Factors affecting lifetimes and resolution of Rydberg states observed in zero‐electron‐kinetic‐energy spectroscopy

Experimental characterization of SiCH+via single-photon ionization of gas-phase SiCH

SiCH and its cation have consistently emerged as predicted species in models of silicon chemistry within the interstellar medium, although they remain unobserved in space. Hindered by their intrinsic instability, no spectroscopic insights have been gleaned concerning the SiCH+ cation. In this study, we present experimental measurements on the SiCH+ cation through single-photon ionization spectroscopy of the SiCH radical within the 8.0-11.0 eV range. Gas-phase SiCH radicals were generated through chemical reactions involving CHx (x = 0-3) and SiHy (y = 0-3) within a microwave discharge flow-tube reactor. Employing a double imaging photoelectron/photoion coincidence spectrometer on the DESIRS beamline at the SOLEIL synchrotron, we recorded mass-selected ion yield and photoelectron spectra. From the analysis of the photoelectron spectrum supported by ab initio calculations and Franck-Condon simulations, the adiabatic ionization energies for the transitions from the X2Π ground electronic state of SiCH toward the X+3Σ- and A+3Π electronic states of SiCH+ have been derived [8.935(6) and 10.664(6) eV, respectively, without spin-orbit correction]. The contribution from the less stable isomer HSiC has been explored in our analysis and ruled out in our experiments.

Electrostatic Trapping of N_{2} Molecules in High Rydberg States

N_{2} molecules traveling in pulsed supersonic beams have been excited from their X ^{1}Σ_{g}^{+} ground electronic state to long-lived Rydberg states with principal quantum numbers between 39 and 48 using a resonance-enhanced two-color three-photon excitation scheme. The Rydberg states populated had static electric dipole moments exceeding 5000 D which allowed deceleration of the molecules to rest in the laboratory-fixed frame of reference and three-dimensional trapping using inhomogeneous electric fields. The trapped molecules were confined for up to 10 ms, with effective trap decay time constants increasing with principal quantum number, and ranging from 450 to 700  μs. These observations, and comparison with the results of similar measurements with He atoms, indicate that the decay dynamics of the trapped Rydberg N_{2} molecules are dominated by spontaneous emission and do not exhibit significant contributions from effects of intramolecular interactions that lead to non-radiative decay.

Refining the thermochemical properties of CF, SiF, and their cations by combining photoelectron spectroscopy, quantum chemical calculations, and the Active Thermochemical Tables approach

Fluorinated species have a pivotal role in semiconductor material chemistry and some of them have been detected beyond the Earth's atmosphere. Achieving good energy accuracy on fluorinated species using quantum chemical calculations has long been a challenge. In addition, obtaining direct experimental thermochemical quantities has also proved difficult. Here, we report the threshold photoelectron and photoion yield spectra of SiF and CF radicals generated with a fluorine reactor. The spectra were analysed with the support of ab initio calculations, resulting in new experimental values for the adiabatic ionisation energies of both CF (9.128 ± 0.006 eV) and SiF (7.379 ± 0.009 eV). Using these values, the underlying thermochemical network of Active Thermochemical Tables was updated, providing further refined enthalpies of formation and dissociation energies of CF, SiF, and their cationic counterparts.

Single-photon ionization of SiC in the gas phase: experimental and ab initio characterization of SiC. Phys Chem Chem Phys 2023;25:23568-23578. [PMID: 37656136 DOI: 10.1039/d3cp02775a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We report the first experimental observation of single-photon ionization transitions of the SiC radical between 8.0 and 11.0 eV performed on the DESIRS beamline at the SOLEIL synchrotron facility. The SiC radical, very difficult to synthesize in the gas phase, was produced through chemical reactions between CHx (x = 0-3) and SiHy (y = 0-3) in a continuous microwave discharge flow tube, the CHx and SiHy species being formed by successive hydrogen-atom abstractions induced by fluorine atoms on methane and silane, respectively. Mass-selected ion yield and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The photoelectron spectrum enables the first direct experimental determinations of the X+ 4Σ- ← X 3Π and 1+ 2Π ← X 3Π adiabatic ionization energies of SiC (8.978(10) eV and 10.216(24) eV, respectively). Calculated spectra based on Franck-Condon factors are compared with the experimental spectra. These spectra were obtained by solving the rovibrational Hamiltonian, using the potential energy curves calculated at the multireference single and double configuration interaction level with Davidson correction (MRCI + Q) and the aug-cc-pV5Z basis set. MRCI + Q calculations including the core and core-valence electron correlation were performed using the aug-cc-pCV6Z basis set to predict the spectroscopic properties of the six lowest electronic states of SiC+. Complete basis set extrapolations and relativistic energy corrections were also included in the determination of the energy differences characterizing the photoionization process. Using our experimental and theoretical results, we derived semi-experimental values for the five lowest ionization energies of SiC.

Photoion Mass-Selected Threshold Photoelectron Spectroscopy to Detect Reactive Intermediates in Catalysis: From Instrumentation and Examples to Peculiarities and a Database

Photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) is a synchrotron-based, universal, sensitive, and multiplexed detection tool applied in the areas of catalysis, combustion, and gas-phase reactions. Isomer-selective vibrational fingerprints in the ms-TPES of stable and reactive intermediates allow for unequivocal assignment of spectral carriers. Case studies are presented on heterogeneous catalysis, revealing the role of ketenes in the methanol-to-olefins process, the catalytic pyrolysis mechanism of lignin model compounds, and the radical chemistry upon C-H activation in oxyhalogenation. These studies demonstrate the potential of ms-TPES as an analytical technique for elucidating complex reaction mechanisms. We examine the robustness of ms-TPES assignments and address sampling effects, especially the temperature dependence of ms-TPES due to rovibrational broadening. Data acquisition approaches and the Stark shift from the extraction field are also considered to arrive at general recommendations. Finally, the PhotoElectron PhotoIon Spectral Compendium (https://pepisco.psi.ch), a spectral database hosted at Paul Scherrer Institute to support assignment, is introduced.

Pulsed-ramped-field-ionisation zero-kinetic-energy photoelectron spectroscopy of the metastable rare-gas atoms Ar, Kr and Xe

The photoionisation of the metastable rare-gas atoms Rg = Ar, Kr and Xe is investigated at the Rg+ […](ns)2(np)5 2P3/2 ← Rg[…](ns)2(np)5((n + 1)s) 13P2 photoionisation threshold (n = 3, 4 and 5 for Ar, Kr and Xe) using the technique of pulsed-ramped-field-ionisation zero-kinetic-energy (PRFI-ZEKE) photoelectron spectroscopy. This technique, which monitors the field ionisation of high Rydberg states induced by a slowly growing electric-field ramp after a prepulse of opposite polarity, was recently introduced to record photoelectron spectra with high resolution and high sensitivity [Harper, Chen, Boyé-Péronne and Gans, Phys. Chem. Chem. Phys., 2022, 117, 9353]. A tunable UV laser system with a bandwidth of 30 MHz is used here to establish the factors determining the resolution and overall accuracy of this new method. In particular, the effects of stray electric fields, and of the magnitude of the prepulse and the field ramp on PRFI-ZEKE photoelectron spectra are studied by combining experiments with numerical simulations of the field-ionisation of high Rydberg states and of the electron flight times from the ionisation spot to the detector. A spectral resolution of 0.05 cm-1 is demonstrated in the photoelectron spectrum of metastable Ar.

Gas phase synthesis of the C40 nano bowl C40H10

Nanobowls represent vital molecular building blocks of end-capped nanotubes and fullerenes detected in combustion systems and in deep space such as toward the planetary nebula TC-1, but their fundamental formation mechanisms have remained elusive. By merging molecular beam experiments with electronic structure calculations, we reveal a complex chain of reactions initiated through the gas-phase preparation of benzocorannulene (C₂₄H₁₂) via ring annulation of the corannulenyl radical (C₂₀H₉^•) by vinylacetylene (C₄H₄) as identified isomer-selectively in situ via photoionization efficiency curves and photoion mass-selected threshold photoelectron spectra. In silico studies provided compelling evidence that the benzannulation mechanism can be expanded to pentabenzocorannulene (C₄₀H₂₀) followed by successive cyclodehydrogenation to the C40 nanobowl (C₄₀H₁₀) - a fundamental building block of buckminsterfullerene (C₆₀). This high-temperature pathway opens up isomer-selective routes to nanobowls via resonantly stabilized free-radical intermediates and ring annulation in circumstellar envelopes of carbon stars and planetary nebulae as their descendants eventually altering our insights of the complex chemistry of carbon in our Galaxy.

Probing the pyrolysis of ethyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O=C(H)-O-C(H₂ )-CH₃ and trans (staggered) configuration of the O=C(H)-O-C(H₂ )-CH₃ dihedral angles. We observed the following ethyl formate pyrolysis products: CH₃ CH₂ OH, CH₃ CHO, C₂ H₆ , C₂ H₄ , HC(O)OH, CH₂ O, CO₂ , and CO, with HC(O)OH and C₂ H₄ pyrolyzing further, forming CO + H₂ O and C₂ H₂  + H₂ . The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH₃ CH₂ O + CHO and CH₃ CH₂  + HC(O)O, were studied computationally at the M06-2X-GD3/aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.

Exploitation of Synchrotron Radiation Photoionization Mass Spectrometry in the Analysis of Complex Organics in Interstellar Model Ices

Unravelling the generation of complex organic molecules (COMs) on interstellar nanoparticles (grains) is essential in establishing predictive astrochemical reaction networks and recognizing evolution stages of molecular clouds and star-forming regions. The formation of COMs has been associated with the irradiation of interstellar ices by ultraviolet photons and galactic cosmic rays. Herein, we pioneer the first incorporation of synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (SVUV-PI-ReTOF-MS) in laboratory astrophysics simulation experiments to afford an isomer-selective identification of key COMs (ketene (H₂C═CO); acetaldehyde (CH₃CHO); vinyl alcohol (H₂C═CHOH)) based on photoionization efficiency (PIE) curves of molecules desorbing from exposed carbon monoxide-methane (CO-CH₄) ices. Our results demonstrate that the SVUV-PI-ReTOF-MS approach represents a versatile, rapid methodology for a comprehensive identification and explicit understanding of the complex organics produced in space simulation experiments. This methodology is expected to significantly improve the predictive nature of astrochemical models of complex organic molecules formed abiotically in deep space, including biorelated species linked to the origins-of-life topic.

Substitution-induced Nonplanarity of 3-Fluorothioanisole in the First Electronically Excited State

Vibronic spectra of 3-fluorothioanisole (3FTA) in the first electronic excited state (S₁) and the cationic ground state (D₀) have been obtained by one-color resonant two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopy. Spectroscopic measurements and theoretical calculations indicate that both cis- and trans-rotamers of the 3FTA molecule are stable and coexist in the S₀ (the electronic ground state) and D₀ states, and the cis-rotamer is shown to be slightly more stable than the trans-rotamer. In the S₁ state, theoretical calculations predict a stable gauche-structure of 3FTA, manifested by the observation of strong activation of the vibrational modes involving the motion of the -SCH₃ group in the low-frequency regions of the 1C-R2PI and MATI spectra. The electronic excitation energy from the S₀ state to the S₁ state (E₁) and the adiabatic ionization energy (IE) are respectively determined to be 34 820 ± 3 and 65 468 ± 5 cm^-1 for cis-3FTA, and those of the trans-rotamer are respectively determined to be 35 047 ± 3 and 65 644 ± 5 cm^-1. The structural properties of the stable rotamers of 3FTA and their comparison with other F- and Cl-substituted thioanisole derivatives are discussed as well.

On the absolute photoionization cross section and threshold photoelectron spectrum of two reactive ketenes in lignin valorization: fulvenone and 2-carbonyl cyclohexadienone

We report the absolute photoionization cross section (PICS) of fulvenone and 2-carbonyl cyclohexadienone, two crucial ketene intermediates in lignin pyrolysis, combustion and organic synthesis. Both species were generated in situ by pyrolyzing salicylamide and dectected via imaging photoelectron photoion coincidence spectroscopy. In a deamination reaction, salicylamide loses ammonia yielding 2-carbonyl cyclohexadienone, a ketoketene, which further decarbonylates at higher pyrolysis temperatures to form fulvenone. We recorded the threshold photoelectron spectrum of the ketoketene and assigned the ground state (X̃⁺²A′′ ← X̃¹A′) and excited state (Ã⁺²A′ ← X̃¹A′) bands with the help of Franck–Condon simulations. Adiabatic ionization energies are 8.35 ± 0.01 and 9.19 ± 0.01 eV. In a minor reaction channel, the ketoketene isomerizes to benzpropiolactone, which decomposes subsequently to benzyne by CO₂ loss. Potential energy surface and RRKM rate constant calculations agree with our experimental observations that the decarbonylation to fulvenone outcompetes the decarboxylation to benzyne by almost two orders of magnitude. The absolute PICS of fulvenone at 10.48 eV was determined to be 18.8 ± 3.8 Mb using NH₃ as a calibrant. The PICS of 2-carbonyl cyclohexadienone was found to be 21.5 ± 8.6 Mb at 9 eV. Our PICS measument will enable the quantification of reactive ketenes in lignin valorization and combustion processes using photoionization techniques and provide advanced mechanistic and kinetics insights to aid the bottom-up optimization of such processes.

The absolute photoionization cross section (PICS) of these crucial ketene intermediates supports their quantification in lignin pyrolysis, combustion and organic synthesis.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C₅H₈O₂) and methyl butyrate (MB, C₅H₁₀O₂) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO₂ is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E₀ = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO₂ loss as 66.6 ± 4.3 kJ mol^-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E₀ = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at Δ_fH^o_0K[CH₂C(OH)OCH₃⁺] = 502 ± 6 kJ mol^-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH₃ and CO₂ loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

Pulsed-ramped-field-ionization zero-kinetic-energy photoelectron spectroscopy: a methodological advance

A new experimental method has been developed to record photoelectron spectra based on the well-established pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy technique and inspired by the data treatment employed in slow photoelectron spectroscopy. This method has been successfully applied to two well-known systems: the X⁺²Π_g,1/2(v⁺ = 0) ← X¹Σ+g(v = 0) and the X⁺¹Σ⁺(v⁺ = 2) ← X²Π_1/2(v = 0) ionizing transitions of CO₂ and NO, respectively. The first results highlight several advantages of our technique such as an improved signal-to-noise ratio without degrading the spectral resolution and a direct field-free energy determination. The data obtained for NO indicate that this method might be useful for studying field-induced autoionization processes.

Innovative mass spectrometer for high-resolution ion spectroscopy

Conventional ion spectroscopy is inapplicable for ions produced in low concentrations or with low spectral resolutions. Hence, we constructed a high-resolution vacuum ultraviolet mass-analyzed threshold ionization (HR VUV-MATI) spectrometer composed of a four-wave frequency mixing cell capable of generating long-lasting and intense VUV laser pulses of ∼1 × 10¹⁰ photons/pulse at wavelengths of 123.6-160.0 nm, a space-focused linear time-of-flight photoionization chamber with a new ion-source assembly, and a compact molecular beam chamber with a temperature-controlled pulsed nozzle for ion spectroscopy. The ion-source assembly and pulsing schemes enabled an ∼15-μs-delayed but extremely weak pulsed-field-ionization of the molecules in the zero-kinetic-energy (ZEKE) states and first-order space focusing of the generated MATI ions. These ZEKE states were effectively generated by a minute electric jitter from the high-lying Rydberg states, which were initially prepared via VUV photoexcitation. The spectral and mass resolutions (∼5 cm^-1 and 2400, respectively) and the signal strength were simultaneously enhanced using this spectrometer. Moreover, it could be used to measure the fine vibrational spectrum from the zero-point level of the cation and the exact adiabatic ionization energy of the neutral molecule. Additionally, it could be used to measure the appearance energies of the photoproducts and elucidate the vibrational structures of the cationic isotopomers, utilizing other pulsing schemes. Furthermore, this spectrometer could be used to analyze the congested vibrational spectrum of a cation with multiple conformations. Thus, the HR VUV-MATI spectrometer-a potential alternative to photoelectron spectrometers-can be used to analyze the conformational structure-dependent reactivities.

Quantum-state-dependent decay rates of electrostatically trapped Rydberg NO molecules

Nitric oxide (NO) molecules travelling in pulsed supersonic beams have been prepared in long-lived Rydberg-Stark states by resonance-enhanced two-colour two-photon excitation from the X 2Π1/2 (v'' = 0, J'' = 3/2) ground state, through the A 2Σ+ (v' = 0, N' = 0, J' = 1/2) intermediate state. These excited molecules were decelerated from 795 ms-1 to rest in the laboratory-fixed frame of reference, in the travelling electric traps of a transmission-line Rydberg-Stark decelerator. The decelerator was operated at 30 K to minimise effects of blackbody radiation on the molecules during deceleration and trapping. The molecules were electrostatically trapped for times of up to 1 ms, and detected in situ by pulsed electric field ionisation. Measurements of the rate of decay from the trap were performed for states with principal quantum numbers between n = 32 and 50, in Rydberg series converging to the N+= 0, 1, and 2 rotational states of NO+. For the range of Rydberg states studied, the measured decay times of between 200 μs and 400 μs were generally observed to reduce as the value of n was increased. For some particular values of n deviations from this trend were seen. These observations are interpreted, with the aid of numerical calculations, to arise as a result of contributions to the decay rates, on the order of 1 kHz, from rotational and vibrational channel interactions. These results shed new light on the role of weak intramolecular interactions on the slow decay of long-lived Rydberg states in NO.

Charge-Transfer-Induced Predissociation in Rydberg States of Molecular Cations: MgAr. J Phys Chem A 2021;125:6681-6696. [PMID: 34319723 PMCID: PMC8775275 DOI: 10.1021/acs.jpca.1c03859]

Very little is known about the Rydberg states of molecular cations, i.e., Rydberg states having a doubly charged ion core. With the example of MgAr+, we present general features of the structure and dynamics of the Rydberg states of molecular cations, which we find are subject to the process of charge-transfer-induced predissociation. Our study focuses on the spectrum of low-n Rydberg states with potential-energy functions associated with the Mg+(3d and 4s) + Ar(1S0) dissociation asymptotes. In particular, we have recorded spectra of the 3dπΩ' (Ω' = 1/2, 3/2) Rydberg states, extending from the lowest (v' = 0) vibrational levels to their dissociation limits. This spectral range encompasses the region where the onset of predissociation by interaction with the mostly repulsive 2Σ and 2Π charge-transfer states associated with the Mg(3s2) + Ar+(2P1/2,3/2) dissociation asymptotes is observed. This interaction leads to very strong perturbations of the 3dπ Rydberg states of MgAr+, revealed by vibrational progressions exhibiting large and rapid variations of the vibrational intervals, line widths, and spin-orbit splittings. We attribute the anomalous sign and magnitude of the spin-orbit coupling constant of the 3dπ state to the interaction with a 2Π Rydberg state correlating to the Mg+(4p) + Ar(1S0) dissociation limit. To analyze our spectra and elucidate the underlying process of charge-transfer-induced predissociation, we implemented a model that allowed us to derive the potential-energy functions of the charge-transfer states and to quantitatively reproduce the experimental results. This analysis characterizes the main features of the dynamics of the Rydberg series converging to the ground state of MgAr2+. We expect that the results and analysis reported here are qualitatively valid for a broader range of singly charged molecular cations, which are inherently prone to charge-transfer interactions.

Spectroscopic characterization of a thermodynamically stable doubly charged diatomic molecule: MgAr2

Although numerous doubly positively charged diatomic molecules (diatomic dications) are known from investigations using mass spectrometry and ab initio quantum chemistry, only three of them, NO²⁺, N₂²⁺ and DCl²⁺, have been studied using rotationally resolved optical spectroscopy and only about a dozen by vibrationally resolved double-ionization methods. So far, no thermodynamically stable diatomic dication has been characterized spectroscopically, primarily because of experimental difficulties associated with their synthesis in sufficient densities in the gas phase. Indeed, such molecules typically involve, as constituents, rare-gas, halogen, chalcogen, and metal atoms. We report here on a new approach to characterize molecular dications based on high-resolution photoelectron spectroscopy of the singly charged parent molecular cation and present the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr²⁺. From the fully resolved vibrational and partially resolved rotational structures of the photoelectron spectra of ²⁴MgAr⁺ and ²⁶MgAr⁺, we determined the potential-energy function of the electronic ground state of MgAr²⁺, its dissociation (binding) energy (D₀ = 10 690(3) cm⁻¹), and its harmonic (ω_e(²⁴MgAr²⁺) = 327.02(11) cm⁻¹) and anharmonic (ω_ex_e(²⁴MgAr²⁺) = 2.477(15) cm⁻¹) vibrational constants. The analysis enables us to explain quantitatively how the strong bond arises in this dication despite the fact that Ar and Mg²⁺ both have a full-shell rare-gas electronic configuration.

We present a new method to study doubly charged molecules relying on high-resolution spectroscopy of the singly charged parent cation, and report on the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr²⁺.

Conformer-specific VUV-MATI spectroscopy of methyl vinyl ketone: stabilities and cationic structures of the s-trans and s-cis conformers

Methyl vinyl ketone (MVK), a volatile compound with photochemical activity, has received considerable attention in the fields of environmental chemistry and atmospheric chemistry. We explored the conformational stabilities of MVK in the neutral S0 and the cationic D0 states using conformer-specific vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy, which provided identifiable vibrational spectra for cationic MVK conformers. Based on the origin bands of the two individual conformers of MVK identified in the MATI spectra under different supersonic expansion conditions, the accurate adiabatic ionization energies of the s-trans and the s-cis conformers were determined to be 77 867 ± 4 (9.6543 ± 0.0005 eV) and 78 222 ± 4 cm-1 (9.6983 ± 0.0005 eV), respectively. The identifiable vibrational spectra of the two cationic conformers were further confirmed using vibrational assignments based on the Franck-Condon fit. Accordingly, precise cationic structures of the MVK conformers could be determined. The structural changes of the two conformers upon ionization could be attributed to the removal of an electron from the highest occupied molecular orbital of each conformer, which consists of nonbonding molecular orbitals on the oxygen atom in the carbonyl group interacting with the σ orbitals in the molecular plane. Consequently, the s-trans conformer was preferred by 48 ± 18 and 403 ± 18 cm-1 in the neutral ground S0 and the cationic D0 states, respectively, which was supported by density-corrected density functional theory calculations and natural bond orbital analysis.

Isomer-Selective Threshold Photoelectron Spectra of Phenylnitrene and Its Thermal Rearrangement Products

The photoionization of phenylnitrene was investigated by photoion mass-selected threshold photoelectron spectroscopy in the gas phase. Flash vacuum pyrolysis of phenyl azide at 480 °C produces the nitrene, which subsequently rearranges at higher temperatures affording three isomeric cyanocyclopentadienes, in contrast to low-temperature trapping experiments. Temperature control of the reactor and threshold photoelectron spectra allows for optimizing the generation of phenylnitrene or its thermal rearrangement products, as well as obtaining vibrational information for the corresponding ions. The adiabatic ionization energies (AIE) of the triplet nitrene (³A₂) to the radical cation in its lowest-energy doublet (²B₂) and quartet (⁴A₁) spin states were determined to 8.29 ± 0.01 and 9.73 ± 0.01 eV, respectively. Vibrational frequencies of ring breathing modes were measured at 500 ± 80 and 484 ± 80 cm^-1 for both the [Formula: see text](²B₂) and [Formula: see text](⁴A₁) cationic states, respectively. The AIE differ from the values previously reported; hence, we revise the doublet-quartet energy splitting of the phenylnitrene radical cation to 1.44 eV, in excellent agreement with composite methods and coupled cluster calculations, but considerably higher than the literature reference (1.1 eV).

Excitation and characterization of long-lived hydrogenic Rydberg states of nitric oxide

High Rydberg states of nitric oxide (NO) with principal quantum numbers between 40 and 100 and lifetimes in excess of 10 µs have been prepared by resonance enhanced two-color two-photon laser excitation from the X ²Π_1/2 ground state through the A ²Σ⁺ intermediate state. Molecules in these long-lived Rydberg states were detected and characterized 126 µs after laser photoexcitation by state-selective pulsed electric field ionization. The laser excitation and electric field ionization data were combined to construct two-dimensional spectral maps. These maps were used to identify the rotational states of the NO⁺ ion core to which the observed series of long-lived hydrogenic Rydberg states converge. The results presented pave the way for Rydberg-Stark deceleration and electrostatic trapping experiments with NO, which are expected to shed further light on the decay dynamics of these long-lived excited states, and are of interest for studies of ion-molecule reactions at low temperatures.

R2PI and MATI spectroscopy of 3-fluoro-5-methylanisole: Combined effect of meta-substituents on molecular conformation

The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH₃) and methoxy group (OCH₃), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S₀ and S₁ states, but eclipsed in the D₀ state. While for trans 3F5MA, the staggered conformer is stable only in the S₁ state and the eclipsed one is stable in the S₀ and D₀ states. The first electronic excitation energies (E₁s) of cis and trans 3F5MA were determined to be 36,709 ± 3 and 36,615 ± 3 cm^-1 by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908 ± 5 and 66,692 ± 5 cm^-1. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH₃ torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-trans → s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH₃ torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D₀ ← S₁ ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH₃ group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S₀ and S₁ states but false for the D₀ state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.

Determination of the highest occupied molecular orbital and cationic structure of 2-chloropyridine by one-photon VUV-MATI spectroscopy and Franck–Condon fitting

Substitution of a chlorine atom for the H in pyridine alters the HOMO of the molecule, which ultimately affects the cationic structure.

Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

The photoionization of thymine has been studied by using vacuum ultraviolet radiation and imaging photoelectron photoion coincidence spectroscopy after aerosol flash vaporization and bulk evaporation. The two evaporation techniques have been evaluated by comparison of the photoelectron spectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922±0.008, 9.851±0.008, 10.30±0.02, and 10.82±0.01 eV. Vibrational features have been assigned for the first three electronic states with the help of Franck-Condon factor calculations based on density functional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite method ab initio calculations, suggests that the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parent ion, with the first step corresponding to a retro-Diels-Alder reaction. The dissociation rate constants were extracted from the photoion time-of-flight distributions and used together with the breakdown curves to construct a statistical model to determine 0 K appearance energies of 11.15±0.16 and 11.95±0.09 eV for the m/z 83 and 55 fragment ions, respectively. These results have allowed us to revise previously proposed fragmentation mechanisms and to propose a model for the final, nonstatistical H-loss step in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.

Cation Vibrations of 1-Methylnaphthalene and 2-Methylnaphthalene through Mass-Analyzed Threshold Ionization Spectroscopy

Vibrationally resolved cation spectra of 1-methylnaphthalene (1MN) and 2-methylnaphthalene (2MN) were obtained using the two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy. MATI spectra were obtained through ionization via several intermediate vibronic levels. Due to hindrance effect, no spectral features related to methyl torsion were observed in the MATI spectra of 1MN. By contrast, most of the in-plane ring deformation vibrations of the 2MN cation were found to couple with methyl torsion because of its small internal rotational barrier. These experimental findings were well supported by our theoretical calculations.

Possible Many-Body Localization in a Long-Lived Finite-Temperature Ultracold Quasineutral Molecular Plasma

We argue that the quenched ultracold plasma presents an experimental platform for studying the quantum many-body physics of disordered systems in the long-time and finite energy-density limits. We consider an experiment that quenches a plasma of nitric oxide to an ultracold system of Rydberg molecules, ions, and electrons that exhibits a long-lived state of arrested relaxation. The qualitative features of this state fail to conform with classical models. Here, we develop a microscopic quantum description for the arrested phase based on an effective many-body spin Hamiltonian that includes both dipole-dipole and van der Waals interactions. This effective model appears to offer a way to envision the essential quantum disordered nonequilibrium physics of this system.

A vacuum ultraviolet laser pulsed field ionization-photoion study of methane (CH4): determination of the appearance energy of methylium from methane with unprecedented precision and the resulting impact on the bond dissociation energies of CH4and CH4+

High-resolution VUV laser PFI-PI detection method for the study of quantum-state-selected unimolecular ion dissociation.

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO)

The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed.

Yields and Time-of-Flight Spectra of Neutral High-Rydberg Fragments at the K Edges of the CO2 Molecule

We have studied the production of neutral fragments in high-Rydberg (HR) states at the C 1s and O 1s edges of the CO2 molecule by performing two kinds of experiments. First, the yields of neutral HR fragments were measured indirectly by ionizing such fragments in a static electric field and by collecting resulting singly charged positive ions as a function of the photon energy. Such measurements reveal not only excitations below the core ionization thresholds but also thresholds for single core-hole and shakeup photoionization. Second, we obtained the mass spectra of neutral HR fragments at selected photon energies by exploiting pulsed field ionization; they show atomic fragments C(HR) and O(HR). We discuss dissociation pathways leading to the production of neutral HR fragments in core excitation and ionization of CO2.

Creation of cold nitric oxide by extraction of the cold fraction of a thermal distribution

We describe a device using the Stark effect to extract the cold fraction of nitric oxide molecules from a warmer thermal distribution. Room temperature NO is cryogenically cooled to 72-82 K and injected into a straight, hexapole guide that uses the Stark effect. By blocking line-of-sight trajectories from the input to the output, primarily the slowest molecules are guided around the obstruction and are transferred into a new chamber. We measure the temperature distribution using a field-stabilized Rydberg time-of-flight technique. A superposition of molecular Rydberg states is excited, sufficiently increasing the lifetime of the excited state for a time-of-flight measurement for cold molecular samples. We produce a continuous source of nitric oxide with temperatures ranging from 7 to 20 K in the lowest ro-vibrational state. The output temperature is controlled by the initial temperature distribution and the guide voltage.