28 research outputs found
Bose-Einstein condensation of metastable helium: some experimental aspects
We describe our recent realization of BEC using metastable helium. All
detection is done with a micruchannel plate which detects the metastables or
ions coming from the trapped atom cloud. This discussion emphasizes some of the
diagnostic experiments which were necessary to quantitatively analyse our
results.Comment: 5 pages, 3 figure
Ionization rates in a Bose-Einstein condensate of metastable Helium
We have studied ionizing collisions in a BEC of He*. Measurements of the ion
production rate combined with measurements of the density and number of atoms
for the same sample allow us to estimate both the 2 and 3-body contributions to
this rate. A comparison with the decay of the number of condensed atoms in our
magnetic trap, in the presence of an rf-shield, indicates that ionizing
collisions are largely or wholly responsible for the loss. Quantum depletion
makes a substantial correction to the 3-body rate constant.Comment: 4 pages, 3 figure
Getting the elastic scattering length by observing inelastic collisions in ultracold metastable helium atoms
We report an experiment measuring simultaneously the temperatureand the flux
of ions produced by a cloud of triplet metastablehelium atoms at the
Bose-Einstein critical temperature. The onsetof condensation is revealed by a
sharp increase of the ion fluxduring evaporative cooling. Combining our
measurements withprevious measurements of ionization in a pure BEC,we extract
an improved value of the scattering length nm. The analysis
includes corrections takinginto accountthe effect of atomic interactions on the
criticaltemperature, and thus an independent measurement of the
scatteringlength would allow a new test of these calculations
Using ion production to monitor the birth and death of a metastable helium Bose-Einstein condensate
We discuss observations of the ion flux from a cloud of trapped metastable
helium atoms. Both Bose-Einstein condensates and thermal clouds were
investigated. The ion flux is compared to time-of-flight observations of the
expanded cloud. We show data concerning BEC formation and decay, as well as
measurements of two- and three-body ionization rate constants. We also discuss
possible improvements and extensions of our results.Comment: 14 pages, 9 figures, submitted to Journal of Optics B (special issue,
cold quantum gases
Prospects for measurement and control of the scattering length of metastable helium using photoassociation techniques
A numerical investigation of two-laser photoassociation (PA) spectroscopy on
spin-polarized metastable helium (He*) atoms is presented within the context of
experimental observation of the least-bound energy level in the scattering
potential and subsequent determination of the s-wave scattering length.
Starting out from the model developed by Bohn and Julienne [Phys. Rev. A
\textbf{60}, (1999) 414], PA rate coefficients are obtained as a function of
the parameters of the two lasers. The rate coefficients are used to simulate
one- and two-laser PA spectra. The results demonstrate the feasibility of a
spectroscopic determination of the binding energy of the least-bound level. The
simulated spectra may be used as a guideline when designing such an experiment,
whereas the model may also be employed for fitting experimentally obtained PA
spectra. In addition, the prospects for substantial modification of the He*
scattering length by means of optical Feshbach resonances are considered.
Several experimental issues relating to the numerical investigation presented
here are discussed.Comment: 9 pages, 7 figure
Purely-long-range bound states of HeHe
We predict the presence and positions of purely-long-range bound states of
HeHe near the atomic
limits. The results of the full multichannel and approximate models are
compared, and we assess the sensitivity of the bound states to atomic
parameters characterizing the potentials. Photoassociation to these
purely-long-range molecular bound states may improve the knowledge of the
scattering length associated with the collisions of two ultracold
spin-polarized He atoms, which is important for studies of
Bose-Einstein condensates.Comment: 16 pages, 5 figure
An extensively validated C/H/O/N chemical network for hot exoplanet disequilibrium chemistry
This is the author accepted manuscript We aimed to build a new and updated C0-C2 chemical network to study the CHON
disequilibrium chemistry of warm and hot exoplanet atmospheres that relies on
extensively validated and recent state-of-the-art combustion networks. The
reliability range of this network was aimed for conditions between 500 - 2500 K
and 100 - 10^-6 bar. We compared the predictions of seven networks over a large
set of experiments, covering a wide range of conditions (pressures,
temperatures, and initial compositions). To examine the consequences of this
new chemical network on exoplanets atmospheric studies, we generated abundances
profiles for GJ 436 b, GJ 1214 b, HD 189733 b, and HD 209458 b, using the 1D
kinetic model FRECKLL and calculated the corresponding transmission spectra
using TauREx 3.1. These spectra and abundance profiles have been compared with
results obtained with our previous chemical network. Our new kinetic network is
composed of 174 species and 1293 reactions mostly reversible. This network
proves to be more accurate than our previous one for the tested experimental
conditions. The nitrogen chemistry update is found to be impactful on the
abundance profiles, particularly for HCN, with differences up to four orders of
magnitude. The CO2 profiles are also significantly affected, with important
repercussions on the transmission spectrum of GJ 436 b. These effects highlight
the importance of using extensively validated chemical networks to gain
confidence in our models predictions. As shown with CH2NH, the coupling between
carbon and nitrogen chemistry combined with radicals produced by photolysis can
have huge effects impacting the transmission spectra.Agence Nationale de la RechercheCentre National d’Études Spatiales (CNES)CNRS/INS
Condensation d'He métastable
Un condensat de Bose-Einstein de l'hélium dans un état métastable a été réalisé. Cet état a une énergie interne de 20 électron-volts par rapport à l'état fondamental, ce qui peut conduire à des collisions inélastiques qui détruirait le condensat. Cependant le taux de ces collisions est réduit de plusieurs ordres de grandeurs du fait de la polarisation des atomes dans le piège magnétique (conservation de spin pendant l'interaction). Cette réduction des collisions permet d'obtenir une densité dans l'espace de phase suffisante pour la condensation. La longueur de diffusion a été estimée à partir des données expérimentales et est trouvée en accord avec la prédiction théorique