| 1. |
- Mohammed, Abdelsalam, et al.
(författare)
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Ab initio study of coherent anti-Stokes Raman scattering (CARS) of the 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) explosive
- 2010
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 485:4-6, s. 320-325
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Tidskriftsartikel (refereegranskat)abstract
- Coherent anti-Stokes Raman scattering (CARS) of the 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) C3H6N6O6 molecule is studied by ab initio methods. The results are compared to available experimental observations and against calculations and experimental observations of the conventional non-resonant Raman spectrum for RDX. It is found that all intense bands in the observed CARS spectrum and all Raman differential cross sections are well reproduced by the calculations. The features of the resonant CARS signal vary strongly from the corresponding Raman signal, and are obtained with a considerably larger cross section, a fact that could further facilitate the use of CARS spectroscopy in applications of stand-off detection of gaseous samples at ultra-low concentrations.
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| 2. |
- Mohammed, Abdelsalam, et al.
(författare)
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Classification of Raman active modes of platinum(II) acetylides : A combined experimental and theoretical study
- 2009
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 481:4-6, s. 209-213
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Tidskriftsartikel (refereegranskat)abstract
- Non-resonance Raman scattering of platinum(II) acetylide molecules is studied both by theory and experiment. Differential cross sections are presented for bis((4-(phenyl-ethynyl)phenyl)ethynyl)bis(tri-n-butylphosphine)platinum( II) (Pt1), and for bis((4-(phenyl-ethynyl) bis(tri-n-butylphosphine) platinum(II) substituted with triazole end group (Z1). All intense bands in the observed Raman scattering are assigned to symmetric vibrations of Pt-C C, C C, and phenyl groups; a reliable interpretation includes some details of relative intensities for the stretching and in-plane bending modes. The Raman spectra verify that the Pt-acetylide dyes remain intact during the material processing, indicating that they retain functionality for applications like in optical power limiting, nonlinear optics, and light emitting diodes.
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| 3. |
- Mohammed, Abdelsalam, et al.
(författare)
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Cluster approximations of chemically enhanced molecule-surface Raman spectra : The case of trans-1,2-bis (4-pyridyl) ethylene (BPE) on gold
- 2013
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 581, s. 70-73
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Tidskriftsartikel (refereegranskat)abstract
- The system, trans-1,2-bis (4-pyridyl) ethylene (BPE) on a Au substrate, is used to examine the applicability of cluster approximations for chemically enhanced molecule-surface Raman spectra. Different size of Au clusters, and adsorption sites, as well as different functionals were studied. In addition, the relative intensity of the two dominant modes of the theoretical spectra have been calculated for the different cluster sizes and adsorption sites. It was found that already a Au-20 cluster leads to excellent agreement with experimentally recorded spectra.
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| 4. |
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| 5. |
- Mohammed, Abdelsalam, et al.
(författare)
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Hyper Raman spectra calculated in a time-dependent Hartree-Fock method
- 2012
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Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 110:19-20, s. 2315-2320
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Tidskriftsartikel (refereegranskat)abstract
- Hyper Raman scattering (HRS) of the benzonitrile (BN) and 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) molecules is studied by means of ab initio calculations. The computational procedure employs a recently developed methodology for the analytic calculations of frequency-dependent polarizability gradients of arbitrary order, including perturbation dependent basis sets. The result are compared to normal Raman scattering (NRS) and coherent anti-Stokes Raman scattering (CARS) that previously have been studied using the same technology. It is found that some suppressed or silent modes in CARS and NRS spectra are clearly seen in HRS, and that although under general excitation conditions the HRS intensities are much lower than for CARS and NRS, HRS provides complementary information useful for target identification.
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| 6. |
- Mohammed, Abdelsalam, et al.
(författare)
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Raman Scattering at Resonant or Near-Resonant Conditions : A Generalized Short-Time Approximation
- 2012
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Ingår i: Chinese Journal of Chemical Physics. - : AIP Publishing. - 1674-0068 .- 2327-2244. ; 25:1, s. 31-47
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the dynamics of resonant Raman scattering in the course of the frequency detuning. The dephasing in the time domain makes the scattering fast when the photon energy is tuned from the absorption resonance. This makes frequency detuning to act as a camera shutter with a regulated scattering duration and provides a practical tool of controlling the scattering time in ordinary stationary measurements. The theory is applied to resonant Raman spectra of a couple of few-mode model systems and to trams-1,3,5-hexatriene and guanine-cytosine (G-C) Watson-Crick base pairs (DNA) molecules. Besides some particular physical effects, the regime of fast scattering leads to a simplification of the spectrum as well as to the scattering theory itself. Strong overtones appear in the Raman spectra when the photon frequency is tuned in the resonant region, while in the mode of fast scattering, the overtones are gradually quenched when the photon frequency is tuned more than one vibrational quantum below the first a,absorption resonance. The detuning front the resonant region thus leads to a strong purification of the Raman spectrum from the contamination by higher overtones and soft modes and purifies the spectrum also in terms of avoidance of dissociation and interfering fluorescence decay of the resonant state. This makes frequency detuning a very useful practical tool in the analysis of the resonant Raman spectra of complex systems and considerably improves the prospects for using the Raman effect for detection of foreign substances at ultra-low concentrations.
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| 7. |
- Mohammed, Abdelsalam, et al.
(författare)
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Resonance enhanced Raman scattering from the complex electric-dipole polarizability : A theoretical study on N-2
- 2009
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 468:4-6, s. 119-123
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Tidskriftsartikel (refereegranskat)abstract
- An expression for resonant molecular Raman scattering in terms of the complex electric-dipole polarizability tensor is presented and implemented with first principles complex polarization propagator theory. The theory reveals the strong role of phase delays of the detected Raman radiation in the resonant case. Application on gaseous nitrogen predicts a Raman scattering enhancement of circa 104 in the resonance regions of the b(1)Pi(u) and b '(1)Sigma(+)(u) states as compared to the nonresonant case.
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| 8. |
- Mohammed, Abdelsalam, 1968-
(författare)
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Theoretical Studies of Raman Scattering
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Different theoretical approaches have been presented in this thesis to study the Raman scattering effect. The first one is response theory applied up to third order of polarization, where the determination of α, β and γ is used to calculate linear Raman scattering (resonance Raman scattering (RRS) and normal Raman scattering (NRS)), hyper Raman scattering (HRS) and coherent anti-Stokes Raman scattering (CARS), respectively. The response theory refers to adiabatic time-dependent density functional theory in the complex domain with applications on RRS and NRS, and to a recently developed methodology (Thorvaldsen et al. [105, 106]) for the analytic calculation of frequency-dependentpolarizability gradients of arbitrary order, here with applications on CARSand HRS. Various systems have been studied with the response theory, such as explosive substances (DNT, TNT, RDX and H2O2), optical power limiting materials (platinum(II) acetylide molecules), DNA bases (methylguanine-methylcytosine) and other systems (Trans-1,3,5-hexatriene and Pyridine). We have explored the dependency of the calculated spectra on parametrization in terms of exchange-correlation functionals and basis sets, and on geometrica loptimization. The second approach refers to time-dependent wave packet methodology for RRS and its time-independent counterpart in the Kramers-Heisenberg equation for the scattering cross section, which reduces the calculation of the RRS amplitude to computation of matrix elements of transition dipole moments between vibrational wave functions. The time-dependent theory has been used to examine RRS as a dynamical process where particular attention is paid to the notion of fast scattering in which the choice of photon frequency controls the scattering time and the nuclear dynamics. It is shown that a detuning from resonance causes a depletion of the RRS spectrum from overtones and combination bands, a situation which is verified in experimental spectra. The cross section of NRS has been predicted for the studied molecules to be in the order of 10−30 cm2/sr. A further increase in sensitivity with a signal enhancement up to 104 to 105 is predicted for the RRS technique, while CARS conditions imply an overall increase of the intensity by several orders of magnitude over NRS. In contrast to RRS and CARS, the HRS intensity is predicted to be considerably weaker than NRS, by about four orders of magnitude. However, silent modes in NRS can be detected by HRS which in turncan provide essential spectroscopic information and become complementary to NRS scattering. With the above mention methodological development for NRS, RRS, CARS and HRS, we have at our disposal a powerful set of modelling tools for the four different Raman techniques. They have complementary merits and limitations which facilitate the use of these spectroscopes in applications of Raman scattering for practical applications, for instance stand-off detection of foreign substances.
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| 9. |
- Mohammed, Abdelsalam, et al.
(författare)
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Time-dependent density functional theory for resonant properties : resonance enhanced Raman scattering from the complex electric-dipole polarizability
- 2009
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 11:22, s. 4539-4548
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Tidskriftsartikel (refereegranskat)abstract
- Based on resonant-convergent-and thereby complex-electric-dipole polarizabilities, formulae for differential cross sections in resonant Raman spectroscopy are presented. In absorptive regions of the spectrum, there are phase delays in the induced polarization that enter the expressions for the cross sections. The theory is exemplified by calculations employing the adiabatic time-dependent density functional theory approximation, with applications on the low-lying, dipole-allowed, electronic states of a sample of molecules, including hydrogen peroxide, pyridine, and trinitrotoluene. Results obtained with the Coulomb attenuated B3LYP exchange-correlation functional are found to be more accurate than those obtained with the conventional B3LYP functional-an observation that holds not only for trinitrotoluene with its transitions of charge-transfer character but also for the other cases. The qualitative features of the resonant Raman spectra for a given molecule vary strongly from one resonance wavelength to another, which is a fact that could further facilitate the use of this spectroscopy in applications of stand-off detection of gaseous samples in ultra-low concentrations.
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| 10. |
- Sen, Pinar, et al.
(författare)
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Fluorescence and FTIR Spectra Analysis of Trans-A(2)B(2)-Substituted Di- and Tetra-Phenyl Porphyrins
- 2010
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Ingår i: Materials. - Basel, Switzerland : MDPI AG. - 1996-1944. ; 3:8, s. 4446-4475
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Tidskriftsartikel (refereegranskat)abstract
- A series of asymmetrically substituted free-base di- and tetra-phenylporphyrins and the associated Zn-phenylporphyrins were synthesized and studied by X-ray diffraction, NMR, infrared, electronic absorption spectra, as well as fluorescence emission spectroscopy, along with theoretical simulations of the electronic and vibration structures. The synthesis selectively afforded trans-A(2)B(2) porphyrins, without scrambling observed, where the AA and BB were taken as donor-and acceptor-substituted phenyl groups. The combined results point to similar properties to symmetrically substituted porphyrins reported in the literature. The differences in FTIR and fluorescence were analyzed by means of detailed density functional theory (DFT) calculations. The X-ray diffraction analysis for single crystals of zinc-containing porphyrins revealed small deviations from planarity for the porphyrin core in perfect agreement with the DFT optimized structures. All calculated vibrational modes (2162 modes for all six compounds studied) were found and fully characterized and assigned to the observed FTIR spectra. The most intense IR bands are discussed in connection with the generic similarity and differences of calculated normal modes. Absorption spectra of all compounds in the UV and visible regions show the typical ethio type feature of meso-tetraarylporphyrins with a very intense Soret band and weak Q bands of decreasing intensity. In diphenyl derivatives, the presence of only two phenyl rings causes a pronounced hypsochromic shift of all bands in the absorption spectra. Time-dependent DFT calculations revealed some peculiarities in the electronic excited states structure and connected them with vibronic bands in the absorption and fluorescence spectra from associated vibrational sublevels.
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