P. K. Ludwig

Fluorescence Lifetimes of Vibronic States of Naphthalene Vapor in the Region of Excitation from 3080–2150 Å

Fluorescence lifetimes of naphthalene vapor have been measured for narrow band excitation ranging from the first to third excited singlet states. It is found that the inverse lifetimes 1 / τ vary linearly with increasing excitation energy when exciting within the first two bands. For excitation within the third band, the variation is approximately exponential. An attempt is made to interpret the results in terms of theories of radiative and nonradiative transitions.

Ultraviolet Lamp for the Generation of Intense, Constant‐Shape Pulses in the Subnanosecond Region

A commonly available mercury contact relay is modified to make a uv lamp which yields a spark with approximately 100 times the intensity of the original relay and with constant characteristic risetime of 0.4 nsec, decay time of 0.55 nsec, and half‐width of 0.5 nsec. The lamp can be used not only for fluorescence decay‐time studies, but also for electrical applications in the subnanosecond region. It has been operated in the range 3 to 50 000 pulses per second.

TIME DEPENDENCE OF ION-PAIR RECOMBINATION RATES IN LIQUIDS OF LOW DIELECTRIC CONSTANT.

An analog computer method is employed to solve the differential equation of diffusion when applied to ensembles of isolated ion pairs. The method permits evaluation of the probability density of the ion pairs as well as relative rates of their neutralization as function of time for a variety of initial conditions. The results are discussed with emphasis on ion neutralization processes in radiation chemistry.

Ionic Processes in High‐Energy‐Induced Luminescence of Liquid Organic Systems

Luminescence of dilute organic scintillators in liquid aliphatic hydrocarbon solution excited by very short pulses of x rays decays nonexponentially. In contrast, excitation by pulsed uv gives luminescence the decay law of which can be described by an exponential function. A satisfactory explanation is that under high‐energy radiation, the mechanism of excitation of the scintillator involves ion recombination, while direct excitation and energy transfer play a minor role. Additives (nitrous oxide, carbon tetrachloride, piperidine) which do not affect the excited scintillator but react with charged species such as ions and electrons show a marked effect on the luminescence of the high‐energy‐excited systems only, a result which supports the ionic mechanism. An attempt is made, on the basis of simplified assumptions, to derive from the luminescence intensity–time curves a distribution function of the secondary electrons released by the interaction of the x rays with the system.

Fluorescence Lifetimes of Vibronic States of Benzene in the Vapor Phase

The time dependence of benzene vapor fluorescence is studied as function of vapor pressure for excitation in the neighborhood of 2600, 2530, and 2470 A. At pressures above about 20 torr the decay appears exponential and independent of excitation wavelength. At lower pressure, deviations from exponential decay are observed for 2530‐ and 2470‐A excitation. The decay times obtained under different conditions are combined with steady‐state data available in the literature and several rate constants derived. The result is related to discussions of the mechanism of deactivation of excited benzene molecules.

Luminescence Quenching and Decay Processes in Multicomponent Systems: n‐Nonane+p‐Xylene+CCl4

A monophoton technique to measure fast luminescence decay and utilizing pulsed uv excitation has been employed to determine specific rates of excitation quenching in systems containing n‐nonane+p‐xylene+CCl4, with n‐nonane serving as an optically inert solvent, p‐xylene as scintillator, and CCl4 as quencher. The decay‐time data are employed to evaluate the characteristic rate parameters kq for quenching of p‐xylene luminescence by CCl4. Values so obtained are in good agreement with values obtained from steady‐state studies. The variation in kq with p‐xylene concentration is consistent with the view that energy migration as such via p‐xylene molecules occurs in addition to material diffusion of excited molecules. Dynamic and steady‐state measurements of self‐quenching in p‐xylene+n‐nonane systems indicate that, if excimer formation in p‐xylene occurs, it is followed by a very rapid nonradiative deactivation of the excimer. Furthermore, differences in the self‐quenching behavior as determined with steady‐st...

Question of Energy Migration in Liquid Benzene and Some of Its Derivatives. I. Self‐Quenching Studies

The luminescence of benzene, toluene, p‐xylene, and mesitylene and their solutions in n‐nonane is studied under conditions of steady‐state and nonsteady‐state uv excitation. It is shown that the often‐used simple reaction mechanism involving only excited monomers cannot account for the results under nonsteady‐state excitation. An analysis of the data, which includes formation and reactions of excimers in these systems, is presented.

Fluorescence Decay of Liquid Scintillator Systems After High-Energy Excitation

Abstract The development of fast electro-optical instrumentation over the last ten years1–9 has not only extended dynamic luminescence studies into the range of a few nanoseconds but also made such studies possible with low light intensities. Measurement of luminescence-decay curves over several decades of intensity variation are now feasible.

Question of Energy Migration in Liquid Benzene and Some of Its Derivatives. II. Studies of Foreign Quenching of Luminescence

Quenching of luminescence of benzene, toluene, p‐xylene, and mesitylene and of their solutions in n‐nonane by CCl4 is studied under conditions of steady‐state and nonsteady‐state uv excitation. An increase of the specific rates of quenching with concentration of aromatic is observed when the data are treated by a simple Stern–Volmer reaction mechanism. Consideration of the presence of excimers in these systems shows, however, that the specific rate of quenching of excited monomers of the aromatics is invariant with concentration. The specific rate of excimer quenching, on the other hand, is found to be concentration dependent. A simple model is suggested to explain the observations, and the validity of the hypothesis regarding energy migration in these systems is discussed.