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A new photoelectron spectrometer has recently been used to analyze the energy and spatial distribution of photoelectrons produced by multiphoton ionization of rare gases. It is based on the analysis of the image obtained by projecting the expanding electron cloud resulting from the ionization process onto a two-dimensional position sensitive detector by means of a static electric field. In this article, we present the principle of this imaging spectrometer and the relevant equations of motion of the charged particle in this device, together with an inversion method that allows us to obtain the energy and angular distribution of the electrons. We present here the inversion procedure relevant to the case where the electrostatic energy acquired in the static field is large as compared to the initial kinetic energy of the charged particles. A more general procedure relevant to any regime will be described in a following article.

The application of single photon ionization in combination with mass-selective detection by time-of-flight mass spectrometry is described for the rapid detection of the nitro-containing explosives and explosives-related compounds nitrobenzene, 1,3-dinitrobenzene, o-nitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene, as well as the peroxide-based explosive triacetone triperoxide in the gas phase. The technique is demonstrated to be a plausible approach for laser-based rapid detection of explosives. The limits of detection for nitrobenzene and 2,4-dinitrotoluene using SPI were also measured and determined to be 17-24 (S/N approximately 2:1) and approximately 40 ppb (S/N approximately 2:1), respectively.

[1] Measurements of the intensity ratio of the 589.0/589.6 nm sodium doublet in the terrestrial nightglow over an 8-year period, involving >300 separate determinations, have established that it is variable, the value R D = I(D 2 )/I(D 1 ) lying between 1.2 and 1.8. Sky spectra from the Keck I telescope with the High-Resolution É chelle Spectrometer (HIRES) échelle spectrograph and the Keck II telescope with the É chellette Spectrograph and Imager (ESI) échelle spectrograph were used in this analysis. The result contrasts with the accepted view, from earlier measurements at midlatitude, that the ratio is 2.0, as expected on statistical grounds. The lack of dependence of the ratio on viewing elevation angle, and hence Na slant column, allows self-absorption to be ruled out as a cause of the variability. The data suggest a semiannual oscillation in the ratio, maximum at the equinoxes and minimum at the solstices. Airborne measurements over the North Atlantic (40°-50°N) in 2002 show an even larger range in the nightglow ratio and no correlation with the upper mesospheric temperature determined from the OH 6-2 bands. A laboratory study confirms that the ratio does not depend on temperature; however, it is shown to be sensitive to the

Spatial distributions of photoelectrons produced by multiphoton ionization of xenon atoms are recorded by projecting the expanding photoelectron cloud onto a two-dimensional position sensitive detector. The projected image provides a direct view of the squared angular wave functions of the free electrons as well as their energy distribution. The results confirm recent observations that intermediate state resonances with 5/and 4/character establish the dominant ionization paths at low intensity, for short pulse excitation at 640 and 620 nm. At higher intensity more complex superpositions occur with formation of electrons with continuous distributions at low energies.PACS numbers: 32.80.RmPhotoelectrons generated at a point source with a discrete energy travel outward on the surface of a sphere that expands with time. For example, electrons produced at time £=0 with an energy of 1 eV can be found 20 ns later on the surface of a sphere of 25 mm diameter. This sphere can be projected onto a flat screen using an external electric field. A circular image results with a diameter that is proportional to square root the electron energy and a filling pattern that reveals the spatial distribution of the electrons on the surface of the sphere. In this way, the squared angular wave function of the free electrons is accessible to direct observation.We have used this approach to investigate multiphoton ionization of xenon atoms in an intense laser field. Simultaneous visualization of the photoelectron energy and angular distributions facilitates the identification and classification of ionization mechanisms.Recent work on multiphoton ionization of atoms and molecules by intense laser fields has shown that the dynamics of ionization are governed by the modification of the electronic structure of the target by the radiation field [1][2][3][4][5][6][7]. The effective energies of the electronic states are shifted (ac Stark effect) so strongly that the laser, which at low intensity was not resonant with any particular multiphoton transition, becomes resonant with individual intermediate states at specific critical intensities. One result is that the photoelectron energy and angular distributions often reflect the nature of the dominant intermediate states.Consistent with previous experiments, we observe that at moderate intensities the photoelectron energies are discrete, appearing as if they had come from photoionization of the intermediate states, E =hv -IP (intermediate), rather than from the ground state, E=nhv -IP (ground state). This apparent nonconservation of energy results from the fact that photoelectrons do not recover the ponderomotive energy under conditions such that the product of the laser pulse duration and the electron velocity is much smaller than the spatial dimensions of the laser focus [1,2]. Our photoelectron angular distributions show that predominantly the m=0 component of the intermediate state is being ionized. This is likely a result of the larger m =0 to m =0 matrix elements in multiphoton transitions. A...

We report two complementary experimental investigations of the absorption spectrum of molecular oxygen between 243 and 258 nm. In the first experiment, excitation of O2 is inferred by detecting oxygen atoms resulting from chemical reaction. In the second experiment, absorption by O2 is observed directly by cavity ring-down spectroscopy. Absorption strengths for the Herzberg I [Formula: see text], Herzberg II [Formula: see text], and Herzberg III [Formula: see text] band systems are modeled with the DIATOM spectral simulation computer program using the best available branch intensity formulas. Absolute oscillator strengths are derived for all three systems and compared with values in the literature.

[1] The atlas of terrestrial nightglow emission lines from spectra of the night sky obtained from the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.2-m UT2 telescope at the Very Large Telescope (VLT), European Southern Observatory, Cerro Paranal, Chile, consists of 2808 line positions, line widths, and intensities over the 314-1043 nm spectral range (Hanuschik, 2003). These lines have been absolute intensity calibrated and measured at a spectral resolution (l/Dl) of $43,000-45,000. Presented here are spectroscopic identifications for 98% of the lines in the atlas, made primarily through comparisons with synthetic spectra of prominent OH and O 2 nightglow emission systems. The ability to simulate these systems successfully has shown that there are many additional lines that could be added to the atlas. We believe that all the O 2 and OH lines in the measured region can now be successfully modeled with an accuracy better than the instrumental spectral resolution.

Green line emission at 557.7 nanometers arising from the O(1S - 1D) transition of atomic oxygen has been observed on the nightside of Venus with HIRES, the echelle spectrograph on the W. M. Keck I 10-meter telescope. We also observe optical emissions of molecular oxygen, consistent with the spectra from the Venera orbiters, but our green line intensity is so high that we cannot explain how it could be inconspicuous in the Venera spectra. An upper limit for the intensity of the O(1D - 3P) oxygen red line at 630 nanometers has also been obtained. The large green/red ratio indicates that the source is not associated with the Venus ionosphere. An important conclusion is that observation of the green line in a planetary atmosphere is not an indicator of an atmosphere rich in molecular oxygen.