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 * 10 8 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 energy Energy = frequency * Planck’s constant E = h f h = 6.63 * 10 -34 J s
Wrap – up So 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 * 10 8 m/sh = 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.
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)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 (H 2 CO) 829.66 MHz Methanol (CH 2 OH) 6668.518 MHz Helium Isotope (3HeII) 8665.65 MHz Cyclopropenylidene (C 3 H 2 ) 18.343 GHz Water Vapour (H 2 O) 22.235 GHz Ammonia (NH 3 ) 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 CH 4, N 2, H 2, and O 2 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).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 AbsorptionEmission or Absorption Relative Abundance Direction Velocity Rotation Temperature & PressureTemperature & Pressure Electric & Magnetic FieldsElectric & 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
Direction Spectral 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. Doppler Shift. Not moving Moving slow Moving fast
Rotation When 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. Doppler
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)learn more Zeeman Effect – Magnetic field (learn more)learn more Paschen-Back Effect – Strong Magnetic Field (learn more)learn more
Probes Spectral lines can also serve as Indirect measures of: Abundance Temperature Density
Abundance The hydrogen molecule (H 2 ) is the most abundant molecule in the galaxy. It does not, however, emit a strong spectral line. Several other molecules (CS, H 2 CO, HC 3 N) are used as probes. When H 2 collides with one of the these molecules, it can produce detectable radiation. So, in order to look for H 2, 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).learn more = change in wavelength = v source original v wave