Archive for the ‘ATOMIC PHYSICS’ Category

New chemical bonds possible in extreme magnetic fields

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Could helium molecules form in very high magnetic fields? (Courtesy: NASA)

In the extreme magnetic fields of white dwarves and neutron stars, a third type of chemical bonding can occur. That is the finding of theoretical chemists in Norway, who have used computer simulations to show that as-yet-unseen molecules could form in magnetic fields much higher than those created here on Earth.
High-school chemistry students are taught that there are two types of chemical bond – ionic bonds, in which one atom donates an electron to another atom; and covalent bonds, in which the electrons are shared. In fact, real chemical bonds usually fall somewhere in between.
When two atoms come together, their atomic orbitals combine to form molecular orbitals. For each two atomic orbitals combined, two molecular orbitals are formed. One of these is lower in energy than either atomic orbital and is called the bonding orbital. The other “anti-bonding” orbital is higher in energy than either atomic orbital. Whether or not the atoms will actually bond is determined by whether the total energy of the electrons in the molecular orbitals is lower than the total energy of the electrons in the original atomic orbitals. If it is, bond formation will be energetically favoured and the bond will be formed.

Bonding and anti-bonding

The Pauli exclusion principle forbids a single orbital from holding more than two electrons (it can hold two if they have opposite spins). If the atomic orbital of each atom contained just one electron, both can go into the bonding orbital when the orbitals combine. Both electrons are therefore lowered in energy and the bond formation is energetically favoured. But if the atomic orbitals contained two electrons each, two of the four electrons would have to go into the anti-bonding molecular orbital. Overall, therefore, two electrons would have their energy lowered by bond formation, while two electrons would have their energy raised.
Under normal circumstances, the anti-bonding orbital is always raised in energy farther above the energy of the higher-energy atomic orbital than the bonding orbital is lowered below the energy of the lower-energy atomic orbital. This means that a chemical bond with both its bonding and its anti-bonding orbitals full would always have a higher energy than the atomic orbitals from which it would be formed. Such a bond would therefore not form. This is why noble-gas atoms, which have full outer atomic orbitals, almost never form molecules on Earth.
But now Kai Lange and colleagues at the University of Oslo have used a computer program developed by their group called LONDON to show this is not always true elsewhere. LONDON creates mathematical models of molecular orbitals under the influence of magnetic fields of about 105 T. This is much stronger than the 30–40 T fields that can be made in laboratories and that have little effect on chemical bonds.

Changing the rules

Large fields could be relevant to those studying astronomical objects such as white dwarves – where magnetic fields can reach 105 T – and neutron stars, where fields could be as high as 1010 T. Under such conditions, the team has shown that the rules of bonding change. In particular, the anti-bonding orbital is lowered in energy when a diatomic molecule is subjected to a strong perpendicular magnetic field. Molecules with full bonding and anti-bonding orbitals, such as diatomic helium, can still be energetically favoured.
Team leader Trygve Helgaker explains the sophistication of LONDON enabled the group to perform calculations that others have found impossible. “We can do accurate calculations with all orientations of the molecule to the magnetic field,” he says. “People have done the same kinds of electronic-structure calculations before, but I believe their calculations were limited to the situation where the field is parallel to the molecular axis.”
The research is published in Science; in an accompanying commentary, Peter Schmelcher of the Institute for Laser Physics at the University of Hamburg, Germany, said “Atoms, molecules and condensed-matter systems exposed to strong magnetic fields represent a fascinating topic, and this work has added a key bonding mechanism.” Interestingly, while he accepts the fields present around a white dwarf will be unachievable in a laboratory in the foreseeable future, he sees an alternative way the group’s models might be tested experimentally. Rydberg atoms are highly excited atoms that can be the size of the dot of an “i”. Because the bond length between Rydberg atoms is so great, the Coulomb interaction is much smaller, and Schmelcher believes it might therefore be possible to use them to produce magnetic fields of comparable strength.
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Written by physicsgg

July 21, 2012 at 7:53 am

Posted in ATOMIC PHYSICS, Chemistry

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Computer that could outlive the universe a step closer

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Can time act like crystals? (Image: WestEnd61/Rex Features)

The heat-death of the universe need not bring an end to the computing age. A strange device known as a time crystal can theoretically continue to work as a computer even after the universe cools. A new blueprint for such a time crystal brings its construction a step closer.

Ordinary crystals are three-dimensional objects whose atoms are arranged in regular, repeating patterns – just like table salt. They adopt this structure because it uses the lowest amount of energy possible to maintain.

Earlier this year, Frank Wilczek, a theoretical physicist at the Massachusetts Institute of Technology, speculated that a similar structure might repeat regularly in the fourth dimension – time.

To translate the spatial symmetry of a regular crystal into the fourth dimension, the atoms in such a “time crystal” would have to constantly rotate and return to their original location. Crucially, they would also have to be in their lowest possible energy state as they do so, meaning that they would naturally continue to rotate even after the universe has succumbed to entropy and cooled to a uniform temperature – a state known as heat-death.

Superconducting ring

Such behaviour would normally violate the laws of thermodynamics, but continuous rotation is allowed in the case of electrons in a superconductor, which flow without resistance. Wilczek originally suggested that a superconductive ring could serve as a time crystal if electrons could be made to flow separately rather than in a continuous stream, ensuring a repeating pattern. But he couldn’t figure out how to do so in practice.

Now Tongcang Li at the University of California, Berkeley, and colleagues at the University of Michigan in Ann Arbor and Tsinghua University in Beijing, China, have an alternative suggestion that may be possible to construct.

First you need an ion trap, a device which holds charged particles in place using an electric field. This causes the ions to form a ring-shaped crystal, as ions trapped at extremely low temperatures repel each. Next, you apply a weak static magnetic field, which causes the ions to rotate.

Quantum mechanics means that the rotational energy of the ions must be greater than zero, even when the ring is cooled to its lowest energy state. In this state, the electric and magnetic fields are no longer needed to maintain the shape of the crystal and the spin of its constituent ions. The result is a time crystal – or indeed a space-time crystal, because the ion ring repeats in both space and time.

Pleasing design

“I’m very pleased with it,” says Wilczek. “They’ve really come up with something that looks like a realisable experimental design.”

Building the crystal will be difficult as it needs temperatures close to absolute zero. “The main challenge will be to cool an ion ring to its ground state,” says Xiang Zhang, a member of the team who is also at Berkeley. He says this should be possible in the near future as ion trap technologies improve.

Wilczek has also theorised that a working time crystal could be made into a computer, with different rotational states standing in for the 0s and 1s of a conventional computer. He says this should be possible with the proposed system. “To make it interesting you want to have different kinds of ions, maybe several rings that affect each other,” he says. “You can start to think about machines that run on this principle.”

Don’t expect to see a time crystal computer any time soon, however. While Wilczek points out that the heat-death of the universe is, in principle, “very user friendly” for this kind of experiment because it would be cold and dark, there are other issues to consider. “We focus on a space-time crystal that can be created in a laboratory,” says Li. “So you need to figure out a method to make a laboratory that can survive in the heat-death of the universe.”
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July 19, 2012 at 8:41 am

Physicists Identify New Quantum State

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Strength in Numbers: Physicists Identify New Quantum State Allowing Three — But Not Two — Atoms to Stick Together

Abstract rendering. (Credit: © John Denison / Fotolia)

A Kansas State University-led quantum mechanics study has discovered a new bound state in atoms that may help scientists better understand matter and its composition.

The yet-unnamed bound state, which the physicists simply refer to as “our state” in their study, applies to three identical atoms loosely bound together — a behavior called three-body bound states in quantum mechanics. In this state, three atoms can stick together in a group but two cannot. Additionally, in some cases, the three atoms can stick together even when any two are trying to repel each other and break the connection.
“It’s really counterintuitive because not only is the pair interaction too weak to bind two atoms together, it’s also actively trying to push the atoms apart, which is clearly not the goal when you want things to stick together,” said Brett Esry, university distinguished professor of physics at Kansas State University and the study’s lead investigator.
Esry, along with Kansas State University postdoctoral researcher Nicolais Guevara and University of Colorado-Boulder colleague Yujun Wang — a Kansas State University graduate — calculated the quantum state in their study, “New Class of Three-Body States,” which was recently published in Physical Review Letters.
The state is similar to Efimov three-body states, a loosely-bound quantum state first predicted by Russian physicist Vitaly Efimov in the early 1970s. Physicists were able to first observe Efimov three-body states more than 30 years later through an experiment with ultracold atomic gases in 2006. These gases are one-billionth of a degree kelvin above absolute zero — a temperature that only exists in a handful of laboratories in the world. Esry said similar ultracold atomic gases are needed to observe their new quantum state as well since it can only exist at this temperature.
While Efimov three-body states only occur in ultracold conditions with atoms classified as bosons, the state found by Esry and colleagues applies to both bosons and fermions — the two particle types that all matter can be classified as.
Additionally, the new quantum state exists in a pocket between short-ranged and long-ranged interactions. Short- and long-ranged interactions — or forces — are the distance at which the particle interactions are effective. With a long-ranged force, the particles have a greater distance between them and do not have to touch to interact and influence each other. With a short-ranged force, however, the particles must be in much closer proximity and interact similar to billiard balls colliding with one another, Esry said. The Efimov three-body states only exist for short-ranged interactions.
“The three-body states that we found are formed by interactions that are neither short- nor long-ranged,” Esry said. “Instead, they lie right at the border between the two. So, more than anything, finding this new quantum state fills in a knowledge gap about three-body systems and quantum mechanics, which have been studied for centuries by physicists — including Sir Isaac Newton studying the Earth, moon and sun.”
Scientists may also find uses for the quantum state in experiments with ultracold atomic gases.
“That’s really the nature of basic research,” Esry said. “We’re trying things that hopefully will pay off for somebody 20 years or longer down the line. Efimov had to wait 35 years to see his states actually be seen and used as a way to understand these three-body systems. We hope we don’t have to wait that long.”
Esry and colleagues will continue exploring this quantum state and to uncover how combinations of bosons and fermions behave in it…
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Written by physicsgg

July 3, 2012 at 5:12 pm

From Photons to Atoms – The Electromagnetic Nature of Matter

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An electron (top) and a positron (bottom). The first one is right-handed, whereas the second one is left-handed.
Except for the orientation of the electric field, they display identical properties.

Daniele Funaro
Motivated by a revision of the classical equations of electromagnetism that allow for the inclusion of solitary waves in the solution space, the material collected in these notes examine the consequences of adopting the modified model in the description of atomic structures. The possibility of handling “photons” in a deterministic way opens indeed a chance for reviewing the foundations of quantum physics. Atoms and molecules are described as aggregations of nuclei and electrons joined through organized photon layers resonating at various frequencies, explaining how matter can absorb or emit light quanta. Some established viewpoints are subverted, offering an alternative scenario. The analysis seeks to provide an answer to many technical problems in physical chemistry and, at the same time, to raise epistemological questions.
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June 16, 2012 at 7:52 pm

Performance of a 229 Thorium solid-state nuclear clock

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Performance of a 229-Thorium solid-state nuclear clock -
CaF2 crystal lattice (a) and Thorium inclusion with charge compensated
by two Fluorine in-line intersitials (b)

G. A. Kazakov, A. N. Litvinov, V. I. Romanenko, L. P. Yatsenko, A. V. Romanenko, M. Schreitl, G. Winkler, T. Schumm
The 7.8 eV nuclear isomer transition in 229-Thorium has been suggested as an etalon transition in a new type of optical frequency standard. Here we discuss the construction of a “solid-state nuclear clock” from Thorium nuclei implanted into single crystals transparent in the vacuum ultraviolet range. We investigate crystal-induced line shifts and broadening effects for the specific system of Calcium fluoride. At liquid Nitrogen temperatures, the clock performance will be limited by decoherence due to magnetic coupling of the Thorium nucleus to neighboring nuclear moments, ruling out the commonly used Rabi or Ramsey interrogation schemes. We propose a clock stabilization based on counting of flourescence photons and present optimized operation parameters. Taking advantage of the high number of quantum oscillators under continuous interrogation, a fractional instability level of 10-19 might be reached within the solid-state approach…
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April 21, 2012 at 6:10 pm

Can GPS find variations in Planck’s constant?

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Artist's impression of a GPS satellite (Courtesy: NASA).

Global Positioning System Test of the Local Position Invariance of Planck’s Constant
J. Kentosh and M. Mohageg
Phys. Rev. Lett. 108, 110801 (2012)
Published March 15, 2012

Pinpointing Planck’s Constant with GPS
GPS is helping drivers find their way and parents track their kids and pets. But now a pair of researchers—reporting in Physical Review Letters—has used the same technology to put new limits on variations in Planck’s constant.

Certain theories allow physical constants, such as the speed of light or the gravitational constant, to vary, and some astronomical observations have been interpreted as suggesting the electromagnetic coupling was different in the past. Testing these hypotheses often requires sophisticated instruments. But James Kentosh and Makan Mohageg of California State University, Northridge, have found a way to use the ubiquitous global positioning system, or GPS, to evaluate the constancy of Planck’s constant, h.

GPS relies on atomic clocks, which are sensitive to Planck’s constant through their ticking frequency, f=E/h, where E is the energy of a specific atomic transition. For a clock orbiting in one of the 32 GPS satellites, this frequency can shift with respect to ground-based clocks because of well-known relativistic effects. The GPS system keeps track of this frequency drift and broadcasts a clock correction with its signal.

Kentosh and Mohageg looked through a year’s worth of GPS data and found that the corrections depended in an unexpected way on a satellite’s distance above the Earth. This small discrepancy could be due to atmospheric effects or random errors, but it could also arise from a position-dependent Planck’s constant. Assuming the latter, the authors derive an upper limit on Planck variation. –

Michael Schirber –

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March 28, 2012 at 12:46 pm

A Determination of the Fine Structure Constant …

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… using Precision Measurements of Helium Fine Structure

4He level diagram

Marc Smiciklas
Spectroscopic measurements of the helium atom are performed to high precision using an atomic beam apparatus and electro-optic laser techniques. These measurements, in addition to serving as a test of helium theory, also provide a new determination of the fine structure constant α. An apparatus was designed and built to overcome limitations encountered in a previous experiment. Not only did this allow an improved level of precision but also enabled new consistency checks, including an extremely useful measurement in 3He. I discuss the details of the experimental setup along with the major changes and improvements. A new value for the J = 0 to 2 fine structure interval in the 23P state of 4He is measured to be 31 908 131.25(30) kHz. The 300 Hz precision of this result represents an improvement over previous results by more than a factor of three. Combined with the latest theoretical calculations, this yields a new determination of α with better than 5 ppb uncertainty, α-1 = 137.035 999 55(64).
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March 17, 2012 at 8:32 am


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Albert Einstein: The Size and Existence of Atoms

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March 16, 2012 at 7:00 am


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