Hippler L4 Flashcards
what is the idea that black body is based on?
an object in thermal equilibrium will emit as much energy as it absorbs. So good emitter and good absorber.
a perfecty absorber absorbs all electromagentic radiation and so appears black.
what is the realisation?
Have an enclosure with a small hole in one wall then there is little chance that the radiation is escaping and so is more of an absorber than emitter. Black body. Increase the temp then you get a faint red glow which becomes more intense upon heating. So in the cavity electromagnetic radiation.
what are the requirements for a 1D box?
Electric field must be zero at the walls.
Only wavelenghts that fit the length allowed (L = n *Wavelenth/ 2)
The modes must not be higher in frequency than the frequency of the radation.
is the energy of a standing wave limited?
Originally it was thought that for a standing wave there could be any energy and every oscillator has a mean energy of KT but it energy is limited to discrete values
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does the total irradiance increase or decrease with Temp?
increases as I is proportional to T^4
what are the three primary processes that occur between matter and radiation
absorption
spontaneous emission
stimulated emission
what are the rate laws for the three primary processes?
B is Einstein coefficient for absorption and A is for emission
Remember units
how is the absorption at the cross section (sigma) related to Einstein coefficient?
only 1,2 absorption counts
Abs are weak as [M] assumed to be constant
when do you get population inversion?
When the population of the excited state is higher than the ground state
what are the two requirements for LASERS
- population inversion so the medium is not quenched
2. A meta stable excited state, no spontaneous emission
How is do two level laser systems work and give an example?
Emission when A returns to ground state
Iodide lasers - simple dissociation
What are the problems with two level laser systems?
They are not efficient, A is not meta stable. Transitions from ground state and vice versa are difficult so are slow
describe three level laser systems and give an example?
Ground state to an intermediate state. I to A and then A to ground state which is slow as it is not allowed. Since ground state to I is fast stimulated emission is only viable route down.
Ruby laser - pumps X - I at 690nm
what is the benefit of three level systems over 2 level systems?
Metastable excited state easier to reach
what are the problems with three level laser system?
population inversion needs to be greater in A than in X but at thermal equilbrium X is heavily populated
describe 4 level laser systems and give an example?
Ais not populated to start with.
X- I - A is very fast so automatically have population inversion, then A`` - X is fast so population inversion condition is maintained
Carbon dioxide lasers - electrical discharge
What are the properties of lasers and how are they different to Black body radiation?
- Lasers have exponential growth
- Photons belong to one wavelength (BB photons have all wavelenghts)
- Only light travelling along an axis is amplified so therefore the laser has a direction and has low divergence. (BBR is divergent)
- all photons belong to 1 electromagnetic wave
What is the laser pulse?
If population inversion is achieved too quickly then laser action will use the excited state and then terminate itself - laser pulse
What is continuous wave laser?
Population inversion is achieved continuously, therefore have a continuous discharge
what are the requirements for high resolution spectroscopy?
Narrow and monochromatic bands
v small spectroscopic splitting line
How are small spectroscopic splitting lines achieved?
using a grating inside the resonator. The resonator only allows one frequency mode to travel in s straight line between the two mirrors, so that is amplified by reflection between to two mirrors
what happen if you coherently couple lots of resonator modes together
Get a large Δ and therefore small Δt, ultrashort laser pulse
how is ultrashort laser pulses achieved, draw a diagram?
Photon is amplified back and fourth and every time it hits the output mirror and fraction is lost. this fraction is the shortest possible laser pulse
why do you need to prevent the build of one frequency laser resonator mode to achieve ultrashort pulses?
A build of up one frequency means Δ will be narrow and therefore Δt will be large.
for multiphoton processes is a high light intensity needed to low light intensity
high light intensity where all photons are absorbed at once
what is dispersive or non dispersive spectroscopy?
whether radiation has been pre filtered to the wavelegth that correspond to the absorbance of the sample
what is NDIR spec
non - dispersive infrared spectroscopy
There is a sensor cell that is split into two compartments by a membrane.
In the first compartment IR falls directly onto the sensor cell as it contains a reference gas. In the second compartment Ir falls after it has passed through the sample gas.
If there is no absorption then both compaartments recieve the same amount of IR. there is equal heating and no pressure difference so no force on membrane.
If there is absorbance (from sample gas) then IR falling onto compartments is different, there is unequal heating and therefore a pressure difference. This exerts a force onto the membrane that is proportional to the concentration of the sample
advantages and disadvantages of NDIR spec?
cheap and simple
sensitive to interference with other molecules that absorb in that spectral range. So only useful if the gas to be analysed dominates absorption.
how is light dispersed in Dispersive spec?
uses a grating which is better and cheaper than a prism
How does a grating work?
slit in the grating which detects a particular wavelength. Rotate the grating to detect all wavelengths q
what is the disadvantage of the grating?
Wasteful as only a small fraction of light intensity is measured (going through the slit)
How does a photoiodide improve dispersive techniques?
Each photoioide detects one colour at a time. the grating doesnt have to be rotated and no light is wasted. Also obtain the entire spectrum at once
What is Fourier transformation?
White light source that is split into two beams by a splitter. Each bean is reflected back by two different mirrors in the detector. One mirror is moving so detector detects a changing interference pattern. FT gives intensity distribution as a function of wavenumber. Light source contains many different wavenumber.
advantages of FT?
All light intensity is measured at once
What is DOAS
Electronic spectrscopy in the UV and Visible Range
what is the problem with DOAS?
Broad overlapping of spectral lines
difficult to find and define baseline
How the problem in DOAS overcome?
Measure the left and right hand side of the absorption peak. Interpolate an effective basline. Take the difference between the baseline and peak and use this in beer lambert law?
what is the experimental conditions in DOAS
Xe light source in focus of a parrabolic mirror. Parallel light travels in air for a X amount of Km. reached a newton telescope which focuses light to the detector.
how does LIDAR work?
An intense laser pulse is sent to the sky. some light is scattered back and detected. The time difference between the laser pulse and receiving the scattering can be used to determine at which distance along the beam path was light scattered. The intensity of the scattered light can be used to determine the conc of scattered light at this distance.
what is the disavantage of LIDAR
Limitation on time resolution and since time and distance are related then you get a limitation on distance resolution
what is the main problem with using the beer lambert law in LIDAR?
Light intensity of the original light is different when scattered so cant use beer Lambert Law if only I is measured
how is the problem of the beer lamber law overcome using DOAS?
measure at the wavelength that correspond to the absorption and a different wavelength. Take the difference
what is DIAL?
Differential Absorption LIDAR - combines LIDAR and DOAS
what is Cavity ring down spec - CRD?
Light is coupled into a optical cavity that is composed of highly reflective mirrors and switched off. Light is reflected thousands of times and intensity decays exponentially due to loses on mirror and absorption from the sample.
In a typical decay curve from CRD how many cirves are there and what do they signify?
There are two decay curves.
The faster decay corresponds to the wavelength at which sample inside cavity absorbs light
The slower decay curve corresponds to the wavelength where it does not absorb light.
what are indirect absorption techniques?
the primary process of photon absorption is not directly observed
What is photoacoustic spec?
PA - After molecular collisions energy is released as heat which creates time independent pressure fluctuations (sound) sound intensity is proportional to absorption cross sections.
What is LIF and REMPI?
absorption of photon in visible/UV region promtes e to excited states,
Either flourseces to ground state by spontaeous emission - intensity is proportional to absorption
or absorbs another photon and is ionised, the ion current is proportional to the first absorption step
what are °2 °1
antisymmetric and symmetric solutions to schrodinger
Give the equations for °2 °1
1/√2 +/- °2 °1
What is W
Coupling matrix, interaction energy
how are stationary eigenstates measured?
high resolution spectroscopy
what does it mean if there is a splitting energy between time independent states
τ is ultrafast
in a double energy minimum potential what seperates the the 2 energy minima?
Barrier
in a double energy minimum potential what is tunneling
Penetrate the barrier
what does tunneling depend on
The barrier - the lower the barrier delta E between symmetric and antisymmetric superposition is large, tunneling is faster
The larger the barrier, tunneling is difficult, delta E is small t is slow
Tunneling depends on mass of particle too. light particle t is fast
heavy particle - t is slow
