Mark O. Trulson

Raman cross section measurements in the visible and ultraviolet using an integrating cavity: Application to benzene, cyclohexane, and cacodylate

A new technique has been developed for the measurement of absolute Raman scattering cross sections. The Raman scattering of a sample in an integrating cavity is compared to the elastic scattering from a reference scatterer of unit yield. The fundamental advantage of this approach is that it permits the measurement of absolute cross sections throughout the visible and UV regions with a simple experimental apparatus and procedure. The 992 cm−1 mode of benzene was measured in the visible region and compared satisfactorily to the values obtained by Kato and Takuma [J. Chem. Phys. 54, 5398 (1971)] and Abe et al. [J. Raman Spectrosc. 6, 38 (1977)]. The scattering cross section of the 608 cm−1 mode of sodium cacodylate, an aqueous intensity standard, was measured from 647 to 240 nm. Analysis within the A‐term formalism provided a functional form for interpolation between measured points and revealed that the 608 cm−1 cacodylate mode derives intensity from states in the vicinity of 83 kK (kilokaiser). The 802 cm−...

Excited state structure and femtosecond ring‐opening dynamics of 1,3‐cyclohexadiene from absolute resonance Raman intensities

Resonance Raman spectra of 1,3‐cyclohexadiene have been obtained with excitation wavelengths ranging from 300 to 253 nm. The absolute excitation profiles were analyzed to determine the initial dynamics that occur following photoexcitation in the lowest energy 260 nm 1B2 absorption band. The initial atomic displacements parallel the Woodward–Hoffmann, symmetry‐allowed, ring‐opening pathway towards all‐cis‐hexatriene. The potential surface displacements in the optically prepared 1B2 excited state are, however, a small fraction of the total distance along the reaction coordinate. The low absolute scattering cross sections indicate an excited state dephasing time of 10−14 s, and the fluorescence quantum yield of 2×10−6 shows that the total dephasing rate is dominated by ultrafast internal conversion. The Gaussian homogeneous line shape indicates a nonexponential excited electronic state population decay. We attribute these observations to an allowed crossing of the 1B2 state with a second excited 2A1 state th...

Quantitation of homogeneous and inhomogeneous broadening mechanisms in trans‐stilbene using absolute resonance Raman intensities

The sensitivity of resonance Raman cross sections to the excited state homogeneous width is exploited to separate the homogeneous and inhomogeneous contributions to the optical absorption linewidth in trans‐stilbene. Absolute Raman cross sections have been measured using excitation at 356, 299, 282, and 266 nm, and calculations have been performed to model the intensities of the eleven strongly Raman‐active modes as well as the absorption spectrum. A simple analysis neglecting explicit temperature effects requires a homogeneous Lorentzian linewidth (Γ) of 310 cm−1 and an inhomogeneous Gaussian distribution of zero–zero energies with a standard deviation (θ) of 500 cm−1. Consideration of thermal excitation in the lowest‐frequency single‐bond torsional mode significantly improves the fit to the experimental absorption by broadening the spectrum asymmetrically to higher energies, while allowing Γ and θ to be reduced to 120 and 450 cm−1, respectively. The spectral broadening generated by thermal excitation of...

Absolute resonance Raman intensities demonstrate that the spectral broadening induced by the β‐ionone ring in retinal is homogeneous

The sensitivity of absolute resonance Raman cross sections to the excited state homogeneous width is used to examine whether the electronic spectral breadth induced by the β‐ionone ring of retinal is primarily a homogeneous or an inhomogeneous effect. Resonance Raman intensities have been measured for both all‐trans retinal and its 5,6‐dihydro analog. The dihydro analog has a structured absorption spectrum because the broadening effect of the C6–C7 bond is lost when the π system does not extend into the ionone ring. While the two molecules have similar relative Raman intensities, the absolute cross sections are about a factor of two lower in all‐trans retinal, indicating that the ‘‘effective homogeneous linewidth’’ is significantly larger in all‐trans retinal than in the dihydro analog. Calculations carried out to model both the absorption spectra and the resonance Raman intensities require an effective homogeneous linewidth that is more than twice as large for all‐trans retinal (∼1200 cm−1) as for the 5,...

Computer-assisted structure elucidation

Program case, an evolving computer model of the structure elucidation process, treats three major tasks: reduction of chemical and spectral properties to their structural implications, generation of complete molecular structures compatible with the structural features identified, and ranking these structures by comparing predicted and observed spectral properties. Current program status and application will be discussed.