Competition between excitation and electronic decay of short-lived molecular states

The nuclear dynamics accompanying the excitation to and the subsequent decay of an electronic state is discussed. Particular attention is paid to cases, in which the whole process cannot be divided into two steps (excitation and decay) since the excitation and the decay times are of the same order of magnitude. The recently introduced time-dependent formulation of the theory describing the wave packets’ dynamics is extended to include the excitation process. The wave packets can be related to the intensity of the emitted particles. Most of the resulting integrals can actually be performed by employing eigenstates of the Hamiltonians corresponding to the involved potential energy surfaces. This leads to the so called “timeindependent” formulation of the theory. Computational details of the implementation of the corresponding “timedependent” and “time-independent” methods are presented. Illustrative applications are given to illuminate both the influence of the excitation process and the lifetime of the decaying state. It emerges that the intuitive interpretation of the spectra (within the above two step model) may fail. Insight into the process is gained by studying the evolution of the spectra as a function of time. The appearance of “atomic lines” due to dissociative decaying and final states is investigated in some detail.     

A “reduced” model theory for molecular decay processes

It is shown how a many-level radiationless model may be replaced by an effective one involving only the states of interest. When the coupling between the states of interest and the dissipative quasicontinuum is assumed to be not a constant one, the effective interaction can exhibit a non-markovian feature even in the statistical limit. Then, irradiation fields the intensities of which are not negligible compared to the excited state linewidth, are shown to reduce the coupling between the system of interest and its effective “thermal bath”. The previsions of the theory are checked by comparison with results obtained directly from the diagonalization of the Morokuma and Freed hamiltonian.     

Mechanism of external heavy-atom enhancement of the radiative transition rate from the π,π* triplet state

The effect of external heavy-atom perturbation is examined for each of the three spin subcomponents of the π,π^* triplet state. It is shown that the radiative decay rate from the subcomponent whose spin wavefunction is within the molecular plane is most effectively enhanced. The main perturbing singlet state is the charge-transfer state arising from the excitation of an electron from a π orbital of the molecule to the σ^* orbital of the perturber.     

The effects of saturation and velocity selective population in two-step 6S1/2 → 6P3/2 → 6D5/2 laser excitation in cesium

Excited states population distributions created by two-step 6S_1/2 → 6P_3/2 → 6D_5/2 laser excitation in room temperature cesium vapor were quantitatively analyzed applying absorption and saturation spectroscopy. A simple method for the determination of the excited state population in a single excitation step that is based on the measurements of the saturated and unsaturated absorption coefficients was proposed and tested. It was shown that only ≈ 2% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-line laser. With complete saturation of the second excitation step, the population amounting to only ≈ 1% of the ground state can be eventually created in the 6D_5/2 state. The fluorescence intensity emerging at 7P_3/2 → 6S_1/2 transition, subsequent to the radiative decay of 6D_5/2 population to the 7P_3/2 state, was used to assess the efficiency of the population transfer in the chosen two-step excitation scheme. The limitations imposed on the sensitivity of such resonance fluorescence detector caused by velocity-selective excitation in the first excitation step were pointed out and the way to overcome this obstacle is proposed.     

Photophysics of aromatic molecules with low-lying pi sigma* states: excitation-energy dependence of fluorescence in jet-cooled aromatic nitriles

Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state.     

The role of memory effects in scholastics approach to collinear diatom—diatom collisions

The role of memory effects in the stochastic approach to vibrational—vibrational transitions in collinear diatom—diatom collisions is studied. It is shown that with the help of a new recurrence relation for Clebsch—Gordan coefficients we were able to solve exactly a non-markovian master equation for a model hamiltonian. The derived solution for probabilities of V—V transitions is compared with markovian as well as exact semiclassical results over a wide range of velocities of the colliding molecules. We have found a substantial improvement of markovian results, both qualitative and quantitative, when the non-markovian effects have been included in the stochastic theory. This is in contrast with a recent study by King and Schatz who got for a restricted V—T model more accurate probabilities from the markovian approach than from the non-markovian one. The reasons for this are also discussed.     

Preparation and decay of excited molecular states: the influence of dephasing relaxation and pulse fall time

It is shown that both a finite excitation pulse fall time and a transverse relaxation process allow a non-markovian radiationless process to exhibit a rigorous exponential behavior, whereas a long duration square pulse in the absence of dephasing relaxation does not succeed in fully eliminating any memory feature.     

Correlation contributions to the radical cation states of linear even polyenes: An open-shell RHF-CNDO/S(CI) analysis of the optical and photoelectron spectra

Configuration interaction wave functions are calculated for the low-lying radical cation states of trans-butadiene, hexatriene, and octatetraene within the open-shell RHF -CNDO /S (CI ) approach. The consequences of various one-electron contributions to the interpretation of existing photoemission and radical cation optical spectra are emphasized. Electron correlation is shown to be essential to achieve adequate energy and intensity profiles assuming photoelectron or optical excitation. The excitation energies and transition amplitudes (optical and photoemission) are also found to be sensitive to the molecular geometry. The present results are consistent with previous interpretations that photoionization measurements probe the neutralmolecule alternating single-double bond-length structure, whereas optical excitation samples an ion-state–state–induced “relaxed” reference configuration having a weakened bond-length alternation. Calculated trends in the spectroscopic properties are extrapolated to extended members of the even-polyene series.     

Structure and Decay of Gaseous Organic Radical Cations Studied by Their Radiative Decay,Exemplified by the 1,3-Pentadiyne Cation

The radiative decay of over a hundred open-shell organic radical cations has now been established. As a result, the spectral structure of such cations in their ground and excited electronic states can be probed with resolutions of the order of ? 1 cm^?1. This is achieved by means of emission and laser-induced fluorescence techniques. The analysis of the emission and excitation spectra provides the vibrational frequencies of many of the totally symmetric fundamentals of the cations in the two electronic states. In order to study the relaxation behavior of these cations under “isolated conditions”, the lifetimes and fluorescence quantum fields can be obtained by means of photoelectron-photon coincidence measurements. These data yield the radiative and non-radiative rate constants as a function of the internal energy of the cations. The structural and decay information obtained from each of these techniques is illustrated using the 1,3-pentadiyne radical cation as example.     

“Proximity effects”: the effect of methylation and vibrational excitation on bi-exponential decay in pyrimidine

Experimental data concerning the effect of methylation and vibrational excitation on bi-exponential decay in pyrimidine is analyzed within the framework of a kinetic scheme. A method for the exact determination of the rate constants is given. It is shown that the effect of methylation and vibrational excitation on the internal conversion and intersystem crossing rates is consistent with the results of model studies dealing with the “proximity effect” in radiationless transitions.     

Description of the radiative properties of unsaturated molecules containing a triple bond based on a quasi π-nodel

In this paper we provide a model for the theoretical calculation of radiative triplet properties of aromatic molecules containing a triple bond. Such molecules have special low-lying electronic states with the symmetry properties of a σπ* -state. These states axe caused by electron promotion from an “in-plane π-system” (quasi π-system), which in highly localized at the triple bond, to the aromatic π-system and vice versa. It is shown that these states lead to one-center spin-orbit integrals and therefore are highly active in radiative triplet deny. Theoretical and spectroscopic investigations of ethynylbenzene show that the radiative properties of the lowest triplet state, such as phosphorescence yield, emission spectrum and degree of polarization can be explained not only qualitatively but also quantitatively by restriction to these states (quasi π-model). The theoretical evaluation of the radiative rate was done with respect to individual promoting modes. It is shown that the first order rate (at the equilibrium position of the molecule), though allowed by symmetry, has nearly no influence on the radiative lifetime. As a consequence the first transition in the phosphorescence emission cannot be assigned as the 0-0 transition.     

Theory of correlation measurements of resonance light scattering

The boson nature of radiation is shown to give rise to a purely quantum mechanical exchange contribution to the intensity-intensity correlation function for resonant light scattering by an atomic or molecular system. The exchange contribution can be decomposed into three components, one involving the intensity correlation for a pair of coherently scattered photons (“resonant Raman” processes), another for a pair of incoherently scattered ones (“resonance fluorescence”), and the last involving the exchange correlation one of each. The intensity correlation measurements of Kimble et al., on optically pumped atomic beams of sodium atoms are interpreted with the theory, producing values of the decay rate of the excited sodium atoms and of the coherence time of the exciting radiation in good agreement with expectations.     

Fluorescence properties of perylyne aggregates in a polymer matrix (PMMA)

Fluorescene and fluorescence excitation spectra as well as fluorescence decay functions of solid solutions of up to 7×10⁻² M perylene in PMMA have been measured upon variable site-selective dopant excitation. Fluorescence spectra are the analogue to the Y emission of the -modification of crystalline perylene. Fluorescence decay is non-exponential, the average decay time being correlated with the appearance of the 1150 cm⁻¹ b_2u mode in the emission spectrum. It is concluded that the polymer matrix generates a distribution of ground state pair conformations. After excitation pairs relax to structures with statistically varying coordinates leading to a distribution of decay times. With increasing pair excitation energy the Stokes shift increases indicating greater stability of the excited pair. Spectral as well as decay time analysis suggest that in the parallel pair structure radiative decay is promoted by the non-totally symmetric 1150 cm⁻¹ molecular vibration.     

Theory of half-collision cross sections

Standard Lippmann–Schwinger theory does not apply to decay of metastable or unstable states due to failure of the adiabatic hypothesis. In the Fock–Tani representation, discrete unstable states can be described by state vectors orthogonal to the asymptotic states representing their decay fragments, and a decay/formation interaction is exhibited explicitly as a portion of the total interaction Hamiltonian. This allows a straightforward derivation of a generalized Lippmann–Schwinger “half-collision” differential decay cross section without the need of projection operators. It reduces in first order to the product of a resonance line shape by a Golden Rule matrix element squared. In the general case the line shape is non-Lorentzian and the matrix element factor contains final-state interaction contributions. The exact expression is expected to be applicable to a variety of processes such as predissociation, autoionization, and Auger effect. The derivation employs the van Hove resolvent formalism to exhibit the dependence of the cross section on the complex self energy of the decaying state.     

Excitation spectrum of neutral molecules adsorbed on dielectric surfaces

It is shown that the excitation spectrum of neutral molecules physisorbed on a dielectric surface consists of two symmetric and two antisymmetric energy modes. The spectral functions of these modes are represented respectively by two lorentzian lines whose spectral widths are described by the radiative decay of the energy modes in question. Numerical results are derived for the energies of excitation and spectral widths for the rare-gas atoms adsorbed on graphite.     

Emission spectroscopy and excited-state dynamics of chromyl-chloride vapor

A tunable dye laser has been used to excite single vibronic features in the low-pressure vapor of CrO₂Cl₂. The fluorescence spectrum, fluorescence excitation spectrum and time-resolved fluorescence decay are discussed. It is shown that the active ν′₄ and ν″₄ modes are the same frequency in the gas phase, thus collapsing the sequence congestion normally observed in gas-phase spectra. This degeneracy makes impossible the excitation of single vibronic levels. It is shown that the fluorescence lifetime of the excited state in all except the vibrationally cold level is severely shortened by unimolecular radiationless decay. This radiationless rate is strongly dependent upon the partitioning of energy into various excited-state modes. The radiative lifetime of the vibrationally cold excited state is (1.34 ± 0.08) μs and the apparent bimolecular quenching rate is (5.9 ± 0.2) × 10^?10 cm³/molecules. No evidence of emission from the lowest-energy excited electronic state recently reported by Spoliti [J. Mol. Spectrosc. 52 (1973) 146] is observed.     

Ab initio configuration interaction study of the B- and C-band photodissociation of methyl iodide

Multireference spin-orbit configuration interaction calculations have been carried out for the valence and low-lying Rydberg states of CH(3)I. Potential energy surfaces along the C-I dissociation coordinate (minimal energy paths with respect to the umbrella angle) have been obtained as well as transition moments for excitation of the Rydberg states. It is shown that the B and C absorption bands of CH(3)I are dominated by the perpendicular (3)R(1),(1)R?(E)←X??A(1) transitions, while the (3)R(2)(E),?(3)R(0(+) )(A(1))←X??A(1) transitions are very weak. It is demonstrated that the bound Rydberg states of the B and C bands are predissociated due to the interaction with the repulsive E and A(2) components of the (3)A(1) state, with the (3)A(1)(E) state being the main decay channel. It is predicted that the only possibility to obtain the I((2)P(3/2)) ground state atoms from the CH(3)I photodissociation in the B band is by interaction of the (3)R(1)(E) state with the repulsive (1)Q(E) valence state at excitation energies above 55,000 cm(-1). The calculated ab initio data are used to analyze the influence of the Rydberg state vibrational excitation on the decay process. It is shown that, in contrast to intuition, excitation of the ν(3) C-I stretching mode supresses the predissociation, whereas the ν(6) rocking vibration enhances the predissociation rate.     

On the low-lying triplet of the water molecule and its luminescent decay

Due to a significant discrepancy between theoretical calculations and experimental excitation data, the position of the low-lying triplet state of water has been much debated in recent years. We now report that the corresponding transition (≈ 4.0 eV) can be observed also in emission upon charge neutralization in γ-irradiated ordinary and heavy ices and also by photosensitization from the mercury ³P₁ or ³P₀ state. The observed radiative decay is characterized by an emission λ_max at 380–385 nm.