The history of molecular photography can be traced back to the 1980s, when scientists at the time proposed a method of molecular snapshots. This advanced technology is called pump spectroscopy and uses laser pointer pulses lasting only a few femtoseconds to trigger a chemical reaction (see “Microworld”) After an instant, the second femtosecond pulse arrives and undergoes a mid-term reaction with the molecules in the specimen. This changes the way the detector detects light and takes a “photo” of molecules. By repeating the experiment again and again and changing the delay between the two pulses, the researchers were able to construct a page-turning animation book to show each stage of the chemical change.
This technique uses femtosecond chemistry to show how the interior of a chemical reaction works, revealing the characteristics of a transient intermediate when a single molecule is transformed into another molecule, which has never been seen before. However, the wavelength of the green laser pointerused in femtosecond chemistry is longer than the distance between atoms, so it cannot directly derive the position of atoms in molecules.
In order to obtain clear images of individual atoms, scientists have long relied on X-ray crystallography or electron diffraction to study how photons or electrons scatter through molecules. At the same time, instruments similar to scanning tunneling microscopes and atomic force microscopes can provide richer images, even containing single molecules and the electron cloud around them. But these techniques usually take a few milliseconds or longer to obtain an image, which is too slow to observe the atom’s round-trip motion.
So in the past few years, the producers of molecular “films” have combined multiple technologies such as femtosecond chemistry, diffraction, and atomic imaging to create a collection of hybrid technology tools that can provide the most detailed microcosm, while Combining time and space resolution, it shows the shape of atoms and molecules under natural conditions.
Last year, researchers at the University of Regensburg in Germany used red laser pointer pulses to significantly increase the shutter speed of scanning tunneling microscopes. The researchers took a quick photo by triggering terahertz (THz) radiation at the tip of the scanning tunneling microscope, generating a voltage difference that exactly distinguishes the microscope tip from the target molecule pentacene, allowing electrons to create “channels” through which molecules can penetrate. Out. This “channel” is opened every cycle of the terahertz pulse, leaving the scanning tunneling microscope with a shutter time of about 100 fs. In this very short period of time, it was enough to take a freeze image of the pentacene electron orbit.