Too fast to capture? Not a problem any more – Observing and controlling the electron dynamics using ultrafast X-ray and attosecond lasers
by By Frank Kuo
An exciting technique generates shock waves and attracts followers. However, for a technique to actually fly high, it has to be applicable to exploring the scientific unknowns. The thirst of the unknown then feeds the desire of having even better techniques. This mutual and healthy interaction paves the way of scientific advance and can be seen in many of today’s technical sessions.
One technique is vigorously following this pattern: lasers with attosecond pulse width. Due to its extremely short width in time domain, it extends the spectral coverage into the X-ray domain, which then is suitable to excite electronic transitions. Its attosecond time resolution, on the other hand, is a perfect fit for probing the electron dynamics (in comparison, femtosecond lasers are suitable for studying the vibrations of the molecular bonds. For example, the OH vibration has the frequency of tens of femtoseconds).
Tuesday is filled with these topics. Here are some of the highlights:
Researchers from
Mauro Nisoli’s group are using attosecond lasers to create and observe the quantum interference of electrons in simple atoms and complicated molecules, such as N
+ atom, phenylalanine, and other amino acids.
In his group, by focusing 4 f s/1 kHz pulses into a gas cell filled with noble gases, extreme ultraviolet (XUV) attosecond pulses are generated. XUV radiation is then focused at the sample. Another IR pulse enters the beamline such that pump-probe measurements with attosecond temporal resolution can be performed. The spectral characteristics of the XUV radiation are measured by a flat-field grazing incidence spectrometer. By the velocity map imaging spectrometer (VMI), the momenta of electrons can also be measured. With this robust experimental setup, interesting results are shown. For example, charge migration within the amino acid is happening with ~30 fs timescale.
Zenhu Chang’s group of
CREOL is able to produce “isolated” ultra-broad band attosecond pulses (~ 67 fs) using a technique called double optical gating (DOG). DOG is composed of polarization gating and two color gating, respectively. By playing this trick, attosecond pulse, instead of attosecond pulse trains, is generated. This isolated pulse is used to excite the molecules. The spectrum of the pulse is spanning from ~10eV-620eV, which is so broad to cover essentially all absorption in atoms, molecules, and solids. The experimental setup is also straightforward: using 4-6 fs 730 nm laser -> focusing it in Xe gas with DOG technique to generate attosecond pulse-> filtering out the unwanted frequency by foil filter. Then you are ready to interrogating the molecules. Dynamic wave packet interference and coupled electron-nuclear dynamics are just a few examples of the excellent work in this group.
Anne L'Huillier’s group, on the other hand, utilizes the powerful energy of the attosecond pulses to knock out multiple electrons from the molecules. This single and double photoionization of electrons in noble gas provides insights on the correlation of the electrons.
Something called “
attoclock” definitely highlights my fruitful day. Developed by Professor
Ursula Keller and her group in ETH, the IR pulse is rendered circularly polarized through the wave-plate. Using this modified pulse, and measuring the electron momentum vector ionized from the gas with an instrument called
COLTRIMS, the revolution of the polarization of the pulse will provide time resolution with attosecond precision. Intuitively, it behaves like a physical clock in the daily life. This new technique is used to investigate the tunneling of the electrons. It is found that the electrons do tunnel, but not instantaneously. This “tunneling delay time” of electrons in helium gas calls for new explanation and does stir the emotion of the audience. Enthusiastic discussion was then taking the stage, which is undeniably kind of fun!
More topics, such as zeptosecond pulse synthesis, attosecond chemistry, coherent control of the electronic wavefunction to suppress ionization, and
controlling ferromagnetism using light, were presenting one after the other.
It is worth to mention that the time scale of the spin dynamics, such as demagnetization (the principle behind magnetism) was thought to be happening in nanosecond to microsecond scale. With the advent of the femtosecond laser, that time scale has been pushed down to femtoseconds. Now, we have the attosecond lasers, and it is further pushed down to attosecond regime. The refinement of the time scale calls for new theories. As a result, new theories such as “super-diffusive spin transport” and “local spin flip scattering” are formulated to explain these phenomena. Similar stores are going on in many other fields.
Therefore, one more important lesson is learned: Nothing in science is set in stone. It evolves, and never stops giving us surprises!
Posted: 9 Oct 2013 by By Frank Kuo