Imaging of alignment and structural changes of carbon disulfide molecules using ultrafast electron diffraction

Abstract

Imaging the structure of molecules in transient-excited states remains a challenge due to the extreme requirements for spatial and temporal resolution. Ultrafast electron diffraction from aligned molecules provides atomic resolution and allows for the retrieval of structural information without the need to rely on theoretical models. Here we use ultrafast electron diffraction from aligned molecules and femtosecond laser mass spectrometry to investigate the dynamics in carbon disulfide following the interaction with an intense femtosecond laser pulse. We observe that the degree of alignment reaches an upper limit at laser intensities below the ionization threshold, and find evidence of structural deformation, dissociation and ionization at higher laser intensities.

Gas-phase electron diffraction from laser-aligned molecules

“…The pulse duration is still short enough to operate in the regime of impulsive alignment. In a separate study, we have also observed that further increasing the laser intensity does not significantly increase the degree of alignment [27]. The center region (s < 1.0 Å -1 ) of the diffraction patterns is blocked by a beam stop, as shown in Figure 6.…”

Ultrafast imaging of isolated molecules with electron diffraction

“…Impulsive alignment can be improved by using a sequence of laser pulses [77], but is ultimately limited by the initial rotational temperature of the molecules and the maximum intensity that can be applied. Recent UED experiments have shown that impulsive alignment reaches a saturation value for intensities well below the ionization threshold, and that at high intensities the structure can be distorted by multiphoton excitation [25]. Another option to improve the alignment is a mixed alignment with shaped laser pulses to do adiabatic turnon and impulsive turn-off of the alignment field [78,79].…”

“…The laser intensity is limited by multiphoton excitation and ionization events that can distort the molecular structure [25]. For small molecules in the gas phase, multiphoton ionization typically becomes significant at laser intensities around 10 13 W cm -2 [68].…”

“…We will cover the progress from the first experiments that were able to detect alignment through electron diffraction [21][22][23][24], to more recent results where molecular structures were retrieved directly from the diffraction data [18,25]. The method of laser-alignment inherently requires ultrafast temporal resolution, in particular when the molecules are impulsively aligned.…”

Imaging population transfer in atoms with ultrafast electron pulses

“…For example, structural evolutions during phase transitions have been mapped using ultrafast electron microscopy and crystallography [5][6][7]. Also, diffraction images have identified transient molecular structures [8] and established evidence of deformation and dissociation of molecules interacting with laser pulses [9]. Nowadays, electron pulses with femtosecond (fs) duration have been reported [10][11][12][13][14].…”