`Making the molecular movie': first frames

Abstract

Recent advances in high-intensity electron and X-ray pulsed sources now make it possible to directly observe atomic motions as they occur in barrier-crossing processes. These rare events require the structural dynamics to be triggered by femtosecond excitation pulses that prepare the system above the barrier or access new potential energy surfaces that drive the structural changes. In general, the sampling process modifies the system such that the structural probes should ideally have sufficient intensity to fully resolve structures near the single-shot limit for a given time point. New developments in both source intensity and temporal characterization of the pulsed sampling mode have made it possible to make so-called 'molecular movies', i.e. measure relative atomic motions faster than collisions can blur information on correlations. Strongly driven phase transitions from thermally propagated melting to optically modified potential energy surfaces leading to ballistic phase transitions and bond stiffening are given as examples of the new insights that can be gained from an atomic level perspective of structural dynamics. The most important impact will likely be made in the fields of chemistry and biology where the central unifying concept of the transition state will come under direct observation and enable a reduction of high-dimensional complex reaction surfaces to the key reactive modes, as long mastered by Mother Nature.

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation

“…S ince the introduction of femto-chemistry [1][2][3][4][5] , electron or X-ray diffraction have been the most commonly used techniques to track nuclear rearrangement in molecules [6][7][8][9] . Unfortunately, these techniques are largely insensitive to the more subtle and irregular structural changes that can occur within a single small molecule undergoing a chemical reaction.…”

Time-dependent resonant scattering: An analytical approach

“…The bound state remains continually populated during the passage time ∼1/η of the wave packet [Eqs. (8) and (19)] and scatters the incoming wave packet inelastically into all channels coupled to it [Eqs. (20) and (21)].…”

“…(22)- (24) offer the opportunity to examine the accuracy of the approximate form of c(t) presented in Eq. (19). In Fig.…”

“…(20) may be obtained directly from the first order expansion of c(t) in Eq. (19). The coefficient of −ı¯− 1 F (E, t)V E 0 α 0 V * Eα is then obtained in the form…”

“…Such pulses (on the picosecond scale) are used in ultrafast electron diffraction (UED). [17][18][19] Particularly relevant to the present treatment is the rapid development of highly coherent sources of cold electrons. [20][21][22] Cold-atom collisions can also be studied in the time domain.…”