Cavity RingDown Absorption Spectroscopy

1
Cavity Ring-Down Absorption Spectroscopy

• Trisha Blood
• December 4, 2007

2
Introduction

• Direct absorption technique governed by Beers
  Law
• A ? c l
• Rate of absorption measured as opposed to
  magnitude
• Sample placed within optical cavity between two
  reflective mirrors
• Laser pulse coupled into cavity with small
  fraction leaking out upon reflection
• Decay time dependent upon sample absorbance

Hollas, J.M. Modern Spectroscopy, 4th ed. John
Wiley Sons, Ltd 2004
3
A Brief History

• 1980 Herbelin proposed use of optical cavity
  for measuring reflectance of mirror
  coatings (CW light beam)
• 1988 Applications to molecular absorption
  measurements described by OKeefe and
  Deacon (pulsed laser)
• Sensitivity demonstrated by measuring absorption
  spectrum of weak forbidden b 1?g X
  3?g transition in gaseous oxygen

OKeefe, A. Deacon, D.A.G. Rev. Sci. Instrum.
1988, 59, 2544-2551
4
Principles of CRDS

• Requirements
• Light pulse with coherence length shorter than
  mirror spacing
• Light traverses cavity with small fraction
  exiting upon each pass
• Light intensity decays as a function of time with
  pulses spaced by round trip time (2L/c)

T ? transmission coefficient L ? cavity mirror
spacing t ? time c ? speed of light

• I(t) Ioe (-Tt2L/c)

Scherer, J.J. Paul, J.B. Collier, C.P.
Saykally, R.J. J. Chem. Phys. 1995, 102, 5190-5199
5
Spectral Features

• Ringdown transient, ?, single exponentially
  decaying function of time
• Allows for direct measurement of total losses
  experienced
• Dependent only on properties of cavity and
  absorbing species, not light amplitude
• Absorption spectrum obtained from plot of cavity
  loss (1/c?) versus frequency (?)

?o L / c? lnR?
6
Mirror Reflectivity

• Higher sensitivity attained with greater mirror
  reflectivity
• Typical values R 0.99 or better
• Contributions of broad-band scattering /
  absorption smaller than reflection losses
• ? ?ln (Reff)? ? 1-
  Reff

Scherer, J.J. Paul, J.B. Collier, C.P.
Saykally, R.J. J. Chem. Phys. 1995, 102,
5190-5199
7
Experimental Set-Up
Berden, G. Peeters, R. Meijer, G. Int. Rev.
Phys. Chem. 2000, 19, 565-607
8
Experimental Design

• Pulsed dye laser used in visible spectrum
• Pulse duration of 5-15 ns
• Moderate pulse energies needed (?1 mJ)
• Ring-down cavity
• Two identical plano-concave mirrors
• Radius of curvature between 25 cm and 1m
• Separation distance d so as to create
  non-confocal cavity
• Time-dependent detection
• Photomultiplier tube (PMT)
• Analogue-to-digital converter
• Computer

9
http//www.arp.harvard.edu/atmobs/sciobj/instrumen
t/cr.html
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Advantages of CRDS

• High sensitivity due to multipass nature of
  detection cell
• Effective absorption pathway very long while
  sample volume small
• Absorption independent of pulse-to-pulse
  fluctuations of laser
• Absorption measured on absolute time scale
• Fluorophore unnecessary

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Applications of CRDS

• Studies performed on molecules in various
  environments
• Cells
• Supersonic jets
• Transient molecules generated in discharges and
  flames
• Frequency-dependent absorption strengths given
• Number density, cross-section and temperature
  determined
• No intrinsic limitation to spectral region
• Range from ultraviolet to infrared radiation

http//lot.astro.utoronto.ca/spectrum.html
12
Integrated absorption intensity and Einstein
coefficients for the O2 a 1?g X 3?g (0,0)
transition A comparison of cavity ringdown and
high resolution Fourier transform spectroscopy
with a long-path absorption cell

• Stuart M. Newman, Ian C. Lane, and Andrew J.
  Orr-Ewing
• David A. Newnham and John Ballard

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Ozone photolysis

• Monitor emission of products formed
• O3 h? ? O2 (a 1 ? g , b 1?g ) O (1D)
• O2 electronic configuration
• ...(2p?g)2(2ppu)4(2ppg)2
• Hartley system absorption lt 310 nm
• O2 (a 1 ? g ) and O (1D) formed
• Short-wavelength side of Hartley system
• O2 (b 1?g ) directly produced

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Review of Selection Rules

• O2 a-X and b-X bands weak
• Violate electric dipole selection rules
• ?S 0 D L 0, ? 1
• g ? g ? ? not allowed
• Transition induced by magnetic-dipole and
  electric-quadrupole interactions

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Ozone Concentration

• Einstein A-coefficient necessary for inference of
  ozone concentration
• Spontaneous emission from a 1?g X 3?g
• O2 (a 1?g ) state metastable with lifetime to
  ground state ? 64-113 min.
• Einstein B-coefficient determined from integrated
  absorption intensity, Sint,B

gl / gr ratio of degeneracies of lower and
upper states (3/2)
nr index of refraction (1.00027)
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Experimental Apparatus

• NdYAG laser supplying 450 mJ pulses of 355 nm
  radiation
• Glass vacuum cell (1.5 m long) bounded by
  ultrahigh reflectivity mirrors
• Near-IR sensitive InGaAs photodiode
• Exponential signal decay converted to linear
  function
• Ringdown rate coefficient, k, obtained by linear
  least-squares analysis

17
Cavity Ring-down Spectra

• Ringdown rate coefficient, ko obtained in
  absence of absorbing species (baseline region of
  spectrum)
• Ringdown time (RDT), ?o time required for light
  intensity to fall to 1/e of initial value
• RDT 7 ?s for empty cavity with R 0.9993
• Ringdown time reduced with O2 absorption

• napierian absorption coefficient

Io initial light intensity
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Cavity Ring-down Spectra

• Complications in determination of ? when laser
  bandwidth comparable to widths of resolved O2 a-X
  (0,0) rotational features
• Laser bandwidth 0.25cm-1
• Pressure- and Doppler-broadened linewidth 0.098
  cm-1

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Absorption Spectra

• a 1?g X 3?g (0,0)

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Absorption Spectra

• Integrated absorption intensity a function of
  summation of absorption coefficients of all lines
  in (0,0) band
• Determined by integrating the area under each
  spectrum over the complete wavenumber range of
  the band
• Value converted to Einstein B- and A-coefficients

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Results
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Summary

• Direct absorption technique following Beers Law
• Rate of absorption measured rather than magnitude
• Large effective path length due to highly
  reflective mirrors allows for detection of weak
  absorptions
• Transitions of low probability or molecules
  present in very low concentrations

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References

• Newman, S.M. Lane, I.C. Orr-Ewing, A.J.
  Newnham, D.A. Ballard, J.
• J. Chem. Phys. 1999, 110, 10749-10757
• OKeefe, A. Deacon, D.A.G. Rev. Sci. Instrum.
  1988, 59, 2544-2551
• Scherer, J.J. Paul, J.B. Collier, C.P.
  Saykally, R.J. J. Chem. Phys. 1995, 102,
  5190-5199
• Berden, G. Peeters, R. Meijer, G. Int. Rev.
  Phys. Chem. 2000, 19, 565-607
• Hollas, J.M. Modern Spectroscopy, 4th ed. John
  Wiley Sons, Ltd 2004

24
Exam Question

• What makes CRDS more sensitive than other direct
  absorption techniques?
• See Slide 10