H. A. Schuessler

A frequency comb in the extreme ultraviolet

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency ‘combs’ (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated ‘oscilloscope’ for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.

Sensitivity and detection limit of concentration and adsorption measurements by laser-induced surface-plasmon resonance

The shot noise limitation as well as other factors that influence the sensitivity of measurements with a surface plasmon resonance (SPR) sensor are considered. It is demonstrated that minute changes in the refractive index of a medium close to the surface of a metal film can be detected owing to a shift in the resonance angle. In particular, changes in the adsorption layer of only a fraction of a biomolecular monolayer could be measured. Data for SPR are presented with adjacent media of air, water, as well as aqueous solutions of ethanol and sodium chloride at different concentrations. The immobilization of the protein bovine serum albumin to a specially prepared surface was monitored with the SPR technique. Specific responses to changes in the concentration and thickness of the adsorption layer were determined. The angular resolution of the present apparatus is approximately 1 millidegree, corresponding to a detection limit for an adsorbed protein layer of 15 pg/mm(2), which is still 2 to 3 orders of magnitude larger than the shot-noise limit, and therefore a further improvement in sensitivity is possible.

Propagation length of surface plasmons in a metal film with roughness.

The propagation of laser-excited surface plasmons along a gold film with surface roughness is directly observed via scattered light. The attenuation length of surface plasmons in a broad wavelength interval is calculated for smooth gold and silver films. The surface roughness, which was characterized with an AFM, introduces corrections to the attenuation length, angular dependence of the surface plasmon resonance, and the effective dielectric constant of the metal film. These corrections are also taken into account and discussed.

Nuclear shape isomerism in 185Hg detected by laser spectroscopy

Abstract Laser spectroscopy has been applied to 191−185 Hg m at the on-line mass separator ISOLDE/CERN. For 185 Hg a huge isomer shift has been found, which is caused by the coexistence of slightly deformed oblate and strongly deformed prolate shapes.

Ion/molecule reactions, mass spectrometry and optical spectroscopy in a linear ion trap

Abstract A linear-geometry, radio-frequency, quadrupole ion trap has been developed to generate, purify, accumulate and study atomic and molecular ions in the gas phase. By employing a trap-based system, both reactant and product ions can be stored for significant time periods, which can both enhance the efficiency of gas-phase reaction processes and create an environment to observe collision products after vibrational and rotational excitations have had time to relax. Relaxation occurs via viscous cooling with a dilute buffer gas or via laser cooling. Furthermore, the setup is particularly useful for performing optical spectroscopy on the trapped ions. Atomic and molecular ovens are used to generate thermal beams of neutral species, which are then ionized by electron bombardment. The ions can be trapped, or they can be collided with neutral molecules (e.g. C60) under well defined experimental conditions. The collision energies are variable over a range from nearly 0 to 200 eV. This feature makes possible studies of complex formation, charge transfer and collision-induced fragmentation as a function of kinetic energy. A wide range of masses of up to 4000 u can be stored and manipulated with this apparatus. Two mass spectrometric techniques for the analysis of trapped ionic species are presented. In one method, parametric excitation of the secular motion is used to generate mass spectra with resolutions as high as 1 part in 800 with a simple experimental setup. The second method is capable of quickly generating mass spectra over the entire range of trapped masses, but has only moderate resolution. These spectra are generated by linearly sweeping the rf-trapping voltage to zero and detecting ions as their trap depth falls below a threshold value. In the trapping volume, which is 10 cm in length and 200 μm in diameter, 106 ions can be loaded and stored, corresponding to an ion density above 108 cm−3. Such densities facilitate spectroscopy of the stored ions. Both laser-induced fluorescence and photodissociation measurements have been carried out with a cw laser system providing near-infrared, visible, and ultraviolet beams. Absolute, total cross-sections and branching ratios of the photodissociation of MgC+60 have been measured.

Surface-plasmon resonance spectrometry and characterization of absorbing liquids

The effect of absorption of the sample medium on the surface-plasmon resonance (SPR) characteristics is analyzed by approximate analytical and exact numerical models. We show that absorption leads to specific changes in the value of reflectivity near the SPR angle and that these can be used for absorbance detection. The strongest absorption-induced change in reflectivity occurs at two values of metal film thickness (28 and 55 nm for a gold film and lambda = 632.8 nm). Using a sample solution of Rhodamine 700 in ethanol, we measured the characteristic changes in the SPR angle and in reflectivity over the wavelength interval encompassing the strong absorption band at 610-680 nm. The possibility of the simultaneous determination of the refractive index and absorption from SPR measurements is demonstrated and has the potential for substance-specific detection.

Interaction of laser-generated surface acoustic pulses with fine particles: Surface cleaning and adhesion studies

The mechanical forces associated with the surface acceleration in high-amplitude surface acoustic waves (SAWs) detach the particles from the surface. The removal of micron sized particles with a nanosecond SAW pulse excited by a focused laser beam in a silicon wafer was quantitatively investigated. Both vertical and horizontal particle displacements have been observed. It is shown that for nanosecond SAW pulses the limit of the surface acceleration of about 1010 m/s2 is set by the fracture of the material and corresponds to the removal of particles larger than about 0.05 μm. In addition, the nonlinear transformation of the excited SAW pulses results in an increase of the surface acceleration and contributes to the cleaning process extending it to even smaller particle dimensions. The technique is applicable in vacuum and improves the energetic effectiveness of the cleaning due to the removal of particles not only in the irradiated region, but also in the wider area covered by the SAW pulse propagation. It...

Effect of varying electric potential on surface-plasmon resonance sensing.

The high sensitivity of surface-plasmon resonance (SPR) sensors allows measurements of small variations in surface potentials to be made. We studied the changes of the SPR angle when an oscillating electric potential was applied to a gold film on which surface plasmons were excited. The shifts of the SPR resonance angle were observed for various aqueous solutions as an adjacent medium. A model that takes into account the redistribution of charges at the double layer near the metal-liquid interface as well as the oxidation of the gold film was developed. It was found that a change in the electronic density at voltages below the oxidation potential and, in addition, the oxidation of the gold surface above this potential are the main mechanisms that account for the observed dependences. It was shown that relatively slow oxidation-reduction processes can explain the observed hysteresis effect. Application of these techniques to studies of dielectric properties and conformational changes of polar biomolecules, such as tubulin, are discussed.

Coherent transfer of optical orbital angular momentum in multi-order Raman sideband generation

Experimental results from the generation of Raman sidebands using optical vortices are presented. By generating two sets of sidebands originating from different locations in a Raman-active crystal, one set containing optical orbital angular momentum and the other serving as a reference, Youngs double slit experiment was simultaneously realized for each sideband. The interference between the two sets of sidebands was used to determine the helicity and topological charge in each order. Topological charges in all orders were found to be discrete and follow selection rules predicted by a cascaded Raman process.

Confinement of pulse-injected external ions in a radiofrequency quadruple ion trap

Abstract The phase-space method is used to evaluate the ion confinement properties of a radiofrequency (r.f.) quadrupole trap with phase-synchronized switching-on of the driving r.f. field for pulsed ion injection from an external source. Singly charged ions of mass 100 u and energy 10 eV are considered. The calculation is performed for a well-collimated pulsed ion beam with a circular crow-section 1 mm in diameter, injected into an r.f. quadrupole trap with characteristic dimension ro = 5.4 mm. Ion injections along the gap and through an aperture in one of the electrodes, including off-axial- and off-equatorial-plane incidence directions, are considered. The time delay interval, determined by the intersection of the unperturbed ion trajectory with the critical volume for ion confinement, is shown to be a measure of the ion confinement property. The most favorable initial r.f. voltage phase-angle corresponds to the value when the time delay interval is greatest. Ion confinement efficiency depends on theion pulse duration. In general a shorter pulse duration leads to a higher trapping efficiency. Off-axial and off-radial injections of the ion beam are superior to the corresponding axial and radial injections. Trapping of the injected ions is shown to be effective, with the efficiency reaching unity for many possible incidence directions and pulse durations of the pulsed ion beam.