Ultraviolet laser ionization studies of 1-fluoronaphthalene clusters and density functional theory calculations

This paper studies supersonic jet-cooled 1-fluoronaphthalene(1FN) clusters by ultraviolet(UV) laser ionization at 281 nm in a time-of-flight mass spectrometer.The(1FN) + n(n=1-3) series cluster ions are observed where the signal intensity decreases with increasing cluster size.The effects of sample inlet pressures and ionization laser fluxes to mass spectral distribution are measured.Using density functional theory calculations,it obtains a planar geometric structure of 1FN dimer which is combined through two hydrogen bonds.The mass spectra indicate that the intensity of 1FN trimer is much weaker than that of 1FN dimer and this feature is attributed to the fact that the dimer may form the first "shell" in geometric structure while the larger clusters are generated based on this fundamental unit.     

IR absorption spectrum (4200–3100 cm) of H2O and (H2O)2 in CCl4. Estimates of the equilibrium constant and evidence that the atmospheric water absorption continuum is due to the water dimer

IR absorption spectra, 4200–3100 cm⁻¹, of water in CCl₄ solutions are presented. It is shown that for saturated solutions significant amounts of water are present as dimer (ca. 2%). The IR spectra of the monomer and dimer are retrieved. The integrated absorption coefficients of the monomer absorption are significantly enhanced relative to the gas phase values. The dimer spectrum consists of 5 bands, of which 4 were expected from data from cold beams and cold matrices. The origin of the “extra” band is discussed. In addition it is argued that the dimer absorption bands intensities must be enhanced relative to the gas phase values. Based on recent calculations of band strengths, and observed frequency shifts relative to the gas phase, the intensity enhancement factors are estimated as well as the monomer/dimer equilibrium constant in CCl₄ solution at T=296 K (K_c=1.29 mol⁻¹ L). It is noted that the observed dimer spectrum has a striking resemblance with the water vapour continuum determined by Burch in 1985 which was recently remeasured by Paynter et al. and it is concluded that the atmospheric water absorption continuum in the investigated spectral region must be due to water dimer. Based on the newly published spectral data a revised value of the gas phase equilibrium constant is suggested (K_p=0.035 atm⁻¹ at T=296 K) as well as a value for the standard enthalpy of formation, ΔH⁰=15.4 kJ mol⁻¹.     

REMPI and IR-UV double resonance spectroscopy of 3-aminophenol·(NH3)1 cluster in the gas phase

The electronic and geometric structures of the cluster consisting of 3-aminophenol (3AP) and one ammonia molecule were investigated using resonance-enhanced multiphoton ionization and IR-UV double resonance spectroscopy in the gas phase. Between the cis and trans isomers found for the bare 3AP molecule, only trans isomer was found for 3AP·(NH₃)₁. It was found that complete quenching of the excited state of cis isomer, e.g. by conformer-specific excited state hydrogen transfer reaction, does not seem to occur, but that efficient conformational cooling drives the ground state population into the most stable trans isomer. The 0-0 band of the trans-3AP·(NH₃)₁ at 34 409 cm⁻¹ is red-shifted by 68 cm⁻¹ from that of the trans-3AP at 34 477 cm⁻¹. The IR depletion spectrum shows that the OH stretching vibration (3296 cm⁻¹) is red-shifted by 362 cm⁻¹ from that of the bare molecule. The large red-shift in OH frequency, combined with MP2 calculation, reveals that the structure of the cluster is the one with the nitrogen atom of ammonia bound to the hydroxyl hydrogen of aminophenol.     

Molecular structure of trimethyl[(3-indole)ethoxy]silane cooled in a supersonic jet

Conformations of He-jet-cooled trimethyl[(3-indole)ethoxy]silane (TIES) have been studied using a laser spectroscopy technique in combination with quantum-chemical computations. Six probable conformers of the molecule were computed, of which only two conformations were observed. Based on an analysis of fluorescence excitation spectra, fluorescence spectra, shapes of rotational band contours at the electronic S₀–S₁ transition of TIES, and theoretical computations, the above conformers were assigned to steric structures. Twisted structures have the lowest energy due to intramolecular hydrogen bonds C - H ?O < _C^Si C - H \cdots O <_C^{Si} between hydrogen atoms of methyl groups and an oxygen atom and C–H···π between H and the π-electron cloud of the indole ring.     

Spectroscopic characterization of structural isomers of naphthalene: 1-Phenyl-1-butyn-3-ene

Laser induced fluorescence (LIF), single vibronic level dispersed fluorescence (DFL) spectra, and high resolution rotationally resolved scans of the S₀–S₁ transition of the C₁₀H₈ isomer 1-phenyl-1-butyn-3-ene have been recorded under jet-cooled conditions. The S₀–S₁ origin of PAV at 34 922 cm⁻¹ is very weak. A vibronic band located 464.0 above the origin, assigned as 30¹₀, dominates the LIF excitation spectrum, with intensity arising from vibronic coupling with the S₂ state. High resolution scans of the S₀–S₁ origin and 30¹₀ vibronic bands determine that the former is a 65:35 a:b hybrid band, while 30¹₀ is a pure a-type band, confirming the role for vibronic coupling and identifying the coupled state as the S₂ state. DFL spectra of all vibronic bands in the first 800 cm⁻¹ of the spectrum were recorded. A near-complete assignment of the vibronic structure in both S₀ and S₁ states is obtained. Herzberg–Teller vibronic coupling is carried by two vibrations, ν₂₈ and ν₃₀, involving in-plane deformations of the vinylacetylene side chain, leading to Duschinsky mixing evident in the intensities of transitions in excitation and DFL spectra. Extensive Duschinsky mixing is also present among the lowest five out-of-plane vibrational modes, involving motion of the side chain. Comparison with the results of DFT B3LYP and TDDFT calculations with a 6-311+G(d,p) basis set confirm and strengthen the assignments.     

CH3I分子的光解离的自旋-轨道从头计算

采用从头计算二阶自旋-轨道多组态准简并微扰理论计算了碘代甲烷CH₃I分子与基态²I⁰_3/2和激发态²I^0*_1/2原子解离极限相关联的势能曲线. 计算了CH₃I分子的吸收谱,分析了CH₃I分子的光解离过程,并估计了激发态碘原子²I^0*_3/2的量子产额. 计算结果表明,该方法可用以解释光解离实验结果. 关键词： 3I分子')" href="#">CH₃I分子 自旋-轨道耦合 量子产额 光解离     

FS and FS(OH) defect centers at the MgO(1 0 0) surface: cluster and periodic calculations

We present a model of a new paramagnetic defect center which results from the interaction of atomic hydrogen with the MgO(1 0 0) surface. DFT calculations have been performed using periodic supercells and embedded cluster models where long-range polarization effects are included explicitly. The H atom promotes the creation of an oxygen vacancy (F center) by formation of the F_S⁺(OH⁻) defect where an hydroxyl group is adsorbed near an electron trapped in an oxygen vacancy. This new center has some characteristics similar to those of the classical F_S⁺ centers but a smaller formation energy; furthermore, being globally neutral, it can be treated also with supercell methods.     

boldmath B3Σu-–X3Σg- transition in selenium dimer: ab initio multireference configuration interaction calculations

Theoretical investigation of low-lying electronic states and B³Σ_u^-–X³Σ_g^- transition properties of selenium dimer using size-extensivity singly and doubly excitation multireference configuration interaction theory with nonrelativistic all-electron basis set and relativistic effective core potential plus its split valence basis set is presented in this paper. The spectroscopic constants of ten low-lying Λ-S bound states have been obtained and compared with experiments. Spin-orbit calculations for coupling between B³Σ_u^- sates and repulsive ¹Π_u, ⁵Π_u states have been made to interpret the predissociation mechanisms of the B³Σ_u^- state. The lifetimes of B³Σ_u^- (v=0～6) have been calculated with scalar relativistic effects included or excluded, respectively, and reasonably agree with experimental values.     

Relativistic calculations of 3s2 1S0-3s3p 1P1 and 3s2 1S0-3s3p 3P1,2 transition probabilities in the Mg isoelectronic sequence

Using the multi-configuration Dirac-Fock self-consistent field method and the relativistic configuration-interaction method,calculations of transition energies,oscillator strengths and rates are performed for the 3s 2 1 S 0-3s3p 1 P 1 spinallowed transition,3s 2 1 S 0-3s3p 3 P 1,2 intercombination and magnetic quadrupole transition in the Mg isoelectronic sequence(Mg I,Al II,Si III,P IV and S V).Electron correlations are treated adequately,including intravalence electron correlations.The influence of the Breit interaction on oscillator strengths and transition energies are investigated.Quantum electrodynamics corrections are added as corrections.The calculation results are found to be in good agreement with the experimental data and other theoretical calculations.     

A Theoretical Description of the Photodissociation Spectrum of the H2O...HCl Complex

The effect of association of the water molecule and hydrogen chloride on the UV absorption bands is studied. Complete ab initio calculations for the H₂O...HCl complex in the S ₁ and S ₂ states are performed. A mathematical model using Airy functions is developed to describe the absorption cross-sections in a continuous spectrum. The form of the potential is determined by accurate ab initio calculations. The cross-sections of potential surfaces of lower electron states are found from ab initio calculations using the Hartree–Fock, configurational interaction, and multi-configurational interaction techniques. A complete vibrational analysis and an analysis of the change in the electron density for the S ₀ S ₁ transition on moving along the reaction coordinate allow a conclusion to be made on the feasibility of applying the model proposed to the H₂O...HCl complex. The results obtained in the framework of the model using Airy functions show reasonably good agreement with the experiment. For the H₂O...HCl heterodimer, the absorption band has the same structureless form as for the water monomer. The absorption band (peaking at 161 nm) is seen to shift towards short wavelengths as compared with the water monomer H₂O ( 167 nm).     

The theoretical calculation of collision-induced predissociation

A theoretical treatment of the collision-induced predissociation of a diatomic molecule caused by collisions with an atom is given in terms of the impact parameter method. Numerical calculations of the collision-induced predissociation of the D ¹Π _u state of the hydrogen molecule due to collisions with helium atoms have been performed using an ab initio calculation of the electronic interaction of the helium atom and the hydrogen molecule. The results show good agreement with the available experimental evidence. The continuum energy of the predissociation fragments is shown to have an energy spread determined by the range of the interaction potential.     

Luminescence spectroscopy of the Bi single and dimer centers in Y3Al5O12:Bi single crystalline films

Luminescence of the Bi³⁺ single and dimer centers in UV and visible ranges is studied in YAG:Bi (0.13 and 0.27 at% of Bi, respectively) single crystalline films (SCFs), grown by liquid phase epitaxy from a Bi₂O₃ flux. The cathodoluminescence spectra, photoluminescence decays, and time-resolved spectra are measured under the excitation by accelerated electrons and synchrotron radiation with energies of 3.7 and 12 eV, respectively. The energy level structure of the Bi³⁺ single and dimer centers was determined. The UV luminescence of YAG:Bi SCF in the bands that peaked at 4.045 and 3.995 eV at 300 K is caused by radiative transitions of Bi³⁺ single and dimer centers, respectively. The excitation spectra of UV luminescence of Bi³⁺ single and dimer centers consist of two dominant bands, peaked at 4.7/4.315 and 5.7/6.15 eV, related to the ¹S₀→³P₁ (A band) and ¹S₀→ ¹P₁ (C-band) transitions of Bi³⁺ ions, respectively. The excitation bands that peaked at 7.0 and 7.09 eV are ascribed to excitons bound with the Bi³⁺ single and dimer centers, respectively. The visible luminescence of YAG:Bi SCF presents superposition of several wide emission bands peaking within the 3.125-2.57 eV range and is ascribed to different types of excitons localized around the Bi³⁺ single and dimer centers. Apart from the above mentioned A and C bands the excitation spectra of visible luminescence contain wide bands at 5.25, 5.93, and 6.85 eV ascribed to the O²⁻→Bi³⁺ and Bi³⁺→Bi⁴⁺ + e charge transfer transition (CTT) in Bi³⁺ single and dimer centers. The observed significant differences in the decay kinetics of visible luminescence under excitation in A and C bands of Bi³⁺ ions, CTT bands, and in the exciton and interband transitions confirm the radiative decay of different types of excitons localized around Bi³⁺ ions in the single and dimer centers.     

Ultrahigh-resolution laser spectroscopy of the S1B2u ← S0Ag transition of perylene

A rotationally resolved ultrahigh-resolution fluorescence excitation spectrum of the S₁ ← S₀ transition of perylene has been observed using a collimated supersonic jet technique in conjunction with a single-mode UV laser. We assigned 1568 rotational lines of the band, and accurately determined the rotational constants. The obtained value of inertial defect was positive, accordingly, the perylene molecule is considered to be planar with D_2h symmetry. We determined the geometrical structure in the S₀ state by ab initio theoretical calculation at the RHF/6-311+G(d,p) level, which yielded rotational constant values approximately identical to those obtained experimentally. Zeeman broadening of each rotational line with the external magnetic field was negligibly small, and the mixing with the triplet state was shown to be very small. This evidence indicates that intersystem crossing (ISC) in the S₁¹B_2u state is very slow. The rate of internal conversion (IC) is also inferred to be small because the fluorescence quantum yield is high. The rotational constants of the S₁¹B_2u state were very similar to those of the S₀¹A_g state. The slow internal conversion (IC) at the S₁ zero-vibrational level is attributed to a small structural change upon electronic transition.     

An explanation of the very low fluorescence and phosphorescence in pyridine: a CASSCF/CASMP2 study

In this work, we applied the multiconfigurational complete active space self-consistent field method and the multiconfigurational second-order perturbation theory CASMP2 to study the fundamental excited states of pyridine and its possible photophysical and photochemical transformations. Our calculations, which are in agreement with the experimental results corresponding to excitations around the 0–0 transition, showed that the very low experimentally observed fluorescence of pyridine is due to the presence of two almost isoenergetic crossings, one of triple character, S₁/T₁/S₀ and the other of S₁/S₀ character. Both crossings are below the minimum of S₁(nπ^*) and have a common transition state (S₁(TS)) with a very low energy barrier (1.85 kcal/mol or 0.08 eV at the CASMP2 level of theory) separating them. A third triple crossing of the type S₁/T₁/S₀ lying lower with respect to the other two elucidates the observed T₁→S₀ radiationless transition. This explains not only pyridine's very low fluorescence and phosphorescence but also its almost negligible photochemistry, showing that photophysics is the prevalent process in this molecule.     

Gas phase electronic spectrum of the HSCCS radical by laser-induced fluorescence spectroscopy

In a discharged supersonic jet of a CS₂ and C₂H₂ mixture, a vibronic band system of a new radical species was observed in the energy region 21 800-23 000 cm⁻¹ by laser-induced fluorescence (LIF) spectroscopy. The LIF excitation spectrum shows progressions with 490 and 80 cm⁻¹ separations. The vibronic structure of a dispersed fluorescence (DF) spectrum, obtained by tuning a probe laser to the vibronic origin band, also consists of progressions with 520 and 100 cm⁻¹ separations. A high-resolution laser scan provided a rotationally resolved LIF excitation spectrum for the vibronic origin band, showing the rotational structure of a-type transitions of a near-prolate top. Several chemical tests indicate that the spectral carrier contains sulfur atom(s), one hydrogen atom and more than one carbon atoms. Electronic transition energy, vibrational frequencies, and rotational constants of this species are similar to those of SCCS⁻ [M. Nakajima, Y. Yoneda, Y. Sumiyoshi, T. Nagata, Y. Endo, J. Chem. Phys. 119(2003)7805-7813.], and the spectral carrier was assigned as an isoelectronic radical, bent HSCCS. Ab initio geometrical optimizations supported the spectral carrier to be HSCCS. The observed electronic transition was assigned to be the transition, which corresponds to the Π-Π transition in the limit of linear geometry. The observed vibrations in the excitation and DF spectra were assigned as the symmetric CS stretching (ν₅) and SCC bending (ν₇) modes by comparing the results of theoretical calculations.     

Electronic excitation and singlet-triplet coupling in uracil tautomers and uracil-water complexes

Electronic spectra of uracil in its diketo (lactam) form and five enol (lactim) tautomeric forms have been investigated by means of combined density functional and configuration interaction methods. We have simulated the effects of hydrogen bonding with a protic solvent by recomputing the spectrum of uracil in the presence of two, four, or six water molecules. Geometries of the electronic ground state and several low-lying excited states have been optimized. Spin-orbit coupling has been determined for correlated wavefunctions employing a non-empirical spin-orbit mean-field approach. In accord with experiment, we find the diketo tautomer to be the most stable one. The calculations confirm that the first absorption band arises from the ¹( π↦π^*) S ₀↦S ₂ excitation. The experimentally observed vibrational structure in this band originates from a breathing mode of the six ring. Complexation with water molecules is seen to cause a significant blue shift of n↦π^* excitations while leaving π↦π^* excitations nearly uninfluenced. Computed radiative lifetimes are presented for the experimentally known weak phosphorescence from the π↦π^* excited T₁ state. Among the uracil lactim tautomers, one is particularly interesting from a spectroscopic point of view. In this tautomer, the π↦π^* excitation gives rise to the S₁ state. Received 18 February 2002 / Received in final form 5 June 2002 Published online 13 September 2002     

The aΔg→XΣg magnetic dipole transition of S2

Emission spectra of the 0-0 band of the a¹Δ_g→X³Σ⁻_g magnetic dipole transition of S₂ have been observed in the near-infrared spectral region near 4400 cm⁻¹. The S₂ molecules were generated in a fast-flow system by passing S_x or S₂Cl₂ vapor in Ar carrier gas through a microwave discharge and were excited by electronic-to-electronic energy transfer from metastable singlet oxygen O₂(a¹Δ_g). Medium-resolution spectra of the b¹Σ⁺_g→X³Σ⁻_g and a¹Δ_g→X³Σ⁻_g transitions of S₂ were measured with a Fourier-transform spectrometer. By comparing the bandshape of the 0-0 band of the a→X system with a computer simulation calculated with literature data of the rotational constants of the X and a states, the origin of the 0-0 band was determined to be ν₀=4394.25±0.2 cm⁻¹.     

Model for the blue photoluminescence of cadmium sulfide

A model for the blue luminescence of polycrystalline cadmium sulfide and cadmium sulfide single crystals is proposed on the basis of a joint study of the absorption, reflection, emission, and excitation spectra of this luminescence at 77 °K. The blue luminescence arises in the transition of an electron from the conduction band to the 5s^{2 1}S₀ ground level of a super-stoichiometric cadmium atom which has trapped a hole from the valence band. The width of the energy gap (2.614 eV) at κ = 0 is estimated on the basis of the oscillatory nature of the excitation spectrum for the blue luminescence, and the effective mass of heavy holes is estimated.     

Ab Initio Study of the Phosphorescence of Nitrite Ions

In order to interpret the phosphorescence spectra of NaNO₂ and similar single crystals, we performed MCSCF geometry optimization in the ground singlet (X ¹ A ₁) and in the first excited triplet (a ³ B ₁) states of the NO ₂ ^- ion and MCSCF quadratic response (QR) calculation of the a ³ B ₁–X ¹ A ₁ transition probability at different bending angles and asymmetric stretch modes. The complete form of the spin–orbit coupling (SOC) operator is accounted for in the QR procedure. Dunning's correlation-consistent polarized valence double- (cc-pVDZ) and triple- (cc-pVTZ) basis sets are imployed. The electric-dipole transition moment from the T ^z spin sublevel (z is the C ₂ axis) oriented along the y direction (the other in-plane axis) is found to be 5 times higher than that from the T ^y sublevel in the ground-state geometry. This is in agreement with polarization measurements and with optical detection of ESR spectra. The T ^z–S ₀ transition moment decreases almost linearly with an increase in the ONO bond angle. The so-called non-Condon effects in the phosphorescence spectra of NaNO₂ crystals are explained on these backgrounds. The long progression of the bending vibrations (v ₂, a ₁) with an anomolous intensity distribution in the T ^z–S ₀ transition and additional involvement of the asymmetric stretch mode (v ₃, b ₂) in the T ^y–S ₀ transition are interpreted by force field and SOC calculations in the MCSCF-response technique. Configuration interaction (CI) calculations of the spin-allowed electric dipole transitions in NO ₂ ^- ions with effective one-electron SOC operator matrix element estimations were done for comparison with the results of the quadratic and linear response methods. Other T _n–S ₀ transitions are also studied. Finally, a short discussion of nonradiative processes is presented.     

Mechanism of H2S molecule adsorption on the GaAs(100) surface: Ab initio quantum-chemical analysis

Ab initio quantum-chemical cluster calculations within the density-functional theory were carried out to study the mechanism of H₂S molecule adsorption on the gallium-rich surface of GaAs(100). It was shown that adsorption can occur in four stages: molecular adsorption; dissociative adsorption, during which an HS radical is adsorbed on a gallium atom comprising a dimer while the detached hydrogen atom is adsorbed on another surface atom of the semiconductor; hydrogen adatom migration between neighboring surface atoms of the semiconductor; and the formation of a Ga-S-Ga bridge bond and of a hydrogen molecule. The stationary-state energies and energy barriers to transitions between these states were determined. The conclusions drawn based on an analysis of calculated diagrams of the potential energy of the processes that occur are in good agreement with the experimental data available in the literature.