An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. SLAC-PUB-12162 Submitted to Applied Physics Letters 2Our understanding about dynamical processes in chemistry, materials science and biology on the picosecond and sub-picosecond time scale stems almost exclusively from time-resolved spectroscopy. Structural changes, on atomic length scales, can only be inferred indirectly from the analysis of spectra. Both x-ray and electron diffraction share the goal of 'imaging' molecular structures with a time resolution that captures the motions as systems evolve, whether they be solids, liquids or gases. Lab scale experiments in both electron diffraction 1,2 and x-ray scattering 3 have produced impressive results. Recently, in anticipation of the construction of the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), an experiment using the electron bunch from the SLAC Linac to produce spontaneous undulator radiation 4 has shown the possibilities for ultrafast x-ray scattering from condensed systems with 100 fs time resolution. 5 This has encouraged us to approach ultrafast electron diffraction (UED) using experimental techniques based on electron sources developed for particle accelerators, with the aim of obtaining single-shot diffraction patterns on a 100 fs time scale.Electron diffraction is complementary to x-ray scattering, but features much larger cross sections that allow the study of surface phenomena, the bulk structures of thin foils and membranes, as well as molecular structures of gas phase samples. 6 As with linac based x-ray sources there has been significant development of electron sources for UED based on the use of photocathodes. 7 Unfortunately, the space-charge interactions of the electrons within a pulse, and the initial kinetic energy distribution with which the electrons are generated, have made it difficult to obtain pulses much shorter than 1 ps 8,9,10 ,in 'conventional' UED experiments using ≈30 keV electron beams. To improve the time resolution one could use fewer electrons per pulse, but that requires longer data acquisition times to obtain the necessary signal-to-noise ratio. 11 Alternatively, it is possible to increase the electric field inside the electron gun, while reducing the flight distance between the gun and the target. 12 Both tend to reduce the time of flight of the electron pulse, thereby giving the electron pulse less time to spread. Even so, this 3 approach is limited because the maximum DC and pulsed electric fields are 12 MV/m and 25 MV/m, respectively. 13,14 In the present work we take a fresh approach to ultrafast time-resolved pump-probe diffraction by using MeV electron be...

We explore how structural dispersion in flexible hydrocarbon chain molecules at very high temperatures is reflected in the photoionization spectra of Rydberg levels. The spectra of N,N-dimethylisopropanamine, N,N-dimethyl-1-butanamine, N,N-dimethyl-2-butanamine, N,N-dimethyl-3-hexanamine, and 1,4-dimethylpiperazine, taken at effective vibrational temperatures of 700-1000 K, show well-resolved features stemming from the 3p and 3s Rydberg states. The line shapes observed in molecules with internal rotation degrees of freedom show that multiple structures are populated. Following up on the discovery that low-lying Rydberg states provide sensitive fingerprints of molecular structures, this work supports Rydberg fingerprint spectroscopy as a tool to probe structural details of molecules in the presence of complex energy landscapes and at high vibrational temperatures. A simple model accounts for the sensitivity of Rydberg fingerprint spectroscopy to the molecular shape, as well as the relative insensitivity of the spectra toward vibrational excitation.

Resonance-enhanced multiphoton ionization photoelectron spectroscopy has been applied to study the electronic spectroscopy and relaxation pathways amongst the 3p and 3s Rydberg states of trimethylamine. The experiments used femtosecond and picosecond duration laser pulses at wavelengths of 416 nm, 266 nm, and 208 nm, and employed two-photon and three-photon ionization schemes. The binding energy of the 3s Rydberg state was found to be 3.087 ± 0.005 eV. The degenerate 3p x,y states have binding energies of 2.251 ± 0.005 eV, and 3p z is at 2.204 ± 0.005 eV. Using picosecond and femtosecond time-resolved experiments we spectrally and temporally resolved an intricate sequence of energy relaxation pathways leading from the 3p states to the 3s state. With excitation at 5.96 eV, trimethylamine is found to decay from the 3p z state to 3p x,y in 539 fs. The decay to 3s from all the 3p states takes place with a 2.9 ps time constant. On these time scales, trimethylamine does not fragment at the given internal energies, which range from 0.42 to 1.54 eV depending on the excitation wavelength and the electronic state.