Trouble-Free Investigation Of Individual Molecules

Researchers at the University of Basel have developed a new method with which individual isolated molecules can be precisely examined - without destroying the molecule or even influencing its quantum state. The highly sensitive method is widely applicable, which opens up a number of new applications in quantum science, spectroscopy and chemistry. The scientists in the Department of Chemistry report this in the journal Science.

Spectroscopic studies are based on the interaction of matter with light and are the most important tool for studying the properties of molecules. As a rule, a sample is irradiated with countless molecules directly. The molecules can only absorb light at well-defined wavelengths that correspond exactly to the difference between two quantum mechanical energy states. One speaks of an excitation of spectroscopic transitions.

The molecules are disturbed by these excitations and change their quantum state. In many experiments, the molecules are even chemically destroyed to detect the excitations. By analyzing the wavelength and the intensity of the transitions, information about the chemical structure and molecular movements such as rotations and vibrations can be obtained.

Inspired by methods of quantum science for manipulating atoms, the research group of Prof. Dr. Stefan Willitsch at the University of Basel developed a new spectroscopic method in which only a single molecule, here a charged nitrogen molecule as an example, is examined indirectly - without destroying the molecule or changing its quantum state.

For this purpose, the molecule is trapped in a radio frequency trap and cooled close to the absolute zero temperature (approx. –273 ° C). This requires a foreign atom - here a charged calcium atom - that is located right next to it. This spatial proximity is also essential for the later spectroscopic examination of the molecule.

Molecule in the optical lattice
Two focused and focused laser beams, which are aimed at the molecule - a so-called optical grating - then generate a force on the molecule. This force is stronger the closer the irradiated wavelength corresponds to a spectroscopic transition of the molecule, without, however, exciting it spectroscopically.

Movement of the lattice causes the molecule to start oscillating in the trap, and the more the optical force acts, the more. This movement is transferred to the neighboring calcium atom and can be detected there. In this way, the same information about the molecule can be obtained as with conventional spectroscopic excitation.

The new method, which the researchers call a type of force spectroscopy, is pursuing several new approaches. On the one hand, it takes place on a single isolated molecule. On the other hand, it is trouble-free because it takes place indirectly (via the neighboring atom) and without directly stimulating spectroscopic transitions. The quantum state of the molecule remains intact, so that the measurement can be repeated as often as required. This means that the measurement method is several orders of magnitude more sensitive than common spectroscopy methods, which are based on the direct excitation and destruction of a large number of molecules.

Building blocks for extremely precise clocks and quantum computers
Willitsch sees numerous potential areas of application of this method: Our force spectroscopy enables extremely precise measurements on molecules that were not possible with the previous methods. With the new method, molecular properties and chemical reactions can be examined very sensitively and under precisely defined conditions at the level of individual molecules. It also allows access to fundamental questions - such as whether the natural constants are actually constant or change over time. Practical applications would be the construction of an extremely precise molecular clock - or the use of molecules as building blocks of a quantum computer.