SINGLE AND DOUBLE PHOTOIONIZATION AND PHOTODISSOCIATION OF TOLUENE BY SOFT X-RAYS IN A CIRCUMSTELLAR ENVIRONMENT

Figure 1 shows the mass spectra of toluene at the C1s resonance energy (285.1 eV). Besides the C₇ group (84–93 amu), the following ionic groups were identified, due to carbon atom loss: methyl (12–16 amu), C₂ (24–28 amu), C₃ (36–41 amu), C₄ (48–53 amu), C₅ (60–66 amu), and C₆ (73–78 amu). Moreover, the appearance of fractional m/z from 42 to 47 amu suggests that doubly ionized molecules from the C₇ group were also produced. The region of the spectra corresponding to the double ionized molecules from the C7 group, of special interest in this work, is enhanced in Figure 1, and the fragment-ion assignments are highlighted. It should be mentioned that such a full sequence of doubly ionized molecules is not usually identified in homocyclic and heterocyclic ring molecules. Small features at masses (m/z) 20, 38.5, and 39.5, corresponding to the ions C₃H${}_{4}^{++}$, C₆H${}_{3}^{++}$, and C₆H${}_{5}^{++}$, were also identified. Small, doubly charged molecules, in general, are metastable: charge separation may occur and the fragments are accelerated by their mutually repulsive characters (Coulomb explosion). It is well established, however, that aromatic molecules resist Coulomb explosion better than other molecules (Burdick et al. 1986).

Zoom In Zoom Out Reset image size

Even though the discrimination of doubly charged ions in mass spectra can be difficult or even impossible, they are very easily identified in the mass spectra of toluene. Shaw et al. (1998) reported the appearance of the ion C₇H${}_{8}^{++}$ with ionization by photons of energy 23.8 eV. The peaks of the double ionized fragments in our mass spectra present a narrower profile than those for small, singly ionized fragments, which reflects the thermal energy of the ions. Doubly ionized peaks of C₇H${}_{n}^{++}$, with n = 2–8, were also produced from the fragmentation of toluene with the impact of 100 eV electrons (Głuch et al. 2005).

As shown by Bakes et al. (2001) in a model under interstellar conditions, the infrared emission from PAH molecules depends on their charge state and temperature distribution function. The production of doubly charged ions from PAHs with more than 24 carbon atoms may be dominant for a G₀/n_e ratio larger than 10³ cm⁻³. It also changes the ionization degree of the cloud. In environments with some contribution of soft X-rays, these dications can be produced by the absorption of photons of energy around the C1s edge, followed by the Auger process of both neutral (emitting two electrons) and cationic molecules (emitting just one electron). Reitsma et al. (2015) studied the production and stability of dications of coronene from X-ray photon interaction with cations. This may be an important route for producing dications, but it still lacks the rate production of doubly charged ions from neutral PAHs, since part of them are destroyed by photoionization. Rosi et al. (2004) conducted a theoretical investigation of the stability, structure, and fragmentation routes of C₆H${}_{6}^{++}$. Toluene is considered to be a small molecule, which has experimental and theoretical advantages for investigating dication stability and production from the neutral parent.

As some peak contributions of fragment-ions are superposed, multiple Gaussian function profiles were fitted to the peaks in order to extract the partial ion yield (PIY) of each fragment-ion. The PIY of a fragment-ion i was obtained as follows:

$$\begin{eqnarray}&&{\mathrm{PIY}}_{i}=\left(\displaystyle \frac{{A}_{i}^{+}}{{A}_{t}^{+}}\right)\times 100\%\pm {\rm{\Delta }}{\mathrm{PIY}}_{i}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\rm{\Delta }}{\mathrm{PIY}}_{i}={\mathrm{PIY}}_{i}\times \sqrt{{\left(\displaystyle \frac{{\rm{\Delta }}{A}_{i}}{{A}_{i}^{+}}\right)}^{2}+{\left(\displaystyle \frac{{\rm{\Delta }}{A}_{t}}{{A}_{t}^{+}}\right)}^{2}},\end{eqnarray} \tag{ 2 }$$

where ${A}_{i}^{+}$ is the area of the Gaussian profile fitted for the fragment-ion peak i, and ${A}_{t}^{+}$ is the total area of the PEPICO spectrum. The estimated determination, ΔPIY_i, reflects the uncertainty in the area ΔA_i of each peak, and ΔA_t is the sum of these uncertainties. The uncertainties of the partial yields are estimated around 10%. The PIYs for all the spectra are listed in Table 1. For the monocations, only fragments with PIYs greater than 1% are shown. Based on the partial yields it can be concluded that the photodissociation of toluene leads preferentially to C₃H${}_{3}^{+}$, C₃H${}_{2}^{+}$, C₄H${}_{2}^{+}$, C₄H${}_{3}^{+}$, C₅H${}_{3}^{+}$, C₂H${}_{2}^{+}$, C₂H${}_{3}^{+}$, and C₇H${}_{7}^{+}$. For the doubly ionized C₇H${}_{n}^{++}$ molecules, the total PIY contribution is 1.8%–2.6%, between 277 and 302.7 eV. The most stable doubly charged ion is the parent molecule, C₇H${}_{8}^{++}$, followed by C₇H${}_{2}^{++}$ and C₇H${}_{6}^{++}$, i.e., closed-shell species with an even number of hydrogen atoms. The most produced dication with an odd number of hydrogen is C₇H${}_{7}^{++}$, followed by C₇H${}_{3}^{++}$.

Table 1.  Partial Ion Yield (PIY) for the Fragmentation of Toluene in the Gas Phase, due to Photons with Energies around the C1s Resonance

Fragments			PIY (%) per Energy (eV)
m/z	Attr.	277.9	285.1	286.6	289.1	302.7
1	H+	6.8	7.9	8.8	10.2	11.6
13	CH+	1.5	1.5	1.5	1.6	1.8
14	CH${}_{2}^{+}$	1.3	1.6	1.6	1.8	2.5
15	CH${}_{3}^{+}$	2.4	2.2	2.1	2.3	3.7
24	${{\rm{C}}}_{2}^{+}$	1.4	1.9	1.7	2.0	2.3
25	C2H+	1.1	1.4	1.5	1.6	2.4
26	C2H${}_{2}^{+}$	4.9	5.3	5.5	5.9	7.2
27	C2H${}_{3}^{+}$	4.8	4.7	4.9	4.5	7.0
36	${{\rm{C}}}_{3}^{+}$	1.2	2.1	1.8	2.8	2.5
37	C3H+	3.8	4.4	5.5	5.0	5.9
38	C3H${}_{2}^{+}$	3.5	4.1	4.2	5.2	4.8
39	C3H${}_{3}^{+}$	8.0	8.0	7.9	7.2	7.4
43	C7H${}_{2}^{++}$	0.4	0.5	0.5	0.5	0.7
43.5	C7H${}_{3}^{++}$	0.05	0.1	0.1	0.1	0.15
44	C7H${}_{4}^{++}$	0.05	0.1	0.1	0.1	0.1
44.5	C7H${}_{5}^{++}$	0.04	0.04	0.06	0.04	0.05
45	C7H${}_{6}^{++}$	0.4	0.4	0.4	0.4	0.5
45.5	C7H${}_{7}^{++}$	0.2	0.2	0.2	0.2	0.1
46	C7H${}_{8}^{++}$	0.9	0.5	0.6	0.5	0.7
48	${{\rm{C}}}_{4}^{+}$	0.8	0.9	1.1	1.6	1.6
49	C4H+	1.7	2.6	2.5	3.1	3.2
50	C4H${}_{2}^{+}$	5.2	5.8	5.9	5.5	5.2
51	C4H${}_{3}^{+}$	4.3	4.7	4.6	5.1	5.5
52	C4H${}_{4}^{+}$	1.6	1.6	1.4	0.7	0.6
61	C5H+	1.2	1.9	2.2	3.1	2.0
62	C5H${}_{2}^{+}$	2.4	3.6	3.6	3.7	2.7
63	C5H${}_{3}^{+}$	3.9	4.5	4.6	4.9	4.2
64	C5H${}_{4}^{+}$	1.8	1.7	1.7	1.0	0.41
65	C5H${}_{5}^{+}$	3.2	2.7	2.3	1.8	1.2
91	C7H${}_{7}^{+}$	10.6	5.8	5.3	3.3	1.4
92	C7H${}_{8}^{+}$	7.8	3.8	3.6	2.5	1.1

Download table as:  ASCIITypeset image

The absolute cross-section determination is described elsewhere (Boechat-Roberty et al. 2005). Briefly, the non-dissociative single ionization (photoionization) cross-section σ_ph-i and the dissociative single ionization (photodissociation) cross-section σ_ph-d of toluene were determined by:

$$\begin{eqnarray}{\sigma }_{\mathrm{ph}‐{\rm{i}}}={\sigma }_{\mathrm{ph}‐\mathrm{abs}}\displaystyle \frac{{\mathrm{PIY}}_{{{\rm{C}}}_{7}{{\rm{H}}}_{8}^{+}}}{100}\end{eqnarray} \tag{ 3 }$$

and

$$\begin{eqnarray}{\sigma }_{\mathrm{ph}‐{\rm{d}}}={\sigma }_{\mathrm{ph}‐\mathrm{abs}}\left(1-\displaystyle \frac{{\mathrm{PIY}}_{{{\rm{C}}}_{7}{{\rm{H}}}_{8}^{+}}}{100}\right),\end{eqnarray} \tag{ 4 }$$

where σ_ph-abs is the photoabsorption cross-section (Hitchcock Gas Phase Core Excitation Database, http://unicorn.mcmaster.ca/corex/cedb-title.html).

The double photoionization cross-section of toluene, σ_ph-ii, and the cross-section of dication production, σ₊₊, were obtained as follows:

$$\begin{eqnarray}{\sigma }_{\mathrm{ph}‐\mathrm{ii}}={\sigma }_{\mathrm{ph}‐\mathrm{abs}}\displaystyle \frac{{\mathrm{PIY}}_{{{\rm{C}}}_{7}{{\rm{H}}}_{8}^{++}}}{100}\end{eqnarray} \tag{ 5 }$$

and

$$\begin{eqnarray}{\sigma }_{++}={\sigma }_{\mathrm{ph}‐\mathrm{abs}}\displaystyle \frac{{\displaystyle \sum }_{n=2}^{8}{\mathrm{PIY}}_{{{\rm{C}}}_{7}{{\rm{H}}}_{n}^{++}}}{100}.\end{eqnarray} \tag{ 6 }$$

The photoabsorption, photoionization, and photodissociation cross-sections of toluene at the C1s edge are listed in Table 2. The photoabsorption cross-section of toluene at the C1s resonance energy is 1.9 × 10⁻¹⁷ cm², almost five times higher than that of benzene. This higher value gives rise to higher photoionization and photodissociation cross-sections for the molecule, and could be responsible for the higher production of doubly charged species of toluene, when compared to benzene.

Figure 2 shows the most stable structures for each C₇H${}_{n}^{++}$ species, with n varying from 2 to 8. It is worth mentioning that, although some of the dications were already studied from the theoretical point of view, to the best of our knowledge no comprehensive study concerning their structures and relative stabilities has been presented. The most stable C₇H${}_{2}^{++}$ isomer is the all-linear heptatriynylidene dication (1), a doubly charged polyyne species. Although several neutral and anionic polyynes have been detected in interstellar clouds, such as C₆H and C₆H₂ (Ziurys 2006), C₆H⁻ (McCarthy et al. 2006), HC₆CN (Nguyen-Q-Rieu & Bujarrabal 1984), C₈H (Cernicharo & Guélin 1996), C₈H⁻ (Brünken et al. 2007), and HC₁₀CN (Bell et al. 1997), no dicationic analog has been detected so far. Polyynes were also suggested to be present in interstellar sources attached as substituents on PAHs (Duley & Hu 2009). The most stable C₇H${}_{3}^{++}$ isomer, on the other hand, is the butadiynylcyclopropenyl dication (2), a planar hydrocarbon species containing a linear polyyne attached to a three-membered ring.

Zoom In Zoom Out Reset image size

For C₇H${}_{4}^{++}$, a structure containing a planar tetracoordinate carbon (ptC, 3b) is only 0.5 kcal mol⁻¹ higher in energy than the cyclic 3a structure. At this level of calculation, it is not possible to discriminate which of the two structures is the ground state for the C₇H${}_{4}^{++}$ system. This problem will be addressed in a future study dealing with the low-lying isomers of the C₇H${}_{n}^{++}$ dications.

For C₇H${}_{6}^{++}$, the ground state structure is usually attributed in the literature to that of the cycloheptatrienylidene dication (Roithová et al. 2009). However, a previously non-reported methylbiscyclopropenyl structure (5) was predicted in this work as the global minimum for the system. Its energy is 6.5 kcal mol⁻¹ smaller than the usual seven-membered ring. Finally, for the the parent dication C₇H${}_{8}^{++}$, the meta-protonated benzyl cation (7) is the most stable structure, highlighting the loss of structural integrity from neutral toluene, in agreement with previous works (Roithová et al. 2006).

The thermodynamics of selected dissociation pathways by Coulombic decay from C₇H${}_{n}^{++}$ dications was also studied. The dissociation pathways were chosen from the PE2PICO analysis, and their experimental yields were compared to the variation of electronic energy corrected by zero point energy (ZPE). From this analysis, the viability of the pathways proposed by the experiments was evaluated.

Table 3 shows the ZPE corrected electronic energy (ΔE₀) of selected dissociation pathways in comparison with the estimated coincidence from the PE2PICO spectra. For more hydrogenated C₇H${}_{n}^{++}$ dications (n = 5–8), there is a reasonable correlation between the lower energy pathways and the coincidence of monocations from the PE2PICO analysis. The loss of correlation for less hydrogenated C₇H${}_{n}^{++}$ dications (n = 2–4) suggests that part of the estimated yield from the PE2PICO spectra could be associated with pathways in which there are also formations of neutral species. Nevertheless, the most exoergic dissociation pathways are associated with the loss of cyclic C₃H₃⁺, which also appears as the most prominent peak in the PEPICO spectra, although the presence of the less stable propargyl isomer cannot be ruled out because it could be formed in the experiment, and once formed, the large activation barrier could prevent its isomerization to the cyclic structure (Ricks et al. 2010). This result suggests that a significant part of the formation of C₃H₃⁺ comes from the decomposition of C₇H${}_{n}^{++}$ dications, previously generated by the Auger mechanism. The cyclopropenyl ion, C₃H₃⁺, is considered to be the simplest Hückel aromatic system, having 2π electrons. It is suggested that C₃H₃⁺ plays an important role in the atmospheric chemistry of Titan (Ali et al. 2013). Since toluene is also found in the upper atmosphere of Titan, part of the production of C₃H₃⁺ must come from the photodissociation of toluene.

Table 3.  Electronic Energy Corrected by ZPE (ΔE₀) of Selected Dissociation Pathways, Compared to the Estimated Coincidence from PE2PICO Spectra

Dication	Dissociation Pathway	ΔE0 (Yield)
C7H${}_{5}^{++}$	C3H3+ + C4H2+	−34.9 (4.06)
C7H${}_{7}^{++}$	C3H3+ + C4H4+	−31.1 (2.09)
C7H${}_{6}^{++}$	C3H3+ + C4H3+	−23.3 (3.91)
C7H${}_{5}^{++}$	C2H2+ + C5H3+	−11.4 (3.16)
C7H${}_{8}^{++}$	C2H3+ + C5H5+	−4.6 (1.86)
C7H${}_{5}^{++}$	C3H2+ + C4H3+	−3.8 (3.56)
C7H${}_{6}^{++}$	C2H3+ + C5H3+	−3.7 (4.47)
C7H${}_{8}^{++}$	CH3+ + C6H5+	2.7 (1.05)
C7H${}_{5}^{++}$	C2H3+ + C5H2+	8.6 (1.41)
C7H${}_{7}^{++}$	C2H3+ + C5H4+	18.1 (1.00)
C7H${}_{7}^{++}$	CH3+ + C6H4+	24.6 (1.05)
C7H${}_{4}^{++}$	C3H2+ + C4H2+	25.2 (4.88)
C7H${}_{4}^{++}$	C2H2+ + C5H2+	41.5 (2.67)
C7H${}_{3}^{++}$	C3H+ + C4H2+	45.2 (2.71)
C7H${}_{3}^{++}$	C2H2+ + C5H+	48.9 (2.00)
C7H${}_{3}^{++}$	C3H2+ + C4H+	68.2 (1.45)
C7H${}_{2}^{++}$	C3H+ + C4H+	93.5 (1.73)

Note. Energy values are in kcal mol⁻¹; yields are in %.

Download table as:  ASCIITypeset image

As mentioned before, the production of large PAHs is mainly attributed to the chemical process occurring in the circumstellar envelope of AGB stars. T Dra, a carbon-rich AGB star, is at a distance of 610 pc, with a bolometric luminosity L = 6300 L_⊙, an effective temperature T_* = 1600 K, a velocity of expanding gas v_exp = 13.5 km s⁻¹, and a mass loss $\dot{M}=1.2$ × 10⁻⁶ M_⊙ yr⁻¹ (Schöier & Olofsson 2001). The soft X-ray spectrum (0.2–2 keV) of the object shows emission centered at about ≲1 keV, observed by ROSAT (Ramstedt et al. 2012). In order to obtain the photon flux as a function of the energy, a value of L_X ≈ 3.2 × 10³¹ erg s⁻¹ was used for the X-ray luminosity, corresponding to a column density of N_H ≈ 4,0 × 10²¹ cm⁻² (Ramstedt et al. 2012).

The main suggestions of Ramstedt et al. (2012) regarding the origin of the X-ray emission of T Dra are that it is due: (1) to the mass accretion onto a compact white dwarf, in the case of binarity, or (2) to a coronal emission from a large-scale magnetic field. The gas density near the stellar surface should be sufficient to absorb the X-rays produced by the surface magnetic field, but morphological asymmetries could affect the photoabsorption and allow X-ray emission. Here, we are considering the case of binarity. In fact, it has been suggested that the morphological structures of CRL 618 and IRC +10216, two well-studied evolved stars, are due to binary companions (Velázquez et al. 2014; Cernicharo et al. 2015). The numerical simulations of de Val-Borro et al. (2009) of mass accretion of wind onto an AGB companion suggest this scenario is reasonable. The position of the companion in the extended expanding envelope of the AGB star, which could result in the X-ray absorption constrained by Ramstedt et al. (2012), does not require any further special morphological structures, and it is used as the scenario in this work (Figure 3).

Zoom In Zoom Out Reset image size

Following the equations of Glassgold (1996) for a spherical symmetry and a uniform gas expansion, with column density α r⁻¹, the distance between the AGB star and the small companion that results in a column density of 4.0 × 10²¹ cm⁻² observed from Earth should be ∼45 au.

The X-ray flux, F_X(E), attenuated while it penetrates the gas envelope, is given by

$$\begin{eqnarray}&&{F}_{X}(E)=\displaystyle \frac{{L}_{X}}{4\pi {r}_{2}^{2}h\nu }{e}^{-\tau }.\end{eqnarray} \tag{ 7 }$$

where r₂ is the distance from the companion, and τ is the optical depth:

$$\begin{eqnarray}&&\tau ={\sigma }_{X}{N}_{{\rm{H}}},\end{eqnarray} \tag{ 8 }$$

where σ_X is the total X-ray photoabsorption cross-section by gas in the interstellar medium, and N_H is the hydrogen column density. Considering the photon flux in the vicinity of the C1s resonance energy obtained from Draine & Tan (2003), we evaluate that σ_X is ∼2.3 × 10⁻²¹ cm². The N_H value at a given distance from the AGB's companion was obtained using Equation (2) of Glassgold (1996):

$$\begin{eqnarray}&&{N}_{{\rm{H}}}({r}_{1})=2.67\times {10}^{36}\left[\displaystyle \frac{1}{d}-\displaystyle \frac{1}{{r}_{1}}\right]{\mathrm{cm}}^{-2},\end{eqnarray} \tag{ 9 }$$

where d is the distance between the stars and r₁ is the distance from the AGB star.

With these parameters, we can evaluate the rates of photodissociation, ionization, and the half-life of toluene, besides the production of doubly charged ions through the T Dra envelope that faces the companion star. The dissociation of a given molecule subjected to a radiation field is given by

$$\begin{eqnarray}-\displaystyle \frac{{dN}}{{dt}}={{Nk}}_{\mathrm{ph}‐{\rm{d}}},\end{eqnarray} \tag{ 10 }$$

where the photodissociation rate, k_ph-d, is defined as

$$\begin{eqnarray}{k}_{\mathrm{ph}‐{\rm{d}}}={\sigma }_{\mathrm{ph}‐{\rm{d}}}(E){F}_{X}(E).\end{eqnarray} \tag{ 11 }$$

On the other hand, the photoionization rate is given by

$$\begin{eqnarray}&&{\varsigma }_{i}={\sigma }_{i}(E){F}_{X}(E).\end{eqnarray} \tag{ 12 }$$

and the rate of dications production is

$$\begin{eqnarray}&&{\varsigma }_{i}={\sigma }_{++}(E){F}_{X}(E).\end{eqnarray} \tag{ 13 }$$

The results are shown in Figure 4. The photoionization, photodissociation, and photoproduction of dications drop sharply in the direction of the inner envelope. A half-life radial profile of toluene can be evaluated from Equation (11) by writing ${t}_{1/2}=\mathrm{ln}2/{k}_{\mathrm{ph}‐{\rm{d}}}$, which gives 10⁶ years of half-life at a distance of 3 × 10¹⁴ cm (21 au) from the AGB star. Further chemical reactions should be inhibited as the shell expands outward, creating a photodissociation radius determined by the molecular half-life. The production of molecules is thus strongly dependent on the side of the envelope that is facing the companion star in a binary case. Nevertheless, if magnetic activity is present throughout the envelope, the ions produced by these photochemical reactions may contribute to ambipolar diffusion. Therefore, the structural diversity of the single and double ions would be shared through the envelope within a new network of ion-molecule reactions in the expanding shells of the entire envelope of the AGB star, which has not yet been explored by any chemical models.

Zoom In Zoom Out Reset image size