Ab Initio Multiple Spawning Simulations Research Articles

Photo-induced structural dynamics of o-nitrophenol by ultrafast electron diffraction.

The photo-induced dynamics of o-nitrophenol, particularly its photolysis, has garnered significant scientific interest as a potential source of nitrous acid in the atmosphere. Although the photolysis products and preceding photo-induced electronic structure dynamics have been investigated extensively, the nuclear dynamics accompanying the non-radiative relaxation of o-nitrophenol on the ultrafast timescale, which include an intramolecular proton transfer step, have not been experimentally resolved. Herein, we present a direct observation of the ultrafast nuclear motions mediating photo-relaxation using ultrafast electron diffraction. This work spatiotemporally resolves the loss of planarity which enables access to a conical intersection between the first excited state and the ground state after the proton transfer step, on the femtosecond timescale and with sub-Angstrom resolution. Our observations, supported by ab initio multiple spawning simulations, provide new insights into the proton transfer mediated relaxation mechanism in o-nitrophenol.

Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2 Probed at the S 2p Edge.

Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS2, a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

Vacuum ultraviolet excited state dynamics of the smallest ring, cyclopropane. II. Time-resolved photoelectron spectroscopy and ab initio dynamics.

The vacuum-ultraviolet photoinduced dynamics of cyclopropane (C3H6) were studied using time-resolved photoelectron spectroscopy (TRPES) in conjunction with ab initio quantum dynamics simulations. Following excitation at 160.8 nm, and subsequent probing via photoionization at 266.45 nm, the initially prepared wave packet is found to exhibit a fast decay (<100 fs) that is attributed to the rapid dissociation of C3H6 to ethylene (C2H4) and methylene (CH2). The photodissociation process proceeds via concerted ring opening and C-C bond cleavage in the excited state. Ab initio multiple spawning simulations indicate that ring-opening occurs prior to dissociation. The dynamics simulations were subsequently employed to simulate a TRPES spectrum, which was found to be in excellent agreement with the experimental result. On the basis of this agreement, the fitted time constants of 35 ± 20 and 57 ± 35 fs were assigned to prompt (i) dissociation on the lowest-lying excited state, prepared directly by the pump pulse, and (ii) non-adiabatic relaxation from higher-lying excited states that lead to delayed dissociation, respectively.

Substituent effects on the nonadiabatic dynamics of ethylene: [formula omitted]-donors and [formula omitted]-acceptors

The energetic and structural trends in the minimum energy conical intersections of a series of substituted ethylenes are explained by the degree to which the chemical substituents polarize electron density across the C=C double bond. The addition of a substituent that significantly polarizes the electron density also reduces the magnitude of the large-amplitude pyramidalization motions required to reach a conical intersection and can thus very effectively “direct” these motions to occur at a specific carbon atom. These observations are summarized via the construction of a reduced dimensionality representation of the relevant potential energy surfaces inspired by a previously reported 3-state model for biradical systems. On-the-fly nonadiabatic dynamics simulations reveal that this effect dictates the dominant excited state decay pathway for acrylonitrile (π-acceptor CN substituent) and vinylamine (π-donor NH2 substituent). The ab initio multiple spawning simulations show that population transfer to the ground electronic state for both molecules occurs primarily in the regions around the substituent-altered minimum energy conical intersections, demonstrating that chemical substitution may be used to selectively alter photochemical pathways.

First-Principles Study of Nonradiative Recombination in Silicon Nanocrystals: The Role of Surface Silanol

The photophysical properties of emissive silicon nanomaterials depend strongly on the chemical composition and structure of their surfaces, and the development of a causal, microscopic understanding of this relationship is highly desirable. One surface-dependent property of interest is the propensity for nonradiative recombination (NRR). In this work, we apply ab initio theoretical methods to investigate the mechanism of NRR in a silicon nanocrystal with a single surface silanol group. Ab initio multiple spawning simulations of an electronically excited cluster model (Si7H11OH) indicate ultrafast nonradiative decay to the electronic ground state. A multireference electronic structure study demonstrates that this nonradiative decay occurs near conical intersections between the ground and first excited electronic states of the cluster. These intersections are accessed after stretching of the bond between the silanol silicon atom and an adjacent silicon atom. The presence of this intersection in a true nanomaterial is confirmed by optimization of a similar conical intersection in a silicon nanocrystal (oblate, major diameter 1.4 nm, minor diameter 1.0 nm) with a silanol group on the surface (Si44H45OH). This intersection was identified using a graphics processing unit accelerated implementation of the configuration interaction singles natural orbital complete active space configuration interaction method. All intersections identified in this work are predicted to be at least 4.3 eV above the ground state minimum energy. This confirms the widely held view that silanol groups do not introduce efficient pathways for nonradiative recombination of excitons created upon absorption of visible light. That such an assignment is made entirely from first-principles underscores the value of conical intersection optimization as a tool for elucidating semiconductor photophysics.

Initial Dynamics of The Norrish Type I Reaction in Acetone: Probing Wave Packet Motion

The Norrish Type I reaction in the S(1) (nπ*) state of acetone is a prototype case of ketone photochemistry. On the basis of results from time-resolved mass spectrometry (TRMS) and photoelectron spectroscopy (TRPES) experiments, it was recently suggested that after excitation the wave packet travels toward the S(1) minimum in less than 30 fs and stays there for more than 100 picoseconds [Chem. Phys. Lett.2008, 461, 193]. In this work we present simulated TRMS and TRPES signals based on ab initio multiple spawning simulations of the dynamics during the first 200 fs after excitation, getting quite good agreement with the experimental signals. We can explain the ultrafast decay of the experimental signals in the following manner: the wave packet simply travels, mainly along the deplanarization coordinate, out of the detection window of the ionizing probe. This window is so narrow that subsequent revival of the signal due to the coherent deplanarization vibration is not observed, meaning that from the point of view of the experiment the wave packets travels directly to the S(1) minimum. This result stresses the importance of pursuing a closer link to the experimental signal when using molecular dynamics simulations in interpreting experimental results.

Ab Initio Multiple Spawning Dynamics of Excited Butadiene: Role of Charge Transfer

Ab initio multiple spawning simulations of the photochemical reaction dynamics of s-trans-1,3-butadiene were performed. It is found that nonadiabatic events involving two low-lying excited states begin as early as 10 fs after excitation, resulting in the population being split between the bright 1(1)B(u) state and the dark 2(1)A(g) state. The molecule subsequently twists about a terminal carbon-carbon bond regardless of whether it is on the 1(1)B(u) or 2(1)A(g) electronic state. This twisting motion leads to conical intersections between S(1) and S(0). Several regions of the intersection seam involving states of differing character are accessed. The regions of the seam involving intersection between a state of charge-transfer character and a state of covalent character dominate the quenching dynamics, but intersections between two covalent states are also accessed a small percentage of the time. The existence and relative energies of these intersections are validated by optimization at the multistate complete active space second-order perturbation level of theory (MS-CASPT2). Our results point to a new mechanism for photoisomerization of butadiene that emphasizes the role of charge-transfer states.

Ab initio equation-of-motion coupled-cluster molecular dynamics with ‘on-the-fly’ diabatization: the doublet-like feature in the photoabsorption spectrum of ethylene

We introduce an ‘on-the-fly’ diabatization in the context of ab initio multiple spawning simulations. Adiabatic potential energy surfaces are computed and diabatized by using the block-diagonalization method simultaneously with nuclear wavepacket dynamics. Because the diabatization is carried out without knowledge of the nonadiabatic coupling matrix elements, we are able to use the electronic structure methods where those matrix elements are difficult to calculate, such as equation-of-motion coupled-cluster singles and doubles theory. The electronic absorption spectra of the lowest Rydberg state of ethylene is studied by applying our new approach, and we elucidate the origin of the doublet feature in the spectrum.

Photodynamics of ethylene: ab initio studies of conical intersections

We have used extended basis sets and configuration interaction wave functions to systematically characterize the excited state potential energy surfaces of ethylene, including conical intersections between electronic states. The results are consistent with our previous ab initio multiple spawning simulations of ethylene photodynamics and electronic spectra. The C–C bond on the optically accessible V state is extended in planar geometries, suggesting a role for C–C stretching in the electronic absorption spectrum. A cascade of conical intersections connecting the V state in the Franck–Condon region to each of the low-lying Rydberg states has been identified, in addition to intersections connecting the excited state manifold back to the ground state. The D 2d twisted geometry of ethylene is found to be a saddle point, not a local minimum. Pyramidalization of one of the methylene units in twisted ethylene is found to be favorable, leading to a conical intersection. We have identified and characterized eight conical intersections involving the V state which are likely to be relevant in the photochemistry of ethylene.