Intermolecular Vibrational Relaxation Research Articles

Hydrogen Bonding Effects on Vibrational Dynamics and Photochemistry in Selected Binary Molecular Complexes

This brief review presents an overview and analysis of the experimental studies performed recently in the laboratory of the authors demonstrating the role of hydrogen bonding as a promotional factor for intermolecular vibrational energy relaxation, and as a driving force for the occurrences of specific reaction channels in binary molecular complexes. Both vibrational and electronic spectroscopic methods have been used, and measurements have been performed in the supersonic jet expansion, in cold inert gas matrixes and also in suitable liquids at room temperature. Pure and mixed dimers of two types of aromatic chromophores, 7-azaindole and phenol, have been used as the model molecular systems, which involve O–H···O, O–H···N and N–H···N types of hydrogen bonds. The promotional effect of vibrational relaxation has been demonstrated showing large broadening of either of the infrared X–H stretching fundamentals of the donor groups in the hydrogen-bonded X–H···Y network of the binary complexes or broadening of the fluorescence spectra upon excitations to single vibronic levels of the isolated complexes in the gas phase and making comparisons with the corresponding spectra of the monomers. A similar approach has been adopted to demonstrate the promotion of photodissociation and charge transfer reactions in binary complexes by hydrogen bonding.

Temperature Dependence of Low-Frequency Spectra in Molten Bis(trifluoromethylsulfonyl)amide Salts of Imidazolium Cations Studied by Femtosecond Raman-Induced Kerr Effect Spectroscopy.

In this study, the temperature dependence of the low-frequency spectra of liquid bis(trifluoromethylsulfonyl)amide salts of the monocations 1-methyl-3-propylimidazolium and 1-hexyl-3-methylimidazolium and the dications 1,6-bis(3-methylimidazolium-1-yl)hexane and 1,12-bis(3-methylimidazolium-1-yl)dodecane has been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The intensity in the low-frequency region below 20 cm(-1) in the spectra of the four ionic liquids increases with rising temperature. From a line-shape analysis of the broadened low-frequency spectra of the ionic liquids, it is clear that the lowest-frequency component, which peaks at approximately 5 cm(-1), contributes to the temperature dependence of the spectra. This implies that the activity of the intermolecular translational vibrational motion is increasing with rising temperature. It is also possible that decoupling in the crossover process between intermolecular vibrational motion and structural relaxation occurs as a result of a deterioration of the non-Markovian feature or the loss of memory caused by the higher temperature. The peak of the highest-frequency component, which is due mainly to the imidazolium ring libration, shifts to lower frequency with increasing temperature. This is attributed to weaker interactions of the ionic liquids at higher temperatures. Temperature-dependent viscosities from 293 to 353 K of the four ionic liquids have also been characterized.

Vibrational Relaxation of Cyanate or Thiocyanate Bound to Ferric Heme Proteins Studied by Femtosecond Infrared Spectroscopy

Femtosecond vibrational spectroscopy was used to measure the vibrational population relaxation time (<TEX>$T_1$</TEX>) of different anions bound to ferric myoglobin (<TEX>$Mb^{III}$</TEX>) and hemoglobin (<TEX>$Hb_{III}$</TEX>) in <TEX>$D_2O$</TEX> at 293 K. The <TEX>$T_1$</TEX> values of the anti-symmetric stretching (<TEX>${\nu}_1$</TEX>) mode of NCS in the <TEX>$NCS^-$</TEX> bound to <TEX>$Mb^{III}$</TEX> (<TEX>$Mb^{III}$</TEX>NCS) and <TEX>$Hb_{III}$</TEX> (<TEX>$Hb_{III}$</TEX>NCS) in <TEX>$D_2O$</TEX> are <TEX>$7.2{\pm}0.2$</TEX> and <TEX>$6.6{\pm}0.2$</TEX> ps, respectively, which are smaller than that of free NCS. in <TEX>$D_2O$</TEX> (18.3 ps). The <TEX>$T_1$</TEX> values of the <TEX>${\nu}_1$</TEX> mode of NCO in the <TEX>$NCO^-$</TEX> bound to <TEX>$Mb^{III}$</TEX> (<TEX>$Mb^{III}$</TEX>NCO) and <TEX>$Hb_{III}$</TEX> (<TEX>$Hb_{III}$</TEX>NCO) in <TEX>$D_2O$</TEX> are <TEX>$2.4{\pm}0.2$</TEX> and <TEX>$2.6{\pm}0.2$</TEX> ps, respectively, which are larger than that of free <TEX>$NCO^-$</TEX> in <TEX>$D_2O$</TEX> (<TEX>$1.9{\pm}0.2$</TEX> ps). The smaller <TEX>$T_1$</TEX> values of the <TEX>${\nu}_1$</TEX> mode of the heme-bound NCS suggest that intramolecular vibrational relaxation (VR) is the dominant relaxation pathway for the excess vibrational energy. On the other hand, the longer <TEX>$T_1$</TEX> values of the <TEX>${\nu}_1$</TEX> mode of the heme-bound NCO suggest that intermolecular VR is the dominant relaxation pathway for the excess vibrational energy in the <TEX>${\nu}_1$</TEX> mode of <TEX>$NCO^-$</TEX> in <TEX>$D_2O$</TEX>, and that intramolecular VR becomes more important in the vibrational energy dissipation of the <TEX>${\nu}_1$</TEX> mode of NCO in <TEX>$Mb^{III}$</TEX>NCO and <TEX>$Hb_{III}$</TEX>NCO.

Solvent-assisted conformational isomerization (SACI) of meta-substituted phenols: Tuning relative stability, isomerization barrier, and IVR rate

Solvent-assisted conformational isomerization (SACI) in meta-substituted phenols with substituents of varying electron-donating/withdrawing properties was studied in the gas phase. SACI of m-anisidine (m-methoxyaniline) occurs when the binding energy of solvent is higher than isomerization barrier, very similarly to the previously reported case of m-aminophenol. Two conformers of m-chlorophenol have similar relative energy, and SACI does not occur because forward/reverse isomerization rates are similar. While SACI does not occur with m-cyanophenol–H2O though the binding energy is higher than the barrier, D2O can induce SACI of m-cyanophenol because of increased intermolecular vibrational relaxation rate. In m-cyanophenol-d1, SACI yield with D2O becomes even greater due to faster intramolecular vibrational energy redistribution.

Resonant IR multi-photon dissociation spectroscopy of a trapped and sympathetically cooled biomolecular ion species

In this work we demonstrate vibrational spectroscopy of polyatomic ions that are trapped and sympathetically cooled by laser-cooled atomic ions. We use the protonated dipeptide tryptophan-alanine (HTyrAla(+)) as a model system, cooled by barium ions to less than 800 mK secular temperature. The spectroscopy is performed on the fundamental vibrational transition of a local vibrational mode at 2.74 μm using a continuous-wave optical parametric oscillator (OPO). Resonant IR multi-photon dissociation spectroscopy (R-IRMPD) (without the use of a UV laser) generates charged molecular fragments, which are sympathetically cooled and trapped, and subsequently released from the trap and counted. We measured the cross section for R-IRMPD under conditions of low intensity, and found it to be approximately two orders smaller than the vibrational excitation cross section. The observed rotational bandwidth of the vibrational transition is larger than the one expected from the combined effects of 300 K black-body temperature, conformer-dependent line shifts, and intermolecular vibrational relaxation broadening (J. Stearns et al., J. Chem. Phys., 2007, 127, 154322-154327). This indicates that as the internal energy of the molecule grows, an increase of the rotational temperature of the molecular ions well above room temperature (up to on the order of 1000 K), and/or an appreciable shift of the vibrational transition frequency (approx. 6-8 cm(-1)) occurs.

Vibrational relaxation pathways in the electronic excited state of carotenoid

The intra- and inter-molecular vibrational relaxation in the electronic excited state (1B u +) of spheroidene derivative (the number of conjugated double bonds, n = 8 ) has been investigated at room temperature by means of femtosecond time-resolved fluorescence spectroscopy based on an optical-Kerr-gate technique. Depending on the photo-excitation either to the 1 B u + ( v = 1 ) or 1 B u + ( v = 2 ) vibronic level, remarkable differences were observed in hot luminescence spectra related to the vibrational relaxation process of high- and low-frequency modes. Under the excitation to the 1 B u + ( v = 2 ) state hot luminescence from the 1 B u + ( v = 1 ) state was observed as a dominant feature of the time-resolved spectra while the dynamic Stokes shift originating from the low-frequency-modes dynamics was clearly observed under the excitation to the 1 B u + ( v = 1 ) . These observations of the excitation energy dependence of time-resolved fluorescence spectra were discussed by analyzing the Franck–Condon factors of transitions from v = 0 , 1 , and 2 levels of high-frequency modes.

Femtosecond dynamics of geminate recombination of radicals upon photodissociation of aromatic disulfides

The dynamics of the geminate recombination of thiyl radicals formed upon photodissociation of aromatic disulfides and the effect of the intramolecular relaxation on this recombination are studied using pico-and femtosecond kinetic spectroscopy. It is shown that, in terms of a phenomenological model, the geminate recombination of phenylthiyl radicals in neutral solvents can be satisfactorily described by a biexponential dependence. The model suggests the occurrence of primary geminate recombination in a solvent cage formed around an original molecule and secondary recombination controlled by the diffusional motion of the radicals of a pair. The primary geminate recombination, whose characteristic time (9 ps) is close to the characteristic times of intermolecular vibrational relaxation of complex molecules in solvents at room temperatures, masks the manifestation of thermalization processes in the time kinetics. The direct geminate recombination of aminophenylthiyl radicals with the formation of original molecules virtually does not occur because of the intramolecular charge transfer. In connection with this, the intermolecular vibrational relaxation manifests itself in the kinetics of the induced optical density in the region of the absorption band maximum of radicals as a growing component with a characteristic time of 6 ps.

Competition between adiabatic and nonadiabatic fragmentation pathways in the unimolecular decay of the ArI2(B) van der Waals complex

The competition between vibrational and electronic predissociations of the ArI2(B) van der Waals complex has been studied using several dynamical computational methods: exact quantum wave-packet propagation, time-dependent golden rule, and quasiclassical trajectory with quantum jumps model. Five electronic states are considered using recent three-dimensional coupled surfaces obtained with a perturbative diatoms-in-molecules method. Final vibrational and electronic populations, predissociation rates, and absorption spectra have been computed for I2(B,v=18-24)<--I2(X,v=0) excitations within the complex. The contribution of vibrational predissociation into the total decay oscillates as a function of vibrational excitation due to intramolecular vibrational relaxation in a sparse-intermediate regime, which induces irregular variations of the total decay rate. Franck-Condon oscillations control the branching ratios of the individual electronic predissociation channels. However, since these oscillations are out of phase as a function of vibrational excitation, they have limited effect on the oscillatory behavior of the total predissociation rate. Comparison between exact quantum and perturbative golden rule calculations shows that vibrational predissociation has some impact on the electronic predissociation process and affects the final electronic distributions. On the contrary, vibrational product distributions are not significantly affected by the electronic predissociation. A classical description of the ArI2 dynamics provides an averaged picture of the competing predissociation processes, being better adapted for treating intermolecular vibrational relaxation in the statistical limit.

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 β1 and the mean energies 〈ΔE〉 transferred per collision in mixtures with H2O and other polyatomic foreign gases have been compared. It has been established that the efficiencies β1 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 β2 for mixtures of benzophenone and anthraquinone with H2O and other polyatomic gases are determined by the change in the reduced mass of interacting molecules.

Intra- and Intermolecular Vibrational Energy Relaxation of C−H Overtone Excited Benzonitrile,para-Difluorobenzene, and Pyrazine in Solution

Femtosecond IR-pump−UV-probe spectroscopy allows us to directly observe the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of selectively excited aromatic molecules in solution (CF2ClCFCl2). In this article, we report global IVR and VET rate coefficients for benzonitrile, p-difluorobenzene, and pyrazine in a weakly interacting (nonpolar) solvent which are excited in overtones or combination bands of C−H stretch vibrations. The experimental findings are compared with recent results for benzene, toluene, and α,α,α-trifluorotoluene. While we have found a characteristic variation of the relaxation dynamics of the aromatic systems upon chemical substitution, the remarkable similarity of the time scales of IVR for a large group of molecules investigated is striking. Furthermore, the intermolecular vibrational relaxation is characteristically different for different chemical substitutions; however, the correlation of VET rates with the presence of ...

The triplet lifetime dependence on vibrational energy for gas-phase carbonyl aromatic compounds: benzophenone and fluorenone

The dependences of the intensities and decay rates of CO2 laser induced delayed fluorescence on vibrational energy and bath gas pressure were used to study the dominant relaxation pathways in the lowest triplet state, T1, over a wide range of vibrational energy Evib. With this in view, the excess energy dependences of nonradiative relaxation processes rates in the T1 state, such as intersystem crossing from the T1 to ground S0 states, reversible intersystem crossing between excited S1 and T1 states and intermolecular vibrational relaxation were compared. It was found that under vibrational equilibrium at high vibrational temperature (Tvib ≫ T) the experimentally observed nonexponential dependence of the gas-phase triplet decay time on Evib reflected the excess vibrational energy (in the T1) dependence of the intersystem crossing rate from T1 to S0 that was well approximated by an Arrhenius' activated type rate equation in contrast to exponential decay for isolated molecules.

Ultrafast Dynamics of Porphyrins in the Condensed Phase: I. Free Base Tetraphenylporphyrin

With femtosecond resolution, using fluorescence up-conversion and transient absorption, we have carried out measurements on free base tetraphenylporphyrin (H_2TPP) in benzene solution, pumping with ∼1300 cm^(-1) of excess vibrational energy in each of the Soret, Q_y, and Q_x bands, and also pumping the lowest vibrational band of Q_y. From these studies, made for different excitations and at different detection wavelengths, we provide a model for describing the elementary intramolecular processes in the Soret, Q_y, and Q_x electronic manifolds, with the following order of time scales and couplings: electronic (femtosecond), vibrational (femtosecond−picosecond), and singlet−triplet (nanosecond). These dynamical electronic and vibrational relaxation pathways in a molecule with small dipole in nonpolar solvents can be studied without interference from solvent reorganization, as indicated by the small Stokes shift of fluorescence. Vibrationally excited Soret → {Q_y,Q_x} and Q_y → Q_x electronic relaxation occurs in less than 100 fs, within our resolution, as evidenced by the immediate rise of Q_x fluorescence after Soret (397 nm) and Q_y (514 and 550 nm) excitation. There are generally three distinguishable ultrafast relaxation time scales within the Q_x state, which are assigned to intra- and intermolecular vibrational relaxation processes leading to thermal equilibrium in Q_x, the lowest excited singlet state. The measured time scales are as follows: 100−200 fs for intramolecular vibrational energy redistribution, 1.4 ps for vibrational redistribution caused by elastic collision with solvent molecules, and 10−20 ps for thermal equilibration by energy exchange with the solvent. Decay of the equilibrated Q_x population occurs on the nanosecond time scale by intersystem crossing to the triplet state.

Intramolecular and intermolecular vibrational energy relaxation of CH 2I 2 dissolved in supercritical fluid

A pump–probe experiment was performed to examine vibrational population relaxation of diiodomethane (CH 2I 2) molecule dissolved in supercritical CO 2. Using an apparatus with femtosecond time resolution, we observed the contributions of intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) separately. IVR and VET rates were measured with varying solvent densities at a constant temperature. It is shown that the IVR rate is not density dependent while the VET rate increases with increasing density from 0.4 to 0.8 g cm −3 . This observation suggests that the rate of the VET process is determined by solute–solvent collisions whereas the IVR rate is not much affected by solute–solvent interaction.

Observation of formation dynamics of solvated aromatic cation radicals following photoionization

We have studied the ultrafast relaxation dynamics of aromatic cation radicals following photoionization by time-resolved absorption and Raman spectroscopic techniques. In nonpolar solvents, the absorption due to the cation radical decreases in subpicoseconds, which arises from the geminate recombination of the ejected electrons with their parent cations. In polar solvents, on the other hand, the absorption of the cation radical first decreases in the subpicosecond range, and then increases in tens of picoseconds. The subpicosecond decay is ascribed to the geminate recombination of the unsolvated cation radical in the polar solvents. The picosecond rise is observed in all the cation radicals treated (naphthalene, 1,4-diphenylbutadiene, and biphenyl), suggesting that the picosecond relaxation process increasing the absorption intensity occurs after the photoionization of the aromatic molecules in the polar solvents. The observed picosecond relaxation is discussed in terms of the thermal excitation of the neighboring solvent molecules due to intermolecular vibrational relaxation.

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 quenching‐rate constants Kq 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 Kq 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 T1 triplet state leading to the establishment of relaxation equilibrium at a vibrational temperature Tvib considerably increasing the medium temperature is the fastest process among the biomolecular processes (rate constants KcolV > 106 sec−1·torr−1 > Kq). The values of Tvib and the vibrational energies Evib 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 T1 state at a vibrational equilibrium characterized by high values of Tvib. The influence of Evib of triplet molecules on the DF quenching rates at a photoinduced electron transfer is considered.

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.

Time-Resolved Raman Studies of Photoionization of Aromatic Compounds in Polar Solvents: Picosecond Relaxation Dynamics of Aromatic Cation Radicals

Picosecond time-resolved Raman spectroscopy has been used to study the ultrafast relaxation dynamics of aromatic cation radicals following two-photon ionization. In acetonitrile, integrated Raman intensities due to the cation radicals rise in tens of picoseconds, and reach their maxima at a delay time of 40−60 ps from the pump pulse. Such a slow-rise component is observed in all the cation radicals treated (biphenyl, trans-stilbene and naphthalene), suggesting that the picosecond relaxation process increasing the cation Raman intensities occurs after the photoionization of aromatic molecules. In weak polar solvents such as ethyl acetate, on the other hand, only an instrumental-limited rise (<5 ps) is observed. The rise time of the cation Raman intensity does not correlate with the dielectric relaxation time but depends on the polarity of the solvent. This result indicates that the picosecond relaxation process is not controlled by the dielectric solvent relaxation alone. The positional changes and the band narrowings of the cation Raman bands occur on a 10−20 ps time scale. These are associated with intermolecular vibrational relaxation of the cation radical toward a thermal equilibrium with solvents. The time scale of the intermolecular vibrational relaxation is the same as that of the rise component of the cation Raman intensity. From these observations, it is suggested that the thermal excitation of the solvent shell disturbs the solvation structure of the cation radical, which causes the observed picosecond change in the cation Raman intensity.

Two-dimensional vibrational spectroscopy. VIII. Infrared optical Kerr effect and two-color infrared pump–probe measurements

The infrared optical Kerr effect (IR-OKE) and two-color infrared pump–probe spectroscopies, which can be directly used to study intermolecular vibrational energy relaxation pathways, are theoretically studied. The IR-OKE method involves an intense IR pulse with controlled frequency to build up large population on a specific intramolecular vibrationally excited state. After a finite mixing time, which is also experimentally controlled, the time evolution of the nonequilibrium density matrix is detected by using the femtosecond optical Kerr effect measurement. Due to the nonequilibrium population distribution initially induced by the intensive infrared pulse, the measured OKE signal differs from that measured for a molecular system initially in thermal equilibrium state. Particularly, it is found that the deviation from the harmonic response can be specifically measured by using this method. By following the same procedure developed in this paper, the two-color IR pump–probe absorption spectroscopy is also considered and the corresponding nonequilibrium IR response function is theoretically investigated with a direct comparison with complementary IR-OKE response function. By using the perturbation theory, the corresponding response functions are obtained in terms of the molecular properties such as linear and nonlinear spatial derivatives of dipole and polarizability and cubic and quartic anharmonic coefficients. Also, it is emphasized that this method is conceptually quite similar to the transient nuclear Overhauser effect nuclear magnetic resonance spectroscopy.

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 Evib. 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 T1. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the Evib dependences of reversible intercombination conversion between the states T1 and S1 and interconversion from T1 to the ground electronic state S0 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 T1 state to S0 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.

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 kV 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.