Laboratory Simulation of 11-15 μm Spectra Associated with Polycyclic Aromatic Hydrocarbon Molecules

Hu, Anming; Duley, W. W.

doi:10.1086/518366

Cited by 15 publications

(9 citation statements)

References 28 publications

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“…Using ZEKE, we have studied four peri-condensed species including pyrene, benzo[ a ]pyrene, benzo[ e ]pyrene, and benzo[ ghi ]perylene as well as four cata-condensed species including triphenylene, tetracene, pentacene, and chrysene. − Our ZEKE data, supplemented by matrix isolation spectroscopy and other excitation and fluorescence spectroscopy, ,− allow for a comprehensive analysis of the vibrational states of cationic PAHs. These experimental results can further benchmark theoretical calculations and astrophysical modeling. , …”

Section: Introductionmentioning

confidence: 69%

“…These experimental results can further benchmark theoretical calculations and astrophysical modeling. 40,41 In this work, we report resonantly enhanced multiphoton ionization (REMPI) spectroscopy of the first electronically excited state S 1 as well as ZEKE spectroscopy of the cationic state D 0 of benzo[h]quinoline (BhQ). We compare the spectroscopic features of BhQ with those of two isoelectronic systems phenanthrene and anthracene and comment on the unique contribution of the nitrogen atom.…”

Section: ■ Introductionmentioning

confidence: 99%

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Zero Kinetic Energy Photoelectron Spectroscopy of Benzo[h]quinoline

2015

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We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[h]quinoline (BhQ) via resonantly enhanced multiphoton ionization (REMPI) through the first electronically excited state S1. From the simulated REMPI spectra with and without Herzberg-Teller coupling, we conclude that vibronic coupling plays a minor but observable role in the electronic excitation to the S1 state. We further compare the S1 state of BhQ with the first two electronically excited states of phenanthrene, noticing a similarity of the S1 state of BhQ with the second electronically excited state S2 of phenanthrene. In the ZEKE spectra of BhQ, the vibrational frequencies of the cationic state D0 are consistently higher than those of the intermediate neutral state, indicating enhanced bonding upon ionization. The sparse ZEKE spectra, compared with the spectrum of phenanthrene containing rich vibronic activities, further imply that the nitrogen atom has attenuated the structural change between S1 and D0 states. We speculate that the nitrogen atom can withdraw an electron in the S1 state and donate an electron in the D0 state, thereby minimizing the structural change during ionization. The origin of the first electronically excited state is determined to be 29,410 ± 5 cm(-1), and the adiabatic ionization potential is determined to be 65,064 ± 7 cm(-1).

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Section: Introductionmentioning

confidence: 69%

Section: ■ Introductionmentioning

confidence: 99%

Zero Kinetic Energy Photoelectron Spectroscopy of Benzo[h]quinoline

2015

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“…Plateaulike features are produced by small particles because inter-and intra-molecular interactions within the material perturb the individual bands arising from the fundamental vibrations of the species making up the particles, shifting and broadening them. This results in band "smearing" across the regions normally associated with the fundamental modes (see, e.g., Morterra & Low 1983;Lee & Wdowiak 1993;Colangeli et al 1995;Baratta et al 1996;Schnaiter et al 1998;Mennella et al 1999Mennella et al , 2003Furton et al 1999;Hu & Duley 2007;Jones 2012aJones , 2012bJones , 2012c. If the particles are small enough, they produce broad emission features in the regions of the fundamental vibrational modes and thus are likely contributors to the plateaus that underly the well-known emission features.…”

Section: Discussionmentioning

confidence: 99%

Properties of Polycyclic Aromatic Hydrocarbons in the Northwest Photon Dominated Region of NGC 7023. I. Pah Size, Charge, Composition, and Structure Distribution

Boersma

Bregman

Allamandola

2013

ApJ

100

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Polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer Infrared Spectrograph spectral map of the northwest photon dominated region (PDR) in NGC 7023 was analyzed exclusively using PAH spectra from the NASA Ames PAH IR Spectroscopic Database (www.astrochem.org/pahdb). The 5-15 μm spectrum at each pixel is fitted using a non-negative-least-squares fitting approach. The fits are of good quality, allowing decomposition of the PAH emission into four subclasses: size, charge, composition, and hydrogen adjacency (structure). Maps tracing PAH subclass distributions across the region paint a coherent astrophysical picture. Once past some 20 seconds of arc from HD 200775, the emission is dominated by the more stable, large, symmetric, compact PAH cations with smaller, neutral PAHs taking over along the lines-of-sight toward the more distant molecular cloud. The boundary between the PDR and the denser cloud material shows up as a distinct discontinuity in the breakdown maps. Noteworthy is the requirement for PANH cations to fit the bulk of the 6.2 and 11.0 μm features and the indication of PAH photo-dehydrogenation and fragmentation close to HD 200775. Decomposition of the spectral maps into "principal" subclass template spectra provides additional insight into the behavior of each subclass. However, the general applicability of this computationally more efficient approach is presently undetermined. This is the first time the spectra of individual PAHs are exclusively used to fit the 5-15 μm region and analyze the spatial behavior of the aromatic infrared bands, providing fundamental, new information about astronomical PAH subpopulations including their dependence on, and response to, changes in local conditions.

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“…They are caused by CH out-of-plane bending modes of PAHs. Laboratory studies of thin HAC films assembled from carbon nanoparticles conducted by Hu & Duley (2007) have shown that the spectra in the range from 11 μm to 15 μm show features similar to the interstellar emission bands of PAHs at 11.3,12,12.8,13.5, and 14.2 μm. They suggested that these bands might be attributed to compact ring structures such as coronene (C 24 H 12 ), which has peripheral substitutions with additional aromatic rings or sp, sp 2 -bonded carbon chains.…”

Section: Introductionmentioning

confidence: 98%

Mid-infrared spectroscopy of UV irradiated hydrogenated amorphous carbon materials

Gadallah

Mutschke

Jäger

2012

A&A

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Context. Mid-infrared (MIR) bands are characteristic for the short-range and medium-range structure of hydrogenated amorphous carbon (HAC) materials that exist in the interstellar medium (ISM) and are sensitive to processing in the harsh interstellar radiation field. Aims. We study the variability of the MIR features from the spectrum of non-processed to that of UV-processed HAC materials and compare them to spectra of interstellar carbonaceous materials. Methods. Nano-sized HAC materials produced by laser ablation were irradiated by vacuum-UV photons with doses comparable to those relevant for interstellar processing. They were subsequently analyzed by IR spectroscopy. Results. In the MIR range, the spectra of HAC materials show many absorption bands such as the sp 3 aliphatic ≡C-H stretching vibration at 3.03 μm, the sp 3 aliphatic -C-H stretching vibration at 3.4 μm and also the sp 3 aliphatic C-H bending vibration at both 6.85 μm and 7.25 μm. All these are recognizable bands of HAC materials. Other absorption bands such as the sp 2 aromatic =C-H stretching vibration at about 3.3 μm and the sp 2 -C=C stretching vibration close to 6.25 μm are observed. The HAC materials also possess bands which represent the aromatic out-of-plane bending at 11.65, 12.46 and 12.9 μm in addition to the aromatic -C-C-C in-plane bending at 15.87 μm. With UV irradiation, the mass absorption coefficient of the 3.03 μm band completely disappears and that of the aliphatic C-H bands (3.4, 6.85 and 7.25 μm) decreases. This reduction shows that the UV radiation destroys most of the aliphatic C-H bonds inside the HAC structure. On the other hand, the strength of the aromatic 6.2 μm band increases, which is evidence of the partial graphitization within UV-irradiated HAC materials. Because UV irradiation is not uniform, this band agrees well with the C-class PAH toward HD 100764. The C-H out-of-plane vibration bands are strongly affected by UV irradiation. Bands at 11.35, 12.14 and 12.64 μm (solo, duo and trio, respectively), which are found for PAHs of many interstellar spectra were observed partially and were compared to those of non-processed materials, in particular, those at 11.35 μm, which represent the aromatic structures. Conclusions. UV irradiation has a variable effect on both aliphatic and aromatic bands in the MIR region. The aliphatic C-H structure decreases while the aromatic C=C structure, which might lead to the graphitic colonies for PAHs, increases. UV irradiation has revealed solo, duo and trio bands that are relatively consistent with those of the A-and B-class PAHs.

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Laboratory Simulation of 11-15 μm Spectra Associated with Polycyclic Aromatic Hydrocarbon Molecules

Cited by 15 publications

References 28 publications

Zero Kinetic Energy Photoelectron Spectroscopy of Benzo[h]quinoline

Zero Kinetic Energy Photoelectron Spectroscopy of Benzo[h]quinoline

Properties of Polycyclic Aromatic Hydrocarbons in the Northwest Photon Dominated Region of NGC 7023. I. Pah Size, Charge, Composition, and Structure Distribution

Mid-infrared spectroscopy of UV irradiated hydrogenated amorphous carbon materials

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