Level%20crossing%20spectroscopy

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Level crossing spectroscopy

• rudresh ghosh
• saurish chakrabarty

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Introduction

• Well known Doppler-free technique
• Pre laser era atomic spectroscopy
• LASERs used for optical pumping which allows
  molecular spectroscopy

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Topics of Discussion

• Basic principle and exptl setup
• Hanle effect
• Quantum mechanical description
• Advantages disadvantages
• Modifications

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Basic principle and exptl setup
Schematic expterimental setup for level crossing
spectroscopy
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Basic principle and exptl setup

• Sample placed in uniform magnetic field
• Illuminated by polarized light
• Emits fluorescence
• Shape of signal Ifl(B) gives gt

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Important equations

• E(t)E(0)exp-(iw12g/2)(t-to)
• wLgjmoB/h
• I(B,a,t)IOexp-g(t-to)cos2wL(t-tO)-a
• I(B,a)R-8?t IOexp-g(t-to)cos2wL(t-tO)-adtO
• IOR1/g(gcos2a2wLsin2a)/(g24wL2)
  /2
• I(B)(I0R/2g)1-(1/(1(2gm0B/hg)2)
• DB1/2hg/gmo
• h/mogt

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Hanle Effect

• In case of Zeeman subl-evels of a degenewrate J
  level, the M levels cross each other at B0,when
  the field is tuned from ve to ve values. This
  is called Hanle effect or zero-field level
  crossing.

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Quantum mechanical description

• Level scheme for optical pumping of Zeeman
  levels by light polarised in y-direction.Each of
  the two excited Zeeman levels can decay into
  three final state sub-levels.Superpsition of two
  different routes (J,M) -gt(Jf,M) generates
  interference effects

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Quantum mechanical description

• I2gtx-1/21/2aM1IM1gtaM-1IM-1gt
• I2gty-i/21/2aM1IM1gt-aM-1IM-1gt
• y2(t)e-g(t-t0)/2 e-iH(t-t0)/h y2(t0)
• e-g(t-t0)/2 e-i(E0mBgMB)(t-t0)/hIy2gtxsinmBgB(
  t-t0)/h Iy2gtycosmBgB(t-t0)/h
• I(Ex,t)cexp(-gt)sin2(mBgBt/h)
• Iy(Ex)(ct/2)(2mBgBt/h)2/1(2mBgBt/h)2

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Advantages

• Compared to other Doppler free techniques needs
  simple exptl arrangement
• Sign of dispersion shaped Hanle curve determines
  the sign of the Lande g factor.

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Disadvantages

• There is change in absorption profile with change
  in magnetic field.Laser bandwidth must be
  sufficiently large so that all Zeeman components
  can absorb the laser radiation independent of the
  field strength B.Should not be so large so that
  simultaneous excitations of different closely
  spaced transitions occur.
• Large magnetic fields are required for Hanle
  signals from short lived molecular levels with
  small g.

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Modifications

• To improve signal to noise ratio a method known
  as stimulated level crossing spectroscopy is used
• Use of electric fields instead of magnetic fields
• Studying molecules with simultaneous electric and
  magnetic level crossing spectroscopy give the
  value of g and t simultaneously.

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Thank you