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  • 1
    Electronic Resource
    Electronic Resource
    The Jahn–Teller effect in triptycene (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7306-7320 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The irregular vibronic structure resolved in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10-dihydro-9,10[1'2']benzenoanthracene) is assigned in terms of a single-mode E'⊗e' Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe' =47.83 cm−1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≈98% of the observed absorption band intensities over the observable range 0–350 cm−1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry-equivalent minima. Emission spectra from 13 vibronic levels in the excited E' state show extended vibrational progressions with up to 25 members in the analogous e' ground state vibration, which is highly harmonic in the electronic ground state. The Franck–Condon factors of the fluorescence emission spectra calculated with the E' state Jahn–Teller parameters fitted to the absorption spectrum also yield a quantitative fit to observed emission intensities. The eigenvectors of the E' state vibronic levels are hence determined to great precision; the lowest five can be classified as radial oscillator and/or hindered rotor states, while higher levels have mixed character. Several eigenvectors are strongly localized in the upper sheet of the adiabatic Jahn–Teller surface, corresponding to "cone'' states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462434
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  • 2
    Electronic Resource
    Electronic Resource
    The Jahn–Teller effect in 9-fluorotriptycene (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4932-4941 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibronic structure in the S1(E)←S0(A1) resonant two-photon ionization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—E⊗e, (A⊕E)⊗e, and (A⊕E)⊗(a2+e). The basic E⊗e interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron-withdrawing effect of the F atom. For the same reason, the exciton splitting between the S1 and the S2 states, as calculated in the (A⊕E)⊗e model, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm−1 are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S0 frequencies, as well as those of triptycene. A 273 cm−1 degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher-energy (200–360 cm−1) part of the R2PI spectrum, and may represent an example of e⊕e multimode coupling within a degenerate electronic state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466042
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  • 3
    Electronic Resource
    Electronic Resource
    A trimer vibronic coupling model for triptycene: The Jahn–Teller and Barnett effects (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 3803-3815 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first singlet electronic excited state of triptycene, as measured by resonant two-photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E'⊗e' Jahn–Teller effect. Here it is shown that this E'⊗e' vibronic coupling can be profitably viewed as a subset of a (A'1⊕E')⊗(a'2⊕e') vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e' mode as well as an unusually large second-order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a'2 vibration which couples through nonzero momentum rather than coordinate matrix elements. A coupling of this type has been termed the "molecular Barnett effect'' and has been predicted to appear in Jahn–Teller systems when certain conditions are fulfilled. This effect has not been previously observed in molecular spectroscopy. It indicates that crude adiabatic basis functions are inadequate to describe the electronic states of the present system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464009
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  • 4
    Electronic Resource
    Electronic Resource
    Intermolecular perturbation of a Jahn–Teller system: The triptycene⋅Nen (n=1–3) van der Waals clusters (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 840-855 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of rare gas complexation on the electronically excited S1(E') state of triptycene (T), which is Jahn–Teller distorted, was investigated by two-color resonant two-photon ionization (2C-R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene⋅Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high-symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1⊕E)⊗e coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T⋅Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V-shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T⋅Ne and T⋅Ne2 were fitted with a considerable uniaxial strain of −19.73 and 19.07 cm−1, respectively, confirming both the predicted C2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V-shaped compartments by ∼0.8°. The spectra of T⋅Ne and T⋅Ne2 are much more complex compared to triptycene and T⋅Ne3 due to the splitting of the E vibronic levels in C2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466567
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  • 5
    Electronic Resource
    Electronic Resource
    The low-frequency vibrations of triptycene (1992)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 96 (1992), S. 10713-10719 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100205a026
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  • 6
    Electronic Resource
    Electronic Resource
    Pyrene-rare gas clusters: anomalous spectral shifts and van der Waals mode intensities (1993)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 13527-13534 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100153a018
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  • 7
    Electronic Resource
    Electronic Resource
    The photodissociation dynamics of OClO between 306 and 370 nm: Fragment translational energy release and recoil anisotropy (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6538-6547 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation OClO(A˜ 2A2)→ClO(X˜ 2Π)+O(3P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the A˜ 2A2 surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v(Cl–O)∼1–15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–particularly at λ〈325 nm–could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly increasing vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first approximation, as vibrational predissociation on the A˜ 2A2 potential energy surface involving a relatively late exit barrier. From the measured translational energies the barrier height is estimated to be about 48 kJ/mol. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473652
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  • 8
    Electronic Resource
    Electronic Resource
    The photodissociation of carbonyl cyanide CO(CN)2 at 193 nm studied by photofragment translational energy spectroscopy (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 923-930 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of carbonyl cyanide CO(CN)2 at 193 nm was investigated by photofragment translational energy spectroscopy. For all the fragments created (CO, CN, OCCN, NCCN), the kinetic energy distributions were measured and two decay channels identified. The radical decay, CO(CN)2+hν→OCCN+CN, dominates with a yield of 94%±2% and shows the available energy mainly (82%) channeled into the internal degrees of freedom of the fragments. A fraction of 18%±6% of the nascent OCCN radicals has sufficient energy to spontaneously decay to CO+CN involving a barrier ≤160 kJ/mol. With a yield of 6%±2% the molecular decay produces the fragments CO+NCCN. These fragments acquire a high available energy owing to the formation of the new C–C bond in NCCN. An average fraction of 70% is partitioned into internal fragment energy. Even the fastest fragments are still internally hot, indicating that with the high barrier expected, a substantial exit channel interaction is operative. The isotropic recoil distribution found for the products CN, OCCN, and NCCN further suggests that both the radical and the molecular decay are, on the time scale of a parent rotation, slow and probably indirect. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479376
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  • 9
    Electronic Resource
    Electronic Resource
    Coupling of a Jahn–Teller pseudorotation with a hindered internal rotation in an isolated molecule: 9-hydroxytriptycene (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10767-10780 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The irregular vibronic structure in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene is a result of a classic E⊗e Jahn–Teller effect [A. Furlan et al., J. Chem. Phys. 96, 7306 (1992)]. This is well characterized and can be used as an effective probe of intramolecular perturbations. Here we examine the S1←S0 R2PI spectrum of 9-hydroxytriptycene and the fluorescence from various excited state vibronic levels. In this system the pseudorotation of the Jahn–Teller vibration is strongly coupled to the torsional motion of the bridgehead hydroxy group. This torsional motion results in a tunneling splitting in both the ground and excited states. The population of the upper level in the ground electronic state results in additional vibronic transitions becoming symmetry allowed in the R2PI spectrum that are forbidden in the bare triptycene molecule. The assignment of the R2PI and fluorescence spectra allows the potential energy surfaces of these vibrational modes to be accurately quantified. The full C3v vibronic point group must be used to interpret the spectra. The time scale of the internal rotation of the–OH group and the butterfly flapping of the Jahn–Teller pseudorotation are of similar magnitude. The tunneling between the nine minima on the three dimensional potential energy surface is such that the Jahn–Teller pseudorotation occurs in concert with the–OH internal rotation. The Berry phase that is acquired during this motion is discussed. The simple physical picture emerges of the angle between two of the three benzene moieties opening in three equivalent ways in the S1 electronic state. This geometry follows the position of the hydroxy group, which preferentially orients itself to point between these two rings. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477775
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  • 10
    Electronic Resource
    Electronic Resource
    Photoproducts ejected from liquid surfaces: The importance of photochemical, diffusional, kinetic, and surface structural effects (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10390-10399 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Gas-phase products of liquid surface photochemistry in high vacuum were analyzed by time-of-flight/quadrupole mass spectroscopy. A thin liquid film of a long-chain alkyl iodide, C18H37I, dissolved in squalane (C30H62) was irradiated with nanosecond laser pulses at 275 nm. The photoproducts leaving the liquid after a low-fluence laser pulse (0.5 mJ/cm2) were I, HI, and I2. Since these species may desorb at times delayed by diffusion in the liquid phase, time-of-flight profiles were also recorded using a chopper wheel in front of the surface. This allowed a reconstruction of the time-dependent flux from the surface. The flight time distributions were compared to model calculations which take into account laser photolysis of C18H37I, diffusion and surface evaporation of I, HI, and I2, and the condensed-phase kinetics of radical reactions, and allow for a component of direct photodissociation of surface layer molecules, leading to hyperthermal I atoms. Simulations based on a comprehensive kinetic scheme are in good agreement with our measurements, with no specific surface processes other than evaporation of thermalized species formed at, or diffusing to the surface. However, compared to results previously found for liquid C2H5I, the caging of the geminate pair is much stronger in our system, and the fraction of I atoms promptly reacting to HI is smaller. The absence of prompt hyperthermal I fragments is interpreted by a preferential orientation of the C18H37I molecules in the topmost liquid layer with the I atom pointing into the liquid. The sensitivity of the method is discussed with respect to surface-specific processes, as well as primary and secondary radical reactions occurring in the bulk liquid. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477694
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