Ändra sökning
Avgränsa sökresultatet
1 - 11 av 11
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1.
    Chen, Tao
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi. Leiden University, Leiden Observatory, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands.
    Formation of Covalently Bonded Polycyclic Aromatic Hydrocarbons in the Interstellar Medium2018Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, nr 113Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Photo-/ion-induced ionization and dissociation processes are commonly observed for polycyclic aromatic hydrocarbon (PAH) molecules. This work performs theoretical studies of PAHs and their fragments. Molecular dynamics simulations in combination with static quantum chemical calculations reveal that following a single hydrogen atom loss, the fragments, PAH-H, are extremely reactive. They catch a neighbor molecule within picoseconds to form a covalently bonded large molecule regardless of orientations/angles and temperatures. We calculate the infrared spectra of the covalently bonded molecules, which indicate that such species could be the carrier of unidentified infrared emission bands. It also implies that regular PAHs might be less abundant in space than what is expected. 

  • 2.
    Chen, Tao
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Temperature effects on anharmonic infrared spectra of large compact polycyclic aromatic hydrocarbons2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, artikel-id A152Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. Large compact polycyclic aromatic hydrocarbon molecules (PAHs) present special interest in the astrochemical community. A key issue in analyses of large PAHs is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the infrared (IR) spectra for molecules under various conditions. Methods. Because of the huge amount of interactions/resonances in large PAHs, no anharmonic IR spectrum can be produced with static/time-independent ab initio method, especially for the molecules with D6h symmetry, e.g., coronene and circumcoronene. In this work, we performed molecular dynamics simulations to generate anharmonic IR spectra of coronene and circumcoronene. Results. The method is validated for small PAHs, i.e., naphthalene and pyrene. We find that the semiempirical method PM3 produces accurate band positions with an error <5 cm(-1). Furthermore, we calculate the spectra at multiple temperatures and find a clear trend toward band shifting and broadening.

  • 3.
    Chen, Tao
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi. Leiden Univ, Leiden Observ, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands.
    The Carrier of 3.3 mu m Aromatic Infrared Bands: Anharmonicity and Temperature Effects on Neutral PAHs2018Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 238, nr 2, artikel-id 18Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Anharmonic infrared (IR) spectra are crucial for the study of interstellar polycyclic aromatic hydrocarbon (PAH) molecules. This work aims to provide a comprehensive study of the features that may influence the accuracy of anharmonic IR spectra of PAHs so that a reliable spectrum that incorporates all necessary features for interpreting the observational IR spectra can be obtained. Six PAHs are investigated: naphthalene, anthracene, pyrene, chrysene, 9,10-dimethylanthracene, and 9,10-dihydroanthracene. The NIST spectra and high-resolution IR absorption spectra are utilized as the reference for the comparisons. The influences of different resonances and resonant thresholds are studied. Four methods for electronic structure calculations are tested. The quantitative comparisons indicate that for the NIST data, B3LYP/NO7D provides the best agreement with measured spectra concerning band positions and B3LYP/cc-pVTZ is superior in the description of the relative intensities. The importance of 1-3 Darling-Dennison resonances, which are required for generating triple combination bands, is investigated through a comparison to a high-resolution experimental spectrum. For interpreting the bandwidths and profiles of the observational spectra, the temperature effects are included through the Wand-Landau random walk technique. The comparisons between calculated high-temperature anharmonic and observational spectra indicate that small and compact PAHs might be responsible for the 3.3 mu m aromatic infrared bands.

  • 4.
    Chen, Tao
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Li, Aigen
    Univ Missouri, Dept Phys & Astron, Columbia, MO 65211 USA..
    Synthesizing carbon nanotubes in space2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 631, artikel-id A54Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. As the fourth most abundant element in the universe, carbon (C) is widespread in the interstellar medium (ISM) in various allotropic forms (e.g. fullerenes have been identified unambiguously in many astronomical environments, the presence of polycyclic aromatic hydrocarbon molecules in space has been commonly acknowledged, and presolar graphite, as well as nanodiamonds, have been identified in meteorites). As stable allotropes of these species, whether carbon nanotubes (CNTs) and their hydrogenated counterparts are also present in the ISM or not is unknown. Aims. The aim of the present works is to explore the possible routes for the formation of CNTs in the ISM and calculate their fingerprint vibrational spectral features in the infrared (IR). Methods. We studied the hydrogen-abstraction and acetylene-addition (HACA) mechanism and investigated the synthesis of nanotubes using density functional theory (DFT). The IR vibrational spectra of CNTs and hydrogenated nanotubes (HNTs), as well as their cations, were obtained with DFT. Results. We find that CNTs could be synthesized in space through a feasible formation pathway. CNTs and cationic CNTs, as well as their hydrogenated counterparts, exhibit intense vibrational transitions in the IR. Their possible presence in the ISM could be investigated by comparing the calculated vibrational spectra with astronomical observations made by the Infrared Space Observatory, Spitzer Space Telescope, and particularly the upcoming James Webb Space Telescope.

  • 5.
    Chen, Tao
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Luo, Yi
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Formation of polyynes and ring-polyyne molecules following fragmentation of polycyclic aromatic hydrocarbons2019Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 486, nr 2, s. 1875-1881Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, we perform molecular dynamic (MD) simulations to investigate the stability and fragmentation processes of vibrationally excited linear polycyclic aromatic hydrocarbons (PAHs). The program of CP2K in combination with the semi-empirical method PM3 is utilized for the MD simulations. The simulations show that the formation of molecular hydrogens (H-2) is different than previous studies, in particular, different than compact PAHs. At high temperatures, linear PAHs tend to open aromatic rings and convert the sp(3) C-C or sp(2) C=C bonds to sp C C bonds by removing H-2; i.e. polyynes are formed in such process. Besides polyynes, PAHs attached with sp-bonded polyyne chains are commonly observed at high temperatures. We notice that due to the addition of flexible tails (polyynes), the ring-polyyne molecules do not dissociate for a long period of time at high temperatures. Such structures facilitate the molecules to survive in the harsh environment of the interstellar medium. In addition, the ring-polyyne structures induce dipole moments that could, in principle, be detected by radio astronomy.

  • 6.
    Chen, Tao
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Luo, Yi
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Li, Aigen
    Univ Missouri, Dept Phys & Astron, Columbia, MO 65211 USA..
    Fragmentation and isomerization of polycyclic aromatic hydrocarbons in the interstellar medium: Coronene as a case study2020Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 633, artikel-id A103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives have been poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study. Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations were performed to model the major processes (e.g., vibrations, fragmentations, and isomerizations) of coronene and its derivatives (e.g., methylated coronene, hydrogenated coronene, dehydrogenated coronene, nitrogen-substituted coronene, and oxygen-substituted coronene) at temperatures of 3000 K and 4000 K. Results. We find that the anharmonic effects are crucial for the simulation of vibrational excitation. For the molecules studied here, H-2, CO, HCN, and CH2 are the major fragments. Following the dissociation of these small units, most of the molecules could maintain their ring structures, but a few molecules would completely break into carbon chains. The transformation from a hexagon to a pentagon or a heptagon may occur and the heteroatomic substitutions (e.g., N- or O-substitutions) would facilitate the transformation.

  • 7.
    Chen, Tao
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Luo, Yi
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi. Univ Sci & Technol China, Sch Chem & Mat Sci, Dept Chem Phys, Hefei Natl Lab Phys Sci Microscale, Hefei 230026, Anhui, Peoples R China.
    Li, Aigen
    Univ Missouri, Dept Phys & Astron, Columbia, MO 65211 USA..
    The infrared bands of polycyclic aromatic hydrocarbons in the 1.6-1.7 mu m wavelength region2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 632, artikel-id A71Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The 3.3 mu m aromatic C-H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at similar to 3.4-3.6 mu m. One of these sources, IRAS 21282 +5050, a planetary nebula, also exhibits a weak band at similar to 1.68 mu m. While the satellite features at similar to 3.4-3.6 mu m are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 mu m feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6-1.7 mu m wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang-Landau random walk technique was employed. Results. All the dominant bands in the 1.6-1.7 mu m wavelength range and in the 3.1-3.5 mu m C-H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6-1.7 mu m region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1-3.5 mu m C-H stretching features to the bands in the 1.6-1.7 mu m region, I3.1-3:5/I1.6-1.7, for both ground and vibrationally excited states. On average, we obtain < I3.1-3.5=/I1.6-1.7 > approximate to 12.6 and < I3.1-3.5=/I1.6-1.7 > approximate to 17:6 for PAHs at ground states and at vibrationally excited states, respectively.

  • 8.
    Chen, Tao
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi. Leiden Univ, Leiden Observ, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands..
    Mackie, Cameron
    Leiden Univ, Leiden Observ, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands..
    Candian, Alessandra
    Leiden Univ, Leiden Observ, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands..
    Lee, Timothy J.
    NASA, Ames Res Ctr, Moffett Field, CA 94035 USA..
    Tielens, Alexander G. G. M.
    Leiden Univ, Leiden Observ, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands..
    Anharmonicity and the infrared emission spectrum of highly excited polycyclic aromatic hydrocarbons2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 618, artikel-id A49Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. Infrared (IR) spectroscopy is a powerful tool to study molecules in space. A key issue in such analyses is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the IR spectra for molecules under various conditions. Methods. We combined second order vibrational perturbation theory and the Wang-Landau random walk technique to produce accurate IR spectra of highly excited polycyclic aromatic hydrocarbons. We fully incorporated anharmonic effects, such as resonances, overtones, combination bands, and temperature effects. Results. The results are validated against experimental results for the pyrene molecule (C16H10). In terms of positions, widths, and relative intensities of the vibrational bands, our calculated spectra are in excellent agreement with gas-phase experimental data.

  • 9.
    Chen, Tao
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Zhen, J.
    Wang, Yin
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Linnartz, H.
    Tielens, A. G. G. M.
    Photodissociation processes of Bisanthenquinone cation2017Ingår i: Proceedings of the International Astronomical Union, ISSN 1743-9213, nr S332, s. 353-359Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A systematic study, using ion trap time-of-flight mass spectrometry, is presented for the photo-dissociation processes of Bisanthenquinone (Bq) cations, C28H12O2+, a ketone substituted Polycyclic Aromatic Hydrocarbon (PAH). The Bq cation fragments through sequential loss of the two neutral carbonyl (CO) units upon laser (626nm) irradiation, resulting in a PAH-like derivative C26H12+. Upon further irradiation, C26H12+ exhibits both stepwise dehydrogenation and C2/C2H2 loss fragmentation channels. Quantum chemistry calculations reveal a detailed picture for the first CO-loss, which involves a transition state with a barrier of ∼ 3.4 eV, which is lower than the energy required for the lowest H-loss pathway (∼ 5.0 eV). The barrier for the second CO-loss is higher (∼ 4.9 eV). The subsequent loss of this unit changes the Bq geometry from a planar to a bent one. It is concluded that the photodissociation mechanism of the substituted PAH cations studied here is site selective in the substituted subunit. This work also shows that an acetone substituted PAH cation is not photo-stable upon irradiation. 

  • 10.
    Zhang, Weiwei
    et al.
    Univ Sci & Technol China, Dept Astron, CAS Key Lab Res Galaxies & Cosmol, Hefei 230026, Anhui, Peoples R China.;Univ Sci & Technol China, Sch Astron & Space Sci, Hefei 230026, Anhui, Peoples R China.;Penn State Univ, Dept Mech & Nucl Engn, University Pk, PA 16802 USA..
    Si, Yubing
    Huanghe Sci & Technol Coll, Inst Nanostruct Funct Mat, Henan Prov Key Lab Nanocomposites & Applicat, Zhengzhou 450006, Henan, Peoples R China..
    Zhen, Junfeng
    Univ Sci & Technol China, Dept Astron, CAS Key Lab Res Galaxies & Cosmol, Hefei 230026, Anhui, Peoples R China.;Univ Sci & Technol China, Sch Astron & Space Sci, Hefei 230026, Anhui, Peoples R China..
    Chen, Tao
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Linnartz, Harold
    Leiden Univ, Leiden Observ, Sackler Lab Astrophys, POB 9513, NL-2300 RA Leiden, Netherlands..
    Tielens, Alexander G. G. M.
    Leiden Univ, Leiden Observ, POB 9513, NL-2300 RA Leiden, Netherlands..
    Laboratory Photochemistry of Covalently Bonded Fluorene Clusters: Observation of an Interesting PAH Bowl-forming Mechanism2019Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 872, nr 1, artikel-id 38Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The fullerene C-60, one of the largest molecules identified in the interstellar medium (ISM), has been proposed to form top-down through the photochemical processing of large (more than 60 C atoms) polycyclic aromatic hydrocarbon (PAH) molecules. In this article, we focus on the opposite process, investigating the possibility that fullerenes form from small PAHs, in which bowl-forming plays a central role. We combine laboratory experiments and quantum chemical calculations to study the formation of larger PAHs from charged fluorene clusters. The experiments show that with visible laser irradiation, the fluorene dimer cation-[C13H9-C13H9](+)-and the fluorene trimer cation-[C13H9-C13H8-C13H9](+)-undergo photodehydrogenation and photoisomerization, resulting in bowl-structured aromatic cluster ions, C26H12+ and C39H20+, respectively. To study the details of this chemical process, we employ quantum chemistry that allows us to determine the structures of the newly formed cluster ions, to calculate the dissociation energies for hydrogen loss, and to derive the underlying reaction pathways. These results demonstrate that smaller PAH clusters (with less than 60 C atoms) can convert to larger bowled geometries that might act as building blocks for fullerenes, because the bowl-forming mechanism greatly facilitates the conversion from dehydrogenated PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment that-in principle-allows one to search for such species using radio astronomy.

  • 11.
    Zhen, Junfeng
    et al.
    Univ Sci & Technol China, Dept Astron, CAS Key Lab Res Galaxies & Cosmol, Hefei 230026, Anhui, Peoples R China.;Univ Sci & Technol China, Sch Astron & Space Sci, Hefei 230026, Anhui, Peoples R China..
    Chen, Tao
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Tielens, Alexander G. G. M.
    Leiden Univ, Leiden Observ, POB 9513, NL-2300 RA Leiden, Netherlands..
    Laboratory Photochemistry of Pyrene Clusters: An Efficient Way to Form Large PAHs2018Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 863, nr 2, artikel-id 128Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, we study the photodissociation processes of small PAH clusters (e.g., pyrene clusters). The experiments are carried out using a quadrupole ion trap in combination with time-of-flight (QIT-TOF) mass spectrometry. The results show that pyrene clusters are converted into larger PAHs under the influence of a strong radiation field. Specifically, pyrene dimer cations (e.g., [C16H10-C16H9](+) or C32H19 (+)), will photodehydrogenate and photo-isomerize to fully aromatic cations (PAHs) (e.g., C32H16 (+)) with laser irradiation. The structure of new formed PAHs and the dissociation energy for these reaction pathways are investigated with quantum chemical calculations. These studies provide a novel efficient evolution routes for the formation of large PAHs in the interstellar medium in a bottom-up process that will counteract the top-down conversion of large PAHs into rings and chains, and provide a reservoir of large PAHs that can be converted into C-60 and other fullerenes and large carbon cages.

1 - 11 av 11
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf