K. D. Carnes

Absolute triple differential cross sections for photo-double ionization of helium - experiment and theory

We have measured absolute triple differential cross sections for photo-double ionization of helium at 20 eV excess. The measurement covers the full ranges of energy sharing and emission angles of the two photoelectrons. We compare our data for selected geometries with the convergent close-coupling (CCC) calculations as well as 2SC calculations by Pont and Shakeshaft and 3C calculations by Maulbetsch and Briggs. In terms of the absolute magnitude and the trend in the shapes of the triple differential cross section for different geometries we find good agreement of the CCC and published 2SC calculations with our measurement, though differences with respect to the observed shape of individual patterns still exist.

Adaptive strong-field control of chemical dynamics guided by three-dimensional momentum imaging

Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the nω+C2D4→C2D3++D interaction is manipulated. Second, isomerization of acetylene (nω+C2H2→C2H2(2+)→CH2++C+) is controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule.

Carrier-Envelope Phase Control over Pathway Interference in Strong-Field Dissociation of H2+

The dissociation of an H2+ molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10(14) W/cm2 with a central wavelength of 730 nm was studied using a coincidence 3D momentum imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission direction of H+ fragments relative to the laser polarization were observed. These asymmetries are caused by interference of odd and even photon number pathways, where net zero-photon and one-photon interference predominantly contributes at H+ + H kinetic energy releases of 0.2-0.45 eV, and net two-photon and one-photon interference contributes at 1.65-1.9 eV. These measurements of the benchmark H2+ molecule offer the distinct advantage that they can be quantitatively compared with ab initio theory to confirm our understanding of strong-field coherent control via the carrier-envelope phase.

Coincidence time-of-flight studies of molecular fragmentation

Abstract Molecular fragmentation of diatomic and small polyatomic molecules caused by collisions with fast highly-charged ions are under study using coincidence time-of-flight spectroscopy. A time-of-flight spectrometer in which all positive ions are measured in coincidence was developed for these studies. In this spectrometer, the fragments, which arrive at the detector sequentially, are detected and recorded event-by-event using a multiple hit electronic system. Using this method, detailed information about the fragmentation process can be gathered. For example, the production cross section of the different breakup channels, the average kinetic energy released in the breakup, and the kinetic energy distribution of the fragments can be measured. The uniform electric fields in the time-of-flight spectrometer and the simple geometry used simplifies data analysis such that Monte Carlo simulations commonly used for these studies are not needed.

Carrier-envelope-phase stabilized terawatt class laser at 1 kHz with a wavelength tunable option

We demonstrate a chirped-pulse-amplified Ti:Sapphire laser system operating at 1 kHz, with 20 mJ pulse energy, 26 femtosecond pulse duration (0.77 terawatt), and excellent long term carrier-envelope-phase (CEP) stability. A new vibrational damping technique is implemented to significantly reduce vibrational noise on both the laser stretcher and compressor, thus enabling a single-shot CEP noise value of 250 mrad RMS over 1 hour and 300 mrad RMS over 9 hours. This is, to the best of our knowledge, the best long term CEP noise ever reported for any terawatt class laser. This laser is also used to pump a white-light-seeded optical parametric amplifier, producing 6 mJ of total energy in the signal and idler with 18 mJ of pumping energy. Due to preservation of the CEP in the white-light generated signal and passive CEP stability in the idler, this laser system promises synthesized laser pulses spanning multi-octaves of bandwidth at an unprecedented energy scale.

Disentangling the volume effect through intensity-difference spectra: application to laser-induced dissociation of H2+.

An intensity-difference spectrum method is developed to disentangle the intensity volume effect inherent in focused laser beam interaction with gas-phase matter. This method is applicable to a Gaussian beam of constant axial intensity, which keeps the exact contribution from a predetermined intensity range and eliminates the contributions from lower intensities. We apply this method to the angularly resolved kinetic energy release spectrum of laser-induced dissociation of H2+. The difference spectrum at higher intensities is found to be dominated by the bond-softening process, and the distribution shifts to lower energy and becomes narrower with increasing intensity.

Ionization and excitation of hydrogen molecules by fast proton impact

The ratio of ionization - excitation to single ionization of hydrogen molecules caused by fast proton impact was measured over a wide velocity range (v = 6 - 24 au) using the coincidence time-of-flight technique. This ratio, %, is independent of the collision velocity at high velocities. It differs from the ratio of total to production mostly due to a large contribution from the dissociation of the electronic ground state of the molecular ion. The dissociation fraction of was measured and compares well with our calculations using the Franck - Condon approximation.

Low divergence low energy recoil ion source

Abstract We report test results for a recoil ion source that was developed at the J.R. Macdonald Laboratory. This new source has the advantage of producing an essentially parallel beam of low energy highly charged recoil molecular and atomic ions with a high ion extraction efficiency. We measured recoil beams of various energies (50–900 eV/ q ). The beam diameter was determined to be approximately 2 mm with a divergence of 5 mrad. The extraction efficiency for recoil ions produced in the source was approximately 30%. The recoil ion source was also operated in a pulsed mode by applying a 100 ns 40 V pulse to provide the extraction field for the recoils. The source was pumped by a 1 MeV/amu F 4+ dc beams. Using this technique we measured pulsed recoil ion beams of Ne q + ( q = 2–5), Ar q + ( q = 3–8), He + and He 2+ .

Determining the absolute efficiency of a delay line microchannel-plate detector using molecular dissociation

We present a method to measure the absolute detection efficiency of a delay-line microchannel-plate detector using the breakup of diatomic molecular ions. This method provides the absolute total detection efficiency, as well as the individual efficiency for each signal of the detector. The method is based on the fact that molecular breakup always yields two hits on the detector, but due to finite detection efficiency some of these events are recorded as single particles while others are detected in pairs. We demonstrate the method by evaluating the detection efficiency for both timing and position signals of a delay-line detector using laser-induced dissociation of molecular ions. In addition, the detection efficiency as a function of position has been determined by dividing the detector into sectors.

Fragmentation of CH4 in collisions with fast highly charged ions

Measurements of the production cross sections for singly charged molecular ions by high energy (1 MeV/amu) Fq+ (q=4,6–9), Siq+ (q=5,6,8–12), and Cq+ (q=3–6) projectiles incident on a CH4 target are reported. Similar measurements for singly charged recoil ions are presented for H+ (E=1–11 MeV) projectiles incident on CH4. We observed good agreement between our proton results and earlier electron measurements. Theoretical calculations for the sum of the cross sections for production of all singly ionized CHn fragments from CH4 are presented using the Born and Glauber approximations, and both calculated cross sections are in reasonable agreement with the measured cross sections. Production of CH+3, CH+2, CH+, and C+ fragments is analyzed both as a function of projectile charge and projectile velocity. Deviation from quasiequilibrium theory is observed in the relatively small cross section ratios of C+ and CH+ to CH+4 as a function of projectile charge.