Robert Sang

Experimental observation of the elusive double-peak structure in R-dependent strong-field ionization rate of H2(+).

When a diatomic molecule is ionized by an intense laser field, the ionization rate depends very strongly on the inter-nuclear separation. That dependence exhibits a pronounced maximum at the inter-nuclear separation known as the “critical distance”. This phenomenon was first demonstrated theoretically in H2+ and became known as “charge-resonance enhanced ionization” (CREI, in reference to a proposed physical mechanism) or simply “enhanced ionization”(EI). All theoretical models of this phenomenon predict a double-peak structure in the R-dependent ionization rate of H2+. However, such double-peak structure has never been observed experimentally. It was even suggested that it is impossible to observe due to fast motion of the nuclear wavepackets. Here we report a few-cycle pump-probe experiment which clearly resolves that elusive double-peak structure. In the experiment, an expanding H2+ ion produced by an intense pump pulse is probed by a much weaker probe pulse. The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other. No structure is seen when the probe is polarized perpendicular to the pump.

Self-focusing in air with phase-stabilized few-cycle light pulses

We investigate the nonlinear optical phenomenon of self-focusing in air with phase-stabilized few-cycle light pulses. This investigation looks at the role of the carrier-envelope phase by observing a filament in air, a nonlinear phenomenon that can be utilized for few-cycle pulse compression [Appl. Phys. B79, 673 (2004)]. We were able to measure the critical power for self-focusing in air to be 18+/-1 GW for a 6.3 fs pulse centered at 800 nm. Using this value and a basic first-order theory, we predicted that the self-focusing distance should deviate by 790 mum as the carrier-envelope phase is shifted from 0 to pi/2 rad. In contrast, the experimental results showed no deviation in the focus distance with a 3sigma upper limit of 180 mum. These counterintuitive results show the need for further study of self-focusing dynamics in the few-cycle regime.

Quantitative comparison of rare-gas cold cathode discharge metastable atomic beam sources

Two commonly used cold cathode discharge sources to create rare gas metastable atomic beams are experimentally compared with respect to the output flux and the most probable velocity of the resulting beams. The systems are characterized with the same detection system thereby enabling an unambiguous performance comparison. The results of the experiments show that there is a clear distinction between the output beam characteristics of the beam sources.

Lithographic pattern formation via metastable state rare gas atomic beams

Atomic beams of argon and neon in excited electronic metastable states have been used to pattern bare and dodecanethiol (DDT) resist coated Au/Si substrates. Positive and negative contrast patterning has been observed for DDT-Au/Si, and negative patterning has been observed for bare Au/Si. Our results provide evidence for the formation of these negative patterns resulting from significant background pump oil contamination, and at significantly lower metastable dosages than previously observed. X-ray photoelectron spectroscopy (XPS) results indicate the growth of a carbonaceous layer as the origin of the negative resists in DDT-Au/Si and bare Au/Si substrates. For DDT-Au/Si, results indicate that the transition from positive to negative resist formation relies both on the metastable dosages and level of background pump oil contamination.

A high flux metastable atomic discharge source with three-dimensional translation

We present a new apparatus for the generation of a high flux metastable atomic beam. This source has provided a high flux of metastable helium and argon atoms via a cold cathode discharge with fluxes for metastable helium of up to 9 × 1014 sr−1 s−1. This new source can be translated in three dimensions whilst under vacuum and cooled to 77 K. Experimental results demonstrate that the distance of the source nozzle from the skimmer is important in obtaining the optimum flux. Under typical operating conditions, this optimal distance is 9–10 mm using He. Time-of-flight spectra show that for Ar the average thermal velocity of the beam is 540 m s−1 and can be reduced to 330 m s−1 by cooling the source with liquid nitrogen.

Benchmarking strong-field ionization with atomic hydrogen

As the simplest atomic system, the hydrogen atom plays a key benchmarking role in laser and quantum physics. Atomic hydrogen is a widely used atomic test system for theoretical calculations of strong-field ionization, since approximate theories can be directly compared to numerical solutions of the time-dependent Schrodinger equation. However, relatively little experimental data is available for comparison to these calculations, since atomic hydrogen sources are difficult to construct and use. We review the existing experimental results on strong-field ionization of atomic hydrogen in multi-cycle and few-cycle laser pulses. Quantitative agreement has been achieved between experiment and theoretical predictions at the 10% uncertainty level, and has been used to develop an intensity calibration method with 1% uncertainty. Such quantitative agreement can be used to certify experimental techniques as being free from systematic errors, guaranteeing the accuracy of data obtained on species other than H. We review the experimental and theoretical techniques that enable these results.

A hexapole magnetic guide for neutral atomic beams

In this paper we present a multiple element magnetic device to guide atoms using a spatially inhomogeneous magnetic field formed by a series of permanent hexapole magnets. The operation of the device is demonstrated using an enhanced beam of neon atoms in the 3P2 metastable state. These atoms are guided around a bend of 30 degrees from their original path. A flux of 4.35 x 10(9) +/- 2 x 10(7) atoms s(-1) was measured after the device yielding a transmission efficiency of approximately 9% of the input flux. Simulations of the center of mass motion of the atoms through the magnetic guide have been performed giving reasonable agreement with the experimental results.

Precise and Accurate Measurements of Strong-Field Photoionization and a Transferable Laser Intensity Calibration Standard.

Ionization of atoms and molecules in strong laser fields is a fundamental process in many fields of research, especially in the emerging field of attosecond science. So far, demonstrably accurate data have only been acquired for atomic hydrogen (H), a species that is accessible to few investigators. Here, we present measurements of the ionization yield for argon, krypton, and xenon with percent-level accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferable calibration standard for laser peak intensity, accurate to 1.3%, that is based on a simple reference curve. In addition, our measurements provide a much needed benchmark for testing models of ionization in noble-gas atoms, such as the widely employed single-active electron approximation.

The current status of superelastic scattering studies for e--Na atom collisions

New data from a superelastic scattering experiment on e--Na atom collisions at 10, 15, 20, 25 and 30 eV incident energy are presented and compared with existing experimental data and theories based respectively on the distorted wave second-order Born approximation, 15 state close-coupling and coupled channel optical potential calculations. Overall, fair to good agreement is found between theory and experiments for individual atomic scattering parameters. The loss of coherence previously observed by some workers at small scattering angles (5-20 degrees ), is confirmed at 20 eV and also observed at 25 eV. This loss of coherence is poorly modelled by existing theories.

Transverse electron momentum distribution in tunneling and over the barrier ionization by laser pulses with varying ellipticity

We study transverse electron momentum distribution in strong field atomic ionization driven by laser pulses with varying ellipticity. We show, both experimentally and theoretically, that the transverse electron momentum distribution in the tunneling and over the barrier ionization regimes evolves in a qualitatively different way when the ellipticity parameter describing polarization state of the driving laser pulse increases.