Rate of radiationless electronic transitions for molecules in the condensed phase

Rate constants of radiationless electronic transitions in a diatomic molecule in a crystal at non-zero temperature are calculated. The electronic terms of the molecule are simulated by the Morse potential. The crystal vibrations are assumed to be harmonic. The calculations are done under the assumption that perturbation theory is applicable to the operator inducing the electronic transitions. The vibrational interaction of the molecule with the medium is not supposed to be small. The results explain certain experimental data on the radiationless electronic transitions in aromatic hydrocarbons.     

On the role of intramolecular vibrations in the nonradiative decay of excited charge-transfer states of molecular complexes

An attempt for a theoretical treatment of radiationless transitions from excited charge-transfer states in molecular complexes is made within the framework of the statistical limit of radiationless transitions theory. This work deals with the S₁ → S₀ internal conversion in charge-transfer complexes of tetracyanoethylene (an electron acceptor) with benzene and toluene and their perdeuterated analogues. A dominant role of the high-frequency totally symmetric intramolecular vibrational modes in the nonradiative decay of excited charge-transfer states is assumed (this was inferred from the experimentally observed deuterium isotope effect on radiationless S₁ → S₀ transitions). Calculated absolute rate constants for internal conversion are found to be in good agreement with experimental ones. The results of our calculations reflect very well the observed moderate deuterium isotope effect.     

Radiative and nonradiative lifetimes in excited states of Ar,Kr and Xe atoms in a neon matrix

Synchrotron radiation with its intense continuum and its excellent time structure has been exploited for time resolved luminescence spectroscopy in the solid state. By selective excitation of n = 1, n′ = 2 exciton states of Xe, Kr and Ar atoms in a neon matrix we were able to identify the emitting states involved. Lifetimes within the cascade of radiative and radiationless relaxation between excited states as well as the radiative lifetimes for transitions to the ground state have been derived from the decay curves. Energy positions and radiative lifetimes of the emitting states correspond quite well with those of the free atoms. Radiative and radiationless relaxation processes take place within the manifold of excited states of the guest atoms. The rate constants for radiationless decay confirm an energy gap law. The order of the radiationless processes reaches in some cases extremely high values. Selection rules for spin and angular momentum are essential to understand the observed radiationless transition rates.     

Recent Development in Radiationless Transitions

Radiationless transitions consist of internal conversion, a spin-allowed electronci relaxation, and intersystem crossing, a spin-forbidden electronic relaxation. The historical development of radiationless transitions will be briefly reviewed.I will then discuss the importence of Duschinsky effect on the radiationless transitions of intermediate size molecules like ethylene. Ab initio calculations of the internal conversion in ethylene will be demonstrated. Sub-picosecond internal conversions have been observed to take place in the photosynthetic bacterial reaction centers. In this case,vibrational relaxation is not faster than internal conversion and thus "temperature" has no meaning in this case. I will show how to theoretically treat these ultrafast nonradiative processes.     

Franck-Condon factors in intermolecular intersystem crossing

It is concluded from the energy dependence of the rate constant for intersystem crossing between guests and host molecules in mixed crystals that Franck-Condon factors for inter- and intramolecular radiationless transitions are virtually identical.     

The importance of higher order vibronic mechanisms in radiationless transitions

It is shown that the magnitude of the higher order vibronic couplings pertinent to radiationless transitions are comparable to that of the first order term.     

Nuclear excitation in atomic processes

We have introduced a model of two indistinguishable quantum oscillators (IQO) and examined several processes of nuclear excitations in atomic transitions. These processes are: (i) nuclear excitation in an electron transition, (ii) radiationless transitions in muonic atoms, (iii) nuclear excitation in positron-electron annihilation, and (iv) inelastic photoelectric effect. Predictions of our IQO model for these processes are in very good agreement with the available experimental data.     

Effect of anion coordination upon radiationless transitions of the uranyl ion

At 80 K, where the deactivation processes in uranyl luminescence in solutions are temperature independent, the radiationless transition rate depends upon the presence of H₂O in the first coordination sphere of the uranyl ion, UO₂²⁺. It is found that such radiationless transitions are due to a photophysical intramolecular process.     

The electronic states of biacetyl

The ordering of the lower electronic states of biacetyl is discussed and the consequences for the mechanism of the radiationless S → T transitions are considered in detail.     

Magnetic-field-induced radiationless transitions

A quantum-mechanical theory of the magnetic field effect on radiationless transitions in an isolated molecule is presented. The mechanism of this phenomenon is a second-order process relying on the simultaneous radiationless relaxation of an excited electronic state and the interaction of the molecule with a constant uniform magnetic field.     

Radiationless decay in exciplexes with variable charge transfer

Rate constants for radiative decay, radiationless decay, and intersystem crossing are reported for a series of excited states formed by reaction of cyanoanthracene acceptors with alkylbenzenes as donors in several solvents of moderate to low polarity. The excited states have widely varying degrees of charge transfer, from essentially pure electron transfer states to pure locally excited states. The data illustrate the fundamental factors that control the contrasting relative efficiencies of radiative and radiationless processes in electron transfer compared to locally excited states. The radiationless decay rate constants can be described quantitatively as a function of the extent of charge transfer using weighted contributions from a locally excited decay mechanism and a pure electron-transfer type mechanism. The factors that control the rate constants for radiationless decay in excited states with intermediate charge-transfer character are discussed.     

Electronic spectroscopy and relaxation of gas phase molecular ions

Techniques for obtaining electronic spectra of singly charged cations are described. Jahn-Teller effects in the spectra of polyhalobenzene cations are discussed. Experimental and theoretical methods for studying radiationless transitions in molecular ions are presented. Recent work on the spectroscopy and intramolecular relaxation of doubly-charged cations is reviewed.     

Quantum control of internal conversion in 24-vibrational-mode pyrazine

Quantum control of the S(2)-->S(1) internal conversion in a complete 24-mode dimensionality model of pyrazine is demonstrated. The fully quantum mechanical study makes use of the recently developed "QP algorithm" for performing accurate computations of projected quantum dynamics and the role of overlapping resonances in control. The results are extremely encouraging, demonstrating active control over internal conversion so as to almost completely suppress the process over time scales of approximately 50-100 fs [well in excess of the natural internal conversion times (approximately 20 fs)] or to accelerate it to complete internal conversion in less than 5 fs. A number of new diagnostics are introduced to demonstrate the significance of overlapping-resonance contributions to control. Control is far better than for a reduced dimensionality model of pyrazine, presumably because of the increased degree of overlap between bound state resonances existing in the full dimensionality case.     

Overview of some quantum effects in chemistry

This article is a brief tutorial dealing with the conceptual aspects of (i) excitation transfer in molecular assemblies, such as occurs in the early steps of photosynthesis, (ii) radiationless transitions, which are ubiquitous participants in the light induced isomerization reactions that support vision and (iii) active control of product selection in a chemical reaction. All of these processes exhibit explicit quantum effects that arise from competition between coherent and incoherent evolution of the initial state of the system expressed in the atom and/or electron dynamics. Topic (iii) is associated with how a molecular assembly can be designed to optimize the use of coherent processes to improve efficiency of conversion of the initial excitation to product under the constraint that there are fluctuations in the surrounding medium.     

Active control of the lifetime of excited resonance states by means of laser pulses

Quantum control of the lifetime of a system in an excited resonance state is investigated theoretically by creating coherent superpositions of overlapping resonances. This control scheme exploits the quantum interference occurring between the overlapping resonances, which can be controlled by varying the width of the laser pulse that creates the superposition state. The scheme is applied to a realistic model of the Br(2)(B)-Ne predissociation decay dynamics through a three-dimensional wave packet method. It is shown that extensive control of the system lifetime is achievable, both enhancing and damping it remarkably. An experimental realization of the control scheme is suggested.     

Resonant scattering in the presence of strong laser fields

Resonant scattering in the presence of strong laser fields is investigated by solving numerically the one-dimensional Schrödinger equation. It is shown that at high intensities of radiation one can observe new effects in free-free transitions like, for instance, the splitting and overlapping of the scattering resonances. The relation of these phenomena to multiphoton ionisation processes is also discussed. The Schrödinger equation is solved by applying the approach proposed by Jung and Taylor [1] or by Varró and Ehlotzky [34]. It is also shown how the second approach can be generalised in order to treat more complicated problems like for instance the relativistic processes.     

THEORY ON THE RADIATIONLESS TRANSITIONS IN LARGE POLYATOMIC MOLECULES

Abstract— The stationary perturbation theory of Robinson and Frosch is shown to be inadequate to describe the radiationless transitions in polyatomic molecules. An alternative theory of radiative transfer of excitation energy proposed by Franck and Sponer is considered. Evidence shows that this theory leads to a general mechanism for many radiationless transitions in semi-isolated molecules.
Empirically, the rate constants of the excitation transfer between states i and f of the same energy can be expressed by a simple equation: k_t-f=10¹³( f_i.f_f )^0.56sec^-1 where f 's are oscillator strengths to the ground state. This equation is accurate to within an order of magnitude for various processes, such as internal conversions and intersystem crossings with rates differing by 14 orders of magnitude.
The radiative transfer is the result of second order perturbation on the molecule by the radiation field. The equation of rate constant derived from the perturbation energy agrees fairly well with the above equation.     

The reinterpretation of the zero-field double resonance spectra of the lowest excited 3T1u state of the F center in CaO in terms of random internal strain of Eg symmetry

The zero-field double resonance spectra of the relaxed excited ³T_1u state of the F center in CaO were determined and interpreted previously. In the present paper the phosphorescence microwave double resonance (PDMR) spectra are reinterpreted in terms of random internal strain of E_g symmetry. The strain variables of the subensemble of F centers responding to a particular microwave-induced transition are exclusively determined by the microwave and the phosphorescence frequency. Consequently, the level structure in this subensemble can be calculated. To each PMDR spectrum, two graphical representations describing the possible spread in the phosphorescence frequency and the vibronic level structures of the F centers contributing to the PMDR signal are related. From these graphical representations, the occurring radiationless transitions can be traced. It is shown that, at the low energy side, the bounds of the PMDR lines are determined by the random strain distribution whereas the upper bounds are determined by fast radiationless transitions or by the lack of a population difference between the spin levels involved in the microwave-induced transition.     

Radiationless transitions. Quantitative interpretation of some experiments.

The Green function method is used to calculate the rate of radiationless transitions. The obtained formula is applied for quantitative interpretation of the fluorescence quenching of anthracene, pyrene, methylanthracene in various solvents. The vibrational relaxation rates of small molecules substituted to the noble gas crystalline hosts are evaluated. The agreement with experiment is satisfactory.     

A Comparison of radiationless transitions involving single-minimum and double minima potential energy surfaces

The quantum-statistical-mechanical (QSM) approach to molecular relaxation phenomena is employed to compare radiationless transitions originating from an electronic state characterized by a single minimum and double minima potential surface for a vibronically active, non-totally symmetric mode. The vibronic level dependence of the decay rates of these two cases has been investigated for both small and large energy gap transitions. It is shown that the behavior of a molecular system is quite different for an initial state possesing a double minima potential surface as compared to the case in which the initial state possesses a single minimum.