astrochemistry – spring 2013
TRANSCRIPT
Astrochemistry – Spring 2013 Lecture 6:
Laboratory astrochemistry
Julien Montillaud23th February 2013
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OutlineI. Astrochemical motivation for laboratory experiments (5 p.)
I.1 Spectroscopic identification of moleculesI.2 Improving chemical networksI.3 Exploring unusual conditions
II. Experimental determination of gas-phase reaction rates (10 p.)II.1 Ion-neutral reactionsII.2 Neutral-neutral reactions
III. Experimental investigation of ice non-thermal desorption (7 p.)III.1 Wavelength dependence of CO photodesorptionIII.2 Microphysics of the CO photodesorption process
IV. Experimental study of interstellar solid state reactivity (6 p.)IV.1 Looking for chirality in spaceIV.2 Experiment
IV. Summary
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I. Astrochemical motivation for laboratory experimentsI.1 Spectroscopic identification of molecules
The only way to prove the existence of a molecule in the ISM
The diffuse interstellarBands (DIBs): still unidentified
→ one of oldest problemin astrophysics
Many successful identifications: e.g. C60
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I. Astrochemical motivation for laboratory experimentsI.2 Improving chemical networks
- More than 150 molecules identified in the ISM(in July 2010)
- Several 1000 of reactions, many (most ?) are not known !
- rate coefficients often known for limited rangeof temperature / pressure
- even uncertainties are poorly known
Wakelam et al. 2010
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I. Astrochemical motivation for laboratory experimentsI.2 Improving chemical networks
Uncertainty on rate coefficients => control of the uncertainties on observables
New measurements => update of predicted abundances(steady state or time-dependent)
Wakelam et al. 2010
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I. Astrochemical motivation for laboratory experimentsI.3 Exploring unusual conditions
Pressure [bar] Astrophysical environment Experimental range
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1e-6
1e-12
Planetary atmosphere
Dust forming outer stellaratmosphere
Dense molecular core
Diffuse ISM
High-vacuum 1e-6 → 1e-12 bar
Ultra High-vacuum P < 1e-12 bar
Low & medium vacuum
- (mean free path ~40 km !!!)
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I. Astrochemical motivation for laboratory experimentsI.3 Exploring unusual conditions
Also:
- wide range of temperature: 10 → 3000 K
- wide spectrum of photons (radio → gamma, but mostly IR, visible, UV)
- irradiation by particles (hot electrons, cosmic rays, …)
- long life-time of metastable species, radicals, ...
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II. Experimental determination of gas-phase reaction rates
I. Astrochemical motivation for laboratory experiments
II. Experimental determination of gas-phase reaction ratesII.1 OverviewII.2 Ion-neutral reactionsII.3 Neutral-neutral reactions
III. Reactivity on/in icy mantles
IV. Summary
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II. Experimental determination of gas-phase reaction ratesII.1 Overview
Smith (2011), ARA&ATypes of reactions in a standard astrochemical model:
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II. Experimental determination of gas-phase reaction ratesII.2 Ion-neutral reactions
The flowing afterglow (FA)-selected ion flow tube (SIFT) instrument in Boulder
Snow & Bierbaum 2008
=> ion + H or O or N
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Production of the ions: with an electron impact ionizer
→ an electron gun creates fast electrons→ collision of fast electrons + ion precursor produces the ions→ electromagnetic acceleration of the ions, sent into the flowing carrier gas (He)
See animation: http://chemistry.clemson.edu/chemdocs/marcusgroup/software/CONCEPT/EI/EI.htm
II. Experimental determination of gas-phase reaction ratesII.2 Ion-neutral reactions
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Definition from http://mass-spec.lsu.edu/msterms/index.php/selected_Ion_Flow_Tube
Device in which m/z selected ions are entrained in an inert carrier gas and subsequently undergo ion/molecule reactions with molecules introduced into the gas flow.
Selected ion flow tube:
= electromagnetic selection of ions carried in a flowing buffer gas (He)
→ control of the laminar flow
→ control of the mass/charge ratio of the ion (excitation of the undesired ions by resonance)
II. Experimental determination of gas-phase reaction ratesII.2 Ion-neutral reactions
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Flowing afterglow:
= Reaction inside a flow of carrier gas (He)
→ control of the laminar flow
→ position <=> time
Definition from http://mass-spec.lsu.edu/msterms/index.php/Flowing_afterglow
“A reactor for observing ion-molecule reactions, in which ions are introduced to a bath gas containing a neutral reactant, and flowing rapidly down a vacuum system, where neutral pressure and distance become the reaction variables. Detection of the ions is by mass spectrometry through a leak at the product end of the system.”
II. Experimental determination of gas-phase reaction ratesII.2 Ion-neutral reactions
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Detection by mass spectrometry:
→ electromagnetic selection of ions carried in a flowing buffer gas (He)
→ selected ions are sent to an electron multiplier to countthe impinging ions
+
+oxygen
II. Experimental determination of gas-phase reaction ratesII.2 Ion-neutral reactions
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II. Experimental determination of gas-phase reaction ratesII.2 Ion-neutral reactions
Example of results:
Smith (2011), ARA&A
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II. Experimental determination of gas-phase reaction ratesII.3 Neutral-neutral reactions
Challenge: radical species at low temperatures – one solution, the CRESU set-up
Canosa et al. 2008
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II. Experimental determination of gas-phase reaction ratesII.3 Neutral-neutral reactions
→ From room temperature reservoir, can reach T ~10 K in the frame of the flow
→ almost collisionless trajectories of particles
→ practically a “wall-less flow tube”
Drawback:
→ require hugh pumping power→ require huge quantities of chemical species
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II. Experimental determination of gas-phase reaction ratesII.3 Neutral-neutral reactions
Important result: deviation from Arrhenius law at low T
Arrhenius law: empirical descriptionof temperature variations in k(T):
k(T) = A exp(-Ea/kT)with Ea = activation barrierA = prefactor
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III. Experimental investigation of icenon-thermal desorption
I. The major role of gas-grain interaction in the ISM evolution
II. Formation and destruction of icy mantles
III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO photodesorptionIII.2 Microphysics of CO photodesorption
IV. Experimental study of interstellar solid state reactivity
V. Summary
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III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO ice photodesorption
Motivation
- star formation regions: observation of larger abundance of gas-phase CO than predicted from purely thermal adsorption/desorption budget => additional desorption processes (photodesorption, cosmic ray sputtering, …)
- CO ice photodesorption rates measured for continuous UV spectra by 2 groups (Oberg et al. 2007, 2009, and Munoz Caro et al. 2010), but discrepancy by a factor of 20
- young stars can have strong emission lines
=> let's go for monochromatic measurements of CO ice photodesorption rate
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III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO ice photodesorption
UV sourceTunable high flux monochromatic source in the UV range:
→ synchrotron SOLEIL (DESIRS beamline)
Energy range: 5 – 40 eV (2500 Å – 300Å) – here: 7-14 eVΔλ = 33 mÅ, 60 mÅ and 720 mÅ – here 40 mÅFlux: ~1e10 → 1e14 ph/sec – 1e12 ph/sec
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III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO ice photodesorption
Control the ices: Example of the SPICES (Surface Processes in ICES) set-up (UPMC, Paris)
- ultrahigh vacuum (UHV)( < 1e-12 bar)
- closed-cycle helium cryostat( ~14 K)
- monitoring of the gas phasecomposition by means of a Quadrupole Mass Spectrometer (QMS)
- solid phase monitored withFourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS)
=> formation of ice controledat the monolayer scale(1 ML ~ 1e15 molecules/cm2)
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III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO ice photodesorption
Fringes of interferenceconverted into spectralInformation by Fourier transform
Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS)
Both vibrational and structural informations
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III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO ice photodesorption
Results: photodesorption of CO ice→ absolute rates
Fayolle et al. 2011
- 10 ML thick- @ 18 K - 9.2 eV photons
Obtained with FT-RAIRS on the CO stretching mode
The integrated area is proportional to the total amount of CO ice.
Uncertainties:- FT-RAIRS conversion factor- 1 ML = 1e15 molec/cm2 +/- ?- UV irradiated area ?- IR probed area ?
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III. Experimental investigation of ice non-thermal desorptionIII.1 Wavelength dependence of CO ice photodesorption
Results: photodesorption of CO ice→ yield spectrum
* Vibronic sequence of CO => DIET process(Desorption Induced by Electronic Transition)
QMS
UV absorption spectra of
CO ice @ 10 K
* Strong variations with wavelength→ effective photodesorption rate dependson the spectrum of the UV source→ explain the discrepancy between previousexperiments→imply variations with the astrophysical environment
Vibronic sequenceof CO
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III. Experimental investigation of ice non-thermal desorptionIII.2 Microphysics of CO photodesorption
Which molecules are actually desorbing ? Surface or volume ?
Bertin et al. 2012
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IV. Experimental study of interstellar solid state reactivity
I. Astrochemical motivation for laboratory experiments
II. Experimental determination of gas-phase reaction rates
III. Experimental investigation of ice non-thermal desorption
IV. Experimental study of interstellar solid state reactivityIII.1 Looking for chirality in spaceIII.2 Experiment
IV. Summary
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IV. Experimental study of interstellar solid state reactivityIV.1 Looking for chirality in space
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IV. Experimental study of interstellar solid state reactivityIV.1 Looking for chirality in space
Herbst & van Dishoeck 2009
- Life on Earth based on chiral molecules with an enantiomericexcess
- Chiral molecules found in meteorites, but not in comets, planetary and interstellar environments
- acetaldehyde CH3CHOdetected in Sag B2 in gas phase, + likely to exist in dust ice mantles
- NH3 abundant in ices
=> reaction to form NH
2CH(CH
3)OH ?
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IV. Experimental study of interstellar solid state reactivityIV.2 Experiment
Gold surface (10K)
High vacuum chamber
H2O
:NH
3(g)
CH
3CH
O (
g)
Mixed ice
Ice monitored by Fourier Transform IR spectrometry
NH3:CH
3CHO (4:1)
CH3CHO
NH3
Formation of ice
N-H N-HN-H C=O
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IV. Experimental study of interstellar solid state reactivityIV.2 Experiment
Gold surface (10K)
High vacuum chamber
Irradiation and desorption
VUV source
Heating resistance
irradiation
reaction
desorption
QMSQMS
Heating: Pure ices
- desorption of pure CH3CHO (120 → 135 K)- desorption of pure NH3 (120 → 125 K)
Ice mixture- T>110 K: “consumption” of NH3 and CH3CHO
and appearance of new IR features- complete sublimation of reactants (130 K)- alpha-aminoethanol NH2CH(CH3)OH found on
the gold surface, and sublimates at 200 K
(a) 10 K(b) 110 K(c) 130 K(d) 185 K
NH
CH3
CH
CO+CN
CC+CN
CH
OH+CH
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IV. Experimental study of interstellar solid state reactivityIV.2 Experiment
Gold surface (10K)
High vacuum chamber
Irradiation and desorption
VUV source
Heating resistance
irradiation
reaction
desorption
QMSQMS
Heating of pure ices- desorption of pure CH3CHO (120 → 135 K)- desorption of pure NH3 (120 → 125 K)
(a) 10 K(b) 110 K(c) 130 K(d) 185 K
NH
CH3
CH
CO+CN
CC+CN
CH
OH+CH
Heating of ice mixture:- T>110 K: “consumption” of NH3 and CH3CHO
and appearance of new IR features- complete sublimation of reactants (130 K)- alpha-aminoethanol NH2CH(CH3)OH found on
the gold surface, and sublimates at 200 K
IR => identification of functional groupsConfirmation from isotopic displacements (14N → 15N)
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IV. Experimental study of interstellar solid state reactivityIV.2 Experiment
Gold surface (10K)
High vacuum chamber
Confirmation from mass spectrometry
VUV source
Heating resistance
irradiation
reaction
desorption
QMSQMS
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V. Summary
Wide pannel of technics
Ions: easy to control
Neutrals: much more challenging
Ices: an increasing share of the experimental efforts
Other fields of interest: evolution ofnanoparticles - clusters of atoms (e.g. C
60)
- clusters of molecules (e.g. (PAH)N)
→ dust grain formation
Theoretical Theoretical chemistrychemistry
Experiments Experiments (on ice analogs)
Astrophysical Astrophysical modellingmodelling
(core structure, radiative transfer, chemical network)
ObservationsObservations
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VI. Bibliography
Main references:
Smith (2011), ARA&A, “Laboratory astrochemistry: gas-phase processes”
Wakelam et al. (2010), Space Sci. Rev., 156: 13–72