Mass-analyzed Threshold Ionization Research Articles

High-resolution photoelectron spectroscopy of the very weakly bound MgNe molecule

ABSTRACT We report on the characterisation of the X + 2 Σ + ground electronic state of MgNe + by photoionisation, mass-analysed threshold-ionisation (MATI) and pulsed-field-ionisation zero-kinetic-energy photoelectron (PFI-ZEKE-PE) spectroscopy and the observation of the metastable a 3 Π 0 state of MgNe. Rotationally cold MgNe was generated in a laser-ablation supersonic-beam source in both the X 1 Σ + ( v ″ = 0 ) ground state and a 3 Π 0 ( v ″ = 0 ) metastable electronic state. PFI-ZEKE-PE spectra with full resolution of the vibrational structure and partial resolution of the rotational structure were recorded from the X 1 Σ + ground state following resonant ( 1 + 1 ′ ) two-photon excitation via selected rovibrational levels of the C 1 Π intermediate state and from the a 3 Π 0 metastable state following single-photon excitation. The lowest six vibrational levels of the X + 2 Σ + state were observed, covering 80% of the X + 2 Σ + potential-well depth. The experimental data were used to determine potential-energy functions, dissociation energies, and molecular constants for the X 1 Σ + , a 3 Π 0 and C 1 Π states of MgNe and the X + 2 Σ + state of MgNe + . With dissociation energies D 0 of 12(3) cm−1 and 10(3) cm−1, respectively, the X 1 Σ + and a 3 Π 0 states of 24 Mg 20 Ne are extremely weakly bound. In contrast, the X + 2 Σ + state of 24 Mg 20 Ne + has a dissociation energy D 0 of 185(3) cm − 1 and 12 bound vibrational levels. From the difference between the adiabatic ionisation energies of the X 1 Σ + (61 498.5(4) cm − 1 ) and the a 3 Π 0 (39 645.1(3) cm − 1 ) states, the term value of the a 3 Π 0 ground vibrational level is determined to be 21 853.4(5) cm − 1 . These results are compared with those of earlier experimental studies and ab initio quantum-chemical calculations.

Effect of a single methyl substituent on the electronic structure of cobaltocene studied by computationally assisted MATI spectroscopy.

Metallocenes represent archetypical organometallic compounds playing key roles in various fields of fundamental and applied chemistry. Many of their unique properties arise from low ionization energies (IE) which can be tuned by introducing substituents into the rings. Here we report the first mass-analyzed threshold ionization (MATI) spectrum of a methylmetallocene, (Cp')(Cp)Co (Cp' = η5-C5H4Me, Cp = η5-C5H5). The presence of a single Me group allows us to study the "pure" effect of methylation without the mutual influence of substituents. The MATI technique provides an extremely high accuracy in determining the adiabatic IE of (Cp')(Cp)Co which equals 5.2097(6) eV. The effect of a Me group on the IE of cobaltocene appears to be 36% stronger than that in bis(η6-benzene)chromium. The MATI spectrum of (Cp')(Cp)Co shows a rich vibronic structure from which vibrational frequencies of the free ion are determined. This information provides a solid basis for testing the quality of quantum chemical calculations. Various levels of the DFT and coupled cluster computations are used to describe the structural and electronic transformations accompanying the detachment of an elctron from (Cp')(Cp)Co. New aspects of the methyl substituent influence on the potential energy surfaces, as well as on the inhomogeneous changes in charge density and electrostatic potential caused by ionization, are discussed.

Spin-orbit coupling of electrons on separate lanthanide atoms of Pr2O2 and its singly charged cation.

Although it plays a critical role in the photophysics and catalysis of lanthanides, spin-orbit coupling of electrons on individual lanthanide atoms in small clusters is not well understood. The major objective of this work is to probe such coupling of the praseodymium (Pr) 4f and 6s electrons in Pr2O2 and Pr2O2+. The approach combines mass-analyzed threshold ionization spectroscopy and spin-orbit multiconfiguration second-order quasi-degenerate perturbation theory. The energies of six ionization transitions are precisely measured; the adiabatic ionization energy of the neutral cluster is 38 045 (5) cm-1. Most of the electronic states involved in these transitions are identified as spin-orbit coupled states consisting of two or more electron spins. The electron configurations of these states are 4f46s2 for the neutral cluster and 4f46s for the singly charged cation, both in planar rhombus-type structures. The spin-orbit splitting due to the coupling of the electrons on the separate Pratoms is on the order of hundreds of wavenumbers.

Unraveling the Conformational Preference of Morpholine: Insights from Infrared Resonant Vacuum Ultraviolet Photoionization Mass Spectroscopy.

The preference for different conformations in morpholine has a notable effect on its behavior and reactivity in organic synthesis. Herein, we explored the intricate conformational properties of morpholines through a combination of advanced mass spectrometric techniques and theoretical calculations. Notably, we employed infrared (IR) resonant vacuum ultraviolet (VUV) mass-analyzed threshold ionization spectroscopy to measure the unique vibrational spectra of the distinct conformers (Chair-Eq and Chair-Ax) in morpholine for the first time. Through precise VUV photon energy adjustments to coincide with the vibrational excitation via IR absorption, we effectively pinpointed the adiabatic ionization thresholds corresponding to the Chair-Eq (65 442 ± 4 cm-1) and Chair-Ax (65 333 ± 4 cm-1) conformers. This allowed us to accurately determine the conformational stability between the two conformers (109 ± 4 cm-1). By shedding light on the conformational properties of morpholine, this study brings far-reaching implications to the fields of organic synthesis and pharmaceutical research.

The stable conformations and vibronic and cation spectroscopy of 2-ethoxybenzonitrile

Ethoxy-substituted benzene has several stable conformations as the single bond in the ethoxy group is capable of internal rotation. Ethoxybenzonitrile may has more conformers due to different orientations of ethoxy with respect to the CN group. In this paper, the potential energy surface (PES) of 2-ethoxybenzonitrile in the ground state was calculated at the level of B3LYP/cc-pvdz, and five different molecular conformers were found on the PES. Only the most stable conformer I (trans) was observed in the supersonic molecular beam experiments. The resonance enhanced multiphoton ionization (REMPI) and mass analyzed threshold ionization (MATI) spectra of 2-ethoxybenzonitrile were measured and Franck-Condon simulations were performed, and the theoretical results are in good agreement with the experimental measurements. The similar molecular structures in S0, S1, and D0 states imply the large Franck-Condon factors. The vibronic structures in S1 and D0 states were analyzed in detail and the vibronic features were assigned. The MATI spectra follow well the propensity rule Δν = 0, indicating that the molecular structures of the cationic and excited states are similar. Most of the observed vibrations are associated with the ring in-plane distortion. The band origin of the S1←S0 transition and the accurate adiabatic ionization energy of 2-ethoxybenzonitrile are determined for the first time to be 34 092 ± 2 and 69 796 ± 5 cm−1, respectively.

Vibronic and Cationic Features of 2-Fluorobenzonitrile and 3-Fluorobenzonitrile Studied by REMPI and MATI Spectroscopy and Franck-Condon Simulations.

Fluorinated organic compounds have superior physicochemical properties than general organic compounds due to the strong C-F single bond; they are widely used in medicine, biology, pesticides, and materials science. In order to gain a deeper understanding of the physicochemical properties of fluorinated organic compounds, fluorinated aromatic compounds have been investigated by various spectroscopic techniques. 2-fluorobenzonitrile and 3-fluorobenzonitrile are important fine chemical intermediates and their excited state S1 and cationic ground state D0 vibrational features remain unknown. In this paper, we used two-color resonance two photon ionization (2-color REMPI) and mass analyzed threshold ionization (MATI) spectroscopy to study S1 and D0 state vibrational features of 2-fluorobenzonitrile and 3-fluorobenzonitrile. The precise excitation energy (band origin) and adiabatic ionization energy were determined to be 36,028 ± 2 cm-1 and 78,650 ± 5 cm-1 for 2-fluorobenzonitrile and 35,989 ± 2 cm-1 and 78,873 ± 5 cm-1 for 3-fluorobenzonitrile, respectively. The density functional theory (DFT) at the levels of RB3LYP/aug-cc-pvtz, TD-B3LYP/aug-cc-pvtz, and UB3LYP/aug-cc-pvtz were used to calculate the stable structures and vibrational frequencies for the ground state S0, excited state S1, and cationic ground state D0, respectively. Franck-Condon spectral simulations for transitions of S1 ← S0 and D0 ← S1 were performed based on the above DFT calculations. The theoretical and experimental results were in good agreement. The observed vibrational features in S1 and D0 states were assigned according to the simulated spectra and the comparison with structurally similar molecules. Several experimental findings and molecular features were discussed in detail.

Conformational diversity and environmental implications of trans-2-pentenal.

This study investigates the conformational intricacies of trans-2-pentenal (trans-2PA), a significant biogenic volatile organic compound. To unveil its potential implications in atmospheric chemistry and environmental pollution, we employ advanced infrared resonant vacuum ultraviolet mass-analysed threshold ionisation spectroscopy. Through this method, we identify the major conformers within trans-2PA, encompassing trans-s-trans (tt-) and trans-s-cis (tc-) structures with planar (cis) and non-planar (gauche) configurations introduced by a methyl group. In a pioneering spectroscopic examination, we analyze trans-2PA in both the neutral and cationic states. This approach allows us to gain a comprehensive understanding of its molecular behavior. Our conformer-specific vibrational spectra not only reveal the relative populations of the main conformers, notably tt-cis and tt-gauche conformers, but also shed light on atmospheric oxidation processes and lower tropospheric organic aerosol formation mechanisms. Our findings expand the understanding of the role of trans-2PA in environmental and biological contexts. Additionally, they contribute to a broader understanding of its influence on air quality, climate, and atmospheric dynamics. The collaboration between advanced experimental techniques and computational methods fortifies the scientific underpinning of this study, opening doors to further exploration in the realms of atmospheric chemistry and environmental science.

Valence molecular orbitals and cationic structures of 2-fluoropyridine by high-resolution ion spectroscopy and Franck-Condon fitting.

The alteration of the valence molecular orbitals' ordering of halopyridine molecules, by the introduction of a halogen atom(s) as substituent on the pyridine ring, has spurred an extensive interest for their investigation. Herein, the effect of a fluorine substituent on the two outermost orbitals of pyridine was elucidated by investigating the photoionization dynamics of 2-fluoropyridine (2-FP), considering that the geometrical changes with respect to the neutral geometry induced by adiabatic ionic transition affect the vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectrum. The adiabatic ionization energy associated with the 0-0 band on the measured high-resolution VUV-MATI spectrum was determined to be 9.6702 ± 0.0004eV (77 995 ± 3cm-1), which differs considerably from the 9.401eV by two-color ionization spectroscopy. Franck-Condon simulation of the MATI spectrum corresponded quantitatively with the experimental results. Interestingly, among the forbidden transitions under CS symmetry, an out-of-plane ring-bending mode resulting from the warped cationic structure of 2-FP with C1 symmetry was discovered. Rigorously, among the unassigned peaks, the first prominent peak at 78 532cm-1 should rather be assigned as the origin of the excited electronic state (D1) of the 2-FP cation, in accordance with time-dependent density functional theory calculations. Natural bond orbital analysis led to the conclusion that such observations could be induced by electron removal from the highest occupied molecular orbital (HOMO) consisting of the π orbital of the pyridine ring and lone-pair orbital of the fluorine atom or from the HOMO-1 of the molecular non-bonding orbitals, to generate the two proximate electronic states of the cation.

Ionization of Decamethylmanganocene: Insights from the DFT-Assisted Laser Spectroscopy.

Metallocenes represent one of the most important classes of organometallics with wide prospects for practical use in various fields of chemistry, materials science, molecular electronics, and biomedicine. Many applications of these metal complexes are based on their ability to form molecular ions. We report the first results concerning the changes in the molecular and electronic structure of decamethylmanganocene, Cp*2Mn, upon ionization provided by the high-resolution mass-analyzed threshold ionization (MATI) spectroscopy supported by DFT calculations. The precise ionization energy of Cp*2Mn is determined as 5.349 ± 0.001 eV. The DFT modeling of the MATI spectrum shows that the main structural deformations accompanying the detachment of an electron consist in the elongation of the Mn-C bonds and a change in the Me out-of-plane bending angles. Surprisingly, the DFT calculations predict that most of the reduction in electron density (ED) upon ionization is associated with the hydrogen atoms of the substituents, despite the metal character of the ionized orbital. However, the ED difference isosurfaces reveal a complex mechanism of the charge redistribution involving also the carbon atoms of the molecule.

Electronic states and transitions of PrO and PrO+ probed by threshold ionization spectroscopy and spin-orbit multiconfiguration perturbation theory.

The precise ionization energy of praseodymium oxide (PrO) seeded in supersonic molecular beams is measured with mass-analyzed threshold ionization (MATI) spectroscopy. A total of 33 spin-orbit (SO) states of PrO and 23 SO states of PrO+ are predicted by second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. Electronic transitions from four low-energy SO levels of the neutral molecule to the ground state of the singly charged cation are identified by combining the MATI spectroscopic measurements with the MCQDPT2 calculations. The precise ionization energy is used to reassess the ionization energies and the reaction enthalpies of the Pr+ O → PrO+ + e- chemi-ionization reaction reported in the literature. An empirical formula that uses atomic electronic parameters is proposed to predict the ionization energies of lanthanide monoxides, and the empirical calculations match well with available precise experimental measurements.

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.

Photoionization Spectroscopic and Theoretical Study on the Molecular Structures of cis- and trans-3-Chlorothioanisole.

Resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra are measured for the cis- and trans-3-chlorothioanisole (3ClTA). The first electronic excitation energy (E1) and the adiabatic ionization energy (IE) of the cis-rotamer are determined to be 33 959±3 and 65 326±5 cm–1, respectively, and those of the trans-rotamer are determined to be 34102±3 and 65 471±5 cm–1, respectively. Density functional theory (DFT) calculations confirm that both the cis- and trans-rotamers of 3ClTA are stable and coexist in their respective S0, S1, and D0 states. Both rotamers adopt planar structures with cis- being slightly more stable than trans- in the respective S0, S1, and D0 states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

Excited states of lutetium oxide and its singly charged cation.

Vibronic spectra of lutetium oxide (LuO) seeded in supersonic molecule beams are investigated with mass-analyzed threshold ionization (MATI) spectroscopy and second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. Six states of LuO and four states of LuO+ are located by the MCQDPT2 calculations, and an a3Π(LuO+) ← C2Σ+ (LuΟ) transition is observed by the MATI measurement. The vibronic spectra show abnormal vibrational intervals for both the neural and cation excited states, and the abnormality is attributed to vibrational perturbations induced by interactions with neighboring states.

Determination of highest occupied molecular orbital and cationic structures of cyclopentanone using high‐resolution vacuum ultraviolet mass‐analyzed threshold ionization mass spectrophotometry

AbstractThe conformational structure of cyclopentanone (CP) and the photoionization dynamics of the corresponding conformer were investigated using the high‐resolution vacuum ultraviolet mass‐analyzed threshold ionization (MATI) spectroscopic technique. To identify the conformer corresponding to the measured MATI spectrum, the potential energy surfaces associated with the twisting and carbonyl wagging motions in the S0 and D0 states were constructed at the B3LYP/cc‐pVTZ level, where the twisted conformer in both states corresponded to the global minimum. Furthermore, the Franck–Condon fitting indicated that the cationic conformer had a warped twisted C1 symmetry rather than C2 symmetry. This implies that the ionization‐induced ring distortion can be principally attributed to the removal of an electron from the highest occupied molecular orbital, which consists of the nonbonding orbital of the oxygen atom in the carbonyl group interacting with the σ orbitals in the ring of the twisted CP conformer.

Vibrational spectra of 2-cyanophenol cation studied by the mass analyzed threshold ionization technique

The ionization energy of 2-cyanophenol has been measured by both photoionization efficient spectroscopy and mass analyzed threshold ionization technique. The precise ionization energy of 2-cyanophenol is determined to be 73118 ± 5 cm−1 for the first time. The vibrational spectra of the cation are measured and analyzed in detail based on DFT calculations. The most active normal modes in the D0 state are assigned to the in-plane ring deformation.

Studies of the First Electronically Excited State of 3-Fluoropyridine and Its Ionic Structure by Means of REMPI, Two-Photon MATI, One-Photon VUV-MATI Spectroscopy and Franck–Condon Analysis

3-Fluoropyridine has been investigated by means of resonance-enhanced multi photon ionization, mass-analyzed threshold ionization (MATI) and one-photon VUV-MATI spectroscopy to study the effect of m-fluorine substitution on the involved states.

Two-color resonance enhanced two-photon ionization and mass analyzed threshold ionization spectroscopy of <i>p</i>-chlorobenzonitrile

The vibrational features of <i>p</i>-chlorobenzonitrile in its first electronically excited state S<sub>1</sub> and cationic ground state D<sub>0</sub> have been investigated by two-color resonance enhanced two-photon ionization and mass analyzed threshold ionization spectroscopy. The excitation energy of S<sub>1</sub> ← S<sub>0</sub> and the ionization energy of <sup>35</sup>Cl and <sup>37</sup>Cl isotopomers of <i>p</i>-chlorobenzonitrile are determined to be 35818 ± 2, and 76846 ± 5 cm<sup>–1</sup>, respectively. These two isotopomers have similar vibrational features. Most of the active vibrations in the S<sub>1</sub> and D<sub>0</sub> states are related to the motions of the in-plane ring deformation. The stable structures and vibrational frequencies of <i>p</i>-chlorobenzonitrile are also calculated by the B3LYP/aug-cc-pVDZ method for the S<sub>0</sub> and D<sub>0</sub> states, and TD-B3LYP/aug-cc-pVDZ method for the S<sub>1</sub> state. The changes in the molecular geometry are discussed in the S<sub>1</sub> ← S<sub>0</sub> photoexcitation process and the D<sub>0</sub> ← S<sub>1</sub> photoionization process. The comparisons between the transition energy of <i>p</i>-chlorophenol, <i>p</i>-chloroaniline, <i>p</i>-chloroanisole, and <i>p</i>-chlorobenzonitrile with those of phenol, anisole, aniline, and benzonitrile provide an insight into the substitution effect of Cl atom.

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 123.6-160.0nm, 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 (∼5cm-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-pointlevel 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.

Development and Verification of Conformer-Specific Vibrational Spectroscopy.

Conformers have similar vibrational structures both in neutral (S0) and cationic (D0) states owing to the comparable force fields between their nuclei. Nevertheless, there is a continuous development of vibrational spectroscopic techniques to rigorously identify individual conformers in the designated molecule but only in the S0 state. We developed an inventive conformer-specific vibrational spectroscopic technique to measure identifiable vibrational spectra of individual conformers in both S0 and D0 states. We measured isomer-specific vibrational spectra in both states for gas-phase acetone and oxetane isomers from a solution with azeotropic composition to verify the proposed techniques that are based on infrared (IR) resonant vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The measured IR dip VUV-MATI and IR hole-burn VUV-MATI spectra for each isomer, which correspond to isomer-specific vibrational spectra in both states, can be represented by IR-resonant VUV photoionization and one-photon VUV-MATI spectra of the binary mixture, respectively, under supersonic expansion conditions. The partial pressures of the individual isomers in the binary mixture with different mole fractions estimated according to the relative peak intensities in the measured spectra provide insights on solute-solvent interactions. We suggest that the verified IR-resonant VUV-MATI spectroscopy can form the basis of effective schemes toward conformational chemistry.

Vibronic transitions and spin-orbit coupling of three-membered metallacycles formed by lanthanide-mediated dehydrogenation of dimethylamine.

Metal-mediated N-H and C-H bond activation of aliphatic amines is an effective strategy for synthesizing biologically important molecules. Ln (Ln = La and Ce) atom reactions with dimethylamine are carried out in a pulsed-laser vaporization supersonic molecular beam source. A series of dehydrogenation species are observed with time-of-flight mass spectrometry, and the dehydrogenated Ln-containing species in the formula Ln(CH2NCH3) are characterized by single-photon mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical calculations. The theoretical calculations include density functional theory for both Ln species and multiconfiguration self-consistent field and quasi-degenerate perturbation theory for the Ce species. The MATI spectrum of La(CH2NCH3) consists of a single vibronic band system, which is assigned to the ionization of the doublet ground state of N-methyl-lanthanaaziridine. The MATI spectrum of Ce(CH2NCH3) displays two vibronic band systems, which are attributed to the ionization of two-pair lowest-energy spin-orbit coupling states of N-methyl-ceraaziridine. Both metallaaziridines are three-membered metallacycles and formed by the thermodynamically and kinetically favorable concerted dehydrogenation of the amino group and one of the methyl groups.