How fast do electrons move between atoms in a molecule? Most of the time, they only need a few attoseconds (10^-18 seconds, or a millionth of a billionth of a second). Such fast processes are difficult to track, and a group of scientists in Australia recently developed a new interference technology that can measure time delays with zeptosecond (10^-21 seconds, or trillionths of a billionth of a second) resolution.

As a test, this technology was used to measure the delay between two light pulses emitted by different isotopes of hydrogen – normal hydrogen (H2) and deuterium (D2), simultaneously exposed to a single pulse of laser light. The value of the measured delay was less than three attoseconds, due to the difference in the dynamics of motion of the lighter and heavier nuclei of hydrogen isotope atoms.

The light was emitted by hydrogen atoms using a process called high harmonic generation (HHG). This process occurs when an electron is ejected from an atom by a strong stream of light, which also accelerates the electron to a higher energy (velocity). When the electron returns to the “chest” of the atom, some hard ultraviolet light (extreme ultraviolet, XUV) is emitted. The frequency, intensity and phase of secondary radiation largely depend on the parameters of the wave functions, so all atoms and molecules emit harsh ultraviolet with their own unique parameters.

If the spectral density of secondary radiation is measured quite simply, the measurement of its phase is a much more difficult problem that conventional spectrometers cannot overcome.

To solve this problem, scientists used the phenomenon called the Gouy phase to their advantage. The measurement of the phase shift of the Gui of the light quantum from hydrogen and deuterium is then equivalent to the measurement of the time delay, and experiments have shown that this value is quite stable and is slightly less than 3 attoseconds. The work of Australian scientists was checked for “scientific purity” by a group of theoretical physicists from the University of Shanghai. Chinese scientists modeled all possible options for the generation of HHG radiation of two hydrogen isotopes, taking into account all possible combinations of the movement of nuclei and electrons.

The simulation results obtained match very well with the experimental data, indicating that in the future this type of technology could be used for investigation and measurements of ultrafast processes in atoms and molecules with unprecedented temporal resolution.