Langmuir-Hinshelwood Model
The Langmuir-Hinshelwood Model is a theoretical framework used to describe the formation of molecular hydrogen (H₂) from individual hydrogen atoms on the surfaces of interstellar dust grains. Developed by chemists Irving Langmuir and Cyril Hinshelwood, this model posits that two hydrogen atoms adhere to a dust grain, where they can then diffuse across the surface. When the atoms come together, they undergo a chemical reaction that results in the formation of H₂, which is subsequently released from the dust grain.
This model is a refinement of the broader dust-grain model of hydrogen formation, which emphasizes the role of solid particles in the interstellar medium (ISM) rather than gas-phase reactions. The ISM, primarily composed of hydrogen gas, also contains tiny dust particles that play a crucial role in chemical processes. In contrast to the Langmuir-Hinshelwood Model, the Eley-Rideal mechanism describes a different interaction where only one hydrogen atom adsorbs to the dust grain and reacts directly with a passing second atom.
Understanding these models is important for astronomers as they seek to uncover the conditions and processes that govern molecular formation in the ISM, which holds significance for theories about the origins of life on Earth and the potential for life elsewhere in the universe.
Langmuir-Hinshelwood Model
FIELDS OF STUDY: Astrochemistry; Astrophysics; Astrometry
ABSTRACT: The Langmuir-Hinshelwood model is one of two models commonly used to explain how two atoms form molecular hydrogen (H2) in the interstellar medium (ISM), the material that fills the space between the stars. According to the Langmuir-Hinshelwood model, both atoms adsorb to, or adhere to the surface of, interstellar dust grains, where they combine to form a molecule. Scientists hope to learn more about the formation of life on Earth from studying dust grains in the ISM.
Modeling the Formation of H2
The Langmuir-Hinshelwood model, also called the Langmuir-Hinshelwood mechanism, is used to explain how two atoms form molecular hydrogen (H2). It is based on the work of American chemist and physicist Irving Langmuir (1881–1957) and English chemist Sir Cyril Norman Hinshelwood (1897–1967).
In the Langmuir-Hinshelwood model, two individual hydrogen atoms adsorb to, or stick to the surface of, a grain of interstellar dust. One or both of the two atoms then diffuse over the surface of the dust grain toward each other. When they meet, they undergo a chemical reaction in which they bond to form H2. The newly formed molecule of hydrogen gas then desorbs, or is released, from the surface of the dust grain.
Grains of Dust
The Langmuir-Hinshelwood model is a refinement of the dust-grain model of H2 formation, which states that the formation of molecular hydrogen gas takes place on the surfaces of dust particles found in the interstellar medium (ISM). This is in contrast to a gas-phase reaction, which would use more energy.
The ISM is the matter that exists in space between the stars. About 99 percent of it is gas, mainly hydrogen. The remainder consists of solid particles, typically smaller than one millionth of a meter. These particles are called interstellar dust or dust grains. In addition to being the site of hydrogen gas formation, the ISM is also where new stars form.
The Langmuir-Hinshelwood model is not the only model of hydrogen gas formation. A second model, known as the Eley-Rideal mechanism, is also based on the dust-grain model. In this model, however, only one hydrogen atom adsorbs to the dust grain. It then reacts directly with a second hydrogen atom as it passes by. The difference between the two is that in the Langmuir-Hinshelwood model, both atoms must adsorb to a dust grain before they react with one another; in the Eley-Rideal model, only one atom adsorbs.
Learning More about the ISM
It is difficult for astronomers to determine for certain how H2 forms in the ISM in part because of its extreme temperatures. What they do know about dust grains and H2 in the ISM stems largely from observations and laboratory experiments. Astronomers have determined that dust grains are composed mainly of silicates and carbon and are coated with a layer of ice. Scientists do not yet know definitively how hydrogen atoms on dust grains combine to form H2. However, they hope to gain more information in the near future. The National Aeronautics and Space Administration’s space probe Voyager 1 left the heliosphere, the bubble that separates the solar system from the ISM, in 2012 The Voyager 2 probe did the same in 2018 in a different direction. Both continued to explore and transmit data, such as plasma oscillations that allow scientists to measure the electron density of the ISM, as of 2022. Researchers learned that the electron density of ISM was increasing as the probes moved away from the sun.
Information about the ISM may help scientists learn more about Earth. Some of the molecules necessary for life come from the ISM. By learning more about the ISM, scientists may discover how life began on Earth and whether it exists anywhere else.
Principal Terms
- dust-grain model: a model of molecular hydrogen formation in which the reaction takes place on the surfaces of interstellar dust grains.
- Eley-Rideal mechanism: a model of molecular hydrogen formation in which one hydrogen atom adsorbs to a grain of dust, then reacts directly with a second hydrogen atom as it passes.
Bibliography
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