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.     

用受激拉曼泵浦方法高效制备v=2振动激发态氘分子

We report the preparation of D₂ molecules in v=2 level in molecular beam condition. A single longitudinal mode laser system was used for excitation of D₂ from (v=0, j=0) to (v=2, j=0) with the scheme of stimulated Raman pumping. An excitation efficiency of 25.2% has been achieved, which was determined by the scheme of resonance-enhanced multiphoton ionization. Dependence of relative excitation efficiency on laser energy has been measured. We found that the increasing rate of excitation efficiency became slower as pulse energy of Stokes laser increase, while the excitation efficiency still increases approximately linearly with pump pulse energies up to 60 mJ. The spectral line shapes of Raman transition was also measured at different laser energies and considerable dynamical Stark effect was observed. A single peak was found on the three dimension surface of relative excitation efficiency, indicating the process occurred in the present study is a process of stimulated Raman pumping instead of stimulated adiabatic Raman passage.     

Direct observation of the vibrational energy redistribution in (CF<Subscript>3</Subscript>)<Subscript>2</Subscript>CCO molecules resonantly excited by femtosecond infrared laser radiation

The dynamics of multiphoton excitation of (CF₃)₂CCO molecules has been investigated under the condition of resonant action of femtosecond infrared laser radiation on the ν₁ vibrational mode of the C=C=O bond. It has been shown that the mode-selective excitation of this vibration occurs up to the ν = 6 level. The kinetics of the subsequent intramolecular vibrational energy redistribution from the ν₁ mode has been measured. A value of 5 ± 0.3 ps has been obtained for the characteristic time of this process.     

Dynamics of photoprocesses induced by femtosecond infrared radiation in free molecules and clusters of iron pentacarbonyl

The dynamics of photoprocesses induced by femtosecond infrared radiation in free Fe(CO)₅ molecules and their clusters owing to the resonant excitation of vibrations of CO bonds in the 5-μm range has been studied. The technique of infrared excitation and photoionization probing (λ = 400 nm) by femtosecond pulses has been used in combination with time-of-flight mass spectrometry. It has been found that an infrared pulse selectively excites vibrations of CO bonds in free molecules, which results in a decrease in the yield of the Fe(CO)₅⁺ molecular ion. Subsequent relaxation processes have been analyzed and the results have been interpreted. The time of the energy transfer from excited vibrations to other vibrations of the molecule owing to intramolecular relaxation has been measured. The dynamics of dissociation of [Fe(CO)₅]_n clusters irradiated by femtosecond infrared radiation has been studied. The time dependence of the yield of free molecules has been measured under different infrared laser excitation conditions. We have proposed a model that well describes the results of the experiment and makes it possible, in particular, to calculate the profile of variation of the temperature of clusters within the “evaporation ensemble” concept. The intramolecular and intracluster vibrational relaxation rates in [Fe(CO)₅]_n clusters have been estimated.     

Detection of SF<Subscript>6</Subscript> molecules sublimating from the surface of (CO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> nanoparticles in a cluster beam by the infrared multiphoton excitation method

It has been found that SF₆ molecules captured by large van der Waals clusters (CO₂) _N (where N ≥ 10² is the number of monomers in a cluster) in intersecting molecular and cluster beams sublimate from the surface of clusters after a certain time and carry information on the velocity and temperature (internal energy) of clusters. Experiments have been carried out for detecting these molecules by means of a pyroelectric detector and the infrared multiphoton excitation method. The multiphoton absorption spectra of molecules sublimating from the surface of clusters have been obtained. The temperature of the (CO₂) _N nanoparticles in the cluster beam has been estimated using these spectra and comparing them with the infrared multiphoton absorption spectra of SF₆ in the initial molecular beam.     

Laser R2PI spectroscopic and mass spectrometric studies of chiral neurotransmitters

One color, mass selected resonant two-photon ionization (1cR2PI) spectra of supersonically expanded bare neurotransmitter, (1S,2S)-(+)-N-methyl pseudoephedrine (MPE), and its complexes with chiral and achiral molecules have been investigated. The excitation spectrum of bare MPE has been analyzed and discussed on the basis of theoretical predictions at the B3LYP/6-31G** level of theory. The results allowed to get information on the possible conformers of MPE molecule and on the intermolecular forces on its cluster formed with a variety of solvent molecules, including chiral alcohols, lactates and water. Further information on intermolecular interactions have been obtained with ESI-CID-MS² technique, applied to chiral biomolecules linked through a metal ion to the neurotransmitter. The experimental results are compared with theoretical predictions.     

An analytic algebraic approach to study the influence of molecular alignment and orientation on multiphoton excitation in intense laser fields

The influence of molecular alignment and orientation on multiphoton vibrational excitation of diatomic molecules H₂, HD, and D₂ is studied by an analytical algebraic approach. The explicit expressions of the time-evolution operator and the excitation probability are given. The long-time average absorbed energy spectra and the time-dependent absorbed energy are obtained. Results show that the impact of molecular orientation on the multiphoton resonant excitation is decided by the molecular type and molecular anharmonicity. This is valuable for controlling the vibrational excitation of molecules, which is relevant to the whole field of molecular physics and physical chemistry. Furthermore, good agreement with numerical simulation is achieved.     

Calculation of the Parameters of the Dimeric Association of Water Molecules and Determination of Their Temperature Dependence

The conditions are determined, and the parameters for the onset of the mode of dimeric molecular association in the water system are estimated. The characteristics of dimeric associates of molecules are determined. The region of anomalous thermal compression water is increased from T ≤ 4°C to T ≤ 66.4°C by introducing the temperature equivalent T₀ of the energy of proton transition from molecule to molecule into the parameter of resonant interaction of atoms of different molecules. The time of transfer of excitation energy correlates with the periods of the valence and deformation vibrations of the molecules. Therefore, a molecule that performs valence vibrations “has time” to store an excitation energy sufficient to provide a parallel orientation of the spins of the nuclei of the hydrogen atoms in the molecules. Molecules that perform deformation vibrations have zero spins because of the smallness of the frequencies of such vibrations.     

Fluorescence anisotropy of indole molecules under two-photon excitation in the spectral range of 485–510 nm

Decay of polarized fluorescence in indole dissolved in propylene glycol under two-photon excitation by femtosecond laser pulses in the wavelength range of 485–510 nm has been studied. It is shown that under the experimental conditions used the fluorescence decay signal can be well described by a single excited state lifetime τ_f and a single rotation diffusion time τ_rot. By processing the data obtained, the times τ_f and τ_rot as well as anisotropy parameter r ₀ characterizing the symmetry of two-photon excitation of indole molecules have been determined. Decreasing of the anisotropy parameter r₀ down to zero under two-photon excitation energy higher than 5.1 eV has been observed. Interpretation of the obtained results have been done on the basis of ab initio quantum-mechanical computations. A model of energy relaxation under the condition of twophoton excitation of indole in a polar solvent has been discussed.     

Molecular fusion within fullerene clusters induced by femtosecond laser excitation

Molecular fusion is induced in clusters of fullerene molecules on excitation with fs laser pulses. The dependence of the mass distributions of the fused products on the initial cluster distribution are studied and results for (C₆₀)_N and (C₇₀)_N clusters are compared. The fused products decay by emitting C₂ molecules and the fragmentation spectrum is used to determine the initial excitation energy of the fused species. The threshold excitation energy needed to induce fusion is consistent with the energetic thresholds for molecular fusion of fullerenes determined previously in single collision experiments.     

The formation of an intense secondary pulsed molecular beam and obtaining accelerated molecules and radicals in it

A method for obtaining an intense secondary pulsed molecular beam is described. The kinetic energy of molecules in the beam can be controlled by vibrational excitation of the molecules in the source under high-power IR laser radiation. A compression shock (shock wave) is used as a source of secondary beams. The shock wave is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beam were studied. Its intensity and the degree of gas cooling in it were comparable with the corresponding characteristics of the unperturbed primary beam. Vibrational excitation of molecules in the shock wave and subsequent vibrational-translational relaxation, which occurs when a gas is expanded in a vacuum, allow the kinetic energy of molecules in the secondary beam to be substantially increased. Intense [≥10²⁰ molecules/(sr s)] beams of SF₆ and CF₃I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases, and SF₆ molecular beams with kinetic energies approximately equal to 2.5 and 2.7 eV with He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) as carrier gases, respectively, were obtained. The spectral and energy characteristics of acceleration of SF₆ molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules. The possibility of accelerating radicals in secondary molecular beams was demonstrated.     

Ion-induced molecular ejection from D2O ice

Studies of molecules ejected from water ice by fast ions provide insight into the electronic relaxation processes and subsequent chemistry occurring in ice at very low temperatures. The ion-induced ejection of D₂O, D₂, and O₂ molecules from thin films of D₂O ice has been measured as a function of the fluence of incident MeV ions at temperatures between 10 and 140 K. For a given beam current, the O₂ yield exhibits initial transients which are slow compared with the prompt ejection of D₂O. We interpret these results as due to the build-up of O₂ in the films following fragmentation of D₂O molecules by incident ions. The fragmens re-form into new molecular species which diffuse to and escape from the surface, aided by subsequent bombardment. The D₂ transient has a prompt component, which we postulate is due to rapid formation during electronic recombination near the surface. A slow component of the D₂ transient is also observed, which may arise through a two-step process similar to that of O₂. Time-of-flight energy spectra of the ejected D₂O molecules have also been measured. Incident ion masses and energies range from those for which nuclear elastic energy deposition dominates (50 keV argon) to those for which electronic energy deposition dominates (1.5 MeV helium). The spectra cannot be described by models typically employed for the sputtering of metals. For instance, the spectra do not have maxima characteristic of the sublimation energy of the solid. In addition, the sputtering yield in the high energy part of the ejection spectrum of D₂O is too large to arise from nuclear elastic energy deposition. It must result instead from relatively energetic non-radiative relaxation of electronic excitation. For incident MeV ions that deposit their energy predominantly in electronic excitation, the low energy part of the D₂O ejection spectrum is greatly enhanced, indicative of a weakly antibinding region formed along an incident particle track. Enhanced ion yields are also found in the collision cascade region which are attributed to electronic processes.     

Threshold dependence of the vibrational excitation of molecules on laser radiation intensity

The collisionless vibrational excitation of a polyatomic molecule in an IR laser radiation field has been theoretically studied. It has been shown that (i) the degree of vibrational excitation (namely, number 0000 of vibrational quanta of a molecular mode near-resonant with the IR laser field that are absorbed by the molecule) is low if laser pulse intensity P (energy flux density in the laser beam) is lower than a certain critical value P _cr; (ii) the degree of excitation abruptly increases after crossing the boundary where P = P _cr; (iii) this effect is attributed to two properties inherent in polyatomic molecules, namely, the anharmonicity of the vibrational mode interacting with the laser field and the energy exchange with other modes; and (iv) at P > P _cr, number 00000 is determined only by energy density Φ = Pτ_P, where τ_P is the laser pulse duration, 00000 monotonically increases with increasing Φ. The model is in good agreement with the experimental data.     

Cooperative and quantum phenomena in experiments involved in the study of ordinary and stimulated Raman scattering of light

Results are presented of experimental studies of Raman scattering (RS) of light and of stimulated Raman scattering (SRS) of light, in which the quantum and cooperative properties of these phenomena are manifested. Experiments are described, revealing for the first time ever the connection between the integral intensity of the RS and SRS spectra and Brownian motion of molecules in condensed media. A number of experimental properties of RS and SRS are interpreted as results of collective behavior of molecules in condensed media. Spatial energy quantization of SRS energy is observed for the first time following excitation by 10^–9- and 10^-12-sec pulses. The experimental results are discussed in the Conclusion in light of the existing theoretical premises. New properties of RS and SRS are demonstrated and an explanation of their physical nature is proposed.Optics Division, lebedev Physics Institute. Translated from Preprint No. 103, Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1991.     

Copper hexadecafluoro phthalocyanine and naphthalocyanine: The role of shake up excitations in the interpretation and electronic distinction of high-resolution X-ray photoelectron spectroscopy measurements

We present the first high-resolution X-ray photoelectron core level spectra of bulk copper hexadecafluoro phthalocyanine (CuFPC) and naphthalocyanine (H₂NPC). The measurements have been performed in UHV onto samples grown in situ. A shake-up satellite assigned to a monopole on-site HOMO–LUMO molecular excitation has been evidenced in the F, C and N core-level spectra measured. In the case of the CuFPC, the shake-up is characteristic of the F atoms, of the four N atoms that are Cu bonded, and of the F- and N-bonded C atoms. The shake-up to main peak relative binding energy has been estimated to be 1.6 eV. In the case of H₂NPC, the outer benzenic C atoms do not show a satellite excitation, which instead is characteristic of the C and N atoms belonging to the inner porphyrin-like central ring of the molecule. The shake-up is less than 1 eV at higher binding energies from the main core line. The localisation of the HOMO level in the central structure of the molecule is confirmed by Hartree–Fock all-electron molecular orbital calculations performed on the metal-free phthalocyanine (H₂PC) and hexadecafluoro phthalocyanine (H₂FPC) molecules.     

Generation of high-energy secondary pulsed molecular beams

A method is suggested for generating high-intensity secondary pulsed molecular beams in which the kinetic energy of molecules can be controlled by an intense laser IR radiation through the vibrational excitation of molecules in the source. High-intensity [≥10²⁰ molecule/(sr s)] SF₆ molecular beams with a kinetic energy of ?1.0 eV without carrier gas and of ?1.9 and ?2.4 eV with carrier He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) gases, respectively, were obtained.     

Dynamics of molecules excited by infrared laser radiation

The CF₂HCl and CF₂Cl₂ molecules upon multiphoton excitation are investigated by the method of Raman light scattering spectroscopy at frequencies of fundamental transitions. It is demonstrated that essentially nonequilibrium energy distribution of molecules is formed in the process of excitation. Physical parameters characterizing this distribution are determined.     

Methane activation on Ni(111) at high pressures

High pressure CH₄ decomposition experiments on Ni(111) have been conducted to discriminate between a direct reaction mechanism involving hot CH₄ molecules and a precursor-mediated mechanism. It has been found that the direct process for CH₄ decomposition is kinetically significant in depositing adsorbed carbon. Using absolute carbon coverage measurements the absolute efficiency of carbon deposition on Ni(111) at 600 K was found to be 4 × 10⁻⁸ C atom per CH₄ collision at 1.0 Torr CH₄ pressure. By the use of He, Ne, and Ar buffer gases, it has been shown that the major process leading to CH₄ dissociation involves heating CH₄ followed by return of excited CH₄ back to the surface where the CH₄ dissociates. These experiments demonstrate that CH₄ translational excitation and vibrational excitation, as studied by molecular beam methods, are the kinetically important CH₄ activation processes leading to dissociation in the high pressure regime.     

TEA CO2 laser-induced photodissociation of UF6 via interspecies V-V energy transfer from multiple photon excited halomethanes

The dissociation of UF₆ sensitized by multiple photon excitation of a series of halomethanes: CF₄, CF₃Cl, and CF₂Cl₂ has been investigated. The roles of various experimental parameters like exciting frequency, fluence and pressures of sensitizer/UF₆ on the dissociation yield were studied to examine (1) the characteristics of the sensitizer/UF₆ system and (2) the coupling of vibrational energy between two molecular systems. The efficiency of the energy transfer process was estimated on the basis of long range dipole-dipole interaction to gain an understanding of the dissociation process.     

Two processes that lower the emission energy of an electric-discharge KrF laser

Results are reported of an experimental investigation of the processes that lower the emission energy of an electric-discharge excimer KrF laser operating on mixtures containing F₂ and NF₃. The existence is demonstrated of two processes, reversible and irreversible, that lower the KrF-laser emission energy as the number of excitation pulses is increased (without continuous replenishment of the mixture) and as the pulse repetition frequency is increased. The irreversible process is connected with the decrease of the concentration of initial halogen-containing gas in the mixture as a result of interaction between the halogen atoms and the chamber material. The reversible process is due to the long reduction time of the halogen-containing molecule (~ 1 sec for F₂) and influences the laser emission energy only at pulse repetition frequencies that exceed the reciprocal time of reduction of these molecules. If complex halogen-containing molecules (NF₃, SF₆, ...) are used, the pulse-repetition regime is realized because of the radicals that are produced. The use of such molecules, however, affects adversely the service life of the excimer gas mixture.Translated from Lazernye Sistemy, pp. 46–57, 1982.