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Explain the formation of emission spectrum and absorbtion spectrum.

please help. very urgent.

  • When a sample of gaseous atoms of an element at low pressure is subjected to an input of energy, such as from an electric discharge, the atoms are themselves found to emit electromagnetic radiation.
  • On passing through a very thin slit and then through a prism the light (electromagnetic radiation) emitted by the excited atoms is separated into its component frequencies.

    • The familiar dispersion of white light is illustrated below:

    Solids, liquids and dense gases glow at high temperatures. The emitted light, examined using a spectroscope, consists of a continuous band of colours as in a rainbow. A continuous spectrum is observed. This is typical of matter in which the atoms are packed closely together. Gases at low pressure behave quite differently.

  • The excited atoms emit only certain frequencies, and when these are placed as discreet lines along a frequency scale an atomic emission spectrum is formed.

    • The spectral lines in the visible region of the atomic emission spectrum of barium are shown below.

  • Spectral lines exist in series in the different regions (infra-red, visible and ultra-violet) of the spectrum of electromagnetic radiation.
  • The spectral lines in a series get closer together with increasing frequency.
  • Each element has its own unique atomic emission spectrum.

The problem was now to explain the observations outlined above...

It was necessary to explain how electrons are situated in atoms and why atoms are stable. Much of the following discussion refers to hydrogen atoms as these contain only one proton and one electron making them convenient to study.

In 1913, it was Neils Bohr who solved many of the problems at the time by proposing that the electron revolves around the nucleus of the atom with a definite fixed energy in a fixed path, without emitting or absorbing energy. The electron in the hydrogen atom exists only in certain definite energy levels. These energy levels are called Principal Quantum Levels, denoted by the Principal Quantum Number, n. Principal Quantum Level n = 1 is closest to the nucleus of the atom and of lowest energy. When the electron occupies the energy level of lowest energy the atom is said to be in its ground state. An atom can have only one ground state. If the electron occupies one of the higher energy levels then the atom is in an excited state. An atom has many excited states.

Here's what happens...

When a gaseous hydrogen atom in its ground state is excited by an input of energy, its electron is 'promoted' from the lowest energy level to one of higher energy. The atom does not remain excited but re-emits energy as electromagnetic radiation. This is as a result of an electron 'falling' from a higher energy level to one of lower energy. This electron transition results in the release of a photon from the atom of an amount of energy (E = hn) equal to the difference in energy of the electronic energy levels involved in the transition. In a sample of gaseous hydrogen where there are many trillions of atoms all of the possible electron transitions from higher to lower energy levels will take place many times. A prism can now be used to separate the emitted electromagnetic radiation into its component frequencies (wavelengths or energies). These are then represented as spectral lines along an increasing frequency scale to form an atomic emission spectrum.

Principal Quantum Levels (n)
for the hydrogen atom.


Comment:

A hydrogen atom in its Ground State.
The electron occupies the lowest possible energy level which in the case of hydrogen is the Principal Quantum Level n = 1.

The Bohr theory was a marvellous success...

The Bohr theory was a marvellous success in explaining the spectrum of the hydrogen atom. His calculated wavelengths agreed perfectly with the experimentally measured wavelengths of the spectral lines. Bohr knew that he was on to something; matching theory with experimental data is successful science. More recent theories about the electronic structure of atoms have refined these ideas, but Bohr's 'model' is still very helpful to us.

For clarity, it is normal to consider electron transitions from higher energy levels to the same Principal Quantum Level. The diagram below illustrates the formation of a series of spectral lines in the visible region of the spectrum of electromagnetic radiation for hydrogen, called the Balmer Series.

 
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The Spectral Lines are in Series...

As referred to above for hydrogen atoms, electron transitions form higher energy levels all to the n = 2 level produce a series of lines in the visible region of the electromagnetic spectrum, called the Balmer Series. The series of lines in the ultra-violet region, called the Lyman Series, are due to electron transitions from higher energy levels all to the n = 1 level, and these were discovered after Bohr predicted their existence.

Within each series, the spectral lines get closer together with increasing frequency. This suggests that the electronic energy levels get closer the more distant they become from the nucleus of the atom.

No two elements have the same atomic emission spectrum; the atomic emission spectrum of an element is like a fingerprint.

The diagram to the right illustrates the formation of three series of spectral lines in the atomic emission spectrum of hydrogen.

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Key points: origin of emission and absorption lines; spectra as a cosmic barcode; Doppler effect
Spectrum: the distribution of intensities of light over wavelength
A continuous spectrumA continuous spectrum has at least some light at all the wavelengths.(From R. J. Lavery, http://www..phy.nau.edu/~lavery/Mypage/Astrostuff/A150WEB1998/main2.html#startnotes)
In normal experience, solids and liquids tend to emit blackbody spectra or spectra close to blackbodies. Gases tend to have more complex emission- and absorption-line spectra, allowing us to learn a lot about their conditions.
Emission- and absorption-line spectra are produced by atoms (and molecules)

sketch of hydrogen and helium atomsAtoms consist of nuclei made of protons and neutrons, and electrons around them. Hydrogen (1 proton) and helium (2 protons) are the simplest. Most aspects of the behavior of an atom (e.g., in chemistry) depend just on the number of protons and electrons; there are atoms with up to about 100 protons, giving 100 elements, each with distinct behavior. (figure by G. Rieke).
The electrons in an atom are held by the electric force, which is proportional to 1/r2 just like gravity. This force attracts positive and negative electric charges, but repels like charges - two positives or two negatives.  The protons in the nucleus of the atom are held together by the "strong force", which is clearly much stronger than the electric one but works only over very small distances.
Permitted and forbidden electron orbits in a hydrogen atomElectrons can only be in certain energy levels in an atom because wave-particle duality means they interfere with themselves in the other levels. This behavior is described by the branch of physics called quantum mechanics.buttonbook.jpg (10323 bytes)(Figure by G. Rieke).
Electron transitions between energy levels lead to absorption or emission of photons of specific energy corresponding to the energy level difference.
If an electron moves from an outer, higher energy orbit to an inner, lower energy orbit, energy is released in the form of photon. The properties of this photon depend on the energy difference between the orbits:
Energy = Eorbit 1 - Eorbit 2 = hnu.jpg (6708 bytes) = hc/lambda.jpg (8443 bytes)
animation of excitation of an atom by absorbing a photonIf a photon of exactly the right energy "hits" an atom, it can be absorbed and cause an electron to jump to an outer, higher energy orbit.(The Amazing World of Electrons and Photons - Thinkquest http://library.thinkquest.org/16468/gather/english.htm)
A photon of the same energy is emitted when the electron falls back down to its original orbit.
animation of exciting an atom by colliding with another atom 
Electrons can also be raised to outer orbits when atoms collide
A photon of the characteristic energy is emitted when the electron falls back to its original orbit.
animation of emission-line and absorption spectrum formationIn astronomical situations, we may see either emission lines in a spectrum or absorption lines depending on the relationships of the the sources and gases involved (animation by G. Rieke)
An absorption line spectrum is produced when cool gas lies between a continuum source and us; the specific wavelengths absorbed by the atoms in the gas are removed from the light that comes to us.
An emission line spectrum is produced when photons are emitted by gas that is thin enough to be transparent in the continuum.

Absorption- and emission-line spectra:
If even more energy is supplied to an electron, it can escape from the atom leaving the positively charged nucleus. Because the electron is no longer transitioning between two specific energy states, the atom can absorb a range of energies in this situation. Electrons over a range of energies can be captured by the positive nucleus, emitting photons over a range of energiesbuttonex.jpg (1228 bytes)

Diagram of probability states of electrons in quantum mechanical atomAlthough it is convenient to draw protons, neutrons, and electrons as little dots, quantum mechanics tells us that they cannot be located accurately and are in fact more like fuzzy little fog clouds. We cannot predict precisely what they will do, leading to a scientific confrontation with the philosophy of determinism: science shows that there is fundamental uncertainty in what will happen in the future ribbon.jpg (3557 bytes)(Figure from The Essential Cosmic Perspective by Bennett et al.)

Significance of Spectra
Spectroscopy of astronomical sources has been a key to our understanding of the Universe because spectra are:
1) Aids in determining temperatures (can be more reliable than looking at the wavelength peak)

The higher the temperature, the more electrons are in high energy orbits or have escaped altogether from their atoms because high-temperature atoms run into each other at high enough speeds to shift the electrons. As a result, in hot gas there is emission and absorption of specific lines associated only with the high energy orbits that are inaccessible at low temperature. In this example, the cold atom can only absorb photons with energy corresponding to the transition between its two lowest energy levels, whereas the hot atom can absorb photons of that energy, plus ones corresponding to the transition between its second and third levels.

 
2) Probes of composition. Because each element (and also each type of molecule) has its own set of permitted orbits for its electrons and hence its own pattern of spectral lines, spectracan be used to determine what an object is made of (here are some examplesfrom A. Larson, http://www.astro.washington.edu/astro101v): 
Argon 
spectrum of argon
 Heliumhelium spectrum
Mercurymercury spectrum
 spectrum of sodium
 Neonneon spectrum
 We can consider spectra to be a "cosmic barcode" that identifies the conditions in the object(Fraunhofer spectrum of the sun, from R. Fosbury, http://www.stecf.org/~rfosbury/home/photography/Eclipse99/csp_description.html)
Fraunhofer spectrum of the sun

animation of Doppler shiftThe frequency of a wave is modified by the motion of a source toward or away from the observer. In the case of electromagnetic radiation:
Toward produces "blueshift" ==> spectral lines are shifted towards shorter wavelengths
Away produces "redshift" ==> spectral lines are shifted towards longer wavelengths
This animation shows why these changes occur. As the source moves toward the right, it "catches up" with the waves it has emitted in that direction and shortens their wavelength, shifting the light to the blue. Similarly, it "leaves behind" the waves it has emitted to the left, shifting the light to the red.(From Univ. of Saskatchewan, http://physics.usask.ca/~hirose/ep225/animation/doppler/anim-doppler.htm)
 
dopplereff.gif (24715 bytes)See how the wavelength of the sound into the boy's left ear is shortened in wavelength because the ambulance is approaching him, while the wavelength of the sound into his right ear is lengthened because the ambulance is moving away. The Doppler effect with light is similar to that with sound. It is how the policeman's radar gun works - it sends out a radio wave of a fixed wavelength and then measures the Doppler change in wavelength when the wave comes back, reflected off your car. The change in wavelength goes as the speed you are driving toward the policeman.
(From Japanese Aerospace Exploration Egency, JAXA, http://spaceinfo.jaxa.jp/note/shikumi/e/shi10_e.html.)

animation of Doppler shifting in a double starAlthough the entire spectrum is shifted, it is easiest to notice the shifts when looking at spectral lines because their wavelengths are so specific.(From R. McCray, http://cosmos.colorado.edu/astr1120/lesson1.html)
Doppler Effect as a Speedometer
The amount of frequency (or wavelength) shift is proportional to an object's velocity

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