Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules

Abstract

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.

Femtosecond photoelectron diffraction: a new approach to image molecular structure during photochemical reactions

“…Contrary to the expectation based on our photoelectron diffraction model presented in section 2, no clear angular structure apart from the p-wave-like anisotropy is visible in the photoelectron intensity. However, as shown in our previous work 10,12 , this is mostly the result of the angular averaging due to the imperfect molecular alignment, which averages out most of the interference structure despite the relatively high degree of alignment that was achieved in the experiment.…”

“…The FEL pulses ionize the aligned molecules by emitting photoelectrons predominantly from a targeted inner-shell level of a specific atom in the molecule. These photoelectrons are then imaged shot-by-shot on one side of the VMI spectrometer, while the resulting fragment ions are simultaneously imaged on the other side of the spectrometer, such that the degree of molecular alignment can be constantly monitored 10,12 .…”

“…In order to extract the remaining, weak interference effects in the photoelectron images, we subtract the electron images recorded without YAG pulses from those recorded with YAG pulses present 10,12 . The resulting photoelectron angular distribution differences (∆PADs) for different photoelectron kinetic energies are shown as polar plots in Fig.…”

“…by recording molecular-frame photoelectron angular distributions (MFPADs) 5,6,7,8 , PD has not played a major role in gas-phase studies to date since far more precise methods, such as microwave spectroscopy, exist to determine the ground-state geometry of molecules in the gas phase. However, with the increased interest in recent years in studying non-equilibrium structures such as reaction intermediates and other transient species, and with new experimental developments that allow studying these structures on ultrafast time scales using femtosecondlasers, laser-based high harmonic generation (HHG) sources, and Free-Electron Lasers (FELs), this situation may change since femtosecond photoelectron diffraction (fs-PD) 9,10,11,12,13 may allow accessing time-resolved structural information on transient states that is not accessible by other, conventional methods of structure determination. *Daniel.Rolles@desy.de…”

Imaging electron dynamics with time- and angle-resolved photoelectron spectroscopy

“…And its advantage over ultrafast electron diffraction is that much higher temporal resolution can be obtained using light pulses than using electron pulses [1,10]. Femtosecond time-resolved photoelectron imaging has already been successfully applied to probe structural dynamics in molecules [11][12][13], but its application to electron dynamics, which is much faster, is limited by temporal resolution. Recent progress in attosecond science makes it feasible to produce isolated attosecond pulses with energies reaching the XUV region [14] that enable overcoming this temporal barrier [15].…”