Effect of Supercollisions on Energy Transfer in Mixtures of Bath Gases and Laser Excited Complex Molecules

Intensities and decay rates of delayed luminescence (DL) initiated by a pulse of N₂ laser were employed to probe collisional relaxation of complex molecules (benzophenone, acetophenone) diluted with bath gases Ar, Kr, Xe, C₂H₄, SF₆, C₅H₁₂. It was shown that vibrational relaxation can be interpreted in terms of two consecutive processes: vibration-vibration (V-V) and vibration-translation (V-T). The results clearly demonstrated that fast component of DL can be used to study V-V energy transfer. It was found that at relatively small internal energy the collisional efficiences of V-V process had the values typical for molecular processes in which supercollisions contribute. The average energies transferred per collision, (ΔE), well correlated with predictions of the simple ergodic collision theory of intermolecular energy transfer.     

Intramolecular vibrational energy transfer in mixtures of anthraquinone with foreign gases

Properties of collision transfer of vibrational energy in the vibrational quasi-continuum of mixed singlet-triplet levels of anthraquinone are studied by the method of time-resolved delayed fluorescence. The two-exponential fluorescence decay is analyzed in the kinetic approximation. It is shown that dependences of the intensities and decay rates of the fast and slow components on pressure can be used for estimating the rates of the establishment of the vibrational (V-V) and thermal (V-T) equilibrium. The efficiency β and the average energy 〈ΔE〉 transferred in collisions are estimated for these processes. It is found that the V-V process is characterized by high values of β, which, however, are lower in the quasi-continuum of mixed singlet-triplet states than the gas-kinetic values (β < 0.2). The transformation of the vibrational energy to the translational energy occurs with the low efficiency (10^?2 > β > 10^?5). The average energy 〈ΔE〉 transferred during a collision in the V-V process is comparable with the energy predicted by the statistical theory of ergodic transfer. The correlation between experimental and theoretical values improves when the time resolution of the experiment is sufficient for the separation of the V-V and V-T processes.     

Intermolecular vibrational relaxation upon multiphoton excitation of triplet molecules by a CO2 laser

Intermolecular vibrational relaxation is studied in mixtures of polyatomic molecules (benzophenone and fluorene) with bath gases after multiphoton excitation of the triplet molecules by CO₂ laser radiation. The dependences of the decay rate and the intensity of laser-induced delayed fluorescence on the laser energy density E _CO2 and pressure P _fg of bath gases are analyzed. They are found to be different for the fast and slow components of delayed fluorescence, which decays nonexponentially. It is shown that a change in the decay rate of the fast fluorescence component with increasing pressure P _fg is governed by the properties of vibration-translation relaxation. The efficiency β of this process is estimated in a broad range of vibrational energies. It is found that β weakly changes with increasing E _vib upon excitation of molecules to high vibrational levels. The features of intermolecular vibrational relaxation at high densities of anharmonically coupled vibrational states are discussed.     

Direct Measurement of Collisional Energy Transfer Between Highly Vibrationally Excited Triplet Fluorenone and Bath Gases

Intensities and decay rates of CO₂-laser induced delayed fluorescence are used to probe collisional relaxation of vibrationally excited fluorenone diluted with bath gases: He, N₂, Kr. The average energies 〈ΔE〉 transferred per collision are found to vary with vibrational energy, the energy dependences of the collisional efficiency eventually level off.     

Temperature Dependence of the Efficiency of Collisional Transfer of Vibrational Energy in Mixtures of Vibrationally Excited Triplet Molecules with Foreign Gases

By the pressure dependences of the decay rates of delayed fluorescence activated by vibrational excitation of triplet molecules of benzophenone and anthraquinone, the efficiencies of collisional transfer of vibrational energy (V–V-transfer) in the vibrational quasi-continuum of the triplet state have been estimated. It is shown that the efficiencies of the process in mixtures with foreign gases increase with increasing dipole moment and polarizability of colliding molecules. In the temperature range 433–513 K, we obtained an inverse temperature dependence of the V–V-transfer efficiency, which is satisfactorily described by empirical relations taking into account long-range attractive forces. The results obtained point to the determining role of long-range attractive forces in quasi-resonance V–V-transfer of vibrational energy by molecules excited in vibrational quasi-continuum.     

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.     

Intermolecular Vibrational Relaxation in Mixtures of Excited Benzophenone and Anthraquinone Molecules with Water Vapor

The intermolecular vibrational energy transfer from triplet molecules of benzophenone and anthraquinone to H₂O molecules has been investigated. To determine the rates of establishment of vibrational (V-V) and thermal (V-T) equilibrium in a vibrational quasi-continuum of mixed singlet-triplet levels, the dependences of the decay rates and intensities of the fast and slow components of delayed fluorescence on the H₂O vapor pressure have been investigated. For V-V relaxation, the efficiencies ₁ and the mean energies E transferred per collision in mixtures with H₂O and other polyatomic foreign gases have been compared. It has been established that the efficiencies ₁ for quasi-resonant vibrational energy transfer (V-V) from benzophenone and anthraquinone to H₂O are an order of magnitude lower than the gas-kinetic ones and lower than those obtained under the same experimental conditions for such foreign gases as C₅H₁₂, SF₆, and CCl₄, and decrease with increasing temperature in the 433–513 K range. It has been concluded that the mechanism of V-V relaxation in mixtures with H₂O are determined by long-range attractive forces. In mixtures with H₂O, no acceleration of V-T relaxation due to the formation of hydrogen bonds has been revealed. The low-efficiency thermalization process (V-T relaxation) is controlled by short-range repulsive forces, and the differences between the intensities ₂ for mixtures of benzophenone and anthraquinone with H₂O and other polyatomic gases are determined by the change in the reduced mass of interacting molecules.     

Role of highly efficient collisions in collisional transfer of vibrational energy in benzophenone-foreign gas mixtures

Collisional losses of vibrational energy in mixtures of benzophenone excited by nitrogen laser radiation (λ=337 nm) and foreign gases (Ar, Kr, SF₆, C₅H₁₂) were studied by time-resolved delayed luminescence. It is established that the intensities and rates of decay of the fast and slow components of delayed luminescence can be used to evaluate the characteristics of V–V– and V–T–transfer of vibrational energy. For the V-V-process, the efficiencies and mean energies transferred in a collision are determined. It is shown that in the mixture with multiatomic gases, vibrational equilibrium is reached after a few collisions, the number of which decreases as the molecule of the foreign gas becomes more complicated. The V–V–process is characterized by high efficiencies of collisions typical for “supercollisions”. The experimental characteristics of V-V-transfer correlate well with ergodic transfer of vibrational energy predicted by statistical theories. 0120 0126 V 3 Institute of Molecular and Atomic Physics of the National Academy of Science of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 340–345, May–June, 1998.     

Triplet-triplet transfer of electronic energy upon nonequilibrium vibrational excitation of triplet molecules

The specific features of the triplet-triplet (T-T) transfer of electronic excitation energy in a gas phase upon nonequilibrium vibrational excitation of the triplet molecules of a donor were studied for an anthraquinone-diacetyl donor-acceptor pair using the time-resolved slow fluoresence of anthraquinone and sensitized phosphorescence of diacetyl. It is shown that in the gas phase, which allows regular control of the number of collisions, competition between the processes of T-T transfer and intermolecular vibrational relaxation is observed for nanosecond time resolution. The T-T transfer rate for the molecular system investigated exceeded the rate of intermolecular vibrational relaxation k_V in the triplet state T₁ of the donor. The effectiveness of the T-T transfer of energy by vibrationally excited molecules turned out to be higher than the effectiveness of transfer by thermalized ones, but even the highest of them was much less than unity. An increase in the equilibrium temperature of vapors led to a decrease in the effectiveness of transfer for both vibrationally excited and thermalized triplet molecules, thus indicating the importance of the collisional complex in the intermolecular process studied. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 474–479, July–August, 2000.     

A molecular dynamics simulation of the destruction of explosive molecules at high-velocity collisions

The infrared spectra and the energies of dissociation of R-NO₂ bonds (R≡C, N, and O) were calculated for explosive molecules (trinitrotoluene, hexogen, octogen, pentaerythrityl tetranitrate, triaminotrinitrobenzene, and nitromethane). The time of kinetic energy redistribution over intramolecular vibrational modes for these molecules (the V-V relaxation time) was calculated by the molecular dynamics simulation method. Molecular dynamics simulations were also used to model the collision-induced destruction of hexogen, octogen, and tri-nitrotoluene molecules. The threshold velocities of collisions at which the destruction of molecules took a time shorter than the V-V relaxation time were determined.     

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.     

Manifestation of quantum statistics in vibrational dynamics of poly(ethylene) crystals

The temperature dependences of the transverse expansion ?_⊥(T) and the longitudinal contraction ?_‖(T) (with respect to the axes of chain molecules) in large-sized poly(ethylene) (PE) crystal grains (100×60×60 nm) are measured using x-ray diffraction in the temperature range 5–380 K. The temperature dependence of the elongation of the molecular skeleton ?_C(T) is obtained by Raman spectroscopy. It is found that the dependences ?_⊥(T), ?_‖(T), and ?_C(T) exhibit a similar specific nonlinear behavior. Analysis of these dependences indicates that the nonlinearity is associated with the quantum statistics of transverse vibrations. The energies and amplitudes of zero-point (at T=0) transverse (torsional and bending) vibrations and the relevant zero-point components ?_‖(0) and ?_C(0) are estimated. It is revealed that the zero-point components make a considerable contribution to the dynamics of the PE crystal up to the melting temperature (～400 K).     

Collisional broadening of the J = 1 ← 0 transition of HC15N by nitrogen,oxygen, helium,and air

Nitrogen-broadened linewidths have been measured for the J = 1 ← 0 rotational transition in the fundamental vibrational state of the hydrogen cyanide isotopic species ¹H¹²C¹⁵N, located at 88 055.01 MHz. The data have been recorded for four temperature levels in the range 234–295 K. The collisional half-widths derived from these measurements allowed us to determine, to our knowledge for the first time, the temperature dependence of the foreign gas broadening parameter. Linewidth measurements at room temperature with oxygen, air, and helium as broadening gases are also reported.     

Excitation dynamics during the multiphoton absorption in SF6+buffer-gas mixtures

A detailed analysis of the total average number of absorbed photons 〈n〉_total during Infrared multiphoton absorption processes in mixtures of SF₆-Ar, N₂ and CH₄ buffer gases is presented. The results for 〈n〉_total are deduced using pulsed photoacoustic technique in collisional regime. Complete analysis is based on the theoretical generalized coupled two-level model (GCT) and its application to different gas mixtures. Evaluation of partial 〈n〉_coll. values is presented too, obtained using the results from time-resolved optoacoustic (TROA) and time-resolved absorption (TRA) methods for V-T relaxation times (τ_V-T) and the saturable absorber (SA) method for R,R-T relaxation times (τ_rot.-rel.), and applying them directly to the GCT model. All methods (TROA and SA) and the GCT model use the same photoacoustic results from our experiment under identical experimental conditions.     

Photochemical reactions of triplet benzophenone and anthraquinone molecules with amines in the gas phase

The intermolecular photoinduced reactions between triplet ketone molecules and aliphatic amines and pyridine are studied by the quenching of delayed fluorescence of anthraquinone and benzophenone vapors by diethylamine, dibutylamine, cyclohexylamine, triethylamine, and pyridine. In the temperature range 423–573 K, the delayed fluorescence quenching rate constants k _q are estimated from changes in the decay rate constant and the intensity of delayed fluorescence upon increasing pressure of bath gases. It is ascertained that, in the gas phase, the mixtures under study exhibit both a negative and a positive dependence of k _q on temperature, which indicates that some photoinduced reactions do not have activation barriers. The rate constant k _q is shown to increase with decreasing ionization potential of the electron donors. This points to the importance of interactions with charge transfer in the photoreaction of triplet ketone molecules with aliphatic amines and pyridine in the gas phase. The relationship between k _q and the change in the free energy ΔG upon the photoinduced intermolecular electron transfer, which is the primary stage of the photochemical reaction, is studied. It is shown that the dependence k _q (ΔG) for the donor-acceptor pairs under study is described well by the Marcus equation, in which the average vibrational energies of the donor and acceptor are taken into account for the estimate of ΔG.     

Electron relaxation mechanisms in n-Bi-Sb semiconducting alloys

The magnetic field dependence of diffusion thermal electromotive force α₂₂(H) (?T ∥ C ₁) in degenerate n-Bi-Sb semiconducting alloys, in which only L electrons participate in transfer phenomena, had a maximum at H ∥ C ₃. The electron relaxation time was determined from the magnetic field value corresponding to this maximum. The dependences of the electron relaxation time on temperature and the concentration of alloy components and the dopant (on the concentration of electrons) were used to separate electron relaxation time components corresponding to scattering by phonons, ionized impurities, and component concentration fluctuations. The latter (“alloy”) mechanism of electron scattering by concentration fluctuations was for the first time considered for Bi-Sb alloys; its contribution was found to be comparable with those of the other scattering mechanisms. The obtained relaxation times were used to calculate theoretical magnetic field dependences of thermal electromotive force and the Nernst-Ettingshausen coefficient. The calculation results were in satisfactory agreement with experiment.     

Heterogeneous relaxation of molecule vibrational energy on metals

A mechanism of heterogeneous relaxation of a molecule vibrational energy on metal via conduction electrons is proposed. The probability of vibrational transition W of a molecule adsorbed on metal surface is calculated. The not very sharp dependence of W on the distance between the molecule and the metal surface, on metal parameters and on the molecular vibrational quantum provides a satisfactory explanation of the experimental data available.     

Semiclassical theory of vibrational,rotational and translational energy exchange in collisions of polyatomic molecules

We derive a theory of inter- and intramolecular transfer of vibrational, rotational and translational energy in collisions of polyatomic molecules, in the case that only short range forces are important. Normal mode vibrations of molecules are treated quantum mechanically whereas translations and rotations are assumed to be classical degrees of freedom. We are able to show that, in case of short range forces, the energy exchange in binary collisions is essentially governed by an effective mass which is given by an algebraic function of the usual reduced mass and moments of inertia and depends also on the relative orientation of the two molecules. As an application of the theory, we calculated the probabilities for collision-induced 1 →0 transitions of the v ₃ mode in pure CH₃I, CH₃Br and CH₃Cl gases. The calculated values are close to the experimental vibrational deactivation probabilities.     

BCl3振动激发弛豫的红外吸收研究

本工作用红外双共振技术测量了BCl₃分子的振动激发态吸收光谱。观察了分子间的多种弛豫过程及能量转移过程。得到关系式Pτ_V-V^(11BCl₃)=3微秒·托。观察到径向声波对双共振信号的调制,声速为2×10⁴厘米/秒,与计算值相符。 关键词：