### Session E6: Photon Interactions With Atoms, Ions and Molecules - Experiment

Chair: Timothy Gay, University of Nebraska
Room: Knoxville Convention Center 301C

 Wednesday, May 17, 2006 1:30PM - 1:42PM E6.00001: The Laser-Assisted Photoelectric Effect on Surfaces L. Miaja , G. Saathoff , C. Lei , M.M. Murnane , H.C. Kapteyn , M. Aeschlimann , J.L. Gland While the laser-assisted photoelectric effect (LAPE) in atoms is well established and extensively used for the characterization of femtosecond EUV pulses, the equivalent process in the original manifestation of the photoelectric effect---photoemission from surfaces---has not heretofore been studied. In the present work, we present the first observation of the LAPE process in two-color surface photoemission. In our experiment, IR pulses of 30 fs duration and 1.5 mJ at 780 nm are produced by a Ti:sapphire laser system and split into probe and pump. The probe beam is upconverted to the EUV using phase-matched high harmonic generation in a hollow fiber. A pair of Si:Mo multilayer mirrors spectrally selects the 27th harmonic (30 nm). The pump beam is directed through an optical delay arm and spatially and temporally overlapped with the EUV beam on a Pt(111) sample. A time-of-flight detector then measures the kinetic energy of the photoemitted electrons. In the presence of the pump pulse, these electrons can either absorb or emit an IR photon leading to sidebands in the EUV photoelectron spectrum. This `dressing' of the continuum states is visible as steps in the Fermi edge. Wednesday, May 17, 2006 1:42PM - 1:54PM E6.00002: H$\alpha$, H$\beta$, and Ly$\alpha$ fluorescence from the photodissociation of H$_{2 }$and D$_{2}$ T.J. Gay , J.R. Machacek , J.E. Furst , A.L.D. Kilcoyne , J.D. Bozek , H. Gould , M.S. Lubell , K.W. McLaughlin We have measured the intensity, corrected for polarization, of H$\alpha$ (656 nm), H$\beta$ (486 nm), and Ly$\alpha$ (122 nm) fluorescence from the photodissociation of H$_{2}$ and D$_{2 }$by linearly-polarized photons with energies between 25 and 150 eV. We observe a low-energy maximum in the intensities for all three transitions between 35 and 40 eV, due to excitation of doubly-excited molecular states. A second shoulder or secondary maximum is attributed to the direct production of excited $H_2^+$ states which subsequently dissociate into the excited atoms. This second maximum is in qualitative agreement with the H$\alpha$ results of Garcia \textit{et al.} [1]. The effects of cascading and the size of the visible collision volume on these data is discussed. [1] Garcia \textit{et al.}, J. Phys. B \textbf{39, }205 (2006). Wednesday, May 17, 2006 1:54PM - 2:06PM E6.00003: Dynamics and Fragmentation of Hydrogen Bonded and van der Waal Clusters upon 26.5 eV Soft X-ray Laser Ionization Feng Dong , Scott Heinbuch , Elliot Bernstein , Jorge Rocca A desk-top soft x-ray laser is applied to the study of water, methanol, ammonia, sulfur dioxide, carbon dioxide, mixed sulfur dioxide-water, and mixed carbon dioxide-water clusters through single photon ionization time of flight mass spectroscopy. Almost all of the energy above the vertical ionization energy is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the mass spectra for the first three systems. The temperatures of the neutral water and methanol clusters can be estimated. In the case of pure SO$_{2 }$and CO$_{2}$, the mass spectra are dominated by (SO$_{2})_{n}^{+}$ and (CO$_{2})_{n}^{+}$ cluster series. When a high or low concentration of SO$_{2}$/CO$_{2 }$is mixed with water, we observe (SO$_{2}$/CO$_{2}$)$_{n}$H$_{2}$O$^{+ }$or SO$_{2}$/CO$_{2}$(H$_{2}$O)$_{n}$H$^{+}$ in the mass spectra, respectively. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated for the protonated water, methanol, and ammonia clusters as well as for SO$_{2}$ and CO$_{2}$ clusters. We find that the 26.5 eV soft x-ray laser is a nearly ideal tool for the study of hydrogen bonded and van der Waals cluster systems and we are currently exploring its usefulness for other more strongly bound systems. Wednesday, May 17, 2006 2:06PM - 2:18PM E6.00004: Angle-resolved Two-Dimensional Photoelectron Spectroscopy of van-der-Waals Clusters Daniel Rolles , Huaizhen Zhang , Antony Wills , Rene Bilodeau , Edwin Kukk , Bruce Rude , Glen Ackerman , John Bozek , Nora Berrah We have performed angle-resolved photoelectron spectroscopy on the outer and inner-valence shells of Ar, Kr and Xe van-der-Waals clusters and have collected an extensive data set using two-dimensional photoelectron spectroscopy. The measurements in small photon energy steps show the evolution of the photoelectron angular distribution, partial cross sections and branching ratios. They allow direct comparison between the photoionization of clusters and that of free atoms. Our results show that while the overall behavior of the partial and differential cross sections of the clusters is, in general, very similar to that of the corresponding independent atoms, some distinct differences in the angular distribution and spin-orbit branching ratios point at cluster-size dependent effects and highlight the influence of the cluster field produced by the surrounding atoms on the valence electrons. Wednesday, May 17, 2006 2:18PM - 2:30PM E6.00005: Precise Double-Photoionization Data for Na and K P.N. Jurani\'c , J.C. Nordberg , R. Wehlitz We have measured precise double-to-single photoionization ratios and double-photoionization cross-sections of sodium and potassium near threshold. A previously discovered scaling law \footnote{R.\ Wehlitz and S.\ B.\ Whitfield, J.\ Phys.\ B {\bf 34}, L719 (2001).} allows us to conveniently compare the energy dependence of the double-to-single photoionization ratio by scaling the energy axis. Recently, we have also found a scaling law that enables us to predict the absolute double-to-single photoionization ratio \footnote{J.B.\ Bluett, D.\ Luki\'c, S.B.\ Whitfield, and R.\ Wehlitz, Nucl. Instrum. Methods B, {\bf 241}, 114 (2005).}. We have applied this scaling law to our new data and found excellent agreement. Previous tests of this scaling law were limited to systems where electrons were emitted from $s$-shells. However, in the cases of Na and K a $p$ electrons is participating in the double-ionization process. Interestingly and in spite of the different orbital, the scaling law is still valid. Wednesday, May 17, 2006 2:30PM - 2:42PM E6.00006: Inner Shell Photodetachment of Atomic Negative Ions R.C. Bilodeau , I. Dumitriu , N. Berrah , N.D. Gibson , C.W. Walter , J.D. Bozek , G.D. Ackerman The qualitatively different spectra and structure observed in atomic negative ions, owing largely to the short-range binding potential and the strong correlation effects present in these systems, make negative ions unique and of fundamental interest. A summary of recent experiments led by our team on BL 10.0.1 IPB at the ALS will be presented. The focus will be on resonances observed in the photoexcitation of inner-shell s-, p-, or f-electrons into partially filled p (in He$^{-}$ [1] and S$^{-}$ [2]) or d (in Ni$^{-}$ and Pt$^{-})$ orbitals. Strong shape or Feshbach resonances were produced in all cases, and observed to couple strongly into decay channels leading to typically many final ionization states. Observed ionic channels were set on a measured absolute scale. \begin{enumerate} \item R.C. Bilodeau \textit{et al.}, Phys. Rev. Lett. \textbf{93}, 193001 (2004). \item R.C. Bilodeau \textit{et al.}, Phys. Rev. A \textbf{72}, 050701(R), (2005). \end{enumerate} Wednesday, May 17, 2006 2:42PM - 2:54PM E6.00007: Photodetachment Spectroscopy Of Evaporatively Cooled Negative Ions John N. Yukich , James E. Wells Spectroscopic resolution of photodetachment from negative ions is limited by thermal broadening phenomena such as the Doppler and the motional Stark effects. To reduce this broadening the ions may be evaporatively cooled. Our experimental apparatus includes a Penning ion trap in which negative ions are created, trapped, and stored. Numerical models show that evaporative cooling, producing a lower average ion temperature, will enhance resolution of photodetachment spectroscopy. We present preliminary evidence of this cooling effect.