Radiative and non-radiative decay processes of the excited state of the colored form of photochromic furylfulgide

The time evolution of the luminescence of the colored form of a furylfulgide dispersed at various concentrations in a poly(methyl methacrylate) film was measured as a function of the luminescence photon energy. The observed decay time of the luminescence is about 1–2 ns and one order of magnitude shorter than the radiative lifetime (14 ns) estimated from the absorption intensity. The decay time is independent of temperature below 77 K. These results suggest that the non-radiative tunneling process from the excited state to the ground state is responsible for the decay.     

Novel aspects of ultrafast non-radiative processes

We outline a new approach to determine theoretically the time-dependent intensity of fluorescence I(t) in systems with intersecting potential energy surfaces and strong non-adiabatic effects. To a good approximation, I(t) is found to be proportional to the probability of occupying the upper electronic state times a suitable combination of potential energy pre-factors. In the limit of weak non-adiabatic effects, and when the initial state is a single zero-order state, this quantity reduces to the usual squared modulus of the autocorrelation function. For systems with strong non-adiabatic effects, the present theory predicts drastic deviations from the autocorrelation-function approximation.     

On the temperature dependence of non-radiative deactivation processes

It is shown that the fluorescence quantum yield depends on temperature rather than on the viscosity of the solvent. The intersystem crossing rate can be described as a sum of a temperature independent and a temperature dependent term. For most of the molecules investigated in this paper the latter can be assumed to have the Arrhenius form. With anthracene, however, significant deviations from the Arrhenius behaviour are observed. With a more general expression involving the density of states the temperature dependence of the fluorescence quantum yield can be satisfactorily described.     

Radiative and non-radiative decay of selected vibronic levels of the state of alkoxy radicals

We have measured the lifetimes of some of the prominent bands observed in the moderate-resolution, jet-cooled laser induced fluorescence excitation spectra of the transition of ethoxy, all the structural isomers of propoxy and butoxy, and 1-pentoxy. Recent high-resolution, rotationally resolved studies on primary alkoxy radicals have given conformer specific assignments for the bands for which lifetimes have been measured. We report observed lifetime trends as a function of vibrational excitation for specific isomers and conformers. The implications of these observations for the dynamics of the state will be discussed.     

Radiative dielectronic recombination and correlated processes

A new quasi-resonant radiative dielectronic recombination (RDR) process is formulated as an off-shell extension of the dielectronic recombination. It is an electron-ion recombination process in which dielectronic-excitation capture is accompanied by simultaneous radiation emission, and can occur at all continuum energies of the electron being captured. The RDR can be the dominant mode of recombination at off-resonance energy regions, and may provide an important distortion correction to the direct recombination. The distinct signature of the RDR is the quasi-discrete low energy x-rays emitted during the capture, shifted down in energy by the excitation energies of the core electrons. The analog of the RDR must also take place in the ion-atom collisions, as radiative transfer excitation (XTE). Evaluation of the RDR cross section is difficult because of the correlated nature of the transition operator, but detailed qualitative discussions of the RDR are presented. A crude estimate of the cross section is obtained and compared with the other recombination modes. The radiative Auger effect is roughly an inverse of RDR, and the charge dependence of its cross section is improved, using a correlated vertex.     

Absolute fluorescence quantum yields for vapour-phase benzene and naphthalene,and comments on the non-radiative processes

Optic—acoustic measurements have been employed in the determination of absolute quantum yields for benzene and naphthalene. Heat yields are measured by a method using oxygen quenching of both triplet and singlet states. For vibrationally relaxed excited singlet states the fluorescence quantum yields, φ^B_f, are 0.16 ± 0.02 and 0.79 ± 0.02 for benzene and naphthalene respectively. For 0.07 torr naphthalene at room temperature with 248 nm excitation, φ_f = 0.35 ± 0.03 and the quantum yield of internal conversion is less than 0.05. The decay of the highly vibrationally excited triplet state is dominated by vibrational relaxation for 0.07 torr naphthalene, but for benzene, even at high pressures, strong competition comes from an indirect coupling process to the ground state.     

Isomerism in jet-cooled 1-cyanonaphthalene complexes

The effect of complexation on the first electronic transition of 1-cyanonaphthalene has been studied for complexes with aprotic polar solvents (acetonitrile, diethylether) or olefins (2-methyl-2-butene, 2,3-dimethyl-2-butene). In both cases, isomerism has been evidenced. With polar solvents, two strong absorption bands appear in the 0 ₀ ⁰ region, one being shifted to the red and the other one to the blue side of the bare cyanonaphthalene transition. These two bands have been assigned to two isomeric forms involving specific interactions between the solvent and the cyanonaphthalene molecule. These isomers have been shown to interconvert at higher vibronic energies. With the 2,3-dimethyl-2-butene a broad absorption band leading to a red shifted fluorescence has been observed and assigned to an exciplex. With the 2-methyl-2-butene, one observes the superposition of narrow and broad bands in the excitation spectrum. These two systems have been assigned to two isomeric forms, one of them crossing to an exciplex in the excited state. At higher vibronic energy, emission from the two conformers is observed.     

Vibrational energy redistribution in jet-cooled hydrogen-bonded phenols

Dispersed fluorescence spectra of jet-cooled hydrogen-bonded phenols have been observed by excitation of intra- and inter-molecular vibrational levels. The spectra show that vibrational energy redistribution occurs from the excited vibronic level into intermolecular vibrational modes. Energy redistribution within the intermolecular vibrational modes was also found.     

Fluorescence spectra and intramolecular vibrational redistribution in jet-cooled perylene

The jet-cooled fluorescence spectra of perylene excited to the S₁ state with E_vib = 0–1600 cm^?1 are recorded and analyzed. For E_vib <800 cm^?1 only the resonant fluorescence was detected. Ground- and excited-state frequencies of 14 low-frequency normal modes are determined. A drastic change in frequency of the “butterfly” modes upon electronic excitation shows that perylene slightly deviates from planarity in its ground state and is more rigid in the excited singlet state. For a number of levels in the E_vib = 800–1600 cm^?1 range, the fluorescence is composed of the resonant emission and of non-resonant (“‘relaxed’”) bands. It is shown that apparently single bands in the fluorescence-excitation spectrum correspond to ovelapping bands pumping different molecular eigenstates resulting from the intrastate coupling. The relative role of the anharmonicity and of the Coriolis interaction are discussed. The data are treated in terms of a selective coupling between doorway and hallway states with the coupling constant rapidly decreasing with the difference in the overall vibrational quantum number between initial and final state.     

Coriolis interactions and intramolecular vibrational redistribution in jet-cooled pyrimidine

Evidence is presented which indicate that the second-order Coriolis interaction play an important role in intramolecular vibrational redistribution.     

Vibrationally selective radiative and non-radiative transitions in gaseous hydrogen molecules

An efficient vibrationally selective technique to build-up the v″=1 vibrational levels in gaseous hydrogen is demonstrated using stimulated Raman pumping (SRP). Both photo-acoustic Raman spectroscopy (PARS) and coherent anti-Stokes Raman spectroscopy (CARS) are used to study non-radiative and radiative (v″=0 and v″=1) transitions in gaseous H(2) molecules. The population fraction in the v″=1 vibrational level has been estimated using combined photo-acoustic and coherent anti-Stokes Raman spectroscopy with stimulated Raman pumping.     

Two-color photoionization spectroscopy of jet-cooled nickelocene

Two-photon ionization of the nickelocene molecules cooled in a supersonic jet was performed for the first time by simultaneous excitation of (⁵–C₅H₅)2Ni with two tunable nano- second dye lasers. The one-photon transition from the HOMO to the Rydberg R4p level was used as the initial step of the multiphoton excitation. In a one-color experiment, the conditions were found for generation of the intact molecular ion, (⁵–C₅H₅)2Ni⁺, as the only ionic product of the multiphoton ionization. The use of an intense pulse of the second dye laser lead to an increase in the yield of the molecular ion.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1604–1606, August, 2004.     

Excitation and emission spectra of jet-cooled naphthylmethyl radicals

Gas phase excitation and emission spectra of three naphthylmethyl radical chromophores are presented. These resonance-stabilized species, 1-naphthylmethyl, 2-naphthylmethyl, and α-acenaphthenyl, each possessing an sp(2) carbon adjacent to a naphthalene moiety, are studied by resonant two-color two-photon ionization, laser induced fluorescence, and dispersed fluorescence spectroscopy. Identification of the radicals is made through a combination of dispersed fluorescence and density functional theory calculations. All three species possess spectra in the 580 nm region. The possible relevance to unidentified spectroscopic features such as the diffuse interstellar bands and emission from the Red Rectangle nebula is discussed.     

Hot bands in jet-cooled and ambient temperature spectra of chloromethylene

Rotationally resolved spectra of several bands lying to the red of the origin of the A(1)A" - X (1)A' band system of chloromethylene (HCCl), were recorded by laser absorption spectroscopy in ambient temperature and jet-cooled samples. The radical was made by excimer laser photolysis of dibromochloromethane, diluted in inert gas, at 193 nm. The jet-cooled sample showed efficient rotational but less vibrational cooling. Analysis showed that the observed bands originate in the (upsilon(1),upsilon(2),upsilon(3)) = (010), (001), and (011) vibrational levels of the ground electronic state of the radical, while the upper-state levels involved were (000), (010), (001), and (011). Vibrational energies and rotational constants describing the rotational levels in the lower-state vibrational levels were determined by fitting to combination differences. The analysis also resulted in a reevaluation of the C-Cl stretching frequency in the excited state and we find E(001)' = 13 206.57 or 926.17 cm(-1) above the A(1)A" (000) rotationless level for HC(35)Cl. Scaled ab initio potential energy surfaces for the A and X states were used to compute the transition moment surface and thereby the relative intensities of different vibronic transitions, providing additional support for the assignments and permitting the prediction of the shorter wavelength spectrum. All the observed upper state levels showed some degree of perturbation in their rotational energy levels, particularly in K(a) = 1, presumably due to coupling with near-resonant vibrationally excited levels of the ground electronic state. Transitions originating in the low-lying a(3)A" were also predicted to occur in the same wavelength region, but could not be identified in the spectra.     

Spectroscopy and dynamics of jet-cooled 1,1′ binaphthyl

Fluorescence excitation and dispersed fluorescence spectra of jet-cooled 1,1′-binaphthyl are reported and analysed. The spectra indicate that in the ground and excited state the naphthalene rings are perpendicular to one another. The spectra can be further interpreted in terms of an exciton model with an exciton splitting of 21 cm^?1 in the origin. From the structureless emission spectrum and lifetime it is concluded that, in the isolated molecule, efficient vibrational relaxation occurs through conversion of vibrational into librationaI energy.     

Rotational spectroscopy of jet-cooled molecular ions and ion complexes

Rotational spectra of the molecular ions HOCO⁺ and HOCS⁺, and the ion complexes, D₃⁺Ar and sym-D₂H⁺Ar were observed in a supersonic-jet expansion by using a Fabry-Perot type Fourier-transform microwave spectrometer cooperated with a pulsed discharge nozzle. Ion-formation efficiency for HOCS⁺ relative to the parent molecule under applied conditions was estimated to be ≈ 10⁻⁴. Tunneling splitting in the lowest rotational transition of D₃⁺-Ar was not resolved within the experimental linewidth of ≈ 100 kHz.     

Effects of anharmonicity on non-radiative transitions in polyatomic molecules

The effect of anharmonicity on the non-radiative transition in large molecules is examined within the Morse potential surface model. The vibrational wavefunctions are assumed to be the product of the harmonic and Morse oscillator wavefunctions. The method of factorization introduced by Gelbart et al. is used for the evaluation of a density weighted Franck-Condon factor. As an example, we choose the intersystem crossing ³ B _1u→¹ A _1g in benzene. The numerical calculation shows that the anharmonicity causes an increase by a numerical factor ～ 10³ in the non-radiative transition rate. The electronic energy distribution over the vibrational modes in the final state is determined and compared with that obtained using the harmonic potential surface model.     

Intramolecular relaxation of excess vibrational energy in jet-cooled perylene

The intramolecular redistribution of excess vibrational energy (IVR) in electronically excited perylene is being studied by fluorescence techniques. Analysis has shown, in agreement with the literature, little evidence of relaxation of fundamental modes up to ? 1100 cm^?1. However, it is also shown, contrary to literature assertions, that combination states from 700 to 1100 cm^?1 do not relax significantly on the time-scale of molecular fluorescence. The picture is simplified by reassignment of several key combination bands in the spectrum. Excitation at higher energies reveals differences in behaviour between combination bands involving high-frequency fundamentals and those only using fundamentals < 800 cm^?1. In the latter case, the persistence of narrow-line emission indicates substantially slower relaxation rates. As an example, the 1600 cm^?1 fundamental state appears to relax substantially faster than the 1603 cm^?1 satellite state, which is assigned to 353³550¹. This kind of disparity has been observed up to 2000 cm^?1. These data provide evidence for the importance of anharmonic interactions in determining the relative rates of IVR over short energy ranges.