Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction



Yüklə 2,68 Mb.
səhifə6/44
tarix29.07.2018
ölçüsü2,68 Mb.
#59552
1   2   3   4   5   6   7   8   9   ...   44
Cesium, like the other alkali metals, readily dissolves in liquid ammonia to produce "solvated electrons" and cesium cations. The solvated electrons possess a characteristic blue color caused by transmitted light and are paramagnetic. At high concentrations the cesium-ammonia solution becomes quite viscous and turns bright gold due to reflected light. On exposure to oxygen, cesium readily forms seven different oxides ranging from Cs7O to CsO3.

Researchers at Bell Labs in the early 1990s reported that doping buckminsterfullerene, C60, with the alkali metal potassium led to room-temperature conductivity and low-temperature superconductivity. In collaboration with physics and chemistry colleagues, my group produced the first pure samples of K3C60 and determined the structure to be face-centered cubic C60 with potassium in all octahedral and tetrahedral sites. K3C60 has a superconducting transition temperature of 19.3 K that surprisingly decreases as pressure is increased. To apply "negative" pressure, we replaced K with Rb, which produced an increase in the transition temperature to 29.6 K for Rb3C60. Unfortunately, substituting three cesiums for the rubidiums led to disproportionation back to C60 and the insulating body-centered cubic phase, Cs6C60, rather than a superconducting material. However, when a small amount of Cs is substituted into Rb3C60, a superconducting transition temperature above 30 K is achieved, the highest known for a carbon-based material.

Therefore, a small amount of cesium goes a long way, whether in doping C60 or in a vial to pass around class. On a warm day, cesium appears as liquid gold. Just watch out for its explosive personality!



Richard Kaner is a professor of chemistry and biochemistry at the University of California, Los Angeles. He is a recipient of the ACS Exxon Fellowship in Solid-State Chemistry; the Buck-Whitney Award from the ACS Eastern New York Section; and Dreyfus, Guggenheim, Packard, and Sloan Fellowships.


CESIUM AT A GLANCE


Name: From the Latin caesius, heavenly blue. The metal is characterized by two bright blue lines in its spectrum.

Atomic mass: 132.91.

History: Discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff.

Occurrence: Primarily obtained from the mineral pollucite.

Appearance: Silvery gold, soft, ductile metal.

Behavior: The most electropositive and alkaline element. Liquid around room temperature. Cesium reacts explosively with cold water, and reacts with ice at temperatures above –116 ºC. Cesium is fairly toxic.

Uses: Used as a catalyst promoter, as a "getter," in radiation monitoring equipment, and in atomic clocks.

FRANCIUM

LUIS A. OROZCO, STATE UNIVERSITY OF NEW YORK, STONY BROOK




Little did I know when I arrived at the State University of New York, Stony Brook, in 1991 and started collaborating with Gene D. Sprouse that on the night of Sept. 27, 1995, we were going to succeed in capturing some thousand atoms of francium in a magneto-optical trap. Francium is an alkali and so is one of the simplest heavy atoms. We are interested in performing precision measurements of its spectroscopic properties to find out more about the weak force--the force of nature responsible for the beginning of the solar cycle: the conversion of a proton into a neutron.

There is much less than an ounce of francium at any given time in the whole Earth. It is the most unstable element of the first 103 in the periodic table, and its longest lived isotope lasts a mere 20 minutes.

We made francium in a nuclear fusion reaction. After many trials, we ended up with a beam of oxygen ions accelerated enough to fuse with gold atoms in a target. Gold is a noble metal and does not form compounds with francium, so we could extract and transport it to the trapping region.

Only in 1978 did the team led by Sylvan Liberman at ISOLDE in CERN succeed in finding the D2 resonant line of the spectrum of francium, opening the road for further spectroscopic studies. Meanwhile, the development of tunable lasers had enabled the cooling and trapping of alkali atoms using resonant light in combination with magnetic fields, so all the parts necessary for trapping and cooling francium were there at Stony Brook in the early 1990s.

Gerald Gwinner and John Behr joined the effort to trap radioactive rubidium on-line with the superconducting linear accelerator at Stony Brook. We succeeded in 1994, opening the way to the more challenging effort to capture francium. Jesse Simsarian and Paul Voytas took over, and after many failed attempts during the summer of 1995, we gave it another try at the end of September.

It was already past midnight, and I was preparing the class that I had to give the following morning. We were in the control room of the accelerator, far from the trapping area. We had set television monitors and computer screens to follow the signal. I was not looking into any of the monitors but was sitting reading and facing the other members of the team. I saw a funny expression on their faces, and I just thought, "Something has failed." But no, there was a signal that increased and increased as we changed the frequency of the laser, just where we expected it, but about a hundred times larger! We were all skeptical and set up to repeat the scan. A few hours later we could not stop celebrating.

We managed to optimize the trap to a point that, later that year, Simsarian pointed to the fluorescing francium on a television screen. We had about 3,000 atoms suspended by a combination of a magnetic field gradient and six laser beams. Wenzheng Zhao joined us, and we started in earnest to learn more about the spectroscopy of francium. We went hunting for the excited states that had not been detected, we measured their lifetimes, and we learned a lot about francium's atomic structure. The small trap was good for many years. Josh Grossman, a new graduate student in 1998, helped measure the change in the nuclear magnetization in the five different isotopes that we could then produce, but we knew that we needed to improve the number of atoms.




TRAPPED

Afluorescence image of 200,000 francium atoms in a magneto-optical trap at SUNY Stony Brook.



Yüklə 2,68 Mb.

Dostları ilə paylaş:
1   2   3   4   5   6   7   8   9   ...   44




Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur ©genderi.org 2024
rəhbərliyinə müraciət

    Ana səhifə