Session U7: Molecular Photophysics

Observation of a novel dissociation path in deuterium molecules fragmented via a two-photon process

A recently developed VUV monochrometor combined with a COLTRIMS detection system is used to study deuterium molecules with the pump-probe method. A single high-order harmonic with photon energy near 17 eV is used to promote D$_{\mathrm{2}}$ via a single-photon process. A delayed 800 nm infrared (IR) pulse feeds the promoted system with an additional photon of 1.54 eV and opens the dissociative ionization channel energetically. The two-photon process was identified from electron-ion coincident measurements and both spectroscopic and time-resolved measurements were made. A previously unobserved channel is found for which the dissociation direction does not follow the polarization of the IR. We interpret this channel as corresponding to VUV excitation of D$_{\mathrm{2}}$ followed by pre-dissociation into two separated atoms (D(\textit{1s})$+$D(\textit{nl})), with the excited atom subsequently ionized by the IR pulse. When the VUV photon is blueshifted to 17.4 eV and above, the more familiar dissociation channel is opened whereby VUV ionization of D$_{\mathrm{2}}$ is followed by electronic excitation (bond softening) by the IR pulse. The dissociation direction does follow the polarization of the IR for this channel.   

Strong-field dissociation of CS$^{2+}$ via a pump/dump-like mechanism

Laser-induced dissociation of the quasi-bound electronic ground state of CS$^{2+}$ is investigated in intense laser pulses ($<$55 fs, $<$10$^{16}$ W/cm$^2$). Photodissociation is observed to be the dominant dissociation pathway; however, a more curious feature in the kinetic energy release spectrum suggests no significant energy gain from the initial states. We propose a pump/dump-like mechanism to explain this observed feature. Contrary to the conventional pump/dump control scheme, this process occurs within a single laser pulse, where the time delay is caused by the molecular structure. The process begins when the vibrational wavepacket in the electronic ground state of CS$^{2+}$ is pumped into the electronic first excited state's continuum by a single photon. After a period of stretching at an energy above the potential barrier, the emission of a second photon is stimulated by the same laser pulse, most likely at the Condon point.   

Discerning the direct and indirect ionization processes in the photo-double-ionization of 1, 1-C$_{2}$H$_{2}$F$_{2}$ near and above threshold

We have studied the photo-double-ionization of 1, 1-C$_{2}$H$_{2}$F$_{2}$ near and above threshold using linearly polarized single photons (40 to 70eV). Kinematically complete experiments were achieved for the nondissociative ionization (NDI) and all ionic two body break up channels by measuring the electrons and recoil ions in coincidence with the COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) method. Using electron-ion and electron-electron energy correlation maps as well as asymmetry parameters and relative angles between the emitted electrons, we were able to trace the electronic states involved and distinguish between the direct and indirect ionization mechanisms of the NDI and the fragmentation processes.   

VUV-Pumped, XUV-Probed Dissociation and Relaxation Dynamics

We present a time-resolved study of ultrafast dynamics near conical intersections in ethylene. Using a bright high harmonic source, we excite the molecular system at 8~eV and probe the excited state wavepacket through photoionization by a second extreme ultraviolet~(XUV) pulse. A Velocity Map Imaging spectrometer measures the kinetic energy release and angular distributions of the resultant photofragments. We also explore controlling the wavepacket dynamics with an infrared pulse preceding the XUV probe pulse.   

XUV Pump - XUV Probe Studies of Bond Forming Processes in Polyatomic Molecules

Extreme-ultraviolet (XUV) high-order harmonics along with strong-field femtosecond near infra-red (NIR) laser pulses have been used to perform time-resolved pump-probe studies of ionization and fragmentation dynamics in atomic and molecular systems. With the availability of high pulse energy femtosecond laser systems it is now possible to generate high-harmonics with enough flux to perform XUV pump -- XUV probe experiments. Here, we use high harmonics generated from a state-of-the-art 30 mJ, 1 KHz femtosecond NIR laser system to study dissociation dynamics in Sulfur hexa-fluoride (SF6). We focus mainly on dissociation channels above the first ionization threshold where neutral molecular Fluorine (F2) is eliminated from SF6. Using photo-ion and photo-electron spectroscopy we time-resolve the formation of a chemical bond.   

Probing Ultrafast Dynamics in Small Molecules Using Vacuum Ultraviolet Pulses

A time-resolved study of ultrafast energy conversion in molecular systems is presented. The mechanisms underlying these ultrafast conversions of energy are studied with a novel VUV~pump XUV~probe spectroscopy. Velocity map imaging and time of flight techniques are used to time resolve the kinetic energy release, total ion yield, and angular distributions of both photoions and photoelectrons. This technique allows us to observe state specific dynamics near conical intersections in ethylene and carbon dioxide.   

Imaging the spatial many-body wave functions of H$_3$

We study state-selected H$_3$ molecules which predissociate into the $H(1s)+H(1s)+H(1s)$ channel of the repulsive ground state. The correlated fragment momentum vectors in lab-frame are recorded in triple coincidence and transformed to center-of-mass momenta. Accumulating $\sim 10^4$ such events yields a probability map of momentum configurations, equivalent to the modulus square of the momentum wave function long after the decay. Recently theory was successful in predicting several details of the maps observed. Here, we present a model which well explains the dominant features of momentum maps in terms of spatial symmetries in the direct product of initial bound-state wave function and the non-adiabatic coupling term which initiates dissociation. Excellent agreement is found between measured asymptotic modulus squared wave function in momentum space and modelled initial modulus squared wave function in position space [P.~C.~Fechner, H.~Helm, Phys. Chem. Chem. Phys., 2014, 16, 453]. On this basis we discuss in general the equivalence of a complete characterization of linear momenta of fragments from a many-body fragmentation process and the many-body spatial wave function, a concept introduced by J.~H.~Macek as \textit{imaging theorem}.   

Theoretical analysis of the double photoionization of the helium dimer

We study the scattering effects in the double photoionization process of the helium dimer by numerically solving the time-dependent Schr\"odinger equation. To this end, we use a planar two-active-electron model and abridged Hamiltonians to distinguish the contributions from different interactions to the photoelectron angular distributions. We are able to identify the role of the Coulomb interactions between the electrons as well as between the electrons and the nuclei on the emission of the primary as well as the knock-off electron in the double ionization process. Furthermore, the role of the initial state (singlet versus triplet) will be discussed.   

Time-Resolved Mid-Infrared Frequency Comb Spectroscopy of Transient Radical Species

Understanding chemical reactions require unambiguous determinations of reactant, intermediate, and product concentrations on time scales faster than the reaction rate. For high detection sensitivities, direct absorption spectroscopy in the mid-infrared can often be desirable due to strongly absorbing fundamental molecular vibrations. Here, we demonstrate time-resolved frequency comb spectroscopy (TRFCS), a mid-infrared broadband technique for the study of chemical reactions on the $\mu $s timescale, to measure an important transient free radicals species, hydroxyformyl radical trans-DOCO. Directly after photolysis of the chemical precursor acrylic acid-$d_{\mathrm{1}}$, we measure absolute trans-DOCO product concentrations as well as its subsequent loss with a time resolution of 25 $\mu $s. In addition to trans-DOCO product formation, we observed unexpected C-H bond fission channels in photoexcited acrylic acid.   

Taple-top imaging of the non-adiabatically driven isomerization in the acetylene cation

One of the primary goals of modern ultrafast science is to follow nuclear and electronic evolution of molecules as they undergo a photo-chemical reaction. Most of the interesting dynamics phenomena in molecules occur when an electronically excited state is populated. When the energy difference between electronic ground and excited states is large, Free Electron Laser (FEL) and HHG-based VUV sources were, up to date, the only light sources able to efficiently initiate those non-adiabatic dynamics. We have developed a simple table-top approach to initiate those rich dynamics via multiphoton absorption. As a proof of principle, we studied the ultrafast isomerization of the acetylene cation. We have chosen this model system for isomerization since the internal conversion mechanism which leads to proton migration is still under debate since decades. Using 266 nm multiphoton absorption as a pump and 800 nm induced Coulomb Explosion as a probe, we have shoot the first high-resolution molecular movie of the non-adiabatically driven proton migration in the acetylene cation [1]. The experimental results are in excellent agreement with high level ab initio trajectory simulations. [1] H. Ibrahim \textit{et al., Nature Communication,} (Under Review)