A few Experimental methods for optical spectroscopy Classical methods Modern methods. Remember class #1 Generating fast LASER pulses

Transcription

1 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

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4 W. Demtröder, Laser Spectroscopy, Springer Series in Chemical Physics 5 Haken, Wolf, atomic and quantum physics, chapter 22 Lecture drafts

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6 Understanding atomic structure Test fundamentals of quantum mechanics

7 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

8 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

9 Technique The very basics

10 LASER Blackbody radiation Synchrotron radiation Gas discharge

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12 Transducer Class Wavelength Range Output Signal phototube photon nm current photomultiplier photon nm current Si photodiode photon nm current photoconductor photon nm change in resistance photovoltaic cell photon nm current or voltage thermocouple thermal m voltage thermistor thermal m change in resistance pneumatic thermal m membrane displacement pyroelectric thermal m current

13 Channeltron = continuous channel electron multiplier ion HV A UHV technique SRS mass spec ground

14 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

15 Technique Diffraction gratings

16 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 ]

17 Diffraction gratings spectrometer PChem Quantum mechanics light source slit lens diffraction gratings lens detector

18 Technique Interferometer

19 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

20 Michelson Interferometer Details in class #10 experimental Figure Engel/Reid

21 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 ]

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23 λ 1 λ 2 λ 1 & λ 2 F-P interferometer only transmits light which closely match the constructive interference condition.

24 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

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26 Technique Quantum beats Coherent spectroscopy

27 1.0 amplitude 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. Simplest example of coherent spectroscopy

28 Technique Lamp dip

29 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

30 some key words Saturation spectroscopy Spectral hole burning Lamb dip Two photon spectroscopy

31 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

32 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 ( ) Austrian mathematician and physicist

33 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

34 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 ν 1/ 2 τ FWHM Lifetime

35 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)

36 Hole width is the natural line width

37 Technique Two-photon spectroscopy

38 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

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40 Technique Level crossing Coherent spectroscopy

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42 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)]

43 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)] I [ E ( t) + E t a + a ( )] [ 1 2 ] Excitation with the same LASER beam Two levels Resonant process

44 Life time of the states g-factor measurement

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47 Technique

48 Technique W. Demtröder, Laser Spectroscopy, Springer Series in Chemical Physics 5

49 Class 11 Raman spectroscopy Class 13 fs spectroscopy LIF MPI Pump & probe

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54 Figure acknowledgement All images shown in this power point presentation were made by the author except the following with are excluded for the copyright of the author: xxx No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means except as permitted by the United States Copyright Act, without prior written permission of the author. Trademarks and copyrights are property of their respective owners., 2016 Publisher and author: Uwe Burghaus, Fargo, ND, USA