Ground State Of Cation Research Articles

Structure of the low-lying electronic states of from rotationally resolved photoelectron spectra

Pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of 132Xe2 have been measured in the range of 90100 to 108400 cm−1, where the lowest electronic states of the cation are located. A (1 + 1′) resonance-enhanced two-photon excitation scheme via selected rovibrational levels of the C 0 state of 132Xe2 was used to resolve the rotational structure of several vibrational levels of the I(1/2u), I(3/2g), I(3/2u) and II(1/2u) electronic states. For the ground cationic state even the Ω-doubling was resolved. A vibrational progression belonging to the II(1/2g) electronic state has been observed for the first time proving that the II(1/2g) state is bound. The analysis of the rotational structure, performed in Hund's angular momentum coupling case (c), enabled the determination of the equilibrium internuclear distances and the dissociation energies of the I(1/2u), I(3/2g), I(3/2u) and II(1/2u) states of Xe and the corresponding adiabatic ionisation energies.

Oscillator strengths and photoionisation cross sections for Rydberg transitions in acetaldehyde

Oscillator strengths for electronic transitions involving Rydberg states of acetaldehyde, as well as cross sections for all the dipole allowed photoionisation channels, all ending in the ground state of the molecular cation, are reported. The molecular quantum defect orbital method, which has proved to be reliable in previous applications to molecular Rydberg states, has been used. Despite its relevance for atmospheric chemistry and astrophysics, only a few data seem to be available in the literature. The continuity of the calculated differential oscillator strength across the ionisation threshold has been adopted as a quality criterion. To our knowledge, predictions of oscillator strengths for transitions to high-lying Rydberg states, as well as of photoionisation profiles on acetaldehyde are made here for the first time and we are not aware of any reported experimental data. We, thus, hope the present results may be useful in the interpretation of the spectrum of acetaldehyde and might be of help in future experimental measurements.

Carbazole endcapped heterofluorenes as host materials: theoretical study of their structural, electronic, and optical properties

By mimicking the molecular structure of 4,4'-bis(N-carbazolyl)-2,2'-biphenyl (CBP), which is a widely used host material, a new series of host molecules (carbazole-endcapped heterofluorenes, CzHFs) were designed by linking the hole-transporting carbazole to the core heterofluorene molecules in either meta or para positions of the heterofluorene. The aromatic cores considered in this study are biphenyl, fluorene, silafluorenes, germafluorenes, carbazole, phosphafluorene, oxygafluorene, and sulfurafluorene. To reveal their molecular structures, optoelectronic properties and structure-property relationships of the proposed host materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of these designed molecules have been studied with emphasis on the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), energy gaps (E(g)), triplet energy gaps ((3)E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ), triplet exciton generation fraction (χ(T)), spin density distributions (SD), and absorption spectra. These photoelectronic properties can be tuned by chemical modifications of the heteroatom and the carbazole substitution at different positions. This study provides theoretical insights into the nature of host molecules, and shows that the designed CzHFs can meet the requirements of the host materials for triplet emitters.

Bipolar Host Molecules for Efficient Blue Electrophosphorescence: A Quantum Chemical Design

On the basis of density functional theory (DFT) calculations, a new series of bipolar host molecules for efficient blue electrophosphorescence devices are designed by linkage of hole-transporting moiety carbazole (CZ) and electron-transporting unit diphenylphosphoryl (ph(2)P horizontal lineO) to the core molecules with high triplet energies. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of the designed molecules have been studied with emphasis on triplet energies, spin density distributions, ionization potentials, electron affinities, and the influence of molecular topology. Designed bipolar host molecules possess the following features: (1) relatively higher highest occupied molecular orbital (HOMO) for hole injection and, relatively lower lowest unoccupied molecular orbital (LUMO) for electron injection; (2) HOMO and LUMO separation and localization in the respective hole- and electron-transporting moieties; (3) dramatic bond length changes in ionic states occurring at different parts of the bipolar molecules with respect to their neutral states; (4) keeping higher triplet energy. The DFT results provide deep insight into the nature of bipolar molecules and show that the designed molecules are feasible to meet the requirements of the host materials for blue triplet emissions.

Theoretical Study of the Tautomeric Reactions of Dinitromethane and Its Radical Cation

Reactions of aci-form and diaci-form formation in dinitromethane and its radical cation have been theoretically studied at DFT B3LYP level of theory with 6-31G(d) basis set. The lowest energy structures of the dinitromethane aci-form and diaci-form were optimized. Analogous theoretical study was carried out for dinitromethane radical cation. In connection with observed conformation transitions in aci- and diaci-form, B3LYP level of theory with the 6-31G(d) basis set was used to investigate the relevant parts of dinitromethane and its radical cation ground state potential energy surfaces.

Ground and Singlet Excited State Pyridinic Protonation of N9-Methylbetacarboline in Water-N,N-Dimethylformamide Mixtures

The ground and the singlet excited state pyridinic protonation of 9-methyl-9H-pyrido[3,4-b]indole, MBC, in water-N,N-dimethylformamide mixtures has been studied by absorption, steady state and time resolved fluorescence measurements. These proton transfer reactions elapse by a stepwise mechanism modulated by different hydrogen bonded adducts and exciplexes formed by water molecules and the pyridinic nitrogen atom of the MBC. Based in the present and previous studies, a general mechanistic Scheme for the ground and the singlet excited state MBC pyridinic protonation has been proposed. Accordingly, in the ground state, upon increasing the water proportion of the water-N,N-dimethylformamide mixtures, a hydrogen bonded complex, HBC, its hydrogen bonded proton transfer complex, PTC, a pre-cationic complex, PC, and the cation, C, are progressively formed. In the excited state, MBC, HBC and PC behave as independent fluorophores. Excited state cations, C*, are mainly formed by direct excitation of the ground state cations and, in minor proportion, by the excited state reaction of the PTC* through the CL* exciplex.

An excited state ab initio and multidimensional Franck–Condon analysis of the A B12←X A11 band system of fluorobenzene

This work combines high level ab initio calculations with multidimensional Franck-Condon calculations to refine and augment previous assignments of the lower wavenumber region of the A (1)B(2) <-- X (1)A(1) band system of fluorobenzene. The strength of the assignment has been greatly assisted by the use of zero electron kinetic energy spectroscopy in a series of pump-probe experiments where the response of the molecule to selective excitation in specific modes prior to ionization has been studied. The net result of this analysis is the reassignment of 7 of the 12 previously assigned bands in the region below about 1000 cm(-1) using a strategy that aims to trace the origins of excited state normal modes of fluorobenzene to the well-known Wilson modes of benzene by taking full account of the Duschinsky mixing that accompanies electronic excitation. Duschinsky normal mode analyses of the ground and first excited states of fluorobenzene as well as the electronic ground state of fluorobenzene cation have shown that the common use of the benzene Wilson notation to describe normal modes of this prototypical benzene derivative is highly questionable, particularly following electronic excitation and ionization.

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.

Interpreting ultrafast molecular fragmentation dynamics with ab initio electronic structure calculations

High-level ab initio electronic structure calculations are used to interpret the fragmentation dynamics of CHBr(2)COCF(3), following excitation with an intense ultrafast laser pulse. The potential energy surfaces of the ground and excited cationic states along the dissociative C-CF(3) bond have been calculated using multireference second order perturbation theory methods. The calculations confirm the existence of a charge transfer resonance during the evolution of a dissociative wave packet on the ground state potential energy surface of the molecular cation and yield a detailed picture of the dissociation dynamics observed in earlier work. Comparisons of the ionic spectrum for two similar molecules support a general picture in which molecules are influenced by dynamic resonances in the cation during dissociation.

Observation and assignment of the first photoelectron band of the CH 3CO (X 2A) radical

HeI photoelectron spectra have been recorded for the reaction of fluorine atoms with acetaldehyde at different reaction times. A structured band associated with a short-lived primary reaction product was recorded at a reactant mixing distance of 6 mm above the photon beam. The vertical ionization energy was measured as (8.39 ± 0.05 eV), and regularly spaced vibrational structure was observed in this band with an average vibrational separation of (1605 ± 30 cm −1). The band was assigned to ionization of the CH 3CO (X 2A) radical on the basis of first vertical ionization energies computed for the two possible primary products, CH 3CO and CH 2CHO, using ab-initio calculations. The calculations carried out on CH 3CO (X 2A), and its singlet ground cationic state, revealed that the observed vibrational structure can be assigned to excitation of the C O stretching mode in the ion. The measured adiabatic ionization energy, 7.21 ± 0.05 eV, is higher than the value expected (6.94 ± 0.03 eV) from evidence in the literature and calculated values in this work. This difference is assigned to poor Franck–Condon factors in the region of the band onset which means that the true adiabatic ionization energy was not observed.

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.

Zero kinetic energy spectroscopy of hydroquinone-water (1:1) complex: A probe for conformer assignment

Zero kinetic energy (ZEKE) photoelectron spectroscopy of the hydroquinone-water (HQW) complex was carried out to characterize its S(1)-S(0) resonantly enhanced multiphoton ionization (REMPI) spectrum in terms of the cis and trans conformers. The ZEKE spectra of the hydroquinone isomers show differences in the Franck-Condon (FC) activity of a few ring modes, viz., modes 15, 9b, and 6b, due to the different symmetries of the two isomers. These modes were used as a "diagnostic tool" to carry out the categorical assignment of the REMPI spectrum of the HQW complex. It was found that the FC activity of these diagnostic modes in the cationic ground state (D(0)) of the water complex is similar as that of the monomer. The two lowest energy transitions in the REMPI spectrum of the water complex, 33,175 and 33,209 cm(-1), were reassigned as the band origins of the cis and trans hydroquinone-water complexes, which is opposite of the previous assignment. The intermolecular stretching mode (sigma) of the complex shows a long progression, up to v(')=4, in the cationic ground state and is strongly coupled to other observed ring modes. The Franck-Condon factors for different members in the progression were calculated using the potential energy surfaces computed ab initio. These agree well with the observed intensity patterns in the progression. The ionization potential of the trans and cis complexes was determined to be 60,071+/-4 and 60,024+/-4 cm(-1), respectively.

Vibrational spectrum of p-fluoroanisole in the first excited state (S 1) and ab initio calculations

The vibronic structure of p-fluoroanisole in the first excited state (S 1) has been investigated with mass selected resonance-enhanced two photon ionization spectroscopy. The band origin of S 1 ← S 0 transition of p-fluoroanisole is measured to be 35149 cm −1, which is red-shifted by 1234 cm −1 with respect to that of anisole. Combining with the ab initio calculations, the measured frequencies 397, 487, 559, 840 and 1150 cm −1 in the S 1 state are assigned as the in-plane ring vibrational mode 9 b, 6 a, 6 b, 1 and 9 a, respectively. The optimized molecular geometries and vibrational frequencies of p-fluoroanisole in the ground state (S 0) and cation ground state (D 0) are also achieved from DFT calculations.

IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters

IR spectra of phenol-Arn (PhOH-Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus pi bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, nuOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH-Ar2 as well as cationic PhOH+-Ar have a pi-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+-Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This pi-->H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The pi-bound nuOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound nuOH band. The analysis of the picosecond IR spectra demonstrates that (i) the pi-->H site switching is an elementary reaction with a time constant of approximately 7 ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100 cm(-1)), (iii) both the position and the width of the H-bound nuOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from pi bonding to H bonding in the PhOH+-Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.

Vibrationally Mediated Photodissociation of C2H4+

We report the vibrationally mediated photodissociation dynamics of C2H4+ excited through the B2Ag state. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (pi, 3s) and (pi, 3p) Rydberg intermediate states in the wavelength range 298-349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C2H3+ and C2H2+ were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state.

Comparison of Photoelectron-Spectroscopy Results to Ab-Initio and Density Functional Calculations: The Ethylbenzene Cation

In this work resonant S 0–S 1 two-photon ionization (R2PI) and high-resolution R(1+1’)PI photoelectron spectroscopy (PES) as well as ab initio and density functional (DFT) calculations of ethylbenzene (EB) are combined. Conformer energies and equilibrium geometries have been calculated for neutral and cationic EB with the HF, UHF, B3LYP and the MP2 methods and different basis sets. In agreement with previous results the tail-to-chromophore orientation of neutral EB is orthogonal. This conformer is also the most stable structure in the cation, but a second local minimum in which all carbons lie in a plane (termed “planar” conformer) lays 325cm-1 higher in energy. R(1+1’)PI PE spectra were recorded by time-of-flight spectrometer with an energy resolution (Δ E) below 8 cm-1 and an absolute accuracy of ± 10 cm-1 for electron energies below 200 meV. Because the experiment starts in the orthogonal conformer and ionization is vertical, the recorded PE spectra show the cation ground state vibrations of this conformer. Beside benzene modes also low-energetic tail-to-chromophore modes are observed and assigned by DFT vibrational mode analysis. The differences of the calculated vibrational frequencies between the two conformers are comparable to the deviation between experiment and theory and a conformer assignment by comparison of theory and experiment would be difficult. R(1+1’)PI PE spectra recorded via selected S 1 vibrations provide vibrational assignments for S 1, qualitative S 1–D 0 geometry changes, vibrational symmetries as well as internal vibrational redistribution dynamics in S 1. Charge and spin densities of the neutral and cation were calculated to elucidate the problem of charge delocalization and electronic tail-to-chromophore coupling.

An examination of structural characteristics of phenylacetylene by vibronic and rovibronic simulations of ab initio data

The structural properties of phenylacetylene have been investigated in the S(0)((1)A(1)) neutral ground and S(1)((1)B(2)) and S(2)((1)A(1)) singlet excited states and the D(0)((2)B(1)) cationic state using both rovibronic and multidimensional Franck-Condon simulations from data determined via correlated ab initio methods. Results are compared to experimental and ab initio data reported in the literature. (10,10)-CASSCF and a hybrid CASSCF/SACCI frequency analysis using the cc-pVDZ Dunning basis set have been employed to produce vibronic simulations of REMPI/FES, dispersed fluorescence, TPES and MATI spectra. Calculated rotational constants are used where appropriate to compare to rotationally resolved experimental studies. Whilst the simulations are of generally good quality, it is apparent that the distortion of the ring along the long axis upon electronic excitation is underestimated, resulting in smaller predicted changes in ipso and para CCC bond angles and weaker activities in the 6a and 9a modes compared with experiment. Simulations of one-photon MATI spectra on the other hand, which do not rely on excited state methodologies, compare very well with experiment, suggesting that the neutral and cationic ground state geometries are quite accurate, as are the predicted changes in geometry accompanying ionisation. Simulated rotational and vibrational profiles, as well as other calculated physical data, show good agreement with the numerous experimental and computational studies of phenylacetylene in the literature.

Vacuum-UV and electron dissociative ionisation studies of SF 5Cl

A complementary study of the interaction of SF 5Cl in the gas phase with vacuum-UV photons and low-energy electrons from the onset of ionisation, ca. 12 eV, up to 20 eV is presented. The photon-induced experiments have used tunable vacuum-UV radiation from a synchrotron and threshold photoelectron photoion coincidence spectroscopy for product ion detection, the electron-induced experiments a trochoidal electron monochromator and a quadrupole mass spectrometer. The strengths and limitations of both experiments are contrasted, the main difference being the absence of state selectivity in the electron-induced study. The parent cation is not observed in either study, suggesting that its ground electronic state is repulsive following Franck–Condon vertical excitation. The fragment cations SF 5 +, SF 4Cl +, SF 4 + and SF 3 + have been observed in both studies, with reasonable agreement in their threshold appearance energies. Using a variant of threshold photoelectron photoion coincidence spectroscopy applicable when the ground state of the parent cation is repulsive, the first dissociative ionisation energy of SF 5Cl is determined to be 12.3 ± 0.2 eV, leading to a value for the adiabatic ionisation energy for the SF 5 radical of 9.92 ± 0.28 eV. The electron-induced experiment is sensitive to ion-pair production, and onsets for F + and Cl + production have been observed which are only possible energetically if the accompanying fragments are the anions SF 4Cl − and SF 5 −, respectively. A lower limit for the electron affinity of the SF 4Cl radical of 4.88 eV is determined, a value confirmed by ab initio calculations. The electron-induced experiment is very sensitive to gas impurities, and the effects of minute quantities of SF 4, FCl, Cl 2 and possibly SF 2 in the gas sample are observed.

Multi-mode vibronic interactions in the five lowest electronic states of the fluorobenzene radical cation

The multi-mode vibronic interactions between the five lowest electronic states of the fluorobenzene radical cation are investigated theoretically, based on ab initio electronic structure data, and employing the linear vibronic coupling model. Low-energy conical intersections, and strong vibronic couplings are found to prevail within the set of X ∼ – A ∼ and B ∼ – C ∼ – D ∼ cationic states, while the interactions between these two sets of states are found to be rather weak (owing to high-energy conical intersections). The overall intensity distribution of the experimental photoelectron spectrum, as well as the line positions observed in the MATI spectrum, are well reproduced. The vibronic interactions in the X ∼ – A ∼ states are found to be a replica of the multi-mode dynamical Jahn–Teller effect in the parent system, the X ∼ 2 E 1 g ground state of the benzene radical cation. Ultrafast internal conversion processes within the electronic manifolds in question demonstrate the strength of the nonadiabatic coupling effects and complement the analogous findings for the electronic spectra. The implications for the fluorescence dynamics of the fluorobenzene radical cation are discussed.

Applying the vibronic coupling model Hamiltonian to the photoelectron spectrum of cyclobutadiene

The vibronic coupling model Hamiltonian has been set up to provide global potential energy surfaces for the lowest three singlet states of the neutral molecule and the doubly-degenerate ground state of the radical cation. Both systems are dominated by non-adiabatic coupling between the states, and the model is able to describe the anharmonic adiabatic surfaces using simple coupled diabatic states. The model is fitted to electronic structure calculations at the CASSCF level. To test the model, the photoelectron spectrum is calculated and compared to experiment. It is found that while the simple model is able to reproduce qualitatively the known topology of the potential energy surfaces, it is unable to accurately reproduce the photoelectron spectrum. It is concluded that this is due to inaccuracies in the electronic structure calculations.