Intramolecular Vibrational Dynamics in Free Polyatomic Molecules with C═O Chromophore Bond Excited by Resonant Femtosecond IR Laser Radiation

Abstract

In nine polyatomic molecules, we have studied the intramolecular redistribution of vibrational energy from chromophore C═O group excited by a resonant femtosecond IR laser radiation at a wavelength of ∼5 μm. All experiments have been performed in the gas phase using the IR-IR pump-probe technique in combination with the spectral analysis of the probe radiation. For molecules with one C═O end group, characteristic times of intramolecular vibrational redistribution (IVR) lie in the range between 2.4 and 20 ps and correlate with the density of four-frequency Fermi resonances. The IVR times in metal carbonyl molecules are anomalously long, being ∼1.0 ns for Fe(CO)5 and ∼1.5 ns for Cr(CO)6. In the CH3(C═O)OC2H5 and H2CCH(C═O)OC2H5 molecules, it has been observed that there are two characteristic IVR times, which differ by an order of magnitude from each other; this was interpreted in terms of the developed model of "accumulating states". For the ICF2COF molecule, it has been revealed that the IVR time decreases with increasing level of the vibrational excitation of the C═O bond of the molecule.

IVR Dynamics of Vibrational Levels of the ν1 Mode in (CF3)2C═C═O Molecules Excited by Resonant IR Femtosecond Radiation

“…We used the method of IR pumping and IR probing of molecules in the gas phase accompanied by the spectral analysis of the probe radiation. 11 As IR radiation sources, two nonlinear independent converters based on TOPAS-C optical parametric amplifiers (OPAs) with subsequent difference frequency generation in a DFG-1 unit (Light Conversion) were employed. One of these converters was used as a pump source, while the other one, as a probe source.…”

“…12 It is likely that this was related to the fact that, for such molecules, fast (2.4−8 ps) IVR times are observed even at the first level of vibrational excitation. 11 According to the predictions of the theory, 3 as the excitation level of an isolated vibrational mode in a molecule increases, the IVR times should decrease. This follows from Fermi's golden rule for the relaxation rate of vibrational energy (see, e.g., ref 3, page 222) (2πcτ IVR ) −1 = 2πV̅ 2 ρ, where V̅ 2 (in cm −1 ) is the average nondiagonal element of the anharmonic interaction between the excited state and the close combination of the bath states of the other modes (it is known that…”

“…The density of states ρ is unknown and depends on the nature of the bath states. If this is all vibrational states near the energy E̅ vib + ν 1 (E̅ vib , the average vibrational thermal energy), then ρ is equal to 10 6 cm (see our previous paper 11 ). In the case that ρ corresponds to all vibrational−rotational states, then ρ vib−rot ≫ ρ vib .…”

“…8,9 It is worth noting that, because of peculiarities of their chemical bonds, these molecules have rather low activation energies, 35−40 kcal/ mol (12000−14000 cm −1 ), and, furthermore, metal carbonyl molecules are characterized by anomalously long IVR times (∼1 ns). 10,11 Attempts to initiate monomolecular decomposition reactions by femtosecond IR irradiation of conventional molecules such as (CF 3 ) 2 CCO and C 4 F 9 COI, the activation energies of covalent bonds of which are higher than 50 kcal/mol (>17500 cm −1 ), were unsuccessful. 12 It is likely that this was related to the fact that, for such molecules, fast (2.4−8 ps) IVR times are observed even at the first level of vibrational excitation.…”

“…Upon multiphoton excitation of a selected vibrational mode in a molecule by femtosecond resonant infrared radiation, it is possible to initiate photochemical processes that are selective with respect to a bond (or group of bonds). However, this possibility is limited by the effect of intramolecular redistribution of vibrational energy (IVR) from the excited mode to other molecular vibrations. − Nevertheless, upon resonant multiphoton excitation by femtosecond IR laser pulses of vibrations of CNN and CO bonds in molecules of diazomethane and metal carbonyls, it became possible to reveal the nonstatistical character of their dissociation in the gas phase. , It is worth noting that, because of peculiarities of their chemical bonds, these molecules have rather low activation energies, 35–40 kcal/mol (12000–14000 cm –1 ), and, furthermore, metal carbonyl molecules are characterized by anomalously long IVR times (∼1 ns). , Attempts to initiate monomolecular decomposition reactions by femtosecond IR irradiation of conventional molecules such as (CF 3 ) 2 CCO and C 4 F 9 COI, the activation energies of covalent bonds of which are higher than 50 kcal/mol (>17500 cm –1 ), were unsuccessful . It is likely that this was related to the fact that, for such molecules, fast (2.4–8 ps) IVR times are observed even at the first level of vibrational excitation .…”