ZEKE Spectra Research Articles

On the Jahn–Teller effect in the [formula omitted] electronic ground state of CH3F+

In view of recent experimental and theoretical developments, we revisit our earlier theoretical studies (Mahapatra et al., 2004, 2005) on the Jahn–Teller effect in the degenerate X∼2E electronic ground state of CH3F+. The electronic potential energy surfaces and the coupling surfaces are re-calculated employing state-of-the-art ab initio quantum chemistry methods. The vibronic Hamiltonian, constructed with the aid of multimode vibronic coupling theory and symmetry selection rules, is systematically extended to higher order in the Taylor series expansion and the parameters are carefully revised in the present study. First principles quantum dynamics study is carried out to calculate the vibronic eigenvalue spectrum of this degenerate electronic state of CH3F+. The vibronic energy levels are assigned and compared with the experimental pulsed-field-ionization zero-electron-kinetic energy (PFI-ZEKE) and one photon ZEKE spectra of CH3F+ and also with the earlier theoretical results reported in the literature.

Angle-resolved metastable fragment yields spectra of N2 and CO in K-edge excitation energy region

Angle-resolved metastable fragments yields spectra have been measured in the N 1s ionization region of the N(2) and C 1s ionization region of CO. These spectra are compared with zero kinetic energy electron and photoelectron spectra. It has been shown that an isotropic metastable fragments yields spectra are almost identical with the ZEKE spectrum, whereas metastable fragments yields spectra with the Σ-Σ transition show similarity with photoelectron spectra. This means that these spectra clearly contain information about two shake-up mechanisms: conjugate and direct shake-up processes. All the peaks in the metastable photofragment spectra can be assigned as either satellite states or double/triple excitation states. Thus, it was shown that angle-resolved metastable photofragment spectroscopy could be used to help characterize multi-electron excitation states in general.

Anion ZEKE-Spectroscopy of the Weakly Bound Iodine Water Complex

Zero kinetic electron energy photodetachment spectroscopy of I(-)·H(2)O and I(-)·D(2)O has been performed from 27 660 to 28 500 cm(-1) and from 27 660 to 35 900 cm(-1), respectively. The I(-)·D(2)O spectral data and theoretical studies resulted in a reassignment of earlier anion-ZEKE spectra of iodide water ( Bässmann , C. ; et al. Int. J. Mass Spectrom. Ion Processes 1996 , 159 , 153 ). In opposite to the I(-)·H(2)O, the I(-)·D(2)O spectrum reveals a regular progression of the iodine-water van der Waals stretching mode and a short progression of even quanta of the van der Waals rocking mode. A rough estimation delivers dissociation thresholds of the anionic and of the lower and the upper spin-orbit component of the neutral van der Waals complex. A high resolution ZEKE spectrum of the van der Waals stretching mode (v = 1) reveals significant fine structure, which is found again in a former photodissociation spectrum of the anionic complex ( Ayotte , P. ; et al. J. Phys. Chem. A 1998 , 102 , 3067 ). Our assignments are supported by theoretical calculations of molecular structures and vibrational motions. Vibrational frequencies and isotope effects are reproduced very satisfyingly by these calculations.

ZEKE-PFI Spectroscopy of Benzocaine

The adiabatic ionization threshold (AIT) of trans- and gauche-benzocaine has been measured by zero electron kinetic energy-pulsed field ionization (ZEKE-PFI) spectroscopy to be 7.8412+/-0.0008 eV (lasers at 34,134.4 and 29,109.3 cm(-1)) and 7.8421+/-0.0004 eV (34,144.8+29105.7 cm(-1)), respectively. AITs computed at the B3LYP/AUG-cc-p-VDZ level for the two conformers are some approximately 2,500 cm(-1) lower than the experimental; in contrast their energy difference is very close. The trans-benzocaine cation ZEKE spectra has been recorded taking a number of S(1) intermediate vibronic states. The spectra provide an energy threshold for the appearance of intramolecular vibrational redistribution (IVR) around approximately 540 cm(-1) in the S(1) state.

Electronic excitation and ionisation of 1-azabicyclo[2.2.2]octane (ABCO): evidence for a twisted structure from experiment and theory

The first excited neutral and ground cationic states of 1-azabicyclo[2.2.2]octane (ABCO) have been studied using a combination of (1 + 1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. In common with the related tertiary alkyl amines, analysis of both REMPI and ZEKE spectra indicate that the overriding impulse upon electronic excitation and ionisation is for a flattening of the nitrogen end of the molecule, albeit hindered by the cage. In addition, the activity of the torsion mode (19) in both REMPI and ZEKE spectra suggests geometry changes along the torsion coordinate between the three electronic states sampled in these experiments. Ab initio molecular orbital calculations conducted at the MP2/cc-pVDZ level of theory yield minimum energy geometries with CNCC dihedral angles of 6.011° and 7.86° for the S 0 and D 0 states, respectively, which indicates that the torsion coordinate is described by a double minimum potential. However, the degree of perturbation to the vibrational eigenvalues suggests that the zero point level lies a few wavenumbers above a small barrier and that ABCO is pseudo-twisted with a vibrationally averaged C 3v symmetry.

The effects of changes in the electronic environment of the 2-aminopyridine–argon van der Waals complex

The first excited neutral and ground cationic states of the 2-aminopyridine–argon van der Waals complex have been studied using a combination of (1 + 1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. In addition to an updated ionisation energy of 65 265 ± 3 cm −1, an analysis of the intermolecular vibrational structure observed in both REMPI and ZEKE spectra supports an assignment to a π-bound complex. The pyramidal conformation of the amino group in the electronic ground state allows for the possibility of anti and syn π-bound conformers: this is discussed in the context of the related molecule, aniline–Ar.

A REMPI and ZEKE Spectroscopic Study of a Secondary Amide Group in Acetanilide

Vibrationally resolved REMPI and ZEKE spectra of acetanilide are presented with assignments based on CASSCF/cc-pVDZ ab initio calculations and symmetry considerations of different methyl rotor states. ZEKE spectra selectively recorded via intermediate rotor states of a particular symmetry allow us to directly measure the splittings of the 0a1 and 1e rotor levels in the S0 state. Methyl group rotational features in the REMPI and ZEKE spectra are assigned from a simulation of methyl internal rotor levels in a one-dimensional model using a periodic V3−V6 potential, and barrier heights for methyl group rotation are extracted for the S0, S1, and D0 states. Additional vibrational features corresponding to displacements of the amide group and side-chain are identified and analyzed by comparison to formanilide which was studied previously (Ullrich, S.; et al. Phys. Chem. Chem. Phys. 2001, 3, 5450; Angew. Chem. Int. Ed. 2002, 41, 166).

The effect of weak interactions on the ring puckering potential in the coumaran–argon van der Waals complex: Experimental and ab initio analysis of the intermolecular and low-frequency intramolecular vibrations

The coumaran–argon van der Waals (vdW) complex has been investigated using a combination of (1+1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. Coumaran (2,3-dihydrobenzofuran) has a puckered, nonplanar equilibrium structure in the electronic ground state which allows for the formation of two energetically nonequivalent π-bound geometrical conformations. The experimental observation of bands attributable to two isomers in the REMPI spectrum is consistent with a significant barrier to planarity existing in both S0 and S1 states. The two isomers are related through the ring-puckering motion but the interaction of the argon atom with the monomer results in an induced asymmetry in the potential which transforms the ring-puckering tunnelling motion in the isolated monomer to a localized vibration near nonequivalent local minima. Both REMPI and ZEKE spectra show rich vibrational structure, characteristic of excitation of the long axis van der Waals bending mode and the stretching mode, as well as combination bands involving the vdW modes with low-frequency out of plane intramolecular vibrations. The vibrational structure is consistent with a shift in the position of the argon atom along the long axis upon excitation and subsequent ionization, as well as a modest reduction in the van der Waals bond length.

Bonding in Transition Metal−Ether Complexes: The Spectroscopy and Reactivity of the Zr Atom−Dimethyl Ether System

The reactivity and bonding of neutral Zr metal atoms with dimethyl ether have been thoroughly characterized using flow tube reactor, PFI-ZEKE, and DFT techniques. Between 275 and 371 K, these species form an association complex with the Zr bound to the ether O atom. Using a flow tube reactor, the equilibrium constant for the association reaction has been found to be 6 ± 2 × 10-14 cm3 at 361 ± 10 K. From this measure the Zr−dimethyl ether bond energy is estimated to be 80 ± 2 kJ mol-1. The ZEKE spectrum of the association complex is reproduced very well by DFT calculations. The calculations indicate that the Zr−dimethyl ether complex is bound by dative bonding involving donation of electron density from the highest occupied dimethyl ether orbital of a1 symmetry to the Zr 4d orbital of σ symmetry directed along the Zr−dimethyl ether bond axis. Dative bonding interactions are much stronger than donor−acceptor bonding as the vacant dimethyl ether antibonding orbital lies too high in energy for back-donation o...

Theoretical prediction of the ArO − anion ZEKE photoelectron spectrum

High-resolution zero electron kinetic energy spectra of the weakly bound ArO − anion are predicted using the method which combines atoms-in-molecule and Rau–Fano models for calculating relative intensities and accurate ab initio interaction potentials. ZEKE spectrum envelope and its temperature variation are shown to be essentially determined by the electronic photodetachment probabilities which are highly selective with respect to the symmetry of the spin–orbit components of anion and neutral electronic states.

Internal Rotation of the CF3 Group in the (Trifluoromethyl)anilines: A Zero-Kinetic-Energy Pulsed-Field-Ionization Study

Internal rotation in the ionic ground states of 1-, 2-, and 3-(trifluoromethyl)aniline (ABTF) have been studied by high-resolution photoelectron spectroscopy (ZEKE-PFI) using the S1 state as the resonant intermediate. Coefficients of the torsional potential of the CF3 group obtained from a one-dimensional rigid rotor model were as follows: 2-ABTF, V3=720 ± 10 cm-1, V6 = −3 ± 3 cm-1; 3-ABTF, V3 = 222 ± 10 cm-1, V6 = −4 ± 2 cm-1; and 4-ABTF, V6 = 12 ± 5 cm-1, with little change in the rotational constant of the rotor. These values display trends consistent with those found for the CH3 rotor in the same environment. In particular, the equilibrium conformation of the rotor changes on ionization from the S1 state by 60° in the 3-isomer, but is unchanged in the 2- and 4-isomers. Symmetry forbidden transitions, leading to marked non-Franck−Condon behavior of intensities in the ZEKE spectra of the meta isomer are observed to be far more extensive than in the corresponding CH3 case. A further factor directly affecting the ZEKE spectrum in the ortho isomer is the coupling between the adjacent CF3 and NH2 rotors, which is far more pronounced than with CH3 and NH2.

Multichannel Quantum Defect Theory (MQDT) Analysis of the (2 + 1‘) Mass Analyzed Threshold Ionization (MATI) Spectroscopy of NH3

MATI spectra of NH3 have been recorded using two-color (2 + 1‘) multiphoton excitation via the B̃ and C̃‘ states and have been simulated using multichannel quantum defect theory. New vibrational bands in the MATI spectra are reported for intermediate levels up to ν2 = 6 in the B̃ state. The spectra obtained should in principle be identical to ZEKE spectra recorded under the same resolution. A good agreement between experiment and theory is obtained by using quantum defects determined previously from optical spectra and optimization of unknown parameters. The effects of pπ−dδ mixing in the core region are included to account for “forbidden” transitions, and the optimization of quantum defect parameters is used to quantify the strength of this mixing. Molecular symmetry group arguments are used to define which channels must be included in the quantum defect matrix. The paper demonstrates that in favorable cases a complete theory of ZEKE/MATI line intensities is achievable including the effects of bound-state couplings.

Ultrafast Dynamics of Chlorine−Water and Bromine−Water Radical Complexes Following Electron Photodetachment in Their Anionic Precursors

Picosecond dynamics initiated by electron photodetachment in Cl-···H2O and Br-···H2O complexes is explored using classical Wigner trajectories which correctly map the initial quantum vibrational state of the systems. The three lowest potential energy surfaces of the neutral clusters reached after electron photodetachment are constructed by the ab initio Fock-Space Coupled Cluster Method and then quantitatively fitted to a Diatomics-in-Molecule Model which also allows for a simple inclusion of spin−orbit interactions. Because of large differences between the shapes of the anionic and neutral potential energy surfaces, and due to the presence of light hydrogen atoms, an unusual dynamical behavior is observed. Although the vertical photodetachment process places the systems above the dissociation threshold, the clusters do not dissociate directly. Instead, relatively long-lived vibrational resonances are observed. This is due to a strong excitation of the nondissociative (internal) water rotation and only a weak excitation of the dissociative intermolecular stretch upon photodetachment. Implications of the observed dynamics for the interpretation of experimental vibrationally resolved ZEKE spectra are discussed.

Evidence for stable Rydberg states in the presence of an electron scavenger: The ZEKE spectrum of the toluene·SF 6 complex

ZEKE and MATI spectroscopies have been used to study the toluene·SF 6 complex, revealing that high- n Rydberg states appear to be stable in the presence of a strong electron scavenger such as SF 6. The lifetimes are significantly longer than predicted by the `free electron model', suggesting that the molecular collisions inherent in previous cross-beam studies are crucial for effecting electron transfer. The study also provided spectroscopic information on toluene·SF 6, including the S 1–S 0 transition energy, 37 485.9±0.6 cm −1, and the adiabatic ionisation energy, 70 688.6±1.2 cm −1. Binding energies of 739±23, 731±23 and 1250±20 cm −1 were determined for the S 0, S 1 and D 0 states, respectively.

The cis and trans structures of the NO dimer cation

The NO dimer cation, (NO) 2 +, has been studied using high-level ab initio calculations. The ground state is determined to be the trans isomer, which has C 2h symmetry. The cis, C 2v, isomer lies ca. 100 cm −1 above this, as calculated at the RCCSD(T)/aug-cc-pVQZ(no g)//UQCISD/6-311+G(3df) level of theory. The equilibrium geometry of the trans isomer has an N–N bond length of 2.25 Å and a NNO bond angle of 112°. A reassignment of the ZEKE spectrum of the NO dimer is postulated on the basis of calculated harmonic vibrational frequencies.

Theoretical Simulation of the Zeke Spectra of Naphthalene From Single Vibronic Levels of S1

We present the simulations and analysis of the two-color ZEKE spectra of naphthalene, performed with the help of quantum chemical calculations of molecular parameters followed by the modelling of vibronic intensities. Ab initio and semi-empirical calculations were carried out to obtain molecular structures of neutral and ionic naphthalene, and vibronic perturbations that couple the electronic states. It is shown that the intensities, simulated with a model based on the perturbative expansion of vibronic states, nicely reproduce the observed spectra and contribute to reassign some of the ground state frequencies of naphthalene cation.

Structure and dynamics of the phenol–water–argon cation radical

The phenol–water–argon system has been studied by ab initio methods and by ZEKE spectroscopy. Two structures of the cation radical were found; one has the argon sitting above the hydrogen bond, and the other has the argon linear with an OH bond in the water. It was found that the argon has little effect on the phenol–water moiety. The only notable change upon argon complexation is a shortening of the hydrogen bond. Two low-frequency vibrational progressions with spacings of 18.5 and 24.9 cm−1 were observed in the ZEKE spectrum. Ab initio calculations reveal that they are due to bending motions of the argon against the phenol–water moiety. The stretching mode of the argon is not seen in the ZEKE spectrum but is predicted to occur around 50 cm−1. The energies of the two minima are strongly dependent on the calculation used. At the UMP2/cc-pVDZ level, the structure with the argon sitting on the hydrogen bond has the lowest energy. On consideration of Franck–Condon factors, this structure is the most likely candidate for the true experimental structure.

Decay Dynamics of the Predissociating High Rydberg States of NO

The dynamics of predissociating high molecular Rydberg states of NO below the lowest ionization threshold is computed in the presence of a weak external dc field using a quantum theory based on an effective Hamiltonian formalism. The core−electron interaction affecting the low l states (l ≤ 2) is taken into account by molecular quantum defect theory, while for the high l states (l ≥ 2), a long range multipolar expansion is used to describe the effect of the anisotropy of the molecular core. Time- and frequency-resolved ZEKE spectra are computed. In the energy range investigated, the decay kinetics of the ZEKE intensity is found to exhibit two time scales, which differ by more than an order of magnitude. The short decay constant typically falls in the submicrosecond range and is in agreement with previous experimental results and computations. In addition, our computations predict a long-time component, which decays in the tens of microseconds range. The two decay times are discussed in terms of the short and long range interseries dynamics, in terms of the strength of the external dc field, and in terms of the nature of the bottlenecks in phase space for a predissociating molecular core with a rather large rotational constant (BNO+ = 1.9842 cm-1). It is found that for this particular case where all series are coupled to the fragmentation channels through a low l bottleneck, the predissociation process does not quench the long time component in the decay kinetics. The reason is that the interplay between the interseries coupling and the external dc field leads to a shift of the decay constants to larger values together with an enhancement of the weight of the long time component in the decay kinetics. Special attention is devoted to the role of the dipolar interaction and its synergy with the external dc field.

Chemical Applications of Zero Kinetic Energy (ZEKE) Photoelectron Spectroscopy.

The extremely improved resolution of ZEKE spectroscopy is one of its main advantages over conventional photoelectron (PE) spectroscopy. A good illustration of this is provided by the comparison of the photoelectron spectrum of NO with the corresponding ZEKE spectrum (shown on the right). ZEKE spectroscopy can be used to characterize large organic molecules such as benzene and para-difluorobenzene, hydroden-bonded complexes such as phenol–water, transition metal clusters such as Nb3O, van der Waals complexes such as benzene–argon, and even reactive intermediates.

Two Classes of ZEKE Spectroscopy

A classification of ZEKE spectra into two classes based on operational criteria is useful for discussion. The proposal is that some of the different and seemingly conflicting effects reported for different molecular (or the same molecule for different excitations), such as the role of an external field, are due to these two distinct classes of states which can be optically accessed. Class A is the direct, "front door" entry where the states excited are those which are detected by the delayed ionization. Class B is a very prevalent but indirect "backdoor" route where it is only the interaction of the Rydberg electron with the core, possibly aided by external perturbations, that allows a signal to be detected upon ionization. The operational criteria for distinguishing between or even exploiting the features of the two classes are discussed. Such attention might be useful as new techniques for class B spectra are developed.