wavelength – frequency measurementswimu/educ/em-sheets-1.pdfwavelength – frequency measurements....
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Masters Course: Experimental Techniques
Wavelength – Frequency Measurements
Frequency: - unit to be measured most accurately in physics- frequency counters + frequency combs (gear wheels)- clocks for time-frequency
Wavelength:- no longer fashionable- unit [m] no longer directly defined- always problem of the medium- index of refraction
Units: exercise convert - Ångstrom -> nm- eV, J, cm-1, Hz
Masters Course: Experimental Techniques
Notes
Laser Spectroscopy, Vol 1W. Demtröder
Chapter 4 Spectroscopic instrumentation (4.1 – 4.4)
Spectrographs, Monochromators, Prisms and Gratings
InterferometersFabry-Perot (etalon), Michelson, Mach-Zehnder
Wave metersMichelson, Sigma, Fizeau, Fabry-Perot
Chapter 9 Frequency measurement/Frequency comb (9.7)
Masters Course: Experimental Techniques
Until 1960
(Wave)Length standard
smc 299792458=
tcL ⋅=Now, since 1983
Krypton (Kr): International Standard of Length
The picture shows a device holding a tube of krypton gas. The isotope Kr-86 contained in the tube can be excited so
that it emits light. The international standard of length is one meter, which is 1,650,763.73 wavelengths of radiation
emitted by Kr-86.
A practical realisation of the metre is usually delineated (not defined) today in labs as 1,579,800.298728(39) wavelengths of helium-neon laser light in a vacuum.
Masters Course: Experimental Techniques
10-13 accuracy
Time standard (and realization)
Cs fountain clock
the duration of 9,192,631,770 periods of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the cesium 133 atom
Masters Course: Experimental Techniques
Classical Spectrometers
Spectral resolution limited by the diffraction determined by total aperture(size of prism or grating) study this
Prisms:how does the dispersion (resolution) depend ongeometry/material of prism
Grating:study the avantages of the “Echelle grating”
Masters Course: Experimental Techniques
Spectroscopy and Calibration
Absolute measurements (wavelength or frequency)
Relative measurements (line separations)
Spectral referencing (use of atlases)I2, Te2, Hollow-cathode lamps, Th-Ar
Scanning vs Multiplex spectroscopy
Masters Course: Experimental Techniques
Example: The Ultraviolet-Visibile Echelle grating spectrometer at VLT
Masters Course: Experimental Techniques
8 m mirror at VLT
Masters Course: Experimental Techniques
Masters Course: Experimental Techniques
Masters Course: Experimental Techniques
Detection of all orders on the CCD
Masters Course: Experimental Techniques
Instant calibration of all orders by illuminating with ThAr lamp
Masters Course: Experimental Techniques
Science CCD’s Calibration CCD’s
Masters Course: Experimental Techniques
Spectral referencing in laser spectroscopy
H2Oabsorption
Masters Course: Experimental Techniques
Frequency conversion and calibration
Masters Course: Experimental Techniques
Referencing against tellurium 130Te2
Masters Course: Experimental Techniques
Precision Doppler Free Spectroscopy
1. Saturation spectroscopy – Lamb dips2. Polarization spectroscopy3. Two-photon spectroscopy4. Molecular beam spectroscopy
Bennet hole
Bennet peak
Burn a fraction out of the velocity distribution
Masters Course: Experimental Techniques
Lamb Dips
Saturation holesLamb dip in scanning
Willis E LambNobel Prize in Physics 1955
Masters Course: Experimental Techniques
Lamb Dips
Saturation inHomogeneous broadening
Saturation inHeterogeneous broadening
Standing wave field
Saturation inHeterogeneous case
Weak fieldprobing
Masters Course: Experimental Techniques
Lamb dip spectroscopy unraveling overlapping lines
Masters Course: Experimental Techniques
Saturated absorption for referencing
+ Phase sensitive detectionlock-in principle
Masters Course: Experimental Techniques
Doppler-free two-photon absorption (excitation)
Doppler shift: vk ⋅−=ωω '
Resonance condition: ( ) ( ) ( )212121 ''/ kkvEE if
+⋅−+=+=− ωωωω
All molecules, independent of their velocities, absorb at the sum frequency
ωωω 221 =+
Masters Course: Experimental Techniques
Sub-Doppler spectroscopy in a beam
xkvvk −=⋅−= 00' ωωω
Reduction of the Doppler width:
εωω sin' DD ∆=∆ 2ln2 0 cvp
D ωω =∆
Masters Course: Experimental Techniques
Molecular beam spectroscopy
Masters Course: Experimental Techniques
Laser-based calibration techniques“Harmonics” + “saturation”
Masters Course: Experimental Techniques
P(3) C-X(1,0)
R(0) B-X(9,0) line
Calibration of H2 spectral lines (in XUV)