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Spectroscopy and Atomic Structure Ch 04. Four essential questions:. What is an atom ? How do atoms interact with light? What kinds of spectra do you see when you look at celestial objects? What can you learn from a star’s spectrum?. Objectives (Ch 04). So far, l tells us ….

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Spectroscopy and

Atomic Structure Ch 04


Four essential questions
Four essential questions:

  • What is an atom?

  • How do atoms interact with light?

  • What kinds of spectra do you see when you look at celestial objects?

  • What can you learn from a star’s spectrum?


Objectives (Ch 04)

  • So far, l tells us ….

  • Color

  • Temperature

  • Energy Flux

  • Some Doppler

The Enigma, to understand how stars work  need to understand how atoms work????

Explain the Formation of Spectral Lines

The Energy Levels of the Hydrogen Atom

The Photoelectric Effect

Chemical Composition

Explain how spectral lines apply to molecules

Explain what information Spectral-Lines provide


Stellar spectra

0

Stellar Spectra

The spectra of stars are more complicated than pure blackbody spectra.

They containcharacteristic lines, called absorption lines.

With what we have learned about atomic structure, we can now understand how those lines are formed.


Kirchhoff s laws of radiation 1

0

Kirchhoff’s Laws of Radiation (1)

  • A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.


Kirchhoff s laws of radiation 2

0

Kirchhoff’s Laws of Radiation (2)

2. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.

Light excites electrons in atoms to higher energy states

Transition back to lower states emits light at specific frequencies


Kirchhoff s laws of radiation 3

0

Kirchhoff’s Laws of Radiation (3)

3. If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

Light excites electrons in atoms to higher energy states

transition energies are absorbed from the continuous spectrum.


The spectra of stars

0

The Spectra of Stars

The inner, dense layers of a star produce a continuous (blackbody) spectrum.

Cooler surface layers absorb light at specific frequencies.

=> Spectra of stars are absorption spectra.


Analyzing absorption spectra

0

Analyzing Absorption Spectra

  • Each element produces a specific set of absorption (and emission) lines.

  • Comparing the relative strengths of these sets of lines, we can study the composition of gases.

By far the most abundant elements in the Universe


Spectral Lines

Emission spectrum can be used to identify elements:


Spectral Lines

An absorption spectrum can also be used to identify elements. These are the emission and absorption spectra of sodium:


Spectral Lines … Bottom Line

Kirchhoff’s laws:

1. Luminous solid, liquid, or dense gas produces continuous spectrum

2. Low-density hot gas produces emission spectrum

3. Continuous spectrum incident on cool, thin gas produces absorption spectrum


Formation of Spectral Lines

Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed.

Bohr model had certain allowed orbits for electron:


Quantized energy
Quantized Energy

  • Continuous energy is like a ramp.

  • Quantized energy is like a stair case.

  • Each stair increases the energy by the value of Planck’s constant

  • h = 6.63x10-34 J-s

  • E = hf

  • C = lf

  • 1eV = 1.602 x 10-19 J

  • En = 13.6 eV (1 – [1/n2])


The dual nature of light
The Dual Nature of Light

Light is a wave

  • Reflection

  • Refraction

  • Interference

  • Polarization

    Light is a particle

  • Photoelectric effect

  • reflection

    E = hf

Light is both a

wave and particle!!


  • http://jersey.uoregon.edu/vlab/elements/Elements.html;

  • Become familiar with the absorption and emission of various elemental spectra. Pay particular attention to H, He, and the noble gases.

  • 1. Set the applet to “absorption.” What do you notice about the spectral lines? What is happening at the atomic level with respect to electron transitions?

  • 2. Set the applet to “emission.” What do you notice about the spectral lines? What is happening at the atomic level with respect to electron transitions?


The Bohr model is a primitive model of the hydrogen atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

However, because of its simplicity, and its correct results for selected systems, the Bohr model is still commonly taught to introduce students to quantum mechanics, before moving on to the more accurate but more complex valence shell atom. The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a full-blown quantum mechanics (1925) is often referred to as the old quantum theory.

In the model above, various electron transitions are shown, where n = energy level. The ground state is given by, n = 1. Transitions starting or ending at the ground state are known as the “Lyman” series, discovered in 1914 by Theodore Lyman. The first is Lyman-a (n = 2 to n = 1), then Lyman-b (n = 3 to n = 1), and so on. When n = ∞, ionization occurs


Formation of Spectral Lines a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

Energy levels of the hydrogen atom, showing two series of emission lines:


Formation of Spectral a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory. Lines … bottom line

  • Absorption can boost an electron to the second (or higher) excited state

  • Two ways to decay:

  • to ground state

  • cascade one orbital at a time


Formation of Spectral Lines a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

(a) Direct decay (b) cascade


Formation of Spectral Lines a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

Absorption spectrum: created when atoms absorb photons of right energy for excitation

Multielectron atoms: much more complicated spectra, many more possible states

Ionization changes energy levels


Formation of Spectral Lines a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

Emission lines can be used to identify atoms:


Molecules a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

Molecules can vibrate and rotate, besides having energy levels

Electron transitions produce visible and ultraviolet lines

b) Vibrational transitions produce infrared lines

c) Rotational transitions produce radio-wave lines


Molecules a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

Molecular spectra are much more complex than atomic spectra, even for hydrogen:

H2

H

(a) Molecular hydrogen (b) Atomic hydrogen


Spectral-Line Analysis a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

  • Information that can be obtained from spectral lines:

  • Chemical composition

  • Temperature

  • Radial velocity:


Spectral-Line Analysis a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

  • Line broadening can be due to Doppler shift

  • from thermal motion

  • from rotation


Zeeman Effect … It’s magnetic a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

http://phys.educ.ksu.edu/vqm/free/zeemanspec.html


The Doppler Effect a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.


The doppler effect

0 a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

The Doppler Effect

Take l0 of the Ha (Balmer alpha) line:

Assume, we observe a star’s spectrum with the Ha line at l = 658 nm. What is the shift and radial velocity?


Spectral-Line Analysis a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.


Summary a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

  • Spectroscope splits light beam into component frequencies

  • Continuous spectrum is emitted by solid, liquid, and dense gas

  • Hot gas has characteristic emission spectrum

  • Continuous spectrum incident on cool, thin gas gives characteristic absorption spectrum


Summary, cont. a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

  • Spectra can be explained using atomic models, with electrons occupying specific orbitals

  • Emission and absorption lines result from transitions between orbitals

  • Molecules can also emit and absorb radiation when making transitions between vibrational or rotational states


The amazing power of starlight

0 a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.

The Amazing Power of Starlight

Just by analyzing the light received from a star, astronomers can retrieve information about a star’s

  • Total energy output

  • Surface temperature/color

  • Radius

  • Chemical composition

  • Velocity relative to Earth

  • Rotation period


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