Publisher Summary This chapter discusses collision experiments using laser excited atoms in crossed beams. With the advent of tunable narrow-band lasers, especially cw dye lasers, the situation has changed and it has become clear that it should be possible to excite atoms optically within the scattering region of an otherwise conventional crossed-beam experiment. In this way a steady-state upper-state population could be reached that may be comparable to the ground-state population. When an atomic beam is excited, it is free of internal collisions and, for right angle intersection with the laser, free of Doppler broadening. Then the laser properties allow selecting the state into which the atom is excited. Specific fine- and hyperfine-structure states may be chosen as well as a particular combination of sub-states. The novel techniques allow preparing states with an angular momentum different from zero and to vary systematically the alignment and orientation of the resulting non-spherical interaction potentials. Frequency doubling of dye lasers can also widen the scale of possible applications. Very high powers are needed and one probably would have to use a pulsed laser. An alternative to frequency doubling is the direct two-photon excitation of atoms.

Collision Experiments with Laser Excited Atoms in Crossed Beams

New diffractive imaging techniques using coherent x-ray beams have made possible nanometer-scale resolution imaging by replacing the optics in a microscope with an iterative phase retrieval algorithm However, to date very high resolution imaging (< 40nm) was limited to large-scale synchrotron facilities Here, we present a significant advance in image resolution and capabilities for desktop soft x-ray microscopes that will enable widespread applications in nanoscience and nanotechnology Using 13nm high harmonic beams, we demonstrate a record 22nm spatial resolution for any tabletop x-ray microscope Finally, we show that unique information about the sample can be obtained by extracting 3-D information at very high numerical apertures

Ultrahigh 22 nm resolution coherent diffractive imaging using a desktop 13 nm high harmonic source

A high energy photon source. A pair of plasma pinch electrodes (8) are located in a vacuum chamber. A working gas supply system supplies a working gas which includes a noble buffer gas and an active gas chosen to provide a desired spectral line. A pulse power source (10) provides electrical pulses at a frequency of at least 100 Hz and voltages high enough to create electrical discharge between the electrodes to provide very high temperature, high density plasma pinches in the working gas providing radiation at the spectral line of the active gas. An external reflection radiation collector-director (4) collects radiation produced in the plasma pinches and directs the radiation in a desired direction. In a preferred embodiment the active gas is lithium and the buffer gas is helium and the radiation-collector is coated with the material used for the electrodes. That material is tungsten.

Plasma focus high energy photon source

Autocorrelation measurement was demonstrated for the first time to our knowledge in the XUV region to measure the width of a high-order harmonic pulse. Two-photon ionization of rare gases was used as a nonlinear process for the autocorrelation measurement. The 27-fs pulse width that was obtained is, to our knowledge, the shortest in the XUV region.

27-fs extreme ultraviolet pulse generation by high-order harmonics.

Mode selective approaches to controlling reactions which are based upon one photon absorption of a fast and/or shaped laser pulses creating localized excitation are formally examined and found deficient. Specifically, we show that if such control is possible with laser pulses, then the same result may be obtained with incoherent sources operating over essentially cw time scales. Since the time dependence imparted by the preparation is irrelevant, related concepts, such as the idea that rapid excitation is necessary to “beat out intramolecular vibrational relaxation” are misguided.

One photon mode selective control of reactions by rapid or shaped laser pulses: An emperor without clothes?

We describe two new experiments which demonstrate laser-induced inelastic collisions. Energy was stored in the Sr $5p^{1}P^{o}$ state and selectively transferred both to the Ca $4{p}^{2}^{1}S$ and $5d^{1}D$ states. The collision cross sections maximized at the interatomic wavelengths of 4977 and 4711 \AA{}, respectively, and had a half-power width of about 14 ${\mathrm{cm}}^{\ensuremath{-}1}$.

Observation of laser-induced inelastic collisions

A simple dichroic beam splitter capable of separating extreme-ultraviolet (XUV) radiation from high-powered visible and UV laser beams is demonstrated. The device has high XUV reflection efficiency (R approximately 44%) and high damage resistance to intense laser radiation and is therefore applicable to laser-generated XUV radiation sources, including harmonic generation, sum-frequency mixing, and anti-Stokes Raman scattering.

Dichroic beam splitter for extreme-ultraviolet and visible radiation.

Doubly excited even-parity lsnpmp /sup 2/P states of neutral Li are metastable against autoionization and have large radiative yields. An emission spectrum from 160 A to 250 A of a high-power-microwave--heated Li plasma is reported. The spectrum, which differs substantially from that obtained in absorption, exhibits several new series as well as the phenomena of radiative autoionization.

Emission Spectra of Core Excited Even-Parity 2 P States of Neutral Lithium

Soft x rays from a plasma generated by a neodymium: yttrium aluminum garnet laser beam (1.06 μm) are used to photoionize neutral Li vapor to produce Li+ metastables. Maximum metastable densities of Li+ (1s2s) 1S=6×1014 ions/cm3 and Li+ (1s2s) 3S=3×1015 ions/cm3 are obtained. The effective lifetimes of these levels are measured to be approximately 5 ns. At 50 mJ of incident laser energy the inferred conversion efficiency from 1.06‐μm radiation to soft x rays is ∼14%.

R. W. Falcone

Papers

Observation of laser-induced inelastic collisions

Dichroic beam splitter for extreme-ultraviolet and visible radiation.

Emission Spectra of Core Excited Even-Parity 2 P States of Neutral Lithium

Soft x‐ray pumping of metastable levels of Li+

Laser Spectroscopy of Metastable Extreme-Ultraviolet Levels in Lithium Atoms and Ions