Strong-field Dissociative Ionization Research Articles

Identifying photoelectron releasing order in strong-field dissociative ionization of H2.

Driven by intense laser fields, the outgoing photoelectrons in molecules possess a quiver motion, resulting in the rise of the effective ionization potential. The coupling of the field-dressed ionization potential with abundant molecular dynamics complicates the laser-molecule interactions. Here, we demonstrate an approach to resolve photoelectron releasing order in the dissociative and non-dissociative channels of multiphoton ionization driven by an orthogonally polarized two-color femtosecond laser pulse. The photoelectron kinetic energy releases and the regular nodes in the photoelectron angular distributions due to the participation of different continuum partial waves allow us to deduce the field-dressed ionization potential of various channels. It returns the ponderomotive energy experienced by the outgoing electron and reveals the corresponding photoionization instants within the laser pulse. Our results provide a route to explore the complex strong-field ionization dynamics of molecules using two-dimensional photoelectron momentum spectroscopy.

Femtosecond Rotational Dynamics of D_{2} Molecules in Superfluid Helium Nanodroplets.

Rotational dynamics of D_{2} molecules inside helium nanodroplets is induced by a moderately intense femtosecond pump pulse and measured as a function of time by recording the yield of HeD^{+} ions, created through strong-field dissociative ionization with a delayed femtosecond probe pulse. The yield oscillates with a period of 185fs, reflecting field-free rotational wave packet dynamics, and the oscillation persists for more than 500periods. Within the experimental uncertainty, the rotational constant B_{He} of the in-droplet D_{2} molecule, determined by Fourier analysis, is the same as B_{gas} for an isolated D_{2} molecule. Our observations show that the D_{2} molecules inside helium nanodroplets essentially rotate as free D_{2} molecules.

Orientation-dependent dissociative ionization of H2 in strong elliptic laser fields: Modification of the release time through molecular orientation

We investigate the photoelectron angular emission distributions obtained by strong field dissociative ionization of H$_2$ using cold target recoil ion momentum spectroscopy. In case of employing laser light with an ellipticity close to 0.9 and an intensity of 1.0 $\times$ 10$^{14}$ W/cm$^2$, we find that the most probable release-time of the electron does not generally coincide with the time when the laser field maximizes. The release-time is affected by the molecular orientation. In addition, we observe that the width of the release-time distribution depends on molecular orientation. We attribute this observation to the two-center-interference.

Strong Field dissociative ionization of the

SynopsisStrong field dissociative ionization of the molecule in the multiphoton regime is investigated employing ab initio calculations for fixed molecular axis orientations. The resulting joint (photoelectron and nuclear kinetic) energy spectra are explored. The number of photons absorbed in each multiphoton peak of these spectra is determined analyzing the angular distribution of the photoelectrons, while the exact path followed by is identified studying the time evolution of the nuclear wave packets.

Strong field dissociative ionization of the [formula omitted]: Nuclear wave packet analysis

Theoretical ab initio investigation of strong field dissociative ionization of the D2+ molecule in the multiphoton regime is reported. The dynamics is initiated by ultrashort laser pulses for fixed molecular axis orientations. Nuclear wave packet calculations are performed to provide the joint energy spectra (JES): ionization-dissociation probability density via electron (Ee) and nuclear (En) kinetic energy. Analyzing the time-dependent nuclear wave packet densities we have successfully identified the exact path followed by the D2+ target for each multiphoton peak.

Inter- and intracycle interference effects in strong-field dissociative ionization

We theoretically study dissociative ionization of H$_2^+$ exposed to strong linearly polarized few-cycle visible, near-infrared and midinfrared laser pulses. We find rich energy-sharing structures in the combined electron and nuclear kinetic energy spectra with features that are a priori at odds with simple energy conservation arguments. We explain the structures as interferences between wave packets released during different optical cycles, and during the same optical cycle, respectively. Both inter- and intracycle interference structures are clearly visible in the joint energy spectra. The shapes of the interference structures depend on the dynamics leading to the double continuum, and carry sub-femtosecond information.

VUV excitation of a vibrational wavepacket in D2 measured through strong-field dissociative ionization

Femtosecond vacuum ultraviolet pulses from a monochromated high harmonic generation source excite vibrational wavepackets in the state of D2. The wavepacket motion is measured through strong field ionization into bound and dissociative ion states yielding and D+ products. The time dependence of the and D+ ion signals provides a sensitive fingerprint of the quantum nuclear wavepacket, due to the different ionization rates for the two channels. The experiments are modelled with excitation and ionization processes included explicitly, with the results of the model showing a very good agreement with the experimental observations. The experiment demonstrates the level of detail attainable when studying ultrafast quantum nuclear dynamics using high harmonic sources.

Multielectron effects in strong-field dissociative ionization of molecules

We study triple-ionization-induced, spatially asymmetric dissociation of ${\mathrm{N}}_{2}$ using angular streaking in an elliptically polarized laser pulse in conjunction with few-cycle pump-probe experiments. The kinetic-energy-release dependent directional asymmetry in the ion sum-momentum distribution reflects the internuclear distance dependence of the fragmentation mechanism. Our results show that for 5--35-fs near-infrared laser pulses with intensities reaching 10${}^{15}$ W/cm${}^{2}$, charge exchange between nuclei plays a minor role in the triple ionization of N${}_{2}$. We demonstrate that angular streaking provides a powerful tool for probing multielectron effects in strong-field dissociative ionization of small molecules.

Ultrafast Relaxation Dynamics of Uracil Probed via Strong Field Dissociative Ionization

We study the ultrafast relaxation dynamics of uracil excited to the first bright ππ* state (S2) by an ultrafast laser pulse in the deep ultraviolet (central wavelength λ0 = 260 nm). With a unique combination of strong field dissociative ionization measurements, state of the art strong field ionization calculations, and high level ab initio calculations of excited neutral and ionic states at critical points along the neutral potentials, we are able to gain a detailed picture of the relaxation dynamics of the molecule, which resolves earlier disagreements regarding measurements and calculations of the relaxation.

Following Ultrafast Radiationless Relaxation Dynamics With Strong Field Dissociative Ionization: A Comparison Between Adenine, Uracil, and Cytosine

We present the application of ultrafast time- and mass-resolved ion yield laser spectroscopy in conjunction with ab initio electronic structure calculations to track molecular excited-state dynamics. We discuss how molecular fragment ions can be associated with conformations the molecule assumes during its relaxation, and how various features of the pump-probe signal for those fragments can be used to infer details of the excited-state dynamics. We present results for radiationless relaxation in DNA and RNA bases adenine, cytosine, and uracil in the gas phase, pumped near a one-photon resonance transition to an excited state, and probed via strong-field near-IR dissociative ionization.

A semi-classical model of attosecond electron localization in dissociative ionization of hydrogen

In the development of attosecond molecular science, a series of experiments have recently been performed where ionic fragment asymmetries in the dissociative ionization of H(2) into H(+) + H and that of D(2) into D(+) + D were used to uncover electron localization processes that occur on the attosecond and few-femtosecond timescale. Electron localization was observed both in strong-field dissociative ionization using carrier envelope phase-stable few-cycle laser pulses [Kling et al., Science, 2006, 312, 246] and in a two-color extreme ultra-violet + infrared attosecond pump-probe experiment [Sansone et al., Nature, 2010, 465, 763]. Here we show that the observed electron localization can be well understood using a semi-classical model that describes the dynamics in terms of quasi-static states that take the interaction of the molecule with the laser field instantaneously into account. The electron localization is shown to be determined by the passage of the dissociating molecule through a regime where the laser-molecule interaction is neither diabatic nor adiabatic.

Ultrafast strong-field dissociative ionization dynamics of CH2Br2 probed by femtosecond soft x-ray transient absorption spectroscopy

Femtosecond time-resolved soft x-ray transient absorption spectroscopy based on a high-order harmonic generation source is used to investigate the dissociative ionization of CH(2)Br(2) induced by 800 nm strong-field irradiation. At moderate laser peak intensities (2.0 x 10(14) Wcm(2)), strong-field ionization is accompanied by ultrafast C-Br bond dissociation, producing both neutral Br ((2)P(32)) and Br(*) ((2)P(12)) atoms together with the CH(2)Br(+) fragment ion. The measured rise times for Br and Br(*) are 130+/-22 fs and 74+/-10 fs, respectively. The atomic bromine quantum state distribution shows that the BrBr(*) population ratio is 8.1+/-3.8 and that the Br (2)P(32) state is not aligned. The observed product distribution and the time scales of the photofragment appearances suggest that multiple field-dressed potential energy surfaces are involved in the dissociative ionization process. At higher laser peak intensities (6.2 x 10(14) Wcm(2)), CH(2)Br(2) (+) undergoes sequential ionization to form the metastable CH(2)Br(2) (2+) dication. These results demonstrate the potential of core-level probing with high-order harmonic transient absorption spectroscopy for studying ultrafast molecular dynamics.

Resonant excitation during strong-field dissociative ionization

We studied dissociative ionization of oxygen by intense femtosecond laser pulses with central wavelengths between 550 and 1800 nm. We measured kinetic energy release spectra and angular distributions of fragments resulting from symmetric dissociation of doubly charged molecular ions (${\mathrm{O}}_{2}^{2+}\ensuremath{\rightarrow}{\mathrm{O}}^{+}+{\mathrm{O}}^{+}$ channel). In the kinetic energy release spectra we identified a number of distinct excited states of the molecular ion. Angular distributions for all but one of those states were consistent with recollision excitation following single ionization of ${\mathrm{O}}_{2}$. For the remaining $(B\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Pi}_{\mathbf{g}})$ excited state we observed a characteristic resonant dependence of its relative yield on the central optical frequency of the pulse, with the yield peaking at around 890 nm. This presents unambiguous evidence in support of importance of resonant excitation channels in strong field ionization of molecules.

Strong-field dissociative ionization of a linear triatomic molecule: Relationship between Coulomb-explosion energies and bond angle

Correlation images of the symmetric six-electron Coulomb-explosion channel of ${\mathrm{CO}}_{2}$ were used to isolate specific geometries (linear and bent) for angular-resolved analysis of the Coulomb-explosion energy in the framework of both the critical radius ${R}_{c}$ and dynamic screening models. We show that it is possible to connect the ${R}_{c}$ and screening pictures through a single parameter, a charge defect $\ensuremath{\sigma},$ which depends on the charge state and the ratio between ${R}_{c}$ and the equilibrium bond length. Our data show that ${R}_{c}$ and hence $\ensuremath{\sigma}$ are slow varying functions of bond angle between $145\ifmmode^\circ\else\textdegree\fi{}$ and $180\ifmmode^\circ\else\textdegree\fi{}$ with ${R}_{c}\ensuremath{\sim}4\mathrm{a}.\mathrm{u}.$ and $\ensuremath{\sigma}\ensuremath{\sim}0.5$ for the symmetric six-electron channel of ${\mathrm{CO}}_{2}.$ In the ${R}_{c}$ picture, the experimental value for ${R}_{c}$ is consistent with a theoretical value associated with ${\mathrm{CO}}_{2}^{3+},$ which is considerably smaller than that associated with ${\mathrm{CO}}_{2}^{+}(\ensuremath{\sim}6\mathrm{a}.\mathrm{u}.).$

Disentangling molecular alignment and enhanced ionization in intense laser fields

Using circular and linear laser polarization for strong-field dissociative ionization we show that iodine molecules are rotationally fixed by their own moment of inertia during the dissociative ionization with a 80-fs pulse. What seems to be alignment of the fragments is created by an enhancement of the ionization rate, which is strongly dependent on the angle between the molecular axis and the laser electric-field vector. By the introduced method we can measure the degree of alignment for ${\mathrm{I}}_{2}, {\mathrm{Br}}_{2}, {\mathrm{Cl}}_{2},$ and ${\mathrm{O}}_{2}$ and independently estimate the enhancement of the ionization rate for ${\mathrm{I}}_{2}.$