Science cover: end point of chemical synthesis? Splicing atoms directly into molecules

Chemical reactions are usually produced by atoms or molecules colliding with the right orientation, but both are random Therefore, traditional chemists often study the average state of the whole reaction system, but can not study the collision of a molecule in the reaction system alone With the development of science and technology, some techniques have been able to detect elementary reactions by reducing the thermodynamic diffusion rate of molecules In order to observe the process of a reaction from the most basic structural unit, people must have the ability to control a single molecule or even a single atom At present, scanning tunneling microscope (STM) is a common single atom manipulation method, but STM technology can only extract and place atoms on the surface of a certain material, so people are more eager for a technology that can operate atoms freely Optical tweezer is a kind of technology that uses the optical trap formed by optical gradient force to operate micro particles Due to its advantages of non-contact and non-destructive, optical tweezers have a wide range of applications in the fields of physics, chemistry, materials, machinery, biology and medicine Recently, Assistant Professor Kang Kuen Ni of Harvard University led the team to control the movement of two atoms through optical tweezers, so that they can combine and react successfully with the help of photons With the help of movable optical tweezers, the team manipulated one sodium (NA) atom and one cesium (CS) atom, finally making the two atoms in the same optical well, and studied the collisions between the atoms In addition, through the excitation of atoms, the team successfully realized the reaction of single sodium atom and cesium atom This achievement was published in science under the title of "building one molecule from aservoir of two atoms", and was selected as science cover article (DOI: 10.1126 / science Aar7797) Figure 1 A) schematic diagram of reaction device; b) single molecule fluorescence image of sodium atom (Orange) and cesium atom (blue); c) histogram of fluorescence image of sodium atom (Orange) and cesium atom (blue) (picture source: Science, 2018, 900 – 903) how to obtain single sodium and cesium atoms and keep them stably is the basis of the whole experiment, and also the first problem faced by the author The temperature of sodium and cesium atoms is reduced to near absolute zero (< 1 MK) by laser cooling technology, and the laser cooled atoms are stored in magneto-optical wells under high vacuum (10-8 PA) to obtain a relatively stable single atom Then the author immediately faced the second problem: how to effectively control the two atoms without mutual interference In principle, atoms can be manipulated by beams of different wavelengths, but how to choose the appropriate wavelength is a time-consuming and labor-consuming work Through literature and experiments, the author found that 700 nm light can attract sodium atom but repel cesium atom, and 976 nm light is five times more attractive to cesium atom than sodium atom Therefore, the manipulation of the two atoms by these two beams of light alone will not be interfered with (Fig 1) In addition, the transfer of atoms from magneto-optical trap to optical tweezers is not 100% successful Through many experiments, the author found that the probability of successfully transferring sodium and cesium atoms to optical tweezers is only 33%, and most of the cases are only successful transfer of one atom or complete failure Figure 2 Cesium atom moving to sodium atom: a) cesium atom moving at different times; b) cesium atom content change centered on sodium atom light pinch during moving (picture source: Science, 2018, 900 – 903) after determining that all atoms can be loaded into the corresponding optical tweezers, how to move them to the same position becomes the next problem that the author needs to solve Since the 700 nm beam will repel the cesium atom, the strategy adopted by the author is to use the 700 nm optical tweezers to keep the sodium atom in place, while the 976 nm optical tweezers control the cesium atom to move slowly to the position of the sodium atom (Fig 2a) When the two coincide, the 700 nm optical tweezers are removed to prevent cesium atoms from escaping After that, the author began to study the collision between sodium and cesium The collisions between atoms usually do not generate a new molecule directly, but the changes of hyperfine states caused by collisions will give the atoms enough kinetic energy to escape from the optical tweezers Therefore, there may be four different results in one experiment: 1) two atoms in the optical tweezers; 2) one sodium atom; 3) one cesium atom; 4) zero atoms The results of many experiments show that when the two atoms are in the hyperfine state with medium energy, the energy generated by the impact will make the atoms in the optical tweezers escape at a faster speed (8 ms, figure 3); while when the two atoms are in the hyperfine state with the lowest energy, the atoms in the tweezers can remain for a long time after the impact (0.65 s, figure 3) Fig 3 Results of collision between sodium atom and cesium atom (top) and photo association reaction (bottom) (picture source: Science, 2018, 900-903) Finally, the author conducted photo Association (PA) reaction research on two atoms, and found that sodium atom and cesium atom will generate an excited molecular NACs * (Fig 3, bottom) In principle, the technique of controlling the reaction of two atoms by optical tweezers is not limited to the reaction of double alkali metals In addition, based on this technology, through the study of more complex molecular formation process, chemists can provide the most detailed reaction information All authors: L R Liu, J D hood, Y Yu, J T Zhang, N R hutzler, t rosenband, K - K Ni science cover (photo: Ken Richardson)