n,π* Fluorescence from selected vibronic levels of pyrimidine-d4 vapour: franck-condon factors,ground and excited state anharmonic coupling

Some “single vibronic level” fluorescence spectra of pyrimidine-d₄ are reported. The progression forming modes are ν″_6a (659 cm⁻¹) and ν″₁₂ (1048 cm⁻¹) and in contrast to pyrimidine-d₀ excitation of up to 7 quanta may be clearly identified. The ground-state Fermi resonance between ν_6a1 and 2ν_16b2, so significant in the fluorescence spectra of pyrimidine-d₀, is absent for pyrimidine-d₄. Analyses of the spectra have enabled comparable studies of Franck-Condon factors and Fermi resonances to be carried out. They have provided confirmation of vibrational assignments and have provided the underlying reason for the much sharper and more discrete nature of the pyrimidine-d₄ spectra.     

Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

The multiphoton ionization (MPI) spectrum of toluene arising from the ¹B₂ (¹L_b) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν₁(a₁), ν₁₂(a₁) and ν₁₄(b)₂). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν₁₂ is vibronic coupling.     

Mass-analyzed threshold ionization spectroscopy of pyrimidine: determining the geometry in the first excited and the ionic ground states

Vibrational spectra of the pyrimidine cation in the electronic ground state were measured via several intermediate states of the first excited state (0⁰,16a¹, 16a², 16a⁴, 16b¹, 10b¹, 6b², 6a¹, 1¹, 4¹, 4² and 12¹) by mass-analyzed threshold ionization spectroscopy. For the first time, several vibrational modes could be assigned in the first excited and the ionic ground states. Anharmonic coupling is shown to occur in the first excited state due to Fermi resonance between the 1¹ and the 16a⁴ vibrations. From the results of the measurements and calculations presented here, pyrimidine is predicted to be planar in the first excited and the ionic ground states, and it belongs to the C_2V point group.     

High resolution infrared analyses of fundamentals and overtones in isotopic ketenes

Rovibrational structure in the ν₅, ν₆, ν₇, ν₈ fundamentals and 2ν₅, 2ν₆ overtones in each of H₂¹²C¹²CO, H₂¹²C¹³CO, H₂¹³C¹²CO has been assigned and analysed. The effects of Fermi resonance between ν₇ and ν₂+ν₈ are corrected for, and a set of accurate unperturbed vibrational frequencies and ¹³C isotopic frequency shifts therefrom is obtained. The changing effects of the major Coriolis interactions involving the out-of-plane fundamentals, ν₅ and ν₆, are manifest in varying isotopic displacements of equivalent Q branch structures between the isotopic species studied, from which accurate isotopic vibrational shifts are obtained. Variable anharmonicity constants associated with the vibration frequencies of 2ν₅ and 2ν₆ give evidence of major Fermi resonance interactions between both overtones and the ν₄ fundamental, which they straddle.     

Excited state geometry of and radiationless processes in the lowest B3u(nπ*) singlet state of s-tetrazine

High resolution absorption and laser induced emission spectra of the lowest B_3u(nπ^*) singlet state of s-tetrazine-h₂ and -d₂ in a benzene crystal at 1.8 K are presented and discussed. The absorption spectrum with origin at 17231 cm^?1 (h₂) is dominated by a progression in ν_6a and a Herzberg-Teller origin which has been assigned as ν₁. The absence of ν₁ in the emission spectrum is explained as being due to a destructive vibronic interference effect. The Franck-Condon envelope of the unique ν_6a progression in emission is used for a determination of the excited state structure and the limitations of this procedure are examined. Direct lifetime measurements using a dye laser and single photon counting techniques show the fluorescence lifetime of s-tetrazine-h₂ and -d₂ to be shorter than 1.5 ns. From a deconvolution of the emission pulse of dimethyl s-tetrazine its fluorescence lifetime in the gas phase is found to be 6.0 ± 0.3 ns. Through a comparison of the fluorescence quantum yield of s-tetrazine-h₂ and dimethyl s-tetrazine we calculate for s-tetrazine-h₂ a fluorescence lifetime of 1.5 ± 0.2 ns and a fluoresence quantum yield of 1.8 × 10^?3. The ratio of the emissive lifetimes of s-tetrazine-d₂ and -h₂ was measured from relative fluorescence yields and found to be 1.18 ± 0.05. Photodissociation quantum yield studies on s-tetrazine-h₂, -d₂ and dimethyl for excitation into the origin of the ¹B_3u(nπ^*) state show this yield to be in the range of 1.3 ± 0.3, and this could explain the low fluorescence yields of the s-tetrazines. The fluorescence quantum yields in the gas phase are found to vary among the vibronic levels of the ¹B_3u state. This finding is in agreement with earlier measurements by Vemulapalli and Cassen, but the report by these authors that such quantum yield variations also occurred in the rovibronic structure is not confirmed.     

Study of the spectrum of CH379Br between 1520 and 1700 cm−1: The ν3 + ν6 and the ν5 rovibrational bands

The ν₃ + ν₆ band of CH₃ ⁷⁹Br has been directly analyzed for the first time, and an r.m.s. standard deviation of 0.0035 cm⁻¹ was obtained over 394 lines of K″ΔK = 2 up to 12, through a least-squares calculation using an unperturbed model. Nevertheless discrepancies occur on sub-bands with K″ ⩾ 9, which remain not yet understood. In particular it seems difficult to explain them by a Fermi resonance with the ν₅ band, since it has been possible to fit properly around 580 lines of this band, belonging to sub-bands K″ΔK = 7 up to 16, taking only into account the Coriolis resonance with the ν₂ band and the l(2,2) resonance of ν₅.     

Vibrational anharmonicity and Fermi resonances in CHF2Cl

The infrared spectrum of CHF₂Cl has been recorded between 15 000 and 350 cm⁻¹. The Fermi resonance between levels involving ν₄ and 2ν₆ is analysed in bands extending from 800 cm⁻¹ to 7000 cm⁻¹ leading to a best value of k₄₆₆ = ± 14.98 cm⁻¹. In conjunction with the recent results of Amrein, Dubal and Quack, Molec. Phys. 56,727 (1985); estimates are reported for 38 out of 45 possible x_ij constants. A variation in the relative intensity of the two Q branches associated with ν₁, on cooling the gas cell, indicates that a hot band contributes to the upper branch at 3024.55 cm⁻¹. However, other evidence suggests that the latter arises also from the combination ν₂ + ν₇ + ν₉, in a very weak, close resonance with ν₁ at 3021.27 cm⁻. A number of anomalous band contours are reported.     

The predissociation of Li2 b3Πu by a3Σ+u

⁶Li₂ 1³Δ_g(F₁) → b³Π_u(F₁v = 0–11) rotationally resolved fluorescence spectra are recorded following perturbation-facilitated optical—optical double resonance excitation of 1³Δ_g via spin—orbit mixed A¹Σ⁺_u ∼ b³Π_u(F₁e) intermediate levels. The f-symmetry Λ-components of b³Π_u(F₁) are broadened above the 0.05 cm⁻¹ detection threshold owing to predissociation by the vibrational continuum of the a³Σ⁺_u state. The observed v = 0–11, N = 31f level widths were used to determine the potential energy curve for the Li₂ a³Σ⁺_u state in the region 2.35 < R < 2.60 Å and 11200 < E < 14900 cm⁻¹ (relative to E = 0 at the minimum of X¹Σ⁺_g). The a³Σ⁺_u ∼ b³Π_u curve crossing is at R = 2.57 Å and E = 11246 cm⁻¹ and the electronic part of the − BN·LL-uncoupling matrix element is 〈b Π¦L⁺ ¦aΣ〉 = 1.216H at an R-centroid Rv_bϵ_a = 2.61Å.     

Rate constants of the near-resonant E-E energy exchange processes SeS(b0+) + O2(X3Σg−) ⇌ SeS(X10+) + O2(a1Δg)

SeS radicals generated in a fast flow system were excited to their b0⁺, ν' = 0 vibronic state by absorption of Raman-shifted dye laser pulses at 1280 nm. From time-resolved measurements of the b0⁺ → X₁0⁺ fluorescence as a function of added gas pressure, the radiative lifetime of the b0⁺ = 0 state (τ₀ = 400 ± 100 μs) and quenching rate constants for H₂, D₂, N₂, CO, O₂, and CO₂ were deduced. Quenching of SeS(b0⁺, ν'= 0) by O₂ is attributed to the near-resonant electronic- to-electronic energy-transfer process (1), SeS(b0⁺, ν'₁ = 0) + O₂(X³Σ_g⁻, ν″₁ = 0) ⇌ SeS(X₁0⁺, ν″_f = 0) + O₂(a¹Δ_g, ν'_f = 0)−77 cm⁻¹, for which (k₁ = (1.4±0.3) × 10⁻¹² cm³ s⁻¹ was obtained. On the assumption of detailed balancing, k₋₁ was calculated to be (3.0 ± 0.7) × 10⁻¹²cm³ s⁻¹.     

Ab initio simulation of benzene Raman intensities

C₆H₆ Raman scattering activities calculated from harmonic model ab initio Hartree–Fock 6–311 ++ G(d, p) polarizability derivatives (and harmonic force fields calculated at the same level) accurately simulate experiment (to within 1% for the a_1g modes). Accurate predictions are also made for the e_2g modes (to within 5% for ν₇ and ν₉, and more poorly for ν₆ and ν₈ [in Fermi resonance with ν₆ + ν₁]) and for the e_1g out-of-plane mode, ν₁₀. Only the ν₆ in-plane CCC bending mode scattering activity is found to be anomalous. Systematic variation of the basis set indicates that the benzene scattering activities and depolarization ratios are strongly dependent on inclusion of both carbon and hydrogen atom diffuse functions in the basis set. Predictions are also made for ¹²C₆D₆ and for unmeasured intensities in ¹³C₆H₆. Measurements of a_1g mode scattering activities in the latter molecule are predicted to be useful in testing the harmonic Hartree–Fock Raman intensity model.     

Vibronic coupling activity in benzene

The absorption spectra of benzene-h₆, -d₃ and -d₆ have been remeasured in the region of the hot band transitions 1⁰₁6⁰₁ and 8⁰₁. Whereas for the first an intense doublet is found, for the latter two only a single intense peak is observed. This is taken as evidence of the inactivity of ν″₈ in inducing intensity in contradiction to some recent calculations. The strong doublet for benzene-h₆ has been attributed to Fermi resonance.     

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     

Density and temperature effects on relative Raman intensities in liquid toluene

Raman spectra of toluene at pressures up to 4.1 kbar and temperatures up to 100°C, have been studied. The frequency and intensity changes of the symmetric (ν_6a) and antisymmetric (ν_6b) ring breathing vibrations have been related principally to changes in density. Increasing density at constant temperature increases I(ν_6b/ν6a) and increases the frequency of (ν_6a) but has little effect on the frequency of ν_6b. Increasing temperature at constant density decreases I(ν_6b/ν_6a) and increases the frequency of ν_6a but has little effect on the frequency of ν_6b. An explanation of the different intensity changes with density for these two bands is suggested in terms of the contrasting volume changes associated with the two modes.     

Study of the ν4-2ν8 Fermi resonance in heavy isotopic species of CH3CN by the Modified Winther Method

The Modified Winther Method is used to calculate the Fermi coupling constant W for the resonance between ν₄ and 2ν₈ vibrational levels in four heavy isotopic species (¹³C, ¹⁵N) of gaseous CH₃CN. The change of isotopic mass is used as a frequency scanning variable. Reliable values of W (5.41 cm⁻¹)and of the anharmonicity of 2ν⁰₈ (−12.5 cm⁻¹) are obtained.     

Picosecond reversible reaction kinetics of excited p-dimethylaminoacetophenone and m-methyl-p-cyanodimethylaniline studied by oxygen quenching technique

Double fluorescence of p-dimethylacetophenone (DMAPh) in CH₃CN and m-methyl-p-cyanodimethylaniline (MCDMA) in CH₂Cl₂ has been observed and analyzed in terms of reversible excited state isomerisation of the primary excited form b^* to the strongly polar rotamer a^*. Using the oxygen quenching technique, the kinetics of the reactions have been solved and all rate constants separated. The “formal” lifetimes of the species b^*, τ_b ≡ (k_bf + k_bd + k_ba)^?1, are 1 ps and 2.2 ps for DMAPh and MCDMA, respectively. The first value fits well to the reorientation relaxation time of acetonitrile.     

On the assignment of SiH and SiD stretching frequencies: A reanalysis of the ν7 bands of Si2H6 and Si2D6 and a harmonic local mode force field for disilane

Assignments of ν(SiH) and ν(SiD) modes in SiHX₃, SiH₂X₂ and SiH₃X compounds are surveyed. With a few exceptions, ν(SiH)/ν(SiD) ratios are found to be those expected on the basis of harmonic local mode calculations, if ν(SiD) values are first divided by a factor R which normally lies in the range 1.010–1.011. Major deviations from this range indicate misassignments or Fermi resonances. Stretch-stretch interaction constants f′_a are tabulated, and agree excellently with ab initio values, where these are available. Where ν^is (SiH) values are known, the frequency sum rule is found to be obeyed closely. The ν₇ bands of Si₂H₆ and Si₂D₆ are reanalysed to yield an R value of 1.010. The ν(SiH) data for disilanes then yield, in a “6 × 6” refinement, values of the three interaction force constants f′_a, f′_g and f′_g, together with predictions of hitherto unlocated stretching fundamentals. Constancy of the R factor for different types of XH bonds is shown to be a consequence of Dennison's rules, and similar anharmonicity factors.     

Non-radiative decay and mode mixing in benzene

The non-radiative decay of mixed vibronic states is studied using the Fermi resonance 6²l⁰−11² states in S₁ benzene as a prototype. A strong dependence on the mixing coefficient is exhibited, and the results are compared with the experimental data of Stephenson et al. to elucidate the nature of the mixed state.     

The importance of ground-state vibronic mixing in molecular Raman scattering

We consider Albrecht's theory for Raman scattering of fundamentals in the far and pre-resonance regions. Destructive interferences inherent to the A and B terms augment the conventionally suppressed C term dramatically. Raman excitation profiles for the ν₁(a_1g) and ν₆(e_2g fundamentals in benzene can be well fitted with theoretical C-term profiles involving the ¹E_1u(π-π^*) state at 1800 A.     

Resonance CARS spectra and vibrational analysis of fluorene-h10 and fluorene-d10 in T1 state

The observation of time resolved resonance CARS spectra of fluorene-h₁₀ and -d₁₀ in their T₁ states in room temperature solution are reported. Observed Raman frequencies are assigned on the basis of a crude normal coordinate calculation of the a₁ fundamentals and a CNDO/S-Cl calculation. The agreement between the calculated and observed values of ν₆ and ν₈ CC stretching frequencies confirms the orbital nature of the T₁ state.