What Are Intermolecular Forces?

Intermolecular forces, also known as van der Waals force. It is a weakly alkaline electrical attraction that exists between neutral molecules or atoms. There are three sources of intermolecular forces (Vandervalls force): the interaction between the permanent dipole moments of polar molecules. A polar molecule polarizes another molecule, which induces a dipole moment and attracts each other. The movement of electrons in a molecule produces a momentary dipole moment, which momentarily polarizes neighboring molecules, which in turn enhances the momentary dipole moment of the original molecule; this mutual coupling produces an electrostatic attraction, and these three forces contribute differently The third role usually contributes the most. ^[1]

Definition: Van der Waals forces (also known as molecular forces) arise from

French scientists first measured the van der Waals force between two atoms for the first time in 2013. The experimental methods used can be used to build quantum logic gates or to perform quantum simulations of condensed matter systems.

Atomic dipole: the first direct measurement of van der Waals forces between atoms

Atomic dipole: For the first time, scientists directly measure van der Waals forces between atoms

(Image source: iStockphoto / Nicemonkey)

Van der Waals forces between atoms, molecules, and surfaces appear in everyday life in a variety of ways. For example, spiders and geckos rely on van der Waals forces to climb up smooth walls. Proteins in our bodies also fold into complex shapes because of van der Waals forces.

Van der Waals force is named after Dutch scientist Johannes Diderik van der Walls, who first proposed the concept of van der Waals force to explain the behavior of gas in 1873. This force is very weak and only makes sense when the atoms or molecules are very close together. The fluctuations of the atomic electron cloud make the atoms have instantaneous electric dipole moments, thereby inducing nearby atoms to produce electric dipole moments, and as a result, mutually attractive dipole interactions will occur.

Indirect measurement

There have been many research results on indirect measurement of van der Waals forces between atoms, such as analyzing the net force between macroscopic objects to obtain empirical values, or using spectroscopy to analyze the long-range force between two atoms in a diatomic molecule. Prior to this, there has been a lack of related research to directly measure van der Waals forces.

The latest research was done by researchers at the Laboratoire Charles Fabry (LCF) in Palazzo and the University of Lille. "What we do is directly measure the van der Waals force between two independent atoms located within a controlled distance, and the distance between the atoms is set by the experimenter. As far as we know, this is the first time that direct measurement has been achieved," said LCF team member Ti Thierry Lahaye said.

When measuring the force between atoms, it is extremely difficult to control the distance between two ordinary atoms, because the related distance is very small. The research team uses Rydberg atoms to solve this problem, which are much larger than ordinary atoms. One of the electrons in the Rydberg atom is in a highly excited state, which means that they have a large instantaneous electric dipole moment, so that even at a relatively long distance, there will be a large Van der Waals force. They also have certain unique properties that allow them to be precisely controlled in the laboratory.

Atomic right and wrong

The experiment first used two highly focused laser beams to capture two plutonium atoms separately, and separated the atoms by a few micrometers. A laser beam of a specific wavelength is then irradiated on the atoms, causing the system to oscillate between the ground state and one or two Rydberg atoms. The research team found that when the conditions are right, the system will oscillate between the ground state and a pair of Rydberg atoms, at which point the two atoms are at the focal points of the two lasers. By measuring these oscillations, the researchers calculated the van der Waals force between the two Rydberg atoms.

By adjusting the capture laser beam, researchers can move Rydberg atoms closer or further apart. When the researchers changed the distance R between the atoms, the force showed a change law inversely proportional to the sixth power of R-this result is exactly the same as the expected van der Waals force.

In addition to measuring van der Waals forces, the research team also found that the evolution of the quantum states of two interacting Rydberg atoms is completely coherent. Antoine Browaeys, a member of the LFC team, said it was "unlikely seen in atomic physics".

Similar to quantum logic

The coherent evolution of two interacting atoms is exactly the same as a quantum logic gate working on two qubits. According to Bravis, this shows that the two atoms that interact through Van der Waals forces are an ideal system for creating high-fidelity quantum gates. "This result takes our vector computer one step further." He said.

In fact, researchers believe that the long-term significance of their experiments is not in measuring Van der Waals forces themselves, but in achieving precise control of Rydberg atoms. "This allows us to design small quantum systems and gradually increase the size of the quantum system. It is hoped that from two Rydberg atoms to gradually increase to dozens, and we can completely control the interaction between atoms." LaHaye Explained.

Such quantum systems are expected to be used in quantum information processing or quantum simulation of condensed matter systems (such as quantum magnets).

Steven Rolston, of the University of Maryland's Joint Quantum Research Institute, who was not involved in the research, called this achievement an important milestone, which he believes will help the development of quantum information equipment and Manufactured because it proves that van der Waals forces between atomic qubits behave as expected.

This study was published in Physical Review Letters. ^[2]

Author Katia Moskovich is a British science writer.

(Translation: Shen Tianyi)