Ab initio quantum direct dynamics simulations of ultrafast photochemistry with Multiconfigurational Ehrenfest approach

The Multiconfigurational Ehrenfest (MCE) method is a quantum dynamics technique which allows treatment of a large number of quantum nuclear degrees of freedom. This paper presents a review of MCE and its recent applications, providing a summary of the formalisms, including its ab initio direct dynamics versions and also giving a summary of recent results. Firstly, we describe the Multiconfigurational Ehrenfest version 2 (MCEv2) method and its applicability to direct dynamics and report new calculations which show that the approach converges to the exact result in model systems with tens of degrees of freedom. Secondly, we review previous “on the fly” ab initio Multiple Cloning (AIMC-MCE) MCE dynamics results obtained for systems of a similar size, in which the calculations treat every electron and every nucleus of a polyatomic molecule on a fully quantum basis. We also review the Time Dependent Diabatic Basis (TDDB) version of the technique and give an example of its application. We summarise the details of the sampling techniques and interpolations used for calculation of the matrix elements, which make our approach efficient. Future directions of work are outlined

Ultrafast photoelectron spectroscopy: Femtosecond pump-probe coincidence detection of ammonia cluster ions and electrons

A new type of experiment is described, in which the femtosecond pump-probe method is combined with the photoelectron-photoion coincidence technique and time-of-flight photoelectron energy analysis. The experimental conditions for observing true coincidences are discussed. The performance of the new time resolved, ultrafast photoelectron spectroscopy is exemplified by studying the excited state dynamics of ammonia molecules and clusters

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH3)m are photo-excited to the electronic $\tilde{A}$ state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH3)$_{m-2}$NH4(3s)NH2 with the subunit NH4 in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH3)$_{m-2}$NH4(3p )NH2 states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m>2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation

Hydrogen atom transfer in indole (NH

The H atom transfer reaction in electronically excited $\mathrm{indole(NH}_{3}\mathrm{)}_{n}$ clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies we are able to detect the dissociation products $\mathrm{(NH}_{3}\mathrm{)}_{n-1}\mathrm{NH}_{4}$ for n = 1–6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster ions due to evaporation of $\mathrm{NH}_{3}$ molecules. The formation times of the products are ca. 140~ps for n=2–4 and about 80~ps for n=5,6