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American Chemical Society National Meeting – August 2002, Boston. Invited talk at the Session: Chemical Studies Important in Astrobiology Division of Physical Chemistry Division. Laboratory Studies of Molecular Synthesis on Surfaces of Interstellar Dust Grain Analogues. Gianfranco Vidali

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American chemical society national meeting august 2002 boston

American Chemical Society National Meeting – August 2002, Boston

Invited talk at the Session:

Chemical Studies Important in Astrobiology

Division of Physical Chemistry Division


Laboratory studies of molecular synthesis on surfaces of interstellar dust grain analogues

Laboratory Studies of Molecular Synthesis on Surfaces of Interstellar Dust Grain Analogues

Gianfranco Vidali

Syracuse University, Physics Dept., Syracuse, N.Y.

J.E.Roser, Valerio Pirronello*, G.Manico’*

*permanent address: Universita’ di Catania, Sicily (Italy)

phy.syr.edu/research/astro

Work supported by NASA (Grants NAG5-6822, NAG5-9033) and CNR 210483088 (Italy)


Outline

Outline

  • Part I: The Interstellar Medium

  • Part II: Application of Surface Science Techniques to Astrophysical Problems: Experimental Challenges

  • Part III: Examples from our Research Program

    • Experiments of molecular synthesis on interstellar dust grain analogues

      • H+HH2 on dust grains

      • CO+OCO2 on icy mantles

    • Theoretical and computational methods connecting laboratory data to actual processes in the ISM


Part i interstellar medium ism

Part I: Interstellar Medium (ISM)

  • Interstellar medium:

    • composition: gas (99%) and dust (1%)

      • Dust: silicates and carbonaceous grains

      • In dense clouds: icy mantles

    • role: birthplace and deathbed of stars


Atoms and molecules in space

How do we know?

absorption of starlight /emission of photons: from FUV (electronic transitions), to near infrared (vibrations) and millimeter waves (rotations)

However, homopolar molecules are hard to detect

Most important molecule: molecular hydrogen:

chemistry

cloud collapse-star formation

detection:

weak quadrupolar transitions

H2 in shocked regions

excitation of CO via collisions

Atoms and Molecules in Space

W33A


The molecular hydrogen problem

H+HH2 on dust grains (Salpeter, Hollenbach ~1970)

1-105 atoms/cm3

1 grain/1012 atoms

0.1 mm

10-20 K

H+HH2 not in the gas phase

Other routes: H+eH-+hn

H-+HH2+e requires ionized medium

The Molecular Hydrogen Problem

b3Su+

X1Sg+


Part ii application of surface science techniques to astrophysical problems

Part II: Application of surface science techniques to astrophysical problems

  • Measurement of hydrogen recombination and hydrogenation/oxidation reactions on surfaces of dust grain analogues

  • Experimental Conditions

    • Low kinetic energy of H atoms (gas phase atoms): ~ 200-300 K

    • Low flux of H atoms <1012 atoms/cm2/sec

    • Low sample temperature (5-40 K)

    • Low background pressure (10-10 torr)

  • Theory: Hollenbach and Salpeter (1970)

    • Fast diffusion

  • Experiment: Schutte et al. (1976), King and Wise (1963)

    • Not in astrophysically relevant conditions


Our research program

Our Research Program

  • Experiments of molecular synthesis on interstellar dust grain analogues

    • H+H  H2

      • Measure H2 formation on:

        • Silicates (olivine) Ap.J. 475, L69 (1997); Ap.J.483, L131 (1997) – first experiments to study H2 formation on dust grains analogues in astrophysically relevant conditions

        • Carbonaceous Materials (amorphous carbon) A&A 344, 681 (1999)

        • Amorphous Water Ice: Ap.J. 548, L253 (2001); Ap.J. accepted (2002)

    • CO+OCO2

      • Measure CO2 formation due to oxidation of CO-ice by atomic oxygen


Experimental challenges

Experimental Challenges

  • Background pressure of H2 and the undissociated fraction of hydrogen

    • Solution: Use twoatomic beams (one with H and the other with D) and look for the formation of HD.


American chemical society national meeting august 2002 boston

Apparatus to study molecule formation on dust grain analogues


Measurement methods

Measurement Methods

  • Irradiation of sample with thermal energy H atoms

  • Measurement of hydrogen recombination events

    • Measurement of H2 formation due to fast processes, due to:

      • Eley-Rideal ("prompt") reaction

      • Fast diffusion on surface of grain analogue

    • Thermal Programmed Desorption, to:

      • Desorb molecules that have already formed on the surface

      • Acceleratethe diffusion of H atoms and favour the recombination process


Example i hydrogen recombination reaction

Example I: Hydrogen recombination reaction

  • Thermal desorption trace: HD from olivine (a silicate) as a function of exposure (sub-monolayer coverage)

  • Ap.J. ’97

  • Learn about reaction kinetics and rates


Example i hydrogen recombination reaction1

Example I: Hydrogen recombination reaction

  • Molecular hydrogen recombination efficiency on different dust grain analogues

water ice

amorphous carbon

olivine


Experiment ism connections

Experiment-ISM connections

  • Theoretical and computational methods connecting laboratory data to actual processes in the ISM

    • Model hydrogen recombination reactions in the ISM using laboratory results:

      • Ap.J. 553, 595 (2001); Ap.J. 522, 305 (1999); MNRAS 296, 869 (1998)

Amorphous carbon


Molecular hydrogen formation on amorphous water ice

Molecular hydrogen formation on amorphous water ice

  • Study of molecular hydrogen formation on amorphous ices found in various interstellar environments.

  • Study of the role of ice morphology and UV processing on H2 formation.

  • Comparison of recombination efficiency due to surface or near-surface processes with competing mechanisms, such as cosmic rays and UV photons. See:Ap.J. 548, L243 (2001).

  • Study of evolution of morphology of icy grains through astrophysical environments (Ap.J., accepted - 2002)


Influence of ice morphology on h surface d surface hd reaction

Desorption of HD from amorphous ice (Roser et al., ApJ ‘02):

high density, low density, gas phase deposited

Recombination efficiency on amorphous ice surfaces (Roser et al., ApJ ‘02):

high density

low density

gas-phase deposited

Influence of ice morphology on Hsurface+DsurfaceHD reaction


Example ii oxidation reaction of co

The Problem:

Solid CO2 more abundant than explained by gas-phase reactions

Solid CO2 can be made by UV in CO- and O2–rich ices

However, solid CO2 is seen in quiescent regions – no UV

Can solid CO2 be made by:

COice + Ogas CO2 ice ?

Example II: Oxidation reaction of CO

Whittet et al, A&A, 1998

Spectrum towards Elias16


Oxidation of co ice by atomic o

Roser et al., Ap.J. 2001

Oxidation of CO ice by atomic O

O

CO

~100 layers CO +O

CO/O ~5.6-21

~100 layers H2O

substrate

CO2

CO

O

heat


Summary of accomplishments and future directions

Summary of accomplishments and future directions

  • We showed that:

    • Measurement of hydrogen recombination and CO oxidation reactions on dust grain analogues can explain processes occurring in the ISM

  • Challenges:

    • Composition, morphology of dust poorly known

    • Partition of reaction energy between new-born molecule and solid

      • Excitation of molecule ejected into the gas phase; theoretical estimates vary greatly

      • Role of energy deposited in the ISM


Current research study of the energetics of h 2 formation

Current Research: Study of the energetics of H2 formation

  • Goal:

  • Measurement of excitation state of molecular hydrogen formed on dust grain analogues

  • Techniques:

    • Time-of-flight (tof) detection to measure the translationalenergy of molecules

    • (2+1) REMPI (Resonance Enhance MultiPhoton Ionization) to measure the roto-vibrational state of molecules leaving the dust grain analogue


Time of flight measurements

Time-of-flight measurements

  • In the time-of-flight experiment, the desorbing flux is chopped by a rotating mechanical wheel, see adjacent sketch. The time that a pulse of molecules takes to go from the chopper to the detector is measured and the kinetic energy calculated.

  • Of the 4.5 eV energy released in the recombination reaction, it is not known quantitatively the partition of the in roto-vibration vs. translation of the molecule. Guess estimates of the amount of translational energy range from thermal energy (~20 K) to 1 eV.

  • The challenge is to measure the velocity distribution of the molecules exiting the surface during the brief time (~ a few tens of sec.) of the TPD run. This imposes stringent requirements on abating the residual gas background pressure.

  • Such experiment has not been done before under these conditions.


Measurement of the roto vibrational energy of h 2

Measurement of the roto-vibrational energy of H2

  • Of the 4.5 eV energy released in the recombination reaction, some is available to the molecule as roto-vibrational energy. Estimates of this energy vary greatly.

  • The experiment consists in probing the quantum state of the desorbing hydrogen molecules. Because vibrationalstates of H2 lie in the UV, the measurement of the roto-vibrational state is challenging.

  • We use the (2+1) REMPI (Resonance Enhanced MultiPhoton Ionization). The molecule is taken to an electronically excited state by the absorption of two photons. Here the molecule absorbs another photon that removes an electron. The molecular ion is then collected by a detector (a channel-plate), see adjacent diagrams.


Specifics of the detection of roto vibrational energy levels

Specifics of the detection of roto-vibrational energy levels

  • The light from a Nd:YAG laser (1089 nm) is doubled and sent to a dye laser for tuning. The ~600 nm light is then sent to a non-linear crystal that convert visible light into a 200 nm and a 300 nm beams. The molecule absorbs a 200 nm photon that takes it to a virtual state. If the molecule absorbs another photon, then it can go in an electronically excited state, see diagram. From there, the absorption of a 300 nm photon ionizes the molecule. That’s the explanation for the (2+1) nomenclature.

  • The challenge is to have a beam of photons intense enough so the molecule can absorb two photons virtually simultaneously. Furthermore, because the generation of tunable laser light at 200 nm requires the use of the non-linearity of special crystals, the process is inherently inefficient and the experiment needs powerful lasers.


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