R Jobe
Electrical Safety Officer, Environmental Safety & Health
All Publications
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Structural dynamics of laser-ionized cis-stilbene studied by ultrafast electron diffraction
JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS
2025; 58 (17)
View details for DOI 10.1088/1361-6455/adfda0
View details for Web of Science ID 001561409900001
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Applying Bayesian inference and deterministic anisotropy to retrieve the molecular structure ÷Ψ(<bold>R</bold>)÷<SUP>2</SUP> distribution from gas-phase diffraction experiments
COMMUNICATIONS PHYSICS
2023; 6 (1)
View details for DOI 10.1038/s42005-023-01420-9
View details for Web of Science ID 001123559000001
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Femtosecond gas-phase mega-electron-volt ultrafast electron diffraction.
Structural dynamics (Melville, N.Y.)
2019; 6 (5): 054305
Abstract
The development of ultrafast gas electron diffraction with nonrelativistic electrons has enabled the determination of molecular structures with atomic spatial resolution. It has, however, been challenging to break the picosecond temporal resolution barrier and achieve the goal that has long been envisioned-making space- and-time resolved molecular movies of chemical reaction in the gas-phase. Recently, an ultrafast electron diffraction (UED) apparatus using mega-electron-volt (MeV) electrons was developed at the SLAC National Accelerator Laboratory for imaging ultrafast structural dynamics of molecules in the gas phase. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63A spatial resolution, and 0.22A-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. In this paper, the design that enables the high spatial-temporal resolution of the SLAC gas phase MeV UED is presented. The compact design of the differential pump section of the SLAC gas phase MeV UED realized five orders-of-magnitude vacuum isolation between the electron source and gas sample chamber. The spatial resolution, temporal resolution, and long-term stability of the apparatus are systematically characterized.
View details for DOI 10.1063/1.5120864
View details for PubMedID 31649964
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Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction.
Science (New York, N.Y.)
2018; 361 (6397): 64-67
Abstract
Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations.
View details for DOI 10.1126/science.aat0049
View details for PubMedID 29976821
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A terahertz pump mega-electron-volt ultrafast electron diffraction probe apparatus at the SLAC Accelerator Structure Test Area facility
JOURNAL OF INSTRUMENTATION
2018; 13
View details for DOI 10.1088/1748-0221/13/06/P06014
View details for Web of Science ID 000435599800007
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Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction
Science
2018; 361 (6397): 64-67
View details for DOI 10.1126/science.aat0049
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A direct electron detector for time-resolved MeV electron microscopy
REVIEW OF SCIENTIFIC INSTRUMENTS
2017; 88 (3)
Abstract
The introduction of direct electron detectors enabled the structural biology revolution of cryogenic electron microscopy. Direct electron detectors are now expected to have a similarly dramatic impact on time-resolved MeV electron microscopy, particularly by enabling both spatial and temporal jitter correction. Here we report on the commissioning of a direct electron detector for time-resolved MeV electron microscopy. The direct electron detector demonstrated MeV single electron sensitivity and is capable of recording megapixel images at 180 Hz. The detector has a 15-bit dynamic range, better than 30-μm spatial resolution and less than 20 analogue-to-digital converter count RMS pixel noise. The unique capabilities of the direct electron detector and the data analysis required to take advantage of these capabilities are presented. The technical challenges associated with generating and processing large amounts of data are also discussed.
View details for DOI 10.1063/1.4977923
View details for Web of Science ID 000397871400027
View details for PubMedID 28372435
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Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules
PHYSICAL REVIEW LETTERS
2016; 117 (15)
Abstract
Observing the motion of the nuclear wave packets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wave packet in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 Å and temporal resolution of 230 fs full width at half maximum. The method is not only sensitive to the position but also the shape of the nuclear wave packet.
View details for DOI 10.1103/PhysRevLett.117.153002
View details for Web of Science ID 000384479300004
View details for PubMedID 27768362
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Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses
NATURE COMMUNICATIONS
2016; 7
Abstract
Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angström spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.
View details for DOI 10.1038/ncomms11232
View details for Web of Science ID 000373622400001
View details for PubMedID 27046298
View details for PubMedCentralID PMC4822053
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Femtosecond gas phase electron diffraction with MeV electrons
FARADAY DISCUSSIONS
2016; 194: 563–81
Abstract
We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.
View details for DOI 10.1039/c6fd00071a
View details for Web of Science ID 000392422200026
View details for PubMedID 27711826
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Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory
REVIEW OF SCIENTIFIC INSTRUMENTS
2015; 86 (7)
Abstract
Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.
View details for DOI 10.1063/1.4926994
View details for Web of Science ID 000358934400053
View details for PubMedID 26233391
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Comparison of film measurements and Monte Carlo simulations of dose delivered with very high-energy electron beams in a polystyrene phantom
MEDICAL PHYSICS
2015; 42 (4): 1606-1613
Abstract
To measure radiation dose in a water-equivalent medium from very high-energy electron (VHEE) beams and make comparisons to Monte Carlo (MC) simulation results.Dose in a polystyrene phantom delivered by an experimental VHEE beam line was measured with Gafchromic films for three 50 MeV and two 70 MeV Gaussian beams of 4.0-6.9 mm FWHM and compared to corresponding MC-simulated dose distributions. MC dose in the polystyrene phantom was calculated with the EGSnrc/BEAMnrc and DOSXYZnrc codes based on the experimental setup. Additionally, the effect of 2% beam energy measurement uncertainty and possible non-zero beam angular spread on MC dose distributions was evaluated.MC simulated percentage depth dose (PDD) curves agreed with measurements within 4% for all beam sizes at both 50 and 70 MeV VHEE beams. Central axis PDD at 8 cm depth ranged from 14% to 19% for the 5.4-6.9 mm 50 MeV beams and it ranged from 14% to 18% for the 4.0-4.5 mm 70 MeV beams. MC simulated relative beam profiles of regularly shaped Gaussian beams evaluated at depths of 0.64 to 7.46 cm agreed with measurements to within 5%. A 2% beam energy uncertainty and 0.286° beam angular spread corresponded to a maximum 3.0% and 3.8% difference in depth dose curves of the 50 and 70 MeV electron beams, respectively. Absolute dose differences between MC simulations and film measurements of regularly shaped Gaussian beams were between 10% and 42%.The authors demonstrate that relative dose distributions for VHEE beams of 50-70 MeV can be measured with Gafchromic films and modeled with Monte Carlo simulations to an accuracy of 5%. The reported absolute dose differences likely caused by imperfect beam steering and subsequent charge loss revealed the importance of accurate VHEE beam control and diagnostics.
View details for DOI 10.1118/1.4914371
View details for Web of Science ID 000352273200015
View details for PubMedID 25832051
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Laser ionized preformed plasma at FACET
PLASMA PHYSICS AND CONTROLLED FUSION
2014; 56 (8)
View details for DOI 10.1088/0741-3335/56/8/084011
View details for Web of Science ID 000340055300012
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Direct measurement of the transverse wakefields of tapered collimators
PHYSICAL REVIEW SPECIAL TOPICS-ACCELERATORS AND BEAMS
2007; 10 (3)
View details for DOI 10.1103/PhysRevSTAB.10.034401
View details for Web of Science ID 000245810500014
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Low-level RF signal processing for the next linear collider test accelerator
17th Particle Accelerator Conference
IEEE. 1998: 3027–3029
View details for Web of Science ID 000079582800983
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SLC interferometer system and phase distribution upgrades
edited by Comyn, M., Craddock, M. K., Reiser, M., Thomson, J.
IEEE. 1998: 3021–23
View details for Web of Science ID 000079582800981
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1ST MEASUREMENT OF THE LEFT-RIGHT CROSS-SECTION ASYMMETRY IN Z-BOSON PRODUCTION BY E+E- COLLISIONS
PHYSICAL REVIEW LETTERS
1993; 70 (17): 2515-2520
View details for Web of Science ID A1993KY50300003
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LENGTH MONITOR FOR 1-MM SLC BUNCHES
1993 Particle Accelerator Conference
I E E E. 1993: 2423–2425
View details for Web of Science ID A1993BA40R00800
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USING TRANSIENT WAVE-FORM RECORDERS TO MEASURE AND STORE BEAM PARAMETERS
edited by Corneliussen, S. T., Carlton, L.
I E E E. 1993: 2234–36
View details for DOI 10.1109/PAC.1993.309279
View details for Web of Science ID A1993BA40R00736
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MULTIBUNCH ENERGY AND SPECTRUM CONTROL IN THE SLC HIGH-ENERGY LINAC
edited by LIZAMA, L., CHEW, J.
I E E E. 1991: 3210–12
View details for DOI 10.1109/PAC.1991.165237
View details for Web of Science ID A1991BV39S01047
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ENERGY FEED FORWARD AT THE SLC
edited by LIZAMA, L., CHEW, J.
I E E E. 1991: 1464–66
View details for DOI 10.1109/PAC.1991.164670
View details for Web of Science ID A1991BV39S00476
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INITIAL MEASUREMENTS OF Z-BOSON RESONANCE PARAMETERS IN E+E- ANNIHILATION
PHYSICAL REVIEW LETTERS
1989; 63 (7): 724-727
View details for Web of Science ID A1989AK17500008
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THE SLAC LINAC AS USED IN THE SLC COLLIDER
edited by BENNETT, F., KOPTA, J.
I E E E. 1989: 983–86
View details for DOI 10.1109/PAC.1989.73322
View details for Web of Science ID A1989BQ56Q00315
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PHASING TOOLS FOR THE KLYSTRONS AT THE SLC
edited by BENNETT, F., KOPTA, J.
I E E E. 1989: 1984–86
View details for DOI 10.1109/PAC.1989.73002
View details for Web of Science ID A1989BQ56Q00644
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RF PHASE DISTRIBUTION-SYSTEMS AT THE SLC
edited by BENNETT, F., KOPTA, J.
I E E E. 1989: 1987–89
View details for DOI 10.1109/PAC.1989.73003
View details for Web of Science ID A1989BQ56Q00645
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HIGH-RESOLUTION BEAM PROFILE MONITORS IN THE SLC
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1985; 32 (5): 2003-2005
View details for Web of Science ID A1985G933200141
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KLYSTRON CONTROL SOFTWARE IN THE SLC
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1985; 32 (5): 2110–11
View details for DOI 10.1109/TNS.1985.4333831
View details for Web of Science ID A1985G933200177
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A MULTICHANNEL PULSER FOR THE SLC THERMIONIC ELECTRON SOURCE
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1985; 32 (5): 1829-1831
View details for Web of Science ID A1985G933200084
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HARDWARE UPGRADE FOR KLYSTRONS IN THE SLC
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1985; 32 (5): 2107-2109
View details for Web of Science ID A1985G933200176
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GENERATION AND ACCELERATION OF HIGH-INTENSITY BEAMS IN THE SLC INJECTOR
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1985; 32 (5): 3160-3162
View details for Web of Science ID A1985ARG7100153
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A NEW CONTROL-SYSTEM FOR THE SLAC ACCELERATOR KLYSTRONS FOR SLC
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1983; 30 (4): 2325–27
View details for DOI 10.1109/TNS.1983.4332804
View details for Web of Science ID A1983RA81800121
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NEW SURGICAL HEADLIGHT REFLECTOR WHICH AVOIDS DISADVANTAGES OF PARALLEL BEAMS
PLASTIC AND RECONSTRUCTIVE SURGERY
1977; 60 (4): 638–39
View details for DOI 10.1097/00006534-197710000-00033
View details for Web of Science ID A1977DY75100029
View details for PubMedID 909977