UV Chapter 4 Photon

Photons

Photons absorbed in a semiconductor release their energies to electrons in the valence band, causing them to rise to the conduction band.

From: Solar Energy Conversion, 1979

Related terms:

Energy Engineering

Semiconductor

Solar Energy

Solar Cells

Conduction Band

Wavelength

Photon Energy

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From Nuclear Fusion to Sunlight

Alexander P. Kirk, in Solar Photovoltaic Cells, 2015

2.15 Photon flux

Photon flux will become important later in Chapter 3 when calculating photogenerated current density in solar photovoltaic cells. Under a clear sky AM1.5G terrestrial spectrum, there are some 1017 photons irradiating a 1 cm2 Sun-facing surface area every second. Spectral photon flux φ(λ) is calculated from the spectral irradiance Iλ (W m–2 nm–1) through the following relationship:

(2.13)φ(λ)=Iλ/E,

where E = hc/λ and h is Planck's constant. The AM1.5G spectral photon flux is shown in Figure 2.7, and the integrated photon flux is shown in Figure 2.8.

Fig. 2.7. ASTM G173 AM1.5G spectral photon flux (plotted out to the full 4000 nm cutoff).

Fig. 2.8. ASTM G173 AM1.5G integrated photon flux (plotted out to the full 4000 nm cutoff).

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Quantum Computing and Communication

Paul E. Black, ... Carl J. Williams, in Advances in Computers, 2002

6.2.5 Photon

Photons are clearly the best way to transmit information, since they move at the speed of light and do not strongly interact with their environment. This near-perfect characteristic for quantum communication makes photons problematic for quantum computation. In fact, early approaches to using photons for quantum computation suffered from a requirement of exponential numbers of optical elements and resources as one scaled the system. A second problem was that creating conditional logic for two-qubit gates appeared very difficult since two photons do not interact strongly even in highly nonlinear materials. In fact, most nonlinear phenomena involving light fields result only at high intensity.

Recently, new approaches for doing quantum computation with photons that depend on using measurement in a "dual-rail" approach to create entanglement have appeared. This approach removes many of the constraints of early approaches, but provides an alternative approach to creating quantum logic. Experimental efforts using this approach are just beginning. The approach will still have to solve the technically challenging problems caused by high-speed motion of their qubits, a benefit in communication and a possible benefit in computational speed, and by the lack of highly efficient, single-photon detectors essential to the success of this approach.

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Simulation of biomedical signals and images using Monte Carlo methods for training of deep learning networks

Navid Mavaddat, ... Kamal Alameh, in Deep Learning Techniques for Biomedical and Health Informatics, 2020

Free path

The free path (sometimes referred to as the photon mean free path) is the average distance a photon travels in a particular direction before it collides with another particle causing it to change direction. In a vacuum, this distance may be extremely long, but within a turbid material using near infrared (NIR) light, this distance is often less than 100 μm [44].

Modeling individual photons is infeasible, due to the huge number of photons emitted by a typical OCT light source. To address this issue the process can be simplified by considering large groups of photons as "photon packets" with an associated total energy or "weight." For some interactions, there is a probability that individual photons might be absorbed. This interaction that results in photon absorption can be modeled as a proportional loss of packet weight, and simulation can then be continued with the remaining photons. The free path covered by a photon packet between the points of interaction within a medium is determined by the following equation [41]:

(9.3)s=−lnξμa+μs

where

s is the free distance covered by a photon package.

μs is the scattering coefficient.

μa is the absorption coefficient.

ξ is discrete character energy absorption—a random number uniformly distributed between 0 and 1.

As the free path is traversed, photons are absorbed by the material it passes through, and to model this, the change in weight of the photon packet can be calculated by the following equation [43]:

(9.4)ΔW=Wμaμa+μs

where

ΔW is the incremental change in the statistical weight of the photon packet at each point.

W is the statistical weight of the photon packet.

μs is the scattering coefficient.

μa is the absorption coefficient.

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Dosimetry

J.W. PostonSr., in Encyclopedia of Physical Science and Technology (Third Edition), 2003

III.C X and Gamma Radiation

Photons interact with matter through three primary mechanisms: the photoelectric effect, Compton scattering, and pair production. The probability of each of these interactions occurring depends on the energy of the radiation and the material through which it is passing.

The photoelectric effect occurs primarily at low photon energies and in high–atomic-number (Z) materials. This interaction should be considered to occur with the entire atom even though the energy transfer is between the photon and an orbital electron. In this interaction a photon strikes a tightly bound electron and transfers its entire energy to the electron. If this energy is greater than the binding-energy of the electron to the atom, then the electron will be knocked out of the atom. The electron (a photoelectron) may possess kinetic energy as a result of this interaction. This energy is the difference between the initial energy of the photon and the binding energy of the electron.

Photoelectric interactions are most probable with the most tightly bound electrons (K shell), and the loss of an electron from the inner shell(s) leaves a vacancy that must be filled. An electron from a higher orbit will drop into the vacancy, but it in turn leaves another vacancy. There is in effect a cascading of electrons as they drop into lower energy states to fill the existing vacancies. As each electron fills a vacancy, a photon is emitted whose energy is equal to the difference between the initial and final energy levels. These photons are called characteristic X-rays because the energy differences between the electron orbits are unique for an atom and the photons are characteristic of the element from which they originate.

As stated previously, photoelectric interactions are most probable at low photon energies. The interaction is relatively unimportant for photons with energies >1 MeV, except in very heavy elements.

Compton scattering is an interaction that occurs between a photon and an essentially "free" electron. That is, the electron is in one of the outer orbits and its binding energy is significantly less than the energy of the photon. In Compton scattering, the requirements for the conservation of momentum and energy make it impossible for complete transfer of the photon energy to the electron. Basically, the photon has a collision with the electron and transfers only a portion of its energy to the electron. The photon is deflected from its original path (scattered) and has less energy (longer wavelength) than the incident photon. The Compton electron has kinetic energy equivalent to the difference between the initial photon and the Compton-scattered photon.

The probability of Compton scattering decreases with increasing photon energy and with increasing Z of the absorber. This interaction is, therefore, more probable in the middle photon energy range (i.e., 0.1–1 MeV) and with light materials.

The third interaction, pair production, may be considered the opposite of the production of annihilation radiation. In this case, a high-energy photon comes into the near vicinity of the nucleus of an atom and has a coulombic interaction in which the photon disappears and two charged particles are produced in its place. These charged particles, a positron and an electron, share (as kinetic energy) any available energy of the photon over and above the threshold energy for the reaction. The rest-mass energy of each of these charged particles is equivalent to 0.511 MeV and, therefore, pair production is not possible below a "threshold" of 1.022 MeV. Even though the threshold for this reaction is just >1 MeV, pair production does not become important until a photon energy of ∼4 MeV is reached.

When the positron has expended its kinetic energy in the medium, it will annihilate with a free electron, as described previously.

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Rare-Earth Doped Upconversion Nanophosphors☆

F. Wang, X. Liu, in Comprehensive Nanoscience and Nanotechnology (Second Edition), 2011

1.18.2.1.1 Absorption processes

Photon absorption processes in UC that populate emitting states are mainly divided into three broad classes: excited-state absorption (ESA), energy-transfer upconversion (ETU), and photon avalanche (PA). All these processes involve sequential absorption of multiple photons (Fig. 1). Thus, UC processes are different from concerted multiphoton processes where the photon absorptions occur simultaneously.

Fig. 1. Principal upconversion (UC) processes: (a) excited-state absorption, (b) energy-transfer UC, and (c) photon avalanche. The dashed-dotted, dashed, and full arrows represent photon excitation, energy transfer, and emission processes, respectively.

In the case of ESA, excitation takes the form of a successive absorption of pump photons by a single ion. The general energy diagram of the ESA process is shown in Fig. 1(a) for a simple three-level system. If the excitation energy is resonant with the transition from the ground level G to an excited metastable level E1, photon absorption occurs and populates E1 from G in a process known as ground-state absorption (GSA). A second pump photon that promotes the ion from E1 to higher-lying state E2 results in UC emission, corresponding to the E2–G optical transition.

ETU is similar to ESA in that both processes utilize sequential absorption of two photons to populate the metastable level. The main difference between ETU and ESA is that the excitation in ETU is realized through energy transfer between two neighboring ions. In the ETU process, each of two neighboring ions can absorb a pump photon of the same energy, thereby populating the metastable level E1 (Fig. 1(b)). A nonradiative energy-transfer process promotes one of the ions to an upper emitting state E2 while the other ion relaxes back to its ground state G. The dopant concentration that determines the average distance between the neighboring dopant ions has a strong influence on the UC efficiency of an ETU process.

PA-induced UC features an unusual pump mechanism that requires a pump intensity above a certain threshold value. The PA process starts with population of level E1 by nonresonant weak GSA, followed by resonant ESA to populate an upper visible-emitting level E2 (Fig. 1(c)). After the metastable-level population is established, cross-relaxation energy transfer (or ion pair relaxation) occurs between the excited ion and a neighboring ground-state ion, resulting in both ions occupying the intermediate level E1. The two ions readily populate level E2 to further initiate cross-relaxation and exponentially increase level E2 population by ESA, thereby producing strong UC emission as an avalanche process.

The UC efficiency in these three processes varies considerably. ESA is the least efficient UC process [1]. Efficient UC is possible in PA with metastable, intermediate levels that can act as a storage reservoir for pump energy. However, the PA process suffers from a number of drawbacks, including pump-power dependence and slow response to excitation (up to several seconds) due to numerous looping cycles of ESA and cross-relaxation processes. In contrast, ETU is instant and pump-power-independent, and thus has been widely used to offer highly efficient UC (~2 orders of magnitude higher than ESA) [1] over the past decade.

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Optical and Electro-Optic Processes

Kwan Chi Kao, in Dielectric Phenomena in Solids, 2004

The Outstanding Differences between Photons and Electrons or Protons

There are several significant differences between photons and electrons or protons.1

•

Photons may be created and annihilated, whereas electrons or protons are conserved.

•

Photons do not interact with each other, and they obey Bose–Einstein statistics; electrons or protons do interact with each other and obey Fermi–Dirac statistics.

•

Photons do not have electrostatic charges, spin moments, or rest mass; these are possessed by electrons or protons.

•

All photons have a common constant velocity c in free space (constant velocity v = c/n in materials), whereas the velocities of electrons or protons are variable, depending on the accelerating voltage.

•

Photons have diffraction wavelength λd equal to their radiation (conversion) wavelength λE, whereas electrons or protons have λd ∞ V−1/2 but λE ∞ V−1, where V is the accelerating voltage.

•

Photons have momentum p and kinetic energy Ek, depending on their frequency, whereas electrons or protons have momentum and kinetic energy, depending on their velocity.

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Radiation Physics

John H. Hubbell, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.B Photons

Photons that have energies in the keV and MeV (megavolt) range (x rays and γ rays) have the ability to penetrate matter deeply and, when absorbed, to produce strong effects. X rays are generated when an electron beam strikes matter; an x-ray generator consists of a powerful electron gun and a metal target in which the photons are generated. For x-ray crystallography applications, the photons will be in the energy range from 5 to 30 keV, including line-energies characteristic of the atomic number of the metal in the target, superimposed on a bremsstrahlung continuum spectrum. For imaging and irradiation applications, the energies commonly range up to 3 MeV, mostly in the form of bremsstrahlung. For research applications, electron accelerators such as synchrotrons and linacs (linear accelerators) produce photons up to the GeV (109 electron volts) region, but for imaging and irradiation purposes the energy is usually kept below 5 MeV to avoid producing radioactivities in the sample due to the photonuclear effect which has a resonance peak in the region 5 to 40 MeV.

γ Rays are high-energy monoenergetic photons. The term is used specifically for photons created during the disintegration of atomic nuclei. A well-known example is the pair of photons created in the spontaneous disintegration of the cobalt-60 atomic nucleus. These photons have energies of 1.1732 and 1.3325 MeV, and cobalt-60 (60Co) is frequently used in plaque and other geometrical configurations in irradiation facilities. γ Rays are created by nuclear chain reactions such as those that occur in a nuclear reactor core or a nuclear explosion. The isotopes contained in nuclear fuel represent concentrated sources of γ rays. These can be used for experimental irradiation in spent-fuel ponds. Table I presents data on 60Co and some of the other radioisotopes useful in medical therapy and industrial irradiation applications, also on 90Sr which can be important as an environmental hazard.

TABLE I. Radioisotopes Important in Medical Therapy and Industrial Irradiation Applications, also as Environmental Hazards (90Sr)a

NuclideHalf-life (year)Type of decayPhotonsParticlesEnergy (MeV)Percentage emitted (%)Energy (MeV)Transition probability (%)60Co5.271.17399.860.31899.9β−1.33399.981.4910.1(av 1.25)192Ir0.5260.29629.6(192 d)0.30830.70.31682.70.53042.6β−0.46847.00.67047.20.6048.20.6125.3137Cs0.66285.190Sr + daughter28β−0.54100——90Y0.176β−2.27100——85Kr10.6β−0.150.70.510.7β−0.6799.3——252Cf2.65Spontaneous fission——Neutrons, 2 MeVγs, 5.9–6.1 MeVFission fragments, 80 and 104 MeV

aMain emission energies.

In recent decades, synchrotron radiation has become a major high-flux source of photons for research and analytical applications. This radiation is produced in high-energy accelerators from the bending of electron orbital trajectories in the confining magnetic field, sometimes by magnetic "wigglers and undulators" interposed in the electron path. The photon energies thus produced range from tens of eV from accelerators in the hundreds of MeV range, to above 100 keV for electron accelerators in the multi-GeV range. Another recently developed source of photons in the γ-ray energy range is by inverse Compton scattering. In such devices, intense laser beams in the visible or ultraviolet (eV range) are collided with GeV-range electrons in an accelerator, boosting the eV laser photons up to MeV energies.

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Electromagnetic Radiation

David L. Andrews, in Encyclopedia of Spectroscopy and Spectrometry, 1999

Photon properties

Mass

Photons are elementary particles with zero rest mass – necessarily so, since, from special relativity theory, no particle with a finite mass can move at the speed of light.

Velocity

The speed of light is normally quoted as speed in vacuo, c0, with refractive corrections applied as appropriate; the free propagation of any photon also has a well-defined direction, usually denoted by the unit vector kˆ.

Energy

Photon energy is linked to optical frequency ν through the relation E = hν (where h = 6.6261 × 10−34 J s). Each photon essentially conveys an energy E from one piece of matter to another, for example from a television screen to a human retina.

Frequency

The optical frequency ν expresses the number of wave cycles per unit time. Also commonly used in quantum mechanics is the circular frequency ω = 2πν (radians per unit time), in terms of which the photon energy is E = ℏω where ℏ = h/2π. The lower the optical frequency, the more photons we have for a given amount of energy; and the larger the number of photons, the more their behaviour approaches that of a classical wave (this is one instance of the 'large numbers' hypothesis of quantum mechanics). It is for this reason that electromagnetic radiation becomes increasingly wave-like at low frequencies, and why we tend to think of radiofrequency and microwave radiation primarily in terms of waves rather than particles.

Wavelength

The wavelength λ of the electric and magnetic waves is given by λ = c/ν. In spectroscopy, common reference is made to its inverse, the wavenumber v¯ = 1/λ, usually expressed in cm−1.

Momentum

Each photon carries a linear momentum p, a vector quantity of magnitude h/λ = ℏk pointing in the direction of propagation. It is then convenient to define a wave vector or propagation vector k = kkˆ such that p = ħk. Since the photon momentum is proportional to frequency, photons of high frequency have high momenta and so exhibit the most particle-like behaviour. X-rays and gamma rays, for example, have many clearly ballistic properties not evident in electromagnetic radiation of lower frequencies.

Electromagnetic fields

The electric and magnetic fields, E and B respectively, associated with a photon are vector quantities oriented such that the unit vectors (Ê, Bˆ, kˆ) form a right-handed orthogonal set.

Polarization

For plane-polarized (also called linearly polarized) photons, the plane within which the electric field vector oscillates can sit at any angle to a reference plane containing the wave vector, as shown in Figures 2A and 2B. Other polarization states are also possible: in the right- and left-handed circular polarizations depicted in Figures 2C and 2D, the electric field vector sweeps out a helix about the direction of propagation. Elliptical polarization states are of an intermediate nature, between linear and circular. Together, the wave vector and polarization of a photon determine its mode.

Figure 2. Polarization states: (A) and (B) plane; (C) and (D) circular.

Spin

Many of the key properties of photons as elementary particles relate to the fact that they have an intrinsic spin S = 1, and so are classified as bosons (particles with integer spin as opposed to half-integer spin particles of matter such as electrons). As such, photons collectively display a behaviour properly described by a Bose–Einstein distribution. At simplest, this means that it is possible for their oscillating electromagnetic fields to keep in step as they propagate. Through this, coherent beams of highly monochromatic and unidirectional light can be produced; this is of course the basis for laser action.

Angular momentum

The intrinsic spin of each photon is associated with an angular momentum, a feature that plays an important role in the selection rules for many spectroscopic processes. Circularly polarized photons have the special property of quantum angular momentum: the two circular polarization states, left- and right-handed, respectively carry +1 or −1 unit of angular momentum, ℏ.

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Radiation Effects in Electronic Materials and Devices

Andrew Holmes-Siedle, Victor A.J. van Lint, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.B Photons

Photons that have energies in the keV and MeV range (X rays and gamma-rays) have the ability to penetrate matter deeply and, when absorbed, to produce strong effects. X-rays are generated when an electron beam strikes matter; an X-ray generator consists of a powerful electron gun and a metal target in which the photons are generated. Energies from 30 keV to 3 MeV are common. Small fluxes of X-rays are also generated naturally in radioisotope samples by the collision of beta rays within the sample itself or with the capsule in which the sample is contained. In electron-beam devices (cathode-ray tubes and electron accelerators), a hazard to humans may be created by the unintentional generation of X-rays when the beam collides with the wall of the chamber.

Gamma-rays are high-energy, often monoenergetic photons. The term is used specifically for photons created during the disintegration of atomic nuclei. A well-known example is the pair of photons created in the spontaneous disintegration of the cobalt-60 atom. These photons have energies of 1.17 and 1.33 MeV. Gamma rays are commonly created during nuclear chain reactions, such as those that occur in a nuclear reactor core or a nuclear explosion. The isotopes contained in nuclear fuels represent concentrated sources of gamma-rays. These can be used for experimental irradiation in spent-fuel ponds. When certain equipment used in reprocessing nuclear fuel is directly exposed to the isotope sample, there is danger that exposed optical and electronic parts may degrade in performance. Thus, the equipment has to be radiation hardened (see later).

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PHOTON PICTURE OF LIGHT

S.J. Bentley, in Encyclopedia of Modern Optics, 2005

Introduction

The photon nature of light is the central topic in the field of quantum optics, which has developed into a leading area of research. The possible technological impacts of this field to many areas, such as imaging, computing, and lithography, have greatly expanded interest into the nature of the photon. A photon description of light goes well beyond the particle nature of light, to include any optical phenomena that cannot be adequately described by classical physical optics. This article will introduce the nature of the photon through a discussion of many such phenomena, covering topics from the beginning of quantum mechanics, including black-body radiation and Compton scattering, to areas of modern interest, such as squeezed light and entangled photons.

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Recommended publications:

Photon

Photon resulting from a transition in an atomic nucleus, either from natural decay of a radioisotope, or from an induced nuclear transition.

From: Encyclopedia of Physical Science and Technology (Third Edition), 2003

Related terms:

Wavelength

Fluorescence

Excitation

Ionization

Neutron

Gamma Radiation

Ion

Photosynthesis

X Ray

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Reconciling the Kinetic Theory of Gas With Gas Transport Data

Anne M. Hofmeister, in Measurements, Mechanisms, and Models of Heat Transport, 2019

5.1.4 The Photon Gas and (Macroscopic) Thermodynamic Laws

Photons are not part of classical thermodynamics, but can explain several of the laws of thermodynamics when radiative transfer is taken into account (Text Box 5.1; Chapter 1). Importantly, inelastic collisions are required for thermal evolution. If instead collisions were perfectly elastic, the gas could maintain its temperature for an indefinite period of time, and so once a gas were heated, the heat source can be shut off and the gas would not cool. It should also be clear that Fourier's equations, which conserve heat-energy (the caloric model), are fully compatible with a moving photon gas.

Text Box 5.1

Thermodynamic Laws Recast in Terms of Macroscopic Photon Behavior.

First LawThird LawPhotons are energy and can do work, or can be produced by work, and thus must be accounted for in conservation laws.Photons are everywhere.First+Third Law CorollaryA photon gas resides inside every assembly of matter due to atomic collisions being inelastic.Second LawBecause the internal photon gas of a medium is not bound by gravity or other forces, it flows outward providing emissions, and as such prohibits reversible processes.The strong growth of emitted flux with temperature constrains the direction of heat flow.Zeroth LawThermal equilibrium requires the same fluxes and thus the same temperature, allowing equilibrium to be communicated among multiple bodies.

Although this section considers monatomic gases, the findings must extend to other types of matter. Ubiquitous occurrence of the photon gas is compatible with Maxwell's (1871) remark: "All heat is of the same kind." A flux of photons on a surface of a material provides heat to that material. A flux of photons from the surface sheds heat to the surroundings. If the inbound flux is less than the outbound flux, the body cools.

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Dosimetry

J.W. PostonSr., in Encyclopedia of Physical Science and Technology (Third Edition), 2003

III.C X and Gamma Radiation

As stated previously, photoelectric interactions are most probable at low photon energies. The interaction is relatively unimportant for photons with energies >1 MeV, except in very heavy elements.

When the positron has expended its kinetic energy in the medium, it will annihilate with a free electron, as described previously.

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Radiation Physics

John H. Hubbell, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.B Photons

TABLE I. Radioisotopes Important in Medical Therapy and Industrial Irradiation Applications, also as Environmental Hazards (90Sr)a

aMain emission energies.

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Molecular Breeding of Woody Plants

Carmen Valero-Aracama, ... Toyoki Kozai, in Progress in Biotechnology, 2001

Effects of CO2 concentration and PPF on growth of plantlets using two-leafed nodal cuttings as explants (Expt. 2)

PPF and CO2 concentration significantly affected the multiplication ratio and growth of plantlets (Table 3). Among photoautotrophic treatments, plantlets in the 100H treatment had a greater fresh mass and leaf area, which were not significantly different from those in the Control, and a greater dry mass and multiplication ratio, which were greater than those in the Control. These results are in accordance with those obtained with other species, such as strawberry, raspberry, asparagus 10 and Rosa 11, in which growth and development of plantlets were promoted under high PPF and high CO2 concentration. Under photoautotrophic conditions, two-leafed nodal cuttings were usable explants for the multiplication stage, where plantlet growth was enhanced with the combination of 100 μmol m- 2 s- 1 PPF and high CO2 concentration.

Table 3. Effects of PPF (150, 100 and 50 μmol m- 2 s- 1: 150, 100 and 50, respectively) and CO2 concentration (2200 and 600 μmol mol- 1: H and L, respectively) on the growth of Rhododendron plantlets on day 40 (Expt. 2)

Treatment CodeFresh mass / Plantlet (mg)Dry mass / Plantlet (mg)Leaf Area / Plantlet (mm2)Multiplication Ratio150Hz29 ± 2.0bcNS10 ± 0.7bc*70 ± 6.8b**2.2 ± 0.2bNS100H41 ± 4.3aNS14 ± 1.4a**117 ± 13.4aNS3.3 ± 0.4a*50H30 ± 4.2abcNS10 ± 1.3abc*100 ± 16.4abNS2.2 ± 0.2bNS150 L24 ± 2.2bcNS8 ± 0.6bcNS67 ± 5.2b*2.2 ± 0.2bNS100 L32 ± 3.4abNS10 ± 1. lab*89 ± 11.5abNS2.2 ± 0.3bNS50 L20 ± 1.8c*7 ± 0.7cNS64 ± 6.4b1.5 ± 0.2bNSControl39 + 6.37 ± 1.192 ± 12.72.1 ± 0.4yANOVAPPF (X)********[CO2] (Y)*******X x YNSNSNSNS

zMean ± SE in the same column followed by the same letter are non-significantly different by the Duncan Multiple Range Test at P≦0.05. NS, *, **Non-significantly or significantly different from Control by t-test at P≦0.05 and P≦0.01, respectively.ySignificance among the photoautotrophic treatments

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Radiation Effects in Electronic Materials and Devices

Andrew Holmes-Siedle, Victor A.J. van Lint, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.B Photons

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SURFACE ANALYSIS | Laser Ionization

L. Van Vaeck, in Encyclopedia of Analytical Science (Second Edition), 2005

Laser Ionization Schemes

Photon absorption is by far the most elegant way to give neutrals a well-defined amount of energy, which is almost exclusively converted into electronic excitation.

Single photon ionization (SPI) requires Eϕ

Figure 2. Survey of laser ionization schemes for gas-phase analytes: single photon ionization (SPI), multiphoton ionization (MPI) with distinction between resonant ionization (RI), resonance enhanced multiphoton ionization (REMPI), and nonresonant multiphoton ionization (NRMPI). Real and virtual states are denoted by R and S, respectively, while C refers to the ionization continuum.

[5]Eϕ=hc/λ

where h is Plank's constant and c the speed of light.

Modern lasers provide monochromatic beams in the UV and VUV range down to ∼200 nm (IP<6 eV) while the excimer line of 157 nm allows transitions of 7.9 eV to be addressed. The overall ionization efficiency for elemental species primarily depends on the absorption probability, which is in turn a function of the density of the neutrals and the photon flux (intensity) in the ionization volume. Current lasers provide sufficient intensity to ionize nearly all atoms in the ionization volume. The situation is different for molecular species. As only little Ekin is given to the departing photoelectron, parent ions M+ obtain an amount of internal energy Eint corresponding to (Eϕ–IP). Unlike ionized atoms, M+ tend to relax their internal stress by fragmentation. Depending on the λ, the ratio of daughter ions over parent can be tuned for maximal yield of either M+ or fragment ions to obtain molecular weight or structural information, respectively. Unfortunately, each daughter costs the life of her parent and generation of fragments lowers the signal intensity of M+z.rad;.

Strictly speaking, distinction must be made between resonant and nonresonant SPI. In the resonant case, the Eϕ exactly matches the transition from the ground state into an autoionizing bond state above the IP. As a result, the high σ allows complete ionization to be achieved with lower laser intensities than when nonresonant SPI is used. In the latter case, the Eϕ drives the analyte 'somewhere' into the continuum above the IP.

Analytes with IP>8 eV require multiphoton ionization (MPI), i.e., consecutive absorption of two or more photons. A first photon transfers the analyte into a specific, electronically excited state and subsequent uptake of photon(s) is required to overcome the IP. The efficiency of this sequential process depends on the decay rate of the intermediate state relative to the photon flux density that promotes its population. The balance between the two competitive effects can be quantified by a quality factor as in electric resonance circuits. The ultimate efficiency is obtained in resonant (R) MPI, often denoted as resonant ionization (RI). In this case, the Eϕ of the photons used exactly corresponds to the energy gaps between the ground level and the long-lived excited state on the one hand, and between that excited state and an autoionizing level on the other. Selectivity is optimized when molecules have low internal excitation. For instance, jet cooling of the gas-phase analytes reduces the population of vibrational states and the σ shows sharp maxima as a function of λ. Even isotope-labeled molecules can be discriminated from unlabelled ones. However, the situation is less ideal for the ion-beam sputtered neutrals because of their vibrational excitation. Therefore, resonance enhanced (RE) MPI is preferred in SALI.

Use of photons with a λ that does not match the transition to a long-lived intermediate state, allows nonresonant (NR) MPI to be performed by the 'quasisimultaneous' absorption of two or more photons. The sensitivity and selectivity of NRMPI is much lower than that of REMPI. The photon flux density (intensity) becomes a prime factor in NRMPI. Using laser pulses with a duration of <10 ns and peak power densities above 1010 and 1013 W cm−2 are required for NRMPI involving two and more photons, respectively. In the case of three-photon MPI of molecules, further increase of the flux density does not really improve the yield as a result of the so-called 'ladder mechanism'. Above a given threshold, the laser intensity primarily determines the extent of fragmentation. Figure 3 illustrates the ladder concept for a neutral ABC of which the ionization requires isoenergetic photons. The first photon brings ABC one step higher on its ladder, where competition occurs between uptake of another photon (ladder climbing, requires high flux density) or fragmentation into AB and C (ladder switching). In the latter case, the fragments AB and C can start their own process of ladder climbing. When a second photon brings the already excited ABC neutrals above the IP, fragmentation into, e.g., BC+ competes with absorption of an additional photon yielding an excited state of ABC+. The ladder concept explains the breakdown of stable molecules, such as benzene, into graphite-like Cn+ ions using photons of 4.8 eV.

Figure 3. The effects of fragmentation and ladder switching in the MPI process of a molecule ABC at high photon fluxes. (Reprinted from Vorsa V, Kono T, Willey KF, and Winograd N (1999) Femtosecond photoionisation of ion beam desorbed aliphatic and aromatic amino acids: fragmentation via α-cleavage reactions. Journal of Physical Chemistry B 103: 7889–7895; © American Chemical Society.)

Ladder switching in organic molecules often involves processes of internal conversion (IC) and intersystem crossing (ISC), which bring molecules in vibrationally excited states and favor their fragmentation. As the rate constant of IC and ISC is typically <106 and 1011 s−1, respectively, use of lasers with a pulse duration in the femtosecond range makes excitation faster than the depopulation of the excited state and increases the yield of M+. The growing availability of the tabletop ultrafast laser systems brings a tremendous potential for the ionization of large fragile organic molecules within the reach of the analytical chemist.

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Missions and Sensors

G. Jaross, in Comprehensive Remote Sensing, 2018

1.12.5 Stray Light

Photons originating from outside the intended field of view or spectral bandpass are collectively referred to as stray light. Typical sources of stray light are optical surfaces that are not perfectly smooth. There are two basic types of stray light in a BUV instrument: light scattered prior to the entrance slit and light scattered in the spectrometer after the slit. Stray light scattered in the entrance baffles or telescope typically results in photons from adjacent bright areas (i.e., clouds) contaminating the darker scene being measured. While undesirable, this spatial stray light does not present a major problem for most BUV product retrievals because the scattered photons that eventually make it to the detector are in-band, meaning they are at the same wavelength as unscattered photons. Most modern retrieval algorithms are relatively insensitive to errors that are independent of wavelength. The spectral structure of the atmospheric species does not vary with scene brightness. The net effect of spatial stray light is a reduction in spatial resolution of the retrieved species.

Conversely, spectrometer stray light can be a significant issue for BUV instruments because it involves photons scattered to the wrong spectral location of the detector. While the signal contrast between two adjacent Earth scenes rarely exceeds a factor of 5 in the UV, the contrast between signals at extreme ends of the BUV spectrum can differ by more than three orders of magnitude. If just 1% of 400 nm photons contaminate the detector location for 300 nm measurements, the error in the 300 nm signal can be 100%. Ruled diffraction gratings are notorious for scattering photons in a part of the sensor where they are especially difficult to deal with. Traditional methods for dealing with the high spectral contrast in the UV have been double monochromators (dual gratings) and splitting the spectral range into multiple spectrometers. While neither of these solutions eliminates spectral stray light, each introduces new problems: low signal levels in double monochromators and calibration differences when measuring with multiple spectrometers.

To make matters worse, two-dimensional detectors add a new dimension of stray light. In older linear array detectors most spectrometer stray light comes from within a single Earth scene. In spectrometers that simultaneously contain spectrally and spatially dispersed photons scattering can occur diagonally, where long wavelength photons from bright scenes contaminate short wavelengths in dark scenes. Fortunately, optical filter technologies matured at about the same time that the CCD detectors were introduced. With the advent of ion-assisted deposition and sophisticated computer-aided design, it has become possible to produce complex multilayer filters that are stable over the long term in the space environment. The transmission or reflection spectrum of these filters can be shaped to counteract the natural spectral gradient of the Earth backscatter radiation. When placed near the entrance slit, these flattening filters reduce the spectral contrast to more manageable levels. This has the effect of not only reducing spectral stray light levels but also optimizes the use of the detector's dynamic range.

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ACTIVATION ANALYSIS | Photon Activation

D. Schulze, C. Segebade, in Encyclopedia of Analytical Science (Second Edition), 2005

Analytical Procedure

Reference Materials

Photon activation analysis, as well as other instrumental techniques, is generally quantified by comparison of activities in the sample with those in a reference material of known elemental composition that was irradiated simultaneously. In photon activation analysis this is necessary particularly because some accelerator parameters and nuclear data for the photoreactions involved are either unknown or not precisely determinable. In addition, some machine parameters of the accelerator cannot be assumed constant throughout the exposure period and might show uncontrolled variation. The use of reference materials irradiated simultaneously with the samples implicitly accounts for these parameters. Primary standards, i.e., pure elements or substances synthesized from stoichiometrically well-determined compounds, are used for quantification through comparison of the activities after activation. Certified reference materials are also analyzed to ascertain the total accuracy of the results. Frequently it is useful to provide a matrix-inherent or additive internal standard that serves as a flux monitor. The integral quality of the analytical data (in terms of accuracy and precision) is thereby significantly improved.

Analysis of Light Elements

One special feature of photon activation analysis is the analysis of light elements such as carbon, nitrogen, oxygen, and fluorine. The analytical procedure in this case is essentially different from that in instrumental multielement analysis. This is due to the fact that the prominent photonuclear reactions of these elements, e.g., 12C(γ,n)11C, yield pure β+-emitters exclusively and thus emit no characteristic photon radiation by which they might be detected. Hence, after activation these elements have to be separated radiochemically from the sample matrix prior to annihilation radiation measurement. In the case of carbon, nitrogen, and oxygen, this is usually performed by heat extraction. Fluorine separations are normally carried out by distillation as hexafluorosilicic acid. Nondestructive or instrumental analyses of the light elements are possible only in exceptionally favorable cases, namely if the activated matrix does not emit positron radiation at significant level at the time of measurement. However, currently photon activation techniques are under development that, with help of improved high-performance hardware and software, will enable instrumental analyses of light elements in multicomponent samples through photon activation.

Analysis of Heavier Elements; Multielement Analysis

High-resolution photon spectrometry with semiconductor detectors has normally been used for multielement photon activation analysis. One generally strives for a purely instrumental or, if possible, nondestructive procedure. In several cases, however, a radiochemical separation step is unavoidable and sometimes advantageous.

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Volume 6

Klaus-P. Gilbertz, ... Mark E. Hotz, in Encyclopedia of Environmental Health (Second Edition), 2019

Physical and Chemical Processes

Photons of optical radiation may be selectively absorbed by atoms or molecules. The energy difference of two electron levels corresponds to that of the photon. Absorption of the UV radiation results in excitation of these atoms and molecules. Critical absorbing molecules are called chromophores. Excited atoms and molecules release their energy either by emission of a quantum (fluorescence or phosphorescence) or by reaction with other molecules. Photochemistry deals with reactions, whose process is influenced or enabled by the interaction of the involved molecules with optical radiation. Photochemical reactions occur extremely fast from the first excitation via intermediate stages (reactive species, free radicals) to stable final compounds. They are translated to photobiological responses that may occur within seconds but can take years or even decades to be fully manifest.

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