Our Research
Our research centers upon measuring and controlling dynamics in atomic and molecular systems using shaped ultrafast laser pulses. We are working to develop new time resolved spectroscopies to follow the quantum dynamics of electrons and nuclei in molecular systems. Our approach is to work on experiments of varying complexity, and to understand our experimental results through both simple intuitive models and detailed calculations.
On Left: Electronic wave packet generated and probed with shaped ultrafast laser pulses. The movie shows the electron density in moleclar thiophene as a function of the relative phase between two electronic eigenstates that are excited by a pump pulse and probed with a second pulse whose relative phase is controlled independent of delay.
Velocity Map Imaging of Strong Field Molecular Ionization
We have developed a coincidence velocity map imaging apparatus for studying strong field molecular ionization using shaped laser pulses. For each laser shot, we measure ions and electrons in coincidence using velocity map imaging to record the vector momentum of the electrons as they leave the molecule. An illustration of our apparatus with a typical measurement is shown below.
Velocity Map Imaging and Timepix Camera
We have integrated our coincidence velocity map imaging apparatus with a Timepix3 Camera for studying strong field molecular ionization. The Timepix3 Camera, which has ~1.5ns time resolution and excellent spatial resolution, enables us to simultaneously measure the mass and 3D momentum of all charged particles in each ionization event. This capability allows us to measure the momentum of two ions and two electrons in coincidence from a single double ionization event. Also, the camera’s large throughput enables straightforward switching between low data-rate coincidence and high data-rate non-coincidence detection modes.
Vacuum Ultraviolet(VUV) Light Source and UV/VUV Pump Probe Experiments
We have developed a new ultrafast light source to conduct UV/VUV pump probe experiments to study excited state dynamics such as internal conversion, isomerization, intersystem crossing, and dissociation. We use a Ti:Sapphire laser to generate UV and VUV pulses. UV(4.8eV) pulses are generated through third harmonic generation in BBO crystals. The VUV(8eV) pulses are generated via non-collinear-four-wave-mixing in Argon. In some experiments, the UV pulse acts as a pump and the VUV as a probe, while in others the role of the pulses are reversed, allowing us to study excited states at both 4.8 eV and 8 eV.
Gas-Phase MeV Relativistic Ultrafast Electron Diffraction
We are collaborating with SLAC national laboratory ultrafast electron diffraction (UED) team and make use of the ultrashort electron bunch studying the gas phase excited state dynamics on polyatomic molecules. The SLAC UED team developed an ultrafast electron diffraction apparatus using MeV electrons (relativistic electrons) for imaging ultrafast structural dynamics of molecules in gas phase. With relativistic electrons, the electron bunches spread much less due to the Coulomb repulsion and travel close to the speed of light, leading to much better group velocity matching with the optical pump pulse over the sample volume, and thus higher time resolution. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63Å spatial resolution, and 0.22 Å−1 reciprocal-space resolution. Such high spatial-temporal resolution has enabled the capturing of real-time molecular movies of fundamental photochemical mechanisms, such as chemical bond breaking, ring opening, and a nuclear wave packet crossing a conical intersection.
With the capability of Time-resolved photoelectron spectroscopy (TRPES) in Weinacht lab, we are particularly interested in combining TRPES (spectroscopic probing) and UED (structural probing) techniques to follow the coupled electronic and nuclear dynamics involved in the internal conversion and photodissociation of polyatomic molecule. UED directly probes the 3D nuclear dynamics, and TRPES is sensitive to electronic energies and configurations. These two measurements are interpreted with trajectory surface hopping calculations, which are capable of simulating the observables for both measurements from the same dynamics calculations. The measurements highlight the non-local dynamics captured by different groups of trajectories in the calculations. More details could be viewed from the literature: Phys. Rev. X 10, 021016 (2020)
Recent Results
Filament Based Spectral Broadening for Sub 10 fs Pulses
We recently developed a filament based spectral broadening source capable of spanning an octave of spectral bandwidth. We compress the pulses using a grating based compressor and have measured pulses as short as 9 fs. We are currently working on compressing the pulses down to 5 fs. The figure below shows the spectrum from our filament source along with the shortest pulse supported by this bandwidth in the inset.
Strong field ionization as a function of pulse duration and shape
Making use of our new broadband light source, we are studying the dynamics of strong field ionization as a function of pulse duration and shape. The figure below illustrates how the photoelectron spectrum for ionization of CH2IBr changes as a function of pulse duration. The measurements are based on velocity map imaging of the photo-electrons and the peaks have been identified using coincidence measurements with photo-ions. The figure illustrates how the ionization changes with pulse duration, with a single ionic state dominating the ionization process for the shortest laser pulse. What is particularly interesting about the measurement is that it demonstrates preferential removal of an electron from a more deeply bound orbital (HOMO-1, leading to the first excited ionic state D1), rather than from the highest occupied orbital (HOMO, leading to the ground ionic state,D0) for the shortest pulse As a complement to the measurement shown above, we also measured the photoelectron spectrum for ionization as a function of central frequency. The measurements were carried out using our filament based supercontinuum, using a slit to select a ~25 nm window over a 200 nm range. The result below indicates that most of the ionization consists of removing an electron from the HOMO, in contrast with the measurements shown below.
UV/VUV Pump Probe Experiment Results
VUV-pump UV-probe Pyrrole results
With our newly developed UV-VUV light source, we have conducted VUV-pump UV-probe experiments in pyrrole(C4H5N). Our measurements, in conjuction with electromic structure calculations, indicate that pyrrole undergoes rapid internal conversion to the ground state in less than 300fs. The internal conversion to the ground state dominates over dissociation. The figure below shows the VUV-pump UV- probe ion yields and their fits proformed on ethylene and pyrrole. We applied a mono-exponential fit to ethylene which yields a decay constant of 37fs, consistant with values reported in literatures. Both mono-exponential(a) and dual-exponential(b) fits are applied to the pyrrole data and residuals are shown in the insets. The residuals of the dual-exponential fit is randomly distributed, indicating that this fit is consistant with the measurements.
UV-pump VUV-probe CH2I2 Results
UV-pump VUV-probe experiments were performed on CH2I2. The figure below shows the pump probe yield of parent and fragment ions, in which the fits show a 20fs difference in the decay constants between them. This highlights the question how does the fragmentatoion happen in the ionization process. The calculations done by our theory collaborator(Philipp Marquetand) point out that there are rich dynamics during the ionization process such as I and I2 dissociation. The movie below shows the I dissociation when the ionization happens. We are working on improving the time resolustion and upgrading the detector to conduct velocity map imaging measurements.
Double Ionization and Electron Rescattering Dynamics
Classical trajectory of electron recollsion dynamics
Rescattering driven dynamics is root of attosecond science. Here is an animation of a classical trajectory simulation of such dynamics.
Double ionization of 1,3-cyclohexadiene(1,3-CHD) and 1,4-cyclohexadiene(1,4-CHD)
By using our broadband light source (duration ~10fs, peak intensity up to ~300TW/cm2) as well as the Timepix3 camera, we performed strong field molecular double ionization experiments on 1,3-CHD and 1,4-CHD. The figures below show the coincident ion-ion and electron-electron momentum correlation (first figure), electron-electron correlation for different fragments of different molecules(second figure), as well as the correlation coefficient for different fragments of different molecules(third figure), respectively. Our results highlight the differences in electron correlation when removing electrons from different orbitals and leaving the molecule in different dication states.
Figure 1: Ion and electron images (top left and right panels respectively) as well as correlation plots for ion and electron pairs (bottom left and right panels respectively) measured in quadruple coincidence for 10 fs duration laser pulses with a peak intensity of about 40 TW/cm2. The ion and electron images show a cross or circle for each particle detected, with only a small fraction of the total hits measured being displayed so that they can be seen clearly. The correlation plots show the number of ion and electron pairs arriving at our detector with the momentum along the laser polarization axis indicated by the x and y axes of the plots.
Figure 2: Comparison of correlation plots (momentum parallel to the laser polarization axis) for electron pairs from dissociative and non-dissociative channels of 1,3-CHD and 1,4-CHD. Left panels show the electron momentum correlation plots for the C3Hx+ dissociative channels of 1,3-CHD and 1,4-CHD, respectively. The right panels show the electron momentum correlation plots for the C6Hx++ non-dissociative channel. The measurements are for a peak intensity of 100 TW/cm2. Black lines mark the positions of zero x-momentum.
Figure 3: Correlation coefficients for electron pairs measured in coincidence with different fragment ions. The left panel shows numbers calculated for 1,3-CHD ion channels. The right panel shows the same for 1,4-CHD. The measurements show that there is a non-trivial anticorrelation for electrons in coincidence with fragment ions having high KER as well as C6Hx++ (all coming from DI). This is in contrast with the electrons measured in coincidence with low KER fragment ions (labeled with SI). The correlations are calculated for measurements with a laser intensity of 100 TW/cm2. Error bars are obtained from a bootstrapping analysis.
CH2I2 Dissociation Dynamics Probed with Time-Resolved Photoelectron Spectroscopy and Ultrafast Electron Diffraction
Combination of UED and TRPES Methods on Probing molecular dynamics
Figure below are schematic diagrams illustrating the experimental methods and calculations to follow the pho-toinduced excited state dynamics of CH2I2.(A)Aschematic diagram of the time-resolved photoelectron spectroscopy experiment.A cartoon of CH2I2 molecule is shownin the bottom left corner.(B)A schematic diagram of the rel-ativistic ultrafast electron diffraction experiment. Two cam-eras are responsible for recording images of scattered and un-scattered electrons.
Pump-Probe Results of TRPES and UED Measurements
Fig. 1 in the lefthand-side shows the measured and simulated time-resolved photo-electron spectra. (A)TRPES from experimental mea-surement.(B)Simulated TRPES including all trajectories.(C)Simulated TRPES for direct (Dir) dissociation trajectories (D) TRPES for indirect (InDir) dissociation trajectories.
Fig. 2 on the righthand-side shows the measured ∆PDF as a function of pump-probe delay.A vertical magenta-colored line marks zero pump-probe delay. The data is normalized by the largest value of ∆PDF. Four regions corresponding to specific molecular motions arehighlighted with dashed lines in different colors. Snapshots on the right side of the graph illustrate the different types ofmotion highlighted in the regions bounded by the dashed lines to the left. A cartoon picture on the top-right of the graph showsthe ground state equilibrium geometry of CH2I2, with black, purple and cyan colored balls representing the carbon, iodine andhydrogen atoms, respectively. The atomic pair distances (blue color) and I-C-I angle (red color) are labeled underneath. The corresponding ground state PDF and atomic pair distances are plotted adjacently.
Fig. 3 above shows the combined spectroscopic and structural views of CH2I2 dissociation dynamics. (A)Potential energy curves andcartoon snapshots of the wavepacket at different time delays for indirect dissociation(blue) and direct dissociation(red) trajectories.(B1)Time-evolved electronic state index for indirect(blue) and direct(red) dissociation trajectories.(B2)Time-evolved potential energy for indirect (blue) and direct (red) dissociation trajectories.(C1)Snapshots of the molecular structureat different time delays for an indirect dissociation trajectory.(C2)Snapshots of the molecular structure at different timedelays for a direct dissociation trajectory. The frames show 2D projections of the 3D molecule onto the I-C-I plane.(D1)Timeevolution of pair distances for indirect (blue) and direct (red) dissociation trajectories.(D2)Time evolution of I-C-I angle fromthe indirect and direct dissociation trajectories.