Alkali—SCl2 chemiluminescence: Vibrational population inversion in the B state of S2

S₂(B-X) chemiluminescence, primarily from ν′ = 0 and 1, has been observed in alkali atom—SCl₂ reactions. The addition of He or Ar buffer gas to the reaction chamber alters the reaction dynamics, and at certain buffer gas pressures the S₂ emission originates exclusively from the ν′ = 6–9 vibrational levels of the B(³Σ_u^?) state.     

Analysis of the Vibrational Structure of the <Emphasis Type="Italic">n</Emphasis>–π* Transition in the High-Resolution UV Absorption Spectrum of Methacryloyl Fluoride in the Gas Phase

The UV absorption spectrum of methacryloyl fluoride molecule in the gas phase is obtained in the wavenumber range of 32300–35900 cm^?1. The resolved vibrational structure of this spectrum consists of 153 absorption bands. The assignment of all bands has been made for the first time. Values ν_00trans = 35670.0 сm^?1 and ν_00cis = 35371.1 cm^?1 are determined. The fundamental frequencies for isomers in the S₀ and S₁ states are found. Several Deslandres Tables (DTs) are constructed for the torsional vibration of the s-trans- and s-cis-isomers of the investigated molecule using the NONIUS program. The origins in these DTs correspond to bands attributed to ν₀₀, and to the fundamental frequencies of each isomer in states S₀ and S₁. These DTs are used to determine harmonic frequencies ω_e, anharmonicity coefficients х₁₁, and the frequencies of torsional vibration 0–v transitions up to high values of vibrational quantum number v for s-trans- and s-cis-isomers in both electronic states. The frequencies of torsional vibrations for the s-trans-isomer and the s-cis-isomer in the S₀ state are ν″₁ = 80.9 сm^?1 and ν″₁ = 59.8 сm^?1, respectively. The frequencies for the s-trans- isomer and the s-cis-isomer in the S₁ state are ν′₁ = 134.1 сm^?1 and ν′₁ = 103.6 cm^?1, respectively.     

Vibration—rotation spectrum of methyl cyanide— analyses of ν2 and ν3 + ν4 bands

Infrared spectrum of methyl cyanide was recorded in the region from 2235–2320 cm⁻¹ with a working resolution of 0.05 cm⁻¹. The transitions of two parallel type bands ν₂ and ν₃ + ν₄ as well as the associated hot bands are assigned. Since the molecular constants for the ground and ν₈ vibrational states are known precisely by microwave study for this molecule, highly accurate molecular constants for the upper vibrational states have been determined from those data by a least-squares procedure.     

The microwave spectrum of dichlorosilylene (SiCl2) in excited vibrational states

The microwave spectrum of dichlorosilylene in excited vibrational states has been measured in the millimetre- and submillimetre-wave regions. Rotational and centrifugal distortion constants were determined for the ν₁, ν₂, 2ν₂ and ν₃ excited states of the Si³⁵Cl₂ isotopic species and for the ν₂ and 2ν₂ states of Si³⁵Cl³⁷Cl. Analysis of the Coriolis resonance between the ν₁ and ν₃ states of Si³⁵Cl₂ yielded values of the D Coriolis interaction constant with F constrained, of two higher-order terms and also an accurate value [5.402338(95) cm⁻¹] of the energy difference between the two excited vibrational states. The rotational constants of Si³⁵Cl₂ in the first excited states of the three normal vibrations were combined with those of the ground vibrational state reported in a previous paper [M. Tanimoto et al., J. Chem. Phys. 91, 2102 (1989)] to obtain the equilibrium structure, harmonic and cubic/third-order anharmonic potential constants.     

The rotational spectrum of the hydrogen-bonded heterodimer H2O…HF in the frequency range 40–80 GHz

The μ_a R-branch rotational spectra of the ground and first excited states of the three lowest vibrational modes of the H₂O…HF heterodimer have been observed in the frequency range 40–80 GHz. Coriolis perturbations between the ground vibrational state (υ = 0) and the first excited state of the out-of-plane bending vibration (υ_β(0) = 1) show that for a given J the K₋₁ = 2 levels of υ_β(0) = 1 lie approximately 3 cm⁻¹ above the K₋₁ = 3 levels of υ = 0. The vibrational separation between these two states is estimated to be 70±3 cm⁻¹. This value is consistent with those determined by other methods and reinforces the conclusion that ν_β(0) is governed by a double-minimum potential energy function with the quantitative form previously published. A perturbation is also observed in the first excited state of the hydrogen-bond stretching vibration υ_σ = 1. This manifests itself as a large, negative centrifugal distortion constant D_JK = −8.5 MHz compared with 2 MHz in the other vibrational states.     

Non-radiative transitions. I. S1 ⇝ S0 internal conversion calculations in benzene and deuterobenzene

Non-radiative rate calculations for the S₁ ⇝ S₀ transition are presented. Complete vibrational basis sets are used. A Morse oscillator potential is assumed, for both the ν₁ and ν₂ vibrations. It is shown that the ν₁ potential has a dominant influence on the excess energy behaviour of the rates similar to that of the ν₂. The increase of the normalized non-radiative rate curves with excess energy is consistent with experimental results. A comparison is made with the non-radiative rates determined using the local-mode approximation for the CH- and CD-stretch vibrations.     

Lifetimes of C-2 in rotational levels of the B̃2Σ+u state in the gas phase

Lifetimes of C^-₂ in rotational levels of the B?²Σ⁺_u:ν′ = 0, ν′ = 1 states have been measured. C^-₂ was produced from bromoacetylene and rare-gas metastables and the B?²Σ⁺_u—X?²Σ⁺_g transition was laser excited. The lifetimes are constant within a vibrational level, 77 = 8 ns for ν′= 0 and 73 = 7 ns for ν′ = 1. The oscillator strength f_νo = 0.044 ± 0.004.     

Stimulated Raman investigation of CO2-laser-excited SF6

Stimulated Raman spectroscopy is used to probe the photoexcitation dynamics of CO₂-laser-excited SF₆ in a molecular free-expansion jet. Time-dependent depletion of rotational levels in the ground state is observed by monitoring the ν₁ Raman spectrum at various delay times after the CO₂-laser excitation pulse. Optical Stark shifts are also seen, at short delay times, arising from resonances of the IR laser pulse with ν₃ transitions from both ground and ν₁ = 1 states. Molecules excited to the ν₃ = 1 level are detected by scanning the ν₁ + ν₃ ? ν₃ hot band. A direct determination of the anharmonicity X₁₃ = ?2.910 ± 0.002 cm^?1 is thus obtained.     

High-Resolution Experimental Study on Photodissocaition of N2O

We study the photodissociation dynamics of nitrous oxide using the time-sliced ion velocity imaging technique at three photolysis wavelengths of 134.20, 135.30, and 136.43 nm. The O(¹S_J=0)+N₂(X¹∑_g⁺) product channels were investigated by measuring images of the O(¹S_J=0) products. Vibrational states of N₂(X¹∑_g⁺) products were fully resolved in the images. Product total kinetic energy releases (TKER) and the branching ratios of vibrational states of N₂ products were determined. It is found that the most populated vibrational states of N₂ products are v=2 and v=3. The angular anisotropy parameters (β values) were also derived. The β values are very close to 2 at low vibrational states of the correlated N₂(X¹∑_g⁺) products at all three photolysis wavelengths, and gradually decrease to about 1.4 at v=7. This indicates the dissociation is mainly through a parallel transition state to form products at lower vibrational states, and the highly vibrational exited products are from a more bent configuration. This is consistent with the observed shift of the most intense rotational structure in the TKER as the vibrational quantum number increases.     

Deactivation of CO(AΠ) in individual vibrational levels

Resonance absorption of CO(A¹Π → X¹Σ⁺) fourth positive bands was used to excite CO molecules selectively into the ν′ = 0 and ν′ = 1 vibrational levels of the A¹Π state. Studies of the fluorescence spectra at different added gas pressures yielded effective cross sections for the vibrational relaxation of A¹Π, ν′ = 1 and for the quenching of A¹π, ν′ = 0 molecules. Very large cross sections up to gas kinetic were measured for the rare gases He, Ne, Ar, and Kr as well as for the molecular species H₂, D₂ and N₂     

Analysis of the molecular bands of D2H and H2D at 5600 Å

Spectral simulation was used to analyze the molecular rovibrational bands of D₂H and H₂D at 5600 Å. These bands were previously measured by the ion beam neutralization method. They were assigned to the electronic 3p²B₁ ? 2s²A₁ and vibrational (ν - ν″) = (0, 0, 0,-0, 0, 0) transitions. Least squares fits to the experimental line-positions were made to determine the asymmetric rotator constants A, B and C for the 2s²A₁ and 3p²B₁ ν = 0 states of D₂H and H₂D, hitherto unknown. Lorentz line-profiles were assumed for the D₂H and H₂D rotational lines, whose widths are mainly governed by the lifetimes of the lower states. The bands at 5600 Å were simulated and the 2s²A₁ state lifetimes were estimated to be σ ≥ 0.5 ± 0.2 ps for D₂H and σ ≥ 0.4 ± 0.2 ps for H₂D. Vibrational constants of D₃ and D₂H in the 2s²A₁ states are determined from the positions of the 0-0 and 0-1 vibrational bands given in respective experimental spectra previously measured. For the first time the vibrational constants ω₁ and ω₂ of the 2s²A₁ state of H₂D were estimated from the positions of the 0-0 and 0-1 band maxima. These vibrational constants are compared with the corresponding vibrational constants of their ions.     

Vibrational analysis of the SO+ (A2Π−χ2Πr) emission system by an isotopic study

Absolute vibrational quantum numbers for the SO⁺ (A²Π—χ²Π_r) emission system have been determined by measurement of isotope shifts between S¹⁶O⁺ and S¹⁸O⁺ bands. It has been found that the tentative ν″ values reported previously should be decreased by one vibrational quantum number, The definite molecular constants for the SO⁺ (A²Π,χ²Π_r) states are determined and compared with photoelectron spectroscopic data.     

Ab initio/empirical potential energy functions and stretching vibrational frequencies of the X̃ 2Π and à 2Π states of the chloroacetylene cation

Anharmonic potential energy functions for the stretching vibrational motions of the two lowest doublet states of chloro-acetylene cation have been constructed using ab initio RHF SCF calculations and some experimental information. Stretching vibrational frequencies are calculated variationally for four different isotopomers. The ν₁ vibrations of H¹²C₂³⁵Cl⁺ are predicted to occur at 3176 (X̃ ²Π) and 3231 (à ²Π) and the ν₂ vibration of the à state at 2081 cm⁻¹.     

Theoretical study of structure,vibrational and electronic spectra of isomers of methyl-3-methoxy-2-propenoate

A systematic quantum mechanical study of the possible conformations, their relative stabilities, vibrational and electronic spectra and thermodynamic parameters of methyl-3-methoxy-2-propenoate has been reported for the electronic ground (S₀) and first excited (S₁) states using time-dependent and time-independent Density Functional Theory (DFT) and RHF methods in extended basis sets. Detailed studies have been restricted to the E-isomer, which is found to be substantially more stable than the Z-isomer. Four possible conformers c′Cc, c′Tc, t′Cc, t′Tc, of which the first two are most stable, have been identified in the S₀ and S₁ states. Electronic excitation to S₁ state is accompanied with a reversal in the relative stability of the c′Cc and c′Tc conformers and a substantial reduction in the rotational barrier between them, as compared with the S₀ state. Optimized geometries of these conformers in the S₀ and S₁ states are being reported. Based on suitably scaled RHF/6-31G^** and DFT/6-311G^** calculations, assignments have been provided to the fundamental vibrational bands of both these conformers in terms of frequency, form and intensity of vibrations and potential energy distribution across the symmetry coordinates in the S₀ state. A complete interpretation of the electronic spectra of the conformers has been provided.     

Vibrational Spectra and Quantum Calculations of Ethylbenzene

Normal vibrations of ethylbenzene in the first excited state have been studied using resonant two-photon ionization spectroscopy. The band origin of ethylbenzene of S₁←S₀ transition appeared at 37586 cm^-1. A vibrational spectrum of 2000 cm^-1 above the band origin in the first excited state has been obtained. Several chain torsions and normal vibrations are obtained in the spectrum. The energies of the first excited state are calculated by the time-dependent density function theory and configuration interaction singles (CIS) methods with various basis sets. The optimized structures and vibrational frequencies of the S₀ and S₁ states are calculated using Hartree-Fock and CIS methods with 6-311++G(2d,2p) basis set. The calculated geometric structures in the S₀ and S₁ states are gauche conformations that the symmetric plane of ethyl group is perpendicular to the ring plane. All the observed spectral bands have been successfully assigned with the help of our calculations.     

Vibrational Spectrum and Gas‐Phase Structure of Disulfur Dinitride (S2N2)

Gas‐phase FTIR spectra of the ν₆ (B‐type) and the ν₄ (C‐type) fundamental bands of S₂N₂ (D_2h) were recorded with a resolution of ≤0.004 cm^?1 and the vibrational spectrum of S₂N₂ (D_2h) in solid Ar has been revisited. All IR‐active fundamentals and four combination bands were assigned in excellent agreement with calculated values from anharmonic VPT2 and VCI theory based on (explicitly correlated) coupled‐cluster surfaces. Accurate experimental vibrational ground‐ and excited‐state rotational constants of ³²S₂¹⁴N₂ are obtained from a rovibrational analysis of the ν₆ and ν₄ fundamental bands, and precise zero‐point average r_z (R_z(SN)=1.647694(95) Å, α_z(NSN)=91.1125(33)°) and semi‐experimental equilibrium structures (R_e(SN)=1.64182(33) Å, α_e(NSN)=91.0716(93)°) of S₂N₂ have been established. These are compared to the solid‐state structure of S₂N₂ and structural properties of related sulfur nitrogen compounds and to results of ab initio structure calculations.     

ℓ-Resonance effects in C2H2 near 13.7 μm. Part H: The two quantum hotbands

This article is the second part on ℓ-resonance effects on the rotation-vibration bands of acetylene observed in the ν₅ fundamental region. While the first part concentrated on the energy level analysis of the fundamental and the seven strongest hotbands originating in the ν₄ and ν₅ excited states for both major isotopes [Spectrochim. Acta 48A, 1203 (1992)], this article summarizes the results of the analysis of the hotbands 2ν₄ + ν₅ ← 2ν₄, ν₄ + 2ν₅ ← ν₄ + ν₅, and 3ν₅ ← 2ν₅ from which improved molecular constants for the 2ν₄ and three quantum energy levels were derived for the major isotope ¹²C₂H₂. The mixing levels within the excited vibrational states due to vibrational and rotational ℓ-resonance effects are discussed which lead to the identification of the strong “forbidden” Δℓ3 band, 2ν₄+ ν³₅←2ν^Oe₄ as a result of ℓ-resonance intensity perturbation.     

Interstate coupling and dynamics of excited singlet states of isolated diphenylbutadiene

In this paper, we report on absolute fluorescence quantum yields from photoselected vibrational states of jet-cooled 1,4-diphenylbutadiene for excess vibrational energies, E_v = 0−7500 cm⁻¹, above the apparent electronic origin of the S₁(2A_g) state. The pure radiative lifetimes, τ_r, of the strongly scrambled S₂(1B_u)—S₁(2A_g) molecular eigenstates (E_v = 1050−1800 cm⁻¹) show a marked dilution effect, (τ_r/τ_r(S₂) ≈ 40), being practically identical with the τ_r values from the S₁(2A_g) manifold (E_v = 0–900 cm⁻¹), which is affected by near-resonant vibronic coupling to S₂(1B_u) and exhibiting the dynamic manifestations of the intermediate level structure. Isomerization rates in the isolated molecule, which do not exhibit vibrational mode selectivity, were recorded over the energy range 0–6600 cm⁻¹ above the threshold.     

β‐Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy

β‐Carotene in n‐hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S₁ excited‐state absorption, a picosecond process is found that is absent when the C₉‐methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S₁ potential energy surface, involving dihedral angles between C₆ and C₉. In Raman studies, electronic states S₂ or S₁ were selected through resonance conditions. We observe a broad vibrational band at 1770 cm^?1 in S₂ already. With 200 fs it decays and transforms into the well‐known S₁ Raman line for an asymmetric C=C stretching mode. Low‐frequency activity (<800 cm^?1) in S₂ and S₁ is also seen. A dependence of solvent lines on solute dynamics implies intermolecular coupling between β‐carotene and nearby n‐hexane molecules.     

Calculations of IR-spectra of metastable bond isomers of ruthenium nitrosocomplexes [Ru(NO)Cl<Subscript>5</Subscript>]<Superscript>2−</Superscript> [Ru(NO)(CN)<Subscript>5</Subscript>]<Superscript>2−</Superscript> by DFT method

IR-spectra of stable and metastable isomers of ruthenium nitrosocomplexes, [Ru(NO)Cl₅]^2? and [Ru(NO)(CN)₅]^2?, have been calculated within the frames of density functional theory. Frequency assignment was refined on the base of analysis of normal vibration in internal vibrational coordinates. Calculation results confirm that spectrally observed metastable states are bond isomers with ν¹-ON and ν²-NO coordination.