Photophysical Processes in the Gas Phase at High Levels of Vibrational Excitation of Triplet Molecules of Benzophenone and Fluorenone

By the delayed fluorescence activated by direct multiphoton excitation of triplet molecules by CO₂–laser radiation we have studied the prevailing deactivation pathways of triplet molecules with a high store of vibrational energy E _vib. The dependences of the kinetic characteristics of delayed fluorescence on the presence of vapors and foreign gases have been used to estimate the rates and efficiencies of intermolecular vibrational relaxation in the vibrational quasi–continuum of the triplet state T ₁. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the E _vib dependences of reversible intercombination conversion between the states T ₁ and S ₁ and interconversion from T ₁ to the ground electronic state S ₀ for both the case of isolated excited molecules and at a steady vibrational temperature. It is shown that at high vibrational temperatures the radiationless transition from the T ₁ state to S ₀ has an activation character and is accomplished through the energy barrier. The conditions for going to an exponential dependence have been determined. It has been found that the obtained dependences are in good agreement with the known experimental results. The influence of molecular and environmental characteristics on the decay rate of triplet molecules is compared.     

Collisional transfer of roto-vibrational energy from quantum calculations: The He-HF system

Summary The problem of simultaneous vibrational and rotational excitations of HF molecules in collision with helium has been approached via a quantum-mechanical treatment of the full dynamics and by employing a very accurate potential-energy surface suggested earlier in the literature. The complicated coupling of rotational and vibrational channels has been partly simplified by taking advantage of their different time scales, thus allowing a reduction of dimensionality for the corresponding multichannel scattering couple equations (IOS approximation). The relative influence of the various modes on preferential energy depositions upon collision is discussedvis à vis specific features of the interaction and specific regions of the range of collision energies employed. The essential inefficiency of (R, T) processes as apposed to (V, T) or (V, R, T) processes is once more pointed out for the present system. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Forbidden vibrational-rotational transitions and Herman-Wallis factors in fundamental IR bands of symmetric-top molecules

Within the theory of coupled schemes of ordering of vibrational-rotational interactions, the operator of the effective dipole moment of single-quantum vibrational transitions is represented in the form of an infinite series in vibrational (normal coordinates and conjugate momenta) or rotational variables (components of the total angular momentum). Mechanisms of activation of infrared-inactive totally symmetric vibrations in molecules of the D _2a , D _3h , C _3h , D _n (n ≥ 3), S ₄, T, T _a , and O symmetries and forbidden vibrational-rotational transitions in IR bands of active vibrations have been studied. The group-theoretic analysis of tensor parameters in higher-order effective dipole moments of single-quantum vibrational transitions in axially symmetric molecules has been performed. The strengths of allowed transitions and forbidden transitions in fundamental and hot IR bands of axially symmetric molecules are calculated with allowance for the Herman-Wallis factors. For effective dipole moments of multiquantum transitions in molecules, models are developed in the form of infinite series in rotational variables and in the form of Padé approximants.     

Influence of Vibrational Excitation of Triplet Molecules of Anthraquinone on the Photochemical Processes in Gas Mixtures with Amines

By the quenching of the delayed fluorescence (DF) of anthraquinone vapors by aliphatic amines (diethylamine, dibutylamine, cyclohexylamine) and pyridine the photoinduced processes proceeding with the participation of vibrationally excited triplet molecules of anthraquinone have been investigated. The DF quenchingrate constants K _q varying from 1·10⁶ sec^–1·torr^–1 in mixtures with diethylamine to 7·10³ sec^–1·torr^–1 in mixtures with pyridine have been estimated. A correlation between the values of K _q and the ionization potentials of foreign gases confirming the important role of interactions with charge transfer in the quenching of triplet molecules in the gas phase has been established. The influence of other relaxation processes on the DF quenching is considered. It is shown that the intermolecular vibrational relaxation in the T ₁ triplet state leading to the establishment of relaxation equilibrium at a vibrational temperature T _vib considerably increasing the medium temperature is the fastest process among the biomolecular processes (rate constants K _col ^V > 10⁶ sec^–1·torr^–1 > K _q). The values of T _vib and the vibrational energies E _vib of the triplet molecules after the energy exchange in the collisional complex have been estimated. It has been concluded that the photochemical reaction yield is determined by the intermolecular processes proceeding in the T ₁ state at a vibrational equilibrium characterized by high values of T _vib. The influence of E _vib of triplet molecules on the DF quenching rates at a photoinduced electron transfer is considered.     

Wave packet dynamics in different electronic states investigated by femtosecond time-resolved four-wave-mixing spectroscopy

This paper reviews results on wave packet dynamics investigated by means of femtosecond time-resolved four-wave-mixing (FWM) spectroscopy. First, it is shown that by making use of the various degrees of freedom which are offered by this technique information about molecular dynamics on different potential-energy surfaces can be accessed and separated from each other. By varying the timing, polarization, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dynamics are coherently excited and probed by the nonlinear spectroscopy. As a model system we use iodine in the gas phase. These techniques are then applied to more-complex molecules (gas phase: benzene, toluene, a binary mixture of benzene and toluene; solid state: polymers of diacetylene matrix-isolated in single crystals of monomer molecules). Here, ground-state dynamics are investigated first without any involvement of electronically excited states and then in electronic resonance to an absorption transition in the investigated molecules. Signal modulations result which are due to wave packet motion as well as polarization beats between modes in different molecules. Phase and intensity changes yield information about intramolecular vibrational energy redistribution, population decay (T₁), phase relaxation (T₂), and coherence times. Received: 12 October 1999 / Published online: 13 July 2000     

Intramolecular interactions and deuteration effect in nonradiative deactivation of lowest triplet state of anthracene and naphthalene

In the approximation of nonadiabatic interactions and considering all out-of-plane vibrational modes to be promoting ones, we calculate changes in the rate constants T _s K _dg ^s of the nonradiative degradation T ₁ ^s ⇝ S ₀ of in-plane spin (s) triplet states as a result of the complete deuteration of anthracene and naphthalene molecules. We examine how the deuteration, the frequency factor, and the shape of promoting vibrational modes affect the squared matrix elements of both nonadiabatic coupling and adiabatic vibronically induced spin-orbit (VISO) coupling of electronic states. The compensation effect of spin-orbit interactions in structural elements of the carbon backbone of the anthracene molecule is ascertained.     

Large angle jumps of small molecules in amorphous matrices analyzed by 2D exchange NMR

The reorientation dynamics of deuterated benzene and hexamethyl benzene as additives to the glass former oligostyrene is studied below the glass transition temperature T_g. By means of 2D ²H NMR, analyzed in the frequency and in the time domain, it is shown that the dynamics of the small molecules is governed by an isotropic large angle reorientation process, which is close to the random jump model. Furthermore, the dynamics is characterized by a broad distribution of correlation times. Even 65 K below T_g, a fraction of small molecules reorients on the timescale of 100 ms. In contrast, small angle reorientation dominates in the neat glass former polystyrene near T_g. As a consequence of the presence of large angle jumps, the 2D spectra can be described by an additive superposition of two sub-spectra—a ridge along the diagonal and a complete exchange pattern—where the weighting factor W(t_m) is directly given by the reorientational correlation function F₂(t_m). Additionally, for a sample with very low benzene concentration (c≈0.5%), the 1D spectra indicate that the same dynamic scenario is present in the single particle limit. Tentatively, we assume that the large angle reorientation of the small molecules is associated with a translational diffusion process of the small molecules within the amorphous matrix.     

Vibrational relaxation in liquids

A new theory is presented for vibrational energy relaxation in a liquid. It is shown that a vibrationally excited probe molecule relaxes through interaction with the density fluctuations in the surrounding solvent fluid. This interaction occurs through a potential V(k), which is expressed in terms of the intermolecular force between the excited probe molecule and the surrounding fluid molecules. By assuming spherically symmetric solvent particles the T ₁ energy relaxation time for direct V-T processes is related to the translational dynamic structure factor for the fluid S(k, ω_v), evaluated at the vibrational resonance frequency. It is shown that this is described by gas-like particle motions on a very short distance scale corresponding to k vectors lying well beyond the first or second peaks of the fluid structure factor S(k). Such motions can be pictured as high-frequency, short-distance distortions of the local equilibrium configuration of the solvent particles around the probe. T ₁ ^-1 is found to be proportional to ρ_e T ^1/2 ω_v ^-3. The V-V energy exchange relaxation time is also calculated. This is found to be proportional to S(k, ω′) evaluated at a frequency ω′, corresponding to the vibrational energy missmatch. An energy gap law for the V-V process is derived.     

Giant magneto-optical response of ferromagnetic EuB<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Intercalation of polyatomic molecules into a superconductor can drastically affect the properties of the compound. A mechanism responsible for a large increase in T _c for such systems is proposed. It explains the recent remarkable observation of high T _c superconductivity in the hole-doped C₆₀/CHX₃ (X≡Cl, Br) compounds and the large shift in their T _c upon Cl↦Br substitution. The increase in T _c is due to contribution to the pairing arising from the interaction of electrons with the vibrational manifold of the molecule. The proposed mechanism opens up the possibility to observe a site-selective isotope effect. We also suggest that intercalating CHI₃ would further increase the critical temperature to T _c≃ 140 K. Received 23 January 2002     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

Configuration interaction study including the effects of spin–orbit coupling for the electronic states of the LiX molecules (X = C,Si, Ge,Sn)

Ab initio multireference configuration interaction calculations including spin–orbit coupling effects have been carried out for four LiX molecules (X?=?C, Si, Ge and Sn). Potential energy curves of the ground and low-lying excited states have been obtained in each case as well as the corresponding spectroscopic constants. Transition moments have also been computed in order to give estimates of the radiative lifetimes of the excited states for each system. Trends in a variety of quantities such as T _e values, spin–orbit splittings, equilibrium bond lengths and vibrational frequencies for this series of molecules are discussed in detail and comparison with the corresponding data reported earlier for the PbLi system is also made.     

强红外场作用下BCl3振动态传能动力学研究

基于文献[1],本工作在较高泵浦激光能通量范围,测量并研究了BCl₃分子振动激发v_3吸收谱及时间演变,观察了v₃激发弛豫的几种能量转移过程,以及对泵浦激光能量BCl₃气压等参数的依赖关系。表明泵浦光脉冲产生一个振动态非热分布的系综,转动能量转移对引起这种非热分布有重要作用。用简化碰撞动力学模型讨论了BCl₃振动激发吸收谱的演变过程,得到振动态再分布的简单关系;Pτ_v-v≌c/K′(T_v,T₀,q)和等效振动温度、平均吸收光子数的分析表达式,与实验结果定性地符合。 关键词：     

Vibrational non-equilibrium in the hydrogen–oxygen reaction. Comparison with experiment

A theoretical model is proposed for the chemical and vibrational kinetics of hydrogen oxidation based on consistent accounting of the vibrational non-equilibrium of the HO₂ radical that forms as a result of the bimolecular recombination H+O₂ → HO₂. In the proposed model, the chain branching H+O₂ = O+OH and inhibiting H+O₂+M = HO₂+M formal reactions are treated (in the terms of elementary processes) as a single multi-channel process of forming, intramolecular energy redistribution between modes, relaxation, and unimolecular decay of the comparatively long-lived vibrationally excited HO₂ radical, which is able to react and exchange energy with the other components of the mixture. The model takes into account the vibrational non-equilibrium of the starting (primary) H₂ and O₂ molecules, as well as the most important molecular intermediates HO₂, OH, O₂(¹Δ), and the main reaction product H₂O. It is shown that the hydrogen–oxygen reaction proceeds in the absence of vibrational equilibrium, and the vibrationally excited HO₂(v) radical acts as a key intermediate in a fundamentally important chain branching process and in the generation of electronically excited species O₂(¹Δ), O(¹D), and OH(²Σ⁺). The calculated results are compared with the shock tube experimental data for strongly diluted H₂–O₂ mixtures at 1000 < T < 2500 K, 0.5 < p < 4 atm. It is demonstrated that this approach is promising from the standpoint of reconciling the predictions of the theoretical model with experimental data obtained by different authors for various compositions and conditions using different methods. For T < 1500 K, the nature of the hydrogen–oxygen reaction is especially non-equilibrium, and the vibrational non-equilibrium of the HO₂ radical is the essence of this process. The quantitative estimation of the vibrational relaxation characteristic time of the HO₂ radical in its collisions with H₂ molecules has been obtained as a result of the comparison of different experimental data on induction time measurements with the relevant calculations.     

Model and calculation of rate constants of nonradiative <Emphasis Type="Italic">T-S</Emphasis> intersystem conversion: Test by example of naphthalene and its chlorine derivatives

For in-plane spin states (s = z, y), the rate constants K _dg ^s of the nonradiative energy degradation T ₁ ⇝ S ₀ of the lowest triplet T ₁ state of naphthalene (NPH) and its dichloro-substituted derivatives at positions 1,4- and 2,3- of the molecule (1,4-NPH and 2,3-NPH) are calculated. A simple model is proposed for calculations that is based on the nonadiabatic approximation and uses all the out-of-plane vibrational modes of the molecule as promoting vibrational modes. As a result of calculations, the dependences of the rate constants K _dg and K _dg ^s on the positions of chlorine atoms in the molecule are obtained, which are consistent with the known data of magnetooptical measurements. The inversion of the ratio K _dg ^z : K _dg ^y in the 1,4-NPH and 2,3-NPH molecules is established.     

Utilization of rotational and vibrational temperatures for plasma diagnostics of high-pressure discharges

In the spectra of high-pressure discharges excited in molecular gases, very intensive molecular spectral bands may usually be observed. We may determine the rotational and vibrational temperatures without difficulty, however, the rotational and vibrational temperatures (T _r, T_v) do not offen equal to the temperature of neutral gas (T ₀) or to that of electrons (T _e). If the collision cross sections of electronic, atomic, and molecular excitation (deexcitation) are known, we may then calculate the dependence of the rotational and vibrational temperatures onT _e,T ₀,N _e and the pressure of the gas. The calculations have been performed for pure N₂ and for an Ar-N₂ mixture at atmospheric pressure. The computed graphs make it possible to determine some of the values 4T _e,T ₀,N _e if the temperaturesT _r andT _v are known.The author wishes to extend his thanks to Prof. V. Truneek for valuable comments and to Mr. A. Struka for the preparation of the diagrams.     

Short-time microscopic dynamics of aqueous methanol solutions

In this paper we present the picosecond vibrational dynamics of a series of methanol aqueous solutions over a wide concentration range from dense to dilute solutions. We studied the vibrational dephasing and vibrational frequency modulation by calculating the time correlation functions of vibrational relaxation by fits in the frequency domain. This method is applied to aqueous methanol solutions xMeOH–(1???x)H₂O, where x?=?0, 0.2, 0.4, 0.6, 0.8 and 1. The important finding is that the vibrational dynamics of the system become slower with increasing methanol concentration. The removal of many-body effects by having the molecules in less-crowded environments seems to be the key factor. The interpretation of the vibrational correlation function in the context of Kubo theory, which is based on the assumption that the environmental modulation arises from a single relaxation process and applied to simple liquids, is inadequate for all solutions studied. We found that the vibrational correlation functions of the solutions over the whole concentration range comply with the Rothschild approach, assuming that the environmental modulation is described by a stretched exponential decay. The evolution of the dispersion parameter α with dilution indicates the deviation of the solutions from the model simple liquid and the results are discussed in the framework of the current phenomenological status of the field.     

Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.?Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T _l=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T _c=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below T _c as well as the two-dimensional decay above T _c.?In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers. Received: 25 March 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

Simulation of Raman spectra of C and C by non-orthogonal tight-binding molecular dynamics

A non-orthogonal tight-binding molecular-dynamics formalism is used to simulate Raman spectra of the fullerene molecules C₆₀ and C₇₀. Two parametrization schemes for the Hamiltonian and the overlap matrix elements are investigated. The considered molecules are excited randomly and the Fourier transform of the displacement autocorrelation function is employed to extract the vibrational properties. Fair agreement with experiment and with force-constant and ab initio calculations is achieved, with comparatively smaller maximum errors in the frequencies than for other molecular dynamics or semi-empirical calculations from the literature. Received 4 February 1999 and Received in final form 28 November 1999     

Rotational and Vibrational Temperature Measurements in a High‐Pressure Cylindrical Dielectric Barrier Discharge (C‐DBD)

The rotational (T_R) and vibrational (T_v) temperatures of N₂ molecules were measured in a high‐pressure cylindrical dielectric barrier discharge (C‐DBD) source in Ne with trace amounts (0.02 %) of N₂ and dry air excited by radio‐frequency (rf) power. Both T_R and T_v of the N₂ molecules in the C ³Π_u state were determined from an emission spectroscopic analysis the 2^nd positive system (C ³Π_u → B³Π_g). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both T_R and T_v also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N₂ to the Ne in the discharge resulted in higher T_R and T_v values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Internal energy distribution of recombinatively desorbing D2 from sulfur covered poly-Pd

Rotational and vibrational population distributions have been determined for D₂ molecules recombinatively desorbing from polycrystalline Pd surfaces by tunable vacuum-ultraviolet laser-induced fluorescence. In the temperature range 550 KT _s 1050 K studied in this work a rotational temperature ofT _rot 450 K was found, nearly independent of the surface temperature. Similarly, the vibrational temperature could be described by a value ofT _vib1100 K, being always higher than the surface temperature.