Papers by Krupa Ramasesha
Excitation of iron pentacarbonyl (IP), a prototypical photocatalyst, at 266 nm causes sequential ... more Excitation of iron pentacarbonyl (IP), a prototypical photocatalyst, at 266 nm causes sequential loss of two CO ligands in the gas phase, creating catalytically active, unsaturated iron carbonyls. Thus far, the electronic states involved in the dissociation have eluded experimental observation, hindering a comprehensive understanding of IP photochemistry. Using femtosecond extreme ultraviolet transient absorption spectroscopy near the Fe M2,3-edge, we present the first spectroscopic characterization of valence electronic dynamics during IP photodissociation. Informed by electronic structure calculations, we uncover the spectroscopic signatures of the intertwined structural and electronic evolution among the manifold of metal-centered excited states during first CO loss from IP on a 100-fs timescale. Furthermore, spectroscopic signals associated with the formation of Fe(CO)4 on its lowest singlet excited state and in structures fluctuating between C2v and C3v geometries, and its subs...
Ultrafast Dynamics of Two- and Three-Body Dissociation Captured by Core-To-Valence Transient Absorption Spectroscopy
Proceedings of the 2022 International Symposium on Molecular Spectroscopy
Excited-State Dynamics during Primary CI Homolysis in Acetyl Iodide Revealed by Ultrafast Core-Level Spectroscopy
Table-Top High Photon Energy Sources for Chemical Dynamics Investigations
PHYS 280-Ultrafast vibrational dynamics of asymmetric proton interfaces in solution
Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, can serve as a... more Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, can serve as a model system for understanding photofragmentation in polypeptides and proteins. Prior studies using ~266 nm and ~248 nm excitation of DMDS have shed light on dissociation pathways involving the lowest excited electronic states (S1), but far less is understood about photodissociation at higher excitation energies. In this work, we characterize the excited states of DMDS with equation of motion coupled cluster theory (EOM-CCSD) and compare computed and experimental UV spectra. Through Natural Transition Orbital analysis of the excited states, we find significant Rydberg character in numerous excited states that are accessed with ~200 nm excitation. One-dimensional potential energy scans along the C-S and S-S bond coordinates reveal novel photodissociation routes resulting from ~200 nm excitation, involving excited state potential energy surfaces S1-S6. Our high-level ab-initio investigati...
Proton Tunable Analog Transistor for Low Power Computing
Dimethyl disulfide dissociation is both ultrafast and very slow
Proposed for presentation at the 2021 SSRL/LCLS Users Meeting held September 20-24, 2021 in Palo Alto, CA.
The Journal of Chemical Physics
We employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of car... more We employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of carbonyl ligands from gas-phase nickel tetracarbonyl following ultraviolet photoexcitation at 261 nm. Here, nickel tetracarbonyl undergoes prompt dissociation to produce nickel tricarbonyl in a singlet excited state; this electronically excited tricarbonyl loses another CO group over tens of picoseconds. Our results also suggest the presence of a parallel, concerted dissociation mechanism to produce nickel dicarbonyl in a triplet excited state, which likely dissociates to nickel monocarbonyl. Mechanisms for the formation of these photoproducts in multiple electronic excited states are theoretically predicted with one-dimensional cuts through the potential energy surfaces and computation of spin–orbit coupling constants using equation of motion coupled cluster methods (EOM-CC) and coupled cluster theory with single and double excitations (CCSD). Bond dissociation energies are calculated with...
Femtosecond Reflectance Spectroscopy for Energetic Material Diagnostics
Dramatic Conformer-Dependent Reactivity of the Acetaldehyde Oxide Criegee Intermediate with Dimethylamine Via a 1,2-Insertion Mechanism
The Journal of Physical Chemistry A, 2021
The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and subst... more The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and substituent dependencies. Through a combination of synchrotron-multiplexed photoionization mass spectrometry experiments (298 K and 4 Torr) and high-level theory [CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ with an added CCSDT(Q) correction], we explore the conformer dependence of the reaction of acetaldehyde oxide (CH3CHOO) with dimethylamine (DMA). The experimental data support the theoretically predicted 1,2-insertion mechanism and the formation of an amine-functionalized hydroperoxide reaction product. Tunable-vacuum ultraviolet photoionization probing of anti- or anti- + syn-CH3CHOO reveals a strong conformer dependence of the title reaction. The rate coefficient of DMA with anti-CH3CHOO is predicted to exceed that for the reaction with syn-CH3CHOO by a factor of ∼34,000, which is attributed to submerged barrier (syn) versus barrierless (anti) mechanisms for energetically downhill reactions.
Butanone (H3CC(=O)CH2CH3) is a potential biofuel derived from biochemical and catalytic processes... more Butanone (H3CC(=O)CH2CH3) is a potential biofuel derived from biochemical and catalytic processes utilizing a range of different platform chemicals including 2,3-butanediol, produced from microbial fermentation, and levulinic acid. The presence of the carbonyl group in butanone introduces resonance stabilization in R and QOOH radicals, a characteristic that impacts combustion-relevant reaction mechanisms. Resonance stabilization alters the competition between chain-branching, chain-propagation, and chain-termination in low-temperature autoignition chemistry, which is initiated by R + O2 reactions. Ignition delay time experiments of Burke et al. (Comb. Flame, 2016) confirm inhibited chain-branching in the lowand intermediate-temperature regimes for butanone relative to n-butane. The present work examines product formation and related mechanisms in butanone oxidation and concludes that chain-propagation pathways remain relevant at temperatures below 700 K.
A New Pathway for Intersystem Crossing: Unexpected Products in the O(3P) + Cyclopentene Reaction
The Journal of Physical Chemistry A, 2021
We investigated the reaction of O(3P) with cyclopentene at 4 Torr and 298 K using time-resolved m... more We investigated the reaction of O(3P) with cyclopentene at 4 Torr and 298 K using time-resolved multiplexed photoionization mass spectrometry, where O(3P) radicals were generated by 351 nm photolysis of NO2 and reacted with excess cyclopentene in He under pseudo-first-order conditions. The resulting products were sampled, ionized, and detected by tunable synchrotron vacuum ultraviolet radiation and an orthogonal acceleration time-of-flight mass spectrometer. This technique enabled measurement of both mass spectra and photoionization spectra as functions of time following the initiation of the reaction. We observe propylketene (41%), acrolein + ethene (37%), 1-butene + CO (19%), and cyclopentene oxide (3%), of which the propylketene pathway was previously unidentified experimentally and theoretically. The automatically explored reactive potential energy landscape at the CCSD(T)-F12a/cc-pVTZ//ωB97X-D/6-311++G(d,p) level and the related master equation calculations predict that cyclopentene oxide is formed on the singlet potential energy surface, whereas propylketene is first formed on the triplet surface. These calculations provide evidence that significant intersystem crossing can happen in this reaction not only around the geometry of the initial triplet adduct but also around that of triplet propylketene. The formation of 1-butene + CO is initiated on the triplet surface, with bond cleavage and hydrogen transfer occurring during intersystem crossing to the singlet surface. At present, we are unable to explain the mechanistic origins of the acrolein + ethene channel, and we thus refrain from assigning singlet or triplet reactivity to this channel. Overall, at least 60% of the products result from triplet reactivity. We propose that the reactivity of cyclic alkenes with O(3P) is influenced by their greater effective degree of unsaturation compared with acyclic alkenes. This work also suggests that searches for minimum-energy crossing points that connect triplet surfaces to singlet surfaces should extend beyond the initial adducts.
Understanding Soot Development and Thermal Stratification in Combustion Engines through Hyperspectral Non-linear Optical Diagnostics Development
The time scale associated with shock-induced detonation is a key property of energetic materials ... more The time scale associated with shock-induced detonation is a key property of energetic materials that remains poorly understood. Herein, we test aspects of one potential mechanism, the phonon up-pumping mechanism, where shock compression excites lattice phonon modes, transferring energy to intramolecular vibrations leading to chemical bond cleavage and reaction. Using ultrafast infrared pump−probe spectroscopy on pentaerythritol tetranitrate (PETN), we reveal sub-picosecond vibrational energy transfer (VET) from the photoexcited band at 1660 cm −1 into every other infrared-active mode in the probed frequency range 800−1800 cm −1. Energy transfer processes remain incomplete at 150 ps. Computational predictions from density functional theory are used in tandem to elucidate VET pathways in PETN.
The Journal of Chemical Physics
We have measured, analyzed, and simulated the ground state valence photoelectron spectrum, x-ray ... more We have measured, analyzed, and simulated the ground state valence photoelectron spectrum, x-ray absorption (XA) spectrum, x-ray photoelectron (XP) spectrum as well as normal and resonant Auger–Meitner electron (AE) spectrum of oxazole at the carbon, oxygen, and nitrogen K-edge in order to understand its electronic structure. Experimental data are compared to theoretical calculations performed at the coupled cluster, restricted active space perturbation theory to second-order and time-dependent density functional levels of theory. We demonstrate (1) that both N and O K-edge XA spectra are sensitive to the amount of dynamical electron correlation included in the theoretical description and (2) that for a complete description of XP spectra, additional orbital correlation and orbital relaxation effects need to be considered. The normal AE spectra are dominated by a singlet excitation channel and well described by theory. The resonant AE spectra, however, are more complicated. While the...
Liquid water consists of a highly dynamic network of hydrogen bonds, which evolves on timescales ... more Liquid water consists of a highly dynamic network of hydrogen bonds, which evolves on timescales ranging from tens of femtoseconds to a few picoseconds. The fast structural evolution of water's hydrogen bond network is at the heart of numerous fundamental aqueous processes, such as proton transport, solvation, the hydrophobic effect and protein folding. In this thesis, I present our efforts in understanding the dynamics governing hydrogen bond switching and vibrational energy dissipation in water, and the transport of excess protons in strong acid solutions. We use ultrafast nonlinear infrared spectroscopy to study hydrogen bond and proton transfer dynamics in water and acids since vibrational frequencies, intensities and line shapes are closely associated with chemical structure and dynamics. We employed and characterized a new source of ultrafast broadband infrared pulses that span the entire mid-infrared region from 4000 cm-1 down to hundreds of cm-I, with <70 fs pulse dur...
The coupling of inter-and intramolecular vibrations plays a critical role in initiating chemistry... more The coupling of inter-and intramolecular vibrations plays a critical role in initiating chemistry during the shock-to-detonation transition in energetic materials. Herein, we report on the subpicosecond to sub-nanosecond vibrational energy transfer (VET) dynamics of the solid energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) using broadband, ultrafast infrared transient absorption spectroscopy. Experiments reveal VET occurring on three distinct timescales: subpicosecond, 5 ps, and 200 ps. The ultrafast appearance of signal at all probed modes in the midinfrared suggests strong anharmonic coupling of all vibrations in the solid whereas the long-lived evolution demonstrates that VET is incomplete, and thus thermal equilibrium is not attained, even on the hundred picosecond timescale. Density functional theory and classical molecular dynamics simulations provide valuable insights into the experimental observations, revealing compressioninsensitive timescales for the initial VET dynamics of high frequency vibrations and drastically extended relaxation times for low frequency phonon modes under compression. Mode-selectivity of the longest dynamics suggests coupling of the N-N and axial NO2 stretching modes with the long-lived, excited phonon bath.
Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy
The Journal of Chemical Physics
SHOCK COMPRESSION OF CONDENSED MATTER - 2019: Proceedings of the Conference of the American Physical Society Topical Group on Shock Compression of Condensed Matter
Shock-induced detonation is a key property of energetic materials (EM) that remains empirically u... more Shock-induced detonation is a key property of energetic materials (EM) that remains empirically understood. One proposed mechanism of shock-initiation in EM is "phonon up-pumping" to initiate chemical reactions, where excitation of lattice phonon modes rapidly transfers energy into intramolecular vibrations, ultimately resulting in the breaking of chemical bonds. We are developing novel ultrafast laser spectroscopy techniques to study vibrational energy transfer from phonon modes to intramolecular vibrations (phonon up-pumping), as well as competing energy transfer pathways from intramolecular vibrations to phonon modes (vibrational cooling). Through combinations of plasmagenerated supercontinuum infrared, tunable near-and mid-infrared, and terahertz pulses in pump-probe spectroscopy, supplemented with ab inito simulations, we can explore the energy transfer processes on a sub-picosecond time scale to elucidate vibrational energy transfer pathways and lifetimes in EM. Herein we highlight recent progress, including the spectral and temporal characteristics of the infrared and THz sources as well as preliminary results on select EM.
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Papers by Krupa Ramasesha