Abstract
Recent advances in pulsed electron gun technology have resulted in femtosecond electron pulses becoming available for ultrafast electron diffraction experiments. For experiments investigating chemical dynamics in the gas phase, the resolution is still limited to picosecond time scales due to the velocity mismatch between laser and electron pulses. Tilted laser pulses can be used for velocity matching, but thus far this has not been demonstrated over an extended target in a diffraction setting. We demonstrate an optical configuration to deliver high-intensity laser pulses with a tilted pulse front for velocity matching over the typical length of a gas jet. A laser pulse is diffracted from a grating to introduce angular dispersion, and the grating surface is imaged on the target using large demagnification. The laser pulse duration and tilt angle were measured at and near the image plane using two different techniques: second harmonic cross correlation and an interferometric method. We found that a temporal resolution on the order of 100 fs can be achieved over a range of approximately 1 mm around the image plane.
Highlights
Recent improvements in electron pulse technology have resulted in tabletop sources capable of delivering femtosecond pulses to a target [1, 2]
For the second harmonic generation (SHG) method, the two pulses overlapped in a thin barium borate (BBO) crystal placed at the image plane
Our results indicate that it is be possible to reach a temporal resolution on the order of 100 fs for gas-phase ultrafast electron diffraction (UED) experiments using tilted laser pulses to compensate for the group velocity mismatch
Summary
Recent improvements in electron pulse technology have resulted in tabletop sources capable of delivering femtosecond pulses to a target [1, 2] This has enabled ultrafast electron diffraction (UED) experiments on solid samples with a resolution on the order of 200 fs [2]. Thin (submicrometer) samples were used to capture diffraction patterns in transmission mode Another important application of UED is investigating ultrafast chemical reactions on isolated molecules. For these experiments, the target is a gas beam with a diameter typically between 0.1 and 1 mm. The problem is that the velocity must be matched throughout the length of the gas target and both the tilt angle and duration of the laser pulse change as it propagates. We investigate this issue in detail and show experimentally that, with the appropriate optical design, it is possible to achieve a resolution on the order of 100 fs
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