arXiv subjects

John F. Stanton

Publications and source records attributed to John F. Stanton.

12 recordsLinked to original sources

arXiv2026

Experimental proof of strong $Π$-$Σ$ mixing in the Renner-Teller and Pseudo-Jahn-Teller affected CCH$^+$ ($^3Π$) ion

The ethynyl radical cation, CCH$^+$ ($^3Π$), offers a unique system for fundamental spectroscopic studies of non-adiabatic effects due to its open-shell linear structure and the presence of a low-lying $^3Σ^-$ state, which induces notable perturbations in the (ro-)vibrational spectrum. To probe these effects, we recorded the broadband vibrational spectrum of CCH$^+$ from 350-3450 cm$^{-1}$ using leak-out spectroscopy. The spectrum reveals a complex splitting pattern in the CCH bending mode, attributed to Renner-Teller and pseudo-Jahn-Teller coupling effects between the $^3 Π$ and $^3 Σ^-$ electronic states. A three-state diabatic model, validated here against high-resolution IR data of the CH stretching mode, facilitated assignments within the broadband infrared (IR) spectrum, including an additional $Π$ vibronic feature observed in the aforementioned high-resolution spectrum. Our results highlight a pronounced sensitivity of the splitting pattern to the $Π$-$Σ$ energy gap, with couplings so large that even the zero-point vibrational motion of the bending vibration is sufficient to disrupt the vibronic structure of this ion. This compact ion, with strong coupling effects and high-quality spectroscopic data, serves as an exemplary system for evaluating non-adiabatic models.

physics.chem-ph↗

arXiv2025

Exploiting a Shortcoming of Coupled-Cluster Theory: The Extent of non-Hermiticity as a Diagnostic Indicator of Computational Accuracy

The fundamental non-Hermitian nature of the forms of coupled-cluster (CC) theory widely used in quantum chemistry has usually been viewed as a negative, but the present letter shows how this can be used to advantage. Specifically, the non-symmetric nature of the reduced one-particle density matrix (in the molecular orbital basis) is advocated as a diagnostic indicator of computational quality. In the limit of full coupled-cluster theory (which is equivalent to full configuration interaction (FCI)), the electronic wavefunction and correlation energy are exact within a given one-particle basis set and the symmetric character of the exact density matrix is recovered. The extent of the density matrix asymmetry is shown to provide a measure of ``how difficult the problem is'' (like the well-known T$_1$ diagnostic), but its variation with level of theory also gives information about ``how well this particular method works'', irrespective of the difficulty of the problem at hand. The proposed diagnostic is described and applied to a select group of small molecules, and an example of its overall utility for the practicing quantum chemist is illustrated through its application to the beryllium dimer (Be$_2$). Future applications of this idea to excited states, open-shell systems, symmetry-breaking problems and extension of the method to the two-particle density are then proposed.

physics.chem-ph↗

arXiv2023

Post-CCSD(T) corrections to bond distances and vibrational frequencies: the power of $Λ$

The importance of post-CCSD(T) corrections as high as CCSDTQ56 for ground-state spectroscopic constants ($D_e$, $ω_e$, $ω_ex_e$, and $α_e$) has been surveyed for a sample of two dozen mostly heavy-atom diatomics spanning a broad range of static correlation strength. While CCSD(T) is known to be an unusually felicitous `Pauling point' between accuracy and computational cost, performance leaves something to be desired for molecules with strong static correlation. We find CCSDT(Q)$_Λ$ to be the next `sweet spot' up, of comparable or superior quality to the much more expensive CCSDTQ. A similar comparison applies to CCSDTQ(5)$_Λ$ vs. CCSDTQ5, while CCSDTQ5(6)$_Λ$ is essentially indistinguishable from CCSDTQ56. A composite of CCSD(T)-X2C/ACV5Z-X2C with [CCSDT(Q)$_Λ$ -- CCSD(T)]/cc-pVTZ or even cc-pVDZ basis sets appears highly effective for computational vibrational spectroscopy. Unlike CCSDT(Q) which breaks down for the ozone vibrational frequencies, CCSDT(Q)$_Λ$ handles them gracefully.

physics.chem-ph↗

arXiv2020

Theoretical Prediction of Magnetic Exchange Coupling Constants from Broken-Symmetry Coupled Cluster Calculations

Exchange coupling constants ($J$) are fundamental to the understanding of spin spectra of magnetic systems. Here we investigate the broken-symmetry (BS) approaches of Noodleman and Yamaguchi in conjunction with coupled cluster (CC) methods to obtain exchange couplings. $J$ values calculated from CC in this fashion converge smoothly towards the FCI result with increasing level of CC excitation. We compare this BS-CC scheme to the complementary EOM-CC approach on a selection of bridged molecular cases and give results from a few other methodologies for context.

physics.chem-ph↗

arXiv2019

Decoherence in Molecular Electron Spin Qubits: Insights from Quantum Many-Body Simulations

Quantum states are described by wave functions whose phases cannot be directly measured, but which play a vital role in quantum effects such as interference and entanglement. The loss of the relative phase information, termed decoherence, arises from the interactions between a quantum system and its environment. Decoherence is perhaps the biggest obstacle on the path to reliable quantum computing. Here we show that decoherence occurs even in an isolated molecule although not all phase information is lost via a theoretical study of a central electron spin qubit interacting with nearby nuclear spins in prototypical magnetic molecules. The residual coherence, which is molecule-dependent, provides a microscopic rationalization for the nuclear spin diffusion barrier proposed to explain experiments. The contribution of nearby molecules to the decoherence has a non-trivial dependence on separation, peaking at intermediate distances. Molecules that are far away only affect the long-time behavior. Because the residual coherence is simple to calculate and correlates well with the coherence time, it can be used as a descriptor for coherence in magnetic molecules. This work will help establish design principles for enhancing coherence in molecular spin qubits and serve to motivate further theoretical work.

physics.chem-ph↗

arXiv2019

Quantum-State-Specific Reaction Rate Measurements for the Photo-induced Reaction Ca$^+$ + O$_2$ $\rightarrow$ CaO$^+$ + O

Atoms and molecules often react at different rates depending on their internal quantum states. Thus, controlling which internal states are populated can be used to manipulate the reactivity and can lead to a more detailed understanding of reaction mechanisms. We demonstrate this control of reactions by studying the excited state reaction reaction Ca$^+$ + O$_2$ $\rightarrow$ CaO$^+$ + O. This reaction is exothermic only if Ca$^+$ is in one of its excited electronic states. Using laser-cooling and electrodynamic trapping, we cool and trap Ca$^+$ at millikevin temperatures for several minutes. We can then change the fraction of time they spend in each of the two excited states by adjusting the detunings of the cooling lasers. This allows us to disentangle the reactions that begin with Ca$^+$ in the $^2$P$_{1/2}$-state from the ones where Ca$^+$ is in the $^2$D$_{3/2}$-state. Using time-of-flight mass spectrometry, we determine independent reaction rate constants for Ca$^+$ in both electronically excited quantum states.

physics.atom-ph↗

arXiv2018

Vibrational Satellites of C$_2$S, C$_3$S, and C$_4$S: Microwave Spectral Taxonomy as a Stepping Stone to the Millimeter-Wave Band

We present a microwave spectral taxonomy study of several hydrocarbon/CS$_2$ discharge mixtures in which more than 60 distinct chemical species, their more abundant isotopic species, and/or their vibrationally excited states were detected using chirped-pulse and cavity Fourier-transform microwave spectroscopies. Taken together, in excess of 85 unique variants were detected, including several new isotopic species and more than 25 new vibrationally excited states of C$_2$S, C$_3$S, and C$_4$S, which have been assigned on the basis of published vibration-rotation interaction constants for C$_3$S, or newly calculated ones for C$_2$S and C$_4$S. On the basis of these precise, low-frequency measurements, several vibrationally exited states of C$_2$S and C$_3$S were subsequently identified in archival millimeter-wave data in the 253--280 GHz frequency range, ultimately providing highly accurate catalogs for astronomical searches. As part of this work, formation pathways of the two smaller carbon-sulfur chains were investigated using $^{13}$C isotopic spectroscopy, as was their vibrational excitation. The present study illustrates the utility of microwave spectral taxonomy as a tool for complex mixture analysis, and as a powerful and convenient `stepping stone' to higher frequency measurements in the millimeter and submillimeter bands.

astro-ph.IM↗

arXiv2018

Spectral analyses of trans- and cis-DOCO transients via comb spectroscopy

We use time-resolved direct frequency comb spectroscopy in the mid-infrared to obtain high-resolution rovibrational spectra of products produced from the OD+CO reaction. In this work, we present spectral analyses for isotopologues of the transient DOCO radicals from this reaction in the OD stretch region. The analyses were performed with aid of two different theoretical approaches based on both perturbation theory and variational calculations used for prediction of rovibrational spectra of polyatomic molecules. We discuss the advantages and challenges of our current approach for studying spectroscopy and dynamics of transient molecules.

physics.chem-ph↗

arXiv2017

Direct measurements of DOCO isomers in the kinetics of OD+CO

Quantitative and mechanistically-detailed kinetics of the reaction of hydroxyl radical (OH) with carbon monoxide (CO) have been a longstanding goal of contemporary chemical kinetics. This fundamental prototype reaction plays an important role in atmospheric and combustion chemistry, motivating studies for accurate determination of the reaction rate coefficient and its pressure and temperature dependence at thermal reaction conditions. This intricate dependence can be traced directly to details of the underlying dynamics (formation, isomerization, and dissociation) involving the reactive intermediates cis- and trans-HOCO, which can only be observed transiently. Using time-resolved frequency comb spectroscopy, comprehensive mechanistic elucidation of the kinetics of the isotopic analogue deuteroxyl radical (OD) with CO has been realized. By monitoring the concentrations of reactants, intermediates, and products in real-time, the branching and isomerization kinetics and absolute yields of all species in the OD+CO reaction are quantified as a function of pressure and collision partner.

physics.chem-ph↗

arXiv2015

Inner-shell photoionization and core-hole decay of Xe and XeF$_2$

Photoionization cross sections and partial ion yields of Xe and XeF$_2$ from Xe 3d$_{5/2}$, Xe 3d$_{3/2}$, and F 1s subshells in the 660--740 eV range are compared to explore effects of the F ligands. The Xe 3d - $ε$f continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF$_2$ cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. The subshell ionization thresholds, the LUMO resonance energies and their oscillator strengths are calculated by relativistic coupled-cluster methods. Several charge states and fragment ions are produced from the atom and molecule due to alternative decay pathways from the inner-shell holes. Total and partial ion yields vary in response to the shape resonances and LUMO resonances. Previous calculations and measurements of atomic Xe 3d core-hole decay channels and our calculated results for XeF$_2$ guide interpretations of the molecular ion products. The partial ion yields of XeF$_2$ are dominated by Xe 3d core-hole decays, but distinct ion products are measured at the F 1s - LUMO resonance. Xe 3d core-hole decays from XeF$_2$ produce lower charge states in comparison with atomic Xe, and energetic F ions are produced by Coulomb explosions of the molecular ions. The measurements support a model of molecular core-hole decay that begins with a localized hole, stepwise Auger electron emission spreads charge across neighboring atoms, and the system fragments energetically.

physics.atom-ph↗

arXiv2013

A New Approach toward Transition State Spectroscopy

Chirped-Pulse millimetre-Wave (CPmmW) rotational spectroscopy provides a new class of information about photolysis transition state(s). Measured intensities in rotational spectra determine species-isomer-vibrational populations, provided that rotational populations can be thermalized. The formation and detection of S0 vinylidene is discussed in the limits of low and high initial rotational excitation. CPmmW spectra of 193 nm photolysis of Vinyl Cyanide (Acrylonitrile) contain J=0-1 transitions in more than 20 vibrational levels of HCN, HNC, but no transitions in vinylidene or highly excited local-bender vibrational levels of acetylene. Reasons for the non-observation of the vinylidene co-product of HCN are discussed.

physics.chem-ph↗

arXiv2011

Reduced Dimension DVR Study of cis-trans Isomerization in the S_1 State of C_2H_2

Isomerization between the cis and trans conformers of the S1 state of acetylene is studied using a reduced dimension DVR calculation. Existing DVR techniques are combined with a high accuracy potential energy surface and a kinetic energy operator derived from FG theory to yield an effective but simple Hamiltonian for treating large amplitude motions. The spectroscopic signatures of the S1 isomerization are discussed, with emphasis on the vibrational aspects. The presence of a low barrier to isomerization causes distortion of the trans vibrational level structure and the appearance of nominally electronically forbidden Ã1A2 \leftarrow X 1Σ+g transitions to vibrational levels of the cis conformer. Both of these effects are modeled in agreement with experimental results, and the underlying mechanisms of tunneling and state mixing are elucidated by use of the calculated vibrational wavefunctions.

physics.chem-ph↗