Physics
Spectral Lines
Spectral lines are specific wavelengths of light emitted or absorbed by atoms or molecules. They are characteristic of the element or compound and can be used to identify its presence and properties. Spectral lines are produced when electrons in an atom move between energy levels, emitting or absorbing photons with specific energies corresponding to the line's wavelength.
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12 Key excerpts on "Spectral Lines"
eBook - PDF- James J. Condon, Scott M. Ransom(Authors)
- 2016(Publication Date)
- Princeton University Press(Publisher)
7 Spectral Lines 7.1 INTRODUCTION Spectral Lines are narrow ( ν ν ) emission or absorption features in the spectra of gaseous and ionized sources. Examples of radio Spectral Lines include recombination lines of ionized hydrogen and heavier atoms, rotational lines of polar molecules such as carbon monoxide (CO), and the λ = 21 cm hyperfine line of interstellar H I . Spectral-line emission and absorption are intrinsically quantum phenomena. Classical particles and waves are idealized concepts like infinitesimal points or perfectly straight lines in geometry; they don’t exist in the real world. Some things are nearly waves (e.g., radio waves) and others are nearly particles (e.g., electrons), but all share characteristics of both particles and waves. Unlike idealized waves, real radio waves do not have a continuum of possible energies. Instead, electromagnetic radiation is quantized into photons whose energy is proportional to frequency: E = h ν . Unlike idealized particles, real particles of momentum p are associated with waves whose De Broglie wavelength is λ = h / p . An electron’s stable orbit about the nucleus of an atom shares a property with standing waves: its circumference must equal an integer number of wavelengths. Planck’s constant h ≈ 6 . 62607 × 10 − 27 erg s in these two equations is a quantum of action whose dimensions are (mass × length 2 × time − 1 ), the same as (energy × time) or (angular momentum) or (length × momentum). Although h has dimensions of energy × time, physically acceptable solutions (the wave functions and their derivatives must be finite and continuous) to the time-independent Schrödinger equation exist only for discrete values of the total energy, so Spectral Lines have definite frequencies resulting from transitions between discrete energy states. A second quantum effect important to Spectral Lines, particularly at radio wavelengths where h ν kT , is stimulated emission (Section 7.3.1).
- Heinrich F. Beyer, Viateheslav P. Shevelko(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
Chapter 3 Spectroscopy The pattern of observed lines of absorption or emission is an intrinsic property of an atom and its state of ionization. The lines may appear as an emission spectrum or as absorption lines depending on whether an atom or ion emits light radiation or absorbs it. Spectroscopic investigation of the emission or absorption atomic spectra allows one to gain detailed information about the light source and its properties. 3.1 Spectral Lines The investigation of the manner in which matter can emit and absorb radiation is known as spectroscopy . This entails the analysis of spectra—the splitting of radiation into its components using spectrometers as tools. For a continuous source, eventually characterized as a black-body emitter, the visible portion reveals the familiar rainbow of colors. In addition, there are light sources emitting narrow well-defined emission lines which are characteristic for the material. By changing the condition of excitation, the intensity of a particular spectral line changes but not its wavelength or frequency. Regarding a whole spectrum of lines as an entity, the pattern of relative intensity may depend on the mode of excitation; however the position of the lines on the wavelength scale remains unchanged. In chapter 2 we learnt about the example of the yellow sodium D lines representing the fingerprint of the element sodium. In general, the pattern of lines observed is an intrinsic property of the element and its state of ionization. The lines may appear as an emission spectrum or as absorption lines, i.e. dark lines in a continuous spectrum as, for instance, that observed from the Sun, explained by the presence of gases in the outer layer of the Sun and in the Earth’s atmosphere. The empiric Kirchhoff’s laws describe the conditions for the formation of the three different spectra which are continuous , emission-line and absorption-line spectra.
eBook - PDF- Andrew Fraknoi, David Morrison, Sidney C. Wolff(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
182 Chapter 5 Radiation and Spectra This OpenStax book is available for free at http://cnx.org/content/col11992/1.13 5.3 Spectroscopy in Astronomy A spectrometer is a device that forms a spectrum, often utilizing the phenomenon of dispersion. The light from an astronomical source can consist of a continuous spectrum, an emission (bright line) spectrum, or an absorption (dark line) spectrum. Because each element leaves its spectral signature in the pattern of lines we observe, spectral analyses reveal the composition of the Sun and stars. 5.4 The Structure of the Atom Atoms consist of a nucleus containing one or more positively charged protons. All atoms except hydrogen can also contain one or more neutrons in the nucleus. Negatively charged electrons orbit the nucleus. The number of protons defines an element (hydrogen has one proton, helium has two, and so on) of the atom. Nuclei with the same number of protons but different numbers of neutrons are different isotopes of the same element. In the Bohr model of the atom, electrons on permitted orbits (or energy levels) don’t give off any electromagnetic radiation. But when electrons go from lower levels to higher ones, they must absorb a photon of just the right energy, and when they go from higher levels to lower ones, they give off a photon of just the right energy. The energy of a photon is connected to the frequency of the electromagnetic wave it represents by Planck’s formula, E = hf. 5.5 Formation of Spectral Lines When electrons move from a higher energy level to a lower one, photons are emitted, and an emission line can be seen in the spectrum. Absorption lines are seen when electrons absorb photons and move to higher energy levels. Since each atom has its own characteristic set of energy levels, each is associated with a unique pattern of Spectral Lines. This allows astronomers to determine what elements are present in the stars and in the clouds of gas and dust among the stars.
eBook - ePubFundamentals of Radio Astronomy
Astrophysics
- Ronald L. Snell, Stanley Kurtz, Jonathan Marr(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
Chapter 4Spectral LinesS pectral lines result from the interaction of light with atoms, ions and molecules. The development of quantum mechanics, describing the behavior of matter and its interaction with light on atomic scales, led to the realization that certain properties (such as angular momentum or energy) of an atom or molecule are restricted to a discrete or quantized set of values. Changes of an atom’s energy, for example, can only occur between these discrete sets of values. Light is also quantized into discrete particles of energy or photons, and unlike the processes we described in Chapter 3 , the interaction of light with electrons bound to atoms and molecules usually involves only a single photon. In the following discussion we assume the reader has some background in modern physics.Let us start our discussion by reviewing the electronic states of the hydrogen atom and its possible photon interactions. With regards to the electron’s orbitals, the allowed energy states of the hydrogen atom, labeled by their principal quantum number n, are given byE n= − 13.60 (1) eV ,n 2(4.1) where 1 eV (electron volt) = 1.60 × 10−12 ergs. The lowest energy state (or the ground state) is denoted by n = 1, while higher values of n indicate higher energy states, often referred to as excited states. Note that the total electronic energy (kinetic and electric potential energy) is negative, as one would expect if the electron is bound to the nucleus. Often the energies of the excited states are measured relative to the energy in the ground state. In this case the relative energy levels of hydrogen are given byE n= 13.60 ( 1 −1) eV .n 2A useful way to depict the allowed energy states of an atom or molecule is an energy level diagram that has energy on the vertical axis and the allowed energy states indicated as horizontal lines. The energy level diagram for the orbital energy of the electron in the hydrogen atom is shown in Figure 4.1 . Note that the electronic energy levels for hydrogen get closer and closer together as the principal quantum number grows larger. If the energy added to an electron in the ground state of the hydrogen atom exceeds 13.60 eV, then the electron will not be bound to the nucleus and the atom is said to be ionized. For this reason, 13.60 eV is called the ionization energy
eBook - PDF- Carlos Jaschek, Mercedes Jaschek(Authors)
- 1995(Publication Date)
- Cambridge University Press(Publisher)
Part Three Content description In this part of the book we shall provide short summaries (but no detailed treatment) on different topics, which are necessary for a better understanding of the book. We have grouped these matters in five chapters, namely 1 Terminology of Spectral Lines 2 The selection of stars 3 Line identification 4 Equivalent widths 5 Abundances We have added also chapter 6, which contains some general thoughts on the matters covered in this book, which constitutes the epilogue. • -1 ' TERMINOLOGY OF Spectral Lines Most of the terminology used by astronomers for Spectral Lines follows the definitions of the physicists. Let us recall briefly the meaning of some terms that are used in this book. For more details, the reader can consult any book on spectroscopy, like Atomic Spectra and Atomic Structure by Herzberg (1945) or Structure and Spectra of Atoms by Richards and Scott (1976). An atom can have all its electrons, in which case it is said to be in the neutral state. If it has lost one electron it is said to be singly ionized and if it has lost n electrons it is said to be n times ionized. Degrees of ionization are indicated by Roman numerals - Nal is neutral sodium, Nail singly ionized sodium and so forth. If one wants to refer to any ionization stage of an atom then one speaks of species. A spectral line is the result of the transition of an electron between two energy levels. Among the different energy levels available for an electron, some transi- tions are permitted by the selection rules. Among these one calls fundamental, resonance or ultimate lines those connecting a level to the lowest energy level. Usually these lines are the most intense ones. 261 Part Three Forbidden lines are formed by transitions between levels that are not allowed by the selection rules. Forbidden lines are indicated by the element enclosed in a square bracket - for example [Fell].
eBook - PDF- Michael Seeds, Dana Backman(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
These types of spectra are described by Kirchhoff ’s laws. When you see one of these types of spectra, you can recognize the arrangement of matter that emitted the light. 2 Photons are emitted or absorbed when an electron in an atom makes a transition from one energy level to another. The wavelengths of the photons depend on the energy dif-ference between the two levels, so (note, this is especially important) each spectral line represents not one energy level but rather an electron transition between two energy levels. Hydrogen atoms can produce many Spectral Lines that are grouped in series such as the Lyman, Balmer, and Paschen 1 2 series. Only three hydrogen lines—all in the Balmer series— are visible to human eyes. The emitted photons coming from a hot cloud of hydrogen gas have the same wave-lengths as the photons absorbed by hydrogen atoms in a cool cloud between an observer and a light source. 3 Most modern astronomy books and articles display spectra as graphs of intensity versus wavelength. Be sure you recog-nize the connection between dark absorption lines, bright emission lines, and the dips and peaks in the graphed spectrum. Imagine you are an astronaut with a handheld spectrograph, approaching a fresh lava flow on a moon with no atmosphere. You aim your spectrograph straight at the lava flow; what kind of spectrum do Kirchhoff’s laws say you will see? You should see a continuous (blackbody) spectrum, with all the colors of the rain-bow present, produced by the opaque, glowing hot lava. And as a bonus, measuring the wavelength of the strongest blackbody emission lets you determine the temperature of the lava using Wien’s law. Suddenly, the lava flow begins to bubble, and gas trapped in the molten rock is released, making a temporary, warm, thin atmosphere right above the lava flow.
eBook - ePub- Mario Bertolotti(Author)
- 2004(Publication Date)
- CRC Press(Publisher)
If, for example, we send an intense light containing all visible wavelengths (a continuum) through a flame in which sodium is burnt, the transmitted light is found to lack the wavelengths corresponding to the two yellow lines of the D-line of sodium. In the spectrum two dark lines appear at the place of the two bright lines that are observed in the emission spectrum. This explanation applies not only to our Sun but to every star. In fact, dark lines similar to the ones Fraunhofer observed in the solar spectrum are observed in the spectra of all stars, and their positions in the spectrum indicate which wavelengths have been absorbed by the substances present in the stellar atmospheres, so allowing us to identify them. Fundamental contributions to this new science of spectroscopy were made by Kirchhoff, David Brewster, John Herschel (1792–1871), William Henry Fox Talbot, Charles Wheatstone (1802–1875), Antoine-Philibert Masson (1806–1880), Anders Jonas Ångström (1814–1874) and William Swan. The fact that spectra of a substance are sometimes composed of a number of discrete lines and sometimes occur in the form of bands was finally explained by George Salet (1875)—after many controversies—by associating the line and band spectra to atoms and molecules respectively. Atoms Already Democritus and Leucippus in the 5th century BC had spoken of atoms. The Latin poet Lucretius (98–55 BC) in De rerum natura, explaining Democritus’ theory, said that air, water, earth and all the other things of this world are made by a number of particles or corpuscles—atoms—engaged in a restless, very rapid movement, and these are so small as to be perfectly invisible to the human eye
eBook - ePub- R. W. Ditchburn(Author)
- 2013(Publication Date)
- Dover Publications(Publisher)
fig. 1.5 . The emission and absorption of spectra which consist of sharp lines suggest that the atom may be regarded as a system of simple harmonic oscillators. Each oscillator emits light of the wavelength corresponding to its natural frequency and thus produces a line in the spectrum. When white light passes through a gas or vapour, the oscillators in the various atoms resonate and absorb light of the wavelengths which correspond to their own natural frequencies of oscillation. Thus the absorption lines coincide with the emission lines. When an atom is subject to strong interaction with its neighbours (as in a gas at high pressure, or in a solid or liquid) the oscillators are continually being disturbed. They emit irregular pulses instead of simple harmonic waves, and these pulses (which have no well-defined frequency) make up a continuous spectrum. Some of the natural periods of atoms or molecules correspond to wavelengths greater or less than those to which the eye is sensitive, and they give lines in the infra-red and ultra-violet respectively.4.7.—This general description of the emission and absorption of radiation by atoms and molecules includes many of the observations, but there are certain difficulties. It is not easy to understand why some atoms should emit so many lines if each line corresponds to a distinct mode of oscillation which has its own natural frequency. This difficulty is greater when we consider that even the hydrogen molecule, which consists of only four particles, emits an extremely complicated spectrum containing tens of thousands of lines. It is also found that normally only some of the emission lines appear in the absorption spectrum.In the emission spectra of atoms some lines appear only in the spark spectrum and not in the arc spectrum. Others appear only in the gaseous discharge. These observations indicate that under a given set of conditions some of the oscillators are not available, and the theory does not suggest any simple reason why this should be so. Although there are many difficulties, the simple picture of the atom as a set of harmonic oscillators is still very useful. At a later stage it will be incorporated in a more detailed theory of the emission and absorption of light. At present it may be regarded as a working hypothesis to be amended when more detailed experimental data are available. It suggests that it is desirable to see whether all the properties of the light belonging to one of the sharp lines in the spectrum are characteristic of the long trains of sine waves which would be emitted by a simple harmonic oscillator. For this purpose it is necessary to isolate one of the lines of the spectrum. This may be done by placing a mask over the spectrum with a slit placed so as to allow only a narrow region, containing one line, to pass. Such an arrangement is called a monochromator
eBook - ePub- Leo Graetz, G. Barr(Authors)
- 2014(Publication Date)
- Routledge(Publisher)
LECTURE V LINE SPECTRA AND BOHR’S MODEL OF THE ATOM P RIOR to the discovery of radio-activity, the most sensitive method available in chemistry for the detection of the smallest traces of substances was, as is well known, that of spectroscopic analysis; and it was by this method that the discovery of a large number of new elements was rendered possible. The principle of it is to cause the light emitted by incandescent vapours or by luminous gases to pass through a slit and a prism, whereby it is spread out into a spectrum. What then appears is not, as with white-hot solids, an extended band which contains all the colours from red to violet merging continuously into one another, i.e. a complete spectrum, but only a few or sometimes many distinct coloured lines; this is what is called a line spectrum. The lines which are produced under these conditions are so characteristic of the chemical elements that are present in the luminous vapour that it is possible to deduce, inversely, the presence of a particular element from the occurrence of certain lines. In the most accurate experiments a diffraction grating is used instead of a prism for the production and examination of the spectrum; the best gratings are those of Rowland, consisting of a plate of speculum metal on which are ruled 1700 fine equidistant lines per millimetre. If the light from a luminous vapour or gas is allowed to pass through a slit and fall on such a grating, diffraction spectra are obtained by reflection on both sides of the direct image of the slit; there are a primary, a secondary, and a tertiary spectrum at increasing angular distances from the direct image
eBook - ePub- David M. Gates(Author)
- 2012(Publication Date)
- Dover Publications(Publisher)
ance throughout this book for all uses.The spectral characteristics of the earth’s atmosphere are described before the spectral distribution of sunlight since the atmosphere has a strong effect on the spectral distribution of sunlight received at the earth’s surface. The basic theory of molecular spectroscopy is given first in order that the absorptive and radiative properties of the earth’s atmosphere can be properly understood.Molecular Spectra
Atoms have absorption and emission spectra corresponding to discrete changes of the energy states of their electrons. The energy state of an electron bound in orbit around a nucleus is dependent upon the orbital number, angular momentum, and spin of the electron. As an electron moves from an inner to an outer orbit, energy is absorbed. In turn, when an electron jumps from an outer to an inner orbit, radiation is emitted by the atom. Higher energy states characteristic of outer electron orbits converge to smaller energy differences from orbit to orbit until ionization occurs and an energy continuum results. The line emissions or absorptions of such well-known spectral sequences as the Balmer, Lyman, Pas-chen, Brackett, and Pfund series of atomic hydrogen are manifestations of transitions between various orbital states and changes in spin and angular momentum.When atoms bind together to form molecules, additional energy states exist because of vibrational and rotational modes within the molecule. A simple diatomic molecule has only one vibrational mode, which is an oscillation of the atoms along the axis of the molecule. This vibrational oscillation modulates the electron energy states, and so the spectra produced are more complex than those of single atoms. The diatomic molecule can tumble or rotate as it is jostled about by collisions in a gas. This rotational motion is quantized and will further modulate the energy states of the electrons and add additional complexity to the absorption and emission spectra. Polyatomic molecules have much more complicated spectra than diatomic molecules. A large portion of the earth’s atmospheric absorption spectrum is caused by polyatomic molecules.
eBook - PDF- Bernhard Welz, Michael Sperling(Authors)
- 2008(Publication Date)
- Wiley-VCH(Publisher)
The spectral range within the half-width is the line core and the ranges to either side are the line wings. t 0 Frequency v Figure 2-4. Line profile and half-width of a spectral line; refer to the text for an explanation. 2.3 Line Width and Line Profile 75 Since an emitting or absorbing atom is not an isolated atom, but is in interaction with its environment, the emitted or absorbed line is broadened in its frequency distribution by a number of mechanisms.
eBook - ePubSpectroscopy
Principles and Instrumentation
- Mark F. Vitha(Author)
- 2018(Publication Date)
- Wiley(Publisher)
1 FUNDAMENTALS OF SPECTROSCOPYAll instruments are designed to take advantage of some molecular property or behavior. For example, chromatography is based on the different strength of intermolecular interactions that molecules have with mobile and stationary phases. Electrochemistry is based on the ability of molecules to gain or lose electrons. In this book, we focus on the fact that atoms and molecules absorb and emit electromagnetic radiation (EMR). By measuring the amount and the characteristics of the EMR absorbed and emitted, we can measure the concentration of particular molecules present in a sample or gain structural information about them. You may already be familiar with several of the instrumental methods used to measure the absorption and emission of electromagnetic radiation, such as UV‐visible, infrared (IR), and fluorescence spectroscopy.In order to better understand the fundamental basis of these techniques, in this chapter we examine the properties of electromagnetic radiation and its effects on atoms and molecules. In subsequent chapters, we examine specific spectroscopic techniques. While all the techniques share common features, the specific instruments required and the information we gain from each are quite different and therefore require individual examination.1.1. PROPERTIES OF ELECTROMAGNETIC RADIATION
Spectroscopic methods ultimately rely on measuring characteristics of electromagnetic radiation, which travels through space as a wave, as shown in Figure 1.1 . As the name implies, it has two components, an electric field and a magnetic field, which are at right angles to one another. Figure 1.1 shows only a single wave with its electric field oriented along the x‐axis, but in reality, most sources of electromagnetic radiation, like light bulbs and car headlights, emit radiation in which the electric field of the waves are randomly distributed around the x
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