arXiv subjects

Namana Venkatareddy

Publications and source records attributed to Namana Venkatareddy.

3 recordsLinked to original sources

arXiv2026

Persistent singlet electronic character in the multiexcitonic triplet-pair state of strongly coupled pentacene singlet fission dimers

Singlet fission converts an optically excited singlet state into a spin-entangled triplet pair state (TT$_1$)$^1$ that can, in principle, yield two free triplets for photovoltaics and/or a polarized high spin state for quantum technologies. Synthetically tunable templates suggest that the above photophysics is governed by a subtle but poorly understood interplay of molecular motifs, geometry and structural fluctuations. Here, we investigate the (TT$_1$)$^1$ state in a library of conformationally flexible pentacenic dimers, where a (TT$_1$)$^1$-specific near-IR spectral feature is readily available. Using a suite of polarization-controlled impulsive optical spectroscopies, we find that (TT$_1$)$^1$ formation is specific to planar conformations and is accompanied by large nuclear reorganization in the (TT$_1$)$^1$ photoproduct. Introducing polarization anisotropy to track the electronic character of the (TT$_1$)$^1$ species, supported by screened configuration interaction based electronic structure theory, we find that significant singlet-triplet electronic mixing is persistent throughout its evolution. This behavior is universal across diverse bridging motifs and indicates that, once the triplet pair is strongly bound, neither substantial nuclear reorganization nor structural fluctuations on longer timescales are sufficient to suppress persistent singlet-triplet electronic mixing, such that triplet-pair decorrelation is outcompeted by its decay. Our observations establish polarization-selective pump-probe and anisotropy as a direct optical probe of triplet pair decorrelation, complementary to spin-selective measurements at longer timescales.

physics.chem-ph↗

arXiv2025

Symmetries in zero and finite center-of-mass momenta excitons

We present a symmetry-based framework for the analysis of excitonic states, incorporating both time-reversal and space-group symmetries. We demonstrate the use of time-reversal and space-group symmetries to obtain exciton eigenstates at symmetry-related center-of-mass momenta in the entire Brillouin zone from eigenstates calculated for center-of-mass momenta in the irreducible Brillouin zone. Furthermore, by explicitly calculating the irreducible representations of the little groups, we classify excitons according to their symmetry properties across the Brillouin zone. Using projection operators, we construct symmetry-adapted linear combinations of electron-hole product states, which block diagonalize the Bethe-Salpeter equation (BSE) Hamiltonian at both zero and finite exciton center-of-mass momenta. This enables a transparent organization of excitonic states and provides direct access to their degeneracies, selection rules, and symmetry-protected features. As a demonstration, we apply this formalism to monolayer MoS$_2$, where the classification of excitonic irreducible representations and the block structure of the BSE Hamiltonian show excellent agreement with compatibility relations derived from group theory. Beyond this material-specific example, the framework offers a general and conceptually rigorous approach to the symmetry classification of excitons, enabling significant reductions in computational cost for optical spectra, exciton-phonon interactions, and excitonic band structure calculations across a wide range of materials.

cond-mat.mtrl-sci↗

arXiv2025

Double excitations in molecules

Double excitations in organic molecules have garnered significant interest as a result of their importance in singlet fission and photophysics. These excitations play a crucial role in understanding the photoexcitation processes in polyenes. To describe photoexcited states with both single and double excitation character, we use a first-principles many-body theory that combines the GW / Bethe-Salpeter equation and the configuration interaction (CI) methods. Specifically, we develop and employ two CI-based methods: screened configuration interaction singles and doubles (scrCISD) and screened configuration interaction singles with perturbative doubles (scrCIS(D)), applied to an effective many-body Hamiltonian that incorporates screening. We apply these methods to Thiel's set of molecules, which exhibit excited states predominantly characterized by single excitations with a partial double excitation character. Our results indicate that the scrCISD method systematically underestimates the excitation energies compared to the best theoretical estimates, while the scrCIS(D) method shows good agreement with these estimates. Furthermore, we used the scrCISD method to calculate the binding energies of the dominantly doubly excited correlated triplet pair states, $\mathrm{TT^1}$, in pentacene dimers, finding that the $\mathrm{TT^1}$ binding energies agree well with empirical calculations.

physics.chem-ph↗