1,721,096 research outputs found
Specific rate constants k(E, J) for the dissociation of NO2. I. Time-resolved study of rotational dependencies
No full textThe effect of rotational excitation on the specific rate constants k(E,J) of the unimolecular decomposion of NO2 has been investigated. Time-resolved pump- and probe experiments with sub-ps time resolution are reported in which NO2 molecules with well-defined rotational and vibrational energy distributions were optically excited near and above the dissociation threshold. The subsequent unimolecular decay of the reacting NO2 molecules was probed by time-resolved laser-induced fluorescence of the disappearing NO2 via excitation to Rydberg states. At constant photolysis wavelength, increasing rotational energy (up to 310 cm(-1)) was found to leave the overall decay rate nearly unaffected. This observation can be rationalized by assuming a compensation of the angular momentum and energy dependences of the specific rate constants when J and E are, changed at the same time. Keeping the total energy E nearly constant and changing J independently, the effect of rotation on the decay rate can be separated and observed more clearly. From the experimental data we conclude that, for sufficiently high J and constant total energy, molecules with larger J dissociate more slowly than molecules with small J, which is in agreement with predictions from statistical unimolecular rate theory. (C) 2001 American Institute of Physics
Real-time probing of intramolecular vibrational energy redistribution and intermolecular vibrational energy transfer of selectively excited CH2I2 molecules in solution
No full textCompetition between intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of excited methylene iodide (CH2I2) in solution has been measured in real time. After excitation of the C-H- stretch overtone and C-H- stretch containing combination bands of CH2I2 between 1.7 and 2.4 mum an increase followed by a decrease in the transient electronic absorption at 400 nm has been monitored. The transient absorption has been attributed to vibrational energy flow from the initially excited degrees of freedom to vibrational states with larger Franck-Condon (FC) factors for the electronic transition (long wavelength wing) and energy loss due to energy transfer to the solvent. A model based upon the dependence of the electronic absorption on the internal energy of CH2I2 has been used to determine the times for intramolecular vibrational energy redistribution and intermolecular energy transfer to the solvent. in the simplest version of our model the internal energy of the molecule probed by the population of the FC-active modes rises and decays exponentially on a picosecond (ps) time scale, which reflects the initial intramolecular vibrational energy redistribution and the subsequent energy transfer to the solvent. This simple approach was able to accurately describe the measured transient absorption for all solvents and excitation wavelengths. Overall time constants for IVR have been found to be on the order of 9-10 ps, almost independent of the excitation wavelength, the excited modes, and the solvent. In contrast, energy transfer to the solvent takes significantly longer. Overall time constants for VET have been determined in the range between 60 and 120 ps depending on the solvent, the excitation energy, but not on the mode which was initially excited
Vibrational energy relaxation of the ND-stretching vibration of NH2D in liquid NH3
No full textThe vibrational energy relaxation from the first excited ND-stretching mode of NH2D dissolved in liquid NH3 is studied using molecular dynamics simulations. The rate constants for inter- and intramolecular energy transfer are calculated in the framework of the quantum-classical Landau-Teller theory. At 273 K and an ammonia density of 0.642 g cm(-3) the calculated ND-stretch lifetime of tau = 9.1 ps is in good agreement with the experimental value of 8.6 ps. The main relaxation channel accounting for 52% of the energy transfer involves an intramolecular transition to the first excited state of the umbrella mode. The energy difference between both states is taken up by the near-resonant bending vibrations of the solvent. Less important for the ND-stretch lifetime are both the direct transition to the ground state and intramolecular relaxation via the NH2D bending modes contributing 23% each. Our calculations imply that the experimentally observed weak density dependence of tau is caused by detuning the resonance between the ND-stretch-umbrella energy gap and the solvent accepting modes which counteracts the expected linear increase of the relaxation rate with density.Deutsche Forschungsgemeinschaft [SCHR 303/1-1, SFB 813
The photodecomposition mechanism of tert-butyl-9-methylfluorene-9-percarboxylate: new insight from femtosecond IR spectroscopy.
Full text linkThe ultrafast photodissociation of tert-butyl-9-methyl. uorene-9-percarboxylate (TBFC) is studied by mid-infrared transient absorption spectroscopy after UV excitation at 266 nm. By means of 13 C-labeled TBFC and additional DFT calculations transient IR bands in the fingerprint region were unambiguously assigned to the methylfluorenyl radical. The experiments show that the fragmentation is controlled by the S(1)-lifetime of TBFC and, dependent on the solvent, within 0.8-2.1 ps leads to tert-butyloxy and methylfluorenyl radicals plus CO(2) via concerted bond breakage of the O-O and the fluorenyl-C(carbonyl) bond. In accordance, the CO(2) quantum yield is determined to be unity
Femtosecond IR Spectroscopy of peroxycarbonate photodecomposition: S1-lifetime determines decarboxylation rate
No full textThe ultrafast photofragmentation of arylperoxycarbonates R-O-C(O)O-O-tert-butyl (R = naphthyl, phenyl) is studied using femtosecond UV excitation at 266 nm and mid-infrared broadband probe pulses to elucidate the dissociation mechanism. Our experiments show that the rate of fragmentation is determined by the S-1-lifetime of the peroxide, i.e., the time constants of S-1 decay and Of CO2 and R-O-center dot formation are identical. The fragmentation times are solvent dependent and for tert-butyl-2-naphthylperoxycarbonate (TBNC) vary from 25 ps in CH2Cl2 to 52 ps in n-heptane. In the case of the tert-butylphenylperoxycarbonate (TBPC) the decomposition takes 5.5 ps in CD2Cl2 and 12 ps in n-heptane. The CO2 fragment is formed vibrationally hot with an excess energy of about 5000 cm(-1). The hot CO2 spectra at high energy can be modeled assuming Boltzmann distributions with initial vibrational temperatures of ca. 2500 K which relax to ambient temperature with time constants of 280 ps in CCl4 and 130 ps in n-heptane. In CCl4 the relaxed spectra at 1.5 ns show 3.5% residual excitation in the n = 1 level of the asymmetric stretch vibration
Multiplex detection of collisional energy transfer using KCSFI
Full text linkA new detection method for obtaining collisional transition probabilities P (E', E) of highly vibrationally excited molecules in the gas phase is presented. The technique employs energy-selective probing of the time-dependent vibrational population distribution by kinetically controlled selective fluorescence (KCSF)". We present experimental results for a test system, the collisional deactivation of toluene by argon, where we use the well-known kinetically controlled selective ionization (KCSI) scheme as a reference for comparison. A newly designed setup is employed that allows simultaneous detection of fluorescence and ionization signals under identical experimental conditions ( kinetically controlled selective fluorescence and ionization = KCSFI"). For the system toluene + argon it is demonstrated that KCSF and KCSI yield identical results. A rate-equation model is presented to understand common features and differences of both approaches. The fluorescence detection scheme shows promise for future investigations on collisional energy transfer. The experimental setup is simpler, because it requires no additional ionization wavelength. This will hopefully give access to the P ( E 0, E) of systems where, e. g., ionization schemes are difficult to implement due to short wavelengths required for the ionization step. A few examples will be outlined briefly
Comment on 'Rate coefficients for photoinitiated NO2 unimolecular decomposition: energy dependence in the threshold regime' [Chem. Phys. Lett. 358 (2002) 71]
Full text linkRecently, Wittig and co-workers have published rate coefficients k(E) for the unimolecular decomposition of photoinitiated NO2 close to the dissociation threshold [Chem. Phys. Lett. 358 (2002) 71]. They found out that k(E) for low angular momentum J exhibits a strong increase within 25 cm(-1) of the reaction threshold. The authors emphasize that their experimental results are surprising and cannot be understood quantitatively on the basis of current theory on NO2. In this Comment we demonstrate that recent quantum mechanical calculations of NO2 resonances on a global 3D-potential energy surface can indeed explain their data close to the dissociation threshold as well as for larger excess energies. (C) 2002 Elsevier Science B.V. All rights reserved
Unravelling the ultrafast photodecomposition mechanism of dibenzoyl peroxide in solution by time-resolved IR spectroscopy.
Full text linkThe ultrafast photo-fragmentation of dibenzoyl peroxide (DBPO) is studied using femtosecond UV excitation at 266 nm and mid-infrared broadband probe pulses to elucidate the dissociation mechanism. With the help of C-13-labeled DBPO it was possible to unambiguously assign transient IR bands in the fingerprint region to the benzoyloxy radical. Our experiments show that the fragmentation is controlled by the S-1-lifetime of DBPO and within 0.4 +/- 0.2 ps leads to a benzoyloxy/phenyl radical pair plus CO2 via concerted bond breakage of the O-O and the phenyl-C(carbonyl) bond. 20% of the radical pairs geminately recombine to phenyl benzoate on a timescale of 70 ps
Mode-specific energy absorption by solvent molecules during CO2 vibrational cooling
No full textNon-equilibrium molecular dynamics (NEMD) simulations of energy transfer from vibrationally excited CO2 to CCl4 and CH2Cl2 solvent molecules are performed to identify the efficiency of different energy pathways into the solvent bath. Studying in detail the work performed by the vibrationally excited solute on the different solvent degrees of freedom, it is shown that vibration-to-vibration (V-V) processes are strongly dominant and controlled by those accepting modes which are close in frequency to the CO2 bend and symmetric stretch vibration
Photodecarbonylation of diphenylcyclopropenone - A direct pathway to electronically excited diphenylacetylene?
Full text linkA comparison of combined IR and UV/Vis pump-probe measurements of diphenylcyclopropenone (DPCP) and diphenylacetylene (DPA) provides clear evidence that the photodecarbonylation of DPCP following excitation to its S(2)-state proceeds non-adiabatically to the electronic ground state of DPA. It is shown that transient absorption of electronically excited DPA is caused exclusively by photoexcitation of ground state DPA generated by preceding DPCP photodecarbonylation.Deutsche Forschungsgemeinschaf
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