Line Spectra Flashcards
What is the ground state?
n = 1
The lowest energy level electrons can exist in
If an electron is in an energy level above n = 1 it is _______
excited
n = ∞
Ionised
What is emitted when electron de-excite?
A photon is emitted each time an electrons moves downwards
What determines the energy of photons emitted in electron transitions
The difference between the initial and final energy levels (the photon conserves energy)
Why are line spectra specific to elements?
- Each element has different discrete energy levels
- Electrons drop specific distances between these levels
- Discrete photons are emitted by each different drop
- So the frequencies of the photons correspond to a certain element
Ionisation energy (physics)
The minimum energy needed to to remove an electron from the ground state to be completely free of the atom.
n=1 → n=∞
How can electrons in gas atoms be ionised?
Through collisions with photons or electrons
Structure of a gas discharge lamp
- Glass tube
- Cathode and anode at opposite ends
- Contains a low pressure gas
How does a gas discharge lamp work?
- High voltage ionises gas atoms
- Positive ions attracted to the cathode cause release of electrons
- Electrons are accelerated towards the anode, colliding and ionising more atoms
- Photons are emitted when positive ions and electrons recombine
- Photons are emitted when gas atoms de-excite
When will a current electron transfer energy to an atomic gas electron? (inelastic collision)
When the current electron has sufficient energy to excite the gas electron.
Any additional energy remains in the current electron as KE
When will a photon transfer energy to an atomic gas electron?
When the photon has exactly the same energy as one of the electron transitions.
It is absorbed by the electron, and another photon is emitted when the electron de-excites
What produces a continuous spectrum?
Hot source (black body radiation)
What produces an absorption spectrum?
A cool gas that absorbs photons and excites
What produces an emission spectrum?
An excited gas de-exciting
Continuous spectrum
All frequencies of light
Absorption spectrum
Specific frequencies are absorbed and re-emitted in random directions
Emission spectrum
Specific frequencies are emitted that match the decrease in energy
How are spectra used in astrophysics?
Identifying gasses present in the photospheres of stars
How are gasses identified from light coming from stars?
- Stars emit a continuous spectrum
- Photons collide with gases in the stars photosphere and are absorbed
- Photons are re-emitted in all directions
- These frequencies appear darker in the spectrum which can be compared with gas emission spectra
What gas is used in fluorescent gas tubes?
Mercury vapour
What range of photons are emitted when mercury atoms de-excite?
Mostly UV
Why is a phosphor coating used in fluorescent lamps?
Phosphor atoms have lots of energy transitions that emit photons in the visible range, this converts the UV light to visible light.
