Femtosecond Electronic and Hydrogen Structural Dynamics in Ammonia Imaged with Ultrafast Electron Diffraction

Directly imaging structural dynamics involving hydrogen atoms by ultrafast diffraction methods is complicated by their low scattering cross sections. Here we demonstrate that megaelectronvolt ultrafast electron diffraction is sufficiently sensitive to follow hydrogen dynamics in isolated molecules. In a study of the photodissociation of gas phase ammonia, we simultaneously observe signatures of the nuclear and corresponding electronic structure changes resulting from the dissociation dynamics in the time-dependent diffraction. Both assignments are confirmed by ab initio simulations of the photochemical dynamics and the resulting diffraction observable. While the temporal resolution of the experiment is insufficient to resolve the dissociation in time, our results represent an important step towards the observation of proton dynamics in real space and time.

Figure 1

(a) ${\mathrm{S}}_0$ and ${\mathrm{S}}_1$ PESs (obtained from Refs. [40, 41]) along the umbrella ($\mathrm{\Theta }$) and ${\mathrm{ND}}_2\text{−}\mathrm{D}$ dissociation coordinates. Photoexcitation (blue arrow) to the predissociative $2{}^1\mathrm{A}$ state activates the umbrella mode and promotes adiabatic or nonadiabatic ${\mathrm{ND}}_2\text{−}\mathrm{D}$ dissociation. The contour plot shows the ${\mathrm{S}}_1/{\mathrm{S}}_0$ energy gap, indicating a smaller gap (blue) along the ${\mathrm{ND}}_2\text{−}\mathrm{D}$ coordinate. Key orbitals involved in the process are shown as insets and correspond to state-averaged natural orbitals ($\text{isovalue}=0.36\mathrm{a}.\mathrm{u}.$). The Rydberg orbital correlates with a ${\sigma }^{*}$ orbital in the distorted ${\mathrm{ND}}_2\text{−}\mathrm{D}$ and eventually becomes the $1s$ H orbital upon dissociation. Additionally, the dominant electron configurations (nonspin adapted) of the states in the Franck-Condon region and in the dissociation limit are shown. (b) Experimental absorption spectrum of ${\mathrm{ND}}_3$ (black line) showing strong vibrational progression in the umbrella mode (${\nu }_2^{\prime }$). The peaks of the progression are labeled with respect to the corresponding vibrational level. The small peak at 217 nm results from a hot band of ${\mathrm{ND}}_3$ [42, 43]. The spectrum of the UV pump laser centered around ${\nu }_2^{\prime }=4$ of the umbrella mode is shown in blue.

Figure 2

Comparison of experimental and simulated signatures. (a)–(c) show $\mathrm{\Delta }I/I_{\mathrm{ref}}$ signals at different delays from (a) the experiment, (b) ab initio scattering calculations based on AIMS simulations, and (c) the same AIMS simulations, but using the independent atom model (IAM) to compute the diffraction signal. The delays for the temporal lineouts in (a)–(c) are marked as color-coded horizontal lines in the corresponding false-color plots of the time-dependent signals from the experiment and the two different simulation approaches in (e)–(g). Additionally, the time dependence of the integrated low-$s$ regions of the three false-color plots is shown in plot (d) where the upper-$s$ integration limits are marked by vertical color-coded dashed lines in plots (e)–(g).

Figure 3

Decomposition of the ab initio scattering signal at $t=0.84\mathrm{ps}$ according to the exit channel. (a) Total scattering signal, and its decomposition into (b) elastic and (c) inelastic components. The vertical dashed lines mark $s=1$ and $3{\AA }^{-1}$. The distance-based cutoffs used to define the photoproducts are indicated in the figure (see also Fig. S5).