John R. Ackerman

s‐cis octatetraene: Photoproduction and spectroscopic properties

Irradiation of solutions of all‐trans octatetraene in n‐octane and cyclohexane at temperatures below 10 K efficiently produces a photoproduct whose fluorescence and absorption spectra are shifted ∼1200 cm−1 to lower energy than the corresponding features in the all‐trans precursor. High resolution excitation and fluorescence spectra and fluorescence lifetimes for this product have been determined. These data establish that the photoproduct is a noncentrosymmetric octatetraene isomer. The spectroscopic properties and thermal lability of this isomer are consistently interpreted if it is assumed that the photoproduct is generated from all‐trans octatetraene by isomerization about one of the single bonds.

Radiationless decay of 1,3,5,7‐octatetraene

The decay kinetics of the 2 1Ag state of the linear polyene 1,3,5,7‐octatetraene in hydrocarbon solutions have been measured by nanosecond and picosecond techniques from 10 to 320 K. The observed decrease in emission lifetime with increasing temperature is well described in terms of a temperature activated intramolecular radiationless decay process proceeding over a ∼4 kcal Arrhenius barrier. This barrier is not significantly affected by intermolecular interactions such as sample viscosity or phase and may be related to rotation about essential single bonds in the excited state.

s‐cis octatetraene: Ground state barrier for s‐cis to s‐trans isomerization

At temperatures below 10 K photolysis of solutions of all‐trans octatetraene in n‐octane efficiently produce a photoproduct which has been identified as an s‐cis isomer. This paper reports quantitative measurements of the relative concentrations of this photoproduct and the all‐trans precursor as a function of irradiation and temperature. These data establish that the primary photochemical channel open to all‐trans octetraene under these conditions leads to the production of the s‐cis isomer which, at temperatures above 50 K, quantitatively reverts to the all‐trans form. This thermal reversion is well described as an activated first order process proceeding over a barrier of 4.0±0.2 kcal/mol. A model for computing isomerization rate constants from a simple one‐dimensional potential also quantitatively fits the data. In this model the barrier height for s‐cis to s‐trans conversion is 3.16 kcal/mol, the enthalpy of the s‐cis species is 2.88 kcal/mol above that of the all‐trans and the internal rotor is a te...

Photochemical site conversion: Octatetraene in n‐octane

Irradiation of n‐alkane solutions of all‐trans octatetraene at temperatures below 10 K produces a number of photoproducts. This paper reports spectroscopic and thermodynamic properties of one of these which has been identified to be the all‐trans isomer in a second, less stable crystallographic site than that produced by slowly cooling through a succession of equilibrium states. This identification follows from the excited state lifetime and high resolution fluorescence and one‐ and two‐photon excitation spectra for this photoproduct in n‐octane which show that: (1) states of this photoproduct are only slightly shifted with respect to those of the all‐trans precursor; (2) the fluorescence lifetime is nearly the same as that measured previously for the all‐trans precursor; (3) the photoproduct is only stable at temperatures below 20 K, at higher temperatures it reverts to the all‐trans isomer in the minimum energy site; and (4) the photoproduct is centrosymmetric. In the ground state the conversion of this...

Optimization of the field of view of a Brillouin-enhanced four-wave-mixing phase conjugator

A Brillouin-enhanced four-wave mixing phase conjugator featuring a field of view that varies by approximately an order of magnitude, i.e., from 2 to 20 mrad, is described. We control this variable field of view by utilizing a slightly noncollinear pumping geometry and by adjusting the pump-pump frequency difference to optimize the phase mismatch for the overall process. A model that describes the interplay between frequency and geometry to the Brillouin-enhanced four-wave-mixing phase mismatch is found to predict the expandable field of view.

Spectral characteristics of Brillouin-enhanced four-wave mixing for pulsed and CW inputs

Optical phase conjugators exhibiting extremely high gains and low thresholds have been developed over the last decade by researchers using Brillouin-enhanced four wave mixing (BEFWM). This phenomenon, based upon the interaction between light and acoustic waves in a nonlinear medium, is still not fully characterized theoretically or experimentally. In particular, the frequency relationships among the four interacting light waves plays a crucial role. In this paper, experiments investigating the spectral nature of BEFWM using two tunable single mode YAG lasers are described.