Mass-analyzed Threshold Ionization Research Articles

An improved experimental scheme for simultaneous measurement of high-resolution zero electron kinetic energy (ZEKE) photoelectron and threshold photoion (MATI) spectra

An improved detection scheme is presented for threshold ionization spectroscopy with simultaneous recording of the Zero Electron Kinetic Energy (ZEKE) and Mass Analysed Threshold Ionisation (MATI) signals. The objective is to obtain accurate dissociation energies for larger molecular clusters by simultaneously detecting the fragment and parent ion MATI signals with identical transmission. The scheme preserves an optimal ZEKE spectral resolution together with excellent separation of the spontaneous ion and MATI signals in the time-of-flight mass spectrum. The resulting improvement in sensitivity will allow for the determination of dissociation energies in clusters with substantial mass difference between parent and daughter ions.

Two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of 4-chlorostyrene

The first electronic excitation and adiabatic ionization energies of 35Cl and 37Cl 4-chlorostyrene were similar, with values of 33,977±2 and 67,972±5cm−1, respectively. The general features in the obtained vibronic and cation spectra of the two isotopologues were similar. A frequency shift of 1–5cm−1 was observed on many active vibrations of the 35Cl and 37Cl isotopologues of 4-chlorostyrene in the S1 and D0 states. This frequency difference at each mode may reflect the degree of Cl atom involvement in the overall vibration.

Structure and vibrations of 2-fluoro-N-methylaniline in the S0, S1 and D0 states: REMPI and MATI spectroscopy and theoretical calculations

Theoretical calculations predicted that there are only two stable conformers, trans and cis, for 2-fluoro-N-methylaniline (2FNMA) in the S0, S1 and D0 states. Compared to the cis conformer, the trans one is more stable, and has a population more than 99% at room temperature. The optimized molecular skeleton of trans and cis 2FNMA are both non-planar in the S0 state, but planar in the S1 and D0 states. The one-dimensional potential energy surface of 2FNMA in the S0 state is obtained. The Resonance-enhanced two-photon ionization (R2PI) and Mass-analyzed threshold ionization (MATI) spectra of trans 2FNMA are obtained. The first electronic excitation energy (E1) and the adiabatic ionization energy (IE) of trans 2FNMA are determined. The substitution effect on the molecular structures, transition energies and vibrations of 2FNMA are discussed.

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by C—C bond cleavage and coupling of propene

La reaction with propene is carried out in a laser-vaporization molecular beam source. Three La-hydrocarbon radicals are characterized by mass-analyzed threshold ionization (MATI) spectroscopy. One of these radicals is methylenelanthanum [La(CH2)] (Cs), a Schrock-type metal carbene. The other two are a five-membered 1-lanthanacyclopent-3-en [La(CH2CHCHCH2)] (Cs) and a tetrahedron-like trimethylenemethanelanthanum [La(C(CH2)3)] (C3v). Adiabatic ionization energies and metal-ligand stretching and hydrocarbon-based bending frequencies of these species are measured from the MATI spectra, preferred structures and electronic states are identified by comparing the experimental measurements and spectral simulations, and reaction pathways for the formation of the metal-hydrocarbon radicals are investigated with density functional theory calculations. All three radicals prefer doublet ground electronic states with La 6s1-based valence electron configurations, and singly charged cations favor singlet states generated by the removal of the La 6s1 electron. The metal-carbene radical is formed via multi-step carbon-carbon cleavage involving metallacyclization, β-hydrogen migration, and metal insertion. The metal-carbene radical formed in the primary reaction reacts with a second propene molecule to form the five-membered-ring and tetrahedron-like isomers through distinct carbon-carbon coupling paths.

REMPI and MATI spectroscopy of cis and trans 4-chloro-3-fluoroanisole

The Resonance-enhanced two-photon ionization (R2PI) and Mass-analyzed threshold ionization (MATI) spectra of 4-chloro-3-fluoroanisole (4Cl3FA) were recorded in detail. The experimental and calculated results revealed that cis and trans 4Cl3FA are the only two stable conformers in each of the S0, S1 and D0 states. The first electronic excitation energies (E1’s) of cis and trans35Cl-4Cl3FA were measured to be 35,326±3 and 35,443±3cm−1 by the R2PI spectroscopy and the corresponding adiabatic ionization energies (IEs) were determined to be 67,330±5 and 67,591±5cm−1 by the MATI spectroscopy. The E1’s and IEs of cis and trans37Cl-4Cl3FA were also obtained and found to be identical to that of the 35Cl-4Cl3FA conformers. Compared with the trans 4Cl3FA, the cis 4Cl3FA is more stable in the S0, S1 and D0 states. The vibrational bands observed in the R2PI and MATI spectra are related to the vibrations involving the in-plane deformation of the benzene ring, the CH3 torsion and the OCH3 inversion. The substituent effects on the molecular structures, transition energies and vibrations of 4Cl3FA are discussed.

Ionization spectroscopies and theoretical calculations of cis and trans 3-fluoro-N-methylaniline-Arn(n = 1,2) van der Waals clusters: Structures and binding energies

The ab initio and dispersion-corrected density functional theory (DFT) calculations of the van der Waals (vdW) clusters of cis and trans 3-fluoro-N-methylaniline-Arn (3FNMA-Arn) (n=1,2) predict that, for cis and trans 3FNMA-Ar1 clusters, the π-bound geometry with the Ar atom sitting over the ring is the global minimum in both neutral and cationic states, while for cis and trans 3FNMA-Ar2 clusters, the [π|π]-bound sandwich structure with two Ar ligands occupying nearly equivalent π-bound positions above and below the ring plane is the global minimum. The vibronic spectra of cis and trans 3FNMA-Ar1 clusters in the S1 state were recorded by using one-color and two-color resonant two-photon ionization (R2PI) techniques, the comparison of which yields an estimate of the binding energy of cluster in the S1 state. It is found that the linear correlation between the redshift of the S1←S0 electronic transition energy (E1) of cluster and the E1 of the monomer also holds for the Ar clusters of hetero-di-substituted aromatics. By recording the photoionization efficiency (PIE) curves and mass-analyzed threshold ionization (MATI) dissociation spectra of cis and trans 3FNMA-Ar1 clusters, the ionization energies (IE) and binding energies of clusters in the D0 state are obtained. The MATI spectra of the cis and trans 3FNMA-Ar1 cations exhibit significant progressions of the vdW bending mode (bx), indicating the structural changes of the clusters in the bending coordinate upon ionization. The combination of the three kinds of spectroscopic methods provides the binding energies of cis and trans 3FNMA-Ar1 clusters in the S0, S1 and D0 states with good accuracy, and the values estimated by dispersion-corrected DFT calculations compare well with the experimental results. From our studies, the ωB97X-D and TD-ωB97X-D methods using high quality basis set are recommended for studying the intermolecular interaction of such vdW clusters in the ground and excited states.

Ring-Puckering Potential Energy Functions for Trimethylene Sulfide and Its Monovalent Cation.

The spectra and ring-puckering potential energy function for trimethylene sulfide cation (TMS+) from vacuum ultraviolet mass-analyzed threshold ionization spectra have recently been reported. To provide an in-depth comparison of the potential function with that of trimethylene sulfide (TMS) itself, we have used ab initio MP2/cc-pVTZ calculations and DFT B3LYP/cc-pVTZ calculations to predict the structures of both TMS and TMS+ and then used these to calculate coordinate-dependent ring-puckering kinetic energy functions for both species. These kinetic energy functions allowed us to calculate refined potential energy functions of the puckering for both molecules based on the previously published spectra. TMS has an experimental barrier of 271 cm-1 and energy minima at ring-puckering angles of ±29°. For TMS+ the barrier is 60 cm-1 and the energy minima correspond to ring-puckering angles of ±21°. The lower barrier for the cation reflects the smaller amount of angle strain in the ring angles for TMS+.

REMPI and MATI spectroscopy of cis and trans 3-chloro-N-methylaniline

The Resonance-enhanced two-photon ionization (R2PI) and Mass-analyzed threshold ionization (MATI) spectra of 3-Chloro-N-methylaniline (3ClNMA) were recorded in detail. The experimental and calculated results revealed that cis and trans 3ClNMA are the only two stable conformers for each of the S0, S1 and D0 states. The first electronic excitation energies (E1s) of cis and trans35Cl-3ClNMA were measured to be 33,003±3 and 32,886±3cm−1 by R2PI spectroscopy. The adiabatic ionization energies (IEs) of cis and trans35Cl-3ClNMA were determined to be 61,531±5 and 61,625±5cm−1 by MATI spectroscopy. The E1s and IEs of cis and trans37Cl-3ClNMA were also obtained and found to be identical to that of the 35Cl-3ClNMA rotamers. Compared with the trans rotamer, the cis rotamer is less stable by 50cm−1 in the S1 state, but more stable by 67 and 161cm−1 in S0 and D0 states. Most of the active vibration modes observed in the R2PI and MATI spectra are in-plane ring deformation and substituent-sensitive modes. Detailed analysis reveals a correlation between the typical ring vibration frequencies and the relative stability of the two rotamers, suggesting that the conformation effect on the ring rigidity plays a key role in determining the relative stability of the conformers. The ab initio and density functional theory (DFT) calculations were performed to analyze the substitution and conformation effects on the properties of 3ClNMA rotamers.

Mass-analyzed threshold ionization spectroscopy of lanthanum-hydrocarbon radicals formed by C-H bond activation of propene.

La(C3H4) and La(C3H6) are observed from the reaction of laser-vaporized La atoms with propene by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. Two isomers of La(C3H4) are identified as methyl-lanthanacyclopropene [La(CHCCH3)] (Cs) and lanthanacyclobutene [La(CHCHCH2)] (C1); La(C3H6) is determined to be H-La(η3-allyl) (Cs), a C-H bond inserted species. All three metal-hydrocarbon radicals prefer a doublet ground state with a La 6s-based electron configuration. Ionization of the neutral doublet state of each of these radicals produces a singlet ion state by removing the La-based 6s electron. The threshold ionization allows accurate measurements of the adiabatic ionization energy of the neutral doublet state and metal-ligand and ligand-based vibrational frequencies of the neutral and ionic states. The formation of the three radicals is investigated by density functional theory computations. The inserted species is formed by La inserting into an allylic C-H bond and lanthanacyclopropene by concerted vinylic H2 elimination, whereas lanthanacyclobutene involves both allylic and vinylic dehydrogenations. The inserted species is identified as an intermediate for the formation of lanthanacyclobutene.

Spectroscopic Characterization of Nonconcerted [4 + 2] Cycloaddition of 1,3-Butadiene with Lanthanacyclopropene To Form Lanthanum-Benzene in the Gas Phase.

The reaction between La atoms and 1,3-butadiene is carried out in a laser-vaporization molecular beam source. Metal-hydrocarbon species with formulas La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6) are observed with time-of-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum-benzene complex [La(C6H6)] is formed by 1,3-butadiene addition to lanthanacyclopropene [La(C2H2)] followed by molecular hydrogen elimination. Lanthanacyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3-butadiene reaction are La[η4-(1-buten-3-yne)] [La(C4H4)] and 1-lanthanacyclopent-3-ene [La(C4H6)]. The La(benzene) complex exhibits distinctive metal-ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations.

Observation of the Ring-Puckering Vibrational Mode in Thietane Cation

We have measured the high-resolution vibrational spectra of a thietane (trimethylene sulfide) cation in the gas phase by employing the vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopic technique. Peaks in the low-frequency region of the observed MATI spectrum of thietane originate from a progression of the ring-puckering vibrational mode (typical in small heterocyclic molecules), which is successfully reproduced by quantum-chemical calculations with 1D symmetric double-well potentials along the ring puckering coordinates on both the S0 and D0 states, the ground electronic states of neutral and cation of thietane, respectively. The values of the interconversion barrier and the ring-puckering angle on the S0 state, the parameters used for the quantum-chemical calculations, were assumed to be 274 cm-1 and 26°. The barrier and the angle on the D0 state, however, are found to be 48.0 cm-1 and 18.2°, respectively, where such small barrier height and puckering angle for the cation suggest that the conformation of thietane cation on the D0 state should be more planar than that of the thietane neutral.

Conformational structures of the tetrahydrofuran cation determined using one-photon mass-analyzed threshold ionization spectroscopy.

One-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy was used to characterize the essential conformations of tetrahydrofuran (THF) and thus determine the stereochemistry of the furanose ring constituting the backbones of DNA and RNA. Since the VUV-MATI spectrum of THF exactly corresponds to the vibrational spectrum of the gas-phase THF cation, the above cation was detected using time-of-flight mass spectrometry featuring the delayed pulsed-field ionization of the target in high Rydberg states by scanning the wavelength of the VUV pulse across the region of the vibrational spectrum. The position of the 0-0 band in the recorded VUV-MATI spectrum was extrapolated to the zero-field limit, allowing the adiabatic ionization energy of THF to be accurately estimated to be 9.4256 ± 0.0004 eV. The above ionization was assigned to a transition between C2-symmetric neutral (S0) and cationic (D0) ground states. The potential energy surfaces associated with molecular pseudorotation in the above states were constructed at the B3LYP/aug-cc-pVDZ level, being in good agreement with experimental observations. The twisted (C2-symmetric) and bent (CS-symmetric) conformers of the S0 state were predicted to be separated by a small interconversion barrier, whereas the D0 state exclusively existed in the C2 conformation. Based on the above, the peaks in the MATI spectrum were successfully assigned based on the Franck-Condon factors and vibrational frequencies calculated by varying the geometrical parameters of the C2 conformation, which determines the precise molecular structure of the THF cation.

Threshold Ionization and Spin-Orbit Coupling of Ceracyclopropene Formed by Ethylene Dehydrogenation.

A Ce atom reaction with ethylene was carried out in a laser-vaporization metal cluster beam source. Ce(C2H2) formed by hydrogen elimination from ethylene was investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The theoretical calculations include a scalar relativistic correction, dynamic electron correlation, and spin-orbit coupling. The MATI spectrum exhibits two nearly identical band systems separated by 128 cm(-1). The separation is not affected by deuteration. The two-band systems are attributed to spin-orbit splitting and the vibrational bands to the symmetric metal-ligand stretching and in-plane carbon-hydrogen bending excitations. The spin-orbit splitting arises from interactions of a pair of nearly degenerate triplets and a pair of nearly degenerate singlets. The organolanthanide complex is a metallacyclopropene in C2v symmetry. The low-energy valence electron configurations of the neutral and ion species are Ce 4f(1)6s(1) and Ce 4f(1), respectively. The remaining two electrons that are associated with the isolated Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a 5d Ce orbital and a π* antibonding orbital of acetylene.

Spectroscopic Characterization of Lanthanum-Mediated Dehydrogenation and C-C Bond Coupling of Ethylene.

La(C2H2) and La(C4H6) are observed from the reaction of laser-vaporized La atoms with ethylene molecules by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. La(C2H2) is identified as a metallacyclopropene and La(C4H6) as a metallacyclopentene. The three-membered ring is formed by concerted H2 elimination and the five-membered cycle by dehydrogenation and C-C bond coupling. Both metallacycles prefer a doublet ground state with a La 6s-based unpaired electron. Ionization of the neutral doublet state of either complex produces a singlet ion state by removing the La-based electron. The ionization allows accurate measurements of the adiabatic ionization energy of the neutral doublet state and metal-ligand and ligand-based vibrational frequencies of the neutral and ionic states. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal-hydrocarbon cycles are lower than that of the neutral La atom. Deuteration has a small effect on the ionization energies of the two cyclic radicals but distinctive effects on their vibrational frequencies.

Threshold ionization spectroscopic investigation of supersonic jet-cooled, laser-desorbed Tryptophan

Tryptophan (Trp) was studied by two-colour Photoionization Efficiency (PIE) and Mass Analysed Threshold Ionization (MATI) spectroscopy using a laser desorption apparatus. Conformer A of Trp was excited into the S1 state (34,878cm−1) and the second laser was scanned around the D0 cation ground and the D1 excited state. No ionization signal into the D0 state could be found, but a clear threshold was observed for the D1 state with an ionization energy of 66,704±3cm−1 (8.27eV). This observation is explained in terms of the electronic configurations of the S1 and cationic states.

Combined theoretical and experimental study of the valence, Rydberg and ionic states of fluorobenzene.

New photoelectron spectra (PES) and ultra violet (UV) and vacuum UV (VUV) absorption spectra of fluorobenzene recorded at higher resolution than previously, have been combined with mass-resolved (2 + 1) and (3 + 1) resonance enhanced multiphoton ionization (REMPI) spectra; this has led to the identification of numerous Rydberg states. The PES have been compared with earlier mass-analyzed threshold ionization and photoinduced Rydberg ionization (PIRI) spectra to give an overall picture of the ionic state sequence. The analysis of these spectra using both equations of motion with coupled cluster singles and doubles (EOM-CCSD) configuration interaction and time dependent density functional theory (TDDFT) calculations have been combined with vibrational analysis of both the hot and cold bands of the spectra, in considerable detail. The results extend several earlier studies on the vibronic coupling leading to conical intersections between the X(2)B1 and A(2)A2 states, and a further trio (B, C, and D) of states. The conical intersection of the X and A states has been explicitly identified, and its structure and energetics evaluated. The energy sequence of the last group is only acceptable to the present study if given as B(2)B2<C(2)B1<D(2)A1, a conclusion which is in agreement with most previous EOM-CCSD and other calculations. However, this symmetry ordering of the B and C states forces reconsideration of the nature of the PIRI spectrum. The coupling between these two states is induced by the a2 modes, ν12 and ν14 and we propose that the 14(1) band is observed in the B(2)B2 band in the PES for the first time, because of the improved resolution. This same assignment is given to the lowest energy band in the PIRI spectrum which was previously assigned as the origin band and further conclude that the entire PIRI spectrum is induced by ν12 and ν14. The relative intensities of the various Rydberg state peaks in the VUV absorption and REMPI spectra of fluorobenzene are very similar to those observed in the equivalent spectra of benzene.

La-Activated Bicyclo-oligomerization of Acetylene to Naphthalene

We report the first example of metal-mediated acetylene bicyclopentamerization to form naphthalene in the gas phase. The bicyclic aromatic compound was observed in a complex with La. The La(naphthalene) complex was formed by the reaction of laser-ablated La atoms with acetylene molecules in a molecular beam source and was characterized by mass-analyzed threshold ionization spectroscopy. The bicyclo-oligomerization reaction occurs through sequential acetylene additions coupled with dehydrogenation. Three intermediates in the reaction have been identified: lanthanacyclopropene [La(C2H2)], La(cyclobut-1-en-3-yne) [La(C4H2)], and La(benzyne) [(La(C6H4)]. The metal-ligand bonding in the three intermediates is considerably different from that in the La(naphthalene) complex, as suggested by accurately measured adiabatic ionization energies.

Determination of the accurate ionization energy and cationic structure of cyanopyrazine by one-photon mass-analyzed threshold ionization spectroscopy

The ionization energy and cationic structure of cyanopyrazine have been determined for the first time by pulsed-field threshold ionization mass spectrometry. The ionization energy was accurately measured to be 80,305±6cm−1 (9.9566±0.0007eV) from the 0-0 band position in the mass-analyzed threshold ionization (MATI) spectrum obtained for the direct S0→D0 transition using a vacuum ultraviolet laser pulse generated by four-wave mixing in Kr. The highest occupied molecular orbital (HOMO) and singly occupied molecular orbital (SOMO) calculations indicated the extent of conjugation of nonbonding orbitals on nitrogen atoms with the π orbital in cyanopyrazine, and clarified the effect of substitution of H with CN in pyrazine on the ionization energy of cyanopyrazine. The observed MATI spectrum was interpreted using the Franck–Condon factors and vibrational frequencies calculated through adjustment of the geometrical parameters of the cyanopyrazine cation in the ground electronic state at the B3LYP/cc-pVTZ level. From rigorous analyses, it has been concluded that upon ionization, the cyanopyrazine cation in the D0 state should retain a planar structure with Cs symmetry, similar to the ground state molecule.

Fine Substituent Effects in Sandwich Complexes: A Threshold Ionization Study of Monosubstituted Chromium Bisarene Compounds.

Mass-analyzed threshold ionization spectra of jet-cooled [(η(6) -PhMe)(η(6) -PhH)Cr] and [(η(6) -Ph2 )(η(6) -PhH)Cr] reveal with unprecedented accuracy the effects of methyl and phenyl groups on the electronic structure of bis(η(6) -benzene)chromium. These "pure" substituent effects allow quantitative experimental determination of the ionization energy changes caused by the mutual substituent influence in bisarene systems. Two types of such influence have been revealed for the first time in bis(η(6) -toluene)chromium.

Ionization energy and active cation vibrations of trans-2-fluorostyrene

We applied the two-color resonant two-photon mass-analyzed threshold ionization (MATI) technique to record the cation spectra of trans-2-fluorostyrene by ionizing via six intermediate vibronic levels. The adiabatic ionization energy was determined to be 69304±5cm−1. The distinct MATI bands at 67, 124, 242, 355, 737, 806, 833, and 993cm−1 were assigned to the active cation vibrations related to out-of-plane substituent-sensitive bending vibrations and in-plane ring deformation and bending motions. Many combination vibrations were also observed. Our experimental results suggest that the molecular geometry and vibrational coordinates of the trans-2-fluorostyrene cation in the D0 state resemble those of the neutral species in the S1 state.