User:Monxus X/sandbox/Attosecond

An attosecond (abbreviated as as) is a unit of time in the International System of Units (SI) equal to 10−18 or 1⁄1 000 000 000 000 000 000 (one quintillion) of a second. An attosecond is to a second as a second is to about 31.71 billion years. The attosecond is a newly discovered "slice of time" that is tiny but has various potential applications: it can observe oscillating molecules, the chemical bonds formed by atoms in chemical reactions, and other extremely tiny and extremely fast things.

On October 3rd, 2023, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2023 to Pierre Agostini, Ferenc Krausz, and Anne L’Huillier for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter.

Common measurements

 * 0.247 attoseconds: travel time of a photon across "the average bond length of molecular hydrogen"
 * 24 attoseconds: the atomic unit of time
 * 43 attoseconds: the shortest pulses of laser light yet created
 * 53 attoseconds: the second-shortest pulses of laser light created
 * 82 attoseconds (approximately): half-life of beryllium-8, maximum time available for the triple-alpha process for the synthesis of carbon and heavier elements in stars
 * 84 attoseconds: the approximate half-life of a neutral pion
 * 100 attoseconds: fastest-ever view of molecular motion
 * 320 attoseconds: the estimated time it takes electrons to transfer between atoms

Historical development
During the 1980s, several research groups produced and studied highly charged atomic ions or atoms with few or no electrons. Researchers used a variety of approaches, including the use of advanced ion sources and high-power lasers. They demonstrated how multi-photon ionization processes could produce multiple charged ions.

In 1991, Anne L'Huillier, Kenneth Schafer, and Kenneth Kulander presented results from a numerical solution of the time-dependent Schr¨odinger equation (TDSE). They also provided a clear understanding of the high harmonic generation (HHG) process. In early 1993, they gave an oral presentation of the three-step semiclassical model of HHG: briefly, the manual laser field causes tunneling ionization; free electrons may recombine the atom; the recombination process converts kinetic energy to an emitted extreme ultraviolet (XUV) photon. This kind of photon is measured in attoseconds.

In 1994, Pierre Agostini and co-workers investigated the principle of frequency modulation in a two-color photon field -- RABBIT (reconstruction of attosecond beating by the interference of two-photon transitions). The RABBIT technique makes it possible to measure the pulse duration of a train of attosecond pulses. In 2001, the Agostini group produced a train of pulses with a duration of 250 attoseconds, as measured with the RABBIT metrology using argon as the target gas.

In 2001, Ferenc Krausz and his team at the Vienna University of Technology created the starting point for attosecond spectroscopy. The Krausz group produced isolated pulses with a duration of 650 attoseconds. They first fired an ultrashort wavelength (7 femtoseconds) red laser pulse into a stream of neon atoms, where the stripped electrons were carried by the pulse and almost immediately re-eject into the neon nucleus.

While capturing the attosecond pulse, the physicists also demonstrated its utility. They aimed attosecond and longer-wavelength red pulses at a type of krypton atom simultaneously: first, the electrons were knocked off; then, the red light pulse hit the electrons; finally, the energy was tested. Judging from the difference in the timing of these two pulses, the scientists obtained a very precise measurement of how long it took the electron to decay (how many attoseconds). Never before have scientists used such a short time scale to study the energy of electrons.

Need for More precise units
The crystal lattice vibrates and molecules rotate on a scale of picoseconds. The creation and breaking of chemical bonds and molecular vibration happen in femtoseconds. Observing the motion of electrons happens on the attosecond scale.

The number of electrons in an atom and their configuration define an element. Because attosecond pulses are faster than the motion of electrons in atoms and molecules, attosecond provides a new tool for controlling and measuring quantum states of matter. These pulses have been used to explore the detailed physics of atoms and molecules and have potential applications in fields ranging from electronics to medicine.

Directly observing the wave oscillations of light
Using a method called attosecond streaking, people can see the electrical components of EM waves. Scientists start with a gas of neon atoms and ionize them with a single ultrashort burst of UV radiation measured in attoseconds. The electric field of the infrared can then strongly influence the motion of the electrons. The electrons will be forced up and down as the field oscillates. Depending on when the electron is released, this process will emit different final energies. The final measurement of the electron's energy, as a function of the relative delay between the two pulses, clearly shows the traces of the electric field of the attosecond pulse.

Directly observing electron motion
Similar to observing the motion of the electric field of light, we can also observe the motion of electrons inside an atom. For an electron in a quantum superposition of two different energy levels, the charge density in the atom will oscillate over time. In 2010, scientists were able to show a clear oscillation in how much a short pulse was absorbed by an oscillating charge distribution caused by spin-orbit interactions.

Watching quantum interference build-up
With the help of the attosecond unit, the details of the energy states without interference will be imprinted into the absorbance spectrum of the light. When ionizing an electron, the electron can either ionize directly, or it can spend some time in a highly-excited autoionizing state that will fall apart after some time. The result is that the electron will go into a superposition of both pathways, which will interfere with its spectrum and cause a wonky, nontrivial shape in the absorption spectrum.

If we have short pulses of radiation, like the attosecond pulses, we can control how long we let the electron sit in that autoionizing state before we come in with a second pulse of light to disrupt it and kill the interference.