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 and non-radiative processes in jet-cooled NCNO

Lifetimes of excited vibronic levels in NCNO are measured both by LIF and by monitoring excited state absorptions. Fluorescence lifetimes are longer than S₁ radiative lifetimes at all wavelengths between the band origin (11339 cm⁻¹) and D_o (17085 cm⁻¹). In the language of radiationless transitions, the behavior below D_o is characteristic of the “small or intermediate molecule lim     

Dissociation of photochromic spiroanthrooxazine dimers in the excited state

The influence of temperature and concentration on the spectral polarization properties of the luminescence and absorption of molecules of the original form of photochromic spiroanthrooxazine (SAO) in petroleum ether and polyethylene was investigated. It was shown that the long-wave part of the absorption spectra of the molecules is formed by at least two -*-type electronic transitions, the oscillators of which are mutually orthogonal. It is suggested that the SAO molecules form dimers in nonpolar media through dipole-dipole interaction, and at high concentrations they also form higher associates. Dimers stable in the ground state dissociate when excited to the second electronic transition with a rate constant of 10¹²–10¹³ s^–1.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Department of the Institute of Solid-State Physics and Semiconductors, Academy of Sciences of Belarus, 211177 Vitebsk. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 99–106, January, 1992.     

Formation and decay of the triplet excited state of pyridine

A pulse radiolysis study of the formation and decay of the triplet excited state of liquid pyridine has been performed using quenching techniques. The pyridine triplet excited state is observed with an absorption band at lambda = 310 nm and has a first-order decay with a lifetime of 72 ns. Stern-Volmer plots of the quenching of the pyridine triplet excited state with anthracene, naphthalene, and biphenyl give its yield to be 1.3 molecules/100 eV. This value is very similar to the previously determined yield of 1.25 molecules/100 eV for dipyridyl, the predominant condensed-phase product in the gamma-radiolysis of liquid pyridine. The rate coefficient for pyridine triplet excited-state scavenging by oxygen is estimated to be 6.6 x 10(9) M(-1) s(-1). Oxygen may also scavenge the electron precursor to the pyridine triplet excited state, whereas nitrous oxide is observed to have little effect. A pyridyl radical-pyridine (dimer) complex produced in the pulse radiolysis of neat liquid pyridine is detected at lambda = 390 nm and is consistent with iodine scavenging effects. Formation of the pyridiniumyl radical cation-pyridine charge-transfer complex is proposed to be insignificant in liquid pyridine.     

Radiative decay of vibrationally excited CH2-

We have observed the presence of vibrationally excited CH₂^- created in a discharge, by measuring the photodetachment from CH₂^- as it radiatively relaxes in a high vacuum ion trap. We used a tunable IR laser to produce photons with energies above and below the expected threshold for removing an electron from the ground state. The time dependence of the photodetachment is consistent with the electron affinity of 5250 cm^-1 (0.65 eV) obtained by Sears and Bunker for the ground state X?³B₁ methylene. We have tentatively assigned radiative lifetimes for the excited bending vibrations of CH₂^-:600 ±300 msec for v₂ = 1,80±40msec for v₂ = 2, and 10± 5 msec for v₂ = 3.     

Direct observation of a sulfonyl azide excited state and its decay processes by ultrafast time-resolved IR spectroscopy

The photochemistry of 2-naphthylsulfonyl azide (2-NpSO(2)N(3)) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO(2)N(3) with 330 nm light promotes 2-NpSO(2)N(3) to its S(1) state. The S(1) excited state has a prominent azide vibrational band. This is the first direct observation of the S(1) state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S(1) state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO(2)) was inconclusive. The singlet sulfonylnitrene (1)(2-NpSO(2)N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl(4)) that decays to the lower-energy and longer-lived triplet nitrene (3)(2-NpSO(2)N). Internal conversion of the S(1) excited state to the ground state S(0) is an efficient deactivation process. Intersystem crossing of the S(1) excited state to the azide triplet state contributes only modestly to deactivation of the S(1) state of 2-NpSO(2)N(3).     

Intermediate case radiationless decay: the excited state dynamics of pyrazine

This paper reports new measurements of the non-exponential fluorescence decay of pyrazine, covering the time range 0.1-200 ns. The restits of our measurements remove the inconsistencies between experiment and the Frad-Lahmani-Tramer-Tric model of the radiationless decay in this molecule. In particular, our data show that the triplet component of the mixed singlet-triplet levels does increase with increasing triplet density of states. The effective density of triplet levels determined from our experimental data exceeds the theoretical density of vibrational levels. We propose that at the excitation levels achieved in the triplet manifold there is extensive scrambling of rotational states, but that conservation of nuclear spin states permits a level with total angular momentum quantum number f to couple to only (2J + 1/4) of the 2J+1 rotational levels built on one vibration. The appropriate theoretical density of states to be compared with experiment is then obtained by multiplying the vibrational density of states by J2. Good agreement is found between experimentally determined and calculated densities of states.     

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.     

Optical gain from the open form of a photochromic molecule in the solid state

This work presents evidence for line-narrowing from the UV photoexcited open form of the photochromic molecule, indolinospiropyran (1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro [2H-1-benzopyran-2,2'-(2H)-indole]) in the solid state. The line-narrowing is attributable to amplified spontaneous emission induced by optical gain and assisted by the waveguiding within the organic film. Optical gain throughout a band as large as 28 nm, with a maximum gain coefficient of 5.6 cm(-1), is observed in the merocyanine emission region (660-730 nm). These results open the way to the realization of hybrid devices based on the coupling between photochromic behavior and stimulated emission from conjugated molecules, such as lasing optical memories, and lasers gated by optical molecular switches.     

Radiative decay of nonstationary system

When a finite quantum system, say a fluorescent molecule is attached to a bulk surface and excited by a short laser pulse, the decay dynamics of the system is modulated by the surface and the signal is enhanced due to the bulk surface. We have considered the decay dynamics of a model of displaced distorted molecule whose excited potential surface is coupled to a continuum and then this first continuum is in turn coupled to a second continuum. In the short time scale there is a coherent exchange of energy between the system molecule and the first continuum states. In the long time scale the energy of the whole system plus first continuum drains out to the final continuum states. A dendrimer nanocomposite with the gold surface shows an enhanced light emission. This can be qualitatively understood from the model we proposed here. We have numerically studied the various potential parameters of the molecule which can affect the signal. When the potential surfaces are flat, the band structure of the first continuum states along with its initial excitation has some nontrivial effect on the profile of the radiative decay.     

Multivariate analysis of emission decay matrices for distinguishing ground state heterogeneity and excited state reactions of tryptophan

The amino acid tryptophan displays emission solvatochromism, an emission maximum that shifts with solvent polarity, which is often used in protein studies to indicate local environment hydrophobicity. Use of tryptophan solvatochromism in time-resolved protein studies has traditionally been complicated due to the undescribed photokinetics that result in a characteristic multiexponential emission decay. For the first time, by application of the photokinetic matrix decomposition (PMD) multivariate curve resolution method to time-resolved emission decay (TRED) data, a distinguishment between ground state heterogeneous (GSH) and excited state reaction (ESR) type photokinetics of tryptophan in solution is made possible. It is found that molecular tryptophan displays two emission spectra that decay independently, suggesting GSH type photokinetics, one at 347 nm with a lifetime of 0.5 ns and one at 363 nm with a lifetime of 3.1 ns. When tryptophan is incorporated into a peptide, mastoparan X, the data similarly contain two emission spectra that decay independently, but are shifted in wavelength. Photobleaching experiments confirm that the PMD method is sensitive to tryptophan emission quenching, and therefore may be applied to determine the photokinetics of tryptophan that occur in proteins. Future applications of PMD analysis of tryptophan TRED data as a bioanalytical tool for further characterizing dynamic protein processes are discussed.     

Effect of solvent on the subnanosecond decay of the second excited singlet state of tetramethylindanethione

The S₂ → S₀ fluorescence spectra and quantum yields and the S₂ lifetimes of 2,2,3,3-tetramethylindanethione (TMIT) have been measured in several solvents using a synchronously pumped picosecond dye laser excitation system. The S₂ nonradiative decay rate is markedly solvent dependent. In inert perfluoroalkane solvents remarkably large S₂-S₀ fluorescence quantum yields (θ_f = 0.14) and long S₂ lifetimes (τ = 880 ps) are measured. Hydrocarbons are efficient excited-state quenchers.     

Preventing the formation of the long-lived colored transoid-trans photoisomer in photochromic benzopyrans

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.     

Precise determination of the intensity of 226Ra alpha decay to the 186 keV excited state

There is a significant discrepancy in the reported values for the emission probability of the 186 keV gamma-ray resulting from the alpha decay of ²²⁶ Ra to 186 keV excited state of ²²² Rn. Published values fall in the range of 3.28 to 3.59 gamma-rays per 100 alpha-decays. An interesting observation is that the lower value, 3.28, is based on measuring the 186 keV gamma-ray intensity relative to the ²²⁶ Ra alpha-branch to the 186 keV level. The higher values, which are close to 3.59, are based on measuring the gamma-ray intensity from mass standards of ²²⁶ Ra that are traceable to the mass standards prepared by HÓNIGSCHMID in the early 1930's. This discrepancy was resolved in this work by carefully measuring the ²²⁶ Ra alpha-branch intensities, then applying the theoretical E2 multipolarity internal conversion coefficient of 0.692±0.007 to calculate the 186 keV gamma-ray emission probability. The measured value for the alpha branch to the 186 keV excited state was (6.16±0.03)%, which gives a 186 keV gamma-ray emission probability of (3.64±0.04)%. This value is in excellent agreement with the most recently reported 186 keV gamma-ray emission probabilities determined using ²²⁶ Ra mass standards.     

Radiative lifetimes of excited W I levels

Time-resolved laser-induced fluorescence spectroscopy has been used to measure the lifetimes of 47 selected W I levels with energies between 27800 and 48200 cm^?1. A high-current hollow cathode produces an effusive beam of tungsten atoms and ions in the ground states and in metastable states. Selected states are populated with tunable dye laser pulses. The subsequent fluorescence is measured by means of a Tektronix 1 GHz transient digitizer. By taking into account the separately measured response function of the system the lifetimes can be determined from the full decay curve. All measurements are performed with linearly polarized laser beams adjusted to the magic angle in order to exclude systematic error sources. The decay curves also have been inspected for quantum beats and for saturation effects. A comparison with the scarce literature data is given.     

Peculiarities of the non-radiative decay of a single vibronic level in polyatomic molecules

Optical selection experiments in the first excited singlet state of the benzophenone molecule in solution confirm a theoretical prediction concerning the retardation of the electronic relaxation rate with increasing the excess vibrational energy in excited electronic states of a large molecule characterized by a small electronic energy gap.     

Nature of non-radiative processes involved in the excited state of 9-cyanoanthracene in presence of 2-methylindole/2-methylindoline quenchers. A laser flash photolysis study to reveal the medium effects

By using steady state and time-resolved (laser flash photolysis and single photon counting) spectroscopic techniques the quenching of the lowest excited singlet (S1) state of 9-cyanoanthracene (9CNA) by the donors (quenchers) 2-methylindole (2MI) and 2-methylindoline (2MIN) in solvents of different polarity has been studied. Both the transient absorption, by laser flash photolysis technique, and photobleaching measurements were made at the ambient temperature both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) solvents. The photobleaching efficiency (alpha) was found to depend significantly on the polarity of surrounding solvents and also on the molecular structures of the quenchers. In NH the values of alpha are found to be larger than the corresponding values observed in ACN for both 2MI and 2MIN which possess highly reactive H atom bound to the heterocyclic N atom. Following the results obtained from the transient absorption spectra of the present donor-acceptor molecules in the different polarity solvents, a scheme describing the overall reaction mechanisms of the different photoreactions involved has been proposed. The probable causes for the changes observed in the mechanisms of the photoreactions involved in the cases of 2MI and 2MIN donors have been discussed in the light of their canonical structures.