formation of molecules · 2014. 10. 27. · interstellar and circumstellar media (ism and csm)...

Formation of molecules on cosmic dust analogs

in conditions relevant to the ISM (from laboratory to observations)

Jean Louis LEMAIRE

[email protected]

ISM-SPP 1573. Laboratory Astrophysics Summer School. Tabarz (D) October 2014

Content 1- Introduction: Formation of molecules in general

The connection between experimental laboratory astrophysics, theoretical modeling and ISM observations.

2- Formation of H2 molecules Formation processes Role of the grains 3- Some significant experimental results Focus on OPR, NSC … H2O formation 4- A few words on the modeling 5- The need for astronomical observations Observing H2 and its OPR Where and how 6- Formation of large organic molecules from prebiotic to amino acids molecules 7- Conclusions and prospects: Need for more experimental work Need for a crucial observing tool for H2


General framework and implications

4) Dust analogs preparation Silicates, Carbonaceous

+ various characterizations

1) Observation of IR rovibrational H2 emission (excited by PDR, Shocks, Formation)

2) Formation of H2

3) Role and nature of dust Butterfly Nebula Ring Nebula

6) Formation mechanisms (LH, ER, HK) supported by theoretical calculations

7) Partition of the H2 formation energy

( Ev,J ? Ek ? Grain heating ?) (included in astrophysics modeling)

Nd: YAG laser λ = 532 nm

Condensation

chamber

10-4 mbar 10-3 mbar

10-6 mbar

Metallic rotating target of

Olivine/Pyroxene stoichiometry

Expansion chamber

Analyzing chamber

Nozzle

Skimmer

5) Laboratory experimental simulations

J v


- A central goal in laboratory astrophysics is to unveil how molecules observed in the interstellar and circumstellar media (ISM and CSM) form.

- Current laboratory experiments are becoming increasingly more oriented to the study of three (OH2, NH3 …) and higher atom assembly (organic and prebiotic molecules).

1. Introduction Formation of molecules, general remarks

- Nonetheless, the mechanisms of formation of the simplest and most pervasive molecule in the universe, H2, remain an unresolved question, despite numerous theoretical and experimental attempts.

- In addition still lack astronomical observations of the single formation process.

Role of molecular hydrogen

* Implication on galaxy formation in a cosmological context (Thompson 2013) * Cooling role of H2 in the formation of galaxies (Halle & Combes 2012) * Connection in galaxies between star formation rate and H2 emission (Hill 2013) * Implication for first star formation (Kreckel 2010) * Critical step in the conversion of interstellar gas into stars (Lee, 2013) * H2 fundamental constituent of the universe acting as the molecular feedstock for much of the chemistry occurring within the ISM (Irving 2012) * Role in the formation and evolution of sunspots (Jaeglli et al. ApJ 2012)


•The H2 formation process is of paramount importance to astrophysics and in particular for the interstellar medium.

* In dense cores, the formation of H2 is an important non-thermal desorption mechanism.

* The most important point is the fate of the 4.5 eV released during formation which directly affects the star formation rate.

* An other point is the O/P ratio at formation (and subsequent nuclear spin conversion) which helps in determining the age of dark clouds.

* D/H segregation

2. Formation of H2 molecules Formation processes


2. Formation of H2 molecules Types of formation processes

Direct process: heterogeneous reactivity of H atoms on a surface Indirect process: processing of ices by high energetic particles processing of ices by light (UV, FUV, X) catalytic role of H bearing molecules (abstraction)

In the case of : Main interests in: Direct process the surface characteristics nature (carbonaceous, silicateous) morphology (crystalline, amorphous, defects) Indirect process the interstellar analog mixtures the molecules involved the reaction channels (reaction rates, branching ratios)

In both case, main interest in the desorption yield in the energy status (and O/P) of the newly formed H2

Pote

ntia

l Ene

rgy

(eV

)

Internuclear distance (Å)

H(1s)+H(1s)

4.476 eV


2. H2 formation via direct processes cosmic dust grains

* Origin of the dust gains: Further processing: Outflows of late-type massive stars Shocks, grain-grain collisions, sputtering and in SN explosions, PNe defects, porosity * Dust Nature: Oxygen-rich stars Silicates Carbon-rich stars carbonaceous materials (Greenberg & Li 1996) In dense clouds grains also covered by ices (Mathis 1993)

* Dust mass estimated to be 30 % carbonaceous spectroscopic signatures: 70% silicates 9.7 and 18 µm bands …

* Composition, morphology, and chemical reactivity of interstellar dust grains are not easy to characterize: * Grain size distribution observations of UV light scattering * Composition, morphology infrared absorption and emission in the infrared * Relative abundance of different types of grains derived depletions in the gas phase * Observation on a line of sight * In the ISM, most of the dust is amorphous, * In the CSM, ISO showed large amount of crystalline silicates (forsterite & enstatite) (de Graauw et al. 1996, Moister 1999)


* Now well established that H2 molecules form at the surface of inter- or circum-stellar grains in the local universe: catalytic role of the grains (or heterogeneous reactivity of H atoms on a surface)

* In the laboratory, large surfaces are used to mimic dust grains. * A large array of experimental methods is used to study the formation processes, involving surface science, non-linear optics, solid state physics, etc.

2. Formation of H2 molecules on grains

H

Adsorption ( flux, sticking ?…) from the gas phase onto the grain

H

Diffusion (mobility ?)

H Desorption

Formation reaction

Desorption (residence time ?)

Formation and sticking (migration time ?)

Grain T ? Composition ? Morphology ?

Coverage ? H

H

H

H

H

Formation process ISM dynamics (results from the balance between internal pressure and gravitational forces)

Nascent H2 with internal energy H2 will cool through radiation of IR photons

The molecular cloud collapse more readily

Nascent H2 with large translational and small internal energy and/or grain heating Heating of the cloud

Slow collapse of the molecular cloud

Thermal accomodation and/or sticking

Formation energy partition 4.48 eV

Ev,J ? Ek ?

Grain heating ?


3. Some significant experiments on the H2 (D2, HD) formation


3. Some significant experiments on H2 (HD, D2) formation

Odense team (Baurichter A. & Hornekaer L., Dk) 2003 TPD Glace d'eau

Formation de HD, désorption de D2

HOPGraphite, Ek(q) de H2 (D2) 2006, 2009 LAAD

UC London team (Price S., UK) 1999 REMPI-TOF Graphite (15K)

Formation de H2 et HD, diagnostic (v,j)

Cergy/Observatoire de Paris team (Lemaire JL, Fr) 2005 TPD, RAIRS, 2008, 2010, 2012a,b, 2013 a,b REMPI-TOF

Formation de molécules simples et complexes sur des glaces d'eau et des silicates Formation de D2, désorption H2, HD, D2 et de mélanges, énergies d'adsorption, diagnostic (v,j), ortho/para

Waterloo team (Govers T., Ca) 1980 Bolométrie Cinétique de physisorption, H2 et D2

Aarhus team (Hornekaer L. & Besenbacher F., Dk) 2006,2010, 2012 STM HOPGraphite, in situ formation de D2 H2 abstraction on PAHs

Bayreuth team (Zecho T., Küppers J., G) 2002 TPD, ELS, HREELS HOPGraphite 2006 STM

Formation de H2 et D2

chemisorbed physisorbed

Menella H2 abstraction on PAH 2010 ISM-SPP 1573. Laboratory Astrophysics Summer School. Tabarz (D) October 2014

Syracuse (USA) + Catania (It) team (Vidali G. + Pirronello V.) TPD Silicates (Olivine), 1997, 2007, 2009, 2010 carbone amorphe, glace d'eau, de CO et de CO2 2001, 2002, 2003, 2005

Formation de HD

3. Experiments Formation of molecular hydrogen

via indirect processes


H2O, CH4, NH3, C6H6, C6H12

CH3OH, NH3, CO, H2O

Hydrides H2O, NH3, CH4, CD4

PAHs

Ices simulating interstellar material

Irradiation by energetic cosmic ray particles: Energetic electrons, Protons, keV/MeV ions

Irradiation simulating nascent stars: UV, Ly a, FUV, X Catalytic role of H bearing molecules (Abstraction)


H2 formation via indirect processes

- Observation of Negative Ion Resonances in Amorphous Ice via Low-Energy (5-40 eV) Electron-Stimulated production of H2 : Kimmel & Orlando 1996, Orlando et al 1999

- Release of H2 from the photodissociation of ASW and polycrystalline ice at 157 and 193 nm: Yabushita et al 2008

-HD formation by abstraction of H/D chemisorbed in carbon grains with D/H atoms under simulated interstellar conditions : Mennella 2008

- Superhydrogenated PAHs: catalytic formation of H2 : Thrower et al 2011

- The catalytic role of coronene for H2 formation : Mennella et al 2012

- Experimental evidence for the formation of highly superhydrogenated PAHs through atom addition and their catalytic role in H2 formation : Thrower et al 2012.

Simulating types of processing

Main interests in the reaction channels, reaction rates, branching ratios …

Polycristalline ice ASW

Observation of Negative Ion Resonances in Amorphous Ice via Low-Energy (5-40 eV) Electron-Stimulated Production of Molecular Hydrogen Kimmel & Orlando 1996 The D2 (1Σ+

g, v = 0-2, J = 0-2) desorbates produced during low-energy (5-40 eV) electron-beam irradiation of amorphous D2O ice were monitored using resonance-enhanced laser ionization spectroscopy. We attribute the structure in the D2 yield as a function of the incident electron energy to core-excited negative ion resonances. These resonances, or the excited states produced after electron autodetachment, decay via molecular elimination to yield D2(1Σ+

g\) directly. D2 is observed with v = 0 or 2 but not v = 1, suggesting a symmetry propensity in the excitation or decay of the resonances. Orlando et al 1999


THE CATALYTIC ROLE OF CORONENE FOR MOLECULAR HYDROGEN FORMATION Mennella et al, ApJL 745:L2 2012 We present the results of an experimental study on the interaction of atomic deuterium with coronene films. The effects of D atom irradiation have been analyzed with infrared spectroscopy. The spectral changes provide evidence for deuteration of the outer edge coronene C sites via a D addition reaction. A cross section of 1.1Å2 is estimated for the deuteration process of coronene. HD and D2 molecules form, through abstraction reactions, on deuterated coronene sites with a cross section of 0.06Å2. The magnitude of both cross sections is in line with an Eley–Rideal type process. The results showthat hydrogenated neutral polycyclic aromatic hydrocarbon molecules act as catalysts for the formation of molecular hydrogen.

EXPERIMENTAL EVIDENCE FOR THE FORMATION OF HIGHLY SUPERHYDROGENATED PAHs THROUGH H ATOM ADDITION AND THEIR CATALYTIC ROLE IN H2 FORMATION J. D. Thrower et al ApJL752:3 2012. Mass spectrometry measurements show the formation of highly superhydrogenated derivatives of the polycyclic aromatic hydrocarbon molecule coronene through H atom addition reactions. The observed product mass distribution provides evidence also for abstraction reactions resulting in H2 formation, in agreement with recent IR measurements. Complementary density functional theory calculations confirm the stability of the observed superhydrogenated species toward spontaneous H and H2 loss indicating that abstraction reactions may be the dominant route to H2 formation involving neutral polycyclic aromatic hydrocarbons (PAHs). The results indicate that highly superhydrogenated PAHs could well be formed and could act as efficient catalysts for H2 formation in the interstellar medium in low UV flux regions.

SUPERHYDROGENATED PAHs: CATALYTIC FORMATION OF H2 J.D. Thrower et al EAS Series, 46 (2011) 453 The possible role of neutral PAHs as catalysts for H2 formation in the interstellar medium is investigated by a combined experimental and density function theory study of the superhydrogenation of coronene (C24H12). The calculations suggest efficient hydrogenation of both edge and centre sites, along with competing abstraction reactions to form H2 in a series of catalytic cycles. Scanning tunneling microscopy and thermal desorption measurements have been used to provide direct evidence of the formation of superhydrogenated coroneneas a result of exposure to D atoms. Lower limit estimates for the crosssections of 1.8×10 −17, 5.5×10 −18 and 1.1×10 −18 cm2 for the formation of singly, doubly and triply hydrogenated coronene are derived. The results suggest that superhydrogenated PAHs may play an important role in H2 formation in the ISM.



on a graphite surface (direct process)


Formation of HD in v=1-7 from recombination

on a cold HOPG surface

H + D HD on graphite at 15 K S. Price team at UCL

Creighan et al JCP 2006 Islam et JCP 2007

Latimer, Islam & Price CPL 2008


Formation of molecular hydrogen on a graphite surface via direct process


on a silicate surface (direct process)


TPD

s (cm-2) Rate equ. model

7 x 1014 Perets et al 2007

Olivine, Polycristalline Silicate 25 32 27 2 x 1014 Katz et al 1999

2007 1997


Combined TPD & REMPI (Resonantly Enhanced Multi Photon Ionization) expts

New results on the detection of ro-vibrational states of the just-formed D2

* as it leaves a silicate surface suggests new and unexpected answers to the formation question Lemaire, Vidali et al, 2010 Gavilan et al, 2012a,b Gavilan et al. 2013a,b

Formation of H2 molecules on a silicate surface

Polycrystalline silicates (telluric olivine) Pirronello, Vidali et al. 1997 Amorphous silicates (Fex, Mg1-x)2SiO4, x=0.5 Vidali et al. 2007 0<x<1 Vidali et al. 2009, 2010

TPD (Thermally Programmed Desorption) experiments (HD Formation)

Bare grains of silicates are (together with carbonaceous material) the main original constituent of interstellar matter in diffuse clouds


How to measure the population in each ro-vibrational quantum states REMPI diagnostic (Resonantly Enhanced Multi Photon Ionization)

- H2 (homonuclear molecule) then electric dipole transitions within the electronic ground state are forbidden, only quadrupole transitions no selection rules for transitions between various v states transitions between ro-vibrational levels must satisfy J=0,± 2. - Allowed electronic dipole transitions from the ground state X1+

g are to the B1+u

and C1u states.

occuring at energies between 11 and 14 eV (i.e., at UV wavelengths, ~100 nm) and known as the H2 Lyman and Werner bands. - One photon to ionization - Then TOF (Time Of Flight) detection

Franck Condon Factors

REMPI 2+1 (through the E,F state)

REMPI 1+1 also possible (through the B or C states)


Silicate sample Cryo generator Ts=6-150K cleaning: 800 K

temperatures measurements and regulations

H/H2

D/D2

UV laser beam focused in front of the surface

Nd:YAG laser + Dye laser 1.064µm x 2 … 600-300-200 nm

REMPI + Time of Flight Mass Spectrometer (TOF-MS)

quantum states discrimination

Excitation 2.45 GHz

Time of flight

HD+, H2+

, D2+ (v,J) signal

Acceler.

Extract.

MCP

Ultra High Vacuum Chamber (<10-10 mbar, n~106cm-3) pumping: primary, turbomlecular, ionic, titanium sublimation. baking up at 150°C for ~100 hours

Experiment scheme Diagnostics methods with state specific resolution


QMS

TOF-MS UV Laser

Silicate sample


The "FORMOLISM"

setup

(FORmation of MOLecules in the

Inter Stellar

Medium)

Obs. PARIS/Uni. Cergy @ JL Lemaire

"FORMOLISM" experimental setup

Atomic hydrogen source


Quadrupole Mass Spectrometer (+ translation + rotation)

Time of Flight Mass

Spectrometer

Cold head + thermal shield

(+translation)

Inside the UHV chamber (seen from the molecular or atomic gas beams entrance port)

UV Laser beam


Movable H2O spray

Asrophysical Journal Letters, 725, L156 (2010)


Tsilicate=30K

70K Deposition TPD

Beam ON OFF

Same experiment with Tsilicate=30K still some formation in the v"=4, J"=2 rovibrationnally excited quantum state of D2

… and lastly the same experiment at Tsilicate=70K almost no signal

Almost no formed D2 stuck on the surface at 30K

23800 24000 24200 24400 24600 24800 25000

sec

0

1000

2000

3000

4000

5000

QMS (M

4)

0

10

20

30

40

50

60

70

Temp

23800 24000 24200 24400 24600 24800 25000-4.00E-010

-6.00E-010

-8.00E-010

-1.00E-009

-1.20E-009

-1.40E-009

-1.60E-009

REMP

I-TOF

(F1)

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

Temp

Tsilicate= 8K

70K Deposition TPD

Beam ON OFF We record the population of D2 in its ground state in the

v"=4, J"=2 rovibrationnally excited quantum state

D2 stuck on the surface after formation is released during TPD in the v"=0 state its formation energy transferred into the substrate

In conclusion: Recording the population of D2 in its ground state in the v"=4, J"=2 rovibrationnally excited quantum state, - we observe that molecular hydrogen formed by recombination of atoms on grains are immediately released with some internal energy at temperature ranging from 5.5 up to ~70K

29400 29600 29800 30000

sec

0

1000

2000

3000

4000

5000

QMS (M

4)

0

10

20

30

40

50

60

70

Temp

29400 29600 29800 30000-4,00E-010

-6,00E-010

-8,00E-010

-1,00E-009

-1,20E-009

-1,40E-009

-1,60E-009

REMP

I-TOF

(F1)

somme_F4

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

Temp


Prospects in 2012

These experiment and results open a vast field of research on bare silicates

- Our work has to be completed by a full range of v",J" measurements - Check differences between crystalline and amorphous silicates - Check different silicate Mg,Fe composition as well as other material: (Mgx,Fe(1-x)SiO3, SiO2, SiC, Graphene , Silicene) - H2 formation on silicates (D2 previously)

This work should also triggers a renewed interest from theoreticians: - Modelers of the dynamics of infall of an interstellar cloud as well as to those working on models of the chemical evolution of a cloud, where the inclusion of gas-surface processes is recognized to be important. - Modelers of the atom/molecule interaction with silicate surfaces

This work should also orientates astronomical observations: Towards specific locations in the ISM or star forming region to look for pure H2 formation emission lines in the J, H and K bands


Work done or started since 2012

The previous experiment and results opened a vast field of research on bare silicates

- Our work has to be completed by a full range of v",J" measurements - Check differences between crystalline and amorphous silicates - Check different olivine family silicate Mg, Fe composition as well as other material: (Mgx,Fe(1-x)SiO3, SiO2, SiC, Graphene, Silicene) - H2 formation on silicates This work should also triggers a renewed interest to theoreticians: - Modelers of the dynamics of infall of an interstellar cloud as well as to those working on models of the chemical evolution of a cloud, where the inclusion of gas-surface processes is recognized to be important. - Modelers of the atom/molecule interaction with silicate surfaces (in progress)

This work should also orientate astronomical observations: - Towards specific locations in the ISM or star forming region to look for pure H2 formation emission lines in the J, H & K bands (in progress with VLT and Keck observations 2013)


3. Then some more experimental details and results

on the H2 (D2, HD) formation


Laboratory synthesis of interstellar dust analogs

Several techniques (extensive review in Colangeli et al. 2003) - laser pyrolysis (Herlin et al. 1998) - reactive thermal evaporation (Rietmeijer et al. 1999) - laser ablation (Brucato et al. 1999, Jäger et al.) - ion sputtering (Hanley & Sinnott 2002) - sol-gel synthesis (Jäger et al. 2003)

For our first experiment in 2010, Silicate sample prepared by electron-beam evaporation in a high vacuum chamber (10−7mbar)

- San Carlos olivine (Mg1.8Fe0.2SiO4) as solid precursor. - Locally transformed into liquid in a crucible. - Vapor kinetically deposited onto the substrate (p<10−6 mbar, mean free path ∼30 cm). - Vapor deposition rate (~0.2 to 0.5 Å/s) both an our sample holder and on a KBr window - Deposition rate and final film thickness controlled with a quartz micro balance.

thin amorphous film (~100 nm at most) with a high surface-to volume ratio analogous to interstellar dust grains.

- Amorphous character checked by IR spectroscopy - Exact chemical composition not well determined from only IR spectroscopy

(broad bands and uncertainties in the band position) Djouadi, d’Hendecourt, Leroux, Jones, Borg, Deboffle, Chauvin, A&A (2005)

Experiments in 2012-2013 Silicate samples prepared in Jena by laser ablation (Fayalite & Forsterite, end-members of Olivine family)

- Set of amorphous + set of crystallized one (Jena + Japan) - local and external characterization (HRTEM & EDX and IR Spectroscopies, FESEM & AFM imaging, XRD, XRF)

Sabri, Gavilan, Jäger, Lemaire, Vidali, Mutschke, Henning (ApJ 2013)


Sabri, Gavilan, C. Jäger, Lemaire, Vidali, Henning; ApJ 2013

Samples 2012-213 Jena-Paris LASSIE Collaboration Fayalite (Fe2SiO4) manufacturing dust grains analogs deposited on a Si (111) disc, both in amorphous and crystalline form by laser ablation Forsterite (Mg2SiO4) Characterization - high-resolution electron microscopy (HRTEM) - energy dispersive x-ray spectroscopy (EDX) Composition - Fourier Transform Infrared (FTIR) transmission spectroscopy -Atomic Force Microscopy (AFM) imaging Both AFM and HRTEM direct view inside the grains and unveil the information on the morphology of the grains such as sizes and shapes, as well as the structure of silicates.

a-Mg2SiO4 c-Mg2SiO4


Transmission IR spectra HRTEM Amorphous grains Crystallized grains


AFM Silicate characterization showing the surface morphology

3D Surface


QMS

TOF-MS UV Laser

Silicate sample


The "FORMOLISM" (FORmation of

MOLecules in the Inter

Stellar Medium)

Setup


OPR


REMPI-TOF signal (at J=1) during H+H2 and H2 deposition and TPD, in black. QMS signal, recorded synchronously, in red. OPR of H2 and D2 in LTE as a function of the local temperature.

The high temperature equilibrium OPR value occurs at T≥100 K for D2 and for T≥200 K for H2.

ApJ, 760, 35, 2012

NSC

''

'' ''

''

)1''2(

)1''2(

J

KTE

N

KTE

NJ J

J

eJg

eJgf

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

Non Convertie

Convertie

33,33%

66,66%

100%

T=10 K

J''

Ortho

Para

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

Convertie

Non Convertie

19% 21%

39%

11%9%

1% 0,00%

J''

T =300K

Non converted states

"2fraction of even states (Ortho)3 PaireJf

"1fraction of odd states (Para)3 ImpaireJf

Converted states

300K

10K


Previous theoretical work examining molecular hydrogen formation on pristine dust surfaces has assumed that a molecule-saturated surface halts recombination due to short atomic residence times.

Our present experiment on a cold silicate surface shows instead a positive correlation between D2 sub-monolayer pre-dosing and D2* (v”=4, J”=2) formation:

up to 35 ± 10 % at Tdust=5.5 K

Are molecule-covered dust grains efficient catalysts of H2 formation in the cold ISM? L. Gavilan, J. L. Lemaire and G. Vidali. MNRAS, 2012

- REMPI-TOF signal of excited D2* forming on a silicate vs. D atoms irradiation on a surface pre-dosed with D2. - In blue: error bars for the largest D2 pre-dose. - In red: error margins for D2* on a clean surface. T=10K T=5.5K


Rotational distribution of D2 and D2* from:

(up) Mg2SiO4 (amorphous) and (down) from Mg2SiO4 and Fe2SiO4 (crystalline).

* Rotational energy distribution of molecules formed on and ejected from crystalline silicates Trot~120 K amorphous silicate > 310 K Same trend for molecules in a room temperature beam scattered on crystalline vs. amorphous silicate interpreted as a result of kinematic scattering events on the surface. Hints that formation mechanism play a negligible role in the rotational distribution.

* Comparison to previous experimental results on crystalline metallic surfaces and theoretical simulations showing that the direction of the angular momentum compared to the surface could play a role (cartwheel vs. helicopter mode). * Our current interpretation deals with rotational inelastic scattering, where flatter grains hinder the "cartwheel" rotational mode of both newly formed and ejected molecules.

Rotational temperatures: amorphous ~310K

and crystalline ~120K.

THE FORMATION OF MOLECULAR HYDROGEN ON SILICATE DUST ANALOGS: THE ROTATIONAL DISTRIBUTION

•First time that the internal structure of hydrogen molecules formed on silicate dust is measured.

* Essential results to understand the chemistry of diffuse clouds containing bare dust grains.

Gavilan, Lemaire, Vidali, Sabri, Jäger ApJ 2013


o-H2O and p-H2O transitions observed in the far IR (or sub-mm) (Herschel HIFI) with the same (temperature dependant) O/P ratio than H2 ground state at: The OPR is sensitive to the formation temperature of H2O 34 K O K ground rotational state transitions in the range ~550GHz ~1100 GHz ~545µm ~270µm

Other molecules and functional groups containing two (or more) H atoms also have ortho- and para- forms. For instance: H2O and methylene radical H2C= (H2C=O: formaldehyde; H2C=CH-CH3: propene) NH3 (I=3/2 and I=1/2: O/P=2) and CH4

The O/P ratio is useful to study the formation mechanism (and particularly the formation temperature) of some important molecules

ortho-H2 and para-H2 transitions observed from the visible to the NIR (<28 µm) Nuclear spin I=1 I=0 Triplet state Singlet state O/P=3 at room temperature (temperature dependant) ground state at: The OPR is sensitive to the formation temperature of H2 175 K O K


The O/P ratio is useful to study the formation mechanism (and particularly the formation temperature) of some important molecules

Schematically: O/P~3 H2O formation in the gas phase in regions where high gas and dust

temperatures (>60-100 K ) and high densities (>10-4 cm-3) Typically in high-mass star-forming regions O/P<3 H2O formation onto dust grains surfaces at low temperatures (<40-60 K) and

low densities (<10-6 cm-3) Typically in low-mass star-forming regions, hot cores and corinos However there is a complex interplay between different mechanisms: - gas chemitry - grain-surface chemistry - freeze-out - desorption mechanisms so that the chemistry of water in the different regions of the ISM (from the dense and cold clouds to the diffuse clouds) is not yet well understood

Chaabouni et al. JCP 2012


FT- RAIRS

IR detector QMS

Triply differentially Pumped beamlines

* 3 mm collimators

- valves

Movable Cryocooler (5.5 – 350 K)

*

*

*

microwave discharge 2.45 GHz

Gas inlet O2 D

1) Quadrupole MS (deposition +TPD)

2) REMPI-TOF MS (not shown)

3) FT – RAIRS

The "FORMOLISM"

setup


Inter Stellar

Medium)


QMS

T (Si diode) (temp. Controller)

+ QMS

Cryocooler (20– 350 K) QMS

TOF-MS Silicate sample

Movable water spray

ISM-SPP 1573. Laboratory Astrophysics Workshop. Tabarz (D) October 2014


Schematic representation of the surface chemistry reaction network leading to the formation of water ice. Green arrows refer to reactions that have been characterized by experiments or through simulations of experimental data. Red arrows refer to reactions that have not yet been studied experimentally or for which the experimental data are inconclusive. Four cases are distinguished: (i) efficient, effectively barrierless (<500 K) reactions (green), (ii) reactions with a barrier above 500 K (dashedgreen), (iii) reactions not studied experimentally (red), (iv) reactions studied experimentally but for which results are inconclusive (red-dashed).

Van Dishoeck , Herbst & Neufeld. Chem. Rev. 2013 (collected from all available sources)

3. Some important recent (and future) developments

on the H2 (D2, HD) formation


H on HOPGraphite

High Coverage

Medium Coverage

80 x 72 Å2

171 x 155 Å2

Low Coverage

103 x 114 Å2

STM: Vt~800mV, It~0.15-0.2nA

Liv Hornekær, iNano Aarhus


- NRs are non-reactive to O2, - reactive to O ~2L

- 0,94 V ; 0,36 nA

O2

O2

O2

Silicene NRs: Reactivity toward atomic oxidation

- 0,94 V ; 0,36

nA

- Then reactive to H ? H2O

STM studies: Silicene Nano-Ribbons (counterpart of graphene for silicon atoms)

R. Tchalala (LASSIE ESR), H. Oughaddou, H. Enriquez, A. Ali, G. Dujardin ISMO-Orsay/Marrakech



Laboratory simulations of the interstellar processing of grains Crystalline Amorphous Crystalline Amorphous

ESRF beamline (Grenoble) offering both: - irradiation with hard X-rays simulating* the interstellar processing on dust grains analogues. [*high energetic particles and/or Cosmic Rays (CRs)] - detection in-situ of the crystallinity/amorphization evolution using simultaneously XRD and XRF (nano-beam and high flux) check changes in dust structure and morphology Experiment on different kind of analogs of interstellar dust (prepared in Jena) - types and compositions - dust grain sizes Work in progress, PI: Dr. L. Gavilan Collaboration with •Jena (C. Jäger) •Grenoble (one of the specialists of the STARDUST samples analysis)

4. (very) Short review on the modeling of atoms & molecules interactions with surfaces

Formation mechanisms Formation with a chemisorbed H on graphite Formation and dissociation of H2 on simulated nano-silicates dust grains


Interaction of atoms and molecules with surfaces: * physisorption (vdW interaction) * chemisorption (covalent bond)

Surfaces, H2 formationTheoretical aspects

Reaction mechanisms - Eley-Rideal (prompt mechanism) occur at high H atom coverage rate creates "hot" H2 - Harris-Kasemo (hot atom mechanism) several bounces before prompt reaction occur at low and high H atom coverage rate creates "hot" H2 - Langmuir-Hinshelwood (migration mechanism by tunneling or thermal hooping) occur at low and high H atom coverage rate creates H2 at surface temperature


H chemisorbed on HOPG

Rauls

Work on graphitic surfaces Chemisorption - basal plane: Jeloica & Sidis, Chem. Phys. Lett. 300, 157 (1999) Chemisorption at defects: Sha et al, Surface Science 496, 318 (2002) Sha et al, J. Am. Chem. Soc. 126, 13095 (2004) Güttler et al, Surface Science 570, 218 (2004) Chemisorbed states on graphite: Cazaux & Tielens, Astrophys. J. 604, 222 (2004)


Dimer formation

Hornekær et al. Phys. Rev. Lett. 97, 186102 (2006) ISM-SPP 1573. Laboratory Astrophysics Summer School. Tabarz (D) October 2014

The mineral olivine is a magnesium iron silicate with the formula

(Mgx,Fe(1-x))2SiO4

It is one of the most common minerals on Earth, and has also been identified in meteorites, the Moon, Mars, in the dust of comet Wild 2, and within the core of comet Tempel.

The ratio of magnesium and iron varies between the two endmembers of the solid solution series: Forsterite (Mg-endmember, x=1) and Fayalite (Fe-endmember, x=0). Minor amounts of Mn and Ni. ~1900° C Melting temperature ~1200° C

Crystal system: orthorhombic

The atomic scale structure of olivine looking along the a axis. Oxygen is shown in red, silicon in pink, and magnesium/iron in blue. A projection of the unit cell is shown by the black rectangle


A small sample of "Neo-silicates" (American Museum of Natural History NYC)

H2 formation and dissociation on nanosilicate dust grains

Forsterite-like composition (MgO)6(SiO2)3 Enstatite-like composition (MgO)4(SiO2)4

Oueslati, Kerkeni & Bromley 2014 (in preparation) Kerkeni & Bromley 2013 Goumans & Bromley 2011, 2012

Nanosilicates could well be a constituent of the diffuse ISM,

either directly from stellar outflows or from processing of larger grains

SiO4 tetrahedra directly

joined by bridging oxygen

SiO4 tetrahedra directly

joined by bridging oxygen

and separated by Mg cation

(a) (b)


H2 formation and dissociation on nanosilicate dust grains


Oueslati, Kerkeni & Bromley 2014 (in preparation) Kerkeni & Bromley 2013 Goumans & Bromley 2011, 2012 Goumans et al. 2009

Recombination barrier height with respect to H2,phys versus the total binding energy of both chemisorbed H atoms

• Linear dependence

• H2,g formation is largely determined by how strongly both H atoms chemisorb with a

nanosilicate dust grain surface

• A general BEP (Brønsted-Evans-Polanyi) relation for H2 dissociation on bare silicate grains

independent of dust grain nature or size or crystallinity

Abstraction Reaction of H atoms from Tetramethylsilane Computational methods

Semi-Classical Approach – Ab intio methods are employed for the electronic structure computations :

•to optimize the fragments geometries, •to investigate barrier height, exothermicity of the reaction and the minimum energy path.

– Calculate thermal rate constant for temperature range [180-4000K] using : • Conventional transition state theory (TST) [1], • Empirical correction: Wigner correction [2], • Semi-classical correction : Zero Curvature Tunneling (ZCT) [3].

[1] Thanh N. Truong, Donald G. Truhlar, J. Chem. Phys. 93 (1990) 1761. [2] E. Wigner, Z. Phys. Chem. B 19 (1932) 203. [3] Rex T. Skodje, Donald G. Truhla, and Bruce C. Garrett, J. Chem. Phys. 85 (1981) 3020.

Quantum approach (in progress) – Potential Energy surface (PES) :

• 2 dimensional state of the art ab intio PES is based on the model chemistry : CCSD(T)/ccp-pVTZ//MP2/cc-pVTZ[4] • Zero point energies are added to the electronic energies to give the effective potential for dynamics calculations.

– Prediction of state-to-state and total cross sections and rate constants using the 2D-effective reduced dimensionality PES [4].

[4] B. Kerkeni, David C. Clary, J. Chem. Phys. 120 (2004) 2309.

I. Oueslati (LASSIE ESR), B. Kerkeni, N. Feautrier, A. Spielfiedel, L. Tchang-Brillet Paris Obs./UPMC/Tunis


5. The need for astronomical observations Observing H2

Where and how


* In addition to experimental laboratory or modeling computerized astrophysics, decisive proofs of the validity of their tentative descriptions of the formation processes, namely astronomical observations, are still missing. They would provide definitive evidence of these key gas-grain interactions.

5. Formation of H2 molecules in the ISM

* We will reveal, as a conclusion, a crucial tool that could assert the importance of this chief reaction in the field of astrochemistry.

* To achieve this, it is essential to look at particular environments where the pure formation spectral signature is not disturbed mainly by photodissociation (PDR) and/or shock waves processes resulting from nearby star formation.


From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H2 emission, which appears to be observational evidence that H2 forms on grain surfaces.


Planetary Nebula

2003

61

Based on previous works of : Farebrother et al., 2000 Meijer et al., 2001, 2002, 2003


Planetary Nebulae ISM-SPP 1573. Laboratory Astrophysics Summer School. Tabarz (D) October 2014

HST: [SII], Vis, [O III] (RGB)

N-S slit (K-band spectrum) Northern lobe (green & brown) Northern knot (blue & red)

Central core (black), Blackbody 830 K (magenta), H2 labels (pink) Atmospheric absorption: green. Triangle black: He I, orange: H I

E-W slit (spatial distribution) VLT-ISAAC: Ks-band

Bipolar Planetary Nebula PN M2-9

Preliminary results


6. Conclusions Summary * The formation of molecular hydrogen on dust grains in the interstellar medium (ISM) remains an unresolved question (suggested almost 50 years ago) despite soon 20 years of serious experimental and theoretical efforts as well as a wealth of observational results.

* Even if now there is no doubt about the catalytic role of dust grains in the formation process, a variety of multi-disciplinary experimental approaches have to be used to examine the large number of parameters involved.

* The first step to simulate this process in the laboratory is to create an analog of the dust grain. Some of the most important parameters of a dust surface are its stoichiometric composition and morphology. Carbonaceous or siliceous grains can be fabricated. Several techniques are used to create and characterize dust surfaces that mimic dust in space.

* The second step covers the formation mechanisms of molecules on a dust surface. We are mainly interested in learning about the fate of the energy released (about 4.5 eV per H2) in the formation process, due to its paramount role in star formation. Important parameters include the temperatures of both the grain and the atomic gas impinging on the surface as well as the number and nature of available adsorption sites on the surface.

* The third step is to establish an astronomical connection to the experimental results. In this case, near-infrared observations of H2 emission in different astronomical environments are used (in our case, we focus on the dusty rings of planetary nebulae) to verify our experimental hypothesis. The main problem here is that others excitation mechanisms often occur in parallel to the formation of H2. Clarifying the source of the IR emission will then clarify the conditions allowing H2 formation to be observed.

3, 4, 5. Intermediate conclusions on H2 formation

Need for more laboratory experimental work * REMPI (2+1)-TOF MS * use of synchrotron radiation for REMPI (1+1)-TOF MS * use of STM and all techniques of solid state physics

Need for observations with as many lines as possible but ground-based drawbacks: * infrared atmospheric windows * telluric lines pollution then the crucial tool: an embarked-laboratory instrument * from 28 to .8 µm SOFIA or …..

The Molecular Hydrogen Explorer: H2EX A space mission to understand the formation of galaxies, stars and giant planets from molecular hydrogen observations F. Boulanger · J. P. Maillard ·and ~70 co-authors, 2009 ….. but was not accepted !! * Proposed to ESA as infrared medium class space mission, * spectro-imager designed to detect H2 at high spectral resolution, * in its ground state pure vibrational and rotational emission lines


6. Prospects: Large organics molecules A topical hot problem in astronomy: * observed molecules in the universe keeps increasing in number and growing in complexity

* one of the main goal: find a link between astrophysics and astrobiology in search of the origin of life in the universe, obviously a key question of paramount interest and general fascination, even if somewhat and somehow controversial

* study the role of dust grains (vs gas phase) in the formation of larger molecules and in particular of prebiotic molecules

* the main interest is that such molecules can be considered as precursors of the formation of complex organic compounds (like amino acids) which are subsequently at the origin of more complex biological material. - Need to establish a connection between observations and experiments.


* A wide variety of organic compounds have been detected in extraterrestrial bodies as meteorites and comets

* Amino acids were identified in extracts from Murchison meteorite (Kvenvolden et al., Nature 1970; Schmitt-Kopplin et al., PNAS 2010) and other carbonaceous chondrites (Burton et al., M&P Sci. 2013) and from the STARDUST mission (Glavin et al., M&P Sci. 2008). Among them, Glycine and Alanine

6. Bridging Astrophysics and Astrobiology

* It is hypothesized that these compounds are originally formed in ice mantles of ISDs (interstellar dust grains) in molecular clouds by cosmic rays and UV * Formation of amino acid precursors by high energy protons or UV irradiation of simulated ISDs ices was reported by several groups (Kobayashi et al, 36th COSPAR, #2715)

* The amino acid precursors were however not well-characterized ISM-SPP 1573. Laboratory Astrophysics Summer School. Tabarz (D) October 2014

* N-bearing molecules are necessary precursors of prebiotic material. * Among them hydroxylamine (NH2OH) is one of the best candidate. * Both experience and theory have shown that in its protonated form it can react with acetic and propanoic acids to yield protonated glycine (the simplest non-chiral amino-acid) and the chiral one alanine. * But the question is still under debate whether such a synthesis may happen in the gas phase or by surface reactions.

* Recently, a NO ice hydrogenation surface reaction at 10-15K has shown an efficient route to NH2OH formation on interstellar relevant ices on sub- or multi-layer substrates. (Paris-Cergy/Leiden, 2012) * Such a process takes place without external energetic input but implies several successive steps of hydrogenation.

* The main problem at the moment is actually the lack of astronomical detection of both hydroxylamine and glycine, a problem obviously correlated to their low predicted abundances. * But such a detection is claimed to be highly plausible nowadays with ALMA in the case luminous protostars, or so-called hot cores.


L or S(+)-alanine D or R(–)-alanine

Alanine (chiral)

Glycine (non-chiral)

C3H7NO

Zwitterionic or protonated form

The two simplest proteinogenic* α-amino acids with respect to molecular structure (both classified as aliphatic amino acid)

H

C2H5NO

*structural units that make up proteins

Non-proteinogenic alanine forms


* Icy dust grains in space act as catalytic surfaces onto which complex molecules form. * These molecules are synthesized through exothermic reactions from precursor radicals and, mostly, hydrogen atom additions. * Among the resulting products are species of biological relevance, such as hydroxylamine -NH2OH- a precursor molecule in the formation of amino acids. * In this Letter, laboratory experiments are described that demonstrate NH2OH formation in interstellar ice analogs for astronomically relevant temperatures via successive hydrogenation reactions of solid nitric oxide (NO). * Inclusion of the experimental results in an astrochemical gas–grain model proves the importance of a solid-state NO+H reaction channel as a starting point for prebiotic species in dark interstellar clouds and adds a new perspective to the way molecules of biological importance may form in space.


FT- RAIRS

IR detector QMS


* 3 mm collimators

- valves


*

*

*

microwave discharges 2.45 GHz

Gas inlet

(H2, D2, O2, N2)


2) REMPI-TOF MS

3) FT – RAIRS

Nd:YAG + Dye

UV Laser

MCPs (signal 1)

TOF-MS

1 2 3 4

Powermeter (signal 3)

Laser trig.

DSO

HVs TOF MCPs

QMS

T (Si diode) (temp. controller) (QMS + signal 2)



UV Laser

Movable water spray

UHV chamber (p<10-10 mbar)


The "FORMOLISM"

setup


Inter Stellar

Medium)


FT- RAIRS

IR detector QMS


* 3 mm collimators

- valves


*

*

*

microwave discharges 2.45 GHz

Gas inlet

(H2, D2, O2, N2)


2) REMPI-TOF MS (not shown)

3) FT – RAIRS

QMS

T (Si diode) (temp. Controller)

+ QMS



Movable water spray


The "FORMOLISM"

setup


Inter Stellar

Medium)


Similar setup: SURFRESIDE in Leiden (Nl)


TPD mass spectra (~120 – 205 K) after sequential depositions of:

(a) ∼1 ML NO + 3 EML H-atoms on a-silicate at 10 K, (b) ∼1 ML NO + 3 EML D-atoms on a-silicate at 10 K, and (c) ∼1 ML NO + 3 EML D-atoms on crystalline H2O ice at 10 K

The peaks at T∼188 K indicates desorption of hydroxylamine (NH2OH, m=33; ND2OD, m=36) as well as of their fragments: NO, OH (OD), and NH2 (ND2) due to the cracking pattern in the QMS head. Peaks at T∼150 K for m=30, 17, and 18 correspond to co-desorption with water of NO molecules and OH (OD) fragments.


(a) spectrum after 60 minutes co-deposition of NO and an H-atom flux of 7×1012 atoms cm−2s−1 (H/NO = 0.2)

(b) spectrum after 120 minutes co-deposition of NO and an H-atom flux of 3×1013 atoms cm−2s−1 (H/NO = 4)

RAIRS of solid NO deposited on a gold substrate at 15 K upon H-atom exposure


Schematic representation of the solid-state NO + H reaction network

Solid squares: species detected in the solid phase (RAIRS) or final products detected in gas phase (TPD-MS) Dashed squares: intermediate products not detected but maybe formed on the surface Solid arrows: efficient (fast) and/or barrierless reactions Dashed arrows: reaction pathways with an activation barrier

NB: Only atom /radical addition reactions are presented here (abstraction reactions omitted for simplicity. e.g., H)

Fedoseev et al , JCP, 2012


Solid lines: reactions with no barrier Dash lines: reactions with a small barrier Dash-dot: reactions with a noticeable barrier

* On gold, graphite, silicate, compact ASW ice * On co-deposited ices [polar (H2O) or non-polar (CO, CO2, N2)]

Reaction pathways leading to the formation of nitrogen oxides (and other products)

NO + O/O2/O3/N/H/H2

Solid state chemistry of nitrogen oxides – Part I: surface consumption of NO Minissale et al PCCP 2014

– Part II: surface consumption of NO2 Ioppolo et al PCCP 2014

More experiments TPD-MS and FT-RAIRS

combined diagnostics


Fractional abundance of species involved in nitrogen chemistry for a full gas-grain astrochemical model (Hassel et al. 2008) including the effect of the NO hydrogenation channel

(dense cloud conditions of TMC1)

Left panel: gas-phase species Right panel: grain-surface species Solid lines: include NO + H surface reaction Dashed lines: do not include

Abundances with respect to the gas-phase density of H nuclei nH=n(H)+2n(H2)

Gas Phase

Grain Surface Fedoseev et al , JCP, 2012


Propanoic acid Acetic acid

Protonated forms

Blagojevic et al , MNRAS, 2003

And new results to be presented at the workshop ...


7. General conclusions on

- H2 formation - H2O formation - Large organic molecules connected to astrobiology

Still a lot to do !!! - with laboratory experiments - with theoretical simulations - with observations (need for both Hi res. & sensitivity As demonstrated by a very interesting recent paper connecting all these fields (if there is some times left !)


The end

formation of molecules · 2014. 10. 27. · interstellar and circumstellar media (ism and csm)...

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