A few Experimental methods for optical spectroscopy Classical methods Modern methods Shorter class Remember class #1 Generating fast LASER pulses, 2017 Uwe Burghaus, Fargo, ND, USA
W. Demtröder, Laser Spectroscopy, Springer Series in Chemical Physics 5 Haken, Wolf, atomic and quantum physics, chapter 22 Lecture drafts http://home.uni-leipzig.de/energy/pdf/freuse7.pdf https://www.saylor.org/site/wpcontent/uploads/2012/07/chapter1011.pdf
Understanding atomic structure Test fundamentals of quantum mechanics
Extremely small peak splitting of spectral lines, small λ Extremely small peak shifts High spectral resolution techniques required Larges values of λ/ λ (resolving power) needed
Δ λ spectral resolution Δ λ smallest difference in wavelengths that can be distinguished at a wavelength of λ Δ λ Resolving power R = λ λ Goal Small Δ λ Large R
Technique The very basics
LASER Blackbody radiation Synchrotron radiation Gas discharge
Transducer Class Wavelength Range Output Signal phototube photon 200 1000 nm current photomultiplier photon 110 1000 nm current Si photodiode photon 250 1100 nm current photoconductor photon 750 6000 nm change in resistance photovoltaic cell photon 400 5000 nm current or voltage thermocouple thermal 0.8 40 m voltage thermistor thermal 0.8 40 m change in resistance pneumatic thermal 0.8 1000 m membrane displacement pyroelectric thermal 0.3 1000 m current
Channeltron = continuous channel electron multiplier ion HV A UHV technique SRS mass spec ground
Classical Devices based on classical linear optics: prisms, diffraction gratings, interferometer, Modern Non-linear optics based devices, quantum beats, fast LASER pulses, saturation spectroscopy, doppler-free spectroscopy,, 2017 Uwe Burghaus, Fargo, ND, USA
Technique Diffraction gratings
Usually discussed in an introductory physics class about classical optics λ/ λ ~ 10 5 (resolving power) Prisms Diffraction gratings λ λ Nm N: number of lines (grating rulings) m: diffraction order Problems: Diffraction limit Intensity of signal [ blazed gratings ]
Diffraction gratings spectrometer PChem Quantum mechanics light source slit lens diffraction gratings lens detector
Technique Interferometer
Usually discussed in an introductory physics class about classical optics λ/ λ ~ 10 5 (resolving power) Prisms Diffraction gratings λ λ Nm N: number of lines (grating rulings) m: diffraction order Problems: Diffraction limit Intensity of signal [ blazed gratings ] λ/ λ > 10 6 Interferometer
Michelson Interferometer Details in class #10 experimental Figure 19.18 Engel/Reid
Usually discussed in an introductory physics class about classical optics λ/ λ ~ 10 5 (resolving power) Prisms Diffraction gratings λ λ Nm N: number of lines (grating rulings) m: diffraction order Problems: Diffraction limit Intensity of signal [ blazed gratings ] λ/ λ > 10 6 Interferometer Parallel-plate interferometer or Fabry Perot interferometer, etalon Trick: multiple reflections on parallel plates give one large diffraction orders m=10 5 [ Wikipedia ] [ try this one ]
λ 1 λ 2 λ 1 & λ 2 F-P interferometer only transmits light which closely match the constructive interference condition.
Classical Devices based on classical linear optics: prisms, diffraction gratings, interferometer, modern Non-linear optics based devices, quantum beats, fast LASER pulses, saturation spectroscopy, doppler-free spectroscopy,, 2017 Uwe Burghaus, Fargo, ND, USA
Technique Quantum beats Coherent spectroscopy
1.0 amplitude 0.5 0.0-0.5-1.0-200 0 200 time E fluorescence Interference of the fluorescence signals ω = E/h time Concept of coherent LASER spectroscopy: use large bandwidt pulse results in coherent excitation of states Two waves are coherent when they have a constant phase difference and the same frequency, and the same waveform. https://en.wikipedia.org/wiki/coherence_(physics) Simplest example of coherent spectroscopy
Technique Lamp dip
If we reach high resolution with a spectrometer we become limited by Line shape functions Homogeneous /inhomogeneous Natural line width Lorentzian line shape function Gaussian line shape Voight line shape Pressure broadening Doppler broadening Transit-time broadening Power broadening
some key words Saturation spectroscopy Spectral hole burning Lamb dip Two photon spectroscopy
f ( V x ) = m 2πkT mvx / 2kT f ( Vx ) e 2 dv x f (c) V x f ( c) = 4πc 2 ( m 2πkT ) 3/ 2 e 2 mc / 2kT dc c Boltzmann Maxwell
Doppler effect Change of wavelength caused by motion of the source movie moving stationary stationary ν = ν ( 1± ν c) Use Maxwell-Boltzmann distribution for speeds Inhomogeneous Gaussian line shape function moving ν ν T T: gas temperature 1/ M 2 M: atomic mass Much larger than natural line width Christian Andreas Doppler (1803 1853) Austrian mathematician and physicist https://en.wikipedia.org/wiki/christian_doppler
of the excited state population excited state, <2 N 2,v ω 0 ground state, <1 v = 0 v # of excited atoms N 2 with velocity v according to Maxwell distribution
For a two-level system including spontaneous emission one would see this natural lifetime broadening. ω = hν = E 1 E 0 Lorentzian line shape function g( ω ω ) = 0 1 γ = τ γ (2π ) 2 ( γ 2) ( ω ω ) 0 2 Width of the Lorentzian line shape function is consistent with Heisenberg uncertainty principle. ν γ 2π 1 / 2 = = 1 2πτ Fundamental limit on linewidth due to transition between the states. We cannot be better than this https://en.wikipedia.org/wiki/spectral_line#line_broadening_and_shift ν 1/ 2 τ FWHM Lifetime
gas at rest ω 0 excited state ground state ω ω 0 < natural line width considering speed distribution (gas/emitter moving) ω ω v 0 + ω < c 0 natural line width frequency of moving photon (with respect to the absorber)
http://www.phys.ufl.edu/courses/phy4803l/group_iii/sat_absorbtion/satabs.pdf Hole width is the natural line width
Technique Two-photon spectroscopy
frequency of moving (with respect to the absorber) photon ω ωleft = ω (1 + right = ω (1 v v 0 c 0 c ) ) moving to the left moving to the right excited state Resonance condition for absorbing both photons ω right ω left ground state E = ω v v left + ωright = ω0( 1+ ) + ω0(1 ) = 2ω c c 0 The clue: independent of v
Technique Level crossing Coherent spectroscopy
Energy E2( t) = a2 cos( ω2t) excited states Resonant emission & excitation E1( t) = a1 cos( ω1t ) ground state Magnetic field I [ E a + a 2 1 ( t) + E2( t)]... 2 1 2 2
Energy E2( t) = a2 cos( ω2t) Energy E2( t) = a2 cos( ωt) E1( t) = a1 cos( ω1t ) E1( t) = a1 cos( ωt) Magnetic field Magnetic field I [ E a + a 2 1 ( t) + E2( t)]... 2 1 2 2 I [ E ( t) + E t a + a 2 2 1 2( )] [ 1 2 ] Excitation with the same LASER beam Two levels Resonant process
Life time of the states g-factor measurement
Technique
Technique W. Demtröder, Laser Spectroscopy, Springer Series in Chemical Physics 5
Class 11 Raman spectroscopy Class 13 fs spectroscopy LIF MPI Pump & probe
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https://en.wikipedia.org/wiki/absorption_spectroscopy
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