Signatures of Light-Induced Potential Energy Surfaces in H2+

Using theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating H+2. © 2020 Journal of Physics: Conference Series. All rights reserved

The development and application of the alteration strength index equation

We have developed an Alteration Strength Index (ASI) equation to address the effect of hydrothermal alteration on mechanical rock properties. This equation can be used to estimate a range of rock strengths, comparable to uniaxial compressive strength (UCS), based on rapid analysis of mineralogy and microstructure. We used rock samples from three geothermal fields in the Taupo Volcanic Zone (TVZ) to represent a range of alteration types. These are sedimentary, intrusive and extrusive rocks, typical of geothermal systems, from shallow and deep boreholes (72 measured Depth (mD) to 3280 mD). The parameters used in ASI were selected based on literature relating these aspects of mineralogy and microstructure to rock strength. The parameters in ASI define the geological characteristics of the rock, such as proportions of primary and secondary mineralogy, individual mineral hardness, porosity and fracture number. We calibrated the ASI against measured UCS for our samples from the TVZ to produce a strong correlation (R2 of 0.86), and from this correlation we were able to derive an equation to convert ASI to UCS. Because the ASI–UCS relationship is based on an empirical fit, the UCS value that is obtained from conversion of the ASI includes an error of 7 MPa for the 50th percentile and 25 MPa for the 90th percentile with a mean error of 11 MPa. A sensitivity analysis showed that the mineralogy parameter is the dominant characteristic in this equation, and the ASI equation using only mineralogy can be used to provide an estimated UCS range, although the error (or uncertainty) becomes greater. This provides the ability to estimate strength even when either fracture or porosity information are not available, for example in the case of logging drill cuttings. This research has also allowed us to provide ranges of rock strengths based solely on the alteration zones, mineralogy, and depth of lithologies found in a typical geothermal field that can be used to update conceptual models of geothermal fields

Manipulating quantum paths for novel attosecond measurement methods

Sources of attosecond-duration light pulses provide the fastest time resolutions available today for observing ultrafast phenomena in atoms, molecules and condensed matter. The measurement of such pulse durations is challenging because the spectrum lies in the vacuum ultraviolet or soft X-ray range. Two classes of pulse duration measurements now exist; they are classified according to whether the measurement is performed in the generating medium or in a second medium. The first measurement class is called 'in situ' and depends on gently perturbing the electron responsible for attosecond pulse formation. The second measurement class, which we refer to as 'ex situ', takes place in a second medium in which photoelectrons are produced by the attosecond pulses; a synchronized laser field perturbs the photoelectron spectrum. This Review compares and contrasts these two approaches for measuring attosecond pulses. \ua9 2014 Macmillan Publishers Limited.Peer reviewed: YesNRC publication: Ye

Ponderomotive effects in zero kinetic energy photoelectron spectroscopy with intense femtosecond pulses

Zero kinetic energy photoelectron (ZEKE) spectroscopy may be an interesting technique for monitoring the time evolution of state selected atomic products in ultrafast pump-probe experiments. We investigate here the effects of laser intensity by studying three-photon non-resonant ZEKE spectra of atomic xenon at the first ionization threshold with tunable femtosecond UV pulses. High intensity introduces a new broadening due to ponderomotive effects in the ZEKE spectrum. It is demonstrated that a three-photon ZEKE spectrum can be recorded at intensities where the broadenign due to these ponderomotive effects (i.e. the ac Stark shift for high-n Rydberg states) is small in comparison with the effective laser bandwidth.NRC publication: Ye

Femtosecond time-resolved zero kinetic energy photoelectron and phtoionization spectroscopy studies of I₂ wavepacket dynamics.

We report the application of femtosecond zero kinetic energy (ZEKE) photoelectron spectroscopy and photoionization to the study of bound wavepacket motion in the electronic B state of iodine by a [1 + 2] multiphoton process. The experimental approach is described in detail. Comparisons of ZEKE photoelectron detection with ion detection are made. The pump wavelength was fixed at 580 nm whereas the probe wavelength was tuned from 350 to 300 nm. Wavepacket motion in the B state with the fundamental vibrational period of 340 fs was observed, as well as wavepacket dephasing and rephasing. Differences in the modulation depths and Fourier transform power spectra were observed. The phase shifts (time offset from [Delta]t = 0) and modulation patterns varied as the location of the Condon point for the probe step changed upon tuning the probe wavelength. Higher-order wavepacket fractional revivals were observed in time-windowed Fourier transform power spectra.NRC publication: Ye

Space charge and plasma effects in zero kinetic energy (ZEKE)photoelectron spectroscopy

In photoelectron spectroscopy experiments it is generally assumed that the Coulomb force between charged particles is small compared with external fields, and that the free kinetic electrons will quickly leave the ions. This is the basis of the ZEKE photoelectron spectroscopy. However as the density of charged particles is increased, plasma physics effects begin to become important, and the kinetic electrons become trapped by the net positive charge and move so as to set up a self-field which can cancel any externally imposed electric fields. For high densities, fewer electrons than expected are able to escape the self-field. The production of self-consistent electric fields is studied by means of particle-in-cell plasma simulations and by N-body trajectory calculations, and simple expressions are derived for when plasma physics effects become significant. An experimental illustration of plasma effects in ZEKE is presented.Peer reviewed: YesNRC publication: Ye

Femtosecond wave-packet dynamics studied by time-resolved zero-kinetic energy photoelectron spectroscopy

Femtosecond pump-probe zero-kinetic-energy (ZEKE) photoelectron spectroscopy is studied using the known wave-packet dynamics of I2 (B state). The 340 fs wave-packet period, wave-packet dephasing and rephasing are observed in the ZEKE signal. The effect of various laser and ZEKE parameters on the wave-packet dynamics is discussed.Peer reviewed: YesNRC publication: Ye

Strong field processes inside gallium arsenide

We demonstrated experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We observed modulation in the ionization rate of a linearly-polarized 1900 nm laser beam directly by measuring its transmission while rotating the crystal, without Fourier analysis. We propose that the modulation in the ionization rate arises from periodic variation in the reduced carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with Keldyshs non-resonant ionization theory for solids. This opens up a novel method for non-invasive crystallography of semiconductor materials. \ua9 2014 IOP Publishing Ltd.Peer reviewed: YesNRC publication: Ye

Attosecond lighthouse driven by sub-two-cycle, 1.8 \u3bcm laser pulses

We generate space-time coupled attosecond pulse trains with a 1.8 \u3bcm wavelength laser pulse using the 'attosecond lighthouse' technique. We show low divergence, spatially well-separated beamlets from low ionization potential gas media. We also find that there is little long trajectory contribution - only the short trajectory contribution is clearly visible for any beamlet. These results open a new route for extending attosecond technology to higher cut off energy and shorter pulse duration. \ua9 2015 IOP Publishing Ltd.Peer reviewed: YesNRC publication: Ye