulates the 7S atom emission for a duration of 20ns. The polarisation of the amplified probe beam is analysed. A seven-fold signature allows discrimination of the parity-violating linear dichroism, and real-time calibration by a similar, known, parity-conserving linear dichroism. The zero-field linear dichroism signal due to the magnetic dipole transition moment is observed for the first time, and used for in situ determination of the electric field. The result, ImE ₁ ^pv = (- 808±21)×10^-14 ea ₀ , is in perfect agreement with the corresponding, more precise measurement obtained by the Boulder group. A transverse field configuration with large probe amplification could bring atomic parity violation measurements to the 0.1% accuracy level.     

Squeezing and entangling atomic motion in cavity QED via quantum nondemolition measurement

We propose a scheme for preparing the squeezing of an atomic motion and an Einstein-Podolsky-Rosen state in position and momentum of a pair of distantly separated trapped atoms. The scheme utilizes the quantum nondemolition measurements with interaction between the cavity field and the motional state of the trapped atom in cavity QED. By illuminating the atoms with bichromatic light, the interaction Hamiltonian of the cross-Kerr effect between the cavity and atomic motion is generated to implement quantum nondemolition measurements.Received: 5 February 2003, Published online: 17 July 2003PACS: 03.67.Hk Quantum communication - 32.80.Lg Mechanical effects of light on atoms, molecules, and ions - 42.50.-p Quantum optics     

On-axis calcium magneto-optical trap loaded with a focused decelerating laser

We report on a simple scheme to efficiently load an on-axis magneto-optical trap (MOT) from a decelerated atomic beam, which avoids perturbation by radiation pressure from the decelerating laser. This has been tightly focused near the MOT center, with a waist size much smaller than the atomic cloud. For comparison, and in order to test the efficiency of this non-optimum deceleration geometry we have employed a second, independent decelerating laser, with a profile mode matched to the atomic beam. Using a Calcium MOT, good performance has been achieved and for an oven temperature of 580 °C we loaded 1.2(2)×10⁷ atoms in 16(1)ms. The technique described here has been essential for the sensitive detection of cold collisions, which represent minor losses in MOTs of alkaline-earth metal elements (R.L. Cavasso-Filho, A. Scalabrin, D. Pereira, F.C. Cruz: Phys. Rev. A, 67, 021402(R) (2003)). PACS 32.80.Pj, 39.25.+k, 39.10.+j     

Continuously transferring cold atoms in caesium double magneto-optical trap

We have established a caesium double magneto-optical trap (MOT) system for cavity-QED experiment, and demonstrated the continuous transfer of cold caesium atoms from the vapour-cell MOT with a pressure of ～ 1×10^-6 Pa to the ultra-high-vacuum (UHV) MOT with a pressure of ～ 8×10^-8 Pa via a focused continuous-wave transfer laser beam. The effect of frequency detuning as well as the intensity of the transfer beam is systematically investigated, which makes the transverse cooling adequate before the atoms leak out of the vapour-cell MOT to reduce divergence of the cold atomic beam. The typical cold atomic flux got from vapour-cell MOT is ～2×10⁷ atoms/s. About 5×10⁶ caesium atoms are recaptured in the UHV MOT.     

Coherent phase control in ionization of magnesium by a bichromatic laser field of frequencies \(\mathsf\omega \) and 2 \(\mathsf\omega \)

We have investigated the coherent phase control on the 3p² autoionizing state (AIS) resonantly coupled with the ground state for Mg through a two- and a four-photon transition simultaneously, using a bichromatic linearly polarized laser field. The frequency is chosen such that the lasers are tunable around resonance with the transition , which implies eV and eV. We are interested in the modification of autoionizing (AI) line shape through the relative phase and laser intensities. A strong phase dependence on the total ionization yield and ionization rate is found. We also performed a time-dependent calculation which takes into consideration all the resonant states of the process.Received: 5 November 2003, Published online: 9 March 2004PACS: 32.80.Qk Coherent control of atomic interactions with photons - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz AutoionizationG. Buic-Zloh: Permanent address: Institute for Space Sciences, P.O. Box MG-23, 77125 Bucharest-Mgurele, Romania.     

Measurement of the absolute total electron impact cross section on Cs atoms using a magnetooptical trap

Laser cooling and trapping of neutral atoms has been employed for the measurement of the total electron impact cross sections of the ground state 6² S _1/2 of ¹³³Cs atoms in the energy range from 80 to 500 eV. The total electron impact cross sections of the cold Cs atoms in the MOT were measured by observing the decay of the MOT after the interaction with the electron beam, and were found to depend only on the electron flux and not on the total number of atoms in the MOT. We confirm and extend the existing data on these measurements.     

Optimization of the focused flux of high harmonics

Following the theoretical predictions [1], the observation of two-photon processes by interaction of vacuum ultraviolet (VUV) radiation with inner-shell levels of atoms requires focused intensities in the 10¹³-10¹⁴ W/cm² range. Our aim is to reach this regime in order to study non-linear optics at these wavelengths. We first optimized the high harmonic conversion efficiency in argon by studying the best experimental conditions for phase-matching, concentrating on focus geometry related to laser energy, cell length and position relative to the focus. We then studied the resulting harmonic beam focusability by a toroidal mirror (f=10 cm) and made an image of the harmonic focus. We conclude with an evaluation of the focused intensity that we are able to reach experimentally.Received: 28 January 2003, Published online: 24 April 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation     

Transition probabilities in Kr II and Kr III spectra

On the basis of the relative line intensity ratio (RLIR) method, transition probability values of the spontaneous emission (Einsteins A values) of 14 transitions in the singly (Kr II) and 7 transitions in doubly (Kr III) ionized krypton spectra have been obtained relatively to the reference A values related to the 435.548 nm Kr II and 324.569 nm Kr III, the most intensive transitions in the Kr II and Kr III spectra. Our Kr III transition probability values are the first data obtained experimentally using the RLIR method. A linear, low-pressure, pulsed arc operated in krypton discharge was used as an optically thin plasma source at a 17 000 K electron temperature and 1.65 x 10²³ m^-3 electron density. Our experimental relative A values are compared with previous experimental and theoretical data.Received: 16 June 2003, Published online: 16 September 2003PACS: 52.70.Kz Optical (ultraviolet, visible, infrared) measurements - 32.70.Cs Oscillator strengths, lifetimes, transition moments - 32.70.Fw Absolute and relative intensities