We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Ultrasound computed tomography (USCT) holds great promise for improving the detection and management of breast cancer. Because they are based on the acoustic wave equation, waveform inversion-based reconstruction methods can produce images that possess improved spatial resolution properties over those produced by ray-based methods. However, waveform inversion methods are computationally demanding and have not been applied widely in USCT breast imaging. In this work, source encoding concepts are employed to develop an accelerated USCT reconstruction method that circumvents the large computational burden of conventional waveform inversion methods. This method, referred to as the waveform inversion with source encoding (WISE) method, encodes the measurement data using a random encoding vector and determines an estimate of the sound speed distribution by solving a stochastic optimization problem by use of a stochastic gradient descent algorithm. Both computer-simulation and experimental phantom studies are conducted to demonstrate the use of the WISE method. The results suggest that the WISE method maintains the high spatial resolution of waveform inversion methods while significantly reducing the computational burden.

We present a study of multiphoton dissociative ionization from molecules. By solving the timedependent Schrödinger equation for H + 2 and projecting the solution onto double continuum scattering states, we observe the correlated electron-nuclear ionization dynamics in detail. We show -for the first time -how multiphoton structure prevails as long as the energies of all fragments are accounted for. Our current work provides a new avenue to analyze strong-field fragmentation that leads to a deeper understanding of the correlated molecular dynamics.PACS numbers: 31.15. 33.20.Wr, 33.20.Xx, 33.80.Rv Despite more than 20 years of scrutiny, strong-field dissociative ionization of molecules is still not completely understood. Understanding this process, however, would provide insight into how the energy deposited in the molecule by an intense laser pulse is shared between the electrons and the nuclei via their correlated motion. A large part of the challenge in investigating this process is that the dynamics of an ionized electron is not easily treated by the usual Born-Oppenheimer (BO) approximation. Complicating the issue further is the fact that the final state lies in at least a double continuum, likely Coulombic, comprised of the free nuclear and electronic motion which raises fundamental questions about the analysis.Because ab initio calculations of dissociative ionization require going beyond the BO approximation, the vast majority of intense field dissociative ionization calculations have been carried out for the simplest molecule: H + 2 (see, for example, Refs. [1][2][3]). Even for this system, however, full-dimensional solutions of the time-dependent Schrödinger equation (TDSE) have not yet been obtained in an intense field as they pose an immense computational challenge. Consequently, the dissociative ionization calculations that have been done simplified the problem even further through ad hoc reductions of the dimensionality or other severe approximations.Nevertheless, intense field processes identified via H [8,9] and applied to other systems [10,11]. More recently, two new models have been proposed to explain unexpected structure measured in the nuclear kinetic energy release (KER) spectrum following ionization of H + 2 . In one model, the interference of two dissociation pathways leads to the modulation of the KER spectrum via CREI [12,13]. The other model, named above threshold Coulomb explosion (ATCE), is based on the Floquet potentials obtained by dressing not only the field-free BO potentials but also the 1/R Coulomb explosion curve with photons from the laser field (see Fig. 4) [14]. The observed structure in the KER spectrum is thus due to absorption of different numbers of photons [14,15]. The predictions of the models deviate for low intensities and there is no consensus on which is correct. So, in addition to answering fundamental questions about electronnuclear correlations, TDSE solutions -along with an accurate way to analyze them -are needed to resolve this controversy.Key to understan...

High-order (three-photon or more) above-threshold dissociation (ATD) of H(2)(+) has generally not been observed using 800 nm light. We demonstrate a strong enhancement of its probability using intense 7 fs laser pulses interacting with beams of H(2)(+), HD(+), and D(2)(+) ions. The mechanism invokes a dynamic control of the dissociation pathway. These measurements are supported by theory that additionally reveals, for the first time, an unexpectedly large contribution to ATD from highly excited electronic states.

The dissociation of an H + 2 molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10 14 W/cm 2 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 1-photon interference predominantly contributes at H + +H kinetic energy releases of 0.2 -0.45 eV, and net-2-photon and 1-photon interference contributes at 1.65 -1.9 eV. These measurements of the benchmark H + 2 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. PACS numbers: XXXOne ultimate goal of ultrafast, strong-field laser science is to coherently control chemical reactions [1][2][3]. A prerequisite to achieving this goal is to understand the control mechanisms and reaction pathways. To this end, tailoring the electric field waveform of few-cycle laser pulses to control reactions and uncover the underlying physics has become a powerful tool [4][5][6]. It has been applied to the dissociative ionization of H 2 and its isotopologues [7][8][9][10][11][12] and has recently been extended to more complex diatomic molecules, such as CO [13][14][15], and to small polyatomic molecules [16,17].Conceptually, one of the most basic features of a fewcycle laser pulse to control is the carrier-envelope phase (CEP). When the laser's electric field is written as E(t) = E 0 (t) cos(ωt + φ), E 0 (t) is an envelope function, ω is the carrier angular frequency, and φ is the CEP. In fact, all of the few-cycle waveform experiments cited above used the CEP as the control parameter.For example, Kling et al. used 5 fs, 1.2×10 14 W/cm 2 pulses with stabilized CEP to dissociatively ionize D 2 and found asymmetries in the emission direction of D + ions for kinetic energy releases (KER) above 6 eV [7,8]. The diminished dissociation signal in a circularly polarized laser field indicated that recollision played a role. Recollision entails a tunnel-ionized electron undergoing a collision with its parent ion after acceleration by the oscillating laser field [18,19]. The energy exchange between the laser-driven electron and the parent ion can promote the D + 2 to the 2pσ u excited state. Coupling of the 2pσ u and 1sσ g states [20] on the trailing edge of the laser pulse during the dissociation of D + 2 was suggested as the explanation for the CEP-dependent asymmetry [7,8].Another example comes from Kremer et al. who exposed an H 2 target to 6 fs, 4.4×10 14 W/cm 2 CEPstabilized laser pulses and observed asymmetries for KER values between 0.4 and 3 eV [9] -energies they attributed to bond softening (BS) [21] and not electron recollision, which has higher KER. They proposed that the initial ionization of H 2 generates a coherent wav...