Session J5: Strong Field Dissociative Ionization: Wave Packets in Molecular Ions

Ultraslow Dissociation of the H$_2^+$ Molecular Ion via Two-color Ultrafast Laser Pulses

We demonstrate a new mode of H$_2^+$ photodissociation in an intense two-color laser field. Bond-softening and vibrational-trapping of the hydrogen molecular ion in intense single-color laser fields are processes that are well studied and understood in detail. In the past few years, two-color fields have received increasing attention. Our experiment shows that H$_2^+$ ions experience ultraslow dissociation when bond-softened by the superposition of 800-nm and 400-nm ultrafast laser pulses in a narrow intensity range. By running theoretical simulations and examining two-color field-dressed adiabatic potential energy curves, we support this interpretation of near-zero kinetic energy release. Furthermore, we show that the shift to lower energy of a known bond-softened peak can be explained by the influence of a two-color field induced potential well.   

Asymmetric momentum distribution of $d$+H and $p$+D in the dissociation of HD$^+$ by ultrashort laser pulses

Many observables depend on the carrier-envelope phase (CEP) of an ultrashort laser pulse in the interaction of atoms and molecules with these pulses. One important observable is the asymmetry of the fragments in molecular dissociation, e.g. H$_2^+$, HD$^+$ and D$_2^+$. Unlike H$_2^+$ and D$_2^+$, where the CEP gives an asymmetry in the momentum distribution of the $p$+H fragments, in HD$^+$ the CEP controls not only the asymmetry in the momentum distribution for each channel, $p$+D and $d$+H, but also the branching ratios to these channels. To calculate the momentum distribution, we solved the time-dependent Schr\"{o}dinger equation for HD$^+$ in the Born-Oppenheimer representation including all nuclear and electronic degrees of freedom for the molecular dissociation. To ensure the correct dissociation limit and dynamics, non-Born-Oppenheimer terms are essential for HD$^+$. Advancements in experimental techniques have made differential measurement of the momentum distribution for both channels of HD+ possible. Thus, our theoretical results can, in principle, be directly compared to experiment.   

Tracking dynamic wave packets in the O$_{2}$ dication using a pump/probe approach

Vibrational wave packet dynamics in the O$_{2}$ dication have been tracked with few-cycle near-infrared laser pulses. Bound and dissociating wave packets were launched and subsequently probed via a pair of 8 fs pulses at 800 nm. Ionic fragments from the dissociating molecules were monitored using a velocity-map imaging apparatus. Pronounced oscillations in both the time-dependent kinetic energy release spectra and in the time-dependent fragment yields were observed. Facilitated by the observation of vibrational revivals, the dynamics of the wave packets on specific potential energy curves of the O$_{2}$ dication are discerned. Quantum and semi-classical calculations are in good agreement with the measured dynamics.   

Vibrationally-resolved structure in O$_{2}^{+ }$ dissociation by intense ultrafast laser pulses

One of the common aspects of intense laser experiments performed on molecules is the lack of vibrational structure in their dissociation spectra. In fact, as far as we are aware, the only observation of vibrational structure has been in ion-beam experiments for the simplest molecule, H$_{2}^{+}$. It is less intuitive to expect to see vibrationally resolved structure in the larger diatomic molecules. In this talk we report a measurement that shows the first observation of vibrational structure in the dissociation of a massive molecule, O$_{2}^{+}$. To demonstrate the persistent nature of this structure, we measure the dissociation spectra from both an O$_{2}^{+}$ ion beam target and also by initially ionizing O$_{2}$ with a laser pulse, which also drives the dissociation of the daughter O$_{2}^{+}$. Thus, our measurements encompass both types of target: O$_{2}^{+}$ populated with an incoherent and coherent ensemble of vibrational states. Supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.   

Triatomic molecular ionization and dissociation in strong laser fields

We extend molecular ionization and dissociation studies in strong laser fields from diatomic molecules to triatomic molecules. In this work, we study two dissociation pathways in double ionized triatomic molecule, CO$_{2}$, one two-body dissociation channel, CO$^{+}$+O$^{+}$, and the other three-body dissociation channel, C + 2O$^{+}$. The two-body dissociation channel shows a stronger nonsequential rate than the three-body dissociation channel. We found that electronic structure plays a key role on the dissociation process, in consistent with our previously established work in diatomic molecules.   

Creation and Characterization of Multi-Hole Molecular Wave Packets via Strong Field Ionization

We use strong field ionization to create superpositions of electronic and vibrational states in small polyatomic molecules (CH2XY, with X, Y = Br, I). We focus on characterizing the relative phase and amplitude of these superposition states. Our pump-probe measurements with a variable pump pulse chirp demonstrate the ability to create superpositions of electronic states with variable amplitudes. By combining velocity map imaging and wave packet holography for momentum resolved interference measurements, we are working toward measuring and controlling the phase between electronic states in the ionic wave packet. The pump and probe pulses are generated from a single ultrafast laser pulse using an AOM based ultrafast pulse shaper, allowing us to create a shaped initial wave packet and to maintain a phase stability between pulses better than lambda/400. This corresponds to a time stability of a few attoseconds, which is suitable for characterizing electronic wave packets having an energy spread on the order of an eV.   

Dynamic control of the fragmentation of CO$^{q+}$ excited states generated by high order harmonics

The dynamic process of fragmentation of CO$^{q+ }$is investigated by using an EUV pump pulse (35-42eV) generated by high-order harmonics. A time-delayed infrared (IR) probe pulse is used to interact with the excited system and trace the wave-packet evolution. The experimental results indicate that two groups of states prepared by the EUV contribute to C$^{+}$-O$^{+}$ fragmentation: (a) a double ionization with a kinetic energy release (KER) above 6eV and (b) a fragmentation of the cation into C$^{+}$-O*, followed by autoionization of the O*. Channel (b) has a KER near 3eV in the absence of IR, increasing to more than 6eV in the presence of IR. This KER increase drops off with EUV/IR time delay and has a lifetime of $\sim $250 fs. A model TDSE solution gives qualitative agreement with experiment.   

Dissociative ionizaiton of $H_2$ in an ultroviolet pulse train and delayed infrared laser pulse

The ionization of H$_2$ in a single attosecond XUV pulse generates a nuclear wavepacket in H$_2^+$ which is entangled with the emitted photoelectron wavepacket. The nuclear wavepacket dynamics can be observed by dissociating H$_2^+$ in a delayed infrared laser pulse. If H$_2$ is ionized by a sequence of XUV pulses of an attosecond pulse train, whether or not the corresponding sequence of nuclear wavepackets in H$_2^+$ is detected as a coherent or incoherent superposition depends on whether and how the photoelectrons are observed. We simulate the nuclear dynamics in this XUV pump - IR probe scenario and analyze our numerical results, suggesting that interference between coherently launched nuclear wavepackets in H$_2^+$ can be neglected in the recent experiment of Kelkensberg \textit{et al.} [\textbf{103}, 123005 (2009)].   

Molecular dissociation with attosecond pulses: A double-slit with moving nuclei

We have performed numerical simulations of the interaction of the hydrogen molecular ion with two attosecond laser pulses. The first pulse initiates the dissociation of the molecule, while the time-delayed second pulse probes the dissociating molecule at large internuclear distances by ionization. Our results show an oscillation of the ionization yield as a function of the time delay between the two pulses. We interpret the results as an interference effect between the electron wave packets emerging from the two protons. We will discuss the feasibility of such an atomic-scale Young-type double-slit experiment with moving nuclei with respect to the parameters of the two pulses.   

IR-assisted dissociation of D$_{2}^{+}$ by attosecond XUV radiation

High harmonic generation produces perfectly synchronized attosecond XUV pulses and femtosecond IR pulses that can be combined together to induce and control molecular dissociation. Here, we spectrally select XUV attosecond pulse trains (APT), and use them to coherently populate the ground and excited states of D$_{2}^{+}$ that lie just below the double-ionization threshold. Two time delayed femtosecond IR/UV pulses are then used to control the dissociation processes. Momenta and yields of dissociating ions from different channels are identified and monitored as a function of the phase and time difference between the XUV and IR pulses. When a weak IR field is phase locked with the APT, the total IR field a molecule experiences at the instant of the APT strobe pulses can be modulated 2.5 times, by varying the time delay of a stronger IR field. As a result, the yield of the bound nuclear wave packet rapidly oscillates with the period of the full IR cycle. Photoelectrons are measured in coincidence with the ions using a COTRLIMS technique. Different control mechanisms and applications to larger molecules will be discussed.