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Laser reduction of graphene oxide is a promising technology for manufacturing advanced devices such as supercapacitors, sensors and transistors, owing to its distinctive advantages in selective and localized GO reduction, direct micro-nanoscale patterning, and no requirement for chemicals. However, the fundamental mechanism underlying the laser induced reduction is still not well understood. In this paper, we demonstrate that by adjusting the power and scanning speed of a 780 nm femtosecond laser, not only can one distinguish, but also effectively tune, two coexisting sub-processes during the laser reduction, namely the direct conversion from sp3 to sp2 carbon and removal of oxygen functional groups. Different oxygen containing groups demonstrate varied degrees of reduction when the power of the laser was varied. Our study provides solid and direct evidence for the coexistence of two sub-processes in the laser induced reduction of graphene oxide, which is essential for both mechanistic understanding and practical adoption of this technique in real word applications.

Atom-Trap Trace-Analysis (ATTA) is a technique aimed at a measuring the relative (in comparison to the common isotope) abundance of exceedingly rare noble gas radio-isotopes (Ar-39, Kr-81 and Kr-85), for the purposes of dating water, ice and other materials that contain trapped gases. These isotopes exist with an extremely low natural abundance and require measurement capability down to parts in 1017. The noble gas dating protocol is similar to that of radio-carbon dating but, the three isotopes together, allow dating of materials in complementary ranges to that possible with the radiocarbon technique. In a facility jointly created by CSIRO, Griffith and Adelaide Universities, we use laser guidance, cooling and trapping to perform isotopic separation using the isotopic dependence of atomic energy levels. The facility is currently undergoing construction but has been shown to be able to produce metastable Ne and Ar, which can then be laser cooled and trapped. When combined with the existing gas separation facility at the CSIRO Waite facility, which efficiently extracts pure noble gases from that are dissolved in water or ice samples, we should be in a position to start measuring the age of real-world samples in the early part of 2020. In addition, to describing the current state of construction, we will also discuss two more efficient approaches for creating metastable atoms that can reduce samples size and avoid unwanted cross-contamination between samples.

We investigate AC Stark-shifted resonances in argon with ultrashort near-infrared pulses. Using 30 fs pulses we observe periodic enhancements of the excitation yield in the intensity regions corresponding to the absorption of 13 and 14 photons. By reducing the pulse duration to 6 fs with only a few optical cycles, we also demonstrate that the enhancements are significantly reduced beyond what is measurable in the experiment. Comparing these to numerical predictions, which are in quantitative agreement with experimental results, we find that even though the quantum-state distribution can be broad, the enhancements are largely due to efficient population of a select few AC Stark-shifted resonant states rather than the closing of an ionization channel. Because these resonances are dependent on the frequency and intensity of the laser field, the broad bandwidth of the 6 fs pulses means that the resonance condition is fulfilled across a large range of intensities. This is further exaggerated by volume-averaging effects, resulting in excitation of the $5g$ state at almost all intensities and reducing the apparent magnitude of the enhancements. For 30 fs pulses, volume averaging also broadens the quantum state distribution but the enhancements are still large enough to survive. In this case, selectivity of excitation to a single state is reduced below 25% of the relative population. However, an analysis of TDSE simulations indicates that excitation of up to 60% into a single state is possible if volume averaging can be eliminated and the intensity can be precisely controlled., 7 pages, 5 figures

The tunnelling of a particle through a potential barrier is a key feature of quantum mechanics that goes to the core of wave–particle duality. The phenomenon has no counterpart in classical physics, and there are no well constructed dynamical observables that could be used to determine ‘tunnelling times’. The resulting debate1–5 about whether a tunnelling quantum particle spends a finite and measurable time under a potential barrier was reignited in recent years by the advent of ultrafast lasers and attosecond metrology6. Particularly important is the attosecond angular streaking (‘attoclock’) technique7, which can time the release of electrons in strong-field ionization with a precision of a few attoseconds. Initial measurements7–10 confirmed the prevailing view that tunnelling is instantaneous, but later studies11,12 involving multi-electron atoms—which cannot be accurately modelled, complicating interpretation of the ionization dynamics—claimed evidence for finite tunnelling times. By contrast, the simplicity of the hydrogen atom enables precise experimental measurements and calculations13–15 and makes it a convenient benchmark. Here we report attoclock and momentum-space imaging16 experiments on atomic hydrogen and compare these results with accurate simulations based on the three-dimensional time-dependent Schrodinger equation and our experimental laser pulse parameters. We find excellent agreement between measured and simulated data, confirming the conclusions of an earlier theoretical study17 of the attoclock technique in atomic hydrogen that presented a compelling argument for instantaneous tunnelling. In addition, we identify the Coulomb potential as the sole cause of the measured angle between the directions of electron emission and peak electric field: this angle had been attributed11,12 to finite tunnelling times. We put an upper limit of 1.8 attoseconds on any tunnelling delay, in agreement with recent theoretical findings18 and ruling out the interpretation of all commonly used ‘tunnelling times’19 as ‘time spent by an electron under the potential barrier’20. Simulation and measurement of the photoionization of atomic hydrogen at attosecond resolution confirm that the tunnelling of the ejected electron is instantaneous.

We present a technique for generating phase-locked two-colour pulses with controllable electric field waveform, whereby the polarization states of the two frequency components, as well as their intensities, relative phase and pulse delay can be completely controlled. To characterize the phase stability and the waveform of the output two-colour pulse in situ, we measured the momentum spectra of protons from dissociative ionization of the hydrogen molecule for different combinations of polarizations.

We present a detailed experimental and theoretical study on the relativistic non-dipole effects in strong-field atomic ionisation by near-infrared linearly-polarised few-cycle laser pulses in the intensity range 1014 -1015 W/cm2. We record high-resolution photoelectron momentum distributions of argon using a reaction microscope and compare our measurements with a truly ab-initio fully relativistic 3D model based on the time-dependent Dirac equation. We observe counter-intuitive peak shifts of the transverse electron momentum distribution in the direction opposite to that of laser propagation as a function of laser intensity and demonstrate an excellent agreement between experimental results and theoretical predictions., 6 pages, 6 figures, Manuscript prepared for submission to Physical Review Letters

Laser reduced graphene has been increasingly attracting broad attention owing to its unique properties and potential applications in energy conversion and storage, flexible electronics, optoelectronics, and nanocomposites. In this study, graphene sheets decorated with Au nanoparticles are fabricated in situ using milliwatt femtosecond laser reduction. The findings reveal an enhancement in both the reduction of graphene oxide sheets and the nucleation and growth of the Au nanoparticles during the in situ laser treatment. Three stages of reactions are considered, namely, (i) the spontaneous redox reaction between HAuCl4 and graphene oxide, (ii) the laser‐induced decomposition of HAuCl4 and reduction of graphene oxide, and (iii) the localized surface plasmon resonance enhanced photoreduction in the presence of Au particles. Moreover, the Au nanoparticles form densely and evenly distributed square‐lattice‐like microcrack networks that ensure a linear resistance change over the tested strain range. This microcrack network architecture enables the development of flexible graphene/Au strain sensors with gauge factors up to 52.5, and linear behaviour up to 25.4% strain. This strain sensor is demonstrated to effectively monitor human motions. The findings leverage the resistive properties of graphene/metal nanoparticle composites with fundamental mechanisms, laying a critical step toward highly functional, low‐cost, flexible, and wearable graphene‐based electronics.

The debate on tunnelling times have always been full of contradictions and the attoclock experiments that measure tunnelling delays in strong-field ionization are no exception. The current review presents the debate and discussions concerning the studies of tunnelling times based only on the attoclock technique. We review them with their implications and pitfalls identified due to lack of accurate strong field models that validate the observations in interpreting the measurements performed on noble gases. In order to provide a complete picture, the review begins with a background on some of the popular tunnelling time definitions, most of them conceived during the late 1980s debate, which are often cited in the attoclock literature. We then discuss various attoclock experiments on noble gas atoms and their interpretations in context of the tunneling time debate. The recently performed attoclock experiment and numerical modelling using atomic hydrogen are also presented as an attempt at resolving the controversy. We conclude with the current status of the debate.

Measurements to control the morphology and characteristics of a picosecond laser produced chromium plasma plume upon double-pulse (DP) irradiation are presented and compared to their single-pulse (SP) counterpart. DP schemes are implemented by employing two geometries where the inter-pulse delay and the spatial separation are the control parameters. The ratio of plume length to plume width decreases upon increasing the inter-pulse delay and/or the energy of the second pulse in the collinear DP scheme. Interestingly, plasmas generated in the DP scheme at lower pressures resemble the expansion features of the plasma generated in the SP scheme at higher pressures. We find that DP schemes are advantageous for applications such as high harmonic generation and the production of quality thin films.

In strong-field ionization, the combined influence of nondipole and Coulomb focusing effects leave unique signatures on the photoelectron momentum distributions, which may have direct implications on many important strong-field phenomena. We present a comprehensive review of the experimental and theoretical investigations on these effects. We classify these investigations by relativistic and non-relativistic modelling based on different theoretical approaches, e.g. classical, semiclassical, strong-field approximation (SFA), time-dependent Schrodinger equation (TDSE), and time-dependent Dirac equation (TDDE). Finally, we discuss the latest experimental and theoretical development on this subject that has undergone so far to deeply understand the underlying physics.

We demonstrate the generation of metastable krypton in the long-lived 1s5 state using laser excitation. The atoms are excited through a two-photon absorption process into the 2p6 state using a pulsed optical parametric oscillator laser operating near 215 nm, after which the atoms decay quickly into the metastable state with a branching ratio of 75 %. The interaction dynamics are modeled using density matrix formalism and, by combining this with experimental observations, we are able to calculate photo-ionization and two-photon absorption cross-sections. When compared to traditional approaches to metastable production, this new approach shows great potential for high-density metastable krypton production with minimal heating of the sample. Here, we show metastable production efficiencies of up to 2% per pulse. The new experimental results gained here, when combined with the density matrix model we have developed, suggest that fractional efficiencies up to 30% are possible under optimal conditions., 6 pages (including references), 6 figures, journal submission v2: formatting updated and corrections made, v3: minor corrections, v4: submitted version, v5: accepted version (minor revisions)

Frustrated tunnel ionization (FTI) is one of the dominant channels in strong field ionization that results in the excitation of atoms. Recent studies have shown that there is a significant number of FTI events for multi-cycle pulses with the theory predicting that the excitation efficiency increases with pulse duration decreasing into the few-cycle regime. Our work concentrates on experimentally investigating the effect of few-cycle pulses on the FTI excitation process. We use pulses with duration 6 fs centred at 800 nm at intensities up to 0.8 PW crossed with an atomic Ar beam. We find that with few-cycle pulses there is more FTI per tunneling event and that for the same pulse energy more FTI is generated.

Time-resolved optical emission spectroscopic measurements of a plasma generated by irradiating a Cr target using 60 picosecond (ps) and 300 ps laser pulses is carried out to investigate the variation in the linewidth ($\delta\lambda$) of emission from neutrals and ions for increasing ambient pressures. Measurements ranging from 10$^{-6}$ Torr to 10$^2$ Torr show a distinctly different variation in the $\delta\lambda$ of neutrals (Cr I) compared to that of singly ionized Cr (Cr II), for both irradiations. $\delta\lambda$ increases monotonously with pressure for Cr II, but an oscillation is evident at intermediate pressures for Cr I. This oscillation does not depend on the laser pulse widths used. In spite of the differences in the plasma formation mechanisms, it is experimentally found that there is an optimum intermediate background pressure for which $\delta\lambda$ of neutrals drops to a minimum. Importantly, these results underline the fact that for intermediate pressures, the usual practice of calculating the plasma number density from the $\delta\lambda$ of neutrals needs to be judiciously done, to avoid reaching inaccurate conclusions.

Atom-trap trace analysis (ATTA) is an ultra-sensitive technique for measurement of noble-gas radio-nuclide ratios at the 10−17 level, with application to environmental science. We explore laser-based metastable excitation techniques to improve measurement efficiency, accuracy and speed.

In this Letter, the statement 'I.I. and A.B. performed computations at the NCI Australia' was missing from the Acknowledgements section. This has been corrected online.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Different ionization processes were observed for acetylene. These time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

We generated and characterized plasma with single and double picosecond laser pulses in order to study the plume dynamics and to control the plasma properties. The double-pulse scheme was found to be superior for the generation of a homogeneous plasma. The lateral expansion was prominent for irradiation schemes wherein energy of the first pulse is lower/equal to that of the second pulse. While the velocities of the fast and slow species were found to be nearly equal, the emission counts corresponding to slow species are larger for single pulse compared to the double pulse.

We report on a systematic investigation of the dissociation dynamics of ${{\mathrm{H}}_{2}}^{+}$ in intense laser fields, and study how the kinetic energy spectrum of the dissociating proton can be modulated by the wavelength and intensity of the driving laser field. In the experiment, ${\mathrm{H}}_{2}$ is dissociatively ionized by an intense laser pulse with varying carrier wavelengths ranging from 800 to 1800 nm and varying peak intensities. A model based on Floquet theory and Landau-Zener theory is adopted to explain the experimental observations. The intensity effect is further explored in a few-cycle pump-probe experiment. We observed a significant intensity-dependent proton kinetic energy shift, which can also be well explained by the theoretical simulation. The wavelength- and intensity-dependent proton spectra reveal the mechanism of selective excitation of vibrational levels of ${{\mathrm{H}}_{2}}^{+}$ in intense laser fields.

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

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 percentlevel accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferrable 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 ionisation in noble-gas atoms, such as the widely employed single-active electron approximation., Article: 5 pages, 2 figures, submitted to PRL (manuscript number LZ14457). Supplementary information: 7 pages, 6 figures, appended to end of main Article

We demonstrate that the coherent control of unimolecular reactions by using a waveform-controlled laser fields can lead to a strong modulation on the yield of the reaction. By using a synthesized \ensuremath{\omega} (1800-nm) and 3\ensuremath{\omega} (600-nm) two-color laser field, the probability of photodissociation of ${\mathrm{H}}_{2}{}^{+}$ can be strongly modulated by varying the relative phase between the two colors. The dissociation probability maximizes at different relative phases for protons with different kinetic energy, and such energy dependence can also be qualitatively reproduced by our simulation. We attribute the observed dissociation probability modulation to the interference between two different dissociation pathways which start from the same electronic states and end with the same kinetic energy.

We studied the dynamics of dissociative double ionization of acetylene using pump-probe technique with few-cycle laser pulses and Reaction Microscope detection system.

We present initial phase shift measurements of high-order harmonic radiation generated from hydrogen isotopes using Gouy Phase interferometer. We will study the nuclear dynamics of molecules after they undergo strong-field ionization by ultrashort laser pulses.

We present accurate measurements of carrier-envelope phase effects on ionisation of the noble gases with few-cycle laser pulses. The experimental apparatus is calibrated by using atomic hydrogen data to remove any systematic offsets and thereby obtain accurate CEP data on other generally used noble gases such as Ar, Kr and Xe. Experimental results for H are well supported by exact TDSE theoretical simulations however significant differences are observed in case of noble gases., Comment: 4 pages, 5 figures, submitted to PRL (manuscript number LH15031)

In this paper we present the first characterisation of growth of a carbonaceous film on a silicon substrate exposed to a metastable atom beam using an in situ rotating polariser ellipsometer. The initial deposition of oil due to a background partial pressure in vacuum is investigated. Subsequent exposure of the deposited oil to a high flux metastable neon (Ne*) beam results in cross-linking of the oil film, creating a polymerised carbonaceous layer. Values for the mean residence time, polymerisation cross-section, and desorption cross-section are calculated and compared to similar studies performed for ion bombardment. Simple estimates can provide reasonable values for application of the theory to other systems.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Vibrating bound acetylene cations or dissociating dications are produced by the first pulse. The second pulse probes the nuclear dynamics by ionization to higher charge states and Coulomb explosion of the molecule. For the bound cations, we observed vibrations in acetylene (HCCH) and its isomer vinylidene (CCHH) along the CC-bond with a periodicity of around 26 fs. For dissociating dication molecules, a clear indication of enhanced ionization is found to occur along the CH- and CC-bonds after 10 fs to 40 fs. The time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

We describe an extreme ultraviolet (XUV) interferometric technique that can resolve ~100 zeptoseconds (10−21 s) delay between high harmonic emissions from two successive sources separated spatially along the laser propagation in a single Gaussian beam focus. Several improvements on our earlier work have been implemented in the advanced interferometer. In this paper, we report on the design, characterization and optimization of the advanced Gouy phase interferometer. Temporal coherence for both atomic argon and molecular hydrogen gases has been observed for several harmonic orders. It has been shown that phase shift of XUV pulses mainly originates from the emission time delay due to the Gouy phase in the laser focus and the observed interference is independent of the generating medium. This interferometer can be a useful tool for measuring the relative phase shift between any two gas species and for studying ultrafast dynamics of their electronic and nuclear motion.

Laser reduction of graphene oxide has attracted significant interest in recent years, because it offers a highly flexible, rapid, and chemical-free graphene fabrication route that can directly write on almost any solid substrate with down to sub-micrometer feature size. Laser-reduced graphene (LRG) is explored for various important applications such as supercapacitors, sensors, field effect transistors, holograms, solar cells, flat lenses, bolometers, thermal sound sources, cancer treatment, water purification, lithium-ion batteries, and electrothermal heaters. This contribution reviews most recent research progress on the aspects of fabrication, properties, and applications of LRG. Particular attention is paid to the mechanism of LRG formation, which is still debatable. The three main theories, including the photochemical process, the photothermal process, and a combination of both processes, are discussed. Strategies for tuning the properties and performance of LRG, such as the laser parameters, chemical doping, structure modulation, and environment control, are highlighted. LRGs with better performance including smaller feature size, higher conductivity, and more flexible morphology design in both 2D and 3D formats will offer tremendous opportunities for advancement in electronics, photonic, and optoelectronic applications. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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.

We study transverse electron momentum distribution (TEMD) in strong field atomic ionization driven by laser pulses with varying ellipticity. We show, both experimentally and theoretically, that the TEMD 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., Comment: 5 pages, 6 figures

Synopsis We present an intensity dependent study of transverse electron momentum distribution obtained from strong field ionization of argon using linearly polarized few-cycle (6 fs) laser pulses in the intensity range 1015W/cm2-1016W/cm2.

We present the results of attosecond angular streaking of atomic Hydrogen using elliptically polarized, 5.5 fs pulses that are centered around 770 nm, within the intensity range of 1.65 to 4 × 1014 W/cm2. We measure angular offsets in the photo-electron momentum distribution relative to the peak of the electric field of light in the polarization plane. We find a strong agreement in these results with the solutions of complete 3D-TDSE simulations. Further, we compute the contribution of coulomb effects (between the ionized electron-parent ion) to the measured angular offsets using Yukawa potential, subtracting which yields the real 'tunneling delays'.

We explore the effect of nuclear mass on the laser-driven electron localization process. We dissociate a mixed H2 and D2 target with intense, carrier-envelope-phase (CEP) stable 6 fs laser pulses and detect the products in a reaction microscope. We observe a very strong CEP-dependent asymmetry in proton and deuteron emission for low dissociation energy channels. This asymmetry is stronger for H2 than for D2. We also observe a large CEP offset between the asymmetry spectra for H2 and D2. Our theoretical simulations, based on a one-dimensional two-channel model, agree very well with the asymmetry spectra, but fail to account properly for the phase difference between the two isotopes.

We present new measurements on the characteristics of a cold cathode, low velocity metastable neon discharge atomic beam source. We have measured both the relative metastable flux and the most probable velocity as a function of the discharge current for four different cathode designs. The results demonstrate that there is a strong dependence of the flux on the design of the cathode, while there is little difference in the velocity of the atomic beams produced.

The quantum electrodynamic description of circularly and linearly polarized, single mode radiation exciting atomic transitions in a hyperfine manifold is extended to the more general case of elliptic polarization. Calculations for the sodium D2 line are tested by observing the line polarization of the resonance flourescence scattered perpendicular to the plane containing laser and atomic beams as a function of the laser polarization. Very good agreement between the calculations and the measurements is achieved. It is demonstrated that predetermined populations and substate coherences can be engineered by optically pumping the transition with the appropriately polarized radiation.

The energy level shifts of Rydberg atoms in confined space were investigated by passing the atoms between two parallel metal plates. The shifts were measured by Doppler-free two-photon absorption in combination with the two-field Ramsey method. Radiative shifts induced by blackbody photons could be observed. At very low temperatures, the shift was determined being on the order of 100 Hz.

We present the results of an experiment in which the stray fields in a niobium probe were measured. This probe simulated the entrance/exit holes of a high- Q cylindrical cavity (micromaser in Garching) where interference effects, attributed to these stray fields, have been observed [Raithel et al., Phys. Rev. Lett. 75 (1995) 3446]. The gradient of the stray electric fields was measured in both the metallic and superconducting phases with ultra high resolution spectroscopy which used a combination of the Doppler free two photon method and the technique of successive oscillatory fields. We obtain a value of 300 mV/cm for the electric field for both phases which is in excellent agreement with the determination of Raithel et al.

We experimentally optimize the yield of attosecond XUV pulses, with pulse duration inferred from carrier-envelope-phase resolved spectral measurements. We also demonstrate generation of dual attosecond pulses with timing resolution of 90 zeptoseconds.

We present the measured photoionization yield of atomic hydrogen as a function of laser intensity for few-cycle pulses. Fits with exact ab-initio simulations produce better agreement than analytical theories and enable accurate intensity calibration.

Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching ${10}^{15}\phantom{\rule{0.28em}{0ex}}\text{W}/{\text{cm}}^{2}$ with an accuracy of 1$%$. This accuracy is achieved by comparing experimental photoelectron yields from atomic hydrogen with predictions from exact numerical solutions of the three-dimensional time-dependent Schr\"odinger equation. Our method can be extended to relativistic intensities and to the use of other atomic species.

We observe the population dynamics within a metastable neon magneto-optical trap (MOT) through the measurement of the average squared Clebsch-Gordan coefficient ${C}^{2}$ over a range of laser detunings. The magnitude of ${C}^{2}$ is dependent on the internal quantum state of an atom interacting with the light field and is found to show a strong dependence on the applied laser detuning. Previously it has been reported [Townsend et al., Phys. Rev. A 52, 1423 (1995)] that trapped atoms in a MOT are pumped towards the states that interact most strongly with the local field and therefore the measured value of ${C}^{2}$ is larger than the average over all possible transitions. For the ${}^{3}{P}_{2}$-to-${}^{3}{D}_{3}$ cooling transition in metastable neon the average ${C}^{2}$ value is equal to 0.46; however, we have measured $0.29\ifmmode\pm\else\textpm\fi{}0.03l{C}^{2}l0.73\ifmmode\pm\else\textpm\fi{}0.09$. We explain this range of values for ${C}^{2}$ by considering the possible transition rates between the different magnetic sublevels in the system. This result has significant consequences when trap populations are measured via fluorescence in a MOT.

This paper reports on the extension of the electron superelastic scattering technique to three new situations. The first considers scattering from the 32P3/2 level of Na that has been excited by two laser modes tuned, respectively, to the transitions from the two hyperfine states of the 32S1/2 ground level. Both coherent and noncoherent modes are treated in a full quantum electrodynamic model of the laser excitation. Under certain conditions, the time-averaged probability of finding an atom in the 32P3/2 level exceeds 0.5. The second situation is electron superelastic scattering from the 32D5/2 level of Na that has been resonantly excited from the ground level via a resonant intermediate level. With the first observation of superelastically scattered electrons from this higher lying level recently recorded, this paper considers the extension of the quantum electrodynamics (QED) model to describe the optical excitation process. Application of superelastic scattering to the 52S1/2–52P3/2 transition of Rb is the third situation considered. The superelastic scattering formalism is extended to allow for a nonzero spin flip cross section for this transition. The resulting optical pumping terms are calculated using the QED model and the method of their determination for the superelastic scattering experiment described. The experimental design necessary to measure all of the collision parameters for this transition is discussed.

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.

Laser produced plasmas are transient in nature, and their properties, which depend on the laser parameters as well as the material properties and the irradiation conditions, can be tailored for different applications. Highly ionized Al plasmas generated using 7 ns and negatively chirped 60 ps pulses are optimized for the purpose of generating Al IV and Al III, respectively. The plasma is optimized spatio-temporally for Al IV or Al III with irradiation energy as the control parameter using time-resolved optical emission spectroscopy. Plasmas attuned for higher charged states could be utilized as a good alternative source for the generation of high order harmonics.

We studied dependence of dissociative ionization in H2 on carrier-envelope phase (CEP) of few-cycle (6fs) near-infrared (NIR) laser pulses. For low-energy channels, we present the first experimental observation of CEP dependence for total dissociation yield and the highest dwgree of asymmetry reported to date (40%). The observed modulations in both asymmetry and total yield could be understood in terms of interference between different n-photon dissociation pathways - n and (n+1) photon channels for asymmetry, n and (n+2) photon channels for yield - as suggested by the general theory of CEP effects (Roudnev and Esry, Phys. Rev. Lett. 99, 220406 (2007), [1]). The yield modulation is found to be Pi-periodic in CEP, with its phase strongly dependent on fragment kinetic energy (and reversing its sign within the studied energy range), indicating that the dissociation does not simply follow the CEP dependence of maximum electric field, as a naive intuition might suggest. We also find that a positively chirped pulse can lead to a higher dissociation probability than a transform limited pulse., 8 pages, 3 figures

We present a new interferometer technique whereby multiple extreme ultraviolet light pulses are generated at different positions within a single laser focus (i.e., from successive sources) with a highly controllable time delay. The interferometer technique is tested with two generating media to create two extreme ultraviolet light pulses with a time delay between them. The delay is found to be a consequence of the Gouy phase shift. Ultimately the apparatus is capable of accessing unprecedented time scales by allowing stable and repeatable delays as small as 100 zs. © 2012 American Physical Society.

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.