T.D.S. Hamilton

The influence of iodide ions on the phosphorescence of aromatic hydrocarbons

Abstract The phosphorescence decay functions and spectra of ethanol solutions of six aromatic molecules (AM), naphthalcne- hg , and - d 8 , phenanthrene- h 10 and - d 10 , and chrysene- h 12 and - d 12 were observed in the presence of different KI concentrations at 77 K. Moment analysis of the decay functions shows that the dependence of the AM phosphorescence rate parameter k PT on its distance from a heavy-atom (HA) iodide ion is described by a Dexter-type relation. The HA enhancement of k PT depends on the AM triplet energy E T . The HA-perturbed AM phosphorescence spectrum P (ν) includes not only an intense component, similar to the unperturbed AM spectrum P o (ν), due to the second-order exchange mechanism, but also a structured subspectrum P x (ν) red-shifted by 600–900 cm −1 relative to P o (ν). P x (ν) originates at an asymmetric out-of-plane CH vibrational mode, and it is attributed to a third-order HA perturbation, involving vibronic coupling.

Assignment of the vibrational bands of the 1B1u ← 1A1g transition in solid benzene films

Abstract Low temperature absorption spectra of benzene films were observed in the 1 B 1u ← 1 A 1g transition region. The origins of the two progressions of the totally symmetric vibration v 2 (a 1g ) are assigned to the crystal-field-induced 0—0 transition and to the false origin 0 + v 1g (e 2g ).

Isosbestic points in emission spectra

Abstract This paper discusses the conditions necessary for the existence, over a certain range of temperature, of an isosbestic point in the fluorescence spectra of a compound which emits from the monomer as well as the excimer state. It is proved that the conditions which give rise to isosbestic points in the absorption spectra are not the same as those which lead to isosbestic points in emission. It is shown that, when there is an overlap between the monomer and excimer spectra, an isosbestic point will be observed in the fluorescence spectra recorded within a certain range of temperature if the rate parameters for the nonradiative relaxation of the monomer and excimer are independent of temperature over that range of temperature. Experimental investigations on pyrene solutions in ethanol indicate that the latter condition is satisfied at temperatures lower than −40°C or higher than 50°C.

Low temperature spectra of the 1B1u ←1A1g transition of C6H6, sym-C6H3D3 and C6D6

The low temperature thin-film absorption spectra of C6H6, sym-C6H3D3 and C6D6 in the 1 B 1u ←1 A 1g transition region are analysed in terms of a 0, 0 crystal-field-induced origin, and v 18(e 2g ), v 7(b 2g ) and v 16(e 2g ) false origins. The 0, 0 + v 2(α1g ) band is clearly resolved in sym-C6H3D3 whilst it merges with 0, 0 + v 7(b 2g ) in C6H6. Recent polarization measurements of the C6H6 2150 A band are shown to be consistent with a 0, 0 assignment. The 0, 0 origin is also seen in the fluorescence excitation spectrum of crystal C6H6.

The benzene—oxygen complex in the vapour phase

Abstract The absorption spectrum of the benzene—oxygen contact complex from 2350 to 2120 A has been derived from the difference in the absorption spectra of benzene vapour (70 torr) in the presence and absence of oxygen (760 torr). The complex has an absorption maximum at 2190 A (5.65 eV), corresponding to a formation energy of 0.42 eV from 1B1U benzene and 3Σ−g oxygen, and of 2.90 eV from the benzene cation and oxygen anion. The excited complex has an electric dipole length of 5.0 A. The data are compared with those for benzene complexes with other electron acceptors.

Deuteration effects on the phosphorescence of aromatic hydrocarbons

Observations of the phosphorescence decay of isotopic mixtures of naphthalene-hs and -ds, phenanthrene-h10 and -d10, and chrysene-h12 and -d12, in ethanol solutions at 77 K are analysed to determine the ratio kPTH/kPTD of the triplet radiative rate parameters of the perprotonated and perdeuterated compounds. The ratio is 1.20 (±0.07) for naphthalene, 1.39 (±0.06) for phenanthrene, and 0.98 (±0.04) for chrysene.

Site induced structures in the lowest electronic transitions of naphthalene-H8 and -D8 in rare-gas matrices

Abstract Phosphorescence spectra and S1 ← S0 excitation spectra of naphthalene-h8 and -d8 in Ar, Kr and Xe matrices have been investigated with medium spectral resolution. Observed site structures in the spectra are depemdent on deposition conditions and annealing. Comparison of calculated and experimentally observed site shifts suggests that the matrix structure is preferentially ccp. Possible assignments of the site structures are given. Geometrical changes of naphthalene in excited states are important factors determining site structure in electronic transitions.

Luminescence excitation study of benzene-doped rare gas crystals

Abstract Luminescence of benzene (fluorescence from 1B2u and phosphorescence from aB1u) was recorded following excitation in an energy range that covered the excitonic absorption bands of the host crystals, Ne, Ar, Kr and Xe as well as the absorption bands of the benzene dopant. In the host exciton region the luminescence quantum yield is considerably higher than in the benzene lB1u, 1E1u and ‘Rydberg’ regions. This high yield demonstrates an efficient transfer of energy from the exciton states to benzene. It is also inferred that non-radiative relaxation competes more effectively with processes following excitation into the singlet manifold than with those within the triplet system. In the host exciton region and for very thin samples, luminescence yield maxima appear at the positions of absorption maxima. In thicker samples, however, dips appear in the luminescence yield spectra which coincide with the absorption maxima. An analysis of this penomenon is given in terms of a model which takes into account...

Photoelectron emission from pure and benzene-doped rare-gas crystals

Photoelectron emission from pure and benzene-doped solid samples of Ne, Ar and Kr was measured following excitation of the range of exciton states (below the direct threshold, i.e. E or=ETH( identical to EG-VO). The photoemission yield in the exciton range (process (a)) shows an efficient transfer of energy from exciton states to C6H6, and provides information about the free exciton kinetics. The drift length of electrons in argon and krypton has been estimated from the thickness dependence of photoemission yield.

Luminescence excitation spectra of toluene and mesitylene in rare-gas matrices

Abstract Total luminescence yield of toluene and mesitylene, isolated in rare-gas matrices, has been measured as a function of excitation energy in the energy range covering the absorption region of both host and guest. Energy transfer from the host to the dopant is apparent. An increase in the luminescence yield is observed at the host exciton state. Luminescence excitation spectrum of pure mesitylene is presented.