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Page 1: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

The Amazing Spectral Line

Begin

Page 2: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Table of Contents

A light review

Introduction to spectral lines

What spectral lines can tell us

Page 3: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

A Light Review

• Light is both a particle and a wave.

• Being a wave, it has both a wavelength and a frequency.

• Wavelength () – distance between peaks

• Frequency (f ) – number of cycles per second

Page 4: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Some formulas

The speed of light (c) is constant in a vacuum 3.00 * 108 m/s

Wavelength x frequency = the speed of light

f = c This means a high frequency wave has a short

wavelength and a low frequency wave has a long wavelength.

Light carries energyEnergy = frequency * Planck’s constant

E = h f h = 6.63 * 10-34 J s

Page 5: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Wrap – upSo with light waves, you can convert between wavelength,

frequency, and energy with two equations:

f = c E = f h And two constants:

c = 3.00 * 108 m/s h = 6.63 * 10-34 J s

In the visible part of the spectrum, different colors correspond to different frequencies, wavelengths and

energies. Blue light has a short wavelength, high frequency and high energy. Red light has a long

wavelength, low frequency, and low energy.

Page 6: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

An Element’s Fingerprint

• When excited by heat or electricity, gases glow with characteristic colors.

• A prism can be used to spread out the light from these hot gases.

• This reveals a series of discrete lines, the element’s fingerprint.

• Chemists use these fingerprints (called spectral lines) to identify elements both in the lab and in space.

Page 7: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Here are some spectral lines

Page 8: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Where do they come from? The Bohr Model

In the Bohr Model of the atom, electrons orbit in discrete energy levels. When an electron jumps to a lower energy

level, the extra

energy is given

off as radiation.

This is where

those color

fingerprints

come from.

(learn more)

Page 9: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

A Molecule’s Fingerprint

Molecules, like atoms, have characteristic spectral lines. Usually scientists look for a few specific lines to identify a molecule. Above is a list of some astronomical chemicals and their corresponding frequencies. Find radiation at one of these frequencies, and you’ve found a molecule.

• Hydroxyl radical (OH) 1612.231 MHz• Methyladyne (CH) 3263.794 MHz

• Formaldehyde (H2CO) 829.66 MHz

• Methanol (CH2OH) 6668.518 MHz

• Helium Isotope (3HeII) 8665.65 MHz

• Cyclopropenylidene (C3H2) 18.343 GHz

• Water Vapour (H2O) 22.235 GHz

• Ammonia (NH3) 23.694 GHz

Page 10: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Where do they come from?Rotation

• Molecules, like atoms, can occupy different energy states. Diatomic (2 atom) molecules can rotate in two different ways

• As the molecule changes rotation states, it emits radiation at a characteristic frequency.

Page 11: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Some Difficulties in Detecting Molecules

Molecules in space are detected through spectral lines, often times the line from one rotational state to another. Symmetric molecules like CH4, N2, H2, and O2 don’t have rotational states, making them harder to detect. Also, in order to identify a molecule, a researcher must identify its spectral line. This is sometimes very difficult because molecules in space are under very different conditions (very low pressure, no container walls) than are found in the laboratory. One example is the “forbidden” line of O+2. Many spectral lines observed in space have not yet been identified (learn more).

Page 12: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

For us, spectral lines look like this:

Remember that we could just as easily use frequency or energy along the x-axis

Page 13: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

What can they tell us?

•Emission or Absorption• Relative Abundance

• Direction• Velocity• Rotation

•Temperature & Pressure•Electric & Magnetic Fields

• Probes

Page 14: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Emission or Absorption

• When the spectral line is emitting by a heated gas, it appears as a spike.

• When the spectral line is absorbed by a cool gas, it appears as a valley.

• Knowing whether your spectral line is an emission or absorption line tells you if the gas it came from is relatively hot or cold.

Page 15: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Relative Abundance

The following graph shows two spectral lines for two different atoms or molecules. The line

on the left is much more intense than the line on the

right. This indicates that the atom or molecule represented by the line on the left is more abundant

Page 16: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

DirectionSpectral lines can tell us the direction in which their source is moving. If the source is moving towards the receiver, the spectral line will be shifted to a shorter wavelength (blue shifted). If the source is moving away from the receiver, the spectral line will be shifted to a longer wavelength (red shifted).

Not moving

Moving toward receiver

Moving away from receiver

Page 17: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Velocity

The relative change in wavelength is related to the velocity of the source. See: Doppler Shift.

Not moving

Moving slow

Moving fast

Page 18: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

RotationWhen an object rotates, part of it moves towards the observer and is blue shifted. Part of it moves away and is red shifted. This leads to Doppler broadening. The degree of broadening reveals the rate of rotation.

Page 19: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Temperature / Pressure Broadening

Thermal motion and high pressure can broaden spectral lines by causing individual molecules to experience significant Doppler shifts. The natural width of a spectral line is very small. It comes from the Heisenberg Uncertainty Principle.

Page 20: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Electric & Magnetic Fields Cause line splitting

Stark Effect – Plasma density (learn more)

Zeeman Effect – Magnetic field (learn more)

Paschen-Back Effect – Strong Magnetic Field (learn more)

Page 21: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Probes

Spectral lines can also serve as

Indirect measures of:

Abundance

Temperature

Density

Page 22: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Abundance

The hydrogen molecule (H2) is the most abundant molecule in the galaxy. It does not, however, emit a strong spectral line. Several other molecules (CS, H2CO, HC3N) are used as probes. When H2 collides with one of the these molecules, it can produce detectable radiation. So, in order to look for H2, we look for a probe molecule.

Page 23: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Temperature

Symmetric top molecules like methane and ammonia are valuable temperature probes. At high temperatures, collisions with other particles excite these molecules to higher energy states. They return to lower energy states by certain spectral lines. Detecting these spectral lines gives information about the temperature of a region of space.

Page 24: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Density

Some spectral lines are only formed in dense environments. Find one of these lines, and you’ve measured the density of the surrounding region. This is especially useful when studying star-forming regions.

Page 25: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Doppler Shift

The Doppler Shift is the change in wavelength of a wave due to the relative motion of the source and receiver. It is the reason why a car seems to change sounds as it passes by.

MORE!

Page 26: The Amazing Spectral Line Begin. Table of Contents A light review Introduction to spectral lines What spectral lines can tell us

Another formula

To calculate the change in wavelength due to the Doppler shift, use the following equation (learn more).

= change in wavelength

= vsource

original vwave