E. Wells

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 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.

Double and single ionization of hydrogen molecules by fast-proton impact

The ratio of double to single ionization of hydrogen molecules caused by fast-proton impact was measured over a wide velocity range (v = 4-24 au) using the coincidence time-of-flight technique. The value of this ratio for hydrogen molecules at the high-velocity limit was determined to be 0.18+0.01-0.02 using the q/v dependence suggested by McGuire. This ratio is smaller by about a factor of 1.8 for hydrogen than for helium over the measured energy range and by about 1.4 at the high-velocity limit. This difference between the two targets is due mainly to the single-ionization cross section, which was measured to be larger by a factor of 1.79 ± 0.05 for hydrogen molecules than for helium. The double-ionization cross section, in contrast, is similar for both helium and hydrogen targets. It is suggested that single ionization of hydrogen molecules is more likely due to its smaller binding energy while the stronger electron-electron interaction in helium compensates for the smaller probability of proton impact ionization and leads to roughly equal double ionization of both targets. For both hydrogen and helium targets, the double- to single-ionization ratio is smaller for proton impact than for equal-velocity electron impact over the measured velocity range.

Velocity dependence of electron removal and fragmentation of water molecules caused by fast proton impact

The yields of the dissociation products of water ionized by fast proton impact were measured relative to H2O+ using the coincidence time-of-flight method. The relative yields following single ionization were found to be independent of the collision velocity over the range of collision velocities measured and similar to the values reported for ionization by equal velocity electron impact, except for a higher rate of H2+ production. Double ionization of water leads predominantly to two dissociation channels, namely H++OH+ and H++O++H, which are equally likely and also independent of the collision velocity. The O2+ fragment was found to be always in coincidence with H+, thus suggesting it is only a product of triple (or higher) ionization of water.

Electron capture and fragmentation in Ar11+ + CO collisions

Collisions between 32.2, 130.5 and 570.5 keV Ar11+ ions and CO molecules have been studied using the Macdonald Laboratory CRYEBIS. Coincidence time of flight was used to detect all recoil ions originating from each molecule and a position sensitive detector was used to determine final projectile charge states. Single-and double-electron capture cross-sections are much larger than those for ionization at these collision energies. The dominant recoil channel associated with the Ar10+ final charge state is the CO+ molecular ion. The main ion-pair channel is the C+ + O+ dissociation of CO2+ while the relative yields of higher charge states of the transient COq+ fall off rapidly. The dissociated ions corresponding to charge states up to CO4+ were detected in coincidence with Ar10+ (and Ar9+), indicating that multielectron capture followed by autoionization occurs.

The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense laser fields

Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C2D2, C2D4 and C2D6 reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields.

Charge exchange at very low collision energies

Two of the simplest collision systems one can study are H++H(1s) and H++D(1s). Electron transfer is resonant in the first and nearly resonant in the latter because of the 3.7 meV gap between H(1s) and D(1s). Once the collision velocity becomes small enough quantum effects become more pronounced. However, these very low energies, of a few meV, are inaccessible using standard collision techniques. We hereby suggest a method in which a dissociating HD+ molecular ion is the “accelerator” used to measure electron transfer in the H++D(1s) collision system down to a few meV. When a HD molecule is ionized quickly about 1% of the HD+(1sσ) is in the vibrational continuum. During the dissociation, the electron initially centered on the D core can make a transition to the H core when the 2pσ and the 1sσ potential energy curves associated with the two dissociation limits get close to each other. It is important to note that during molecular dissociation the “avoided crossing” is crossed only once in contrast to twice ...

Photon-ion collisions and molecular clocks

The timing of molecular rearrangements can be followed in the time domain on a femtosecond scale by using momentum imaging techniques. Three examples are discussed in this paper: first, the diffraction of electrons ejected from the K-shell of one of the atomic constituents of the molecule takes a ‘picture’ of the molecule, and the correlation between the momentum vector of the photoelectron and the subsequent fragmentation pattern is used to estimate the time delay which accompanies the latter process. Second, the kinetic energy release of proton pairs from the double ionization of hydrogen by fast laser pulses is timed using the optical cycle as a clock. The mechanisms of rescattering, sequential and enhanced ionization are clearly identified in the momentum spectra. Third, the operation of rescattering double ionization in the case of nitrogen and oxygen molecules is discussed.

Comparison of single-ionization of hydrogen molecules to that of helium in collisions with fast protons

The ratio of single ionization (SI) of hydrogen to that of helium was measured using the time of flight technique for 1–12 MeV proton impact ionization. A gas target composed of a 1:1 mixture of D2 and 3He was used to determine the ratio of single ionization of these two-electron targets relative to each other. Using the known double- to single-ionization ratios for both of these targets, we have also evaluated the ratio of their double ionization relative to each other. Preliminary results show that the ratio D2+/3He+ is 1.79±0.05. In contrast, double ionization of these two targets is approximately equal. It is suggested that single ionization of hydrogen molecules is more likely due to its smaller binding energy while the stronger electron-electron interaction in helium compensates for the smaller probability of proton impact ionization and leads to roughly equal double ionization of both targets.The ratio of single ionization (SI) of hydrogen to that of helium was measured using the time of flight technique for 1–12 MeV proton impact ionization. A gas target composed of a 1:1 mixture of D2 and 3He was used to determine the ratio of single ionization of these two-electron targets relative to each other. Using the known double- to single-ionization ratios for both of these targets, we have also evaluated the ratio of their double ionization relative to each other. Preliminary results show that the ratio D2+/3He+ is 1.79±0.05. In contrast, double ionization of these two targets is approximately equal. It is suggested that single ionization of hydrogen molecules is more likely due to its smaller binding energy while the stronger electron-electron interaction in helium compensates for the smaller probability of proton impact ionization and leads to roughly equal double ionization of both targets.

Charge dependence of one and two electron processes in collisions between hydrogen molecules and fast projectiles

The ratio of double- to single-ionization (DI/SI) as well as the ratio of ionization-excitation to single-ionization (IE/SI) in hydrogen molecules was studied by examining the effect of the projectile charge on these processes. The DI/SI and IE/SI ratios were measured using the coincidence time of flight technique at a fixed velocity (1 MeV/amu) over a range of projectile charge states (q = 1-9,14,20). Preliminary results indicate that for a highly charged F{sup 9+} projectile the DI/SI and IE/SI ratios are 6.8% and 24.7%, respectively, a large increase from the ratios of 0.13% and 1.95%, respectively, for H{sup +} projectiles. For low charge states, the DI/SI is negligible relative to the IE/SI ratio, while for more highly charged projectiles the DI/SI ratio becomes comparable to the IE/SI ratio. This indicates that double-ionization increases much more rapidly with projectile charge than ionization-excitation.