For the first time, chemists have succeeded in measuring vibrational motion of a single molecule with a femtosecond（飞秒） time resolution. The study reveals how vibration of a single molecule differs from the behaviour of larger molecular groups. The study was performed at the University of California, Irvine, where post-doctoral researcher Eero Hulkko from the University of Jyväskylä works as a visiting fellow under professor Vartkess A. Apkarian, whose team participated in the study. The second team was lead by Professor Eric O. Potma. The results of the study made the cover of the August issue of Nature Photonics magazine.

The scientists employed ultra-short femtosecond (10-15 s) laser pulses in the visible light spectrum to measure the motion of individual molecules.

Seeing a single organic bipyridylethylene (BPE) molecule vibrate as a function of time was possible through the scattering of the light pulses. The method is known as time-resolved coherent anti-Stokes Raman scattering (tr-CARS).

In conjunction with the study, the scientists developed a new method for identifying single molecules by optical means.

Aided by gold nanoantennas

The observations of single BPE molecule vibrations was possible using so called "plasmonic nanoantennas," which consist of two gold nanoparticles of approximately 90 nanometres. A nanometre is one billionth of a meter. The nanoantennas amplify the radiation from a single molecule to a detectable level.

"Detecting one molecule through scattering of light is extremely difficult," says Doctor Hulkko, who participated in the study, "which is why we needed to amplify the signal."

Nanoparticle amplification of molecular signals is employed in various spectroscopic（光谱学的） techniques, such as surface-enhanced Raman scattering (SERS), which is used for routine detection of single molecules.

Vibrations in a single molecule are strictly governed by quantum mechanics.. In order to be detectable with the method, a molecule has to be in two (or more) quantum states at once. In quantum mechanics, this is known as coherent superposition of vibrational states -- a wave packet. In molecular groups, the states within the wave packet usually lose their phase correlation within a short period of time. This is called dephasing（移相，退相位）.