Spectroscopy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Spectroscopy 

A prism separates white light by dispersing it into its component colors, which can then be studied using spectroscopy.

Spectroscopy is the field of study that measures and interprets electromagnetic spectra as it interacts with matter. In narrower contexts, spectroscopy is the precise study of color as generalized from radiated visible light to all bands of the electromagnetic spectrum.

Spectroscopy, primarily in the electromagnetic spectrum, is a fundamental exploratory tool in the fields of astronomy, chemistry, materials science, and physics, allowing the composition, physical and electronic structure of matter to be investigated at the atomic, molecular and macro scale, and over astronomical distances.

Historically, spectroscopy originated as the study of the wavelength dependence of the absorption by gas phase matter of visible light dispersed by a prism. Current applications of spectroscopy include biomedical spectroscopy in the areas of tissue analysis and medical imaging. Matter waves and acoustic waves can be considered forms of radiative energy, and recently gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Introduction

Spectroscopy is a branch of science concerned with the spectra of electromagnetic radiation as a function of its wavelength or frequency, as measured by spectrographic equipment and other techniques, in order to obtain information concerning the structure and properties of matter. Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers. Most spectroscopic analysis in the laboratory starts with a sample to be analyzed. A light source is sent through a monochromator to spatially separate the colors before passing a selected frequency band through the sample, then the output is captured by a photodiode. For astronomical purposes, the telescope must be equipped with the light dispersion device. There are various versions of this basic setup that may be employed.

High resolution spectrum of the Sun, showing the discrete line pattern created by elements in the stellar atmosphere

Spectroscopy began with Isaac Newton splitting light with a prism; a key moment in the development of modern optics. Therefore, it was originally the study of visible light that we call color. Following the contributions of James Clerk Maxwell, this study later came to include the entire electromagnetic spectrum. Although color is involved in spectroscopy, it is not equivalent to the absorption and reflection of certain electromagnetic waves that give objects or elements a sense of color to our eyes. Rather, spectroscopy involves the splitting of light by a prism, diffraction grating, or similar instrument, to display a particular discrete line pattern called a "spectrum", which is unique for each different type of element or molecule. Most elements are first put into a gaseous state to allow the spectra to be examined, although today other methods can be used for different phases of matter. Each element that is diffracted by a prism-like instrument displays either an absorption spectrum or an emission spectrum depending upon whether the element is being cooled or heated.

Until recently all spectroscopy involved the study of line spectra and most spectroscopy still does. Vibrational spectroscopy is the branch of spectroscopy that studies the spectra, which are caused by vibrations of molecules. However, the latest developments in spectroscopy can sometimes dispense with the dispersion technique. In biochemical spectroscopy, information can be gathered about biological tissue by absorption and light scattering techniques. Light scattering spectroscopy is a type of reflectance spectroscopy that determines tissue structures by examining elastic scattering. In such a case, it is the tissue that acts as a diffraction or dispersion mechanism.

Spectroscopic studies were central to the development of quantum mechanics. The first useful quantum atomic models, including Bohr model, the Schrödinger equation, and Matrix mechanics, reproduced the spectral lines of hydrogen. These equated discrete quantum jumps of the bound electron in a hydrogen atom to the discrete hydrogen spectrum. Max Planck's explanation of blackbody radiation involved spectroscopy because he was comparing the wavelength of light using a photometer to the temperature of a Black Body. Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect, identify and quantify information about the atoms and molecules.

Spectroscopy is used in astronomy and remote sensing on Earth. Most research telescopes have spectrographs. The measured spectra are used to determine the chemical composition and physical properties of astronomical objects, such as their temperature, elemental abundances, velocity, rotation, magnetic field, and more. An important use for spectroscopy is in biochemistry. Molecular samples may be analyzed for species identification and energy content.

Theory

The underlying premise of spectroscopy is that light is made of different wavelengths and that each wavelength corresponds to a different frequency. The importance of spectroscopy is centered around the fact that every element in the periodic table has a unique light spectrum described by the frequencies of light it emits or absorbs consistently appearing in the same part of the electromagnetic spectrum when that light is diffracted. This opened up an entire field of study with anything that contains atoms. Spectroscopy is the key to understanding the atomic properties of all matter. As such spectroscopy opened up many new sub-fields of science yet undiscovered. The idea that each atomic element has its unique spectral signature enabled spectroscopy to be used in a broad number of fields each with a specific goal achieved by different spectroscopic procedures. The National Institute of Standards and Technology maintains a public Atomic Spectra Database that is continually updated with precise measurements.

With an absorption spectrophotometer, the level of absorption of a light source is determined by the Beer-Lambert Law: $${\displaystyle \ln \left(\frac{I_0}{I}\right)=\epsilon \ell c}$$ where ${\displaystyle I_0}$ is the light intensity before passing through the sample, ${\displaystyle I}$ is the output intensity, ${\displaystyle \epsilon }$ is the extinction coefficient, ${\displaystyle \ell }$ is the path length through the sample, and ${\displaystyle c}$ is the concentration of the sample. The extinction coefficient depends on the wavelength selected and the molecule being sampled.

Resonances by the frequency were first characterized in mechanical systems such as pendulums, which have a frequency of motion noted famously by Galileo. In quantum mechanical systems, the analogous resonance is a coupling of two quantum mechanical stationary states of a system, such as two atomic orbitals, via an oscillatory source of energy such as a photon. The coupling of the two states is strongest when the source energy matches the energy difference between the two states. That is, a photon at the right energy is more likely to cause an electron to jump between two orbitals, a process called electron excitation. The energy E of a photon is related to its frequency ν by E = hν where h is the Planck constant, and so a spectrum of the system response vs. photon frequency will peak at the resonant frequency or energy.

Any part of the electromagnetic spectrum may be used to analyze a sample from the infrared to the ultraviolet telling scientists different properties about the very same sample, a discovery that led to a broadening of the field of spectroscopy. For instance in chemical analysis, the most common types of spectroscopy include atomic spectroscopy, infrared spectroscopy, ultraviolet and visible spectroscopy, Raman spectroscopy and nuclear magnetic resonance. In nuclear magnetic resonance (NMR), the theory behind it is that frequency is analogous to resonance and its corresponding resonant frequency.

Classification of methods

A huge diffraction grating at the heart of the ultra-precise ESPRESSO spectrograph

Spectroscopy is a sufficiently broad field that many sub-disciplines exist, each with numerous implementations of specific spectroscopic techniques. The various implementations and techniques can be classified in several ways.

Type of radiative energy

The types of spectroscopy are distinguished by the type of radiative energy involved in the interaction. In many applications, the spectrum is determined by measuring changes in the intensity or frequency of this energy. The types of radiative energy studied include:

• Electromagnetic radiation was the first source of energy used for spectroscopic studies. Techniques that employ electromagnetic radiation are typically classified by the wavelength region of the spectrum and include microwave, terahertz, infrared, near-infrared, ultraviolet-visible, X-ray, and gamma spectroscopy.
• Particles, because of their de Broglie waves, can be a source of radiative energy. Both electron and neutron spectroscopy are used. For a particle, its kinetic energy determines its wavelength.
• Acoustic spectroscopy involves radiated pressure waves.
• Dynamic mechanical analysis can be employed to impart radiating energy, similar to acoustic waves, to solid materials.

Nature of the interaction

The types of spectroscopy can be distinguished by the nature of the interaction between the energy and the material. These interactions include:

• Absorption spectroscopy: Absorption occurs when energy from the radiative source is absorbed by the material. Absorption is often determined by measuring the fraction of energy transmitted through the material, with absorption decreasing the transmitted portion.
• Emission spectroscopy: Emission indicates that radiative energy is released by the material. A material's blackbody spectrum is a spontaneous emission spectrum determined by its temperature. This feature can be measured in the infrared by instruments such as the atmospheric emitted radiance interferometer. Emission can be induced by other sources of energy such as flames, sparks, electric arcs or electromagnetic radiation in the case of fluorescence.
• Elastic scattering and reflection spectroscopy determine how incident radiation is reflected or scattered by a material. Crystallography employs the scattering of high energy radiation, such as X-rays and electrons, to examine the arrangement of atoms in proteins and solid crystals.
• Impedance spectroscopy, where impedance is the ability of a medium to impede or slow the transmittance of energy. For optical applications, this is characterized by the index of refraction.
• Inelastic scattering phenomena involve an exchange of energy between X-ray radiation and the matter that shifts the wavelength of the scattered radiation. These include Raman and Compton scattering.
• Coherent or resonance spectroscopy are techniques where the radiative energy couples two quantum states of the material in a coherent interaction that is sustained by the radiating field. The coherence can be disrupted by other interactions, such as particle collisions and energy transfer, and so often requires high intensity radiation to be sustained. Nuclear magnetic resonance (NMR) spectroscopy is a widely used resonance method, and ultrafast laser spectroscopy is possible in the infrared and visible spectral regions.
• Nuclear spectroscopy are methods that use the properties of specific nuclei to probe the local structure in matter, mainly condensed matter, molecules in liquids or frozen liquids and bio-molecules.
• Quantum logic spectroscopy is a general technique used in ion traps that enables precision spectroscopy of ions with internal structures that preclude laser cooling, state manipulation, and detection. Quantum logic operations enable a controllable ion to exchange information with a co-trapped ion that has a complex or unknown electronic structure.

Type of material

Spectroscopic studies are designed so that the radiant energy interacts with specific types of matter. These studies can be divided into three broad categories: electronic spectroscopy, which measures the transition of electrons between different energy states through absorption or emission of visible or ultraviolet energy; vibronic spectroscopy of molecules induced by the absorption of infrared energy; and rotational spectroscopy of molecules caused by microwave energy. The last two can be combined into rotational–vibrational spectroscopy of a gas.

Atoms

Atomic spectra comparison table, from "Spektroskopische Methoden der analytischen Chemie" (1922)

Atomic spectroscopy was the first application of spectroscopy. Atomic absorption spectroscopy and atomic emission spectroscopy involve visible and ultraviolet light. These absorptions and emissions, often referred to as atomic spectral lines, are due to electronic transitions of outer shell electrons as they rise and fall from one electron orbit to another. Atoms have distinct X-ray spectra that are attributable to the excitation of inner shell electrons to excited states.

Atoms of different elements have distinct spectra and therefore atomic spectroscopy allows for the identification and quantitation of a sample's elemental composition. After Robert Bunsen and Gustav Kirchhoff invented the spectroscope, Bunsen discovered cesium and rubidium by observing their emission spectra. Atomic absorption lines are observed in the solar spectrum and referred to as Fraunhofer lines after their discoverer. A comprehensive explanation of the hydrogen spectrum was an early success of quantum mechanics and explained the Lamb shift observed in the hydrogen spectrum, which further led to the development of quantum electrodynamics.

Modern implementations of atomic spectroscopy for studying visible and ultraviolet transitions include flame emission spectroscopy, inductively coupled plasma atomic emission spectroscopy, glow discharge spectroscopy, microwave induced plasma spectroscopy, and spark or arc emission spectroscopy. Techniques for studying X-ray spectra include X-ray spectroscopy and X-ray fluorescence.

Molecules

The combination of atoms into molecules leads to the creation of unique types of energetic states and therefore unique spectra of the transitions between these states. Molecular spectra can be obtained due to electron spin states (electron paramagnetic resonance), molecular rotations, molecular vibration, and electronic states. Rotations are collective motions of the atomic nuclei and typically lead to spectra in the microwave and millimetre-wave spectral regions. Rotational spectroscopy and microwave spectroscopy are synonymous. Vibrations are relative motions of the atomic nuclei and are studied by both infrared and Raman spectroscopy. Electronic excitations are studied using visible and ultraviolet spectroscopy as well as fluorescence spectroscopy.

Studies in molecular spectroscopy led to the development of the first maser and contributed to the subsequent development of the laser.

Crystals and extended materials

The combination of atoms or molecules into crystals or other extended forms leads to the creation of additional energetic states. These states are numerous and therefore have a high density of states. This high density often makes the spectra weaker and less distinct, i.e., broader. For instance, blackbody radiation is due to the thermal motions of atoms and molecules within a material. Acoustic and mechanical responses are due to collective motions as well. Pure crystals, though, can have distinct spectral transitions, and the crystal arrangement has an effect on the observed molecular spectra. The regular lattice structure of crystals scatters X-rays, electrons, or neutrons, allowing for crystallographic studies.

Nuclei

Nuclei have distinct energy states that are widely separated and lead to gamma ray spectra. Distinct nuclear spin states can have their energy separated by a magnetic field, and this allows for nuclear magnetic resonance spectroscopy.

Other types

Other types of spectroscopy are distinguished by specific applications or implementations:

• Acoustic resonance spectroscopy is based on sound waves primarily in the audible and ultrasonic regions.
• Auger electron spectroscopy is a method used to study surfaces of materials on a micro-scale. It is often used in connection with electron microscopy.
• Cavity ring-down spectroscopy enables measurement of absolute optical extinction by samples that scatter and absorb light.
• Circular dichroism spectroscopy measures the differential absorption of left- and right-handed circularly polarized light.
• Coherent anti-Stokes Raman spectroscopy is a recent technique that has high sensitivity and powerful applications for in vivo spectroscopy and imaging.
• Cold vapour atomic fluorescence spectroscopy is a subclass of the atomic emission spectroscopy technique that measures trace amounts of volatile heavy metals in the air, such as mercury.
• Correlation spectroscopy encompasses several types of two-dimensional NMR spectroscopy.
• Deep-level transient spectroscopy measures concentration and analyzes parameters of electrically active defects in semiconducting materials.
• Dielectric spectroscopy measures the dielectric properties of a medium as a function of frequency.
• Dual-polarization interferometry measures the real and imaginary components of the complex refractive index.
• Electron energy loss spectroscopy in transmission electron microscopy.
• Electron phenomenological spectroscopy measures the physicochemical properties and characteristics of the electronic structure of multicomponent and complex molecular systems.
• Electron paramagnetic resonance spectroscopy is similar to nuclear magnetic resonance (NMR), except it measures the spin excitement of unpaired electrons.
• Force spectroscopy is a set of techniques for the study of the interactions and the binding forces between individual molecules, although the name is somewhat misleading because there is no true matter-radiation interaction.
• Fourier-transform spectroscopy is an efficient method for processing spectra data obtained using interferometers. Fourier-transform infrared spectroscopy is an implementation of infrared spectroscopy. NMR also employs Fourier transforms.
• Gamma spectroscopy measures the gamma ray emissions from high energy processes including radioactive and astrophysical sources.
• Hadron spectroscopy studies the energy/mass spectrum of hadrons according to spin, parity, and other particle properties. Baryon spectroscopy and meson spectroscopy are types of hadron spectroscopy.
• Multispectral imaging and hyperspectral imaging is a method to create a complete picture of the environment or various objects, each pixel containing a full visible, visible near infrared, near infrared, or infrared spectrum.
• Inelastic electron tunneling spectroscopy uses the changes in current due to inelastic electron-vibration interaction at specific energies that can also measure optically forbidden transitions.
• Inelastic neutron scattering is similar to Raman spectroscopy, but uses neutrons instead of photons.
• Laser-induced breakdown spectroscopy, also called laser-induced plasma spectrometry, is a type of atomic emission spectroscopy which uses a highly energetic laser pulse as the excitation source
• Laser spectroscopy uses tunable lasers and other types of coherent emission sources, such as optical parametric oscillators, for selective excitation of atomic or molecular species.
• Light scattering spectroscopy (LSS) is a spectroscopic technique typically used to evaluate morphological changes in epithelial cells in order to study mucosal tissue and detect early cancer and precancer.
• Mass spectroscopy is a historical term used to refer to mass spectrometry. The current recommendation is to use the latter term. The term "mass spectroscopy" originated in the use of phosphor screens to detect ions.
• Mössbauer spectroscopy probes the properties of specific isotopic nuclei in different atomic environments by analyzing the resonant absorption of gamma rays. See Mössbauer effect.
• Neutron spin echo spectroscopy measures internal dynamics in proteins and other soft matter systems.
• Nuclear quadrupole resonance is a chemical spectroscopy method mediated by NMR of the electric field gradient (EFG) in the absence of magnetic field
• Perturbed angular correlation (PAC) uses radioactive nuclei as probe to study electric and magnetic fields (hyperfine interactions) in crystals (condensed matter) and bio-molecules.^{[citation needed]}
• Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy on matter by means of acoustic detection.
• Acoustic emission spectroscopy is the measurement of acoustic waves as a material is deformed.
• Photoemission spectroscopy measures the energy or spin of electrons emitted from materials by the photoelectric effect.
  • X-ray photoelectron spectroscopy is a surface analysis technique that uses a beam of X-rays to perform photoemission spectroscopy
• Photothermal spectroscopy measures heat evolved upon absorption of radiation.
• Pump-probe spectroscopy can use ultrafast laser pulses to measure reaction intermediates in the femtosecond timescale.
• Raman optical activity spectroscopy exploits Raman scattering and optical activity effects to reveal detailed information on chiral centers in molecules.
• Raman spectroscopy is used to determine vibrational modes of molecules, providing a structural fingerprint by which molecules can be identified.
• Saturated spectroscopy uses a dual laser technique to remove the Doppler profiles from spectroscopic signals of hot atoms. It does so by measuring the spectral signal of atoms moving perpendicular to the laser.
• Scanning tunneling spectroscopy uses a scanning tunneling microscope to measure the current–voltage characteristics of a surface and build a map of the electronic structure.
• Spectrophotometry combines a spectrometer with a photometer to measure the energy intensity at different wavelengths. This can be used to determine the reflection or transmission properties of a substance.
• Spin noise spectroscopy traces spontaneous fluctuations of electronic and nuclear spins.
• Time-resolved spectroscopy measures the decay rates of excited states using various spectroscopic methods.
• Time-stretch spectroscopy
• Thermal infrared spectroscopy measures thermal radiation emitted from materials and surfaces and is used to determine the type of bonds present in a sample as well as their lattice environment. The techniques are widely used by organic chemists, mineralogists, and planetary scientists.
• Transient grating spectroscopy measures quasiparticle propagation. It can track changes in metallic materials as they are irradiated.
• Ultraviolet photoelectron spectroscopy
• Ultraviolet–visible spectroscopy
• Vibrational circular dichroism spectroscopy
• Video spectroscopy

Applications

UVES is a high-resolution spectrograph on the Very Large Telescope.

There are several applications of spectroscopy in the fields of medicine, physics, chemistry, and astronomy. Taking advantage of the properties of absorbance and, with astronomy, emission, spectroscopy can be used to identify certain states of nature. The uses of spectroscopy in so many different scientific fields and for so many different applications has led to the creation of specialized subfields. Such examples include:

• Determining the atomic structure of a sample
• Studying spectral emission lines of the sun and distant galaxies
• Space exploration
• Cure monitoring of composites using optical fibers.
• Estimating weathered wood exposure times using near infrared spectroscopy
• Measurement of different compounds in food samples by absorption spectroscopy both in visible and infrared spectrum
• Measurement of toxic compounds in blood samples
• Non-destructive elemental analysis by X-ray fluorescence
• Electronic structure research with various spectroscopes
• Redshift to determine the speed and velocity of a distant object
• Determining the metabolic structure of a muscle
• Monitoring dissolved oxygen content in freshwater and marine ecosystems
• Altering the structure of drugs to improve effectiveness
• Characterization of proteins
• Respiratory gas analysis in hospitals
• Finding the physical properties of a distant star or nearby exoplanet using the Relativistic Doppler effect.
• In-ovo sexing: spectroscopy allows to determine the sex of the egg while it is hatching. Developed by French and German companies, both countries decided to ban chick culling, mostly done through a macerator, in 2022.
• Process monitoring in Industrial process control

History

Main article: History of spectroscopy

The history of spectroscopy began with Isaac Newton's optics experiments (1666–1672). According to Andrew Fraknoi and David Morrison, "In 1672, in the first paper that he submitted to the Royal Society, Isaac Newton described an experiment in which he permitted sunlight to pass through a small hole and then through a prism. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow." Newton applied the word "spectrum" to describe the rainbow of colors that combine to form white light and that are revealed when the white light is passed through a prism.

Fraknoi and Morrison state that "In 1802, William Hyde Wollaston built an improved spectrometer that included a lens to focus the Sun's spectrum on a screen. Upon use, Wollaston realized that the colors were not spread uniformly, but instead had missing patches of colors, which appeared as dark bands in the spectrum." During the early 1800s, Joseph von Fraunhofer made experimental advances with dispersive spectrometers that enabled spectroscopy to become a more precise and quantitative scientific technique. Since then, spectroscopy has played and continues to play a significant role in chemistry, physics, and astronomy. Per Fraknoi and Morrison, "Later, in 1815, German physicist Joseph Fraunhofer examined the solar spectrum, and found about 600 such dark lines (missing colors), are now known as Fraunhofer lines, or Absorption lines."

Spectra of atoms and molecules often consist of a series of spectral lines, each one representing a resonance between two different quantum states. The explanation of these series, and the spectral patterns associated with them, were one of the experimental enigmas that drove the development and acceptance of quantum mechanics. The hydrogen spectral series in particular was first successfully explained by the Rutherford–Bohr quantum model of the hydrogen atom. In some cases spectral lines are well separated and distinguishable, but spectral lines can overlap and appear to be a single transition if the density of energy states is high enough. Named series of lines include the principal, sharp, diffuse and fundamental series.

Hobbyist

Spectroscopy has emerged as a growing practice within the maker movement, enabling hobbyists and educators to construct functional spectrometers using readily available materials. Utilizing components like CD/DVD diffraction gratings, smartphones, and 3D-printed parts, these instruments offer a hands-on approach to understanding light and matter interactions. Smartphone applications along with open-source tools facilitate integration, greatly simplify the capturing and analysis of spectral data. While limitations in resolution, calibration accuracy, and stray light management exist compared to professional equipment, DIY spectroscopy provides valuable educational experiences and contributes to citizen science initiatives, fostering accessibility to spectroscopic techniques.

Carbon-dioxide laser

From Wikipedia, the free encyclopedia

(Redirected from Carbon dioxide laser)

A test target bursts into flame upon irradiation by a continuous-wave kilowatt-level carbon-dioxide laser.

The carbon-dioxide laser (CO₂ laser) was one of the earliest gas lasers to be developed. It was invented by Kumar Patel of Bell Labs in 1964 and is still one of the most useful types of laser. Carbon-dioxide lasers are the highest-power continuous-wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO₂ laser produces a beam of infrared light with the principal wavelength bands centering on 9.6 and 10.6 micrometers (μm).

Amplification

The active laser medium (laser gain/amplification medium) is a gas discharge which is air- or water-cooled, depending on the power being applied. The filling gas within a sealed discharge tube consists of around 10–20% carbon dioxide (CO
₂), around 10–20% nitrogen (N
₂), a few percent hydrogen (H
₂) and/or xenon (Xe), with the remainder being helium (He). A different mixture is used in a flow-through laser, where CO
₂ is continuously pumped through it. The specific proportions vary according to the particular laser.

The population inversion in the laser is achieved by the following sequence: electron impact excites the {v1(1)} quantum vibrational modes of nitrogen. Because nitrogen is a homonuclear molecule, it cannot lose this energy by photon emission, and its excited vibrational modes are therefore metastable and relatively long-lived. N
₂{v1(1)} and CO
₂{v3(1)} being nearly perfectly resonant (total molecular energy differential is within 3 cm⁻¹ when accounting for N
₂ anharmonicity, centrifugal distortion and vibro-rotational interaction, which is more than made up for by the Maxwell speed distribution of translational-mode energy), N
₂ collisionally de-excites by transferring its vibrational mode energy to the CO₂ molecule, causing the carbon dioxide to excite to its {v3(1)} (asymmetric stretch) vibrational mode quantum state. The CO
₂ then radiatively emits at either 10.6 μm by dropping to the {v1(1)} (symmetric-stretch) vibrational mode, or 9.6 μm by dropping to the {v20(2)} (bending) vibrational mode. The carbon dioxide molecules then transition to their {v20(0)} vibrational mode ground state from {v1(1)} or {v20(2)} by collision with cold helium atoms, thus maintaining population inversion. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the laser discharge tube. In flow-through lasers, a continuous stream of CO₂ and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by pumps.

The addition of helium also plays a role in the initial vibrational excitation of N
₂, due to a near-resonant dissociation reaction with metastable He(2³S₁). Substituting helium with other noble gases, such as neon or argon, does not lead to an enhancement of laser output.

Because the excitation energy of molecular vibrational and rotational mode quantum states are low, the photons emitted due to transition between these quantum states have comparatively lower energy, and longer wavelength, than visible and near-infrared light. The 9–12 μm wavelength of CO₂ lasers is useful because it falls into an important window for atmospheric transmission (up to 80% atmospheric transmission at this wavelength), and because many natural and synthetic materials have strong characteristic absorption in this range.

The laser wavelength can be tuned by altering the isotopic ratio of the carbon and oxygen atoms comprising the CO
₂ molecules in the discharge tube.

Construction

See also: Laser construction

Because CO₂ lasers operate in the infrared, special materials are necessary for their construction. Typically, the mirrors are silvered, while windows and lenses are made of either germanium or zinc selenide. For high power applications, gold mirrors and zinc selenide windows and lenses are preferred. There are also diamond windows and lenses in use. Diamond windows are extremely expensive, but their high thermal conductivity and hardness make them useful in high-power applications and in dirty environments. Optical elements made of diamond can even be sand blasted without losing their optical properties. Historically, lenses and windows were made out of salt (either sodium chloride or potassium chloride). While the material was inexpensive, the lenses and windows degraded slowly with exposure to atmospheric moisture.

The most basic form of a CO₂ laser consists of a gas discharge (with a mix close to that specified above) with a total reflector at one end, and an output coupler (a partially reflecting mirror) at the output end.

The CO₂ laser can be constructed to have continuous wave (CW) powers between milliwatts (mW) and hundreds of kilowatts (kW). It is also very easy to actively Q-switch a CO₂ laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers of up to gigawatts (GW).

Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule, the rotational structure of the P and R bands can be selected by a tuning element in the laser cavity. Prisms are not practical as tuning elements because most media that transmit in the mid-infrared absorb or scatter some of the light, so the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected. The finest frequency selection may also be obtained through the use of an etalon. In practice, together with isotopic substitution, this means that a continuous comb of frequencies separated by around 1 cm⁻¹ (30 GHz) can be used that extend from 880 to 1090 cm⁻¹. Such "line-tuneable" carbon-dioxide lasers are principally of interest in research applications. The laser's output wavelength is affected by the particular isotopes contained in the carbon dioxide molecule, with heavier isotopes causing longer wavelength emission.

Applications

A medical CO₂ laser

Industrial (cutting and welding)

Because of the high power levels available (combined with reasonable cost for the laser), CO₂ lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. In selective laser sintering, CO₂ lasers are used to fuse particles of plastic powder into parts.

Medical (soft-tissue surgery)

Carbon-dioxide lasers have become useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery and skin resurfacing ("laser facelifts", which essentially consist of vaporizing the skin to promote collagen formation). CO₂ lasers may be used to treat certain skin conditions such as hirsuties papillaris genitalis by removing bumps or podules. CO₂ lasers can be used to remove vocal-fold lesions, such as vocal-fold cysts. Researchers in Israel are experimenting with using CO₂ lasers to weld human tissue, as an alternative to traditional sutures.

The 10.6 μm CO₂ laser remains the best surgical laser for the soft tissue where both cutting and hemostasis are achieved photo-thermally (radiantly). CO₂ lasers can be used in place of a scalpel for most procedures and are even used in places a scalpel would not be used, in delicate areas where mechanical trauma could damage the surgical site. CO₂ lasers are the best suited for soft-tissue procedures in human and animal specialties, as compared to laser with other wavelengths. Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications include gynecology, dentistry, oral and maxillofacial surgery, and many others.^{[medical citation needed]} In veterinary medicine, 10.6 μm CO₂ lasers are utilized for a variety of soft-tissue procedures, including onychectomy, neutering, spaying, oncological tumor removals, and specialized ophthalmic, aural, and dermatological surgeries.

A CO₂ dental laser at the 9.25–9.6 μm wavelength is sometimes used in dentistry for hard-tissue ablation. The hard-tissue is ablated at temperatures as high as 5,000 °C, producing bright thermal radiation.

Other

The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 μm wavelength band, so CO₂ lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers.

Because the atmosphere is quite transparent to infrared light, CO₂ lasers are also used for military rangefinding using LIDAR techniques.

CO₂ lasers are used in spectroscopy and the Silex process to enrich uranium.

In semiconductor manufacturing, CO₂ lasers are used for extreme ultraviolet generation.

The Soviet Polyus was designed to use a megawatt carbon-dioxide laser as an in-orbit weapon to destroy SDI satellites.

Objective (optics)

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Objective_(optics)

Several objective lenses on a microscope.

Objective lenses of binoculars

In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, binoculars, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.

Microscope objectives

Two Leica oil immersion microscope objective lenses; left 100×, right 40×.

The objective lens of a microscope is the one at the bottom near the sample. At its simplest, it is a very high-powered magnifying glass, with very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube. The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample.

Magnification

One of the most important properties of microscope objectives is their magnification. The magnification typically ranges from 4× to 100×. It is combined with the magnification of the eyepiece to determine the overall magnification of the microscope; a 4× objective with a 10× eyepiece produces an image that is 40 times the size of the object.

A typical microscope has three or four objective lenses with different magnifications, screwed into a circular "nosepiece" which may be rotated to select the required lens. These lenses are often color coded for easier use. The least powerful lens is called the scanning objective lens, and is typically a 4× objective. The second lens is referred to as the small objective lens and is typically a 10× lens. The most powerful lens out of the three is referred to as the large objective lens and is typically 40–100×.

Numerical aperture

Numerical aperture for microscope lenses typically ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 mm to 2 mm, respectively.

Mechanical tube length

Historically, microscopes were designed such that the objective lens would form an image in a specific plane near the eyepiece, which the eyepiece would re-image. Such microscopes were characterized by the mechanical tube length; the distance between the mounting locations for the objective and the eyepiece. Early English microscopes used a mechanical tube length of 10 inches (250 mm). In the 20th century most microscopes used the Royal Microscopical Society standard of 160 millimeters, while many Leitz microscopes used 170 millimeters. Objectives had to be chosen to match the mechanical tube length of the microscope.

Modern microscopes are often designed to use infinity correction, in which the light coming out of the objective lens is focused at infinity. This is denoted on the objective with the infinity symbol (∞).

Objective pupil diameter

The objective pupil diameter, also known as entrance pupil diameter or back aperture diameter, refers to the diameter of the rear opening of an objective lens. In dry infinity corrected objectives, this diameter ${\displaystyle D}$ is

${\displaystyle D=2\times NA\times f_{obj}}$

where ${\displaystyle NA}$ is the numerical aperture, and ${\displaystyle f_{obj}}$ is the effective focal length. Magnification ${\displaystyle M}$ and effective focal length are related by

${\displaystyle f_{tube}=M{f}_{obj}}$

where ${\displaystyle f_{tube}}$ is the tube lens focal length. Tube lens focal lengths vary by manufacturer: Leica and Nikon typically use 200 mm, Olympus uses 180 mm, and Zeiss uses 165 mm.

Cover thickness

Particularly in biological applications, samples are usually observed under a glass cover slip, which introduces distortions to the image. Objectives which are designed to be used with such cover slips will correct for these distortions, and typically have the thickness of the cover slip they are designed to work with written on the side of the objective (typically 0.17 mm).

In contrast, so called "metallurgical" objectives are designed for reflected light and do not use glass cover slips.

The distinction between objectives designed for use with or without cover slides is important for high numerical aperture (high magnification) lenses, but makes little difference for low magnification objectives.

Lens design

Basic glass lenses will typically result in significant and unacceptable chromatic aberration. Therefore, most objectives have some kind of correction to allow multiple colors to focus at the same point. The easiest correction is an achromatic lens, which uses a combination of crown glass and flint glass to bring two colors into focus. Achromatic objectives are a typical standard design.

In addition to oxide glasses, fluorite lenses are often used in specialty applications. These fluorite or semi-apochromat objectives deal with color better than achromatic objectives. To reduce aberration even further, more complex designs such as apochromat and superachromat objectives are also used.

All these types of objectives will exhibit some spherical aberration. While the center of the image will be in focus, the edges will be slightly blurry. When this aberration is corrected, the objective is called a "plan" objective, and has a flat image across the field of view.

Working distance

The working distance (sometimes abbreviated WD) is the distance between the sample and the objective. As magnification increases, working distances generally shrinks. When space is needed, special long working distance objectives can be used.

Immersion lenses

Some microscopes use an oil-immersion or water-immersion lens, which can have magnification greater than 100, and numerical aperture greater than 1. These objectives are specially designed for use with refractive index matching oil or water, which must fill the gap between the front element and the object. These lenses give greater resolution at high magnification. Numerical apertures as high as 1.6 can be achieved with oil immersion.

Mounting threads

The traditional screw thread used to attach the objective to the microscope was standardized by the Royal Microscopical Society in 1858. It was based on the British Standard Whitworth, with a 0.8 inch diameter and 36 threads per inch. This "RMS thread" or "society thread" is still in common use today. Alternatively, some objective manufacturers use designs based on ISO metric screw thread such as M26 × 0.75 and M25 × 0.75.

Photography and imaging

Camera photographic objective, focal length 50 mm, aperture 1:1.4

Camera lenses (usually referred to as "photographic objectives" instead of simply "objectives") need to cover a large focal plane so are made up of a number of optical lens elements to correct optical aberrations. Image projectors (such as video, movie, and slide projectors) use objective lenses that simply reverse the function of a camera lens, with lenses designed to cover a large image plane and project it at a distance onto another surface.

Telescopes

The segmented hexagonal objective mirror of the Keck 2 Telescope

In a telescope the objective is the lens at the front end of a refracting telescope (such as binoculars or telescopic sights) or the image-forming primary mirror of a reflecting or catadioptric telescope. A telescope's light-gathering power and angular resolution are both directly related to the diameter (or "aperture") of its objective lens or mirror. The larger the objective, the brighter the objects will appear and the more detail it can resolve.