Warren D. Lawrance

The S1–S0(1B2–1A1) transition of jet‐cooled toluene: Excitation and dispersed fluorescence spectra, fluorescence lifetimes, and intramolecular vibrational energy redistribution

The fluorescence excitation spectrum of the S1–S0(1B2–1A1) transition in jet‐cooled toluene has been measured up to 2000 cm−1 above the origin band. Dispersed fluorescence spectra of the major features have been recorded and used to assign the levels observed in excitation. Collisional energy transfer experiments have been used to confirm assignments for some of the lower lying S1 fundamentals that were not accessible via direct optical pumping. The number of known S1 fundamentals has been extended to 13. The dispersed fluorescence spectra reveal the onset of intramolecular vibrational energy redistribution (IVR) at low S1 vibrational energies. Fluorescence lifetimes of all of the major bands observed in the excitation spectrum have been measured. The lifetimes decrease from 86 ns for 00 to 48 ns at an S1 vibrational energy of 1900 cm−1. To alleviate the confusion that exists over the mode numbering in toluene a new scheme is proposed which obviates this problem. This system is similar to that used for ot...

Phenylgermane as a suitable precursor for laser flash photolysis measurements of germylene kinetics

Abstract R. Becerra, S.E. Boganov, M.P. Egorov, O.M. Nefedov and R. Walsh [Chem. Phys. Lett. 260 (1996) 433] reported the first direct kinetic measurements of germylene, GeH2, using two precursors, 3,4-dimethylgermacyclopentene and phenylgermane. They report that phenylgermane produces anomalously low reaction rate constants. We have re-examined this issue and measure rate constants using phenylgermane that agree with the values reported by Becerra et al. using the precursor 3,4-dimethylgermacyclopentene. The first direct rate constant measurement for GeH2+phenylgermane is reported. All rate constants are found to be independent of the total pressure (measured to 50 Torr). Photolysis of phenylgermane at 248 and 193 nm yields the same values for the rate constants within the experimental uncertainty, suggesting that relaxation of vibrationally excited germylene is not altering the measured reaction rate constants.

The Dissociation Energy of the Benzene–Argon van der Waals Complex Determined by Velocity Map Imaging

The technique of velocity map imaging has been used to determine the dissociation energy of the C6H6+–Ar van der Waals complex. From the change in the ionization energy between the complex and free benzene and the spectroscopic shift of the S1←S0 transition, the dissociation energies in the S0 and S1 states of the neutral complex were determined, being 314 ± 7 and 335 ± 7 cm−1 for the S0 and S1 states of the neutral complex, respectively, and 486 ± 5 cm−1 for the cation ground (D0) state.

The dissociation energy of van der Waals complexes determined by velocity map imaging: values for S0 and S1p-difluorobenzene–Ar and D0 (p-difluorobenzene–Ar)+

Abstract The technique of velocity map imaging, an enhanced resolution variant of ion imaging, is shown to be a useful method for determining the dissociation energy of van der Waals complexes. The method is demonstrated by measuring the dissociation energy of p -difluorobenzene–Ar in the S 1 state. From the spectroscopic shift of the S 1 ←S 0 transition and the change in the ionisation energy between the complex and free p -difluorobenzene, the dissociation energies in the ground state of the neutral and cationic complexes are determined. The values so determined are 339±4 and 369±4 cm −1 for the S 0 and S 1 states of the neutral complex, respectively, and 576±4 cm −1 for the cation ground (D 0 ) state.

Two dimensional laser induced fluorescence spectroscopy: A powerful technique for elucidating rovibronic structure in electronic transitions of polyatomic molecules

We demonstrate the power of high resolution, two dimensional laser induced fluorescence (2D-LIF) spectroscopy for observing rovibronic transitions of polyatomic molecules. The technique involves scanning a tunable laser over absorption features in the electronic spectrum while monitoring a segment, in our case 100 cm(-1) wide, of the dispersed fluorescence spectrum. 2D-LIF images separate features that overlap in the usual laser induced fluorescence spectrum. The technique is illustrated by application to the S(1)-S(0) transition in fluorobenzene. Images of room temperature samples show that overlap of rotational contours by sequence band structure is minimized with 2D-LIF allowing a much larger range of rotational transitions to be observed and high precision rotational constants to be extracted. A significant advantage of 2D-LIF imaging is that the rotational contours separate into their constituent branches and these can be targeted to determine the three rotational constants individually. The rotational constants determined are an order of magnitude more precise than those extracted from the analysis of the rotational contour and we find the previously determined values to be in error by as much as 5% [G. H. Kirby, Mol. Phys. 19, 289 (1970)]. Comparison with earlier ab initio calculations of the S(0) and S(1) geometries [I. Pugliesi, N. M. Tonge, and M. C. R. Cockett, J. Chem. Phys. 129, 104303 (2008)] reveals that the CCSD∕6-311G∗∗ and RI-CC2∕def2-TZVPP levels of theory predict the rotational constants, and hence geometries, with comparable accuracy. Two ground state Fermi resonances were identified by the distinctive patterns that such resonances produce in the images. 2D-LIF imaging is demonstrated to be a sensitive method capable of detecting weak spectral features, particularly those that are otherwise hidden beneath stronger bands. The sensitivity is demonstrated by observation of the three isotopomers of fluorobenzene-d(1) in natural abundance in an image taken for a supersonically cooled sample. The ability to separate some of the (13)C isotopomers in natural abundance is also demonstrated. The equipment required to perform 2D-LIF imaging with sufficient resolution to resolve the rotational features of large polyatomics is available from commercial suppliers.

The binding energies of p-difluorobenzene–Ar,–Kr measured by velocity map imaging: Limitations of dispersed fluorescence in determining binding energies

The technique of velocity map imaging has been used to determine the dissociation energies of the van der Waals complexes p-difluorobenzene–Ar and p-difluorobenzene–Kr. The values determined for the S0, S1, and D0 states, respectively, are 337±4, 367±4, and 572±6 cm−1 for p-difluorobenzene–Ar and 398±7, 445±7, and 720±6 cm−1 for p-difluorobenzene–Kr. An ionization potential of 73 549±4 cm−1 for p-difluorobenzene–Kr has been determined by velocity map imaging of photoelectrons. The dissociation energies determined here are inconsistent with dispersed fluorescence spectra of the complexes when these are assigned in the usual way. The issue is that spectra for levels below dissociation show bands where free p-difluorobenzene emits, suggesting that dissociation is occurring from these levels. For the dispersed fluorescence and velocity map imaging results to be consistent, these fluorescence bands must arise from transitions of the van der Waals complexes shifted such that they appear at the free p-difluorobe...

Temperature dependence of germylene reactions with acetylene, trimethylsilane, and phenylgermane

Abstract Gas-phase reaction rate constants have been determined over the temperature range 295–436 K for the reactions of germylene, GeH2, with acetylene (GeH2 addition across a triple bond), trimethylsilane (GeH2 insertion into a Si–H bond), and phenylgermane. The room-temperature rate constant for germylene reacting with benzene has been measured and is found to be a factor of ∼300 smaller than that for phenylgermane, indicating that the latter reacts by GeH2 insertion into the Ge–H bonds. A negative temperature dependence is observed in all cases. The activation energies, obtained from weighted linear fits to the data over the experimental temperature range, are −3.5±0.3, −11.0±0.4, and −3.6±0.3 kJ mol−1 for acetylene, trimethylsilane, and phenylgermane, respectively, while the respective frequency factors, log(A/cm3 molecule−1 s−1), are −10.5±0.1, −11.8±0.1, −10.1±0.1.

Fluid dynamic lateral slicing of high tensile strength carbon nanotubes.

Lateral slicing of micron length carbon nanotubes (CNTs) is effective on laser irradiation of the materials suspended within dynamic liquid thin films in a microfluidic vortex fluidic device (VFD). The method produces sliced CNTs with minimal defects in the absence of any chemical stabilizers, having broad length distributions centred at ca 190, 160 nm and 171 nm for single, double and multi walled CNTs respectively, as established using atomic force microscopy and supported by small angle neutron scattering solution data. Molecular dynamics simulations on a bent single walled carbon nanotube (SWCNT) with a radius of curvature of order 10 nm results in tearing across the tube upon heating, highlighting the role of shear forces which bend the tube forming strained bonds which are ruptured by the laser irradiation. CNT slicing occurs with the VFD operating in both the confined mode for a finite volume of liquid and continuous flow for scalability purposes.

Direct measurement of methylene removal rates by species containing the OH functional group

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene (1CH2) with N2, ketene (CH2CO), H2O, D2O, methanol (CH3OH), ethanol (C2H5OH), and n-propanol (n-C3H7OH) at 296 ± 2 K. The rate constants were found to be (0.130 ± 0.012), (2.38 ± 0.18), (1.60 ± 0.17), (1.05 ± 0.13), (2.20 ± 0.19), (2.61 ± 0.23) and (4.15 ± 0.44)× 10–10 cm3 molecule–1 s–1, respectively. The removal rate constants for the alcohols are consistently greater than those for the corresponding alkanes.

Direct observation of methyl rotor and vib-rotor states of S0 toluene: a revised torsional barrier due to torsion-vibration coupling.

We report a two dimensional, laser induced fluorescence study of the lowest 345 cm(-1) region of S0 toluene. Methyl rotor levels of 00 up to m = 6 and of 201 up to m = 4 are observed. The rotor levels of 00 and 201 have quite different energy spacings that are well fit by a model that includes strong torsion-vibration coupling between them. The model requires that the rotor barrier height be revised from -4.84 cm(-1) (methyl hydrogens in a staggered conformation) to +1.57 cm(-1) (eclipsed conformation). However, the 3a2″ state lies below the 3a1″ state as expected for a staggered conformation due to energy shifts associated with the torsion-vibration coupling. It is shown that the rotor wave-functions exhibit little localization at the torsional energy minima. The variation in the m = 0 wavefunction probability distribution with torsional angle is shown to be very similar for the previously accepted negative V6 value and the torsion-vibration coupling model as this coupling shifts the phase of the wavefunction by 30° compared with its phase for V6 alone. The presence of a strong Δυ = ± 1 torsion-vibration coupling involving the lowest frequency vibrational mode provides a potential pathway for rapid intramolecular vibrational energy redistribution at higher energies.