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The phenol-ammonia 1:1 complex has been investigated by mass resolved hole burning spectroscopy and ab initio methods at the HF/6-31G(d,p) and HF/6-31ϩϩG(d, p) levels of theory. By means of spectral hole burning four bands in the region of intermolecular vibrations could be assigned to the 1:1 complex. The ab initio computed cluster structure and its normal vibrations are reported and compared to the experimental results. Anharmonic calculations were carried out for the ammonia torsion. The results are compared to structurally related complexes.

The scientific investigation of fundamental problems plays a decisive role in understanding the mode of action and the consequences of the use of lasers on biological material. One of these fundamental aspects is the investigation of the ablation threshold of various laser wavelengths in dental enamel. Knowledge of the relationships and influencing factors in the laser ablation of hard tooth tissue constitutes the basis for use in patients and the introduction of new indications. The present paper examines the ablation threshold of an Er:YAG laser (lambda=2.94 micro m) and an Er:YSGG laser (lambda=2.79 micro m) in human dental enamel. To this end, 130 enamel samples were taken from wisdom teeth and treated with increasing energy densities of 2-40 J/cm(2). The sample material was mounted and irradiated on an automated linear micropositioner. Treatment was performed with a pulse duration of tau(P(FWHM)) approximately 150 micro s and a pulse repetition rate of 5 Hz for both wavelengths. The repetition rate of the laser and the feed rate of the micropositioner resulted in overlapping of the single pulses. The surface changes were assessed by means of reflected light and scanning electron microscopy. On the basis of the results, it was possible to identify an energy density range as the ablation threshold for both the Er:YAG and the Er:YSGG laser. With the Er:YAG laser, the transition was found in an energy density range of 9-11 J/cm(2). The range for the Er:YSGG laser was slightly higher at 10-14 J/cm(2).

The rate of the electrocyclic ring opening of 7-dehydrocholesterol to previtamin D is investigated by transient absorption spectroscopy with a time resolution better than 300 fs. The dehydrocholesterol, which is a derivative of 1,3-cyclohexadiene, is excited around 267 nm. The primary product, the s-cis,Z,s-cis conformer of the triene previtamin D, appears with a time constant of 5.2 ps. This result is consistent with literature data. We found that it is temperature independent. So there is no activation energy for this electrocyclic ring opening. A barrierless process, on the other hand, is expected to proceed much faster (in about 10 -13 s), unless there is an entropy of activation. The results suggest that the molecule reaches within a few femtoseconds the lowest excited state (2A 1 ) of the product, from where it goes on through an entropic bottleneck. The primary product isomerizes thermally to the stable s-cis,Z,s-trans conformer of previtamin D within 125 ps in ethanol at room temperature. This time is much longer than the reported corresponding time in the cyclohexadiene/ hexatriene system. We found that it depends on the solvent viscosity and on the temperature. The activation energy was determined to be 15.5 ( 1.0 kJ/mol.