For the first time ever, Finnish chemists were able to measure the vibrational motion of a single molecule with a time-resolved femtosecond.
![Illustration of the tested molecule and the gold nanoantenna attached to it. The reaction of the nanoparticles to the light creates an electric field between the particles that increases the fluorescence and makes it possible to observe the single molecule. The image on the right shows the microscope image (TEM) of the molecular structure. [Courtesy of the Academy of Finland]](https://www.hayadan.org.il/images/content3/2014/11/511715b0-b65b-49bf-88da-b55af155f1b6.jpeg)
For the first time ever, Finnish chemists were able to measure the vibrational motion of a single molecule with a time-resolved femtosecond.
The study, conducted at the University of California, Irvine, and its findings were published in the scientific journal Nature Photonics, reveals how the vibration of a single molecule differs from the behavior of larger molecular collections.
The scientists used laser pulses with extremely short time intervals of femtoseconds in the visible light spectrum to measure the movement of individual molecules. The ability to observe the vibration of a single organic molecule [in this case, bipyridylethylene, BPE] as a function of time was made possible by the scattering of light pulses. This method is known as 'time-resolved coherent anti-Stokes Raman scattering' (tr-CARS). As part of the research, the scientists also developed a new method for detecting single molecules using optical means.
The observation of the vibration of a single BPE molecule was made possible with the help of a method known as 'plasmonic nanoantennas' (plasmonic nanoantennas) consisting of two gold nanoparticles that are about 90 nanometers apart. The nanoantenna amplifies the radiation emitted from a single molecule to a measurable level. "The ability to measure a single molecule using light scattering is a particularly challenging task," says one of the scientists involved in the research, "and therefore we had to amplify the signal."
Amplification of molecular signals with the help of nanoparticles is widespread in various spectroscopic methods, for example within the 'surface-enhanced Raman scattering' (SERS) method, which is used for the routine detection of single molecules. The vibration of a single molecule is completely controlled by the laws of quantum mechanics. In order to get a measurement result within this method, the molecule needs to be in two quantum states at the same time. In quantum mechanics, this phenomenon is known as 'coherent superposition of vibrational states' - a wave packet. In molecular collections, the modes within a wave bundle usually lose their phase correlation within a very short period of time, a phenomenon known as dephasing.
Using laser pulses, the scientists were able to create a wave bundle of vibrations in a single molecule and observe them for 10 picoseconds. The time-dependent movement of the wave bundle corresponds to the practical vibrations of the molecule, that is, to its molecular "breathing". The tests proved that the vibrations of a single molecule did not lose its phase correlation. With the help of computer simulations, the scientists were able to explain the observations and show that exiting an event is a property of molecular collections and not of a single molecule, when this property is extremely basic in this case and was not known to science until now.
"Our ability to observe the vibration of a single molecule advances us one step closer to a situation where we can see real chemistry in action at the level of individual molecules," notes the lead researcher. The ability to create, change and measure the quantum states of a single molecule opens a window to new possibilities in the field of quantum computing based on the transfer of molecules and quantum information. In practical terms, observing single molecules means that we are observing individual photons. As a result, the research findings pave the way for new avenues of research and applications in the field of photonics components based on a single molecule, for example - the production of single photons in the state we want.
