127 research outputs found
Magnetic Field Effects on the 1083 nm Atomic Line of Helium. Optical Pumping of Helium and Optical Polarisation Measurement in High Magnetic Field
The structure of the excited S and P triplet states of He
and He in an applied magnetic field B is studied using different
approximations of the atomic Hamiltonian. All optical transitions (line
positions and intensities) of the 1083 nm S-P transition are
computed as a function of B. The effect of metastability exchange collisions
between atoms in the ground state and in the S metastable state is
studied, and rate equations are derived, for the populations these states in
the general case of an isotopic mixture in an arbitrary field B. It is shown
that the usual spin-temperature description remains valid. A simple optical
pumping model based on these rate equations is used to study the B-dependence
of the population couplings which result from the exchange collisions. Simple
spectroscopy measurements are performed using a single-frequency laser diode on
the 1083 nm transition. The accuracy of frequency scans and of measurements of
transition intensities is studied. Systematic experimental verifications are
made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine
decoupling in high field are observed to be in good agreement with the
predictions of the simple model. Based on adequately chosen absorption
measurements at 1083 nm, a general optical method to measure the nuclear
polarisation of the atoms in the ground state in an arbitrary field is
described. It is demonstrated at 0.1 T, a field for which the usual
optical methods could not operate.Comment: 33 pages, 31 figures, 17 tables, 61 references. Revised version
(typos corrected, figure 11 replaced by the proper one) Accepted for
publication in EPJ
Manipulation of ultracold atomic mixtures using microwave techniques
We used microwave radiation to evaporatively cool a mixture of of 133Cs and
87Rb atoms in a magnetic trap. A mixture composed of an equal number (around
10^4) of Rb and Cs atoms in their doubly polarized states at ultracold
temperatures was prepared. We also used microwaves to selectively evaporate
atoms in different Zeeman states.Comment: 9 pages, 6 figure
Weâve Come a Long Way Maybe: Reflections of Women in The Academy
This essay provides narratives of five women in academy â specifically within the communication studies professoriate. They share experiences of equity (or lack thereof), motherhood, identity management, work-life balance, youthfulness, and illness. Overall, the stories presented seek to problematize ongoing difficulties for women in the academy with the hopes of sparking discussion and ongoing debate
Asymmetric Landau-Zener tunneling in a periodic potential
Using a simple model for nonlinear Landau-Zener tunneling between two energy
bands of a Bose-Einstein condensate in a periodic potential, we find that the
tunneling rates for the two directions of tunneling are not the same. Tunneling
from the ground state to the excited state is enhanced by the nonlinearity,
whereas in the opposite direction it is suppressed. These findings are
confirmed by numerical simulations of the condensate dynamics. Measuring the
tunneling rates for a condensate of rubidium atoms in an optical lattice, we
have found experimental evidence for this asymmetry.Comment: 5 pages, 3 figure
Ion detection in the photoionization of a Rb Bose-Einstein condensate
Two-photon ionization of Rubidium atoms in a magneto-optical trap and a
Bose-Einstein condensate (BEC) is experimentally investigated. Using 100 ns
laser pulses, we detect single ions photoionized from the condenstate with a
35(10)% efficiency. The measurements are performed using a quartz cell with
external electrodes, allowing large optical access for BECs and optical
lattices.Comment: 14 pages, 7 figure
Structural and electronic determinants of lytic polysaccharide monooxygenase reactivity on polysaccharide substrates
Lytic polysaccharide monooxygenases (LPMOs) are industrially important copper-dependent enzymes that oxidatively cleave polysaccharides. Here we present a functional and structural characterization of two closely related AA9-family LPMOs from Lentinus similis (LsAA9A) and Collariella virescens (CvAA9A). LsAA9A and CvAA9A cleave a range of polysaccharides, including cellulose, xyloglucan, mixed-linkage glucan and glucomannan. LsAA9A additionally cleaves isolated xylan substrates. The structures of CvAA9A and of LsAA9A bound to cellulosic and non-cellulosic oligosaccharides provide insight into the molecular determinants of their specificity. Spectroscopic measurements reveal differences in copper co-ordination upon the binding of xylan and glucans. LsAA9A activity is less sensitive to the reducing agent potential when cleaving xylan, suggesting that distinct catalytic mechanisms exist for xylan and glucan cleavage. Overall, these data show that AA9 LPMOs can display different apparent substrate specificities dependent upon both productive proteinâcarbohydrate interactions across a binding surface and also electronic considerations at the copper active site
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