Experimental set-up

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67 The THz pulse energy directly after the sLN crystal was ~4 µJ, measured with a calibrated pyroelectric detector from Microtech Instruments. The polarization of the outgoing THz pulse was rotated by 90 degrees, using a periscope, in order to be horizontal and parallel to the axis of the time-of-flight detector that was used to collect the photoelectrons. The THz beam was coupled into the vacuum chamber through a 7 mm thick polymer window. A 275 mm focal length Teflon lens was used to collimate the THz radiation and a 3-inch effective focal length, 90-degree off-axis parabolic mirror, was used to focus the THz in the interaction region, which is precisely defined by the position of the TOF spectrometer entrance aperture and the gas target position. The NIR probe beam and the FEL beam were passed through a 2mm hole in the parabolic mirror and collinearly overlapped with the THz beam. All three beams were adjusted to be spatially and temporally overlapped. For the coarse alignment of the THz beam and the NIR probe beam a MCT detector was placed at the interaction region, while fine alignment was achieved by the EOS measurement. The spatial overlap of the NIR and the XUV was adjusted by observing the fluorescence from a ZnTe (or Ce:YAG) crystal induced by the NIR and XUV pulses.

Figure 5.3: Electro optic sampling of the THz pulse used for the streaking experiments. The measurement was performed in a 500 µm ZnTe crystal using balanced electro-optic detection. The single-cycle pulse is ~2 ps in duration with a carrier frequency of ~ 0.6 THz. Due to the low repetition rate of the pump-probe laser at FLASH (10 Hz) the EOS measurement suffers from a high noise level.

Therefore, a high frequency filter was applied to the measured data. The oscillations for negative times arises from absorption and reemission of the THz by water vapor (corresponding to 1.7 THz), as during the beam time the EO sampling could only be performed in air. The THz amplitude spectrum, displayed on the right panel, is calculated by a numerical Fast Fourier Transformation (FFT) from the measured THz waveform.

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The temporal shape of the THz pulse at the interaction region was characterized by EOS in a 500 µm thick ZnTe crystal, which was mounted on a manipulator arm. The measured THz waveform is shown in Figure 5.3, it consists of a ~ 2 ps long single-cycle, with a central frequency at ~0.6 THz. The oscillations at negative time delays in the measured THz electric field originate from THz absorption and re-radiation by water vapor, as the EO measurement was performed in atmospheric ambience.

For the streaking measurement a 100 µm gas nozzle, mounted on a xyz-manipulator, providing the gas target for the photoemission could be placed in the interaction region. Gas streaming with a continuous flow rate was introduced into the experimental chamber and the background pressure was adjusted to be ~3x10^-6 mbar (for helium). This value was chosen based on the signal level of the measured TOF trace and was kept constant during the streaking measurement. If the electron number that built up the signal is to low, the measured signal will be dominated by statistical fluctuations of the discrete electron counts.

On the other hand, space charge effects could deteriorate the measurement if the number of electrons is too high. Typically, the number of electrons in the TOF traces is about 50-500 electrons. Furthermore, the background pressure in the experimental chamber is determined by the vacuum conditions for the MCP detector and the entire FEL beamline.

The distance between the tip of the gas nozzle and the XUV beam should be minimized to reduce the interaction length between the gas and the XUV, which in turn decreases the influence of the Gouy phase shift. This shift, results in a phase change between the THz streaking wave and the envelope of the XUV pulse while passing the interaction region, leading to additional broadening of the acquired spectra (in Section 5.3.3 this effect is explained in more detail). It should be noted that the nozzle tip cannot be moved arbitrarily close to the THz beam, as this could lead to a locally enhanced THz-field.

Field free 2p (21.6 eV, the splitting of 0.1 eV due to spin-orbit coupling can be neglected) and 2s (48.5 eV) photoemission peaks from neon (Ne) were used to calibrate the TOF spectrometer. Helium was used (24.6 eV) in the pulse characterization measurements, due to its isolated photoemission line, leading to a clean streaked photoelectron spectrum.

This avoids the possibility of photoelectrons from different binding energies to overlap with each other when they are streaked. Soft X-ray FEL pulses with approximately 10 µJ pulse energy at 4.8 nm (258 eV photon energy) with an independently measured bandwidth of

~2.5 eV full-width-maximum (FWHM) [¹¹⁰] were used to generate the He 1s electrons with an initial kinetic energy of ~233 eV.

The THz pulses were polarized along the direction of detection (which is horizontal), such that the streaking effect directly coupled to the observed photoelectron kinetic energy.

69 The FEL pulses are also polarized along the observation direction; this is beneficial as the photoelectron emission probability along the direction of the FEL polarization is increased.

Coarse temporal overlap between the NIR probe pulse (thus THz pulse) and the FEL pulse was achieved with a fast photodiode. Precise temporal overlap was then established by monitoring the changes in the photoelectron spectrum induced by the THz streaking field, as a function of time delay between the THz pulse and the FEL. The NIR pulse generating the THz streaking pulse and the FEL pulse were synchronized electronically to a common radiofrequency (RF) distribution network at the accelerator facility to with ~100 fs r.m.s. precision, which is significantly shorter than the streaking field half cycle of ~600 fs.

As a result, once the timing between the FEL pulse and the THz pulse is established, all single-shot acquisitions occur on a uniquely defined, nearly linear portion of the streaking ramp. The recorded data gives access to the temporal shape and arrival time of the FEL pulses on a single-shot basis.