Recent experimental and computational gas-phase studies have brought to light a new type of unimolecular decomposition called a “roaming mechanism. It has only been observed in the gas phase, and whether it also occurs in solution is an intriguing question. Using ultrafast transient absorption spectroscopy, we report direct isomerization of CHBr₃, BBr₃, and PBr₃ geminal tribromides in solution within the first 100 fs after S₁-excitation. The gas-phase conditions do not affect the earliest course of similar isomerization of CHBr₃. High-level ab initio simulations on XBr₃ (X = B, P, and CH) suggest that isomerization is governed by an energetically and dynamically accessible S₁/S₀ conical intersection and can be best described as a roaming-mediated pathway. Following the initial relaxation from the Franck-Condon point, “wandering” of the central atoms and migration of Br atom starts on a planar region of the S₁ surface, and in the vicinity of the conical intersection ( 40 fs) the XBr₂ and Br fragments become separated to ≥ 3 Å. After passage through the conical intersection, the partially dissociated bromine atom slips off the XBr₂ bisector plane, and forms the Br–Br bond of the BrXBr–Br isomer ( 60 fs). We give examples of similar roaming isomerization in several other di- and polyhalogenated alkanes.

The nonraddiative dececy route involving trans → cis photo-isomerization from the S₁ (ππ*) state has been investigated for several trans-cinnamate derivatives, which are known as sunscreen reagents. We examined two types of substitution effects. One is structural isomer such as ortho-, meta-, and para-hydroxy-methylcinnmate (o-, m-, p-HMC). The S₁ lifetime of p-HMC is less than 8 ps at zero-point level, and it undergoes rapid S₁ → ¹nπ* → T₁ decay via multiple conical intersections. Finally, the trans → cis isomerization proceeds in the T₁ state. On the other hand, both o- and m-HMC show very slow decay. Their S₁ lifetimes are in the order of 100 ps even at the excess energy of 2000-3000 cm⁻¹. The other is the effect of the complexity of ester group in para-subsitituted species, such as para-methoxy-methyl, -ethyl and -2ethylhexyl cinnamate (p-MMC, p-MEC, p-M2EHC). p-MMC and p-MEC show sharp S₀ → S₁ (ππ*) vibronic bands, while p-M2EHC shows only broad structureless feature even under the jet-cooled condition. In addition, we found that the S₀ → ¹nπ* absorption appears at 1000 cm⁻¹below the S₀ → S₁ (ππ*) transition in p-MEC and p-M2EHC, but not in p-MMC. Thus, the complexity of the ester group is very important for the appearance of the ¹nπ* state.

Transition-metal complexes of rare earth metals including ruthenium and iridium are most commonly employed as visible-light photocatalysts. Despite their highly important and broad applications, they have many disadvantages including high cost associated with low abundance in earth crust, potential toxicity, requirement of specialized ligands for desired activity, and difficulty in recycling of metal contents as well as associated ligands. Polymer-based organophotoredox catalysts are promising alternatives and possess unique advantages such as easier synthesis from inexpensive starting material, longer excited state life time, broad range of activity, sustainability, and recyclability. In this research talk, time-resolved photoluminescence and femtosecond transient absorption (TA) spectroscopy measurements of three novel polymer-based organophotoredox catalysts will be presented. By our synthetic team, their catalytic activity has been proven in some highly valuable chemical transformations, that otherwise require transition metal complexes. Time-resolved spectroscopic investigations have demonstrated that photoinduced processes in these catalysts are similar to the transition metal complexes. Especially, intramolecular vibrational relaxation, internal conversion, and intersystem crossing from the S1 state to the T1 state all occur on a sub-picosecond timescale. The long lifetime of the T1 state ( 2-3 microsecond) renders these polymers potent oxidizing and reducing agents. A spectroscopic and kinetic model has been developed for global fitting of TA spectra in both the frequency and time domains. Implication of the current ultrafast spectroscopy studies of these novel molecules to their roles in photocatalysis will be discussed.

The optically populated excited state wave packet propagates along multidimensional intramolecular coordinates soon after photoexcitation. This action occurs alongside an intermolecular response from the surrounding solvent. Disentangling the multidimensional convoluted signal enables the possibility to separate and understand the initial intramolecular relaxation pathways over the excited state potential energy surface. Here we track the initial excited state dynamics by measuring the fluorescence yield from the first excited state as a function of time delay between two color femtosecond pulses for several cyanine dyes, having different electronic configurations. We find that when the high frequency pulse precedes the low frequency one and for timescales up to 200 fs, the excited state can be depleted through stimulated emission with efficiency that is dependent on the molecular electronic structure. A similar observation at even shorter times was made by scanning the chirp (frequencies ordering) of a femtosecond pulse. These changes reflect the rate at which the nuclear coordinates of the excited state leave the Franck-Condon (FC) region and progress towards achieving equilibrium. Through functional group substitution, we explore these dynamic changes as a function of dipolar change following photoexcitation. We show that with proper knowledge and control over the phase of the excitation pulses, we can extract the relative energy relaxation rates through which the early intramolecular modes are populated at the FC geometry soon after excitation

Nonadiabatic processes, dominated by dynamic passage of reactive fluxes through conical intersections (CIs) are considered to be appealing means for manipulating reaction paths. One approach that is considered to be effective in controlling the course of dissociation processes is the selective excitation of vibrational modes containing a considerable component of motion. Here, we have chosen to study the predissociation of the model test molecule, methylamine and its deuterated isotopologues, excited to well-characterized quantum states on the first excited electronic state, S₁, by following the N-H(D) bond fission dynamics through sensitive H(D) photofragment probing. The branching ratios between slow and fast H(D) photofragments, the internal energies of their counter radical photofragments and the anisotropy parameters for fast H photofragments, confirm correlated anomalies for predissociation initiated from specific rovibronic states, reflecting the existence of a dynamic resonance in each molecule. This resonance strongly depends on the energy of the initially excited rovibronic states, the evolving vibrational mode on the repulsive S₁ part during N-H(D) bond elongation, and the manipulated passage through the CI that leads to radicals excited with C-N-H(D) bending and preferential perpendicular bond breaking, relative to the photolyzing laser polarization, in molecules containing the NH₂ group. The indicated resonance plays an important role in the bifurcation dynamics at the CI and can be foreseen to exist in other photoinitiated processes and to control their outcome.

Potential energy surfaces for electronic states of molecules in strong electromagnetic fields can be described in the dressed-state formalism, which introduces light-induced potentials. A light-induced conical intersection (LICI) [1] appears when two electronic states intersect due to the presence of an external electric field and when the dipole coupling between the field and the molecule vanishes. There are several aspects of quantum dynamics near LICIs, which still require a thorough investigation. How do non-adiabatic effects manifest themselves in polyatomic molecules in strong electromagnetic fields? Are the natural conical-intersections (NCI) and the light-induced conical intersections identical in nature? Do topological effects (Berry phase) [2] influence the nuclear dynamics around NCIs and LICIs? To answer these questions, a computer code for time-propagation of the ro-vibronic wavefunction on multiple coupled potential energy surfaces has been developed. The time-independent zero-order basis is taken from the DUO suite [3], which solves the full ro-vibronic Schrödinger equation for diatomic molecules. Non-adiabatic nuclear dynamics near LICIs will be presented on the examples of NaH and CaF molecules, with a perspective for extension to polyatomics.

We present the results of surface hopping studies of the photodissociation of ICN⁻ and BrCN⁻ following UV and visible excitations to states that can dissociate to X⁻ + CN or X^* + CN⁻ (X = I or Br). Based on previous quantum dynamics studies carried out on the anions, the electronic states that are accessed by the initial UV or visible excitation were found to be coupled, and both photoproducts were observed in the dissociation process. The calculated branching ratios indicated stronger non-adiabatic interactions between the adiabatic electronic states in BrCN⁻ than in ICN⁻.B. Opoku-Agyeman, A. S. Case, J. H. Lehman, W. Carl Lineberger and A. B. McCoy, J. Chem Phys. 141, 084305 (2014).A. S. Case, A. B. McCoy, W. C. Lineberger, J. Phys. Chem. A, 117(50), 13310(2013).A. S. Case, E. M. Miller, J. P. Martin, Y. J. Lu, L. Sheps, A. B. McCoy, W. C. Lineberger, Angew. Chem., Int. Ed. 51(11), 2651 (2012).n this work, we employ Tully's surface hopping algorithmJ. C. Tully, J. Chem Phys. 93, 1061 (1990)mplemented in classical dynamics simulations to investigate the fragmentation processes of these anions. We calculate the branching ratios and the partitioning of the energies among the various degrees of freedom during the dynamics. The results of the surface hopping algorithm are then compared to the reported quantum dynamics results after which the surface hopping approach can then be applied to investigate the dynamics of argon clusters of the ICN⁻ and BrCN⁻. In addition to comparison between the quantum and classical dynamics calculations, preliminary results for the semi-classical calculations on the ICN⁻ show that during the dissociation process, some of the anions live longer than others. Once the anions are solvated, we expect the presence of the argon atoms to stabilize the complexes as reported in previous work, resulting in longer lifetimes. Since experimental studies carried out on the solvated anions show the existence of recombined products even for clusters as small as ICN⁻(Ar) or ICN⁻(Ar)₂, when the clusters are initially excited with visible or UV radiations, respectively,^{b c} the observations of these long-lived anions provide a step towards understanding the dynamics processes that lead to the recombined products in the clusters.

---

Footnotes:

B. Opoku-Agyeman, A. S. Case, J. H. Lehman, W. Carl Lineberger and A. B. McCoy, J. Chem Phys. 141, 084305 (2014).

---

Footnotes:

A. S. Case, E. M. Miller, J. P. Martin, Y. J. Lu, L. Sheps, A. B. McCoy, W. C. Lineberger, Angew. Chem., Int. Ed. 51(11), 2651 (2012).I J. C. Tully, J. Chem Phys. 93, 1061 (1990)i

r0pt Figure To completely characterize photodissociation mechanisms with time-resolved spectroscopy, it is essential to obtain unequivocal experimental information about the fragmentation dynamics induced by the laser pulse. We apply time-resolved photoelectron-photoion coincidence (PEPICO) detection in combination with different excitation schemes to obtain a mechanistic picture of the fragmentation process. For gas phase acetone molecules excited to high lying Rydberg states we are able to disentangle different ionization channels and investigate the fragmentation behavior of each channel separately. In particular, the high differentiability of PEPICO allows to distinguish channels where fragmentation proceeds after ionization from channels with fragmentation in the neutral. We show that excited Rydberg state population undergoes internal conversion due to coupling to valence states, which takes place within (150 ± 30) fs. The corresponding non-adiabatic, ultrafast relaxation dynamics to lower lying states causes conversion of electronic to vibrational energy and is found to play a crucial role in the fragmentation process (see figure 1). By studying the influence of photon energy, pulse duration, chirp and intensity of the laser pulses, we are able to determine the energy-threshold that is required for fragmentation, as well as corresponding fragmentation ratios. Surprisingly, for excitation with pulses possessing a strong negative chirp we observe significantly reduced fragmentation, indicating different internal conversion pathways and the associated intramolecular vibrational redistribution.

Dissociative photoionization of vinyl chloride (C2H3Cl) in the 11.0 14.2 eV photon energy range was investigated with the method of threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging. Three electronic states, A2A', B2A" and C2A', of C2H3Cl+ cation were prepared and their dissociation dynamics were investigated respectively. A unique fragment ion, C2H3+, was observed within the present excitation energy range. From the TPEPICO 3-dimensional time-sliced velocity map images of C2H3+, kinetic energy release distributions (KERD) and anisotropy parameters in dissociation of internal energy-selected C2H3Cl+ cation were obtained. At 13.14 eV, the total KERD showed a bimodal distribution consisting of a Boltzmann and a Gaussian-type component, indicating competing statistical and non-statistical dissociation mechanisms. An additional component of Gaussian-type was found in KERD at 13.65 eV with a center located at lower kinetic energy. With the aid of re-calculated Cl-loss potential energy curves with time-dependent density functional theory, the overall dissociative photoionization mechanisms of C2H3Cl+ cation in the B2A" and C2A' states are proposed. The inconsistence of the previous conclusions on dissociation mechanism of C2H3Cl+ is stated.

The Cyanomethyl Anion, CH₂CN-, and neutral radical have been studied extensively, with several findings of autodetachment about the totally symmetric transition, as well as high resolution experiments revealing symmetrically forbidden and weak vibrational features. We report photoelectron spectra using the Velocity-Mapped Imaging Technique in 1-2 cm⁻¹increments over a range of 13460 to 15384 cm⁻¹that has not been previously examined. These spectra include excitation of the ground state cyanomethyl anion into the direct detachment thresholds of previously reported vibrational modes for the neutral radical. Significant variations from Franck-Condon behavior were observed in the branching ratios for resolved vibrational features for excitation in the vicinity of the thresholds involving the ν₃ and ν₅ modes. These are consistent with autodetachment from rovibrational levels of a dipole bound state acting as a resonance in the detachment continuum. The autodetachment channels involve single changes in vibrational quantum number, consistent with the vibrational propensity rule but in some cases reveal relaxation to a different vibrational mode indicating coupling between the modes and/or a breakdown of the normal mode approximation.

Recent experimental interest of the collaborating group of M. Koch on the dynamics of electronic excitations of indium in helium droplets triggered a series of computational studies on the group 13 elements Al, Ga and In and their indecisive behavior between wetting and non wetting when placed onto superfluid helium droplets. We employ a combination of multiconfigurational self consistent field calculations (MCSCF) and multireference configuration interaction (MRCI) to calculate the diatomic potentials. Particularly interesting is the case of indium with an Ancilotto parameterF. Ancilotto, P. B. Lerner and M. W. Cole, Journal of Low Temperature Physics, 1995, 101, 1123-1146 J. H. Reho, U. Merker, M. R. Radcliff, K. K. Lehmann and G. Scoles, The Journal of Physical Chemistry A, 2000, 104, 3620-3626t

n→π* interaction is a newly discovered non-covalent interaction which involves delocalization of lone pair (n) electrons of an electronegative atom into π* orbital of a carbonyl group or an aromatic ring. It is widely observed in materials, biomolecules (protein, DNA, RNA), amino acids, neurotransmitter and drugs. However, due to its weak strength and counterintuitive nature its existence is debatable. Such weak interactions are often masked by solvent effects in condense phase or physiological conditions thereby, making it difficult to prove the presence of such weak interactions. Therefore, we have used isolated gas phase spectroscopy in combination with quantum chemical calculations to study n→π* interaction in several molecules where, our molecular systems are free from solvent effects or any external forces. Herein I will be discussing two of the molecular systems (phenyl formate and salicin) where, we have observed the significance of n→π* interaction in determining the conformational specificity of the molecules. We have proved the existence of n→π* interaction for the first time through IR spectroscopy by probing the carbonyl stretching frequency of phenyl formate. Our study is further pursued on a drug named salicin where, we have observed that its conformational preferences is ruled by n→π* interaction even though a strong hydrogen bonding interaction is present in the molecule. Our results show that n→π* interaction, in spite of its weak strength, should not be overlooked as it existence can play an important role in governing the structures of molecules like other strong non-covalent interactions do.