Impact of aliphatic bonds on the stability of VUV photo-processed PAHs under relevant astrophysical conditions

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Impact of aliphatic bonds on the stability of VUV photo-processed PAHs under relevant astrophysical

conditions

Alexandre Marciniak, Anthony Bonnamy, Venkat Rao Mundlapati, Giacomo Mulas, Christine Joblin

To cite this version:

Alexandre Marciniak, Anthony Bonnamy, Venkat Rao Mundlapati, Giacomo Mulas, Christine Joblin.

Impact of aliphatic bonds on the stability of VUV photo-processed PAHs under relevant astrophysical conditions. MD-GAS WG1 & WG2 Conference 2021, Mar 2021, Virtual Meeting, France. �hal- 03188564�

VUV photo-processing in astro-relevant conditions A kinetic model to extract rates and ratios

Impact of aliphatic bonds on the stability of VUV photo-processed PAHs under relevant astrophysical conditions

A. Marciniak ^{1, †} , A. Bonnamy ¹ , V. Rao Mundlapati ¹ , G. Mulas ^1,2 and C. Joblin ¹

1 IRAP, Université de Toulouse (UPS), CNRS, CNES, Toulouse, France

2 Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy

†Correspondence to : alexandre.marciniak@irap.omp.eu

 Isomerization of Coronene is the first step before dissociation at 10.5 eV

 Common species are produced during the VUV photo-processing of alkylated PAHs

 Pentagonal rings formation stabilizes the structure

 Two carriers for the 3.4 µm : methyl-PAHs and H-shifted PAHs

Take-home messages and Perspectives

Studying the interaction of polycyclic aromatic hydrocarbons (PAHs) with VUV light is crucial for the understanding of the physical and chemical evolution of photodissociation regions (PDRs). In particular, this interaction induces the well-known aromatic infrared bands (AIB) emission [1] but other mechanisms such as fragmentation [2] can be involved, leading to selection effects favoring specific sizes and structures. In this context, despite their lower stability, PAHs containing aliphatic C-H bonds are considered as good candidates for the 3.4 μm AIB [3,4].

We have recently combined a 10.5 eV photon source with our dedicated cryogenic ion trap PIRENEA [5] in order to investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations with conditions applicable for PDRs [6]. The coronene cation (C

₂₄

H

₁₂⁺

) and its alkylated derivatives (with methyl or ethyl sidegroups) were VUV photo-processed over long timescales (~1000 s). Their fragmentation cascades were analyzed with a simple kinetics model which enabled to derive fragmentation pathways, rates and branching ratios.

Alkylated coronene derivatives are found to have a higher fragmentation rate and carbon loss compared to the bare coronene. However, their VUV fragmentation cascade also leads to the formation of species carrying a peripheral pentagonal cycle, which is as stable as coronene. The stability of each detected species is quantified and the most stable ones, for which there is an effective competition of fragmentation with isomerization and radiative cooling, are identified.

This work supports a scenario in which the evaporation of nanograins with mixed aliphatic and aromatic composition followed by VUV photo-processing results in both the production of the carriers of the 3.4µm AIB by methyl sidegroups and in an abundant source of smaller hydrocarbons at the border of PDRs. This process also results in the formation of PAHs containing pentagonal ring. Finally, our study supports the role of isomerization processes in PAH photofragmentation, including the H-migration process [7], which could lead to an additional contribution to the 3.4 µm AIB.

Abstract

Retrieved fragmentation maps of the studied species

[1] A. Leger, L. D’Hendecourt, D. Defourneau, A&A, 216, 148 (1989).

[2] J. Montillaud, C. Joblin, D. Toublanc, A&A, 552, A15 (2013).

[3] C. Joblin, et al., ApJ, 458, 610 (1996).

[4] M. P. Bernstein, S. A. Sandford, L. J. Allamandola, ApJ, 472, L127 (1996).

[5] C. Joblin et al. EAS Pub. Series, 4, 73-77 (2002).

[6] A. Marciniak et al., arXiv:2103.03890 (2021).

[7] G. Trinquier et al. Mol. Astrophys., 7, 27-36 (2017).

Bibliography

PAHs production

Cryogenic ion trap (~10^-11 mbar, 30 K)

VUV screen Xe:Ar tripling cell

(250 mbar) 355 nm laser

(~10 mJ, 6 ns @ 10 Hz)

VUV beam (~10¹¹ photons.s^-1)

Desorption-ionization laser (266 nm)

Cryo-cooler head

Q Sample target

Cryogenic shields

VUV Generation

MgF₂ lens

UV beam-blocker

UV VUV

Photophysics in a collisionless environment High-resolution mass spectrometry (FT-ICR-MS)

~1.5 mm

𝐵₀ = 5 T

MeCor⁺

C₂₃H₁₁⁺

Cor⁺*

Cor⁺

CH₂Cor⁺

[Cor-2H]⁺ EtCor⁺

Fragmentation E.A. ~2- 3 eV

Radiative cooling E.A. ~4- 5 eV -C₂H₂

-H

-H -H

1. Search of parent-daughter relation between the observed fragments to determine the population and depopulation terms in the system of equations :

1

^𝑠𝑡

𝑑𝐼

_𝑀parent

𝑑𝑡 = − ෍

𝑀_𝑖<𝑀_parent

𝑘

_frag^{𝑀parent→𝑀𝑖}

𝐼

_𝑀parent

𝑡

⋮ ⋮

𝑖

^𝑡ℎ

𝑑𝐼

_𝑀𝑖

𝑑𝑡 = ෍

𝑀_𝑛>𝑀_𝑖

𝑘

_frag^{𝑀𝑛→𝑀𝑖}

𝐼

_𝑀𝑛

𝑡 − ෍

𝑀_𝑗<𝑀_𝑖

𝑘

_frag^{𝑀𝑖→𝑀𝑗}

𝐼

_𝑀𝑖

𝑡

2. Fit of the kinetics data points in order to extract the 𝑘

_frag^{𝑀𝑖→𝑀𝑗}

3. Feedback loop in order to remove or add population/depopulation terms with educated guesses 4. Check of the fit quality and coherence of the map

Method

1. VUV beam generated by tripling a 355 nm laser

2. Cationic PAHs produced by laser desorption-ionization 3. Isolation of the parent cations

4. 10.5 eV irradiation of the ion cloud up to 1000 s 5. Measurement of the photoproducts by FT-ICR-MS

Experimental conditions

• Low photon flux (estimated to ~10

¹³

photons.s

^-1

.cm

^-2

)

• Ions have time to relax between two VUV photons absorption

• Sequential cascades of fragmentation

𝐶₂₅𝐻₁₂ ⁺

(M312)

𝐶₂₅𝐻₁₃ ⁺

(M313)

𝐶₂₃𝐻₁₂ ⁺

(M288)

𝐶₂₅𝐻₁₁ ⁺

(M311)

𝐶₂₃𝐻₁₀ ⁺

(M286)

𝐶₂₃𝐻₉ ⁺

(M285)

k = 2.5

k ≈ 1 8% ~20% ~75%

k ≈ 3.3 k ≈ 0.1

𝐶₂₅𝐻₁₄ ⁺

(M314)

𝐶₂₃𝐻₁₁ ⁺

(M287)

~52% ~48%

𝐶₂₆𝐻₁₆ ⁺

(M328)

𝐶₂₆𝐻₁₅ ⁺

(M327)

𝐶₂₆𝐻₁₄ ⁺

(M326)

𝐶₂₆𝐻₁₃⁺

(M325)

𝐶₂₆𝐻₁₂ ⁺

(M324)

28%

k = 5.8

-H

-C₂H_x -2H/H₂ -CH₃

72%

95%

k = 3.1 Common fragments to

MeCor⁺ and EtCor⁺

MeCor⁺ fragments EtCor⁺ fragments

5%

Competition of relaxation processes

𝐶₂₄𝐻₁₁ ⁺

(M299)

𝐶₂₄𝐻₉ ⁺

(M297)

𝐶₂₄𝐻₈ ⁺

(M296)

k = 2.8

k = 0.08

k = 2.2 𝐶₂₄𝐻₁₀ ⁺

(M298)

-H iso.

Hypothesis n°1

k⁽¹⁾ = 0.12 𝐶₂₄𝐻₁₂ ⁺

(M300)

𝐶₂₄𝐻₁₂ ^+∗

(M300)

𝑌₀⁽¹⁾= 96% 𝑌₀⁽²⁾= 4%

k⁽²⁾ = 3.8

Hypothesis n°2

k = 0.17

k = 3.9 𝑌₀⁽¹⁾= 90%

26%

74%

𝐶₂₄𝐻₁₂ ^+∗

(M300)

𝐶₂₄𝐻₁₂ ⁺

(M300)

𝑌₀⁽²⁾= 10%

Cor⁺ Cor⁺ isomers

 VUV photo-processing to differentiate isomers

 Exploration of the fragmentation kinetics of larger alkylated or superhydrogenated PAHs

Coronene

• Sequential loss of H atoms with a global low rate (k = 0.16×10

^-3

s

^-1

)

• Need of an isomer population to fit the data (several hypotheses)

• Isomers can be formed by H-migration on the peripherical backbone

• [Cor-2H]

⁺

is also a stable and highly populated species

Methyl-coronene and Ethyl-coronene

• Strong fragmentation rate with respect to coronene (~2.8×10

^-3

s

^-1

)

• First step mostly corresponds to production of the methylene-coronene

• Additional sequential H-loss path for ethyl-coronene leading to ethynyl-coronene

• C

₂₃

H

₁₁⁺

is a stable and highly populated species

• Fragmentation for each absorbed photon for Cor

⁺

*, MeCor

⁺

and EtCor

⁺

• Cooling fraction is predominant in Cor+, [Cor-2H]+ and C

₂₃

H

₁₁

+

• The fragmentation cascade of alkylated species leads to stable fragments containing a pentagonal ring

Comparison of experimental fragmentation rates with photoabsorption rates from TD-DFT calculations

k = 3.1

Poster for the COST MD-GAS WG1 & WG2 Virtual Meeting, 15-19 March 2021