Two-photon spectra of gases by three-wave mixing

We show that the two-photon resonances in the third-order susceptibility can be exploited to yield two-photon spectra of molecular gases at moderately high spectral resolution. This form of spectroscopy does not depend on the occurrence of processes (such as fluorescence of photoionization) leading to indirect methods of two-photon absorption. The method is direct and leads in principle to values for a two-photon cross section. Comparisons of two-photon and coherent anti-Stokes Raman resonances leads to ratios of Raman and two-proton cross sections independent of the laser powers and spatial characteristics. The technique is documented with rotationally resolved spectra of SO₂ and NO. A value for |α_xxα_yy| of 1.5 × 10^?52 cm⁶ was measured for the O₁₂ (6$\text{built:1}\text{2}$) component of the (A)²Σ ← (X)² Π_{$\text{1}\text{2}$} transition of nitric oxide.     

Vibronic mixing and vibronic coupling of nearby electronic states. III. Stark spectra of 2,6-p-benzoquinone-d2

The Stark effect on the origin of the lowest triplet and single state of 2,6-p-benzoquinone-d₂ single crystals has been measured in absorption. Previous work had shown that the corresponding bands of the centrosymmetric pure proto and deutero compounds can be analysed as inversion doublets in a symmetric double well potential. The asymmetric substitution is shown to distort the double well such that the excited molecule becomes polar; the effect is greatest in the singlet state, where we find dipole moments of about 2 D. The results are analysed in considering the asymmetric isotopic substitution as a perturbation, the magnitude of which is related to parameters describing the double well potential.     

Pathways of spontaneous and photoinduced hydrogen tunnelling in organic crystals

Optical excitation at liquid helium temperature of pentacene molecules embedded in benzoic acid crystals gives rise to defects in the environment of pentacene. By deuteration of the acid proton of the matrix and/or selective deuteration of the pentacene molecules, it has been proven that these defects are due to the displacement of acid protons of the matrix and are produced by reversible proton transfer between the benzoic acid host and the pentacene guest. The defect sites are unstable and decay with lifetimes ranging from subseconds to hours. Different defect sites also interconvert spontaneously in the dark or after absorption of a second photon.     

Assignment of the lowest triplet state of 9,10-anthraquinone single crystals

The lowest triplet state of 9,10-anthraquinone single crystals is assigned as ³B_1g located at 22153 cm^?1. The pure electronic transition to this state is forbidden in the centrosymmetric crystal, but is induced in an electric field. Combined Stark—Zeeman measurements provide an unambiguous symmetric assignment of this transition and clear up the difficulties of previous assignments.     

Untersuchung von Milch und Molkereiprodukten

Ohne Zusammenfassung     

Optical dephasing in a glass-like system: a photon echo study of pentacene in benzoic acid

Optical absorption and picosecond photon echo experiments are used to study the dephasing of pentacene in benzoic acid. It is shown that, while the absorption spectrum of pentacene is effected by proton transfer in the benzoic acid dimer, the dephasing is caused by elastic and inelastic phonon scattering processes.