Laser Magnetic Resonance Spectroscopy Zeeman Spectroscopy

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Laser Magnetic Resonance Spectroscopy (Zeeman
Spectroscopy)

• By Breland Smith

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Introduction

• Discovered in 1968 by Evenson and colleagues
• Allows for high resolution spectra of free
  radicals and other transient paramagnetic
  molecules
• Rotational
• vibration-rotational
• electronic
• Utilizes fixed frequency lasers
• mid infrared CO and CO2 lasers
• far infrared ?28-900 µm

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Introduction

• First technique able to provide structural
  details of free radicals in the gas phase
• OF
• HO2
• CH3O
• Initial experiments performed with stable
  paramagnetic gases
• O22
• NO3
• NO24
• Enhanced sensitivity over microwave spectroscopy
• allowed for detection of a spectrum of the CH5
  radical in 1971

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Origin of LMR Transitions

• Transitions allowed by the dipole selection
  rules
• ?J1
• ?MJ0,1
• Zeeman effect
• loss in Degeneracy of magnetic sublevels (MJ) in
  the presence of a magnetic field

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Spectral Features

• Line widths and Resolution
• Line widths determined by
• Doppler broadening (Plt0.5 torr)
• Pressure broadening (Pgt0.5 torr)
• Mid infrared region
• Saturation (Lamb dip) spectroscopy
• high resolution
• sub-Doppler line widths as small as 5 Gauss
• Far infrared Region
• line widths at half maxima between 2-10 Gauss
• resolution is slightly lower than in microwave
  spectroscopy
• Accuracy
• determined by multiple factors
• laser stability
• Absolute field
• Absolute frequency
• Overall experimental error
• Mid Infrared 10 MHz
• Far Infrared lt10 MHz

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Spectroscopic Results

• Sensitivity
• Improved over EPR
• Allows detection of spectra from electronic fine
  structure transitions with weak magnetic dipole
  intensity
• Sensitivity for a transient radical such as OH is
  106 cm-3 with a far-IR laser and estimated to be
  105 cm-3 with a mid-IR laser
• High Resolution
• Allows for observation of hyperfine patterns in
  many free radical spectra
• Gives important information about electronic
  structure
• Signalnoise ratio
• improved by molecular modulation
• 30001 for weak transitions of O2 at 699.5 µm
  which represents a sensitivity of 2 x 1013 cm-3
• Accuracy
• Slightly lower than microwave spectroscopy
• Not limited to rotational transitions
• Includes
• vibrational
• fine structure
• spin-rotation
• spin-orbit
• hyperfine effects

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Basic Features of Far Infrared Spectrometer
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Far Infrared LMR Spectrometer

• Absorption
• Extracavity absorption
• absorption cell is external to the laser cavity
• used in the original experiment
• Intracavity absorption
• 103 times more sensitive
• Intracavity beam splitter
• determines the polarization of the laser relative
  to the magnetic field
• perpendicular polarization induces ?MJ1
  electric dipole transitions
• parallel polarization induces ?MJ0 transitions
• separates gain and sample mediums which are
  pumped separately
• Detector
• Golay Cell
• Used in first LMR spectra
• able to operate at room temperature
• Low modulation frequencies
• Slow response time
• Helium-cooled Ge bolometer
• More sensitive

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Basic Features of Far Infrared Spectrometer

• Advantage
• More efficient overlap of CO2 beam resulting in
  more effective optical pumping
• Disadvantage
• CO2 radiation is focused along the axis of the
  laser which is separated by a thin propylene beam
  splitter that can break even at low CO2 laser
  powers

• Transverse optimal pumping
• Line tunable CW CO2 laser
• Longitudinal pumping
• radiation introduced along the laser axis through
  a hole in the laser mirror
• Pump radiation
• multiply reflected between 2 gold-coated flats
  parallel to the laser axis

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Molecules Studied by LMR

• Developed for spectra of molecules
• sensitive enough to detect paramagnetic atoms
• Oxygen Atom transitions in the ground state
  (3PJ)
• Diatomic free radicals
• 3S
• 3NH
• ND
• PH
• 2p ground states
• OH
• OF
• 1?
• Triatomic molecules
• HO2
• NH2 (X2B1)

• Polyatomic molecules CH3O
• largest free radical ever detected and analyzed
  by LMR
• determined to be a symmetric top
• far IR spectra characterized by two- or four-line
  hyperfine patterns due to presence of three
  equivalent protons

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Far IR LMR Spectrum of Ground State OH Classic
Example of 2p diatomic free radical

• Observed transitions
• strongly allowed rotational transitions in 2p1/2
  and 2p3/2 states
• This spectra shows 2p1/2, J1/2 ? 2p3/2, J3/2
  transition for vibrational levels of ?1-3
• weak transitions between O fine structure states
  that are forbidden by the ?O0 selection rule

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3S Ground State of ND

• Resolved nine-line hyperfine pattern in a far IR
  transition in v0 for ND(X3S-)
• Pattern results from two triplets
• one splitting from the 14N I1 nucleus
• another from the deuterium nuclear spin

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Far IR LMR Spectrum of HO2 in Parallel (p) and
perpendicular (s) polarization

• Knowledge of structure and reactivity greatly
  enhanced by discovery of spectrum of 2A ground
  vibronic state of HO2 in 1974 by Evanson and
  Howard
• Evidence towards near prolate structure
• Interaction between rotational motion and single
  unpaired electron results in doubling of each
  rotational level which permits calculation of
• rotational parameters
• spin splitting
• components of spin rotation tensor

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Applications of LMR

• Molecular Ion Spectroscopy
• Detection of molecular ions is made possible due
  to high sensitivity of LMR
• Accurate molecular parameters derived from LMR
  spectra of OH, NH, HCl
• Laboratory Astrophysics
• Discovery of LMR spectrum of CH led to
  predictions several transitions in lower
  rotational levels with enough accuracy for
  radioastronomy searches
• Initial measurements of fine structure
  transitions in 3PJ ground state multiplet for 12C
  and 13C allowed detection of far IR emission of
  12C atoms from interstellar sources

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LMR Application to Reaction Kinetics

• Significant potential in study of free radical
  kinetics due to high sensitivity
• Much more sensitive than UV or EPR (Microwave)
  methods
• Molecules such as HO2 are particularly difficult
  to detect by other methods
• Sensitivity to HO2 109 cm-3 for the ground
  state and is similar for HO and other related
  species
• Accurate linear relationship between
  concentration of a species and peak height of its
  spectrum
• Far more gaseous species have identifiable
  Infrared spectra than have UV spectra
• First studies of reaction kinetics done on HO
  radicals by Howard and Evanson
• Subsequent studies are now focused on the
  reactivity of HO2

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LMR Applications to Atmospheric Chemistry

• Important to make accurate predictions about
  effect of nitrogen oxides and chlorine species on
  ozone (O3)
• provide insight on the various catalytic cycles
  that destroy ozone layer
• Nitrogen oxides (NO, NO2)
• released into stratosphere by high flying
  aircraft
• Chlorine species (Cl, ClO)
• formed from decomposition of clorofluoro-methanes
  (CFCl3, CFCl2) used in aerosol propellants,
  refrigerants, and foam-blowing agents

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Ozone

• Formation
• O2 hv (?lt242 nm) ? O O Photolysis O
  O2 M ? O3 M
• Photolysis of O3 is rapid and occurs in presence
  of UV light
• O3 hv (?lt242 nm) ? O2 O
• Simultaneous photolysis of O3 and O2 causes a
  steady state between the two odd forms of Oxygen
  (O, O3)
• Odd Oxygen Removal
• O O3 ? O2 O2 Slow Reaction 20
• Catalytic Cycles
• XO O ? X O XNO, Cl, H, HO XO O3
  ? XO O2
• Net Reaction O O3 ? O2 O2

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Role of HO in Destruction of Odd Oxygen Species

• HO converts catalytically active NO and NO2 to
  inactive HNO3
• HO converts inactive HCl to active Cl and ClO
• Not yet possible to measure concentrations of HO
  in parts of the stratosphere where ozone is the
  most dense
• Altitudes 20-30 km

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Atmospheric Balance of Odd Hydrogen Radicals

• Odd hydrogen radicals
• H
• HO
• HO2
• Rapidly interconverted but only slowly removed by
  radical-radical reactions
• Knowledge of HO2 radical reaction rates
• important in understanding balance between HO and
  HO2 and their removal rate

• A high rate coefficient (8.4 x 10-12 cm-3
  molecule-1s-1) was measured by LMR for the
  reaction HO2 NO ? HO NO2
• Conclusion additional nitrogen oxides in the
  lower stratosphere may increase stratospheric
  ozone concentrations

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Conclusions

• Utilized in over a dozen laboritories worldwide
• Continuous discoveries of new laser lines
  provides almost full frequency coverage in
  far-infrared region
• Tuning transitions across 100-1000µm range
  possible in near future
• Only technique presently available for
  spectroscopy of free radicals in Far-infrared
  region
• Very narrow line widths and intracavity detection
  responsible for enhanced sensitivity for free
  radicals over other spectroscopic techniques

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References

• P.B. Davies, J. Phys. Chem. 1981, 85, 2599-2607
• B.A. Thrush, Acc. Chem. Res. 1981, 14, 116-122

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• Question What type of molecules are studied
  by Laser Magnetic Resonance Spectroscopy?
• Answer Transient Paramagnetic Molecules such as
  Free Radicals