A precise determination of the first ionization potential of benzene

The first ionization potentials of benzene and benzene-d₆ have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (-h₆) and 15 (-d₆) establish adiabatic thtesholds of 74573.0 ± 2.0 cm^?1, and 74592.5 = ± 1.2 cm^?1, respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R? series of Wilkinson is derived from aπ(e_1g)→ nf±1 Rydberg excitation.     

Specific fragmentation of the K‐shell excited/ionized pyridine derivatives studied by electron impact: 2‐, 3‐ and 4‐methylpyridine

Fragmentation of the pyridine ring upon K‐shell excitation/ionization has been studied with gaseous 2‐, 3‐ and 4‐methylpyridine by the electron‐impact method. Ab initio molecular orbital (MO) calculations were also carried out to explore electronic states correlating with specific fragments. Some specific fragmentation channels were identified from the ionic fragments enhanced characteristically at the N 1s edge. Yields of the C₂HN⁺ and C₅H₅⁺/C₅H₆⁺ ions show that the fission of the N? C2 and C4? C5/C5? C6 bonds of the ring is likely to occur after the N 1s excitation and ionization. Ab initio MO calculations for the 2‐methylpyridine molecule indicate that the dissociation channels to produce these ions are only accessible through the excited states of the parent molecular dication, which can be formed by Auger decays after the N 1s ionization. Fragment ions via hydrogen rearrangement are produced as well, but the rearrangement is not a phenomenon specific to the K‐shell excitation/ionization. Copyright © 2010 John Wiley & Sons, Ltd.     

Excited electronic and ionized states of the nitramide molecule, H2NNO2, studied by the symmetry-adapted-cluster configuration interaction method

Symmetry-adapted-cluster configuration interaction (SAC-CI) wave functions were employed to compute 16 singlet and 13 triplet vertical transitions, and 14 ionized states including relative intensities of the nitramide molecule, H₂NNO₂. This molecule is the simplest neutral closed-shell molecule which has an N–NO₂ bond and is a member of the nitramine family, R,R′N(NO₂), an important class of energetic materials with practical applications. The present nitramide results showed strong similarities with the ones of the N, N-dimethylnitramine molecule, which has also an N–NO₂ bond and was previously studied using the SAC-CI method. Experimental ultraviolet and photoelectron band spectra of the nitramide molecule could be successfully assigned. All the singlet transitions have valence character. The computed singlet and triplet transitions, excepting a singlet one, result from excitation originating in the four highest occupied molecular orbitals, which have close energies. Most of the singlet and triplet transitions involved mixing of singly excited configurations. The strongest computed transition, at 6.8 eV, is a mixture of two nπ_NO2 → π^* configurations corresponding to excitations from the highest occupied molecular orbital (HOMO) to the first two virtual orbitals and has an optical oscillator strength value of 0.2665. The computed ionized states described the whole measured spectrum, have excellent agreement when compared with the measured ionization potentials and revealed an inversion of the ordering of the first states not expected according to Koopmanns’ theorem, thereby showing the limitations of the latter.     

Lifetime measurements of highly excited Rydberg states of strontium I

Lifetimes of Rydberg states of the triplet-series 5s ns ³ S ₁ withn = 19–23, 35 and 5s nd ³ D ₃ withn = 18–20, 23–28 in the spectrum of neutral strontium have been determined. Observation of the exponential decay after excitation by a pulsed laser in a fast atomic beam and subsequent state-selective field ionization was employed. The lifetimes of the states of the³ S ₁-series show the expectedn*³ dependence on the effective principal quantum number, while the³ D ₃-series is disturbed by configuration mixing. Furthermore, state re-populations induced by black-body radiation have been observed.     

Ab initio MRD-CI study of ethane: The 14–25 eV PES region and Rydberg states of positive ions

Ab initio calculations of the MRD-CI type are reported for various states of the C₂H₆⁺ ion in two different nuclear geometries and the results are compared with the experimentally observed ethane PES in the 14–25 eV region. The calculated vertical IP values for ionization out of the 1e_u, 2a_2u and 2a_1g MO's respectively agree well with the locations of the three ionization maxima in this spectral range. The analogous findings for excitation out of the relaxed ionic ground state find several relatively low-lying species which occupy a 2pσ* MO in addition to states resulting from simple ionization of the neutral molecule. A number of Rydberg states are also calculated at the relaxed-ion geometry, from which results it is determined that the quantum defects for such species are from 0.40–0.45 units smaller than for their counterparts in neutral systems; these findings are clearly consistent with a decrease in the core penetrability of the Rydberg electron as the effective charge is increased to Z = 2.     

Theory of single and double ionization of helium through two-photon excitation of the1 S 0 e (1) doubly excited resonance

We present a theory and calculations of two-photon-resonant three-photon ionization of He via the lowest even parity doubly excited state¹ S ₀ ^e (1). We assess the importance of double ionization relative to single ionization into excited ionic states. Although double ionization is found to be quite small in the present context, our calculations reveal the importance of autoionizing doubly excited states as virtual intermediate states and suggest contexts in which double ionization may be relatively more efficient.     

Two-color excitation of NO in a supersonic free jet. Autoionization of high rydberg states

High Rydberg states of NO above the ionization limit have been measured for the isolated molecule in a supersonic free jet by two-color multiphoton ionization. Three Rydberg series (ns, np and nf) were identified, which appeared by rotational and the vibrational autoionization. The rotational structures of the 13s(υ = 1), 13p(υ = 1) and 12f(υ = 1) states have been analyzed in detail. The fluorescence dip spectra for the intermediate A²Σ⁺(3sσ) state have been measured simultaneously and the cross sections of the one-photon absorption to the high Rydberg states from the A²Σ⁺(υ = 1) state have been determined.     

The excited states of cycloporane. MCD spectrum,and CD spectrum of an optically active derivative

The MCD spectrum of gas phase cyclopropane and the CD spectrum of trans 1,2-dimethyl cyclopropane were measured in the spectral region 210–140 nm. The absorption spectra are also reported. Comparison of CD and absorption spectra of dimethyl cyclopropane and consideration of the anisotropy factors g = Δε/ε as a guide to the assignment of magnetic dipole allowed transitions led to the ordering of the states of the first excited configuration as A′₁, A′₂, E′ in order of increasing energy. The magnetic moments of the two observed allowed excited states of cyclopropane were determined from the MCD spectrum. LCAO MO level calculations of the MCD parameters $\text{A}$₁/$\text{D}$₀ for the lowest three excited E′ states were carried out and the results were used to discuss the assignment of the allowed transitions. It is concluded that configuration interaction is of considerable importance in the low energy excited states of cyclopropane.     

Propagator quantum chemical study of <Emphasis Type="Italic">S</Emphasis>-<Emphasis Type="Italic">cis</Emphasis>-(<Emphasis Type="Italic">Z</Emphasis>)-2-(2-formylethenyl)pyrrole: electronic structure and aspects of intramolecular hydrogen bond manifestation in ionization spectra

For the purpose of investigation of the electronic structures of functionalized pyrroles with potential biological activity the electronic structures and ionization spectra of S-cis-(Z)-2-(2-formylethenyl)pyrrole (FP) were calculated by the propagator quantum chemical method. The calculations were performed using the third-order algebraic diagrammatic construction method (ADC(3)) for one-particle Green´s function (electronic propagator) and the 6–31G** basis set. Going from FP (possessing the intramolecular hydrogen bond H?O) to its conformation FPR (without H?O bonding), the O1s-ionization energy and the ionization energy of the σ-type lone electron pair orbital of the O atom decrease by ~0.2 eV, which is a consequence of stopping the electron density transfer from the O atom. A strong electron density transfer through the hydrogen bond from the O atom to the NH group occurs in the nitrogen core level ionization spectrum as evidenced by a lower N1s-ionization energy of FP (by ~0.7 eV ) compared to that of FPR. The valence shell ionization spectra of FP and FPR calculated using the ADC(3) method are characterized by a high density of the satellite states. The results obtained indicate that the electronic structures of the compounds of the considered class are characterized by pronounced effects of electron correlation.     

Resonant two-photon ionization of LiNa. Observation and preliminary characterization of five new singlet states

Supersonic molecular beams containing rotationally and vibrationally cold LiNa were probed by one- and multi-photon ionization. Results include determination of a vertical ionization potential (5.05 ± 0.04 eV) as well as first observation of five new singlet states. Preliminary spectroscopic constants (Te, w_e and w_ex_e) and term symbols are reported for these five states (A ¹Σ⁺, C¹ Σ⁺, D ¹Π, E¹ Σ⁺ and F¹ E⁺).     

A study of aluminium monofluoride and aluminium trifluoride by high-temperature photoelectron spectroscopy

The Hel photoelectron spectrum of gaseous AIF(X¹Σ⁺) has been recorded and the first three cationic states have been assigned with the aid of PNO/CEPA calculations. The first band shows vibrational structure and analysis of the component separations and relative intensitives leads to values of ω_c = 1040 ± 40 cm^?1 and r_c = 1.59 ± 0.01 Å in the AIF⁺ (X²Σ⁺) state; the corresponding theoretical values are 960 cm^?1 and 1.60 Å respectively. The first adiabatic ionization potential, 9.73 ± 0.01 eV, allows a determination of the quantum defect, δ, in a number of previously observed Rydberg states of AIF. The Hel photoelectron spectrum of gaseous AIF₃ has also been obtained. It is assigned on the basis of ab initio molecular orbital calculations and comparison with the corresponding BF₃ spectrum.     

Enantiomer assays of amino acid derivatives using tertiary amine appended trans-4-hydroxyproline derivatives as chiral selectors in the gas phase

A pair of pseudo-enantiomers, tertiary amine appended trans-4-hydroxyproline derivatives were designed, synthesized, and evaluated as chiral selectors for enantiomer analysis of DNB-amino acid and their amides, in single-stage electrospray ionization/mass spectrometric experiments. The chiral selectors were designed to remove the interaction of the hydroxyl group of trans-4-hydroxyproline as well as separate the ionization site from the sites required for effective chiral recognition. Addition of a chiral analyte to a solution containing two pseudo-enantiomeric chiral selectors, affords selector-analyte complexes in the electrospray ionization mass spectrum where the ratio of these complexes is dependent on the enantiomeric composition of the analyte. The relationship between the ratio of the selector-analyte complexes in the electrospray ionization mass spectrum and the enantiomeric composition of the analyte can be used to relate the extent of the measured enantioselectivity and for quantitative enantiomeric composition determinations. Effects of acid modifiers (ammonium chloride, acetic acid, formic acid and hydrochloric acid) and instrument conditions on the selector-analyte ion intensity and the enantioselectivity (α_MS) were investigated. The largest α_MS was observed using ammonium chloride at a concentration around 0.5-1 mM at desolvation temperature of 150 °C. Capillary voltage has little effects on α_MS values. The sense of chiral recognition by MS is consistent with what is observed chromatographically. Quantitative enantiomeric composition determinations for N-(3,5-dinitrobenzoyl) leucinyl butylamide were performed. A comparison to the enantioselectivities towards a scope of analytes observed by chiral HPLC using a 3,5-dimethylanilide-proline-derived chiral stationary phase, is presented.     

A quasipotential theory for the relativistic hydrogen atom

Using a previously described relativistic quantum theory for two particles, the spectrum is calculated of an electron with spin 1/2, which is bound by Coulomb interaction to a spinless point nucleus of finite mass and chargeZe. Only states with positive energy are used in the theory. The spectrum is calculated to order α⁴ and includes recoil effects. In the static limit it coincides with the spectrum as calculated by the Dirac equation. Terms of order α⁵ lift the degeneracy between the 2S _1/2 and 2P _1/2 states. The critical valueZ _cr of the nuclear charge above which the 1S _1/2 state gets a binding energy larger than twice the mass of the electron, is found to be 142. This value will increase when also the nuclear structure is taken into account.     

Synthesis,spectral characterization of the zinc(II) mixed-ligand complexes of N(4)-allyl thiosemicarbazones and N,N,N′,N′-tetramethylethylenediamine,and crystal structure of the novel [ZnL2(tmen)] compound

Mixed-ligand zinc complexes with N,N,N′,N′-tetramethylethylenediamine (tmen) and R-salicylaldehyde N(4)-allyl thiosemicarbazones (R: 3-OCH₃ (L₁), 5-Br(L₂)), [ZnL_1,2(tmen)], were synthesized and the complexes were characterized by elemental analysis, atomic absorption spectrometer, magnetic susceptibility, molar conductivity, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) mass spectra and IR, UV–Vis, ¹H NMR and ¹⁵N spectroscopies. Crystal of [ZnL₂(tmen)] have a slightly distorted square pyramid involving O, N, S atoms of thiosemicarbazone and one N atom of tmen in basal plane and the other N atom of tmen in apex of the pyramid. The non-coordinated allyl group is disordered.     

Mass Spectra of New Heterocycles. XVII. Main Fragmentation Routes of Molecular Ions of 4-Alkoxy-5-amino-3-methylthiophene-2-carbonitriles under Electron and Chemical Ionization

For the first time decomposition was investigated of 4-alkoxy-5-amino-3-methylthiophene-2-carbonitriles under the conditions of electronic (70 eV) and chemical (reagent gas methane) ionization. At the electronic ionization the compounds under study [except for 4-(1-ethoxyethoxy) and 4-(ferrocenylmethoxy) derivatives] form stable molecular ions that decompose mainly by the cleavage of an alkyl radical from the alkoxy-substituent. Further fragmentation of the arising ion [M–Alk]⁺ depends on the substituent nature in the amino group. In the mass spectrum of 4-(ferrocenylmethoxy)-substituted thiophene peaks of the ion [FcCH₂]⁺ and its fragmentation products prevail. In the mass spectra of chemical ionization predominant peaks belong to ions M^+·, [M + H]⁺ and [M + C₂H₅]⁺, and fragment ions are absent.     

Photoelectron Photoion Coincidence Spectroscopy of NCl3 and NCl2

We investigate NCl₃ and the NCl₂ radical by photoelectron-photoion coincidence spectroscopy using synchrotron radiation. The mass selected threshold photoelectron spectrum (ms-TPES) of NCl₃ is broad and unstructured due to the large geometry change. An ionization energy of 9.7±0.1 eV is estimated from the spectrum and supported by computations. NCl₂ is generated by photolysis at 213 nm from NCl₃ and its ms-TPES shows an extended vibrational progression with a 90 meV spacing that is assigned to the symmetric N−Cl stretching mode in the cation. An adiabatic ionization energy of 9.94 ± 0.02 eV is determined.     

Microwave ionization and excitation of Ba Rydberg atoms

We have investigated ionization and excitation of the Ba 6s n s ¹ S ₀ and 6s n d ^1,3 D ₂ series in strong microwave fields. The observed microwave ionization threshold fields, scaling as 0.28n ^?5, and the state mixing fields cannot be completely explained in terms of a single cycle Landau-Zener model. However, by taking into account multiphoton resonant transitions driven by many cycles of the microwave field we have been able to interpet the data. In particular, multi-photon transitions have been found to be responsible for apparent resonance structures and for the unexpectedly low mixing fields. Not surprisingly, doubly excited valence states introduce irregularities into both the microwave ionization and the state mixing field values.     

The excited-state geometry of [Re2Cl8]2−

We report the luminescence spectrum of a single crystal of [Bu₄N]₂[Re₂Cl₈] at ≈20 K. The spectrum shows progressions in the Re-Re stretching vibration v₁ based on an electronic origin at 14161 cm⁻¹ and on false origins. The luminescence spectrum exhibits a “mirror-image” relationship to the absorption spectrum and is interpreted as showing that the transition originates from the ¹δδ¹ excited state of the eclipsed ion.     

Benzene clusters in a supersonic beam

A supersonic beam is employed to produce benzene clusters (C₆H₆) _n up ton=40. Mass analysis is achieved after two-photon ionization in a reflectron mass spectrometer. Photon energy is chosen so that the internal energy of the cluster ions is less than 700 meV and a slow decay on the µs time scale is observed. By an energy analysis with the reflecting field it is found that the elimination of one neutral benzene monomer is the favoured dissociation process of the cluster ions. Information about the dissociation pathways of the cluster ions is essential if one is to obtain neutral cluster abundances from the ion mass spectrum. Furthermore an experimental method is presented to obtain pure intermediate state (S ₁←S₀) spectra of selected clusters without interferences from the other clusters present in the molecular beam. This method is based on the observation of the metastable decay of the corresponding cluster ion. When the metastable signal is recorded as a function of photon energy it reflects theS ₁←S ₀ intermediate state spectrum. Spectra are presented for the benzene dimer, trimer, tetramer and pentamer.