High current table-top setup for femtosecond gas electron diffraction

Abstract

We have constructed an experimental setup for gas phase electron diffraction with femtosecond resolution and a high average beam current. While gas electron diffraction has been successful at determining molecular structures, it has been a challenge to reach femtosecond resolution while maintaining sufficient beam current to retrieve structures with high spatial resolution. The main challenges are the Coulomb force that leads to broadening of the electron pulses and the temporal blurring that results from the velocity mismatch between the laser and electron pulses as they traverse the sample. We present here a device that uses pulse compression to overcome the Coulomb broadening and deliver femtosecond electron pulses on a gas target. The velocity mismatch can be compensated using laser pulses with a tilted intensity front to excite the sample. The temporal resolution of the setup was determined with a streak camera to be better than 400 fs for pulses with up to half a million electrons and a kinetic energy of 90 keV. The high charge per pulse, combined with a repetition rate of 5 kHz, results in an average beam current that is between one and two orders of magnitude higher than previously demonstrated.

Solving the jitter problem in microwave compressed ultrafast electron diffraction instruments: Robust sub-50 fs cavity-laser phase stabilization

“…We measure an upper limit IRF of τ e = 45 ± 2 fs (106 fs full-width at half-maximum). We note that this is an improvement by over a factor of 3 when compared to previously reported instrument performance 7,8,11 . We attribute this enhancement in performance to the direct generation of phase-locked microwaves, the use of stable continuouswave high-power microwave amplification in conjunction with active phase stabilization.…”

“…Unmodified by external fields, these space-charge dynamics result in a trade-off between pulse fluence and time resolution that is detrimental to ultrafast electron diffraction and imaging experiments. As a result, there have been a number of efforts to correct such broadening through the addition of electron pulse compression strategies that employ microwave [6][7][8][9][10][11] , terahertz 12 and DC electric fields [13][14][15] . These approaches work by inverting the space-charge driven expansion that occurs naturally in the pulse, leading to a temporal focus downstream from the pulse-field interaction.…”

“…Unfortunately, the stability of the cavity-laser synchronization systems that have been employed to date have been insufficient to provide pulse duration limited timeresolution in ultrafast electron diffraction instruments over longer data acquisition times (several hours). Published reports have all concluded that time-resolution in microwave compressed instruments has been closer to 400 fs due to "time-zero" drift that results from various cavity-laser phase syncrhonization instabilities 7,8,11 that are evident in the frequency range from kHz to µHz. As a result of these drifts, the primary benefit of microwave a) Electronic mail: martin.otto@mail.mcgill.ca b) Electronic mail: bradley.siwick@mcgill.ca pulse compression to date and been an increase in bunch charge rather than a dramatic improvement in time resolution.…”

Terahertz Oscilloscope for Recording Time Information of Ultrashort Electron Beams

“…It is worth mentioning that in this measurement the dielectric tube has a large aperture that allows all the electrons to pass through. Together with the large dynamic range, this THz oscilloscope is well suited as an on-line timing tool to measure and correct the timing jitter to improve the temporal resolution in pump-probe experiments, in particular rf buncher based keV UED [36][37][38] and MeV UED [39][40][41][42][43][44][45][46]. The large dynamic range of this THz oscilloscope also allows us to unambiguously record the slow drift of the timing over a much longer time.…”

Time-resolved photoelectron angular distributions from nonadiabatically aligned CO2 molecules with SX-FEL at SACLA

“…Recent developments of x-ray free electron lasers (XFELs) [1][2][3][4] and table-top laser sources [5] are bringing off ultrafast imaging of molecules with femtosecond temporal and sub-Ångstrom spatial resolutions [6][7][8][9]. The first imaging of atomic motions during photochemical processes has been achieved by the ultrafast electron diffraction (UED) [10][11][12] and the ultrafast x-ray diffraction (UXD) [13][14][15][16]. Other imaging techniques such as the Coulomb explosion imaging (CEI) [17][18][19][20], the laser-induced electron diffraction (LIED) [21][22][23], and the ultrafast x-ray photoelectron diffraction (UXPD) [24][25][26][27][28][29][30][31][32][33][34][35][36] are also in progress.…”