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Complex Refractive Indices of Thin Films of Secondary Organic Materials by Spectroscopic Ellipsometry from 220 to 1200 nm
journal contribution
posted on 2013-12-03, 00:00 authored by Pengfei Liu, Yue Zhang, Scot T. MartinThe
complex refractive indices of three different types of secondary
organic material (SOM) were obtained for 220 to 1200 nm using a variable
angle spectroscopic ellipsometer. Aerosol particles were produced
in a flow tube reactor by ozonolysis of volatile organic compounds,
including the monoterpenes α-pinene and limonene and the aromatic
catechol (benzene-1,2-diol). Optically reflective thin films of SOM
were grown by electrostatic precipitation of the aerosol particles
onto silicon substrates. The ellipsometry analysis showed that both
the real and imaginary components of the refractive indices decreased
with increasing wavelength. The real part n(λ)
could be parametrized by the three-term form of Cauchy’s equation,
as follows: n(λ) = B + C/λ2 + D/λ4 where λ is the wavelength and B, C, and D are fitting parameters. The real
refractive indices of the three SOMs ranged from 1.53 to 1.58, 1.49–1.52,
and 1.48–1.50 at 310, 550, and 1000 nm, respectively. The catechol-derived
SOM absorbed light in the ultraviolet (UV) range. By comparison, the
UV absorption of the monoterpene-derived SOMs was negligible. On the
basis of the measured refractive indices, optical properties were
modeled for a typical atmospheric particle population. The results
suggest that the wavelength dependence of the refractive indices can
vary the Angstrom exponent by up to 0.1 across the range 310 to 550
nm. The modeled single-scattering albedo can likewise vary from 0.97
to 0.85 at 310 nm (UV–B). Variability in the optical properties
of different types of SOMs can imply important differences in the
relative effects of atmospheric particles on tropospheric photochemistry,
as well as possible inaccuracies in some satellite-retrieved properties
such as optical depth and mode diameter.
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flow tube reactorUV absorptionThin Filmsindicesparticle populationrange 310550 nmtropospheric photochemistryangle spectroscopic ellipsometerSpectroscopic Ellipsometrywavelength dependence1200 nmTheAngstrom exponent1200 nm310 nmAerosol particles1000 nmellipsometry analysismode diameteraerosol particlesComplex Refractive IndicesSOMsilicon substratesSecondary Organic Materials
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