Mass-analyzed Threshold Ionization Spectroscopy Research Articles

Dissociation energetics of the phenol+⋯Ar2 cluster ion: The role of π→H isomerization

The dissociation energetics in the phenol(+)⋯Ar(2)(2π) cluster ion have been investigated using photoionization efficiency and mass analyzed threshold ionization spectroscopy. The appearance energies for the loss of one and two Ar atoms are determined as ∼210 and ∼1115 cm(-1), respectively. The difference between the appearance energy for the first Ar ligand in phenol(+)⋯Ar(2)(2π) and the dissociation energy of the phenol(+)⋯Ar(π) dimer (535cm(-1)) is explained by the isomerization of one π-bound Ar ligand to the OH binding site (H-bond) upon ionization. The energy difference between phenol(+)⋯Ar(2)(2π) and phenol(+)⋯Ar(2)(H/π) could also be estimated to be around 325cm(-1), which corresponds roughly to the difference of the binding energy of a π-bound and H-bound Ar ligands. The binding energy of the H-bound Ar atom in phenol(+)⋯Ar(2)(H/π) is derived to be ∼905cm(-1).

Fragmentation Energetics of the Phenol+···Ar3 Cation Cluster

The various dissociation thresholds of phenol(+)···Ar(3) complexes for the consecutive loss of all three Ar ligands were measured in a molecular beam using resonant photoionization efficiency and mass analyzed threshold ionization spectroscopy via excitation of the first excited singlet state (S(1)). The adiabatic ionization energy is derived as 68077 ± 15 cm(-1). The analysis of the dissociation thresholds demonstrate that all three Ar ligands in the neutral phenol···Ar(3) tetramer are attached to the aromatic ring via π-bonding, denoted phenol···Ar(3)(3π). The value of the dissociation threshold for the loss of one Ar ligand from phenol(+)···Ar(3)(3π), ∼190 cm(-1), is significantly lower than the binding energy measured for the π-bonded Ar ligand in the phenol(+)···Ar(π) dimer, D(0) = 535 ± 3 cm(-1). This difference is rationalized by an ionization-induced π → H isomerization process occurring prior to dissociation, that is, one Ar atom in phenol(+)···Ar(3)(3π) moves to the OH binding site, leading to a structure with one H-bonded and 2 π-bonded ligands, denoted phenol(+)···Ar(3)(H/2π). The dissociation thresholds for the loss of two and three Ar atoms are also reported as 860 and 1730 cm(-1). From these values, the binding energy of the H-bound Ar atom can be estimated as 870 cm(-1).

Mass-Analyzed Threshold Ionization Spectroscopy of Ortho Fluorinated 2-Phenylethanol: Identification of an Additional Gauche Conformer

The cationic ground state of the ortho fluorinated 2-phenylethanol has been investigated by combination of mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemistry ab initio density functional theory (DFT) calculations employing the hybrid functional M05 with cc-pVDZ basis set. The MATI spectra measured via vibronic bands in the S1 intermediate state of the most stable gauche conformer stabilized by an intramolecular OH···π hydrogen bond are structureless. The MATI spectrum recorded via a small band blueshifted by 3 cm−1 from the 0(0)(0) electronic origin of the gauche conformer features well-resolved peaks and is assigned to a cationic gauche structure without an OH···π bond. The ab initio calculations are qualitatively consistent with the experimental observations and show that the presumable conformer giving rise to the observed MATI spectrum retains its structure during ionization, whereas the lowest-energy gauche conformer undergoes a significant structural change resulting in a break of the OH···π bond, thus leading to unfavorable Franck−Condon factors for the threshold ionization.

Rotamers of o- and m-Dimethoxybenzenes Studied by Mass-Analyzed Threshold Ionization Spectroscopy and Theoretical Calculations

We applied two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy to investigate the molecular properties of the selected rotamers of o-dimethoxybenzene (ODMB) and m-dimethoxybenzene (MDMB). The present experimental results show that only one stable configuration is involved in the photoexcitation and ionization processes of ODMB, as predicted by theoretical calculations. The adiabatic ionization energy (IE) of ODMB is measured to be 61 617 ± 5 cm(-1). In the case of MDMB, both our experimental and calculated results suggest that there are three stable rotamers coexisting in the sample. The adiabatic IEs of rotamers a, b, and c are determined to be 63 523 ± 5, 64 491 ± 5, and 63 758 ± 5 cm(-1). Analysis on the MATI spectra shows that most of the active cation vibrations of these isomeric species result from in-plane ring motions. In addition, different orientations of the two OCH(3) groups have little effect on these vibrations.

Aromatic π–π interaction mediated by a metal atom: structure and ionization of the bis(η6-benzene)chromium–benzene cluster

Aromatic pi-pi interaction in the presence of a metal atom has been investigated experimentally and theoretically with the model system of bis(eta(6)-benzene)chromium-benzene cluster (Cr(Bz)(2)-Bz) in which a free solvating benzene is non-covalently attached to the benzene moiety of Cr(Bz)(2). One-photon mass-analyzed threshold ionization (MATI) spectroscopy and first principles calculations are employed to identify the structure of Cr(Bz)(2)-Bz which adopts the parallel-displaced configuration. The decrease in ionization potential for Cr(Bz)(2)-Bz compared with Cr(Bz)(2), resulting from the increase of the cation-pi stabilization energy upon ionization, is consistent with the parallel-displaced structure of the cluster. Theoretical calculations give the detailed cluster structures with associated energetics, thus revealing the nature of pi-pi-metal or pi-pi-cation interactions at the molecular level.

Mass-analyzed Threshold Ionization Spectroscopy of Rotamers ofp-ethoxyphenol Cations and Configuration Effect

Two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy was used to record the vibrationally resolved cation spectra of the selected rotamers of p-ethoxyphenol. The adiabatic ionization energies of the trans and cis rotamers are determined to be 61565 5 and 61670 5 cm1, which are less than that of p-methoxyphenol by 645 and 643 cm1, respectively. Analysis on the MATI spectra of the selected rotamers of p-ethoxyphenol cation shows that the relative orientation of the ethoxy group has little effect on the in-plane ring vibrations. The low-frequency OC2H5 bending vibrations appear to be active for both forms of the cation.

Mass-Analyzed Threshold Ionization Spectroscopy of 2-Phenylethanol: Probing of Conformational Changes Caused by Ionization

The vibrational structure of the ionic ground state of different conformers of the biologically relevant molecule 2-phenylethanol has been investigated by combination of two-photon two-color mass-analyzed threshold ionization spectroscopy (MATI) and quantum chemical calculations at M05, MP2, and coupled cluster (CC) levels of theory with extended basis sets. MATI spectra recorded via gauche vibronic bands are with poor structure and increasing background, whereas the ones measured via vibronic bands of the anti conformers feature well-resolved vibronic structure in the cation. Ab initio computations predict three stable conformers for the 2-phenylethanol cation out of five initial neutral structures. None of the theoretical structures in the cation features a nonclassical OH...pi hydrogen bond in conjunction with the analysis of the MATI spectra. This provides clear evidence that the OH...pi hydrogen bond stabilizing the lowest-energy gauche conformer in the neutral breaks upon ionization.

Mass-Analyzed Threshold Ionization (MATI) Spectroscopy of Atoms and Molecules Using VUV Synchrotron Radiation

Mass-analyzed threshold ionization (MATI) spectroscopy using synchrotron radiation (Advanced Light Source, Lawrence Berkeley National Laboratory) has been performed for Ar, N(2), O(2), N(2)O, H(2)O, C(2)H(2), and C(6)H(6). MATI allows for a better determination of ionization energies compared to those derived from photoionization efficiency curves traditionally used in synchrotron photoionization mass spectrometry. The separation of the long-lived Rydberg state from the directly formed prompt ion, essential for a meaningful MATI spectrum, has been accomplished by employing an arrangement of ion optics coupled to unique electric field pulsing schemes. For Ar, a number of resolved bands below the ionization energy are observed, and these are ascribed to high-n,l Rydberg states prepared in the MATI scheme. The first vibrational state resolved MATI spectra of N(2) and O(2) are reported, and spectral characteristics are discussed in comparison with previously reported threshold photoelectron spectroscopic studies. Although MATI performed with synchrotron radiation is intrinsically less sensitive compared to laser-based sources, this work demonstrates that MATI spectroscopy performed with widely tunable vacuum ultraviolet (VUV) radiation is a complementary technique for studying the ionization spectroscopy of polyatomic molecules.

Conformationally Specific Vacuum Ultraviolet Mass-Analyzed Threshold Ionization Spectroscopy of Alkanethiols: Structure and Ionization of Conformational Isomers of Ethanethiol, Isopropanethiol, 1-Propanethiol, tert-Butanethiol, and 1-Butanethiol

Conformational isomers of alkanethiols are isolated in the molecular beam, and the conformer-specific ionization dynamics have been investigated using vacuum ultraviolet mass-analyzed threshold ionization (MATI) spectroscopy. Only a single conformer of ethanethiol is observed to give the adiabatic ionization potential (IP) of 9.2922 +/- 0.0007 eV for the gauche conformer. For isopropanethiol, IP is found to be 9.1426 +/- 0.0006 for the trans conformer and 9.1559 +/- 0.0006 eV for the gauche conformer. Only two major conformational isomers are identified for 1-propanethiol, giving an IP of 9.1952 +/- 0.0006 for the trans-gauche conformer and 9.2008 +/- 0.0006 eV for the gauche-gauche conformer. The tert-butanethiol, as expected, has a single conformer with an IP of 9.0294 +/- 0.0006 eV. For 1-butanethiol, there are a number of conformers, and the assignment of the MATI bands to each conformer turns out to be nontrivial. The spectral simulation using the Franck-Condon analysis based on the density functional theory (DFT) calculations has been used for the identification of each conformational isomer in the MATI spectrum. Each conformer undergoes its unique structural change upon ionization, as revealed in the vibration resolved MATI spectrum, providing the powerful method for the spectral identification of a specific conformational isomer. The conformer specificity in the ionization-driven structural change reflects the role of the electron of the highest occupied molecular orbital (HOMO) in the conformational preference.

Cation Spectroscopy and Binding Energy Determination for 1,4-Benzodioxan−Ar1 and −Ar2 Complexes

Cation vibronic spectra are measured for 1,4-benzodioxan (BZD) and van der Waals complexes of BZD with one and two Ar atoms using zero electron kinetic energy and mass analyzed threshold ionization spectroscopy. The spectra of the monomer cation were used to measure the frequencies of the two key low-frequency modes which had previously been extensively studied in the neutral S0 and S1 states. The aliphatic ring twisting mode, nu25, has an energy of 146 cm(-1) in the cation, intermediate between the values found in the S0 and S1 states. The bending, butterfly-like mode nu48 has an energy of 125 cm(-1), which is of higher frequency than either of the neutral states. The S1 spectra of the BZD-Ar1 and BZD-Ar2 complexes are recorded and observed to have modest red shifts from the monomer. The cation spectra of the complexes are also measured using mass analyzed threshold ionization spectroscopy including scans at higher energy which are used to determine the Ar binding energies. The energies for the loss of one Ar atom were determined to be 630 +/- 10 and 650 +/- 10 cm(-1) for BZD-Ar and BZD-Ar2, respectively. The similar cation spectra and similar binding energies indicate that each Ar atom in BZD-Ar2 has a similar binding geometry. Quantum chemical calculations were performed which had fair agreement with the measured binding energies and give some insight into the specific binding geometry.

Effect of Noncovalent Interactions on the n-Butylbenzene···Ar Cluster Studied by Mass Analyzed Threshold Ionization Spectroscopy and ab initio Computations

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB...Ar clusters, and red shifts for anti-BB...Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I...Ar and BB-V...Ar were determined as 70052 and 69845 +/- 5 cm (-1), respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I...Ar and BB-V...Ar were obtained as 650 and 558 cm (-1), respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB...Ar in the excited neutral S 1 and the D 0 cationic ground states.

Effect of Noncovalent Interactions on Conformers of the n-Butylbenzene Monomer Studied by Mass Analyzed Threshold Ionization Spectroscopy and Basis-set Convergent ab initio Computations

Two conformational isomers of the aromatic hydrocarbon n-butylbenzene have been studied using two-color MATI (mass analyzed threshold ionization) spectroscopy to explore the effect of conformation on ionization dynamics. Cationic states of g auche-conformer III and anti- conformers IV were selectively produced by two-color excitation via the respective S 1 origins. Adiabatic ionization potentials of the gauche- and anti-conformations were determined to be 70146 and 69872 +/- 5 cm (-1) respectively. Spectral features and vibrational modes are interpreted with the aid of MP2/cc-pVDZ ab initio calculations, and ionization-induced changes in the molecular conformations are discussed. Complete basis set (CBS) ab initio studies at MP2 level reveal reliable energetics for all four n-butylbenzene conformers observed in earlier two-color REMPI (resonance enhanced multiphoton ionization) spectra. For the S 0 state, the energies of conformer III, IV and V are above conformer I by 130, 289, 73 cm (-1), respectively. Furthermore, the combination of the CBS calculations with the measured REMPI, MATI spectra allowed the determination of the energetics of all four conformers in the S 1 and D 0 states.

Ionization Spectroscopy of Conformational Isomers of Propanal: The Origin of the Conformational Preference

Two different conformational isomers of propanal, cis and gauche, are investigated by the vacuum-UV mass-analyzed threshold ionization (VUV-MATI) spectroscopy to give accurate adiabatic ionization potentials of 9.9997 +/- 0.0006 eV and 9.9516 +/- 0.0006 eV, respectively. cis-Propanal, which is the more stable conformer in the neutral state, becomes less stable in the cation compared to gauche-propanal. Vibrational structures revealed in the MATI spectra indicate that cis and gauche isomers undergo their unique structural changes upon ionization. The ionization of gauche-propanal induces a geometrical change along the conformational coordinate, suggesting that the steric effect in the ground state is diminished upon ionization. Natural bonding orbital (NBO) calculations provide the extent of hyperconjugation in each conformational isomer of propanal.

Binding energies and dissociation pathways in the aniline-Ar2 cation complex

Mass analyzed threshold ionization spectroscopy is used to measure the Ar binding energy for the cationic aniline-Ar (An(+)-Ar) and aniline-Ar(2) (An(+)-Ar(2)) complexes. Since the experiments begin with the neutral species, photoexcitation creates the cations in the pi-bonding configuration with the Ar located above the phenyl ring. The binding energy in this conformation of the An(+)-Ar complex is determined to be 495+/-15 cm(-1). Measurements of An(+)-Ar(2) revealed the production of a lower energy dissociation product which is assigned to the An(+)-Ar H-bonding configuration. Combinations of measurements allow determination of the dissociation energy of this complex to be 640+/-20 cm(-1). The observation of a more stable H-bonded conformer is consistent with recent infrared experiments on An(+)-Ar complexes created by complexing An(+) with Ar, rather than creation through the neutral complex. Calculations are presented which closely reproduce the binding energy of the pi bound Ar but underestimate the stability of the H-bonded species.

Detailed analysis of the cation ground state of three dichlorobenzenes by mass analyzed threshold ionization spectroscopy

Two color resonant mass analyzed threshold ionization (MATI) spectroscopy was applied in order to investigate the ionic properties of the structural isomers of dichlorobenzene above the ionization threshold. A detailed analysis of the vibrational properties of the two main chlorine isotopomers of each isomer was performed resulting in a precise picture of the vibrational states in the cation ground states. The general features of the two investigated isotopomers turned out to be quite similar, although some vibrational modes are slightly different up to a few wavenumbers, except for the 6b normal mode of p-dichlorobenzene. There the spectrum of the (35)Cl(2)-isotopomer shows several additional peaks compared to the one of the (35)Cl(37)Cl-isotopomer. An explanation for the extraordinary observance is discussed. In addition to the examination of the ionic ground state the measured and calculated frequencies were used to confirm the former assignment of the vibrations in the S(1) state, especially with respect to a Herzberg-Teller coupling as observed in benzene.

One-photon ionization spectroscopy of jet-cooled oxazole and thiazole: the role of oxygen and sulfur in the π-conjugation of heterocyclic compounds

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of jet-cooled oxazole and thiazole has been carried out to give the precise adiabatic ionization energies of 9.5959+/-0.0006 and 9.3633+/-0.0009 eV, respectively. The structural change upon ionization has been revealed in the vibrationally resolved one-photon MATI spectra. Simulations based on the Franck-Condon analysis using the molecular structures calculated by the density functional theory reproduce the experiment very well for both molecules. The ionization-driven structural change of thiazole is quite different from that of oxazole in terms of the detailed geometrical shape, ascribed to the difference in the pi-conjugation nature of two molecules. The role of oxygen and sulfur in the stabilization of heterocyclic systems is discussed through the inspection of the calculated molecular orbitals involved in the photoionization.

A theoretical analysis on the vibronic spectra with mass analyzed threshold ionization spectroscopy

Abstract We have theoretically studied the absorption vibronic spectra with the resonance two-photon (R2P) and non-resonance two-photon (NR2P) mass analyzed threshold ionization (MATI) spectroscopy. The theory uses the inverse Born–Oppenheimer approximation (IBOA) to establish a proper basis set. To analyze the MATI vibronic spectra, we have calculated the Franck–Condon factors involved in the vibronic transitions. Several experimental spectra are analyzed using this theory with emphasis on the importance of the resonance intermediate states. The long vibrational progression in a MATI spectrum can be partly attributed to the result of including the anharmonic correction in the calculated Franck–Condon factors. The experimentally observed isotope effect is also analyzed.

Direct Detection of Individual Bis(arene) Rotational Isomers in the Gas Phase by Mass‐Analyzed Threshold Ionization Spectroscopy

It's a gas! Mass-analyzed threshold ionization (MATI) spectroscopy provides high-resolution ionization potentials and low-energy vibrational frequencies of individual conformers of jet-cooled [(?6-PhMe)2Cr]. The signals arising from distinct structural isomers were identified on the basis of DFT calculations and comparison of the [(?6-PhMe) 2Cr] and [(?6-1,3-Me2C6H 4)(?6-PhH)Cr] spectra (see picture). © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

Resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of the selected rotamers of m-methoxyaniline and o-methoxyaniline

We report the resonant two-photon ionization and mass-analyzed threshold ionization (MATI) spectra of m-methoxyaniline and o-methoxyaniline. The vibronic features of m-methoxyaniline are built on 34308 ± 2 and 34495 ± 2 cm −1 corresponding to the origins of the S 1 ← S 0 electronic transition ( E 1’s) of the cis and trans rotamers. Analysis of the MATI spectra gives the adiabatic ionization energies (IEs) of 59983 ± 5 and 60879 ± 5 cm −1 for these two species. o-Methoxyaniline is found to have only one stable structure whose E 1 and IE are 33875 ± 2 and 58678 ± 5 cm −1, respectively. Most of the active vibrations of m- and o-methoxyaniline in the electronically excited S 1 and cationic ground D 0 states result from the in-plane ring vibrations. Comparing these data with those of p-methoxyaniline allows us to learn about the vicinal substitution effects resulting from the relative locations of the NH 2 and OCH 3 substituents.

Mass-Analyzed-Threshold-Ionization-Spectroscopy of Pyrazine and Pyrazine-Ar

Vibrational spectra of the pyrazine and the pyrazine-Ar cation in the electronic ground state (D 0) have been measured via several intermediate states of the first excited state (S 1) by mass-analyzed threshold ionization spectroscopy. Additionally, ab-initio calculations at the DFT, CIS and MP2 level of theory have been conducted to compare experimental findings with theory. For the pyrazine monomer a negative anharmonic behavior for combination bands containing the 16a or the 1 vibration could be observed for the first time. It was found that the 16a vibration has to be considered when discussing the nature of the vibronic coupling in the first excited state (S 1) as well as in the ionic ground state (D 0). For some vibrations a new assignment is given in the first excited state (S 1) and the ionic ground state (D 0). An earlier discussed appearance of internal vibrational redistribution (IVR) by activating the 516a1 vibrational state in the first electronically excited state (S 1) is rejected due to the results of the presented measurements. For the pyrazine-Ar complex the ionization energy, as well as the binding energy in the ionic and the neutral ground state could be determined. These values are given by 74632 ± 5 cm-1, 630 cm-1 ± 20 cm-1 and 349 cm-1 ± 20 cm-1, respectively. The van-der-Waals vibrations in the first excited state could be reassigned and the frequencies of these vibrations have been found to be 12 cm-1, 27 cm-1 and 39 cm-1.