Isabel Tanarro, Miguel Jiménez‐Redondo, Germán Molpeceres, Belén Maté, Víctor J. Herrero

Inst. Estructura de la Materia (IEM‐CSIC), Serrano 123, 28006, Madrid, Spaini.tanarro@csic.es

Infrared spectroscopy of carbonaceous interstellar dust analogues deposited by PECVD: 

Effects of processing by high energy electrons. 

Funding: FIS2013-48087-C2-1-P, CDS2009-00038 “ASTROMOL”, ERC-2013-Syg 610256 “NANOCOSMOS”

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INSTITUTO DE ESTRUCTURA                                                                     DE LA MATERIA

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• Carbonaceous compounds are found in very diverse astronomical media.

• A significant amount, in small dust grains.

• Characteristic IR absorption bands reveal the presence of aliphatic and aromatic functional groups.

Interstellar hydrocarbon dust

Spectra toward the Galactic Center Quintuplet Cluster (UKIRT, Mauna Kea)

J. Chiar et al. ApJ , 2013, 770, 78Weighted average of the

spectra and decomposition in its components 2900 cm-1

Wavelength (m) IR image of the GC Quintuplet Cluster

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E. Dartois et al. A&A, 2007, 463, 635

• Aliphatic features are more prominent, their absorption strengthsare much higher than aromatic ones.

• Structure estimates suggest an important contribution of H richaliphatic carbon.

Interstellar hydrocarbon dust

CH aromatic

(upper limit)

CCaromatic+ olefinic

CH aliphaticstretchings

CHaliphaticbendings

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The mechanisms of production of carbon dust and its evolution inastronomical media, processed by UV radiation and cosmic rays(i.e. energetic e- and ions), are presently a subject of intensiveinvestigation.

Interstellar hydrocarbon dust

J. Chiar et al. ApJ 2013, 770, 78A. P. Jones ASP Conf. Ser. 414, 2009

Carriers of the4.3 m band

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SEM, dust particle

As possible carriers of these bands, amorphous hydrogenated carbon grown by PECVD in laboratories (a‐C:H or HAC) show the best agreement with astronomical observations. 

E. Kovacevic et al. ApJ 2005, 623, 242

Left: IR spectrum of the dust grown in a capacitivelycoupled RF discharge of Ar+C2H2. Right: comparison of the 2900 cm‐1 band with thecharacteristic fingerprint in diffuse ISM.

Hydrocarbon dust in laboratory plasmas 

RF CC‐Plasma Reactor5

M. Godard et al. A&A 2011, 529, A146

Previous studies: Irradiation by ions of interstellarcarbonaceous analogs. Effects on the 3.4 m band

Decrease of thealiphatic C-H bands

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• Characterization of the effects on the structure and composition of the deposits by FTIR and AFM.

Aim of this talk (work in progress)Study of the stability of carbonaceous dust 

analogues irradiated by electrons.• Generation of dust analogue deposits in He+CH4 RF inductively 

coupled plasma reactors. 

• Simulation of the processing of dust in the interstellar environments by 5 KeV electron bombardment. 

Inductively coupled RF discharge& mass spectrometric diagnostics(IEM-CSIC)

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Sample thickness selection? 

W. Jacob, Thin Solid Films 326 (1998) 1–42Drouin, D. (2011), http://www.gel.usherbrooke.ca/casino

• Desired conditions: The samples should be processed practically in all their thickness and the electrons should left practically all its energy on them.

• Simulation of the passage of 5 keV electron beam through a 500 nm a-C:H layer (CASINO code)

H/C = 25/75 Density = 1 g cm-3

H/C = 25/75Density = 1.5 g cm-3

500 nm

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a‐C:H film deposition in CH4/He plasma

Growth conditions:• RF Plasma: 40 W, 13.56 MHz, 15 min • CH4(5 sccm)/He(10 sccm), 0.3 mbar• Substrate (Si crystal) located in the afterglow

on a glass plate (deposition on one face).Mass spectrometric plasma diagnostics:• CH4 dissociation 70 %.• Large amount of H2, and heavier hydrocarbons

like C2H2 are produced.a‐C:H deposit on Si crystal Thickness ~ 500 nm in the center 

Gas Flow

25 mm

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AFM characterization of the deposits

Granular distribution: Average diameters 170 nmAverage roughness 30 nm

2.521.510.50

50

40

30

20

10

0

X[µm]

Z[nm

]

FTIR spectrum of the deposits

3000 2500 2000 1500 1000 500

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Abso

rban

ce

wavemeter (cm-1)

CH stretch C=Cstretch

CHx bend

AromaticsAliphatics

6.9 µm6.2 µm

11.3 µm

7.2 µm

CH3

CH2

CH3

3.4 µm

CH

CH2

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Irradiation with 5 KeV electrons

Electron Gun

HV Ices Chamber + FTIR Spectrometer

• 5 keV, 1 A/cm2 4.7 1014 e/cm2min (200-500 min)• The sample is alternatively irradiated and analyzed• e- irradiation at room & liquid N2 temperatures

Sample Holder

B. Maté et al, ApJ, 806 (2015) 151-16112

Structural changes of the deposits (AFM) 

Irradiated Region (8 h, 300 K)Average diameter 250 nmAverage roughness 50 nm

External RegionAverage diameter 170 nmAverage roughness 30 nm

The size of the grains increases under irradiation

Effects on the IR spectra

• Remarkable decrease of the global optical depth under irradiation.• Pronounced effect on the aliphatic part of the 3.4 m band, which is

more strongly depleted (the 6.9 m and 7.2 m bands are depleted too).

3100 3000 2900 2800

0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

0,045

0,050

Abs

orba

nce

Wavemeter (cm-1)

EXPERIMENT 3010 CH 2960 CH3 asym 2922 CH2 asym 2875 CH3 sym 2845 CH2 sym

T = 0

3100 3000 2900 2800

0,000

0,005

0,010

0,015

0,020

Abs

orba

nce

Wavemeter (cm-1)

EXPERIMENT 3010 CH 2960 CH3 asym 2922 CH2 asym 2875 CH3 sym 2845 CH2 sym

T = 200 min

3000 2500 2000 1500 1000 500

0,00

0,01

0,02

0,03

0,04

0,05

0,06

Abso

rban

ce

wavemeter (cm-1)

a-C:H deposit , 500 nm Before irradiation After 8 h irradiation

at 300 K

CH stretch C=Cstretch

CHx bend

AromaticsAliphatics

6.9 µm6.2 µm

11.3 µm7.2 µm

CH3

CH2

CH3

3.4 µm

CH

CH2

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0 1 2 3 4 50,0

0,2

0,4

0,6

0,8

1,0

457nm, 85 K 460nm, 85 K

465nm, 300 K 471nm, 300 K

a-C:H density = 1 g/cm3

I/I0

Deposited Energy x 1015 MeV/mg

Evolution of the integrated optical depth of the 2900 cm‐1 band with deposited electron energy

• Reproducibility of the results.

• Different behavior with irradiation temperature.

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M. Godard et al. A&A 2011, 529, A146

460nm, 85 K

471nm, 300 K

a-C:H density = 1 g/cm3

Comparison with a‐C:H irradiation by ions

PresentWork

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Concluding remarks

• Carbonaceous deposits have been grown in inductively coupled RFdischarges of He+CH4.

• Their IR spectra show that they are good analogues of interstellardust.

• Processing with energetic electrons decreases the optical depthand affect the aliphatic component of the carbonaceous material.

• The size and roughness of the grains increases under electronirradiation.

• Different behavior with temperature during irradiation.

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INSTITUTO DE ESTRUCTURA                                                                     DE LA MATERIA 17