An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light, they are used in optical parametric oscillators and some interferometers. Light confined in the cavity reflects multiple times producing standing waves for certain resonance frequencies; the standing wave patterns produced. Different resonator types are distinguished by the focal lengths of the two mirrors and the distance between them; the geometry must be chosen. Resonator types are designed to meet other criteria such as minimum beam waist or having no focal point inside the cavity. Optical cavities are designed to have a large Q factor. Therefore, the frequency line width of the beam is small indeed compared to the frequency of the laser. Light confined in a resonator will reflect multiple times from the mirrors, due to the effects of interference, only certain patterns and frequencies of radiation will be sustained by the resonator, with the others being suppressed by destructive interference.
In general, radiation patterns which are reproduced on every round-trip of the light through the resonator are the most stable, these are the eigenmodes, known as the modes, of the resonator. Resonator modes can be divided into two types: longitudinal modes, which differ in frequency from each other; the basic, or fundamental transverse mode of a resonator is a Gaussian beam. The most common types of optical cavities consist of two facing spherical mirrors; the simplest of these is the plane-parallel or Fabry–Pérot cavity, consisting of two opposing flat mirrors. While simple, this arrangement is used in large-scale lasers due to the difficulty of alignment. However, this problem is much reduced for short cavities with a small mirror separation distance. Plane-parallel resonators are therefore used in microchip and microcavity lasers and semiconductor lasers. In these cases, rather than using separate mirrors, a reflective optical coating may be directly applied to the laser medium itself; the plane-parallel resonator is the basis of the Fabry–Pérot interferometer.
For a resonator with two mirrors with radii of curvature R1 and R2, there are a number of common cavity configurations. If the two radii are equal to half the cavity length, a concentric or spherical resonator results; this type of cavity produces a diffraction-limited beam waist in the centre of the cavity, with large beam diameters at the mirrors, filling the whole mirror aperture. Similar to this is the hemispherical cavity, with one plane mirror and one mirror of radius equal to the cavity length. A common and important design is the confocal resonator, with mirrors of equal radii to the cavity length; this design produces the smallest possible beam diameter at the cavity mirrors for a given cavity length, is used in lasers where the purity of the transverse mode pattern is important. A concave-convex cavity has one convex mirror with a negative radius of curvature; this design produces no intracavity focus of the beam, is thus useful in high-power lasers where the intensity of the intracavity light might be damaging to the intracavity medium if brought to a focus.
A transparent dielectric sphere, such as a liquid droplet forms an interesting optical cavity. In 1986 Richard K. Chang et al. demonstrated lasing using ethanol microdroplets doped with rhodamine 6G dye. This type of optical cavity exhibits optical resonances when the size of the sphere or the optical wavelength or the refractive index is varied; the resonance is known as morphology-dependent resonance. Only certain ranges of values for R1, R2, L produce stable resonators in which periodic refocussing of the intracavity beam is produced. If the cavity is unstable, the beam size will grow without limit growing larger than the size of the cavity mirrors and being lost. By using methods such as ray transfer matrix analysis, it is possible to calculate a stability criterion: 0 ⩽ ⩽ 1. Values which satisfy the inequality correspond to stable resonators; the stability can be shown graphically by defining a stability parameter, g for each mirror: g 1 = 1 − L R 1, g 2 = 1 − L R 2,and plotting g1 against g2 as shown.
A directed-energy weapon is a ranged weapon that damages its target with focused energy, including laser and particle beams. Potential applications of this technology include weapons that target personnel, missiles and optical devices. In the United States, the Pentagon, DARPA, the Air Force Research Laboratory, United States Army Armament Research Development and Engineering Center, the Naval Research Laboratory are researching directed-energy weapons and railguns to counter ballistic missiles, hypersonic cruise missiles, hypersonic glide vehicles; these systems of missile defense are expected to come online no sooner than the mid to late-2020s. Russia, China and the United Kingdom are developing directed-energy weapons. After decades of R&D, directed-energy weapons are still at the experimental stage and it remains to be seen if or when they will be deployed as practical, high-performance military weapons. Directed energy weapons could have several main advantages over conventional weaponry: Direct energy weapons can be used discreetly.
Light is only slightly altered by gravity, giving it an perfectly flat trajectory. It is practically immune to both windage and Coriolis force; this makes aim much more precise and extends the range to line-of-sight, limited only by beam diffraction and spread, absorption or scattering by intervening atmospheric contents. Lasers travel at light-speed and have near infinite range and therefore, are suitable for use in space warfare. Although some devices are labelled as microwave weapons, the microwave range is defined as being between 300 MHz and 300 GHz, within the RF range—these frequencies having wavelengths of 1–1000 milimeters; some examples of weapons which have been publicized by the military are as follows: Active Denial System is a millimeter wave source that heats the water in a human target's skin and thus causes incapacitating pain. It was developed by the U. S. Air Force Research Laboratory and Raytheon for riot-control duty. Though intended to cause severe pain while leaving no lasting damage, concern has been voiced as to whether the system could cause irreversible damage to the eyes.
There has yet to be testing for long-term side effects of exposure to the microwave beam. It can destroy unshielded electronics; the device comes in various sizes including attached to a humvee. Vigilant Eagle is a proposed airport defense system that directs high-frequency microwaves towards any projectile, fired at an aircraft; the system consists of a missile-detecting and tracking subsystem, a command and control system, a scanning array. The MDT is a fixed grid of passive infrared cameras; the command and control system determines the missile launch point. The scanning array projects microwaves that disrupt the surface-to-air missile's guidance system, deflecting it from the aircraft. Bofors HPM Blackout is a high-powered microwave weapon, said to be able to destroy at short distance a wide variety of commercial off-the-shelf electronic equipment, it is said to be not lethal to humans. The effective radiated power of the EL/M-2080 Green Pine radar makes it a hypothetical candidate for conversion into a directed-energy weapon, by focusing pulses of radar energy on target missiles.
The energy spikes are tailored to enter missiles through antennas or sensor apertures where they can fool guidance systems, scramble computer memories or burn out sensitive electronic components. AESA radars mounted on fighter aircraft have been slated as directed energy weapons against missiles, however, a senior US Air Force officer noted: "they aren't suited to create weapons effects on missiles because of limited antenna size and field of view". Lethal effects are produced only inside 100 metres range, disruptive effects at distances on the order of one kilometre. Moreover, cheap countermeasures can be applied to existing missiles. Counter-electronics High Power Microwave Advanced Missile Project An electrolaser first ionizes its target path, sends a powerful electric current down the conducting track of ionized plasma, somewhat like lightning, it functions as a high-energy, long-distance version of the Taser or stun gun. Pulsed Energy Projectile or PEP systems emit an infrared laser pulse which creates expanding plasma at the target.
The resulting sound and electromagnetic waves stun the target and cause pain and temporary paralysis. The weapon is under development and is intended as a non-lethal weapon in crowd control though it can be used as a lethal weapon. A dazzler is a directed-energy weapon intended to temporarily blind or disorient its target with intense directed radiation. Targets can include human vision. Dazzlers emit infrared or invisible light against various electronic sensors, visible light against humans, when they are intended to cause no long-term damage to eyes; the emitters are lasers, making what is termed a laser dazzler. Most of the contemporary systems are man-portable, operate in either the red or green areas of the electromagnetic spectrum. Developed for military use, non-military products are becoming available for use in law enforcement and security; the personnel halting and stimulation response rifle is a prototype non-lethal laser dazzler developed by the Air Force Research Laboratory's Directed Energy Directorate, U.
S. Department of Defense, its purpose is to temporarily blind a target. Blinding laser weapons have been tested i
The micrometre or micrometer commonly known by the previous name micron, is an SI derived unit of length equalling 1×10−6 metre. The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, for grading wool by the diameter of the fibres; the width of a single human hair ranges from 10 to 200 μm. The longest human chromosome is 10 μm in length. Between 1 μm and 10 μm: 1–10 μm – length of a typical bacterium 10 μm – Size of fungal hyphae 5 μm – length of a typical human spermatozoon's head 3–8 μm – width of strand of spider web silk about 10 μm – size of a fog, mist, or cloud water droplet Between 10 μm and 100 μm about 10–12 μm – thickness of plastic wrap 10 to 55 μm – width of wool fibre 17 to 181 μm – diameter of human hair 70 to 180 μm – thickness of paper The term micron and the symbol μ were accepted for use in isolation to denote the micrometre in 1879, but revoked by the International System of Units in 1967; this became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960.
In the SI, the systematic name micrometre became the official name of the unit, μm became the official unit symbol. In practice, "micron" remains a used term in preference to "micrometre" in many English-speaking countries, both in academic science and in applied science and industry. Additionally, in American English, the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is invariably stressed on the second syllable, whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable; the plural of micron is "microns", though "micra" was used before 1950. The official symbol for the SI prefix micro- is a Greek lowercase mu. In Unicode, there is a micro sign with the code point U+00B5, distinct from the code point U+03BC of the Greek letter lowercase mu.
According to the Unicode Consortium, the Greek letter character is preferred, but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters. Metric prefix Metric system Orders of magnitude Wool measurement The dictionary definition of micrometre at Wiktionary
Infrared radiation, sometimes called infrared light, is electromagnetic radiation with longer wavelengths than those of visible light, is therefore invisible to the human eye, although IR at wavelengths up to 1050 nanometers s from specially pulsed lasers can be seen by humans under certain conditions. IR wavelengths extend from the nominal red edge of the visible spectrum at 700 nanometers, to 1 millimeter. Most of the thermal radiation emitted by objects near room temperature is infrared; as with all EMR, IR carries radiant energy and behaves both like a wave and like its quantum particle, the photon. Infrared radiation was discovered in 1800 by astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. More than half of the total energy from the Sun was found to arrive on Earth in the form of infrared; the balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.
Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines transmission of photons in the infrared range. Infrared radiation is used in industrial, military, law enforcement, medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, detect objects such as planets, to view red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, to detect overheating of electrical apparatus. Extensive uses for military and civilian applications include target acquisition, night vision and tracking.
Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication and weather forecasting. Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 millimeter; this range of wavelengths corresponds to a frequency range of 430 THz down to 300 GHz. Below infrared is the microwave portion of the electromagnetic spectrum. Sunlight, at an effective temperature of 5,780 kelvins, is composed of near-thermal-spectrum radiation, more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, 32 watts is ultraviolet radiation. Nearly all the infrared radiation in sunlight is shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight.
However, black body or thermal radiation is continuous: it gives off radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, fires produce far more infrared than visible-light energy. In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors collect radiation only within a specific bandwidth. Thermal infrared radiation has a maximum emission wavelength, inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. Therefore, the infrared band is subdivided into smaller sections. A used sub-division scheme is: NIR and SWIR is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared". Due to the nature of the blackbody radiation curves, typical "hot" objects, such as exhaust pipes appear brighter in the MW compared to the same object viewed in the LW.
The International Commission on Illumination recommended the division of infrared radiation into the following three bands: ISO 20473 specifies the following scheme: Astronomers divide the infrared spectrum as follows: These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, hence different environments in space; the most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used. These letters are understood in reference to atmospheric windows and appear, for instance, in the titles of many papers. A third scheme divides up the band based on the response of various detectors: Near-infrared: from 0.7 to 1.0 µm. Short-wave infrared: 1.0 to 3 µm. InGaAs covers to about 1.8 µm. Mid-wave infrared: 3 to 5 µm (defined by the atmospheric window and covered by indium antimonide and mercury cadmium telluride and by lead
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, tasteless, non-toxic, monatomic gas, the first in the noble gas group in the periodic table, its boiling point is the lowest among all the elements. After hydrogen, helium is the second lightest and second most abundant element in the observable universe, being present at about 24% of the total elemental mass, more than 12 times the mass of all the heavier elements combined, its abundance is similar in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium; this helium-4 binding energy accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, the vast majority of, formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars. Helium is named for the Greek Titan of the Sun, Helios, it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson, Lieutenant John Herschel, was subsequently confirmed by French astronomer Jules Janssen.
Janssen is jointly credited with detecting the element along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868 while Lockyer observed it from Britain. Lockyer was the first to propose; the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, by far the largest supplier of the gas today. Liquid helium is used in cryogenics in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in airships; as with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice.
In scientific research, the behavior of the two fluid phases of helium-4 is important to researchers studying quantum mechanics and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. On Earth it is rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements, as the alpha particles emitted by such decays consist of helium-4 nuclei; this radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Terrestrial helium—a non-renewable resource, because once released into the atmosphere it escapes into space—was thought to be in short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases having been released by volcanic activity.
The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India; this line was assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 because it was near the known D1 and D2 Fraunhofer line lines of sodium, he concluded. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος. In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material, sublimated during a recent eruption of Mount Vesuvius. On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun.
These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, who collected enough of the gas to determine its atomic weight. Helium was isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, attributed the lines to nitrogen, his letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science. In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of
Deuterium is one of two stable isotopes of hydrogen. The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutron in the nucleus. Deuterium has a natural abundance in Earth's oceans of about one atom in 6420 of hydrogen, thus deuterium accounts for 0.0156% of all the occurring hydrogen in the oceans, while protium accounts for more than 99.98%. The abundance of deuterium changes from one kind of natural water to another; the deuterium isotope's name is formed from the Greek deuteros, meaning "second", to denote the two particles composing the nucleus. Deuterium was named in 1931 by Harold Urey; when the neutron was discovered in 1932, this made the nuclear structure of deuterium obvious, Urey won the Nobel Prize in 1934. Soon after deuterium's discovery and others produced samples of "heavy water" in which the deuterium content had been concentrated. Deuterium is destroyed in the interiors of stars faster. Other natural processes are thought to produce only an insignificant amount of deuterium.
Nearly all deuterium found in nature was produced in the Big Bang 13.8 billion years ago, as the basic or primordial ratio of hydrogen-1 to deuterium has its origin from that time. This is the ratio found in the gas giant planets, such as Jupiter. However, other astronomical bodies are found to have different ratios of deuterium to hydrogen-1; this is thought to be a result of natural isotope separation processes that occur from solar heating of ices in comets. Like the water cycle in Earth's weather, such heating processes may enrich deuterium with respect to protium; the analysis of deuterium/protium ratios in comets found results similar to the mean ratio in Earth's oceans. This reinforces theories; the deuterium/protium ratio of the comet 67P/Churyumov-Gerasimenko, as measured by the Rosetta space probe, is about three times that of earth water. This figure is the highest yet measured in a comet. Deuterium/protium ratios thus continue to be an active topic of research in both astronomy and climatology.
Deuterium is represented by the chemical symbol D. Since it is an isotope of hydrogen with mass number 2, it is represented by 2H. IUPAC allows 2H, although 2H is preferred. A distinct chemical symbol is used for convenience because of the isotope's common use in various scientific processes, its large mass difference with protium confers non-negligible chemical dissimilarities with protium-containing compounds, whereas the isotope weight ratios within other chemical elements are insignificant in this regard. In quantum mechanics the energy levels of electrons in atoms depend on the reduced mass of the system of electron and nucleus. For the hydrogen atom, the role of reduced mass is most seen in the Bohr model of the atom, where the reduced mass appears in a simple calculation of the Rydberg constant and Rydberg equation, but the reduced mass appears in the Schrödinger equation, the Dirac equation for calculating atomic energy levels; the reduced mass of the system in these equations is close to the mass of a single electron, but differs from it by a small amount about equal to the ratio of mass of the electron to the atomic nucleus.
For hydrogen, this amount is about 1837/1836, or 1.000545, for deuterium it is smaller: 3671/3670, or 1.0002725. The energies of spectroscopic lines for deuterium and light hydrogen therefore differ by the ratios of these two numbers, 1.000272. The wavelengths of all deuterium spectroscopic lines are shorter than the corresponding lines of light hydrogen, by a factor of 1.000272. In astronomical observation, this corresponds to a blue Doppler shift of 0.000272 times the speed of light, or 81.6 km/s. The differences are much more pronounced in vibrational spectroscopy such as infrared spectroscopy and Raman spectroscopy, in rotational spectra such as microwave spectroscopy because the reduced mass of the deuterium is markedly higher than that of protium. In nuclear magnetic resonance spectroscopy, deuterium has a different NMR frequency and is much less sensitive. Deuterated solvents are used in protium NMR to prevent the solvent from overlapping with the signal, although deuterium NMR on its own right is possible.
Deuterium is thought to have played an important role in setting the number and ratios of the elements that were formed in the Big Bang. Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at the point that the universe cooled enough to allow formation of nuclei; this calculation indicates seven protons for every neutron at the beginning of nucleogenesis, a ratio that would remain stable after nucleogenesis was over. This fraction was in favor of protons primarily because the lower mass of the proton favored their production; as the universe expanded, it cooled. Free neutrons and protons are less stable than helium nuclei, the protons and neutrons had a strong energetic reason to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. Through much of the few minutes after the big bang during which nucleosynthesis could have occurred
Hydrogen fluoride is a chemical compound with the chemical formula HF. This colorless gas or liquid is the principal industrial source of fluorine as an aqueous solution called hydrofluoric acid, it is an important feedstock in the preparation of many important compounds including pharmaceuticals and polymers. HF is used in the petrochemical industry as a component of superacids. Hydrogen fluoride boils near room temperature, much higher than other hydrogen halides. Hydrogen fluoride is a dangerous gas, forming corrosive and penetrating hydrofluoric acid upon contact with moisture; the gas can cause blindness by rapid destruction of the corneas. French chemist Edmond Frémy is credited with discovering anhydrous hydrogen fluoride while trying to isolate fluorine. Although Carl Wilhelm Scheele prepared hydrofluoric acid in large quantities in 1771, this acid was known in the glass industry before then. Although a diatomic molecule, HF forms strong intermolecular hydrogen bonds. Solid HF consists of zigzag chains of HF molecules.
The HF molecules, with a short H–F bond of 95 pm, are linked to neighboring molecules by intermolecular H–F distances of 155 pm. Liquid HF consists of chains of HF molecules, but the chains are shorter, consisting on average of only five or six molecules. Hydrogen fluoride does not boil until 20 °C in contrast to the heavier hydrogen halides which boil between −85 °C and −35 °C; this hydrogen bonding between HF molecules gives rise to high viscosity in the liquid phase and lower than expected pressure in the gas phase. Hydrogen fluoride is miscible with water, whereas the other hydrogen halides have large solubility gaps with water. Hydrogen fluoride and water form several compounds in the solid state, most notably a 1:1 compound that does not melt until −40 °C, 44 °C above the melting point of pure HF. Unlike other hydrohalic acids, such as hydrochloric acid, hydrogen fluoride is only a weak acid in dilute aqueous solution; this is in part a result of the strength of the hydrogen–fluorine bond, but of other factors such as the tendency of HF, H2O, F− anions to form clusters.
At high concentrations, HF molecules undergo homoassociation to form polyatomic ions and protons, thus increasing the acidity. This leads to protonation of strong acids like hydrochloric, sulfuric, or nitric when using concentrated hydrofluoric acid solutions. Although hydrofluoric acid is regarded as a weak acid, it is corrosive attacking glass when hydrated; the acidity of hydrofluoric acid solutions varies with concentration owing to hydrogen-bond interactions of the fluoride ion. Dilute solutions are weakly acidic with an acid ionization constant Ka = 6.6×10−4, in contrast to corresponding solutions of the other hydrogen halides, which are strong acids. Concentrated solutions of hydrogen fluoride are much more acidic than implied by this value, as shown by measurements of the Hammett acidity function H0; the H0 for 100% HF is estimated to be between −10.2 and −11, comparable to the value −12 for sulfuric acid. In thermodynamic terms, HF solutions are non-ideal, with the activity of HF increasing much more than its concentration.
The weak acidity in dilute solution is sometimes attributed to the high H—F bond strength, which combines with the high dissolution enthalpy of HF to outweigh the more negative enthalpy of hydration of the fluoride ion. However, Paul Giguère and Sylvia Turrell have shown by infrared spectroscopy that the predominant solute species is the hydrogen-bonded ion pair, which suggests that the ionization can be described as a pair of successive equilibria: The first equilibrium lies well to the right and the second to the left, meaning that HF is extensively dissociated, but that the tight ion pairs reduce the thermodynamic activity coefficient of H3O+, so that the solution is less acidic. In concentrated solution, the additional HF causes the ion pair to dissociate with formation of the hydrogen-bonded hydrogen difluoride ion. + HF ⇌ H3O+ + HF−2The increase in free H3O+ due to this reaction accounts for the rapid increase in acidity, while fluoride ions are stabilized by strong hydrogen bonding to HF to form HF−2.
This interaction between the acid and its own conjugate base is an example of homoassociation. At the limit of 100% liquid HF, there is self-ionization 3 HF ⇌ H2F+ + HF−2which forms an acidic solution; the acidity of anhydrous HF can be increased further by the addition of Lewis acids such as SbF5, which can reduce H0 to −21. Dry hydrogen fluoride dissolves low-valent metal fluorides, as well as several molecular fluorides. Many proteins and carbohydrates can be recovered from it. In contrast, most non-fluoride inorganic chemicals react with HF rather than dissolving. Hydrogen fluoride is produced by the action of sulfuric acid on pure grades of the mineral fluorite and as a side-product of the extraction of the fertilizer precursor phosphoric acid from various minerals. See hydrofluoric acid; the anhydrous compound hydrogen fluoride is more used than its aqueous solution, hydrofluoric acid. HF serves. A component of high-octane petrol called "alkylate" is generated in alkylation units that combine C3 and C4 olefins and iso-butane to generate petrol.
HF is a reactive solvent in the electrochemical fluorination of organic compounds. In this approach, HF is oxidized in the presence of a hydrocarbon and the fluorine replaces C–H bonds with C–F bonds. P