Vuv Photo-Processing of Pah Cations: Quantitative Study on the Ionization Versus Fragmentation Processes

Jiang, Zhen; Castillo, Sarah Rodriguez; Joblin, C.; Mulas, G.; Sabbah, Hassan; Giuliani, Alexandre; Nahon, Laurent; Martin, S.; Champeaux, Jean-Philippe; Mayer, Paul M.

doi:10.3847/0004-637x/822/2/113

Cited by 78 publications

(84 citation statements)

References 36 publications

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“…Thus, the relative spacing in distance of different emission processes in the clumpy cloud interior behind the ionization front is controlled by the nebular opacity for the photons needed to excite the different emissions. As explained earlier, the spatial distributions of the emission from ionized and neutral PAHs will always be largely overlapping, because the ionization yield of PAHs increases only gradually with energy, once the ionization potential has been exceeded (Verstraete et al 1990;Zhen et al 2016). Therefore, ionized and neutral PAHs largely coexist spatially, with the degree of ionization gradually declining with depth, well behind the region with maximum ERE intensity .…”

Section: Comparison With Ngc 7023mentioning

confidence: 94%

Extended red emission in IC59 and IC63

Lai¹,

Witt²,

Crawford³

2017

Monthly Notices of the Royal Astronomical Society

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We analysed new wide-field, wide-and narrow-band optical images of IC 59 and IC 63, two nebulae which are externally illuminated by the early B-star γ Cas, with the objective of mapping the extended red emission (ERE), a dust-related photoluminescence process that is still poorly understood, in these two clouds. The spatial distribution of the ERE relative to the direction of the incident radiation and relative to other emission processes, whose carriers and excitation requirements are known, provides important constraints on the excitation of the ERE. In both nebulae, we find the ERE intensity to peak spatially well before the more extended distribution of midinfrared emission in the unidentified infrared bands, supporting earlier findings that point toward far-ultraviolet (11 eV < E photon < 13.6 eV) photons as the source of ERE excitation. The band-integrated absolute intensities of the ERE in IC 59 and IC 63 measured relative to the number density of photons available for ERE excitation are lower by about two orders of magnitude compared to ERE intensities observed in the high-latitude diffuse interstellar medium (ISM). This suggests that the lifetime of the ERE carriers is significantly reduced in the more intense radiation field prevailing in IC 59 and IC 63, pointing toward potential carriers that are only marginally stable against photo-processing under interstellar conditions. A model involving isolated molecules or molecular ions, capable of inverse internal conversion and recurrent fluorescence, appears to provide the most likely explanation for our observational results.

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Section: Comparison With Ngc 7023mentioning

confidence: 94%

Extended red emission in IC59 and IC63

Lai¹,

Witt²,

Crawford³

2017

Monthly Notices of the Royal Astronomical Society

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“…New experimental data on the photofragmentation and ionisation probabilities of PAHs is becoming available (Zhen et al 2015(Zhen et al , 2016. In general, the key characteristics of a photoabsorption cross section are:…”

Section: General Propertiesmentioning

confidence: 99%

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

Heays

Bosman

Dishoeck

2017

A&A

442

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A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and for other wavelength-dependent radiation fields, including cool stellar and solar radiation, Lyman-α dominated radiation, and a cosmic-ray induced ultraviolet flux. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. Comparison with other databases such as PHIDRATES is made. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of these shielding types depends on the atom or molecule in question and the assumed dust optical properties. All of the new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of the calculated rates to variation of temperature and isotope, and uncertainties in measured or calculated cross sections, are tested and discussed. Tests were conducted on the new rates with an interstellar-cloud chemical model, and find general agreement (within a factor of two) in abundances obtained with the previous iteration of the Leiden database assuming an ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H 2 O and NH 3 photodissociation forming OH, O, NH 2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

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“…This fragmentation follows the available pathways (i.e., dehydrogenation or loss of C 2 or C 2 H 2 units), producing PAH derivative species along a top-down chemical scenario (Zhen et al 2014a;Bouwman et al 2016;Petrignani et al 2016). Photoionization and photofragmentation compete, and their relative importance depends on both the geometry of PAH molecules and the excitation wavelengths used (Zhen et al 2015(Zhen et al , 2016). …”

Section: Introductionmentioning

confidence: 99%

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

Jiang

Castellanos

Bouwman

et al. 2017

ApJ

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We present the first infrared (IR) gas-phase spectrum of a large and astronomically relevant polycyclic aromatic hydrocarbon (PAH) cation (C 42 H 18 + , HBC + ) and its dication (C 42 H 18 + 2 , HBC 2+ ). The spectra are recorded via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense IR radiation of a free electron laser in the 530-1800 cm −1 (5.6-18.9 μm) range. HBC + shows main intense absorption peaks at 762 (13.12), 1060 (9.43), 1192 (8.39), 1280 (7.81), 1379 (7.25), and 1530 (6.54) cm −1 (μm), in good agreement with density functional theory calculations after scaling to take the anharmonicities effect into account. HBC 2+ has its main absorption peaks at 660 (15.15), 766 (13.05), 1054 (9.49), 1176 (8.50), 1290 (7.75), 1370 (7.30) and 1530 (6.54) cm −1 (μm). Given the similarity in the cationic and dicationic spectra, we have not identified an obvious diagnostic signature to the presence of multiply charged PAHs in space. While experimental issues associated with the IRMPD technique preclude a detailed comparison with interstellar spectra, we do note that the strong bands of HBC + and HBC 2+ at ∼6.5, 7.7, 8.4, and 13.1 μm coincide with prominent aromatic infrared bands (AIBs). HBC has only trio CH groups, and the out-of-plane CH bending mode of both HBC cations is measured at 13.1 μm, squarely in the range predicted by theory and as previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11-14 μm range as a diagnostic tool for the edge topology of large PAHs in space.

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Vuv Photo-Processing of Pah Cations: Quantitative Study on the Ionization Versus Fragmentation Processes

Cited by 78 publications

References 36 publications

Extended red emission in IC59 and IC63

Extended red emission in IC59 and IC63

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

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Vuv Photo-Processing of Pah Cations: Quantitative Study on the Ionization Versus Fragmentation Processes

Cited by 78 publications

References 36 publications

Extended red emission in IC59 and IC63

Extended red emission in IC59 and IC63

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC+ and HBC2+

Contact Info

Product

Resources

About

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺