I am currently an associate staff scientist in SLAC LCLS SRD Chemical Science Department. My research interest falls in excited state dynamics of small organic molecules, and I am particularly interested in using novel experimental techniques probing the ongoing dynamics in real time and space. The excited state dynamics in these systems usually take place in attoseconds to picoseconds time scales. The strongly-coupled electronic and nuclear dynamics often result in ultrafast energy redistribution as well as structure transformation, and facilitate many phenomenons in physics, chemistry, and biology.
My research builds on my extensive experience with ultrafast optical laser science and technology and time resolved spectroscopies. I am currently focusing on developing experiments utilizing multiple time-resolved spectroscopy or diffraction techniques probing molecular dynamics. These included time-resolved valence-ionization spectroscopy, Soft X-ray core-ionization spectroscopy, and ultrafast electron and hard X-ray diffraction. Most of my experiments are built upon the LCLS FEL X-ray beamline, MeV-UED facility in SLAC national lab, and our own tabletop ultrafast laser lab in Stanford PULSE institute.
Education & Certifications
PhD, Stony Brook University, Physical Chemistry (2021)
B.S., Ocean University of China, Optical Information (2013)
Rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction imaged in real-time.
2023; 14 (1): 2795
Electrocyclic reactions are characterized by the concerted formation and cleavage of both sigma and pi bonds through a cyclic structure. This structure is known as a pericyclic transition state for thermal reactions and a pericyclic minimum in the excited state for photochemical reactions. However, the structure of the pericyclic geometry has yet to be observed experimentally. We use a combination of ultrafast electron diffraction and excited state wavepacket simulations to image structural dynamics through the pericyclic minimum of a photochemical electrocyclic ring-opening reaction in the molecule alpha-terpinene. The structural motion into the pericyclic minimum is dominated by rehybridization of two carbon atoms, which is required for the transformation from two to three conjugated pi bonds. The sigma bond dissociation largely happens after internal conversion from the pericyclic minimum to the electronic ground state. These findings may be transferrable to electrocyclic reactions in general.
View details for DOI 10.1038/s41467-023-38513-6
View details for PubMedID 37202402
- Spectroscopic and Structural Probing of Excited-State Molecular Dynamics with Time-Resolved Photoelectron Spectroscopy and Ultrafast Electron Diffraction PHYSICAL REVIEW X 2020; 10 (2)
Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis.
Physical chemistry chemical physics : PCCP
We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (12 ions shot-1). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals.
View details for DOI 10.1039/d2cp03029b
View details for PubMedID 36106844
Nonadiabatic Excited State Dynamics of Organic Chromophores: Take-Home Messages.
The journal of physical chemistry. A
Nonadiabatic excited state dynamics are important in a variety of processes. Theoretical and experimental developments have allowed for a great progress in this area, while combining the two is often necessary and the best approach to obtain insight into the photophysical behavior of molecules. In this Feature Article we use examples of our recent work combining time-resolved photoelectron spectroscopy with theoretical nonadiabatic dynamics to highlight important lessons we learned. We compare the nonadiabatic excited state dynamics of three different organic molecules with the aim of elucidating connections between structure and dynamics. Calculations and measurements are compared for uracil, 1,3-cyclooctadiene, and 1,3-cyclohexadiene. The comparison highlights the role of rigidity in influencing the dynamics and the difficulty of capturing the dynamics accurately with calculations.
View details for DOI 10.1021/acs.jpca.2c04671
View details for PubMedID 36069531
Multichannel photodissociation dynamics in CS2 studied by ultrafast electron diffraction.
Physical chemistry chemical physics : PCCP
The structural dynamics of photoexcited gas-phase carbon disulfide (CS2) molecules are investigated using ultrafast electron diffraction. The dynamics were triggered by excitation of the optically bright 1B2(1Sigmau+) state by an ultraviolet femtosecond laser pulse centred at 200 nm. In accordance with previous studies, rapid vibrational motion facilitates a combination of internal conversion and intersystem crossing to lower-lying electronic states. Photodissociation via these electronic manifolds results in the production of CS fragments in the electronic ground state and dissociated singlet and triplet sulphur atoms. The structural dynamics are extracted from the experiment using a trajectory-fitting filtering approach, revealing the main characteristics of the singlet and triplet dissociation pathways. Finally, the effect of the time-resolution on the experimental signal is considered and an outlook to future experiments provided.
View details for DOI 10.1039/d2cp01268e
View details for PubMedID 35707953
Time Resolved Photoelectron Spectroscopy as a Test of Electronic Structure and Nonadiabatic Dynamics
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2021; 12 (21): 5099-5104
We compare different levels of theory for simulating excited state molecular dynamics and use time-resolved photoelectron spectroscopy measurements to benchmark the theory. We perform trajectory surface hopping simulations for uracil excited to the first bright state (ππ*) using three different levels of theory (CASSCF, MRCIS, and XMS-CASPT2) in order to understand the role of dynamical correlation in determining the excited state dynamics, with a focus on the coupling between different electronic states and internal conversion back to the ground state. These dynamics calculations are used to simulate the time-resolved photoelectron spectra. The comparison of the calculated and measured spectra allows us to draw conclusions regarding the relative insights and quantitative accuracy of the calculations at the three different levels of theory, demonstrating that detailed quantitative comparisons of time-resolved photoelectron spectra can be used to benchmark methodology.
View details for DOI 10.1021/acs.jpclett.1c00926
View details for Web of Science ID 000661113400008
View details for PubMedID 34028278
Effect of dynamic correlation on the ultrafast relaxation of uracil in the gas phase
2021; 228: 266-285
The photophysics and photochemistry of DNA/RNA nucleobases have been extensively investigated during the past two decades, both experimentally and theoretically. The ultrafast relaxation of the canonical nucleobases following photoexcitation is of significant interest when it comes to understanding how nature has ensured their photostability. Here we study the excited state dynamics of uracil which is a nucleobase found in RNA. Although theory and experiment have shed significant light on understanding the photoexcited dynamics of uracil, there are still disagreements in the literature about specific details. In order to examine how the dynamics is influenced by the underlying electronic structure theory, we have performed non-adiabatic excited state dynamics simulations of uracil using on-the-fly trajectory surface hopping methodology on potential energy surfaces calculated at different electronic structure theory levels (CASSCF, MRCIS, XMS-CASPT2, TD-DFT). These simulations reveal that the dynamics are very sensitive to the underlying electronic structure theory, with the multi-reference theory levels that include dynamic correlation, predicting that there is no trapping on the absorbing S2 state, in contrast to predictions from lower level electronic structure results. The dynamics are instead governed by ultrafast decay to the ground state, or trapping on the dark S1 state.
View details for DOI 10.1039/d0fd00110d
View details for Web of Science ID 000664793300013
View details for PubMedID 33566040
Excited-state dynamics of CH2I2 and CH2IBr studied with UV-pump VUV-probe momentum-resolved photoion spectroscopy
JOURNAL OF CHEMICAL PHYSICS
2020; 153 (18): 184304
We perform time-resolved ionization spectroscopy measurements of the excited state dynamics of CH2I2 and CH2IBr following photoexcitation in the deep UV. The fragment ions produced by ionization with a vacuum-ultraviolet probe pulse are measured with velocity map imaging, and the momentum resolved yields are compared with trajectory surface hopping calculations of the measurement observable. Together with recent time-resolved photoelectron spectroscopy measurements of the same dynamics, these results provide a detailed picture of the coupled electronic and nuclear dynamics involved. Our measurements highlight the non-adiabatic coupling between electronic states, which leads to notable differences in the dissociation dynamics for the two molecules.
View details for DOI 10.1063/5.0026177
View details for Web of Science ID 000591895800004
View details for PubMedID 33187419
Excited state dynamics of cis,cis-1,3-cyclooctadiene: UV pump VUV probe time-resolved photoelectron spectroscopy
JOURNAL OF CHEMICAL PHYSICS
2020; 153 (7): 074301
We present UV pump, vacuum ultraviolet probe time-resolved photoelectron spectroscopy measurements of the excited state dynamics of cis,cis-1,3-cyclooctadiene. A 4.75 eV deep UV pump pulse launches a vibrational wave packet on the first electronically excited state, and the ensuing dynamics are probed via ionization using a 7.92 eV probe pulse. The experimental results indicate that the wave packet undergoes rapid internal conversion to the ground state in under 100 fs. Comparing the measurements with electronic structure and trajectory surface hopping calculations, we are able to interpret the features in the measured photoelectron spectra in terms of ionization to several states of the molecular cation.
View details for DOI 10.1063/5.0006920
View details for Web of Science ID 000563480100001
View details for PubMedID 32828099
Excited state dynamics of cis,cis-1,3-cyclooctadiene: Non-adiabatic trajectory surface hopping
JOURNAL OF CHEMICAL PHYSICS
2020; 152 (17): 174302
We have performed trajectory surface hopping dynamics for cis,cis-1,3-cyclooctadiene to investigate the photochemical pathways involved after being excited to the S1 state. Our calculations reveal ultrafast decay to the ground state, facilitated by conical intersections involving distortions around the double bonds. The main distortions are localized on one double bond, involving twisting and pyramidalization of one of the carbons of that double bond (similar to ethylene), while a limited number of trajectories decay via delocalized (non-local) twisting of both double bonds. The interplay between local and non-local distortions is important in our understanding of photoisomerization in conjugated systems. The calculations show that a broad range of the conical intersection seam space is accessed during the non-adiabatic events. Several products formed on the ground state have also been observed.
View details for DOI 10.1063/5.0005558
View details for Web of Science ID 000532296800001
View details for PubMedID 32384830
Liquid-phase mega-electron-volt ultrafast electron diffraction
2020; 7 (2): 024301
The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which occur in solution. To understand the structure-function relationships mediating these processes, a technique with high spatial and temporal resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water and its network were resolved up to the 3 rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.
View details for DOI 10.1063/1.5144518
View details for Web of Science ID 000531214100001
View details for PubMedID 32161776
View details for PubMedCentralID PMC7062553
Simultaneous observation of nuclear and electronic dynamics by ultrafast electron diffraction.
Science (New York, N.Y.)
2020; 368 (6493): 885–89
Simultaneous observation of nuclear and electronic motion is crucial for a complete understanding of molecular dynamics in excited electronic states. It is challenging for a single experiment to independently follow both electronic and nuclear dynamics at the same time. Here we show that ultrafast electron diffraction can be used to simultaneously record both electronic and nuclear dynamics in isolated pyridine molecules, naturally disentangling the two components. Electronic state changes (S1→S0 internal conversion) were reflected by a strong transient signal in small-angle inelastic scattering, and nuclear structural changes (ring puckering) were monitored by large-angle elastic diffraction. Supported by ab initio nonadiabatic molecular dynamics and diffraction simulations, our experiment provides a clear view of the interplay between electronic and nuclear dynamics of the photoexcited pyridine molecule.
View details for DOI 10.1126/science.abb2235
View details for PubMedID 32439793
Excited state dynamics of CH2I2 and CH2BrI studied with UV pump VUV probe photoelectron spectroscopy
JOURNAL OF CHEMICAL PHYSICS
2019; 150 (17): 174201
We compare the excited state dynamics of diiodomethane (CH2I2) and bromoiodomethane (CH2BrI) using time resolved photoelectron spectroscopy. A 4.65 eV UV pump pulse launches a dissociative wave packet on excited states of both molecules and the ensuing dynamics are probed via photoionization using a 7.75 eV probe pulse. The resulting photoelectrons are measured with the velocity map imaging technique for each pump-probe delay. Our measurements highlight differences in the dynamics for the two molecules, which are interpreted with high-level ab initio molecular dynamics (trajectory surface hopping) calculations. Our analysis allows us to associate features in the photoelectron spectrum with different portions of the excited state wave packet represented by different trajectories. The excited state dynamics in bromoiodomethane are simple and can be described in terms of direct dissociation along the C-I coordinate, whereas the dynamics in diiodomethane involve internal conversion and motion along multiple dimensions.
View details for DOI 10.1063/1.5086665
View details for Web of Science ID 000467255500023
View details for PubMedID 31067867
- Strong-field-versus weak-field-ionization pump-probe spectroscopy PHYSICAL REVIEW A 2018; 98 (5)
Real-time adjustable, 11 mu s FWHM, > 5 kHz, piezo electric pulsed atomic beam source
REVIEW OF SCIENTIFIC INSTRUMENTS
2018; 89 (10): 103115
This paper provides a detailed description of how to construct a pulsed atomic beam source [including a fast ionization gauge (FIG) for characterization] with a unique combination of characteristics. We include technical drawings for a real-time adjustable piezo electric actuated pulsed valve capable of generating a 11 μs duration pulse of gas at a repetition rate of >5 KHz, with a shot-to-shot stability of 0.6%, and maximum densities of 1015 particles/cm3. We also include details on how to construct a FIG, with a 4 μs rise time, to measure the pulse. We report a 3D density map of a supersonic expansion of helium gas with a speed ratio S = 46 and a calculated longitudinal temperature of 0.3 K. Finally, the results of a laser ionization test are provided in order to verify the performance of the pulsed valve in a typical experimental configuration.
View details for DOI 10.1063/1.5038013
View details for Web of Science ID 000449144500279
View details for PubMedID 30399851
- Vibrationally assisted below-threshold ionization PHYSICAL REVIEW A 2017; 95 (6)
Ultrafast internal conversion dynamics of highly excited pyrrole studied with VUV/UV pump probe spectroscopy
JOURNAL OF CHEMICAL PHYSICS
2017; 146 (6): 064306
We study the relaxation dynamics of pyrrole after excitation with an 8 eV pump pulse to a state just 0.2 eV below the ionization potential using vacuum ultraviolet/ultraviolet pump probe spectroscopy. Our measurements in conjunction with electronic structure calculations indicate that pyrrole undergoes rapid internal conversion to the ground state in less than 300 fs. We find that internal conversion to the ground state dominates over dissociation.
View details for DOI 10.1063/1.4975765
View details for Web of Science ID 000394577400024
View details for PubMedID 28201903