Photon-correlation Spectroscopy

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Cosmic lasers and
Photon-correlation Spectroscopy
Claudio Germanà and Dainis Dravins INAF
Observatory of Padua Lund Observatory
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OUTLINE
1. Laser Emission in astrophysical sources 2.
Photon-Correlation Spectroscopy Resolving
narrow spectral lines 3. Signal to Noise
ratio
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Thermal Equilibrium conditions
Energy level populations described by
Boltzmanns statistics
Medium acts as an absorber
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NON-Equilibrium
Population inversion
Medium acts as an amplifier
Light amplification by stimulated emission of
radiation LASER
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Astrophysical laser emission
Lasers may be observed if 1) Population
inversion is feasible 2) Pumping mechanism for
population inversion 3) Structures allow
amplification (e.g., clouds)
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...laser emission might be observed in
Fe II and O I lines in ? Carinae (Johansson
Letokhov 2004, 2005)
Wolf-Rayet stars He II He I lines (Varshni
Nasser 1975,1986)
Mass loosing stars
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S. Johansson V.S. Letokhov Astrophysical
lasers operating in optical Fe II lines in
stellar ejecta of Eta Carinae Astron.Astrophys.
428, 497 (2004)
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Model of a compact gas condensation near ? Car
with its Strömgren boundary between photoionized
(H II) and neutral (H I) regions
S. Johansson V. S. Letokhov Laser Action in a
Gas Condensation in the Vicinity of a Hot
Star JETP Lett. 75, 495 (2002) Pisma
Zh.Eksp.Teor.Fiz. 75, 591 (2002)
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at 9997 Å
A microsecond bottle-neck creates a population
inversion in the 3 ? 2 transition of Fe II
S. Johansson V.S. Letokhov Astrophysical
lasers and nonlinear optical effects in space
New Astron. Rev. 51, 443 (2007)
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...how to confirm Laser emission?
Expected extremely narrow linewidth lt 1 mÅ (0.1
pm) (Johansson Letokhov 2004)

by Dravins et al. 2007
Spectral resolution ? 100 million!!
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What about a spectral line?
Electric field emitted from one atom which
undergoes collisions
E n(t) E0 cos(?0t fn (t))
fn (t) is a Gaussian (chaotic process)
Total electric field from the system of n atoms
(Loudon 1973)
a(t) is a Gaussian
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... signal in Fouriers notation...
exp(i?t) Fourier component
E(t)TOT thermal light
a(t) ? cost (Gaussian) E(t)TOT laser light
a(t) cost
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...spectral line profile...
a(t) cost
a(t)? cost (Gaussian)
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...FWHM and time scale of intensity fluctuations

Fouriers temporal domain
Fouriers energy domain
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Photon (intensity) correlation Spectroscopy
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Intensity interferometry
Narrabri stellar intensity interferomter
(R.Hanbury Brown, R.Q.Twiss et al., University of
Sydney)
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Required Telescope diameters
has been set
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S/N for laser spectral lines
If there is laser emission, the coherence time
of light is three or more orders of
magnitude greater and so the S/N.
The required telescope diameter is smaller!!