An official website of the United States government
Organic Polymer-based photovoltaic systems offer a viable alternative to more standard solid-state devices for light-harvesting applications. In this study, we investigate the electronic dynamics of a model organic photovoltaic (OPV) heterojunction consisting of polyphenylene vinylene (PPV) oligomers and a [ 6,6 ] -phenyl C61-butyric acid methyl ester (PCBM) blend. Our approach treats the classical molecular dynamics of the atoms within an Ehrenfest mean-field treatment of the π - π ⁎ singly excited states spanning a subset of donor and acceptor molecules near the phase boundary of the blend. Our results indicate that interfacial electronic states are modulated by C=C bond stretching motions and that such motions induce avoided crossings between nearby excited states thereby facilitating transitions from localized excitonic configurations to delocalized charge-separated configurations on an ultrafast time-scale. The lowest few excited states of the model interface rapidly mix allowing low frequency C-C out-of-plane torsions to modulate the potential energy surface such that the system can sample both intermolecular charge-transfer and charge-separated electronic configurations on sub-100 fs time scales. Our simulations support an emerging picture of carrier generation in OPV systems in which interfacial electronic states can rapidly decay into charge-separated and current producing states via coupling to vibronic degrees of freedom.
more »
« less
Characterizing the origin band spectrum of isoquinoline with resonance enhanced multiphoton ionization and electronic structure calculations
We report the experimental resonance enhanced multiphoton ionization spectrum of isoquinoline between 315 and 310 nm, along with correlated electronic structure calculations on the ground and excited states of this species. This spectral region spans the origin transitions to a π–π* excited state, which previous work has suggested to be vibronically coupled with a lower lying singlet n–π* state. Our computational results corroborate previous density functional theory calculations that predict the vertical excitation energy for the n–π* state to be higher than the π–π* state; however, we find an increase in the C–N–C angle brings the n–π* state below the energy of the π–π* state. The calculations find two out-of-plane vibrational modes of the n–π* state, which may be brought into near resonance with the π–π* state as the C–N–C bond angle increases. Therefore, the C–N–C bond angle may be important in activating vibronic coupling between the states. We fit the experimental rotational contour with a genetic algorithm to determine the excited state rotational constants and orientation of the transition dipole moment. The fits show a mostly in-plane polarized transition, and the projection of the transition dipole moment in the a-b plane is about 84° away from the a axis. These results are consistent with the prediction of our electronic structure calculations for the transition dipole moment of the π–π* excited state.
more »
« less
- Award ID(s):
- 2150871
- PAR ID:
- 10503680
- Publisher / Repository:
- The Journal of Chemical Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 159
- Issue:
- 13
- ISSN:
- 0021-9606
- Subject(s) / Keyword(s):
- isoquinoline excited states resonance enhanced multiphoton ionization REMPI electronic spectroscopy
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The D 5 Π–X 5 Δ (0,0) band of vanadium hydride at 654 nm has been recorded by laser excitation spectroscopy and represents the first analyzed spectrum of VH in the gas phase. The molecules were generated using a hollow cathode discharge source, with laser-induced fluorescence detected via the D 5 Π–A 5 Π (0,0) transition. All five main (ΔΩ = ΔΛ) subbands were observed as well as several satellite ones, which together create a rather complex and overlapped spectrum covering the region 15 180–15 500 cm −1 . The D 5 Π state displays the effects of three strong local perturbations, which are likely caused by interactions with high vibrational levels of the B 5 Σ − and c 3 Σ − states, identified in a previous multiconfigurational self-consistent field study by Koseki et al. [J. Phys. Chem. A 108, 4707 (2004)]. Molecular constants describing the X 5 Δ, A 5 Π, and D 5 Π states were determined in three separate least-squares fits using effective Hamiltonians written in a Hund’s case (a) basis. The fine structure of the ground state is found to be consistent with its assignment as a σπ 2 δ, 5 Δ electronic state. The fitted values of its first-order spin–orbit and rotational constants in the ground state are [Formula: see text] and B = 5.7579(13) cm −1 , the latter of which yields a bond length of [Formula: see text] Å. This experimental value is in good agreement with previous computational studies of the molecule and fits well within the overall trend of decreasing bond length across the series of 3d transition metal monohydrides.more » « less
-
null (Ed.)Abstract Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. Here, we demonstrate a broadband optical approach to pump trapped SiO + molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state N = 67, corresponding to the peak of a 9400 K distribution, had a narrow N spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C 2 Π state. Significant centrifugal distortion of super rotors pumped up to N = 230 allowed probing electronic structure of SiO + stretched far from its equilibrium bond length.more » « less
-
High-resolution direct absorption infrared spectra of metastable cis-formic acid (HCOOH) trapped in a cis-well resonance behind a 15 kcal/mol barrier are reported for the first time, with the energetically unstable conformer produced in a supersonic slit plasma expansion of trans-formic acid/H 2 mixtures. We present a detailed high-resolution rovibrational analysis for cis-formic acid species in the OH stretch ( ν 1 ) fundamental, providing first precision vibrational band origin, rotational constants, and term values, which in conjunction with ab initio calculations at the couple-cluster with single, double, and perturbative triple [CCSD(T)]/ANOn (n = 0, 1, 2) level support the experimental assignments and establish critical points on the potential energy surface for internal rotor trans-to-cis isomerization. Relative intensities for a- and b-type transitions observed in the spectra permit the transition dipole moment components to be determined in the body fixed frame and prove to be in good agreement with ab initio CCSD(T) theoretical estimates but in poor agreement with simple bond-dipole predictions. The observed signal dependence on H 2 in the discharge suggests the presence of a novel H atom radical chemical mechanism for strongly endothermic “up-hill” internal rotor isomerization between trans- and cis-formic acid conformers.more » « less
-
In this work, we report the results of a combined ab initio and experimental investigation of highly excited Σg+1 and Πg1 states of the cesium dimer in a previously unobserved energy region of the molecule. The structure of these high-lying electronic states was predicted via calculations in the framework of the pseudopotential method and observed via the optical–optical double resonance technique. Understanding the rovibronic structure of the cesium dimer at this high energy regime has significance for the formation of ultracold Cs2 ground state molecules and their detection using resonantly enhanced multiphoton ionization (REMPI) techniques. Using the ab initio results and the selection rules for dipole allowed transitions, the experimentally observed rovibrational levels were identified as belonging to the 11Σg+1 and 61Πg electronic states. The Dunham–RKR method was utilized to generate experimental potential energy curves for the two electronic states.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0168421
