A. A. Makarov

Intramolecular vibrational dynamics of propyne and its derivatives: The role of vibrational-rotational mixing

The dynamics of intramolecular vibrational energy redistribution from the initially excited νHC mode in the propyne molecule (H-C≡C-CH3), as well as in three its derivatives that are obtained by replacing one of the hydrogen atoms of the methyl group with the chlorine atom (propargylchloride), the OH radical (propargyl alcohol), or with the NH2 radical (propargylamine), has been studied. Probing was performed by anti-Stokes spontaneous Raman scattering. The measured values of the deexcitation rate W of the νHC mode lie in the range 109−1010 s−1. A significant feature of the dynamics—an incomplete energy redistribution from the νHC mode—is especially clearly pronounced for the H-C≡C-CH3 and H-C≡C-CH2Cl molecules, for which the values of the relative level σ of the residual energy in the νHC mode are approximately equal to 0.54 and 0.25, respectively. A theoretical analysis performed made it possible to relate the parameters W and σ, on the one hand, and the density ρ of the so-called bath states, which are responsible for the vibrational energy redistribution, on the other hand. It is shown that, for all the four molecules considered, the required values of ρ can be accounted for solely by a strong vibrational-rotational mixing in the bath, as a result of which the projection of the total angular momentum onto the axis of the molecule ceases to be “good” quantum number.

A novel feature of intramolecular vibrational redistribution in propargyl alcohol and propargyl amine

Intramolecular vibrational redistribution (IVR) from the terminal acetylene mode nu(HC) has been studied for four molecules: H-C[Triple Bond]C-CH(3) (propyne), H-C[Triple Bond]C-CH(2)Cl (propargyl chloride), H-C[Triple Bond]C-CH(2)OH (propargyl alcohol), and H-C[Triple Bond]C-CH(2)NH(2) (propargyl amine). The experiments were performed with the room-temperature gases. The transition mid R:0-->mid R:1 in the mode nu(HC) was pumped by a short laser pulse. Anti-Stokes spontaneous Raman scattering was used as a probe. The measured parameters were the de-excitation rate W and the dilution factor sigma defined as the relative level of the residual energy in the nu(HC) mode at long pump-probe delay times. The pair of these values {W,sigma} allowed us to determine the density rho(eff) of those vibrational-rotational states, which are involved in IVR from state mid R:1. For two molecules, HCCCH(3) and HCCCH(2)Cl, the experimental results were consistent with the suggestion that all close vibrational-rotational states with the same total angular momentum J and symmetry participate in the IVR regardless of the other rotator quantum number K (in the case of HCCCH(3)) or K(a) (in the case of HCCCH(2)Cl) and the vibrational quantum numbers as well. For the other two molecules, HCCCH(2)OH and HCCCH(2)NH(2), this effect was also present, yet the experimental results revealed certain restrictions. We have obtained a satisfactory theoretical fit with the assumption that the low-frequency torsion vibration of the hydrogen atom in the hydroxyl group (in the case of HCCCH(2)OH) or hydrogen atoms in the amine group (in the case of HCCCH(2)NH(2)) does not participate in the IVR. This assumption can be treated as a challenge to future studies of these molecules by high-resolution spectroscopy and various double-resonance and pump-probe techniques.

Fast collision-induced redistribution of vibrational energy in halogenated methanes

Abstract Abnormally fast relaxation of the high-frequency CH stretch mode of CHF 2 Cl and CHF 3 molecules in mixtures with different buffer gases is observed. The probability of this process with respect to hard-sphere collision rate reaches 0.38 for the noble buffer gas (CHF 2 Cl:Kr mixture) and 1.03 for the polar buffer gas (CHF 2 Cl:SO 2 mixture). This result is referred to the collision-induced intramolecular vibrational relaxation (CIIVR) leading to the redistribution of vibrational energy from the initially excited mode among the nearby optically ‘dark’ states. The mechanisms resulting in CIIVR are discussed. It is shown that the high efficiency of this process results from the joint action of intra- and intermolecular interactions. The number of optically ‘dark’ vibrational states involved in CIIVR is experimentally measured for the CHF 2 Cl molecule. The obtained value coincides with the number of three-frequency combination states in the vicinity of the initially excited state.

Slow intramolecular vibrational redistribution: the latest results for trifluoropropyne, a comparison with the other terminal acetylenes and the mechanism*

We studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode ν1≈3330 cm−1 (acetylene-type H–C bond) in molecules in the gaseous phase by means of time-resolved anti-Stokes spontaneous Raman scattering. The time constant of this process was estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. We have compared this result with earlier results on the other terminal acetylene molecules, and give an explanation of this low IVR rate. Our suggestion for it follows from an assumption that the most probable doorway state leading to IVR from to the bath of all vibrational–rotational states consists of one quantum of the stretch and two quanta of the bend, and the matter is that the energy defect of Fermi resonance is essentially larger in trifluoropropyne than in other similar molecules. In addition, we have obtained the rate of collision-induced IVR for trifluoropropyne from experiments with various gas pressures and have shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational–rotational mixing.

Real-time observation of the dynamics of vibrational-energy redistribution within an isolated polyatomic molecule by spontaneous raman spectroscopy

The dynamics of intramolecular vibrational-energy redistribution from an initially excited mode ν1 (the acetylene-type H-C bond) to the other modes of the H-C≡C-Si(CH3)3 molecule was studied experimentally. Probing was performed by spontaneous anti-Stokes Raman scattering. The measured deexcitation time of ν1 was 128±10 ps.

Extremely slow intramolecular vibrational redistribution: Direct observation by time-resolved raman spectroscopy in trifluoropropyne

We have studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode v1 ≈ 3330 cm−1 (acetylene-type H-C bond) in H-C≡C-CF3 molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The time constant of this process is estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. It is suggested that so long IVR time with respect to the other propyne derivatives can be explained by a larger defect, in this case, of the Fermi resonance of v1 with v2 + 2v7—the most probable doorway state leading to IVR from v1 to the bath of all vibrational-rotational states with the close energies. In addition, it is shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational-rotational mixing.

Direct observation of the vibrational energy redistribution in (CF3)2CCO 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.

Observation of the ν1 fundamental band in sulphur trioxide induced by symmetry breaking upon 18O substitution

Abstract The infrared (IR) absorption spectrum of gaseous SO 3 enriched with 18 O is studied. The ν 1 band of S 16 O 2 18 O 1040±2 cm −1 is observed in the IR spectrum, whereas it is inactive for S 16 O 3 . The isotope shift measured in ν 1 is 28±2 cm −1 . The integrated intensity of this band was measured to 5.7±1.6 atm −1 cm −2 . These results agree with our calculations, carried out within the framework of the valence force model. The opportunity to realize isotopically selective photolysis of S 16 O 2 18 O molecules with excitation of the ν 1 mode by combined IR+UV laser radiation is discussed.

Study of femtosecond dynamics in vibrationally excited free bis(trifluoromethyl)ketene and metal carbonyl molecules induced by CO bond resonant excitation

The dynamics of relaxation processes in free bis(trifluoromethyl)ketene (CF3)2CCO and Fe(CO)5 and Cr(CO)6 metal carbonyl molecules after multiphonon excitation of the C=C=O and C=O vibrations by femtosecond laser infrared radiation was studied. The temporal and spectral dependences of the relaxation of the excited vibrational states were measured. Kinetics with a characteristic decay time of about 5 ps was obtained for (CF3)2CCO molecules. Its behavior is interpreted as a manifestation of the intramolecular relaxation of the excited vibration states of the resonance mode. Kinetic curves with characteristic times of about 250–500 fs were observed for Fe(CO)5 and Cr(CO)6. The behavior of these curves depends on the mutual orientation of the polarizations of the pump and probe pulses.

Time-resolved spectroscopy in study of the intramolecular vibrational dynamics: Recent results and future prospects

Applications of spectroscopic methods in real-time investigations of the vibrational energy redistribution in an isolated polyatomic molecule are reviewed. The results of the latest experiments with propyne and its derivatives according to the pump-probe scheme with the use of Raman spectroscopy are reported. The main result is the key role of the energy transfer between the vibrational and rotational degrees of freedom in the molecules studied.