The photofragment spectrum of NO2 has been measured in the near ultraviolet at 28 810 cm−1. A molecular beam of NO2 is crossed with brief pulses of polarized laser light and measurements are made on the distributions of speed and direction of the recoiling O and NO fragments produced by photodissociation. The average translational energy of the fragments is about 60% of the available energy. There are at least two prominent peaks in the translational energy distribution. We conclude that the two peaks most likely correspond to nearly equal probability of recoil with the NO fragment in the v=0 and v=1 vibrational states. Such vibrationally excited NO fragments produced by photodissociation in polluted atmospheres could perhaps react with different rates than ground state fragments. The positions and widths of the peaks indicate that there is a significant rotational distribution. Statistical and direct models for photodissociation energy partitioning are briefly explored, and their predictions compared with the experimental measurements. A model is tentatively endorsed in which the nascent NO fragment is formed with vibrational excitation already present in the NO2 excited state. Dissociation is then assumed to occur quickly before statistical equilibration is reached. This is the first of a series of four papers on the photofragment spectra of triatomic molecules, using NO2 and NOCl as examples.

The angular distribution of recoil of O atoms from the photodissociation of NO2 at 28 810 cm−1 is presented and analyzed to obtain information about the lifetime and symmetry of the excited dissociative state. The theoretical effects on photofragment angular distributions of excited state symmetry, lifetime, angular momentum, and angular recoil distribution with respect to internal coordinates are considered. The actual angular distribution measured by photofragment spectroscopy, peaks in the direction of the electric vector of the light, indicating that the predominant upper state is of 2B2 symmetry. Some absorption leading to states of different symmetry cannot be excluded. An upper limit for the lifetime of the excited NO2 molecule before dissociation is ∼ 2 × 10−13 sec, indicating that break-up is too rapid to be affected by collisions in the atmosphere where NO2 photodissociation is an important step in photochemical air pollution.

We have excited detergent-solubilized bovine rhodopsin at room temperature with 530-nm light pulses from a mode locked laser, and have observed the appearance and decay of a transient species that absorbs more strongly at 560 nm than does ground-state rhodopsin. Our data show that the absorbing intermediate appears in a time that is at least as short as the experimental resolution (about 6 psec) and decays with a life time of about 30 nsec. The extremely fast risetime supports the hypothesis that prelumirhodopsin is the product of the primary photoprocess.Several intermediates after excitation of bovine rhodopsin have been detected by following changes in the visible and near ultraviolet absorption spectrum, which is due to the retinyl prosthetic group (1, 2). Spectral changes that occur very rapidly near physiological temperatures have been observed by cooling the preparations to low temperatures where the rates are compatible with conventional flash photolysis techniques, or by freezing preparations in glass-forming solvents at temperatures where interconversion rates are so slow that ordinary absorption spectroscopy suffices.The first spectral change during photobleaching of rhodopsin (3) has been interpreted in terms of the formation of an intermediate, prelumirhodopsin (4), having a maximum absorption at 543 nm. This process is considered to be the primary photochemical step for the geometrical isomerization of the polyene chromophore (1,2,5).Experimental data concerning the rate of formation of prelumirhodopsin have not been reported. The decay of prelumirhodopsin in preparations of bovine rhodopsin has been measured at temperatures between -50 and -67°(6, 7).Extrapolation of these values gives an expected lifetime at 370 of <1 Asec (1). Only recently has there been a report of a measurement of the decay near room temperatures (8). In fact, even the existence of prelumirhodopsin as a discrete intermediate at physiological temperatures has been questioned (1, 9).We have used psec spectroscopy (10, 11) at physiological temperatures to detect transients absorbing at longer wavelengths than rhodopsin on time scales from a few psec to 0.1 psec. The results indicate that prelumirhodopsin is an intermediate in the bleaching of bovine rhodopsin excited with 530-nm light at physiological temperatures and are consistent with the notion that prelumirhodopsin is the primary photochemical product. In this paper, we describe the methods and results obtained concerning the kinetics of prelumirhodopsin at physiological temperatures. Abbreviations: ROS, rod outer segments; LDAO, lauryldimethylamine oxide; SHG, second harmonic. PREPARATION OF BOVINE RHODOPSINFrozen bovine retinas were obtained from G. Hormel Co., Austin, Minn. and stored at -60°. Samples were prepared in total darkness or in dim red light near 4°. Bovine retinas were ground to a smooth paste, diluted with cold buffer [0.1 M NaCl-10 mM imidazole-Cl (pH 7)1, and homogenized with a Potter-Elvehjem homogenizer. The homogenized retinas were centrifuged f...

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Tests of the electron beam injector for the Boeing/Los Alamos Average Power Laser Experiment have demonstrated first time operation of a photocathode radio frequency gun accelerator at 25% duty factor, exceeding previous photocathode operating parameters by three orders of magnitude. The macropulse format was 30 Hz and 8.3 ms with a micropulse frequency of 27 MHz. Average beam currents of up to 32 mA have been accelerated to 5 MeV for an average beam power of 160 kW. The macropulse peak current was 128 mA. The 32 mA average beam current exceeds previous cathode performance by a factor of 1000. Emittance measurements demonstrate excellent electron beam quality.