Observation of transient resonance raman spectra of the S1 state of trans-stilbene

Transient resonance Raman spectra of trans-stilbene in n-hexane have been obtained using two pulsed lasers at 266 and 585 nm. The former was used to pump the molecule to the first excited singlet state (S₁) and the latter to proble spontaneous Raman scattering in resonance with the S_n ← S₁ electronic transition. The dependence of the pump and probe laser power, the temporal behavior, and the excitation profile of the spectra clearly indicate that they are due to the S₁ state of trans-stilbene.     

Laser excited S2 → S1 and S1 → S0 emission spectra and the S2 → Sn absorption spectrum of azulene in solution

We present the S₁ → S₀ fluorescence spectrum, between 740 and 940 nm, of azulene solutions (10^?3 M in methanol) excited with a Q-switched ruby laser. The nitrogen-laser excited S₂ → S₁ fluorescence spectrum, between 700 and 930 nm, is also reported. The transient S₁ → S_n spectrum between 500 and 650 nm was studied, using synchronous nitrogen laser and dye laser excitation. The S₅ (¹B₁(3)) state of azulene was found to be located at 45500 cm^?1 and the cross section σ₂₅ of the transient absorption S₂ → S₅ is estimated to be 3 × 10^?18 cm²/molecule.     

Theoretical study of the force field and vibrational assignments of thioacetamide and its deuterated analogues

The harmonic force field of thioacetamide has been obtained by Hartree-Fock level ab initio calculations using the 4-31G(d) basis set. The force constants have been scaled to reproduce the infrared fundamental bands of argon matrix isolated CH3CSNH2 and CD3CSNH2. Normal coordinate calculations based on the scaled force field suggested the co-presence of cis-CH3CSNHD and trans-CH3CSNHD in the infrared spectrum measured for CH3CSND2. The prediction of the fundamental bands was made for CD3CSND2, cis-CD3CSNHD and trans-CD3CSNHD.     

Distinction of the delayed fluorescences S1 → S0 And S2 → S0 of pyrene by selective quenching Of S1 → S0

In the spectrum of the delayed fluorescence (DF) of pyrene, caused by triplet-triplet annihilation T₁ + T₁ → S_n + S_o (n = 1,2), a strong DF S₁ → S_o and a very weak DF S₂ → s₀ are observed. The DF S₁→ S_o is quenched selectively by compounds like N-diethylanine or triethylamine which do not quench T₁ of pyrene.     

Theoretical study of the force field and vibrational assignments of acetamide and deuterated analogues

The complete harmonic force constants of acetamide have been evaluated by ab initio calculations at the Hartree-Fock level with the 4–31G(d) basis set. The force field was scaled to compensate for the systematic overestimations of the Hartree-Fock-level force constants by empirical factors using the matrix isolation IR spectra of CH₃CONH₂ and CD₃CONH₂. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH₃CONH₂. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH₃COND₂, cis-CH₃CONHD and trans-CH₃CONHD. The effect of cis/trans isomerism of CH₃CONHD on the fundamental bands was well reproduced by the calculations. The fundamental vibrations were also predicted for CD₃COND₂, cis-CD₃CONHD and trans-CD₃CONHD.     

Quantum-chemical analysis of vibrational intensity distributions in the S0-S1 absorption spectra and raman excitation profiles of azulene

The vibronic, Franck—Condon and Dushinsky activity of the totally modes of azulene in S₀-S₁ transition is investigated by quantum-chemical methods. It is found that the 900 and 1400 cm⁻¹ mode mixing proposed earlier to explain the relative intensities of Raman fundamentals in the region of the S₁ state is not supported by the calculations. The alternative mechanism arising from vibronic activity of the a₁ modes produces signs of the non-Condon parameters in agreement with those found from fitting the Raman excitation profiles; however, the absolute value of this parameter for the 900 cm⁻¹ mode is almost one order of magnitude too small. Thus, although quantum-chemical calculations favour non-Condon effects over Dushinsky effects in shaping the relative intensities of Raman fundamentals, they do not give a conclusive answer. It is argued that such an answer can be obtained from measurements of the 900 + 1400 cm⁻¹ combination band Raman excitation profiles.     

Structures of 1,2,3-trihydroxybenzene in the S0 and S1 states

In this paper we report on the structure and vibrations of gaseous pyrogallol (1,2,3-trihydroxybenzene) in the electronic ground state (S₀) and its first electronically excited state (S₁). Both ab initio CASSCF/CASMP2 calculations as well as R2PI spectroscopy have been performed. From the ab initio calculations three minimum energy structures are obtained and the vibrations of two structures are observed in the R2PI spectra. The minimum energy structures differ by their OH torsional angles. The full three-dimensional potential energy surface of the coupled torsional motions is investigated and the three-dimensional eigenvalues are calculated. The most stable structure of pyrogallol contains two intramolecular hydrogen bonds and turns out to be planar in the S₀ state. In the S₁ state the free OH group is rotated out of the plane of the aromatic ring by about 40°. The strong change in geometry of this structure is predicted by the CASSCF calculations and confirmed by the R2PI spectra of pyrogallol and its deuterated species. The low frequency region of the R2PI spectra can be explained by a torsional motion and the out of plane vibration 17b.     

Effect of spin-orbit coupling in para-dihalogenobenzenes on the environment-induced frequency changes in their S1 ← S0 spectra

The consequences of spin-orbit coupling in p-dihalogenobenzenes for solvent-induced shifts or S₁ ← S₀ spectral transitions, and for changes of S₁-state vibrational frequencies compared with the S₀ state, are assigned to mixing low-energy triplet states into the singlet-state system.     

Electronic and vibrational resonance Raman spectra of octamethyluranocene

The Raman spectrum of bis(tetramethylcyclo-octatetraene)uranium (U(TMCOT)₂), excited in resonance with its visible charge-transfer transitions shows an anomalously polarized electronic band at 473 cm^?1, twice as broad as the analogous band of uranocene, at 466 cm^?1. The broadening is attributed to crystal-field splitting associated with the lowered symmetry introduced by the methyl group, and/or a distribution of rotamer populations. Totally symmetric vibrational modes are observed at 879, 750, 580, 513 and 95 cm^?1; possible assignments are discussed.     

Theory of the S0 → S1 two-photon transition in benzene

The symmetry forbidden two-photon excitation spectrum recently observed in benzene vapour gave intensities inverted from those expected by current predictions based on the Herzberg-Teller theory for vibronic induced processes. We show that an extension of the HT approximation which allows us to correlate the movement of electrons and nuclei leads to an agreement with the experimental results.     

Investigation of the internal conversion time of the chlorophyll a from S3, S2 to S1

By using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS-TR-SEP-FD), we present a fluorescence depletion study of chlorophyll a (Chl a) in various solvent environments. Internal conversion times (IC) in different solvents are observed. It is found that all the S₃ and S₂ to S₁ transition processes are extremely fast, and that the time constants of these processes range from 100 to 260 fs. In aprotic solvent, the rate of IC decreases with an increasing of solvent polarity.     

Origin of the variation with vibrational and rotational state of the fine-structure constants in O2, SO and S2

Calculations of the spin—orbit part λ_SO of the fine-structure parameter λ have been performed at several internuclear distances for the molecules O₂, SO and S₂ in their ground (X ³Σ^?) states. Only the interaction with the lowest ¹Σ⁺ state was considered and two-centre integrals were neglected. λ_SO is found to make the dominant contribution to λ. The variation of the matrix elements in λ_SO with internuclear distance has been derived and when combined with expressions for the variation of the energy denominator in λ_SO enables the coefficients for the vibrational and rotational dependence of λ to be estimated. Excellent agreement with experiment is found.     

SVL fluorescence and duschinskii effect in the S1 state of pyridine

SVL fluorescence spectra from principal bands in the S₁ state of pyridine h₅ and -d₅ were measured. A large displacement of the normal coordinate origin of v_6a and the Duschinskii rotation of v_10a and v_16a were established.     

FT-IR and Raman spectra vibrational assignments and density functional calculations of 1-naphthyl acetic acid

This work deals with the vibrational spectroscopy of 1-naphthyl acetic acid. The molecular vibrations of 1-naphthyl acetic acid (NAA) is investigated in polycrystalline sample, at room temperature, by Fourier transform infrared (FT-IR) and FT-Raman spectroscopy. In parallel, ab initio and various density functional (DFT) methods were used to determine the geometrical, energetic and vibrational characteristics of NAA. On the basis of B3LYP/6-311+G** method and basis set combinations, a normal mode analysis was performed to assign the various fundamental frequencies according to the total energy distribution (TED). Simulation of infrared and Raman spectra, utilizing the results of these calculations led to excellent overall agreement with observed spectral patterns by refinement of scale factors.     

Vibronic spectroscopy of jet-cooled p-cyanobenzyl radical: D1→D0 spectra and vibrational mode assignments

The jet-cooled p-cyanobenzyl radical has been generated from p-tolunitrile and vibronically excited with a carrier gas, He, using a pinhole-type nozzle in a corona excited supersonic expansion. The vibronic emission spectrum of the p-cyanobenzyl radical in the transition of D₁(2 ²B₂)→D₀(1 ²B₂) has been recorded with a long-path monochromator in the visible region. The spectrum was analyzed to obtain the vibrational mode frequencies of the p-cyanobenzyl radical in the ground electronic state with the aid of an ab initio calculation as well as those of p-tolunitrile.     

Resonance Raman spectra and vibrational assignments of Azulene-d0 and Azulene-d8

The observed resonance effects in the Raman spectra of Azulene (C₁₀H₈) and its completely deuterated analog are utilized to assign the totally symmetric modes of these molecules. In conjunction with the data on the i.r. spectra and the normal coordinate analysis a complete assignment of the vibrational modes of these molecules is presented.The experimental excitation profiles are analyzed in the light of some current theoretical approaches. The experimental data strongly suggest that the variation of transition moment with the vibrational coordinate gives a dominant factor in determining the resonance Raman intensity of a mode.     

The vibrational spectra of maleic anhydride and its deuterated species

The infrared spectra of maleic anhydride and its mono- and dideuterated species were recorded in the region 5000-50 cm⁻¹ in the vapour phase and in solution. Raman spectra of the solids and supercooled melts were obtained. Assignments of the fundamental vibrational frequencies are given, based upon semiquantitative polarization data and the infrared vapour contours as well as on the product rule and complete isotopic rule for the three compounds.     

Unraveling anharmonic effects in the vibrational predissociation spectra of H5O2(+) and its deuterated analogues

The nature of anharmonic couplings in the H(5)O(2)(+) "Zundel" ion and its deuterated isotopologues is investigated through comparison of their measured and calculated vibrational spectra. This follows a recent study in which we reported spectra for H(5)O(2)(+), D(5)O(2)(+), and D(4)HO(2)(+) from ～600 to 4000 cm(-1), as well as H(4)DO(2)(+) in the OH and OD stretching regions [ J. Phys. Chem. B 2008 , 112 , 321 ]. While the assignments of the higher-energy transitions associated with the fundamentals of the exterior OH and OD motions are relatively straightforward, difficulties arise in the assignment of the lower-frequency regions that involve displacement of the bridging proton, especially for the isotopically mixed species. Here we revisit the Ar-tagged isotopomers, and report the low energy action spectrum of H(4)DO(2)(+) for the first time, as well as present significantly improved spectra for the D(4)HO(2)(+) and D(5)O(2)(+) systems. Band assignments are clarified in several cases using IR-IR hole-burning. We then investigate the physical origin of the anharmonic effects encoded in these spectra using a recently developed technique in which the anharmonic frequencies and intensities of transitions (involving up to two quanta of excitation) are evaluated using the ground state probability amplitudes [ J. Phys. Chem. A 2009 , 113 , 7346 ] obtained from diffusion Monte Carlo simulations. This approach has the advantage that it is applicable to low-symmetry systems [such as (HDO)H(+)(OH(2))] that are not readily addressed using highly accurate methods such as the multiconfigurational time-dependent Hartree (MCTDH) approach. Moreover, it naturally accommodates an intuitive evaluation of the types of motion that contribute oscillator strength in the various regions of the spectrum, even when the wave function is intrinsically not separable as a product of low-dimensional approximate solutions. Spectra for H(5)O(2)(+), D(5)O(2)(+), H(4)DO(2)(+), and D(4)HO(2)(+) that are calculated by this approach are shown to be in excellent agreement with the measured spectra for these species, leading to reassignments of two of the bands in the intramolecular bending region of D(4)HO(2)(+).     

Infrared,Raman spectra and vibrational assignment of (PF2)2O

The IR spectra of gaseous and solid (PF₂)₂O has been recorded from 80 to 1200 cm^?1. The Raman spectra of gaseous, liquid and solid (PF₂)₂O have also been obtained (30–1000 cm^?1). The spectra of the fluid phases indicate the presence of at least two conformers. The spectrum of the solid phase can readily be interpreted on the basis of a single conformer possessing a symmetry lower than C_2v. A vibrational assignment is proposed for all normal modes except the PF₂ torsions. The results are compared with similar data of related compounds. There appear to be two molecules per primitive cell based upon the low-frequency Raman data.     

Reactions of CO2 with H, H2 and deuterated analogues

Combining a temperature variable 22-pole ion trap with a cold effusive beam of neutrals, rate coefficients k(T) have been measured for reactions of CO₂⁺ ions with H, H₂ and deuterated analogues. The neutral beam which is cooled in an accommodator to T_ACC, penetrates the trapped ion cloud with a well-characterized velocity distribution. The temperature of the ions, T_22PT, has been set to values between 15 and 300 K. Thermalization is accelerated by using helium buffer gas. For reference, some experiments have been performed with thermal target gas. For this purpose hydrogen is leaked directly into the box surrounding the trap. While collisions of CO₂⁺ with H₂ lead exclusively to the protonated product HCO₂⁺, collisions with H atoms form mainly HCO⁺. The electron transfer channel H⁺ + CO₂ could not be detected (<20%). Equivalent studies have been performed for deuterium. The rate coefficients for reactions with atoms are rather small. Within our relative errors of less than 15%, they do not depend on the temperature of the CO₂⁺ ions nor on the velocity of the atoms (k(T) lays between 4.5 and 4.7 × 10⁻¹⁰ cm³ s⁻¹ with H as target, and 2.2 × 10⁻¹⁰ cm³ s⁻¹ with D). For collisions with molecules, the reactivity increases significantly with falling temperature, reaching the Langevin values at 15 K. These results are reported as k = α (T/300 K)^β with α = 9.5 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.15 for H₂ and α = 4.9 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.30 for D₂.