Michael R. Topp

Activation-controlled hydrogen abstraction by benzophenone triplet

Abstract Through nanosecond flash photolysis measurements, activation energies for hydrogen abstraction by benzophenone triplet have been shown to correlate with solvent molecular structure. We demonstrate that two successive processes result in photoproduct generation. One, triplet quenching, is independent of macroscopic viscosity, and is not simply reflected in the observed quantum yield. The second process involves separation of two caged radicals, and does not occur in rigid media.

Spectroscopy of benzene complexes with perylene and other aromatic species

Abstract The fluorescence excitation spectra are reported of complexes of benzene with perylene and anthracene. Some correlations are established between the results of simple potential energy calculations and the experimental data, in regard to the structures and binding energies of a number of different complexes. In particular, while the anthracenebenzene complex is somewhat similar to one postulated form of the benzene dimer where parallel, but displaced rings are found, benzene appears to occupy a center-of-mass position on perylene. The spectrum of perylenebenzene also shows evidence of strong coupling between internal modes of the two molecules near 1600 cm −1 . The other major perturbation of the spectrum involves damping of the out-of-plane “butterfly” motion of perylene by the adsorbed benzene molecule. The principal low-frequency mode, known to be a v = 0 → v = 2 transition in the bare molecule, at 95 cm −1 , is replaced in the benzene 1:1 and 2:1 complexes by a transition, at 68 cm −1 . Furthermore, unlike the bare molecule, the ground state frequency of the perturbed out-of-plane mode is very similar to that of the excited state. Indications from these data support the idea that the equilibrium out-of-plane distortion of perylene in a complex with benzene is rather different from that observed in the bare perylene molecule.

PICOSECOND LUMINESCENCE DETECTION USING TYPE-II PHASE-MATCHED FREQUENCY CONVERSION

Abstract In order to improve on standard Kerr light gate methods for subnanosecond fluorescence measurements with solid-state lasers, we have used a method for sum and difference frequency generation in a type-11 phase-matched KDP crystal. The result is to generate up- and down-converted radiation from molecular fluorescence with a pulse-correlated time-resolution of less than 10 ps using multiple shots. The wavelength range for fluorescence detection, using KDP alone, is continuous from about 240 nm to greater than 1 micron. The method has a considerably higher signal-to-background ratio than the optical Kerr-cell, easily isolating 5 ps segments of a 10 ns fluorescence profile with high signal-to-noise ratio. Some examples are presented of the application of the method.

Isomer effects on vibrational energy relaxation in perylene-argon complexes

Abstract Fluorescence spectroscopic measurements have been carried out on jet-cooled complexes of perylene with a number of guest species ranging from argon to small hydrocarbons. In the case of argon, a sequence of red-shifted absorption bands is assigned to a group of aggregates involving up to five guest atoms. Calculations using empirical atom-atom pair potentials have allowed unambiguous assignment of eight different perylene-argon complexes. Spectrally dispersed fluorescence measurements have studied the effect of internal energy on the IVR and predissociation processes. In particular, the two perylene diargon isomers are shown to exhibit different rates for IVR. It has been shown that arson forms a series of organised, readily identifiable complexes with perylene in a supersonic jet. On excitation of the different isomeric forms of the 2:1 complex, different rates for vibrational energy redistribution have been found. The greatest difference was observed at low vibrational energies of the parent, which confirmed that low-frequency van der Waals modes were responsible. Although more detailed calculations will appear elsewhere, the most obvious difference between the two isomeric forms is the exchange of two low-frequency x-y plane vibrations (≈5 cm −1 ) for a hindered rotation and a substantially higher-frequency (15–30 cm −1 ) argon-argon stretching mode. Photodissociation is also readily observed, confirming the computed values for the argon-perylene binding energies. Finally, strong resonance fluorescence is observed, accompanied by (relaxed) emission from the singly dissociated species when either the 1:1 or either 2:1 isomer is excited with a (parent) vibrational energy of 705 cm −1 . Thus, in spite of the presence of the dissociative pathway, all observable emission from the undissociated species appears to originate from a state in the “small molecule” limit, which survives for about 4 ns.

Direct time-resolution of the stokes fluorescence shift of a polar molecular in a polar solvent

Abstract Direct picosecond-time-resolved measurements have been made of the temperature-dependent Stokes shift of the fluorescence spectrum of 2-amino,7-nitrofluorene in 2-propanol. Frequency-conversion gating was used to obtain time-resolution ≈ 10 ps and to maintain a high signal-to-noise ratio. Fluorescence decay times measured at different wavelengths were found to vary by up to an order of magnitude. These findings are interpreted in terms of independent relaxation processes in 2-propanol: solvent polarisation and solvent-assisted fluorescence quenching.

Group dispersion effects in picosecond spectroscopy

Abstract Pulse-dispersion effects are shown to be an important consideration in picosecond spectroscopy. Frequency-dispersion is re-emphasized, since it accounts for certain recent misinterpretations in the literature. Transverse dispersion is shown to have advantages in the production of high-intensity, controllable-duration pulses.

Spectral hole burning in free perylene and in small clusters with methane and alkyl halides

Abstract Pump-probe experiments have been used to sample hole-burning phenomena in the spectra of molecular clusters isolated under supersonic-jet conditions. By using two dye-laser pulses of −1 bandwidth, spaced by ≈60 ns, the spectra of different components of a supersonic expansion can be readily separated. In particular, details such as low-frequency mode structure can be examined for different structural isomers at the same level of aggregation. Data are given for methane/perylene clusters up to 3:1 and for different isomers of perylene/1-chlorobutane at the 1:1 level of aggregation.

Picosecond time-resolved S2→S0 fluorescence of xanthione in different fluid solvents

Time-correlated single-photon counting experiments at ≈ 25 ps resolution have been used to measure the S2 fluorescence decay time of xanthione in different fluid solvents. Systematic differences in S2 lifetime in the range 20–30 ps are revealed for a homologous series of n-alkane solvents, and an increase in lifetime for extensive chain branching is also noted. Solutions of xanthione in aromatic solvents and in alcohols are found to exhibit fluorescence decay limes < 20 ps. Little evidence is seen which would support a nonradiative relaxation mechanism involving Franck-Condon selective energy transfer to high-energy vibrational modes of the solvent. Instead, the current results provide support for a mechanism in which a reversible solvent-solute chemical reaction, probably involving bond insertion, is responsible for the solvent dependence of the S2 fluorescence decay time.

Biphotonic laser excitation of upper singlet state fluorescence in organic dyes

Abstract Fluorescence spectra are presented, originating from upper excited singlet states in three xanthene dyes. The experimental method, using biphotonic pulsed laser excitation, is shown to have advantages over single-photon ultraviolet irradiation. It is further shown that the observed spectra depend on the nature and timing of the excitation frequencies.

Low quantum-yield molecular fluorescence: excitation energy dependence and fluorescence polarisation in xanthene dyes

Abstract Consecutive two-photon laser excitation has been used to populate elevated singlet states of the xanthene dyes rhodamine 6G and rhodamine B. Fluorescence has been observed from several states in each molecule, terminating in the ground state, in the range 240-400 nm. It is demonstrated that the shape of the spectrum depends on both excitation energy and polarisation, and the hydrocarbon rubrene is used as an example of the excitation intensity dependence of the weak emission spectrum from a fluid solution.