Computers crunch quantum collisions
1
of the week Computers Crunch Quantum Collisions With two of the world’s most powerful supercomputers and a bit of mathemati- cal sleight of hand, physicists have ac- complished a longstanding and yet re- markably humble goal. They have described what happens when an elec- tron collides with an atom. Underlying everything from the glow of fluorescent light bulbs to the chemistry of stellar gases, such submicroscopic im- pacts are a cosmic commonplace. How- ever, even after boiling down the problem to just three particles- incoming elec- tron, the proton of the target hydrogen atom, and the atom’s own orbiting elec- tron-physicists have failed to describe the event in full mathematical detail. “This is the simplest problem of im- pact ionization you can do, and it’s ex- tremely difficult,”says molecular physi- cist Lee A. Collins of Los Alamos (N.M.) National Laboratory. A team of California physicists de- scribes a solution, however, in the Dec. 24. 1999 SCIENCE. “It’s a real break- < ... 0 .- 0 P d A component of the wave function that describes a subset of the outcomes pos- sible when an electron with 1 Z 6 electron volts of energy hits a hydrogen atom. The horizontal axes represent the distances of each of the two electrons from the nucleus. through,” comments Collins. It’s been nearly threequarters of a cen- tury since physicists first wrote down the relevant quantum mechanical equation, an application of Schrodinger’s equation tailored to the collision between an elec- Smoggy Asian air enters United States Atmospheric scientists have detect- ed a previously unknown Asian import smuggling its way into the United States. Smoggy air, rich in ozone pollu- tion and other contaminants, sailed clear across the Pacific and then drift- ed over northwestern states in the spring of 1999. On April 9, sensors on board an air- plane flying off the coast of Washington detected enhanced ozone concentra- tions between 3,000 and 6,000 meters above sea level. At its worst, the ozone reached 85 parts per billion, reported Daniel Jaffe of the University of Wash- ington at Bothell and his colleagues last month in San Francisco at a meeting of the American Geophysical Union. A triplet of oxygen atoms, ozone can cause respiratory distress and other health problems, and it can also attack vegetation. The concentration detected by the aircraft is significant, says Jaffe, because the pending federal standard for ozone is an &hour average of 80 parts per billion. Using meteorological data, Jaffe and his colleagues ran models to calculate where the plume of pollution had origi- nated. They traced the dirty air back- ward across the Pacific to Southeast Asia 5 days earlier. Whether or not the Asian ozone r e p resents a threat remains unclear. Ground-based sensors throughout the Puget Sound area did not pick up any spike in ozone concentrations at the time the Asian plume was passing over- head, says Naydene Maykut of the Puget Sound Clean Air Agency, nor did an in- strument stationed at 1,700 m above sea level in the Cascade mountains. “It clearly is not of great concern at the moment,” says Maykut. Evidence has indicated that influxes of Asian pol- lution occur most often in spring, when the Puget Sound area has its cleanest air. “lf this [pollution] increases be- cause pollution in the Third World is in- creasing, it could become something that we need to be more concerned about,” she says. Jaffe sees the ozone as a more imme- diate threat. As upper-level air passes over mountains, it mixes with air from the ground, transporting pollution downward. “Potentially,you could see a strong effect from Asia at the Rocky Mountains,” he says. As this imported air becomes diluted near the ground, however, it becomes harder to detect, he adds. lnstead of causing a sudden spike, it may raise the general ozone concentration in an area. A year ago, Jaffe and other re- searchers reported finding a cocktail of noxious chemicals coming clear across the Pacific from Asia (SN: 12/12/98, p. 374). At the time, they were using ground-based measurements and did not detect enhanced concentrations of ozone. -R. Monastersky 4 SCIENCE NEWS, VOL. 157 tron and hydrogen. In that time, they’ve succeeded in calculating the probability of just a few types of outcomes. For example, they’ve worked out the case where the incoming electron knocks the resident electron loose and the two fly off in exactly opposite directions, says An- thony F. Starace, a theoretical physicist at the University of Nebraska at Lincoln. Other geometries and results have proved to be tougher mathematical nuts, Starace says. Theorists made predic- tions, but experimenters who fired beams of electrons into atomized hydro- gen have found the estimates inaccurate. In the simpler case of an undisturbed electron orbiting a nucleus, scientists have calculated the full, detailed solu- tion, or wave function, and from it de- rived the so-called orbital shapes of the electron cloud. These shapes have been a source of revolutionary insights about chemical bonding and the energy- absorption properties of atoms, says C. William McCurdy, a computational chemist at Lawrence Berkeley (Calif.) Na- tional Laboratory and a collaborator in the solution to the collision problem. Un- til now, physicists had no comparable vi- sual image for how electrons move away from a nucleus after an impact, he says. Using supercomputers at the Berkeley and Lawrence Livermore National Labo- ratories, McCurdy and his colleagues cal- culated the wave function for the sim- plest such collision. The task required solving about 5 million linked equations, each containing about 5 million terms. Only a mathematical trick kept the cal- culation that simple. Employed previously by others, the scheme makes the wave function shrink gradually to zero outside a sphere of a certain radius. The re- searchers found that the calculated wave function’s undulations settled into a pat- tern within that radius. This enabled them to predict its shape to infinite distance. The theorists calculated the probabili- ties for different scattering outcomes us- ing the newly derived wave function and found that the predictions agreed well with experimental data. Theoretical atomic physicist Colm T. Whelan of the University of Cambridge in England says that experimenters deserve at least half the credit for solving the collision prob- lem since, without their detailed obser- vations, theorists wouldn’t have been able to confirm that their recent calcula- tions got the answer right. With the wave function in hand, says Collins, physicists now have a bench- mark against which to test potential se- quences of interactions between an atom and an approaching electron. -0. Baker JANUARY 1,2000
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Transcript of Computers crunch quantum collisions
of the week
Computers Crunch Quantum Collisions With two of the world’s most powerful
supercomputers and a bit of mathemati- cal sleight of hand, physicists have ac- complished a longstanding and yet re- markably humble goal. They have described what happens when an elec- tron collides with an atom.
Underlying everything from the glow of fluorescent light bulbs to the chemistry of stellar gases, such submicroscopic im- pacts are a cosmic commonplace. How- ever, even after boiling down the problem to just three particles- incoming elec- tron, the proton of the target hydrogen atom, and the atom’s own orbiting elec- tron-physicists have failed to describe the event in full mathematical detail.
“This is the simplest problem of im- pact ionization you can do, and it’s ex- tremely difficult,” says molecular physi- cist Lee A. Collins of Los Alamos (N.M.) National Laboratory.
A team of California physicists de- scribes a solution, however, in the Dec. 24. 1999 SCIENCE. “It’s a real break-
< ... 0
.- 0 P d
A component of the wave function that describes a subset of the outcomes pos- sible when an electron with 1 Z 6 electron volts of energy hits a hydrogen atom. The horizontal axes represent the distances of each of the two electrons from the nucleus.
through,” comments Collins. It’s been nearly threequarters of a cen-
tury since physicists first wrote down the relevant quantum mechanical equation, an application of Schrodinger’s equation tailored to the collision between an elec-
Smoggy Asian air enters United States Atmospheric scientists have detect-
ed a previously unknown Asian import smuggling its way into the United States. Smoggy air, rich in ozone pollu- tion and other contaminants, sailed clear across the Pacific and then drift- ed over northwestern states in the spring of 1999.
On April 9, sensors on board an air- plane flying off the coast of Washington detected enhanced ozone concentra- tions between 3,000 and 6,000 meters above sea level. At its worst, the ozone reached 85 parts per billion, reported Daniel Jaffe of the University of Wash- ington at Bothell and his colleagues last month in San Francisco at a meeting of the American Geophysical Union.
A triplet of oxygen atoms, ozone can cause respiratory distress and other health problems, and it can also attack vegetation. The concentration detected by the aircraft is significant, says Jaffe, because the pending federal standard for ozone is an &hour average of 80 parts per billion.
Using meteorological data, Jaffe and his colleagues ran models to calculate where the plume of pollution had origi- nated. They traced the dirty air back- ward across the Pacific to Southeast Asia 5 days earlier.
Whether or not the Asian ozone r e p resents a threat remains unclear. Ground-based sensors throughout the Puget Sound area did not pick up any
spike in ozone concentrations at the time the Asian plume was passing over- head, says Naydene Maykut of the Puget Sound Clean Air Agency, nor did an in- strument stationed at 1,700 m above sea level in the Cascade mountains.
“It clearly is not of great concern at the moment,” says Maykut. Evidence has indicated that influxes of Asian pol- lution occur most often in spring, when the Puget Sound area has its cleanest air. “lf this [pollution] increases be- cause pollution in the Third World is in- creasing, it could become something that we need to be more concerned about,” she says.
Jaffe sees the ozone as a more imme- diate threat. As upper-level air passes over mountains, it mixes with air from the ground, transporting pollution downward. “Potentially, you could see a strong effect from Asia at the Rocky Mountains,” he says.
As this imported air becomes diluted near the ground, however, it becomes harder to detect, he adds. lnstead of causing a sudden spike, it may raise the general ozone concentration in an area.
A year ago, Jaffe and other re- searchers reported finding a cocktail of noxious chemicals coming clear across the Pacific from Asia (SN: 12/12/98, p. 374). At the time, they were using ground-based measurements and did not detect enhanced concentrations of ozone. -R. Monastersky
4 SCIENCE NEWS, VOL. 157
tron and hydrogen. In that time, they’ve succeeded in calculating the probability of just a few types of outcomes.
For example, they’ve worked out the case where the incoming electron knocks the resident electron loose and the two fly off in exactly opposite directions, says An- thony F. Starace, a theoretical physicist at the University of Nebraska at Lincoln.
Other geometries and results have proved to be tougher mathematical nuts, Starace says. Theorists made predic- tions, but experimenters who fired beams of electrons into atomized hydro- gen have found the estimates inaccurate.
In the simpler case of an undisturbed electron orbiting a nucleus, scientists have calculated the full, detailed solu- tion, or wave function, and from it de- rived the so-called orbital shapes of the electron cloud. These shapes have been a source of revolutionary insights about chemical bonding and the energy- absorption properties of atoms, says C. William McCurdy, a computational chemist at Lawrence Berkeley (Calif.) Na- tional Laboratory and a collaborator in the solution to the collision problem. Un- til now, physicists had no comparable vi- sual image for how electrons move away from a nucleus after an impact, he says.
Using supercomputers at the Berkeley and Lawrence Livermore National Labo- ratories, McCurdy and his colleagues cal- culated the wave function for the sim- plest such collision. The task required solving about 5 million linked equations, each containing about 5 million terms.
Only a mathematical trick kept the cal- culation that simple. Employed previously by others, the scheme makes the wave function shrink gradually to zero outside a sphere of a certain radius. The re- searchers found that the calculated wave function’s undulations settled into a pat- tern within that radius. This enabled them to predict its shape to infinite distance.
The theorists calculated the probabili- ties for different scattering outcomes us- ing the newly derived wave function and found that the predictions agreed well with experimental data. Theoretical atomic physicist Colm T. Whelan of the University of Cambridge in England says that experimenters deserve at least half the credit for solving the collision prob- lem since, without their detailed obser- vations, theorists wouldn’t have been able to confirm that their recent calcula- tions got the answer right.
With the wave function in hand, says Collins, physicists now have a bench- mark against which to test potential se- quences of interactions between an atom and an approaching electron. -0. Baker
JANUARY 1,2000
