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

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- 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