Spectroscopic characterization of organic cations: emission and laser-excitation spectra of rotationally cooled CH3CCX+, X  Cl,Br

The gas phase emission and laser-excitation spectra of the òE ↔ X̃²E transition of rotationally and vibrationally cooled 1-chloro- and 1-bromopropyne cations, and their fully deuterated analogues, have been obtained. The emission was excited by electron impact on a seeded helium supersonic free jet and the fluorescence by laser-excitation of cations formed by Penning ionization and collisional relaxation of liquid-nitrogen temperature. Comparison of the two sets of data locates the band origins and enables the prominent spectral features to be vibrationally assigned. The vibrational frequencies of many of the fundamentals could be inferred for these cations in their X̃²E and òE states.     

à 2A1 → X̃ 2B1 emission spectrum of cis-1,2-difluoroethylene cation and the non-radiation decay of the à states of the haloethylene cations in the gaseous phase

The results of the search for the à → X? radiative relaxation of haloethylene cations in the gaseous phase are reported. Only in the case of cis-1,2-difloroethylene cation was an emission spectrum detected. It is identified as the à ²A₁ → X? ²B ₁ band system on the basis of photoelectron spectroscopic measurements. An assignment of the emission bands yields the vibrational frequencies of four of the A₁ fundamentals (under C₂, symmetry) for the X? state and one for the à state. Well resolved Ne(I) photoelectron spectra of cis- and trans-1,2-difluoroethylene are presented, from which some vibrational frequencies for these cations in the X? and à states are also obtained. The lifetimes of cis-1,2-difluoroethylene cation in the lowest vibrational levels of the à ²A₁ state have been measured. The decay of this cation is unusual as these levels are depleted both by, radiative, and pathways leading to fragment ions (C₂HF⁺). The lack of detectable emissions with other fluoro-, chloro- and bromo-ethylene cations is discussed and the likely symmetries of the à states are proposed.     

Laser Induced Excitation Spectra of the Cations of Some Fluoro-substituted Phenols (B̃2 A″ ↔ X̃2 A″) in the Gaseous Phase

Excitation spectra of the cations of pentafluorophenol, 2,3,4-trifluorophenol, 2,4,5-trifluorophenol, 2,5-difluorophenol and 3,5-difluorophenol have been obtained by pumping the B?² A″ ← X?²A″ transition with a dye laser. The cations were first produced in their ground states by Penning ionisation using argon metastables. Vibrational frequencies of some of the fundamentals have been inferred for these cations in the B?² A″ state from the excitation spectra, and in the X?² A″ state from the emission spectra. Possible applications of the reported results are suggested.     

Ã2Σ+g → X̃2Πu emission band system of dicyanoacetylene radical cation in the gaseous phase

The emission spectrum of the dicyanoacetylene radical cation has been observed in the 580–720 nm wavelength region as a result of low energy electron impact excitation in a crossed-beam arrangement. The band system is attributed to the òΣ⁺_g → X?²Π_u electronic transition by comparison with the photoelectron spectroscopic and calculated data on the states of dicyanoacetylene cation. The frequencies of the three Σ⁺_g vibrational fundamentals in the ground cation state have been deduced from the emission spectrum. The lifetime of the lowest vibrational level of the òΣ⁺_g state of dicyanoacetylene cation was determined to be 13 ± 2 ns. Emission could not be detected from the corresponding states, òΣ⁺, of fluorocyanoacetylene and cyanoacetylene cations, and these results are discussed.     

The Electronic States of the Pentatetraene Radical Cation

HeIα excited photoelectron spectra of pentatetraene and the inferred ionization energies are reported. The first band has a characteristic Franck-Condon envelope similar to the first photoelectron bands of allene and butatriene. The four bands found below 16 eV ionization energy have been assigned to the X?²E, òE, B?²E and C?²B₂ states of the radical cation of pentatetraene by comparison with STO-3G and SPINDO calculations on the cumulene series. The correlation scheme includes the 2s shell ionization energies of ethylene, allene and butatriene. The π-orbital ionization trends of the cumulenes are discussed in the framework of localized orbitals calculated with the STO-3G basis set.     

Emission spectra of supersonically cooled chloroacetylene cations

The ā²Π_Ω → X?²Π_Ω, Ω = $\text{3}\text{2}$, $\text{1}\text{2}$ emission spectra of rotationally cooled chloro- and deuterochloro-acetylene cations have been obtained by electron-impact ionisation of a seeded helium supersonic free jet. The resultant spectral improvements lead to the identification of the spin-orbit components and isotope splittings and to a vibrational assignment of the prominent bands. The vibrational frequencies could be determined to within ±1 cm^?1 for many of the fundamentals for the cations in the X?²Π_Ω and ā²Π_Ω electronic states.     

Lifetimes of the vibronic Ã2A1 states of H2S+

Lifetimes have been measured for the Σ and Π vibronic òA₁ states of H₂S⁺ by studying the decay curves of the òA₁ (0, υ′₂, 0) → X? ²B₁ (0, υ″₂, 0) emission bands. The vibronic òA₁ states are produced via excitation of H₂S molecules by 150 eV electrons. The Σ sublevels 1 ? υ′₂ ? 7 and the Π sublevels 3 ? υ′₂ ? 6 have been considered. Predissociation occurs in the Σ sublevels for υ′₂ ? 7 and in the Π sublevels for υ′₂ ? 6. The obtained radiative lifetimes for the non-predissociated Σ and Π sublevels are around 4.2(±0.4) × 10^?6 s and 5.6(±0.5) × 10^?6 s respectively. For the predissociated Σ(0, 7, 0) and Π(0, 6, 0) levels the corresponding lifetimes are 2.3(±0.3) × 10^?6 s and 1.6(±0.3) × 10^?6 s respectively. The rate constant for collisional deactivation (quenching) of the vibronic òA₁ states by H₂S molecules was found to equal 2.3(±0.3) × 10^?9 cm³ mol^?1 s^?1.     

The geometries of the X̃ and à states of N2O+ from photoelectron spectroscopy compared with those derived from the emission spectrum

The geometries of the X?² and òΣ⁺ states of N₂O⁺ are obtained from Franck-Condon factors and vibration wavenumbers in the photoelectron spectrum of N₂O⁺. The geometries agree well with those obtained by Callomon and Creutzberg from the emission spectrum of N₂O⁺. This agreement confirms the validity of the method used in calculating geometries of ions from photoelectron spectra.     

Die Photoelektron-Spektren der Monohalogenacetylene

The photoelectron spectra of the four monohaloacetylenes X? C?C? H with X ? F, Cl, Br, I have been recorded. The first four bands of these spectra are assigned (in order of increasing ionization potentials) to the following states: band 1: ²Π_3/2(1), ²Π_1/2(1); band 2: ²Π_3/2(2), ²Π_1/2(2); band 3: ²Σ⁺(3); band 4: ²Σ⁺(4). A correlation diagram based on a simple ZDO-MO model shows that the observed band positions and the size of the splits due to spin-orbit coupling can be satisfactorily explained in terms of such a model. It is found that the orbital energies A_X of the postulated halogen n p λ(X)-basis orbitals are a linear function of the ionization potentials I(X) of the free atoms X. The validity of the ZDO-MO-model is confirmed by the excellent qualitative agreement between the observed and predicted spacings of the vibrational fine structure of the π-bands.     

The emission spectrum of AsH2(Ã2A1 → X̃2B1) via uv photolysis of AsH3

The AsH₂(òA₁ → X̃²B₁) emission spectrum (402–650 nm) following ArF laser photolysis of AsH₃ is reported. Seven bands are assigned and the relation ν = 19928 + (868v'₂-3v'₂²)-(987v″₂-6v″₂²) is obtained. The AsH₂ (òA₁) radiative lifetime is 130 ± 20 ns. Emission spectra from nascent As and a multiphoton ionization signal were also obtained.     

Ã2Σ+ → X̃+Π Emission spectra of the phosphaethyne cations,HCP+ and DCP+

The electron impact excited òΣ⁺ → X?⁺Π emission spectra of HCP⁺ and DCP⁺ have been observed. The spin-orbit split 0-0 band has maxima at 593.7 and 599.0 nm for HCP⁺ and 593.6 and 598.8 nm for DCP⁺. Short progressions in the V₃(CP) vibration are observed. a₀, v₃ and the upper-state lifetime are determined.     

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.     

Lifetimes of the vibronic Ã2A1 states of H2O+ and of the 3Πi (υ′ = 0) state of OH+

Using the delayed coincidence technique, lifetimes have been measured for some Σ and Π vibronic òA₁ states of H₂O⁺ and for the ³Π_i (υ′ = 0) state of OH⁺ by analysing the decay curves of the òA₁(0, υ′₂, 0) ? X?²B₁ (0, υ″₂, 0) and the ³Π_i(υ′ = 0) ? ³Σ^?(υ″ = 0) emission intensities respectively. The excited molecular ionic states are produced via excitation of H₂O molecules by 200 eV electrons. For H₂O⁺(òA₁) the vibronic Σ levels with υ′₂ = 13 and 15 and the vibronic Π levels with υ′₂ = 12 and 14 have been considered. The radiative lifetimes obtained for these levels have about the same value, namely 10.5(±1) × 10^?6 s. The radiative lifetime for the OH⁺(³Π_iυ′= 0) state is 2.5(±0.3) × 10^?6 s. The lifetimes found in this work for H₂O⁺(òA₁) and OH⁺(³Π_i,υ′= 0) are about ten and three times longer respectively than the corresponding lifetimes given by other investigators [1,2]. The probable reason for this discrepancy is that in the other experiments no attention has been paid to the presence of a large space charge effect. This effect is caused by the positive ions which are created by the primary electron beam.     

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene in the gas phase

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene-h₄ (and -d₄), and 1,3,5-trichlorobenzene, excited in the gas phase by controlled electron impact, are presented. The band systems, which lie in the 500–750 nm wavelength region, are attributed to the B?(π^?1) → X?(π^?) electronic transition of the cations on the basis of photoelectron spectroscopic data. The NeI excited photoelectron spectra and the ionisation energies of chloro-,o-,m-,p-dichloro- and 1,3,5-trichlorobenzene have been obtained. The information acquired from the emission and photoelectron spectra is discussed and compiled to deduce the symmetry of the B? states. Emission, with quantum yield > 10^?5, could not be detected with electronically excited radical cations of chloro-,o-dichloro-, 1,2,4- and 1,2,3-trichloro- and tetrachloro-benzenes. This is attributed to the nature of the B? states, which arise by σ^?1 ionisation processes. The lifetimes of the zeroth and some vibrationally excited levels of the B?(π^?1) states were also measured and found to be 22 ± 2 ns for 1,3,5-trichlorobenzene cation and < 6 ns for 1,3- and 1,4-dichlorobenzene cations. The lifetimes of the latter two electronically excited cations are estimated to be two orders of magnitude shorter than 6 ns from the measurement of the relative emission intensities of the B? → B? band systems of the three title cations.     

Spectroscopic Studies of Cyclometallated Pd(II) and Pt(II) Complexes: Optical absorption and emission of single crystals of [Pd(thpy)2] and [Pt(thpy)2]

The polarized optical absorption and emission (spectra, decay times) of single crystals of [Pd(thpy)₂] and [Pt(thpy)₂] (thpy ≡ C(3′)-deprotonated form of 2-(2-thienyl)pyridine) at temperatures 1.9 K ? T ? 80 K are reported. The emission of [Pt(thpy)₂] can be influenced strongly by applied magnetic fields (0 ? H ? 6 T). Depending on the central ions Pd and Pt, the lowest excited electronic states of the single complexes are ligand-centered (LC) states and metal-to-ligand charge transfer (MLCT) states, respectively. This difference leads to distinctly dissimilar properties of the emission of both compounds. The experimental data show that the emission of single crystals of [Pd(thpy)₂] and [Pt(thpy)₂] at T ? 30 K originates from several types of traps (defect states of symmetry ³B₂?stabilized below the exciton band) with LC and MLCT character, respectively. In the Pt compound, the ³B₂ is split by spin-orbit coupling into three states. The states B and A, which determine the emission properties, are separated by Δv ～ 13 cm^?1. Both states can mix under the influence of an applied magnetic field yielding an increase of the emission intensity by a factor of ～ 1.5 at H = 6 T.     

The emission spectrum of the 2,4-hexadiyne cation in a supersonic free jet

The òE_u→X?²E_g emmion band system of the vibrationally and rotationally cooled 2,4-hexadiyne cation has been obtained by electron-impact excitation of the molecule seeded into a supersonic helium free jet. The Spectrum could be recorded at high resolution, enabling vibrational frequencies to be determined with an accuracy of 0.5–1.0 cm^?1.     

Molecular orbital studies of the spectral and structural properties of chlorosilylidyne,SiCl

Si-Cl bond distances for the X̃² Π ground and low lying b̃⁴ Σ, ò Σ and B̃'² Δ excited states of SiCl have been optimized at the SCF and CI level with 6–31G basis set. Optimized bond distances are in good agreement with experimental values. Computed electronic excitation energies for the X̃²Π-Ã²Σ and X̃²Π-B̃'² Δ transitions compare well with the observed spectrum. The calculated harmonic vibrational frequency for the ground state, 525.2 cm⁻¹, also agrees with the experimental value 535.6 cm⁻¹.     

Femtosecond induced transparency and absorption in the extreme ultraviolet by coherent coupling of the He 2s2p (P) and 2p (S) double excitation states with 800 nm light

Femtosecond high-order harmonic transient absorption spectroscopy is used to observe electromagnetically induced transparency-like behavior as well as induced absorption in the extreme ultraviolet by laser dressing of the He 2s2p (¹P^o) and 2p² (¹S^e) double excitation states with an intense 800 nm field. Probing in the vicinity of the 1s² → 2s2p transition at 60.15 eV reveals the formation of an Autler–Townes doublet due to coherent coupling of the double excitation states. Qualitative agreement with the experimental spectra is obtained only when optical field ionization of both double excitation states into the N = 2 continuum is included in the theoretical model. Because the Fano q-parameter of the unperturbed probe transition is finite, the laser-dressed He atom exhibits both enhanced transparency and absorption at negative and positive probe energy detunings, respectively.     

Elemental sulfur as a versatile low-mass-range calibration standard for laser desorption ionization mass spectrometry

Facile generation of series of singly charged radical anions (S _n ^−· ; n=1–15) and cations (S _n ^+· ; n=2–11) by direct laser ionization renders elemental sulfur an excellent material for the low-mass-region calibration of time of flight (TOF) mass spectrometers. Upon irradiation by a 337-nm UV laser, elemental sulfur undergoes facile ionization without the need of an additional laser-absorbing matrix. An intense and evenly spaced set of peaks is obtained in both modes.     

Laser-induced fluorescence of the CHF radical

Laser-induced fluorescence of the à ¹A - X? ¹A' transition in the gas-phase CHF radical has been observed, following the formation of the ground-state radical by IR multiple-photon dissociation. Radiative lifetimes of four vibronic states of CHF (à ¹A) are reported.