The photosensitized crosslinking of polyenes with polythiols has been demonstrated to be a free‐radical chain reaction whose apparent quantum yield is in the range of 2 × 102–4 × 102 mole/einstein. Initiation occurs by abstraction of a hydrogen atom from the thiol group by the excited n,π* triplet of the ketonic photosensitizer. Kinetic studies of model systems have shown that electron‐donating substituents attached or close to the double bond accelerate the thiol addition while electron‐withdrawing groups decrease the rate. With a given olefin, slight differences in reaction rates are observed depending on the structure of the thiol. Mercaptopropionate esters are more reactive than mercaptoacetates which, in turn, are more reactive than alkane thiols. Cure rates for polyene–polythiol systems are in accord with results found for the model systems. Aromatic carbonyl compounds are the most effective sensitizers for this reaction. To a first approximation, the effectiveness of the sensitizer depends only upon the absorbance. No correlation was found between photoactivity and the triplet energies or lifetimes of the sensitizers.

We have performed the first excitation of high-rt states of positronium using resonant, two-photon excitation applied to the «*1 to 2 and 2 to n transitions. Absolute values for the line positions were quantitatively determined for n ~14 and 15 and compared well with calculated predictions. The prediction of the n ~3 scaling of the relative transition rates was observed for n = 13 to 19.PACS numbers: 36.10.Dr, 32.80.Pj Since the discovery of positronium (Ps), spectroscopic studies have been performed on sublevels of the ground and n =2 energy levels of the system. These include the singlet-triplet splitting in the ground state, 1 the twophoton \S-2S transition, 2 and the 2S-2P splitting in the first excited state. 3 These measurements represent a sensitive test of quantum electrodynamics due to the precision of the experimental results, lack of importance of strong forces, and high calculational accuracy available from the theory. In this Letter, we report the first observation in Ps of the excitation of high-H states, « -13-15, a preliminary measurement of the energy of two of these levels, ^ = 14 and 15, and preliminary observations on the relative transition probabilities for n = 13-19. Excitation of high-rt states was performed by a resonant, twophoton excitation n = 1 to 2 and n sss 2to 13-19 and using an extension of our previous investigations into the production of an optically saturated population of n =2 Ps. 4 The basic quantum electrodynamic interactions in Ps, particularly energy shifts induced by perturbative external fields, may be tested in new ways by measurements of the energies and transition properties of high-/z states. Excitation to high-/i states results in a population of long-lived, neutral Ps because the deexcitation and annihilation lifetimes scale as n~3. Such excited populations of Ps can be used to study areas such as antihydrogen production or exotic many-body states of Ps. 5 We can also tune through the resonant excitation profile using narrow laser linewidths allowing us to map the velocity distribution of the ground-and (n =2)-state populations. Since these transitions leave the Ps system in a bound state, they can be of particular use in laser-cooling experiments. 5 In this experiment we excite high-rt states in Ps through a two-step excitation process using the n = 2 state as an intermediate state. The experiment was conducted using extensions of the techniques previously employed to observe the population of the n = 2 levels of Ps. 4 Ps and laser light interacted in the ultrahigh-vacuum, experimental chamber used with the intense, low-energy positron beam at the 100-MeV electron linac at Lawrence Livermore National Laboratory. Thermal Ps was formed by guiding a 1-keV pulsed positron beam with a 200-G magnetic field onto a hot, 1000-K, clean copper single crystal cut along the 100 face. The target was biased with respect to a grid to attract any reemitted positrons. Pulses of 15-ns duration typically contained 10 5 positrons which were converted into Ps with an overall effici...

This investigation evaluaLed the channel-specific absolute photodissociation quantum yields for the nitrate free radical,

We demonstrate that molecular ions with mass-to-charge ratios (m/z) ranging from a few hundred to 19 050 can be desorbed from whole bacterial spores using infrared laser desorption and no chemical matrix. We have measured the mass of these ions using time-of-flight mass spectrometry and we observe that different ions are desorbed from spores of Bacillus cereus, Bacillus thuringiensis, Bacillus subtilis, and Bacillus niger. Our results raise the possibility of identifying microorganisms using mass spectrometry without conventional sample preparation techniques such as the addition of a matrix. We have measured the dependence of the ion yield from B. subtilis on desorption wavelength over the range 3.05-3.8 microm, and we observe the best results at 3.05 microm. We have also generated mass spectra from whole spores using 337-nm ultraviolet laser desorption, and we find that these spectra are inferior to spectra generated with infrared desorption. Since aerosol analysis is a natural application for matrix-free desorption, we have measured mass spectra from materials such as ragweed pollen and road dust that are likely to form a background to microbial aerosols. We find that these materials are readily differentiated from bacterial spores.