A cold, strange and short molecule created in an experiment at the Weizmann Institute of Science following a collision between particles may shed light on chemical reactions at extremely low temperatures
Like balls in a pool game, two particles colliding with each other will normally fly in two opposite directions. But in a recent experiment at the Weizmann Institute of Science, colliding particles exhibited a move more suited to the dance floor than to the pool table: the two - one an atom and the other an ion, that is, an atom with an electric charge - alternately approached and moved away from each other, as if in repetitive dance steps, and behaved as if they were connected by a long, inexhaustible spring visible. "This strange choreography was already revealed in the models we built on the computer in preparation for the experiment, but we were convinced that it was a real phenomenon, only after we saw it 'with our own eyes,'" says doctoral student Mirav Pankas, wholed the research in the laboratory of Prof. Roy Ozeri In the Department of Physics of Complex Systems.
Pancas explains that the original goal of the research was to observe new quantum effects, meaning physical phenomena that do not obey the laws of classical physics. Many studies have already been devoted to quantum effects created as a result of particle collisions in extreme cold conditions, but these experiments mostly focus on atoms separately or ions separately, and not both together, as this is a particularly challenging task from a technical point of view. Prof. Ozari's laboratory, on the other hand, is actually well prepared to deal with this exact challenge, in light of its many years of experience in studying atoms and ions at extremely low temperatures.
Pankas and the other members of the group - Dr. Or Katz, Yonatan Vengerovitz and Dr. Nitzan Akerman - planned an experiment at a temperature of less than 1 millikelvin or one thousandth of a degree above the absolute zero temperature. Strange as it may sound, to reach such cold conditions, they use laser beams that hit atoms or ions and freeze them in place. As part of the current experimental setup, they cooled an ion of the metal strontium, trapped it in an ion trap and exposed it to a stream of about half a million rubidium atoms, which they also cooled with a laser ahead of time. When the ion collided with one of the atoms, another laser beam helped map the results of the event.
The colder the experimental system, the greater the chance of observing quantum effects. For example, the quantum threshold of the system created by the scientists is about one tenth of a millionth of a degree above absolute zero. But in the current experiment, the surprising findings were recorded even before the system reached its quantum threshold, that is, when it was still relatively "warm" - a millionth of a degree above absolute zero, and even hotter - a thousandth of a degree above absolute zero - after the use of the ion trap. Although these were too hot conditions to observe quantum effects, the surprise was provided this time by classical physics: the strange dance of the ion and the atom can be fully explained using Newton's laws. "We went looking for quantum phenomena and found a surprising phenomenon of classical physics," says Pankas.
Quantum chemistry
The scientists recognized that something unusual was happening when they looked at the quantum property called spin. They saw that after the particles collided, the spin of the strontium ion changed in such a way that it could be concluded that it remained somehow bound to the rubidium atom. Only later did they realize that the collision of the particles actually led to the formation of a kind of molecule; Although it was not a stable molecule over time and its components were very far apart, it nevertheless behaved like a molecule.
"It was very strange because two particles that produce a molecule together lose some of their energy in the process. But in our system the excess energy apparently had nowhere to go, so no molecule was supposed to be formed," Pankas explains the magnitude of the surprise. "In the end we found the explanation: the excess energy was absorbed in the ion trap, and this is exactly what prevented the paths of the atom and the ion from separating. If we return to the billiard table, it is as if the edges of the table will curve upwards for a while, creating a kind of bowl that prevents the balls from moving too far apart."
To examine the strontium-rubidium molecule they created, the scientists used quantum tools. They discovered that changing the strength of the magnetic field in the ion trap changes the spin of the particles and makes it possible to examine the effect of this property on the formation of the molecule. These tests may make it possible to understand not only how the molecule is formed, but also how it can be broken down. "We want to achieve as precise a control as possible of the unique phenomenon we discovered," says Pankas. Such control may contribute, for example, to the study of quantum chemistry, including the chemical reactions that occur in interstellar space under extreme cold conditions. In fact, similar to the experimental setup in the study, the most common chemical reaction in interstellar space is the formation of molecular ions as a result of collisions between atoms and ions.
"These are the directions we can imagine today," says Prof. Ozeri, "but the beauty of surprising discoveries is that they may lead us to completely new territories in ways we cannot even imagine now."
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