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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Threshold Ionization Spectroscopy and Theoretical Calculations of LnO (Ln = La and Ce)

Mass-analyzed threshold ionization (MATI) spectroscopy was used to measure the vibronic spectra of LnO (Ln = La and Ce). Single-reference coupled cluster and relativistic multireference configuration calculations were carried out to compare with the measured vibronic energies. The spectrum of LaO displays a single vibronic band system, while that of CeO shows multiple ones. The ionization energies of LaO and CeO are measured as 5.2446 (6) and 5.3332(6) eV, respectively, which are a hundredfold improvement over the literature values. The vibrational energies of the neutral molecule and corresponding ion reveal the charge effect on the metal-oxygen bond of both species. The single band system in the spectrum of LaO arises from the transition of the ground state of the neutral molecule with the La(6s 1 )O(2p 6 ) valence configuration to the ground state of the singly charged ion with the La(6s o )O(2p 6 ) configuration. Here, the multiple band systems in the spectrum of CeO are attributed to the spin-orbit coupling for the Ce(4f 1 6s 1 )O(2p 6 ) configuration of the neutral molecule and an excited state for the Ce(4f 1 )O(2p 6 ) configuration of the ion.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Conical intersections shed light on hot carrier cooling in quantum dots

Experimental observations of vibronic coherences in electronically excited colloidal semiconductor nanocrystals offer a window into the ultrafast dynamics of hot carrier cooling. In previous work, we showed that, in amine-passivated quantum dots (QDs), these coherences arise during relaxation through a cascade of conical intersections between electronically excited states. Here, in this work, we demonstrate the generality of this framework by application to QDs with surface-bound carboxylate ligands. A model involving a similar cascade of conical intersections accurately reproduces the frequencies of vibronic coherences observed with broadband multidimensional spectroscopy. The impact of ligands on the relaxation dynamics is attributed to two distinct mechanisms involving either electronic or vibrational coupling between the core and ligands. Compared to the amine-passivated QDs studied previously, the electronic coupling mechanism is less prominent in carboxylate-passivated QDs. Furthermore, comparison of acetate and formate ligands reveals that truncating the ligand alkyl chains alters the relaxation behavior predicted by the model.

14 SOLAR ENERGY↗
NASA NTRS1994

Porphyrins in the interstellar medium (in grains)

Spectral sensitivity of the chromophores to their immediate chemical environment establishes some of the chemical constituents of the grains in which they reside. These are: (1) Paraffins, such as, octane, nonane, decane, and others...(needed for Shpolskii matrices and producing quasilines); and (2) Pyridine. The presence of pyridine is required not only to produce the spectral DIB matching, but also to produce the 36 cm(sup -1) crystal field splitting of the S(sub 1) electronic state. The presence of pyridine in the grains can be confirmed spectroscopically. Pyridine produces a transmission window at 2175 A, matching exactly the well known UV hump. On grain reflection, some of the incoming UV radiation is absorbed into the grain's outer layers. Spikes in the lab and in the astronomical data are due to vibronic transitions in pyridine. The lab spectroscopy reported here clearly establishes the presence of MgTBP, H2TPB, and pyridine in the interstellar grains. The high fluorescence efficiency of MgTBP (being optically pumped in the visible) apparently accounts for all the observed UIR emissions.

Johnson, Fred M.↗
DOE OSTI2021

Analytic and numerical vibronic spectra from quasi-classical trajectory ensembles

The truncated Wigner approximation to quantum dynamics in phase space is explored in the context of computing vibronic line shapes for monomer linear optical spectra. We consider multiple model potential forms including a shifted harmonic oscillator with both equal and unequal frequencies on the ground and excited state potentials as well as a shifted Morse potential model. For the equal-frequency shifted harmonic oscillator model, we derive an analytic expression for the exact vibronic line shape that emphasizes the importance of using a quantum mechanical distribution of phase space initial conditions. For the unequal-frequency shifted harmonic oscillator model, we are no longer able to obtain an exact expression for the vibronic line shape in terms of independent deterministic classical trajectories. We show how one can rigorously account for corrections to the truncated Wigner approximation through nonlinear responses of the line shape function to momentum fluctuations along a classical trajectory and demonstrate the qualitative improvement in the resulting spectrum when the leading-order quantum correction is included. Finally, we numerically simulate absorption spectra of a highly anharmonic shifted Morse potential model. We find that, while finite quantization and the dissociation limit are captured with reasonable accuracy, there is a qualitative breakdown of the quasi-classical trajectory ensemble’s ability to describe the vibronic line shape when the relative shift in Morse potentials becomes large. The work presented here provides clarity on the origin of unphysical negative features known to contaminate absorption spectra computed with quasi-classical trajectory ensembles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. Furthermore, these findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Chirped Laser Pulse Control of Vibronic Wavepackets and Energy Transfer in Phycocyanin 645

Photosynthetic organisms use light-harvesting complexes to increase the spectrum of light that they absorb from solar photons. Recent ultrafast spectroscopic studies have revealed that efficient (sub-ps) energy transfer is mediated by vibronic coherence in the phycobiliprotein phycocyanin 645 (PC645). Here, we report studies that employ broadband pump–probe spectroscopy with linearly chirped excitation pulses to further investigate the relationship between vibronic state preparation and energy transfer dynamics in PC645. Negatively chirped pulse excitation is found to enhance wavepackets of a high-frequency mode (1580 cm –1 ) and increase the rate of downhill energy transfer, while on the other hand, positively chirped pulses suppress these oscillatory features and decrease this rate. Model calculations incorporating the influence of the chirped pump pulse are used to understand its effect on initial state preparation. Furthermore, these results provide mechanistic insight into how the overall nonequilibrium rate of energy transfer is influenced by initial state preparation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Conical Intersection Passages of Molecules Probed by X-ray Diffraction and Stimulated Raman Spectroscopy

Conical intersections (CoIns) play an important role in ultrafast relaxation channels. Their monitoring remains a formidable experimental challenge. We theoretically compare the probing of the S 2 → S 1 CoIn passage in 4-thiouracil by monitoring its vibronic coherences, using off-resonant X-ray-stimulated Raman spectroscopy (TRUECARS) and time resolved X-ray diffraction (TRXD). The quantum nuclear wavepacket (WP) dynamics provides an accurate picture of the photoinduced dynamics. Upon photoexcitation, the WP oscillates among the Franck–Condon point, the S2 minimum, and the CoIn with a 70 fs period. A vibronic coherence first emerges at 20 fs and can be observed until the S 2 state is fully depopulated. The distribution of the vibronic frequencies involved in the coherence is recorded by the TRUECARS spectrogram. Here, the TRXD signal provides spatial images of electron densities associated with the CoIn. In combination, the two signals provide a complementary picture of the nonadiabatic passage, which helps in the study of the underlying photophysics in thiobases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The role of mixed vibronic Q y -Q x states in green light absorption of light-harvesting complex II

The importance of green light for driving natural photosynthesis has long been underappreciated, however, under the presence of strong illumination, green light actually drives photosynthesis more efficiently than red light. This green light is absorbed by mixed vibronic Q y -Q x states, arising from chlorophyll (Chl)-Chl interactions, although almost nothing is known about these states. Here, we employ polarization-dependent two-dimensional electronic-vibrational spectroscopy to study the origin and dynamics of the mixed vibronic Q y -Q x states of light-harvesting complex II. We show the states in this region dominantly arise from Chl b and demonstrate how it is possible to distinguish between the degree of vibronic Q y versus Q x character. We find that the dynamics for states of predominately Chl b Q y versus Chl b Q x character are markedly different, as excitation persists for significantly longer in the Q x states and there is an oscillatory component to the Q x dynamics, which will be discussed. Our findings demonstrate the central role of electronic-nuclear mixing in efficient light-harvesting and the different functionalities of Chl a and Chl b .

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

In this paper, we report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB - ) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB - reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB - (A u symmetry) to the QBS (Ag symmetry) is triply-forbidden by the electric and magnetic dipoles, and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for near-threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB - . The current study shows a rare example where symmetry selection rules, rather than the Franck-Condon principle, govern vibronic transitions to a non-valence state in an anion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Vibronic Relaxation Pathways in Molecular Spin Qubit Na 9 [Ho(W 5 O 18 ) 2 ]·35H 2 O under Pressure

In order to explore how spectral sparsity and vibronic decoherence pathways can be controlled in a model qubit system with atomic clock transitions, we combined diamond anvil cell techniques with synchrotron-based far infrared spectroscopy and first-principles calculations to reveal the vibrational response of Na 9 [Ho(W 5 O 18 ) 2 ]·35H 2 O under compression. Because the hole in the phonon density of states acts to reduce the overlap between the phonons and f manifold excitations in this system, we postulated that pressure might move the HoO 4 rocking, bending, and asymmetric stretching modes that couple with the M J = ±5, ±2, and ±7 levels out of resonance, reducing their interactions and minimizing decoherence processes, while a potentially beneficial strategy for some molecular qubits, pressure slightly hardens the phonons in Na 9 [Ho(W 5 O 18 ) 2 ]·35H 2 O and systematically fills in the transparency window in the phonon response. The net result is that the vibrational spectrum becomes less sparse and the overlap with the various M J levels of the Ho 3+ ion actually increases. These findings suggest that negative pressure, achieved using chemical means or elongational strain, could further open the transparency window in this rare earth-containing spin qubit system, thus paving the way for the use of device surfaces and interface elongational/compressive strains to better manage decoherence pathways.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Observation of bound valence excited electronic states of deprotonated 2-hydroxytriphenylene using photoelectron, photodetachment, and resonant two-photon detachment spectroscopy of cryogenically cooled anions

Polycyclic aromatic hydrocarbons (PAHs) are common atmospheric pollutants, and they are also ubiquitous in the interstellar medium. Here, we report the study of a complex O-containing PAH anion, the deprotonated 2-hydroxytriphenylene (2-OtPh–), using high-resolution photoelectron imaging and photodetachment spectroscopy of cryogenically cooled anions. Vibrationally resolved photoelectron spectra yield the electron affinity of the 2-OtPh radical as 2.629(1) eV and several vibrational frequencies for its ground electronic state. Photodetachment spectroscopy reveals bound valence excited electronic states for the 2-OtPh– anion, with unprecedentedly rich vibronic features. Evidence is presented for a low-lying triplet state (T1) and two singlet states (S1 and S2) below the detachment threshold. Single-color resonant two-photon photoelectron spectroscopy uncovers rich photophysics for the 2-OtPh– anion, including vibrational relaxation in S1, internal conversion to the ground state of 2-OtPh–, intersystem crossing from S2 to T1, and a long-lived autodetaching shape resonance about 1.3 eV above the detachment threshold. The rich electronic structure and photophysics afforded by the current study suggest that 2-OtPh– would be an interesting system for pump–probe experiments to unravel the dynamics of the excited states of this complex PAH anion.

Chemistry↗
DOE OSTI2023

Electronic spectroscopy and ionization potentials for YbOH and YbOCH 3

The polyatomic molecules YbOH and YbOCH 3 have been recognized as being of potential value for spectroscopic experiments that explore charge-parity and time reversal symmetry violation effects. These measurements require very high precision, which, in turn, will necessitate that the molecules be manipulated at ultra-cold temperatures. Both YbOH and YbOCH 3 have electronic transitions that appear suitable for laser cooling ($\tilde{A}$ 2 Π 1/2 -$\tilde{X}$ 2 Σ + and $\tilde{A}$2E 1/2 -$\tilde{X}$ 2 A 1 , respectively) but the currently available spectroscopic data is not sufficient to determine the extent to which population leaks may compromise the optical cooling processes. A further complication is that the quantum states of interest for these measurements will need to be selectively populated. The $\tilde{A}$-$\tilde{X}$ band systems of both YbOH and YbOCH 3 show evidence of vibronic perturbations, such that there are unassigned vibronic features at energies that are just above the origin bands. Here, in the present study we have recorded spectra for the $\tilde{A}$ 2 Π 1/2 -$\tilde{X}$ 2 Σ + transition of jet-cooled YbOD to facilitate the vibronic assignments. In addition, spectra for the $\tilde{B}$ 2 Σ + - $\tilde{X}$ 2 Σ + transition of YbOH were recorded, establishing the origin band at 20473.8 cm -1 . Previously, the reaction of Yb with CH 3 OH has been used to generate gas-phase YbOCH 3 . As this reaction also yields YbOH, there have been complications in spectroscopic studies of YbOCH 3 due to overlap of the $\tilde{A} -\tilde{X}$ band systems. To identify specific regions of overlap, resonantly enhanced two-photon ionization spectra were recorded using mass-resolved detection of the YbOH + and YbOCH 3 + ions. These data confirmed the overlap of vibronic bands near 17640 and 17680 cm -1 . Two-photon ionization spectroscopy also provided accurate ionization energies of IE(YbOH)=45788(10) and IE(YbOCH 3 )=45283(10) cm -1 . The IE for YbOH is relevant to problems encountered in previous attempts to determine the bond dissociation energy of YbOH + .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Mesoscale interplay between phonons and crystal electric field excitations in quantum spin liquid candidate CsYbSe 2

In this report CsYbSe 2 , a recently identified quantum spin liquid (QSL) candidate, exhibits strong crystal electric field (CEF) excitations. Here, we identify phonon and CEF modes with Raman spectroscopy and observe strong CEF-phonon coupling resulting in a vibronic bound state. Complex, mesoscale interplay between phonon modes and CEF modes is observed in real space. Additionally, an unexpected resonant Raman excitation condition is satisfied, yielding up to third-order combination modes, with a total of 17 modes identified in the spectra. This study paves the way to coherent control of possible QSL ground states with optically accessible CEF-phonon manifolds and mesoscale engineering of CEF-phonon interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

X-ray dichroism as a probe of orbital quenching and unquenching induced by optically excited Jahn-Teller modes

The use of soft-X-ray spectroscopy to probe changes in the ground-state due to vibronic excitations is studied. The changes in L-edge X-ray absorption on divalent cobalt in the presence of a vibrating Jahn-Teller distortion are calculated. For linear dichroism, a strong dependence on the distortion is obtained reflecting the change in the local quadrupole moment due to the changes in the xy versus yz/zx hole densities during the oscillation. Here, in the presence of a magnetic moment, significant variations in the circular dichroic signal are obtained related to the quenching and unquenching of the orbital angular momentum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1993

Novel infrared vibron absorption of solid hydrogen at megabar pressures

We report new phenomena associated with the infrared-active vibrons in hydrogen at megabar pressures. We find a striking 3 order of magnitude increase in vibron absorbance at the 150 GPa phase transition at 85 K. A discontinuity in the frequency of the infrared vibron is observed which is identical to that measured by Raman spectroscopy at the same temperature. The results indicate there is a significant change in electronic properties at the transition. In addition, the infrared measurements provide evidence for a phase transition at 110 GPa at low temperature.

Hanfland, Michael↗
NASA NTRS2023

Cavity Ring-Down Spectroscopy of Anthracene, 9-Methylanthracene, and 2 Methylanthracene in Supersonic Expansion

The S0–S1 absorption spectra of anthracene (C14H10), 9-methylanthracene (C15H12), and 2-methylanthracene (C15H12) are measured in the ultraviolet region between 330 and 375 nm (26,666 to 30,303 cm–1) with cavity ring-down spectroscopy in supersonic free-jet expansions of argon. The associated vibronic band systems and their spectroscopic assignments are discussed and compared to previous studies performed using fluorescence excitation and dispersed fluorescence techniques. Density functional theory (DFT) calculations were carried out to study the structures and evaluate the vibrational transitions of the ground and excited states. Time-dependent DFT calculations of the first electronic excited states and Franck–Condon factor calculations were carried out to assist in the assignment of the experimentally measured vibronic bands. The vibronic spectra obtained in absorption agree well with fluorescence excitation spectra in terms of peak positions but exhibit different relative band intensities. We find a very good match between experimentally obtained vibronic line positions and the peak positions of the quantum chemically calculated Franck–Condon excitation lines.

Salma Bejaoui↗
DOE OSTI2022

Probing the electronic structure and spectroscopy of pyrrolyl and imidazolyl radicals using high-resolution photoelectron imaging of cryogenically cooled anions

High-resolution photoelectron imaging and photodetachment spectroscopy of cryogenically cooled pyrrolide and imidazolide anions are used to probe the electronic structure and spectroscopy of pyrrolyl and imidazolyl radicals. The high-resolution data allow the ground state vibronic structures of the two radicals to be completely resolved, yielding accurate electron affinities of 2.1433 ± 0.0008 eV and 2.6046 ± 0.0006 eV for pyrrolyl and imidazolyl radicals, respectively. Fundamental frequencies for eight vibrational modes of pyrrolyl and ten vibrational modes of imidazolyl are measured, including several nonsymmetric Franck–Condon-forbidden modes. Two electronic excited states are also observed for the two radicals, displaying diffuse spectral features in both systems. The observations of nonsymmetric vibrational modes in the ground states and the diffuse excited state features provide strong evidence for vibronic couplings between the ground state and the two close-by excited states. The 2-pyrrolide isomer is also observed as a minor species from the electrospray ionization source and the electron affinity of 2-pyrrolyl is measured to be 1.6690 ± 0.0030 eV along with five vibrational frequencies. Even though the HOMOs of both pyrrolide and imidazolide anions are p orbitals, photodetachment spectroscopy reveals completely different threshold behaviors for the two anions: a d-wave-dominated spectrum for pyrrolide and an s-wave-dominated spectrum for imidazolide. Furthermore, the current study provides a wealth of electronic and spectroscopic information, which is ideal to compare with more accurate vibronic coupling calculations for these two important radicals, as well as interesting information about the photodetachment dynamics of the two anions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗