- 2570 - PHOTOELECTRIC EFFECT - light comes from atoms? The interaction between light and matter is the basis of many fundamental phenomena and various practical technologies. The photoelectric effect causes electrons to be emitted from a material that is exposed to light of suitable energy. The advent of quantum theory and the genius of Albert Einstein was how the effect became understood.
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-------------------- 2570 - PHOTOELECTRIC EFFECT - light comes from atoms?
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- Everyone is familiar with the photoelectric effect in our every day lives. It is used in your cameras to adjust the aperture for light conditions. It is used to dim headlights from approaching cars. It is used to open doors. The technology is everywhere.
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- This photoelectric effect was first explained by Albert Einstein. He received the Nobel Prize in 1921 for explaining the photoelectric effect in its most intuitive form. Quantized energy is released when a single atom that is irradiated with light. According to Einstein, light consists of particles (photons) that transfer this quantized energy to the electron in the atom.
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- If the photon's energy is sufficient, it knocks the electrons out of the atom. The photoelectric effect is the creation of photoelectrons through this process of ionization. It is one of the most fundamental processes in the interaction between light and matter. But questions still remain about exactly how photons transfer their linear momentum to electrons.
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- The key principle is the transfer not of energy but of linear momentum, or, impulse, from photons to electrons. This is the case, for instance, when laser light is used to cool microscopic and macroscopic objects, or to understand the phenomenon of radiation pressure.
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- Despite the fundamental importance of momentum transfer, the precise details of how light passes its impulse on to matter are still not fully understood. One reason is that the transferred impulse changes during an optical cycle are on extremely fast, sub-femto seconds timescales.
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- The discovery that free electrons can move asymmetrically provides a deeper understanding of one of the basic processes in the photoelectric effect. An electron that has just been released from an atom via the photoelectric effect can change its wave motion. This change can be studied using a laser field. The free electron can both absorb and emit laser light, which changes the electron's rotation in an asymmetrical way.
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- To study this phenomenon, the researchers used ultra short laser pulses with a time precision on an attosecond scale, which is staggeringly short: 0.000,000,000,000,000,001 seconds.
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- The discovery of the asymmetry in combination with the high time resolution gave the researchers the opportunity to disrupt the electrons' ingrained behavior. From only moving up and down along the laser field, the researchers succeeded in getting the electrons to also spread sideways.
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- In classical physics, particles move in a deterministic way from one point to another via Newton's laws. In contrast to this, quantum mechanics says that a particle can move to several places simultaneously. These researchers have been able to take advantage of the this effect.
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- When we change the direction of the electron wave, we are using quantum mechanical interference. That is, the electron takes several paths towards its changed wave form. In the classical physics the electron can only go one way.
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- The phenomenon of the asymmetrical movement pattern has been proved both in experiments and in theory. The results are based on the knowledge that electrons increase their rotating movements when they absorb light.
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- The research aims to control electrons in atoms and molecules with greater precision.
Studies revealed mainly information on time-averaged behavior, missing time-dependent aspects of the linear-momentum transfer during photo ionization.
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- Investigations are using high laser intensities, where multiple photons are involved in the ionization process, and measurements on how much momentum is transferred in the direction of laser propagation.
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- To achieve sufficient time resolution, they employed the so-called attoclock technique. In this method, attosecond time resolution is achieved without having to produce attosecond laser pulses. Instead, information about the rotating laser-field vector in close to circular polarized light is used to measure time relative to the ionization event with attosecond precision. Very similar to the hand of a clock that is rotating through a full circle within one optical cycle of 11.3-femto-seconds duration.
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- Physicists were able to determine how much linear momentum electrons gained depending on when the photoelectrons were 'born'. They found that the amount of momentum transferred in the propagation direction of the laser does indeed depend on when during the oscillation cycle of the laser the electron is 'freed' from the matter, in their case xenon atoms.
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- The classical model had to be extended to take into account the interaction between the outgoing photoelectron and the residual xenon ion. This interaction induces an additional attosecond delay in the timing of the linear momentum transfer compared to the theoretical prediction for a free electron born during the pulse.
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- The device in this experiment is 3 meters long and 2.5 meters high is an extremely high performing laser. Its photons collide with individual argon atoms in the apparatus, and thereby remove one electron from each of the atoms. The momentum of these electrons at the time of their appearance is measured with extreme precision in a long tube of the apparatus.
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- When numerous photons from a laser pulse bombard an argon atom, they ionize it. Breaking up the atom partially consumes the photon's energy. The remaining energy is transferred to the released electron. The question of which reaction partner , electron or atom nucleus, conserves the momentum of the photon has occupied physicists for over 30 years.
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- The simplest idea is as long as the electron is attached to the nucleus, the momentum is transferred to the heavier particle, i.e., the atom nucleus. As soon as it breaks free, the photon momentum is transferred to the electron.
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- This would be analogous to wind transferring its momentum to the sail of a boat. As long as the sail is firmly attached, the wind's momentum propels the boat forward. The instant the ropes tear, however, the wind's momentum is transferred to the sail alone.
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- The electron not only receives the expected momentum, but additionally one third of the photon momentum that actually should have gone to the atom nucleus. The sail of the boat therefore "knows" of the impending accident before the cords tear and steals a bit of the boat's momentum.
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- The electrons tunnel through a small energy barrier. In doing so, they are pulled away from the nucleus by the strong electric field of the laser, while the magnetic field transfers this additional momentum to the electrons.
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- To ensure that the small additional momentum of the electron was not caused accidentally by an asymmetry in the apparatus the laser pulse hit the gas from two sides, either from the right or the left, and then from both directions simultaneously, which was the biggest challenge for the measuring technique.
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- This new method of precision measurement promises deeper understanding of the previously unexplored role of the magnetic components of laser light in atomic physics.
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- 100 years later and we are still learning how the photoelectric effect is actually working.
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- Our eyes have a type of photoelectric effect. Seeing is the most amazing thing. It takes over one forth of the brain and your calorie to make seeing work. Everyone knows that our eyes see with light photons entering the eye and striking the retina in the back of the eye ball. Our brain creates an image from the pattern of chemical excitations created by these light photon energy swaps.
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- Here are previous Reviews that are available for more background learning about these amazing photons:
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- 2229 - When we approach the speed of light, 186,000 miles per second, Clocks run at different rates, distances appear to shrink, and objects themselves change color depending on their speed relative to your speed. Yet, at the same time, relativity declares that the laws of physics are the same and invariant for all observers, regardless of their motion. So what does this mean for a photon, which itself moves at the speed of light?
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- 2181 - The photon is the force carrier for all electric charge repulsive and attractive forces. These forces exit between particles because photons travel between them. One eye blink contains as many erg-seconds as there are centimeters across the Observable Universe.
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- 2113 - The transfer of energy from a photon is dependent on its frequency. The higher the frequency the higher the energy. Ultraviolet light has higher energy than red light. It can give you a sunburn. The human eye has 18 powers of 10 dynamic range. The number of photons entering the eye in daylight is 79,000,000,000,000,000 photons per second.
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- 1863 - From starlight to star dust. How the stars create the light?
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- 1810 - How many photons enter your eye? How many photons exist in the Universe?
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- 1074 - Physics the way I learned it.
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- 738 - The science of physics.
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- December 29, 2019 2570
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