I. Bocharova

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

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.

Resonant Auger decay driving intermolecular Coulombic decay in molecular dimers

In 1997, it was predicted that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen-bonded or van-der-Waals-bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low-energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon, raising questions about its role in DNA damage induced by ionizing radiation, in which low-energy electrons are known to play an important part. It was recently suggested that ICD can be triggered efficiently and site-selectively by resonantly core-exciting a target atom, which then transforms through Auger decay into an ionic species with sufficiently high excitation energy to permit ICD to occur. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant-Auger-driven ICD cascade, we find that ICD occurs in less time than the 20 femtoseconds it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X-ray excitation schemes for more localized and targeted cancer radiation therapy.

IR-assisted ionization of helium by attosecond extreme ultraviolet radiation

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experi- ment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolu- tion in both domains simultaneously. We show that resonant absorption is impor- tant in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660-94).

Following dynamic nuclear wave packets in N{sub 2},O{sub 2}, and CO with few-cycle infrared pulses

We study the evolution of nuclear wave packets launched in molecular nitrogen, oxygen, and carbon monoxide by intense 8-fs infrared pulses. We use velocity map imaging to measure the momentum of the ion fragments when these wave packets are interrogated by a second such pulse after a variable time delay. Both quasibound and dissociative wave packets are observed. For the former, measurements of bound-state oscillations are used to identify the participating states and, in some cases, extract properties of the relevant potential-energy surfaces. Vibrational structure is resolved in both energy and oscillation frequencies for the cations of oxygen and carbon monoxide, displaying the same quantum wave-packet motion in both energy and time domains. In addition, vibrational structure is seen in the dication of carbon monoxide in a situation where the energy resolution by itself is inadequate to resolve the structure.

Dynamic modification of the fragmentation of autoionizing states of O{sub 2}{sup +}

The dynamic process of fragmentation of excited states of the molecular oxygen cation is investigated in a two-part study. First, using monochromatic 41.6 eV radiation and cold-target recoil-ion momentum spectroscopy detection of O{sup +} + O{sup +} ion pairs and associated electrons, we establish that this channel is populated only by an indirect process enabled by autoionization of excited oxygen atoms and identify the final active potential curves. Second, we probe the dynamics of this process using an attosecond pulse train of 35-42 eV EUV followed by an intense IR laser pulse. The results are compared with a model calculation.

Effects of laser pulse duration and intensity on Coulomb explosion of CO2: Signatures of charge-resonance enhanced ionization

We studied laser-induced Coulomb explosion of CO2 by full triple-coincidence momentum resolved detection of resulting ion fragments. From the coincidence momentum data we can reconstruct molecular geometry immediately before explosion. We observe the dynamics of Coulomb explosion by comparing reconstructed CO2 geometries for different Ti:Sapphire laser pulse durations (at the same intensity) ranging from few cycles (7 fs) to 200 fs. We conclude that for longer pulse durations (?100 fs) Coulomb explosion proceeds through the enhanced ionization mechanism taking place at the critical O-O distance of 8 a.u., similarly to well known charge-resonance enhanced ionization (CREI) in H2.

Probing excited states dynamics in CO cations using few-cycle IR and EUV laser pulses

We have used few-cycle IR and EUV laser pulses in pump-probe arrangement to trace out the dissociation pathways in CO when exploded by strong laser fields. We present two preliminary sets of data of different pump pulses. In these sets, different excited state of CO cations are populated using (< 10 fs) IR, and EUV pulses respectively. We followed the time evolution of these states using the time-resolved Coulomb explosion imaging technique. We compare the time evolution of IR- and EUV-induced excited states by measuring the KER of the fragment ions as a function of the time delay between the pump and the IR probe pulse.

IR-assisted ionization of He by attosecond XUV radiation

In this work we characterize the underlying processes behind the IR-assisted ionization of helium by extreme ultra violet (XUV) attosecond radiation. We show that, if used in the form of an attosecond pulse train, the XUV radiation can have a good spectral resolution as well as good temporal resolution. Our experimental data and associated theoretical calculations show how to use the properties of attosecond XUV radiation to induce a desired excited state of helium, and how to tune the IR laser electric field to control the ionization pathways.

Inelastic rescattering processes in molecules measured with few-cycle laser pulses

We study electron recollision processes induced in D2 by the nonlinear interaction with 800 nm few-cycle laser pulses. We show that sequential double ionization is suppressed at intensities below 2 ? 1014?W?cm?2, and the inelastic rescattering processes (electronic excitation and double ionization) become dominant and can be investigated as a function of intensity and ellipticity. At 1 ? 1014?W?cm?2, the D+ kinetic energy spectrum arises from recollision-induced electronic excitation (D2+?D++D) and is explained by the contributions of two dissociative excited electronic states of D2+ (2?u+ and 2?u).