Valence and lowest Rydberg electronic states of phenol investigated by synchrotron radiation and theoretical methods

Cyclohexane Vibronic States: A Combined VUV Spectroscopy and Theoretical Study

In this work, we provide results from a joint experimental and theoretical study of the vibronic features of cyclohexane (C6H12) in the photon energy range of 6.8-10.8 eV (182-115 nm). The high-resolution vacuum ultraviolet (VUV) photoabsorption measurements, together with quantum chemical calculations at the time-dependent density functional theory (TDDFT) level, have helped to assign the major electronic excitations to mixed valence-Rydberg and Rydberg transitions. The C6H12 photoabsorption spectrum shows fine structure which has been assigned to CH2 scissoring, v3'a1g, CH2 rocking, v4'a1g, C-C stretching, v5'a1g, and CCC bending/CC torsion, v24'eg, modes. Molecular structure calculations at the DFT level for the neutral and cationic electronic ground-states have shown the relevant structural changes that are operative in the higher-lying electronic states. Photolysis lifetimes in the Earth's atmosphere are shown to be irrelevant, while the main atmospheric sink mechanism is the reaction with the •OH radical. Potential energy curves have been obtained at the TDDFT level of theory, showing the relevance of interchange character mainly involving the CH2 scissoring, v3'a1g, and CH2 rocking, v4'a1g, modes, while Jahn-Teller distortion yields weak vibronic coupling involving the non-totally symmetric CCC bending/CC torsion, v24'eg, mode.

Dissociation of adsorbates via electronic energy transfer from aromatic thin films

Photofragment translational spectroscopy has been used to characterize the energetics and the cross sections for photodissociation of CH3I and CF3I adsorbed on thin films of a variety of aromatic molecules, initiated by near-UV light. Thin films (nominally 10 monolayers) of benzene, five substituted benzenes and two naphthalenes have been employed to study systematic changes in the photochemical activity. Illumination of these systems with 248 nm light is found to result in a dissociation process for the CH3I and CF3I mediated by initial absorption in the aromatic thin film, followed by electronic energy transfer (EET) to the dissociating species. The effective cross sections for dissociation are found to be substantially increased via this mechanism (from 1.8×-20×), amounts differing depending on the aromatic molecule thin film used, and is connected to the aromatic photabsorption profile and the particular excited states being accessed. Distinctive translational energy distributions for the CH3 and CF3 photofragments are found to vary systematically for the different aromatic molecule thin film used, and are related to the energy of the lowest electronic excited singlet state of the aromatic molecule. The CH3 and CF3 photofragment kinetic energy distributions found for the aromatic thin films suggest that the dissociation occurs via EET to the 3Q1 excited state of CH3I and CF3I.

Near-Ultraviolet Circular Dichroism and Two-Dimensional Spectroscopy of Polypeptides

A fully quantitative theory of the relationship between protein conformation and optical spectroscopy would facilitate deeper insights into biophysical and simulation studies of protein dynamics and folding. In contrast to intense bands in the far-ultraviolet, near-UV bands are much weaker and have been challenging to compute theoretically. We report some advances in the accuracy of calculations in the near-UV, which were realised through the consideration of the vibrational structure of the electronic transitions of aromatic side chains.

Nuclear Ensemble Approach with Importance Sampling

We show that the importance sampling technique can effectively augment the range of problems where the nuclear ensemble approach can be applied. A sampling probability distribution function initially determines the collection of initial conditions for which calculations are performed, as usual. Then, results for a distinct target distribution are computed by introducing compensating importance sampling weights for each sampled point. This mapping between the two probability distributions can be performed whenever they are both explicitly constructed. Perhaps most notably, this procedure allows for the computation of temperature dependent observables. As a test case, we investigated the UV absorption spectra of phenol, which has been shown to have a marked temperature dependence. Application of the proposed technique to a range that covers 500 K provides results that converge to those obtained with conventional sampling. We further show that an overall improved rate of convergence is obtained when sampling is performed at intermediate temperatures. The comparison between calculated and the available measured cross sections is very satisfactory, as the main features of the spectra are correctly reproduced. As a second test case, one of Tully's classical models was revisited, and we show that the computation of dynamical observables also profits from the importance sampling technique. In summary, the strategy developed here can be employed to assess the role of temperature for any property calculated within the nuclear ensemble method, with the same computational cost as doing so for a single temperature.