Integrated Oscillator Strength Research Articles

Optical properties of a small-particle composite

Room-temperature reflectance measurements over frequencies between the far infrared and the ultraviolet have been made on composites of silver small particles and potassium chloride grains. The optical properties of these composites were obtained by Kramers-Kronig analysis. Comparison of the data with the Maxwell-Garnett theory and the effective-medium approximation showed that the effective-medium approximation better described the data. A sum-rule analysis indicates that the expected linear relationship between the volume fraction of metal and the integrated oscillator strength is obeyed.

Excited-state dynamics of the isolated ultracold ovalene molecule

In this paper we report the results of an experimental study of the energetics and the dynamics of the first two electronically excited singlet states S1 and S2 of the ovalene molecule (C32H14) seeded in supersonic beams of Ar and of Kr. By an optimal choice of the stagnation pressure (100–300 Torr) of the Ar and Kr carrier gases expanded through a 200 m nozzle, efficient rotational–vibrational cooling of the large molecule was accomplished, while the spectrum was not obscured by the formation of van der Waals complexes. We have interrogated the energy-resolved fluorescence action spectrum, the time-resolved fluorescence, as well as the energy-resolved fluorescence of the bare, ultracold, isolated, large molecule. The first spin-allowed 1S0(1A1g)→1S1(1B3u−) transition with an origin at 21 449 cm−1, exhibits a sparse, well-resolved, vibrational level structure at low excess vibrational energies, EV, while the higher energy range of S1 reveals a congested level structure corresponding to overlapping resonances within a single electronic manifold. The time-resolved decay lifetimes of photoselected single vibronic levels of S1 are in the range 1.7–2.2 msec, exhibiting a very weak energy dependence on EV and being close to the pure radiative lifetimes of S1. The second S0(1A1g)→S2(1B2u) transition of ovalene reveals an irregular, closely-spaced structure, resulting from strong interstate coupling between the S2 and the S1 electronic states, whose electronic origins are separated by ∼1800 cm−1. The strength function for the scrambling of the S2 origin with the background S1 manifold was extracted from the spectroscopic data, providing information on the energetics and on the energy dependence of interstate coupling. The time-resolved decay lifetimes of the scrambled S2 molecular eigenstates are longer by about two orders of magnitude than the pure radiative lifetimes of S2, as estimated from the integrated oscillator strength, manifesting the dynamic consequences of the intermediate level structure. The energy-resolved fluorescence resulting from excitation into the S2 state exhibits emission in the S1→S0 energy region, which is in accord with the large dilution factor for the S2–S1 scrambling. The structure of the energy-resolved emission spectrum may provide some information regarding intrastate vibrational energy redistribution in this large molecule.

Master equations for photochemistry with intense infrared light (III) the influence of molecular parameters in URIMIR

AbstractThe influence of molecular parameters on the rate of unimolecular reactions induced by monochromatic infrared radiation (URIMIR) is considered in the range of validity of a Pauli master equation. Solutions of the master equation are obtained for a model series of bond fission reactions. A two parameter activation equation for the time dependent rate coefficient is established. The dependence of the steady state rate coefficient, the activation time, and further relevant rate parameters upon the threshold energy for reaction, the excitation quantum, the number of vibrational modes, and the integrated oscillator strength of the fundamental transition is established and summarized in three simple rules. Quantitative calculations for the case B intensity fall‐off are presented. The possibility of an absolute estimate of the rate parameters is discussed with the example of the URIMIR of CF3I.

The absolute photoabsorption cross section of atomic sodium in the region above the 2p threshold (45-250 eV)

The photoabsorption cross section of sodium has been determined in the energy range 45-250 eV using a heat-pipe absorption cell and a synchrotron light source. The results are compared with previous relative values and with a single-electron calculation. The integrated oscillator strength for the n=2 shell is compared with that of neon. The problems of determining accurate cross sections for metal vapours in the extreme ultraviolet spectral region are discussed in some detail.

Transition moments for the B 3Σ−u–X 3Σ−g and 3Πu–X 3Σ−g transitions in O2

Multiconfiguration self-consistent field wavefunctions are used to calculate the transition moments for the two lowest dipole allowed transitions in O2. The transition matrix element for the B–X transition calculated at Re of the ground state corresponds to an oscillator strength of 0.18, in good agreement with the experimental integrated oscillator strength 0.16 of the Schumann–Runge continuum. The B–X transition moment is calculated as a function of internuclear separation. The transition matrix element to the repulsive valence 3Πu state calculated at Re of the ground state corresponds to an oscillator strength of 0.003. Although the weak 3Πu absorption continuum is masked by the strong Schumann–Runge continuum below 1750 Å, this 3Πu absorption may contribute to the background continuum in the Schumann–Runge band region at low temperature.

Electron energy-loss spectroscopy of naphthalene vapor

The naphthalene energy-loss spectrum from 3 to 16 eV obtained for 100 eV incident electrons closely resembles photoabsoprtion results. The need to include Rydberg configurations for any theoretical analysis of the spectrum is emphasized. The data yield an integrated oscillator strength of 8.6 below 15.1 eV.

Long Radiative Lifetimes of Small Molecules

In this paper, we apply a theory of electronic relaxation in polyatomic molecules for the study of the anomalously long radiative lifetimes of NO2, CS2, and SO2. We have classified medium-size molecules which exhibit intramolecular vibronic coupling into two intermediate cases, which we call the sparse intermediate case and the dense intermediate case, characterized by low and high spacing of the vibronic levels relative to the radiative width. The radiative decay in the sparse intermediate case was considered in detail, taking advantage of the coarse level spacing and the extremely short intramolecular recurrence time encountered in this case. From our model calculations, we conclude that: (a) in the sparse case, the radiative decay rate is characterized by a superposition of slowly varying exponentials; (b) the mean radiative lifetime is expressed as a radiative lifetime calculated from the integrated oscillator strength and “diluted” by the number of states within the half-width of the manifold of coupled levels; (c) no intramolecular electronic relaxation processes are encountered in this case; (d) a qualitative interpretation of the complex molecular spectra is provided.