Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules

Abstract

Observing the motion of the nuclear wavepackets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wavepacket in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 Å and temporal resolution of 230 fs full-width at half-maximum (FWHM). The method is not only sensitive to the position but also the shape of the nuclear wavepacket.Photo-induced reactions are of particular interest for understanding the fundamental mechanisms driving the conversion of light into chemical and kinetic energy on ultrafast time scales. The coherent nuclear motion is particularly important to study the reaction pathway and energy conversion efficiency in processes that cannot be described using the Born-Oppenheimer approximation. Diffraction-based techniques, such as ultrafast electron and x-ray diffraction offer a unique advantage for imaging the molecular geometry as those measurements are directly sensitive to the spatial distribution of atoms, and are thus complementary to spectroscopic methods that are sensitive to energy differences between electronic states. The nuclear motion in

Comment on “Self-Referenced Coherent Diffraction X-Ray Movie of Ångstrom- and Femtosecond-Scale Atomic Motion”

“…The excited-state diffraction from a gas thus comes linear in the excitation fraction and the amplitude boost from heterodyne detection is neither necessary nor possible. Equation (2) and equivalents obtained from the independent atom approximation and rotational averaging have been known in the literature on time-resolved X-ray scattering for many years and appear also in electron diffraction [3,5,6].The possibility of heterodyne-detected diffraction in crystals (and other systems with long-range order) can be seen by partitioning the total charge density as a sum of molecular charge densitiesσ gas = ασ α in S = σ * (q)σ(q) . The diagonal terms in this doublesum generate Eq.…”

“…The excited-state diffraction from a gas thus comes linear in the excitation fraction and the amplitude boost from heterodyne detection is neither necessary nor possible. Equation (2) and equivalents obtained from the independent atom approximation and rotational averaging have been known in the literature on time-resolved X-ray scattering for many years and appear also in electron diffraction [3,5,6].…”

Fundamental Limits on Spatial Resolution in Ultrafast X-ray Diffraction

“…The recent development of ultrashort pulsed electron and X-ray beams has now expanded the scope of structure determination to the time domain [4,5] (and references therein). Important applications include the determination of molecular structures in excited states and the observation of structural molecular dynamics, i.e., the time-resolved determination of transient molecular structures during chemical reactions [6][7][8][9][10][11][12][13]. The advent of ultrafast pulsed X-ray Free-Electron Lasers (XFELs) in particular has increased the intensity of X-rays while decreasing pulse durations to below 30 fs [14].…”

Ultrafast nanoscale magnetic switching via intense picosecond electron bunches

“…This reduces dramatically the velocity mismatch between the electromagnetic pump and electron probe pulses in time-resolved diffraction experiments [9]. This has proven indispensable in probing ultrafast processes in fields ranging from gasphase photochemistry [10] over lattice transformation in low-dimensional systems [11] to the excitation of transient phonons [12].…”