[en] The technique of resonance enhanced multi-photon ionization (REMPI) of atomic and molecular species produced from a photofragmentation event combined with time-of flight (TOF) detection is used to examine scalar and vector properties following photodissociation. This technique is applied to the study of methyl bromide dissociation in a product state specific manner. We report measurements of the angular distributions and kinetic energy releases of the resulting bromine atoms in the ground and first spin-orbit excited state. Additionally we report measurements of the angular distributions and kinetic energy releases of the methyl fragment in the ground vibrational state, and also the excited state with one quanta in the ν₂ vibrational modes. These studies were carried out in the red wing of the absorption band at several wavelengths. For these measurements we were able to resolve the spin orbit state of the partner bromine fragment. From our observations we find new evidence for enhanced nonadiabatic curve crossing active in methyl bromide dissociation in comparison with earlier studies of methyl iodide. The atomic polarization produced following photodissociation of a diatomic molecule was investigated both theoretically and experimentally. We develop theoretical expressions relating the lab frame and molecular frame atomic polarization to the photoexcitation and subsequent dissociation of a diatomic molecule. This treatment includes both incoherent, coherent and non-adiabatic processes which may be active in the photodissociation process. We treat the general case of a polarized diatomic molecule yielding two fragments with non zero angular momentum. Experimentally, an investigation of the polarization of atomic Cl(²P_3/2) photofragments from the ∼330 nm photolysis of molecular chlorine using the REMPI-TOF technique is reported. We present a theoretical framework in which to treat such experiments allowing the extraction of parameters with direct physical significance to the coherent and incoherent pathways active in photodissociation. (author)

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No abstract available

[en] The photodissociation of expansion-cooled HI monomer by using 266 nm radiation yields H atoms having 12 830 and 5287 cm-1 of translational energy in the HI center-of-mass system for the I(²P_3/2) and I(²P_1/2) (i.e., I and I^*, respectively) co-fragments. Irradiating HI clusters [i.e., (HI)_n, with n=2 being the dominant cluster] with 266 nm radiation produces, among other things, some H atoms whose translational energies are peaked at 20 285 cm-1, which is 7455 cm-1 higher in energy than the more energetic of the monomer peaks. These very fast H atoms arise from sequential photodissociation within the clusters. Namely, a weakly bound I*·(HI)_n-1 complex is first created by the photodissociation of an HI moiety within (HI)_n, and then the photodissociation of a second HI moiety [within I*·(HI)_n-1] produces a fast H atom that scatters from the nearby I*, in some cases deactivating it in the process. Thus, the latter superelastically scattered H atom acquires, as translational energy, nearly all of the I* energy (7603 cm-1). For example, for the dimer, the first dissociation event, (HI)₂+hv→H+I(I*)·HI, is followed by I*·HI+hv→H_superelastic+I-I. High quality potentials for the relevant HI excited states have been calculated recently, and coupling between ³Π_0⁺ (which correlates with I^*) and ¹Π (which correlates with I) has been shown to be due to spin-rotation interaction. There is a high degree of separability between the photodissociation of the second HI moiety and the subsequent H+I^* scattering (within a given cluster). This is due mainly to the shape of the ³Π_0⁺ potential; specifically, it has a shallow well that persists to small r. The shape of the ³Π_0⁺ potential is influenced by relativity; i.e., strong spin-orbit coupling maintains the I* spherical electron density to relatively small r. The ³Π_0⁺→¹Π transition probabilities are calculated for H+I^* collisions having different values of the collisional orbital angular momentum quantum number, l, by scaling the spin-rotation matrix elements by [l(l+1)]^1/2 and using the Landau-Zener model to treat the electronically nonadiabatic dynamics. It is shown that large l values (l_max=52) play a dominant role in the quenching of I* by H. For example, the partial superelastic scattering cross section is six orders of magnitude larger for l=52 than for l=1, underscoring the dramatic role of angular momentum in this system. It is noted that HI photodissociation (which is dominated by low l) proceeds almost entirely along the diabats with little transfer of flux between them, whereas H+I* intracluster 'collisions' take place with sufficiently large l to facilitate the electronically nonadiabatic process

[en] The room-temperature ultraviolet absorption spectrum of H₂Te has been recorded. Unlike other group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics have been examined at photolysis wavelengths of 266 nm (which lies in the main absorption feature) and 355 nm (which lies in the long-wavelength tail) by using high-n Rydberg time-of-flight spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield H atoms. Photodissociation at 355 nm yields TeH(²Π_1/2) selectively relative to the TeH(²Π_3/2) ground state. This is attributed to the role of the 3A^' state, which has a shallow well at large R_H-TeH and correlates to H+TeH(²Π_1/2). Note that the ²Π_1/2 state is analogous to the ²P_1/2 spin-orbit excited state of atomic iodine, which is isoelectronic with TeH. The 3A^' state is crossed at large R only by 2A^'', with which it does not interact. The character of 3A^' at large R is influenced by a strong spin-orbit interaction in the TeH product. Namely, ²Π_1/2 has a higher degree of spherical symmetry than does ²Π_3/2 (recall that I(²P_1/2) is spherically symmetric), and consequently ²Π_1/2 is not inclined to form either strongly bonding or antibonding orbitals with the H atom. The 3A^'<-X transition dipole moment dominates in the long-wavelength region and increases with R. Structure observed in the absorption spectrum in the 380-400 nm region is attributed to vibrations on 3A^'. The main absorption feature that is peaked at ∼240 nm might arise from several excited surfaces. On the basis of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis, as well as high-level theoretical studies, the main contribution is believed to be due to the 4A^'' surface. The 4A^''<-X transition dipole moment dominates in the Franck-Condon region, and its polarization is in accord with the experimental observations. An extensive secondary photolysis (i.e., of nascent TeH) is observed at 266 and 355 nm, and the corresponding spectral features are assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D₀. For H₂Te and TeH, these are 65.0±0.1 and 63.8±0.4 kcal/mol, respectively, in good agreement with predictions that use high-level relativistic theory

[en] A lacquer-coated, diamond turned Cu ellipsoid has been used to micro-focus undulator light from Beam Line V at SPEAR down to a half-power diameter of about 13 microns. This spot was source-size limited, as has been demonstrated with ray tracings of the optical system. The symmetry of the image, as well as its size and power density, clearly make this optic ideal for many soft x-ray applications, in particular for x-ray microscopy, microprobes, and for pumping soft x-ray transitions in various media

[en] A long undulator installed at a low emittance storage ring, generates quasi-monochromatic beams of high brightness and partial coherence properties; however, this also raises concerns regarding high heat loads on beam line components. There have been intensive research efforts to develop beam line optics to exploit brightness and coherence properties from undulators. These components must withstand high heat loads produced by intense synchrotron radiation beams impinging on their surface which could degrade beam line performance. The effects of high flux undulator radiation on beam line optics for EUV interferometry and photoemission microscopy will be discussed. Specifically, beam line schematics, design considerations of indirectly side cooled mirror and grating assemblies developed at the Center for X-Ray Optics (CXRO) and recent data of performance under undulator radiation load from beam line BL12.0 being commissioned at the ALS will be presented in this study

[en] Magnetic effects on X-ray scattering (Bragg diffraction, specular reflectivity or diffuse scattering) are a well known phenomenon, and they also represent a powerful tool for investigating magnetic materials since it was shown that they are strongly enhanced when the photon energy is tuned across an absorption edge (resonant process). The resonant enhancement of the magnetic scattering has mainly been investigated at high photon energies, in order to match the Bragg law for the typical lattice spacings of crystals. In the soft X-ray range, even larger effects are expected, working for instance at the 2p edges of transition metals of the first row or at the 3d edges of rare earths (300-1500 eV), but the corresponding long wavelengths prevent the use of single crystals. Two approaches have been recently adopted in this energy range: (i) the study of the Bragg diffraction from artificial structures of appropriate 2d spacing; (ii) the analysis of the specular reflectivity, which contains analogous information but has no constraints related to the lattice spacing. Both approaches have their own specific advantages: for instance, working under Bragg conditions provides information about the (magnetic) periodicity in ordered structures, while resonant reflectivity can easily be related to electronic properties and absorption spectra. An important aspect common to all the resonant X-ray scattering techniques is the element selectivity inherent to the fact of working at a specific absorption edge: under these conditions, X-ray scattering becomes in fact a spectroscopy. Results are presented for films of iron and cobalt

[en] The MAXIMUM scanning x-ray microscope, developed at the Synchrotron Radiation Center (SRC) at the University of Wisconsin endash Madison, was implemented on the Advanced Light Source (ALS) in August of 1995. The microscope close-quote s initial operation at SRC successfully demonstrated the use of a multilayer-coated Schwarzschild objective for focusing 130 eV x-rays to a spot size of better than 0.1 micron with an electron energy resolution of 250 meV. The performance of the microscope was severely limited because of the relatively low brightness of SRC, which limits the available flux at the focus of the microscope. The high brightness of the ALS is expected to increase the usable flux at the sample by a factor of 1000. We will report on the installation of the microscope on bending magnet beamline 6.3.2 at the ALS and the initial measurement of optical performance on the new source, and preliminary experiments on the surface chemistry of HF-etched Si will be described. copyright 1996 American Institute of Physics

[en] We use x-ray spectromicroscopy in order to investigate current problems in microelectronics. In particular, we report here the results obtained by MAXIMUM, a scanning photoemission microscope recently installed at the Advanced Light Source (ALS), at the Lawrence Berkeley National Laboratory. MAXIMUM is a spectromicroscope based on multilayer optics at 130 eV, where the images are achieved by collecting the photoelectron emitted from the sample by means of an electron energy analyzer. The spatial resolution is 100 nm, and the spectral resolution is 200 meV. In order to show the capabilities of this instrument, we will discuss the formation of TiSi₂ in patterned structures and the electromigration in AlCu lines. In the case of TiSi₂, we imaged lateral variations in the local chemistry of patterned samples with feature from 100 to 0.1 mm, which are attributed to the formation of different phases (C54 and C49). We studied the effect of electromigration in Al-Cu lines with different Cu content and width in situ, in order to distinguish different phases of Cu and follow its evolution electromigration proceeded