Sydney Leach

Quantum yields and lifetimes of molecular ion fluorescence

Abstract A new method has been developed to detect molecular ion emissions and to measure their quantum yields and lifetimes Unit quantum yields are found for N + 2 (B), N 2 O + ( A ), C 6 F + 6 ( A ) and C 6 F 5 H + ( B ). Yields smaller than unity are CO + 2 ( B ),CS + 2 ( B ), COS + ( A ) and 1:2:4 C 3 F 3 H + 3 ( B ), and greater than unity for CO + 2 ( A ). Preliminary results on the emissions of CICN + , BrCN + and ICN + , discovered in the course of the work, are presented.

Pulse radiolysis study of buckminsterfullerene in benzene solution. Assignment of the C60 triplet-triplet absorption spectrum

Abstract Pulse radiolysis and, to a lesser extent, laser flash photolysis have been used to determine properties of the lowest triplet of C 60 in benzene at room temperature, including its absorption spectrum between 300 and 1600 nm, its molar absorption coefficient (ϵ T = 20200 ± 2000 dm 3 mol −1 cm −1 at 750 nm) and its quantum yield of formation (Φ T = 0.88 ± 0.15). The features of the triplet spectrum are assigned according to the triplet transition energies based on molecular orbital calculations previously reported; the strong band at 750 nm is attributed to the 2 3 G u -1 3 T 2g transition. Implications of the results are discussed, including those related to C 60 as an optical limiting material.

Determination of the Symmetry of the First Excited Electronic State of Benzyl by Rotational Contour Analysis of Vibronic Bands of the Emission Spectra of C6H5CH2, C6H5CD2, and C6D5CD2

Theoretical calculations have differed in their predictions as to the symmetry of the first excited state of benzyl. Furthermore, conflicting interpretations have been given to previous experimental results. Rotational band contour analysis of high resolution gas phase emission spectra of C6H5CH2, C6D5CD2, and C6H5CD2 are here used to determine this electronic symmetry. A discussion is given of the treatment of new problems which arise in the band contour analysis of emission spectra and of free radicals as compared with previous contour work on absorption spectra of stable molecules, in particular, as regards the determination of molecular geometry, temperature effects and contour program modifications. Qualitative and quantitative criteria for matching experimental and calculated contours are made explicit. Nonmatching features are assigned to hot bands, using supporting assignments. The effect on calculated contours of the interpolation procedure used in Parkins program is discussed in the Appendix. G...

Emission spectra of seven fluorobenzene cations

The emission spectra of C6F6 +, C6F5H+, the three C6F4H2 + and two C6F3H3 + (1, 2, 4 and 1, 3, 5) cations were obtained in the gas phase, in a special discharge tube. Symmetry and mode correlation of the fundamental frequencies of the parent neutral molecules enabled the isodynamic molecule method to be used for achieving vibrational analysis of the ion spectra. The vibronic structure in each emission spectrum is interpreted. Dynamic Jahn-Teller effects are shown to occur in the degenerate ground state of C6F6 + and of 1, 3, 5-C6F3H3 +, in particular irregular ground-state progressions in mode 6 are observed. Extensive Jahn-Teller calculations were made for 1, 3, 5-C6F3H3 +; frequencies and intensities in its progressions are in good agreement with the results of calculations based on the linear coupling model of Longuet-Higgins et al., using a deperturbed frequency ω = 477–484 cm-1 and a dimensionless interaction parameter D = 0·13-0·10. A review of the vibrational data of photoelectron spectra and Rydbe...

Fluorescence quantum yields of isotopic CO2+ ions

Abstract Fluorescence quantum yields of the A 2 Π u and B 2 Σ u + states of the isotopic ions 12 C 16 O 2 + , 13 C 16 O 2 + and 12 C 18 O 2 + , formed by 584 A photoionization of CO 2 , have been measured by a photon—photoion coincidence technique. The corresponding 584 A photoelectron spectra, measured for 12 C 16 O 2 and 13 C 16 O 2 , showed that the relative photoelectron branching ratios for forming the A 2 Π u and B 2 Σ u + states of CO 2 + are isotope independent. The fluorescence quantum yields are greater than unity for the A 2 Π u state and less than unity for B 2 Σ u + for the three isotopic ions but the values are isotopic dependent, the difference from unity correlating with the degree of spectral perturbation in the B 2 Σ u + − X 2 Π g 0 0 band. Calculations are made to identify the vibronic levels of A 2 Π u capable of interacting with the B 2 Σ u + (0, 0, 0) level. The discrepancy concerning the apparent population ratio of the A 2 Π u and B 2 Σ u + states of CO 2 + , formed by photoionization of CO 2 , between the results of photoelectron spectroscopy and those of CO 2 + , ion fluorescence intensities is explained as due to interelectronic state coupling. The effects of this discrepancy on the determination of cross sections for forming the B 2 Σ u + and A 2 Π u states by various excitation mechanisms are discussed.

Triplet state absorption studies of C70 in benzene solution

Abstract Pulse radiolysis and laser flash photolysis have been used to determine properties of the lowest triplet state of C70 in benzene at room temperature, including its absorption spectrum between 300 and 1620 nm, its molar absorption coefficients (ϵT=3800 dm3 mol−1 cm−1 at 970 nm) and its quantum yield of formation in benzene T=0.76±0.15. Measurements were also made of the quantum yield Δ=0.82±0.15 for the C70 sensitised formation of O2(1Δg). Transition assignments, oscillator strengths, and optical limiting properties of C70 are discussed.

Jahn—Teller effects in substituted benzene cations. II. Gas phase emission spectrum of 1, 3, 5-C6F3D+3 and comparison with 1, 3, 5-C6F3H+3

Abstract The emission spectrum of 1, 3, 5-C 6 F 3 D + 3 in the 4376–5435 A region has been photographed and vibronically analyzed. The transition is B 2 A″ 2 -X 2 E″. Vibrational frequencies were assigned using the isodynamic molecule method as previously done in analyzing the 1, 3, 5-C 6 F 3 H + 3 spectrum. Out-of-plane a″ 2 and e″ modes are active in even quanta only and are associated with sequence bands a′ 1 and e′ modes are active in both odd and even quanta. The e′ mode activity is shown to be due to dynamic Jahn—Teller effects in the X 2 E″ state. A brief review of the relevant theory leads to a discussion of criteria for identifying Jahn—Teller spectral effects. Two importants parameters enter into the linear coupling model calculations: D , the interaction constant, and ω, the deperturbed frequency of the Jahn—Teller active mode. These parameters are determined by fitting calculated band positions and intensities to experimental results. It is shown how to resolve possible ambiguities in these determinations, D and ω values were obtained for four e′ modes of 1, 3, 5-C 6 F 3 D + 3 and 1, 3, 5-C 6 F 3 + 3 , modes 6, 7, 8 and 9. The corresponding Jahn—Teller barrier heights for interconversion between the lowest energy asymmetric configurations were evaluated for the two ions.

Transmembrane electron transport mediated by photoexcited fullerenes

Abstract Experiments were caried out to confirm the existence of nanoampere mm-2 currents photoinduced in bilayer lipid membranes (≈40 A width) containing C60 or C70 fullerenes and to explore possible mechanism of electron transport in these model light harvesting and conversion systems. Thermodynamic considerations indicate the electron transfer occurs via reduction of fullerene triplets. This supported by triplet quenching experiments. No photocurrent change was observed when an ionic gradient was imposed on the membrane or when its ionic permeability was modified. This is consistent with an eletron-hopping mechanism. The following parameters were varied: membrane thickness, fullerenes concentration, applied membrane potential, light intensity and intermittency. The results indicate that electron transport is mediated by fullerene aggregates that span the membrane and that light excitation can either enhance or diminish fullerene aggregation and electron transfer, according to its intensity.

Involvement of C60 fullerene monomers and aggregates in the photoconductivity of ultrathin bilayer lipid membranes

Abstract Due to its hydrophobic character, the hydrocarbon core of ultrathin phospholipid bilayer membranes lends itself to the insertion of fullerenes. When doped with C 60 these membranes are photoconducting, the photoconductivity being affected by the presence of quenchers of the excited triplet state such as oxygen. The photoconductivity involves trans-membrane electron transfer; its dependence on C 60 concentration and on the excitation wavelength suggests the participation of small aggregates in the membrane. As derived from time-resolved triplet absorption and fluorescence quenching measurements, the suggested mechanism makes more precise (i) the role of the aggregates which might act as a collecting antenna for the benefit of the monomers, and (ii) the role of the monomers which have a much higher yield of photoinduced triplet state formation than the aggregates.

Non-radiative transitions. I. S1 ⇝ S0 internal conversion calculations in benzene and deuterobenzene

Abstract Non-radiative rate calculations for the S 1 ⇝ S 0 transition are presented. Complete vibrational basis sets are used. A Morse oscillator potential is assumed, for both the ν 1 and ν 2 vibrations. It is shown that the ν 1 potential has a dominant influence on the excess energy behaviour of the rates similar to that of the ν 2 . The increase of the normalized non-radiative rate curves with excess energy is consistent with experimental results. A comparison is made with the non-radiative rates determined using the local-mode approximation for the CH- and CD-stretch vibrations.