of solid phase molecules - Radboud Universiteit · 2012-06-13 · Thickness measurement Laser...
Transcript of of solid phase molecules - Radboud Universiteit · 2012-06-13 · Thickness measurement Laser...
Infrared spectroscopy
M.E. Palumbo and G.A. BarattaINAF – Ossevatorio Astrofisico di Catania, ITALY
of solid phase molecules
Why IR spectroscopyTielens and Allamandola, 1987
Why IR spectroscopy Tielens and Allamandola, 1987
Characteristic vibrationalfrequency range of variousmolecular groups made upout of the abundantelements H, C, N, and O
Optical depth
The optical depth expresses the quantity of electromagnetic radiation removedfrom a beam by scattering or absorption during its path through a medium. If I0 isthe intensity of radiation at the source and I is the observed intensity after a givenpath, then optical depth is defined by the following equation
I0 I
I/I0=e-τ
Beer-Lambert law
The law states that there is a logarithmic dependence between the transmission, T,of electromagnetic radiation through a substance and the product of theabsorption coefficient of the substance, α, and the distance the radiation travelsthrough the material (i.e., the path length), x.
T=I/I0=e-αx
α(λ)=4πk/λ where k is the imaginary part of the complex refractive index
where τ=αx
Absorbance
where I is the intensity of electromagnetic radiation at a specified wavelength λ
that has passed through a sample (transmitted intensity) and I0 is the intensity ofthe radiation entering the sample.
In spectroscopy, the absorbance A is defined as
Aλ= -log10(I/I0)
Optical depth vs Absorbance
Given the definitions above the optical depth and the absorbance are related by
τ= ln(10) A = 2.303 A
Vacuum chamber
High vacuum P 10-7 mbar residual gas H2O Ultra high vacuum P 10-9 mbar residual gas H2
Cryostat Liquid nitrogen, T=77 KClosed cycle helium cryostat, T=10 KLiquid helium, T=3 K
Substrate in thermal contact with the cryostat
Reflection Absorption Infrared Spectroscopy (RAIRS)and Transmittance Spectroscopy
School of Chemistry, University of Nottingham
NASA/Ames Research Center
Substrates
Transmittance
transparent to IR radiation
KBr
CsI
ZnSe
Si
Reflection Absorption IR spectra
reflect IR radiation
Au
Al
KBr and CsIHigh IR transmittanceNo IR bandsHygroscopic Fragile
Potassium Bromide (KBr)
Caesium Iodide (CsI)
SiliconLower transmittanceIR bands Not hygroscopicChemically very resistantResist thermal and mechanical shocks
Deposition angle
Directed deposition
clean chamber
very low deposit on the back of the
substrate
non-uniform film (unless an array of
capillaries is used)
Substrate T=15 K
Substrate T=15 K
Background deposition
uniform film
molecules everywhere
deposit on the back of the substrate
How to protect the back of the substrate
see Sicilia et al. 2012
Laboratorio di Astrofisica Sperimentale, Catania
Laboratorio di Astrofisica Sperimentale, Catania
Experimental procedure
Substrate (Si, KBr, CsI) T=10-300 K
IR beam
Background (mid-infrared) at 16 K (KBr substrate)
Experimental procedure
Sample T=10-150 K
IR beam
Mid-IR spectrum of the sample as deposited (CH3OH at 16 K)
Experimental procedure
Continuum normalization
τ = - ln (I/I0)
Band’s profile
Profile = shape, width and peak position
Band’s profile may strongly depend on the optical set-up geometry
IR spectra of solid CO
RAIRS
Transmittance at normal incidence
Transmittance at oblique incidence
e.g. Sandford et al. 1988Tielens et al. 1991Gerakines et al. 1996Collings et al. 2003Palumbo et al. 2006Fuchs et al. 2009
TO-mode
TO-mode
LO-mode
LO-mode
LO-TO splitting
Almeida 1992, Phys. Rev. B 45, 161
LO-TO splitting
A band due to the LO mode CANNOT be present in the spectra taken in
transmittance at normal incidence.
The bands due to the LO-TO modes are observed in transmittance at
oblique incidence.
In RAIR spectra of thin films on a metal surface (e.g. Au) only the LO
mode band is observed. The band due to the TO mode is not observed
because in a metal the electric field close to the surface is normal to
the surface.
IR spectra of solid CO2
Baratta et al. 2000, A&A 357, 1045
TO-mode
TO-mode
TO-mode
LO-mode
LO-mode
RAIR spectra of solid CO
The profile of the CO banddepends on the thicknessof the underlying ice layer
Palumbo et al. 2006
Transmittance spectra of solid CO
The profile of the CO banddepends on the thicknessof the underlying ice layer
Palumbo et al. 2006
Transmittance spectra of solid CO
The profile of the CO banddepends on the thicknessof the underlying ice layer
Palumbo et al. 2006
4000 3500 3000 2500 2000 1500
0.8
0.9
1.0
1.1
1.2
1.3
TRANSMITTANCE SPECTRA
CO on N2
S polarization
Si substrate
T=16 K
N2 thickness
0 m
1.1 m
2.2 m
transm
itta
nce
wavenumber (cm-1)
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
Baratta et al. 2002
LO mode
At oblique incidence a band due to the LO modeCANNOT be present in the spectra taken whenyou select the S component with the polarizer. Itcan be observed when you select with thepolarizer the P component.
The LO-TO splitting is usuallyobserved in strong bands (i.e.fundamental modes).
Solid CH4
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania LO mode
LO mode
Fulvio et al. 2009
Fulvio et al. 2009
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
Palumbo 2006
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
13C16O
13C18O
Loeffler et al. 2005
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
Sicilia et al. 2012
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
Sicilia et al. 2012
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
Abdulgalil et al., submitted
CH3CN When band’s profiles in transmittance spectra at oblique incidence in P and S polarization are similar, band’s profile in transmittance and RAIR spectra are similar as well.
CH3CNAbdulgalil et al., submitted
Use of a polarizer
Laboratorio di Astrofisica SperimentaleCatania
CO2 formed after ion bombardment of solid COIoppolo et al. 2009
Amorphous vs crystalline samples
It was formerly believed that the LO-TO slitting is onlyobserved in crystalline samples.
Several experimental results indicate that amorphoussamples (glasses) show the LO-TO splitting (e.g. Almeida1992; Wackelgard 1996; Trasferetti et al. 2001)
Quantitative analysis
Given an IR spectrum (both experimental and astronomical) we want to know how many molecules are present in the sample.
Column density (molecules cm-2) = band area (cm-1) / band strength (cm molecule-1)
Available band strength values
Yamada and Person, 1964, J. Chem. Phys., 41, 2478 CO2,
Jiang et al., 1975, J. Chem. Phys., 64, 1201 CO
D’Hendecourt and Allamandola ,1986, A&AS 64, 453 H2O, CH4, NH3, CH3OH, CH3CN, hydrocarbons
Hudgins et al., 1993, ApJSS, 86, 713 H2O, CH3OH, CH4, CO2, OCS, mixtures
Gerakines et al. 1995, A&A, 296, 810 H2O, CO, CO2
Kerkhof et al., 1999, A&A, 346, 990 CH3OH, NH3, CH4
Gerakines et al., 2005, ApJ, 620, 1140 CO, CO2, C3O2, CH4, H2O, CH3OH, NH3
Brunetto et al., 2008 ApJ, 686, 1480 CH4, mixtures
Oberg et al. 2007, A&A, 462, 1187 H2O, CO2, mixtures
Fulvio et al., 2009, Spectroch. Acta A, 72, 1007 N2O, NO2
Modica and Palumbo, 2010, A&A, 519, A22 HCOOCH3
etc., etc.
Band strength values
Uncertainties as large as 30% (e.g., Hudgins et al. 1993);
Depend on the substrate (e.g., Fulvio et al. 2009; Modica and Palumbo 2010);
Depend on the structure (crystalline or amorphous) of the sample (e.g. Gerakines et al. 1995).
Measurements of band strength values
Usually measured form IR spectra when an independent measurement of the column density is available.The column density can be obtained form the knowledge of the thickness of the sample if the density is known.
Thickness measurement
Laser interference fringe method
laser beam
substrate
sample
vacuum
Experimental interference curve
For thick ice layers the damping in the interference curve is due to: absorption by the sample, diffusion by the bulk or surface, loss of coherence of the reflected light (non-uniform growth of the sample) Palumbo et al. 2008
Solid CO
Thickness measurement
Laser interference fringe method
laser beam
substrate
sample
vacuum
0 100 200 300 400 500 600
0.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
6.0x10-6
7.0x10-6
8.0x10-6
inte
nsity (
a.u
.)
time (s)
Experimental interference curve
Baratta & Palumbo 1998, JOSA 15, 3076Westley et al. 1998, J. Chem. Phys. 108, 3321Fulvio et al. 2009, Spectr. Acta A 72, 1007Modica & Palumbo 2010, A&A 519, A22
Solid HCOOCH3Modica and Palumbo 2010
Thickness measurement
Laser interference fringe method
0 100 200 300 400 500 600
0.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
6.0x10-6
7.0x10-6
8.0x10-6
inte
nsity (
a.u
.)
time (s)
Experimental interference curve
The distance between two maxima or two minima is given by
0 = laser wavelengthnf = refractive index of the filmi = incidence angle
Solid HCOOCH3Modica and Palumbo 2010
Thickness measurement
Laser interference fringe method
The distance between two maxima or two minima is given by
0 = laser wavelength (known)nf = refractive index of the filmi = incidence angle (geometrically measured)
i = arctg a/2b
Thickness measurement
Laser interference fringe method
The distance between two maxima or two minima is given by
0 = laser wavelength (known)nf = refractive index of the filmi = incidence angle (geometrically measured)
0 100 200 300 400 500 600
0.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
6.0x10-6
7.0x10-6
8.0x10-6
inte
nsity (
a.u
.)
time (s)
Experimental interference curve
If the substrate is opaque (e.g. Si, Au) the refractiveindex can be obtained by a numerical method fromthe ratio between the first maximum and the firstminimum because the amplitude depends on nf .
Solid HCOOCH3Modica and Palumbo 2010
Thickness measurement
Laser interference fringe method
From the knowledge of0 = laser wavelength (known)nf = refractive index of the film (numerical method)i = incidence angle (geometrically measured)ns = refractive index of the substrate (database)it is possible to draw the theoretical interference curve
0 100 200 300 400 500 600
0.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
6.0x10-6
7.0x10-6
8.0x10-6
inte
nsity (
a.u
.)
time (s)
Experimental interference curve
0.0 0.2 0.4 0.6 0.8
0.0
0.2
0.4
0.6
0.8
1.0
no
rma
lize
d r
efle
cta
nce
thickness (m)
Normalized theoretical interference curve
Solid HCOOCH3Modica and Palumbo 2010
Solid HCOOCH3Modica and Palumbo 2010
Thickness measurement
Laser interference fringe method
The absolute accuracy of the thickness measured in this way is about 5%and it is mainly limited by the uncertainties in the knowledge of therefractive index of the substrate at low temperature and by the error inmeasuring the incidence angle of the laser.
0 100 200 300 400 500 600
0.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
6.0x10-6
7.0x10-6
8.0x10-6
inte
nsity (
a.u
.)
time (s)
Experimental interference curve
0.0 0.2 0.4 0.6 0.8
0.0
0.2
0.4
0.6
0.8
1.0
no
rma
lize
d r
efle
cta
nce
thickness (m)
Normalized theoretical interference curve
Solid HCOOCH3Modica and Palumbo 2010
Solid HCOOCH3Modica and Palumbo 2010
Thickness measurement
Laser interference fringe method
The choice of the substrate and laser wavelength
Lorentz-Lorenz relation
For a given species the Lorentz-Lorenz coefficient, L, is nearly constant for afixed wavelength regardless of the material phase and temperature.If the density and the refractive index of a molecular species are known at roomtemperature, the density at low temperature can be derived when therefractive index at low temperature is also known.The Lorentz-Lorenz relation is valid as far as the sample is opticallyhomogeneous.
= densitynf = refractive index
Column density
The column density can be obtained using the following relation
d = thickness (cm) = density (g cm-3) = molecular weight (g)
Methyl formate
Modica and Palumbo, 2010
Methyl formate
Plot of the band area fromspectra taken in P and Spolarization vs. the columndensity for the most intensebands of HCOOCH3. Theslope of the fit (solid line)gives the value of the bandstrength A.
(Modica and Palumbo, 2010)
Methyl formate
Plot of the band area fromspectra taken in P and Spolarization vs. the columndensity for the most intensebands of HCOOCH3. Theslope of the fit (solid line)gives the value of the bandstrength A.
(Modica and Palumbo, 2010)
Methyl formate
(Modica and Palumbo, 2010)
We note differences due to the substrate in the obtained A values between 6−25%.
These differences can be explained by the limits of applicability of the Beer-Lambert law.
Saturated bandsoptical depth > 1
Fulvio et al. 2009
The profile of a non-saturated band does not depend on the thickness of the sample.
Saturated bandsBand area could not be proportional to column density
Fulvio et al. 2009
Quantitative analysis from RAIR spectra
A-values obtained from transmittancespectra cannot be used for quantitativeanalysis from RAIR spectra.
Quantitative analysis from RAIR spectra
Oberg et al. 2009
Optical constants
There are two sets of quantities, both known as optical constants, that can be used todescribe the intrinsic optical properties of matter:
the real and imaginary parts of the complex refractive indexN = n + ik
the real and imaginary parts of the complex dielectric function = ’+ i’’
These sets of quantities are not independent: if the material is non-magnetic therelationship between the two is:
= N2
’ = n2 - k2 and ’’ = 2n k
Available mid-IR optical constants of molecules in the solid phase
Wood and Roux, 1982, J. Opt. Soc. Am. ,72, 720 H2O, NH3, CO2
Tielens et al. 1991, ApJ, 381, 181 CO
Hudgins et al., 1993, ApJSS, 86, 713 H2O, CH3OH, CH4, CO, CO2, OCS, mixtures
Trotta, 1996, PhD Thesis, Univ. J. Fourier, Grenoble, F CO, H2O, CO2, NH3, CH4, C2H4, C2H6, H2S, SO2, CH3OH
Ehrenfreund et al., 1997, A&A, 328, 649 CO, CO2, mixtures
Elsila et al., 1997, ApJ, 479, 818 CO, mixtures
Baratta and Palumbo, 1998, J. Opt. Soc. Am. 15, 3076 CO, CO2
Dartois, 2006, A&A 445, 959 CO
Palumbo et al., 2006, PCCP, 8, 279 CO
Moore et al., 2010, ApJSS, 191, 96 HCN, C2N2, CH3CN, C2H5CN, HC3N
etc. etc.
A few examples solid CO2
Strong bandsk > 0.5
Weak bandsk 0.5
α(λ)=4πk(λ)/λ
where α is the absorption
coefficient and k is the
imaginary part of the complex
refractive index
Baratta and Palumbo 1998
Baratta and Palumbo 1998
A few examples solid CO
Strong band k > 0.5
Solid COPalumbo et al. 2006
A few examples solid CO
Real and imaginary part of the dielectric function in the CO stretching region for solid CO
Tielens et al. 1991
Dartois 2006
A few examples solid CO
Real and imaginary part of the dielectric function in the CO stretching region for solid CO
Palumbo et al. 2006
The wave numbers of the LO and TO modescan be approximated either by theminimum and maximum of || respectively(solid lines in Fig. 3), or by the maximum ofthe energy-loss function given by Im(1/)and maximum of ’’ respectively (dashedlines in Fig. 3).
When the real part of the complex dielectricfunction has negative values, the reflectivityis very high. This spectral region is confinedby the LO and TO mode frequency.
A few examples solid CH3CN
weak bands k < 0.5
A few examples solid CH3CN
In this case the real partof the dielectric functionis always positive then noLO-TO splitting can beobserved.CH3CN
Moore et al. 2010
Comparison between different sets of optical constants
Ehrenfreund et al. 1997
Solid CO
Solid CO2
stretching mode and bending mode
With correct optical constants it is possible to reproduce experimental spectra
IMD is a free IDL application (http://www.rxollc.com/idl/imd/index.html)
With correct optical constants it is possible to reproduce experimental spectra
IMD is a free IDL application (http://www.rxollc.com/idl/imd/index.html)
With correct optical constants it is possible to reproduce experimental spectra
IMD is a free IDL application (http://www.rxollc.com/idl/imd/index.html)
With correct optical constants it is possible to reproduce experimental spectra
IMD is a free IDL application (http://www.rxollc.com/idl/imd/index.html)
With correct optical constants it is possible to reproduce experimental spectra
IMD is a free IDL application (http://www.rxollc.com/idl/imd/index.html)
With correct optical constants it is possible to reproduce experimental spectra
IMD is a free IDL application (http://www.rxollc.com/idl/imd/index.html)
With correct optical constants it is possible to reproduce experimental spectra
All these sets of optical constants, except one, have been obtained from spectra taken at normal incidence. In these experimental conditions the LO mode is not observed and an important constraint is missing.
Baratta and Palumbo, 1998
With correct optical constants it is possible to reproduce experimental spectra
With correct optical constants it is possible to reproduce experimental spectra
Baratta and Palumbo, 1998
stretching mode bending mode
Not all optical constants canproperly reproduce experimentalspectra (in transmittance at obliqueincidence and RAIR spectra).
Warning!
Why should astronomers worry about optical constants?
Particle size and shape effects
In the laboratory we take spectra of thin films
In space we observe ice grain mantles
Is it possible to compare the profile of absorption bands ofthin films (bulk) to the band’s profiles in astronomical spectra?
SOMETIMES!
Particle size and shape effects
Shift of peak position with respect to laboratory (bulk) spectraSub-peaks appearDepend on optical constants (n and k)
Particle size and shape can have very important effects on the profile ofabsorption features. For strong transitions in small particles, this class ofelectromagnetic modes is referred to as “surface modes” (van de Hulst1957; Bohren and Huffman 1983).
Particle size and shape effects
These effects can be understood in the Rayleigh limit (2πa << λ, a being the radius ofthe particle and λ the wavelength of the incident photon) since the particle may beconsidered placed in a homogeneous electric field and electrostatic applies. Because ofits electric polarization, the applied homogeneous electric field induces a dipole in theparticle the strength of which depends on the complex dielectric constant = ’+ i’’of the particle as well as its shape. The field outside the particle is then a superpositionof the uniform field and the particle’s dipole field, while inside the particle the field ishomogeneous. In the spectral range of an absorption feature where variesconsiderably and its real part may even be negative, the internal field of the particlecan be very large. In these instances an oscillator inside the particle is strongly affectedby this field and the spectral feature can be shifted with respect to the bulk materialfrom about the maximum of ’’ to frequencies where ’ is small or negative. Inaddition spheroidal particles give rise to subpeaks not present in the laboratoryspectra.
Particle size and shape effects
Sphere (a/b = 1 ; c = b)
Prolate (b/a < 1; c = b)
Oblate (a/b < 1; c = b)
Rods (b/a = 0; c = b)
Disks (a/b = 0; c = b)
Mie scattering calculations
Tielens et al. 1991
Calculated solid CO absorption cross sections (solid lines) for various solid mixturescontaining CO are compared to bulk absorption spectra (dashed lines). Homogeneousspheres have been assumed in the calculations.
Solid CO
Mie scattering calculations
Tielens et al. 1991
Absorption cross sections calculated in theRayleigh limit for various homogeneousspheroids consisting of pure CO. The prolateand oblate spheroids have axial ratio of 1/5.
Solid CO
Mie scattering calculations
Tielens et al. 1991Calculated absorption cross sections for acontinuous distribution of ellipsoid shapes(CDE). Upper panel: homogeneous particlesconsisting of a mixture CO/H2O=10/1. Lowerpanel: silicate core-mantle (CO/H2O=10/1)spheroids.
Solid CO
Mie scattering calculations
Palumbo et al. 1995
Solid OCS
Ehrenfreund et al. 1997
Solid CO
Different band’s profilesresult from different setsof optical constants.
Ehrenfreund et al. 1997
Solid CO2
Different band’s profilesresult from different setsof optical constants.
Laboratorio di Astrofisica SperimentaleCatania
Baratta et al. 2000CH3OH:CO2 deposited at 12.5 K and warmed-up to 105 K
Baratta et al. 2000
Quoting Bohren & Huffman (1983): “All of thisillustrates a general rule, which we can state butnot prove: if there is an interesting effect in athin film, there will be a corresponding effect insmall particles”.
In summary
Transmittance spectra
Relatively easy quantitative analysis
First choice to obtain optical constants (better at oblique incidence)
Low sensitivity
RAIR spectra
Difficult quantitative analysis (not impossible)
Second choice to obtain optical constants
High sensitivity (sub-monolayer)