the amazing spectral line begin. table of contents a light review introduction to spectral lines...
TRANSCRIPT
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
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
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.
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.
Here are some spectral lines
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)
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
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.
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).
For us, spectral lines look like this:
Remember that we could just as easily use frequency or energy along the x-axis
What can they tell us?
•Emission or Absorption• Relative Abundance
• Direction• Velocity• Rotation
•Temperature & Pressure•Electric & Magnetic Fields
• Probes
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.
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
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
Velocity
The relative change in wavelength is related to the velocity of the source. See: Doppler Shift.
Not moving
Moving slow
Moving fast
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.
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.
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)
Probes
Spectral lines can also serve as
Indirect measures of:
Abundance
Temperature
Density
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.
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.
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.
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!
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