University of São Paulo: Experiment uses light to manipulate and alter atoms and molecules

A new way of precisely manipulating atoms and molecules, unimaginably small components that make up matter, was developed in a research with the participation of USP’s Faculty of Philosophy, Sciences and Letters (FFCLRP). Scientists used different frequencies of light to cause changes in the states of the particles. The technique can be applied to facilitate the understanding of chemical reactions and to identify substances by means of light (spectroscopy), increasing the accuracy of analyzes on combustion processes, atmospheric chemistry and forensic chemistry, where it is necessary to detect compounds in small quantities.
The experiment is described in an article published on the website Nature Communications on April 13. “In classical physics, knowing the state of a system implies knowing everything that is necessary to predict the future of the system. For example, for a free classical particle, its position and speed determine its trajectory ”, explains professor Antonio Gustavo Sampaio de Oliveira Filho, from FFCLRP, who participated in the research. “The behavior of atoms and molecules, on the other hand, is governed by quantum mechanics, in which it is not possible to know precisely the position and speed simultaneously.”

As a consequence, knowing the quantum state of atoms and molecules involves as much knowledge about the system as possible, points out the professor. “For a molecule, that means knowing its energy and how it is distributed in translational energy (how it moves in space), vibrational (how the atoms that make up the molecule move in relation to each other), rotational (how the molecule spins in itself) and electronics (what is the configuration of electrons in the molecule). ”

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According to Oliveira Filho, there are other ways to carry out optical control of atoms and molecules. “In fact, a method of preparing molecules with high rotational energies, optical centrifuges, is already known,” he says. “But the method used in our work is different, producing distributions of speed of rotation quite narrow, that is, with the well-defined rotational energy, which can be important for eventual applications.”

Optical control
Optical control was observed in a spectroscopy experiment. “Firstly, electrically charged molecules of silicon monoxide are generated and introduced into a trap made with electric fields, where they are also cooled by laser to a temperature close to absolute zero”, describes the professor. “This causes the translational movement to decrease, without changing its speed of rotation, keeping the energy of the particles distributed in various rotational states. Then, the optical control generates, through a sequence of excitations with lasers, narrow rotational distributions around a certain desired state. ”

“Once prepared, the molecule is excited, by means of light, to an electronic state in which the speed of rotation is sufficient to break the chemical bond, allowing the detection of particles by mass spectrometry”, concludes Oliveira Filho. “The characterization of electronic states, transitions and the dissociation process was also carried out by mechanical-quantum calculations and an excellent agreement was observed between the theoretical and experimental results.”

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One of the possible applications, demonstrated in the work, is the determination of the molecular structure away from the equilibrium distance. “The control over molecular properties allows us to understand and manipulate chemical reactions at the most fundamental level”, says the professor. “Another possible application is in the processing of quantum information, since the molecule studied in the work has a large number of quantum states, making it possible to have an interest in using it in quantum computers.”

According to Oliveira Filho, the fact that the system remains in a controlled rotational state and in a practically collision-free environment means that the sensitivity of spectroscopic experiments, which identify substances by means of light, is greatly increased, allowing it to work with a very small number or even perform the identification of a single molecule ”, he points out. “This allows new possibilities for analytical methods where the detection of species present in small quantities is necessary, as in the study of combustion processes, atmospheric chemistry and forensic chemistry, for example.”

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