Structural dynamics and energy flow in Rydberg-excited clusters of N,N-dimethylisopropylamine (original) (raw)
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The Journal of Physical Chemistry A, 2003
In this paper, we report our experimental studies, using femtosecond-resolved mass spectrometry, and theoretical calculations of the potential energy surfaces, using density functional theory, for 10 aliphatic amines (substituted ammonia) excited directly near the ionization continuum. By probing parent and fragment masses, we are able to decipher pathways of the reactive R-cleavage and the nonreactive internal conversion, a precursor for the following chemistry. The bifurcation in these channels is related to the structure, as evidenced by the dramatic effect of R-substitution vs N-substitution on the time scale of the dynamicssfemtosecond vs picosecond rates. For all molecules studied, the observed branching is dependent on the change of character of the state, ionic and/or neutral, along the reaction coordinate, and this change blurs the distinction between Rydberg and Superexcited states in their subsequent reactivity.
Excited State Energetics and Dynamics of Large Molecules, Complexes and Clusters
New techniques for spectroscopy in supersonic expansions and in the development of specific supersonic sources were developed, including vacuum ultraviolet absorption spectroscopy in supersonic expansions, development of conical nozzles for supersonic jets and pulse extraction mass spectrometer. Research included studies of energetics of rydberg states of jet-cooled molecules, rydberg states of anthracene, intramolecular relaxation of rydberg states and interference effects between extravalence and intravalence molecular excitations. The rotational state dependence of intramolecular dynamics was investigated as well as the coupling between intrastate vibrational energy redistribution and interstate electronic relaxation and the coriolis rotation-vibration coupling and intramolecular dynamics. Mediated intersystem crossing phenomenology, inverse isotope effects and microscopic level shifts were explored. Studies of fluorescence quantum yields for highly-excited states of large molecu...
Dynamics of Very High Molecular Rydberg States: The Intramolecular Processes
1994
Classical trajectory computations are used to document and examine the purely intramolecular decay dynamics of very high Rydberg states of an isolated cold molecule. The Hamiltonian is that of an anisotropic ionic core about which the Rydberg electron revolves. The equations of motion are integrated using action angle variables in order to ensure numerical stability for many orbits of the electron. Examination of individual trajectories verifies that both "up" and "down" intramolecular processes are possible. In these, the electron escapes from the detection window by a gain or loss of enough energy. Either process occurs in a diffusive like fashion of many smaller steps, except for a very small fraction of prompt processes. The results for ensembles of trajectories are examined in terms of power spectra of the different modes of motion and in terms of the decay kinetics.
Electronic Structures and Photoevaporation Dynamics of Benzene Cluster Ions
The Journal of Physical Chemistry A, 1997
The electronic spectra of benzene cluster ions, (C 6 H 6) n + with n) 3-6, are measured through mass-selected photodissociation spectroscopy. The spectra in the 400-1100-nm region show three distinct absorption maxima centered around 430, 590-620, and 950 nm, which are analogous to the spectrum of (C 6 H 6) 2 +. The 950-nm band is assigned to a charge resonance (CR) band characteristic of the dimer ion, while the other two bands are attributed to local excitation bands. The position of the CR band is found to be almost independent of cluster size. The result suggests that the cluster ions have a charge-localized structure involving a strongly bound dimer ion core. In addition, the number and translational energy of neutral molecules ejected following photoexcitation are measured for (C 6 H 6) n + with n) 3-8 in the photon energy range of 0.5-3.0 eV. The average number of ejected molecules increases linearly with increasing photon energy, suggesting that the fragmentation proceeds via the sequential ejection of neutral monomers. The average translational energy carried by one monomer is determined to be 50-70 meV, which is comparable with the calculated value according to a statistical theory. A large part of the imparted photon energy is partitioned into internal energies of the products. These results indicate that the photofragmentation of (C 6 H 6) n + can be regarded as a unimolecular decay of vibrationally hot clusters, despite the promotion of the chromophoric dimer core to the repulsive excited state.
The Journal of Chemical Physics, 1995
The effect of an electrical field on the dynamics and decay kinetics of a high Rydberg electron coupled to a core is discussed with special reference to simulations using classical dynamics and to experiment. The emphasis is on the evolution of the system within the range of Rydberg states that can be detected by delayed pulsed ionization spectroscopy ͑which is nϾ90 for both the experiment and the computations͒. The Hamiltonian used in the computations is that of a diatomic ionic core about which the electron revolves. The primary coupling is due to the anisotropic part of the potential which can induce energy and angular momentum exchange between the orbital motion of the electron and the rotation of the ion. The role of the field is to modulate this coupling due to the oscillation of the orbital angular momentum l of the electron. In the region of interest, this oscillation reduces the frequency with which the electron gets near to the core and thereby slows down the decay caused by the coupling to the core. In the kinetic decay curves this is seen as a stretching of the time axis. For lower Rydberg states, where the oscillation of l is slower, the precession of the orbit, due to the central but not Coulombic part of the potential of the core, prevents the oscillation of l and the decay is not slowed down. Examination of individual trajectories demonstrates that the stretching of the time axis due to the oscillatory motion of the electron angular momentum in the presence of the field is as expected on the basis of theoretical considerations. The relation of this time stretch to the concept of the dilution effect is discussed, with special reference to the coherence width of our laser and to other details of the excitation process. A limit on the principal quantum number below which the time stretch effect will be absent is demonstrated by the computations. The trajectories show both up and down processes in which the electron escapes from the detection window by either a gain or a loss of enough energy. Either process occurs in a diffusive like fashion of many smaller steps, except for a fraction of trajectories where prompt ionization occurs. The results for ensembles of trajectories are examined in terms of the decay kinetics. It is found that after a short induction period, which can be identified with the sampling time of the available phase space, the kinetics of the decay depend only on the initial energy of the electron and on the magnitude of the field, but not on the other details of the excitation process. The computed kinetics of the up and down channels are shown to represent competing decay modes. A possible intramolecular mechanism for long time stability based on the sojourn in intermediate Rydberg states is discussed. The available experimental evidence does not suffice to rule out nor to substantiate this mechanism, and additional tests are proposed. The theoretical expectations are discussed in relation to observed time resolved decay kinetics of high Rydberg states of BBC ͑bisbenzenechromium͒ and of DABCO ͑1,4-diazabicyclo͓2.2.2͔octane͒. The experimental setup allows for the imposition of a weak ͑0.1-1.5 V/cm͒ electrical field in the excitation region. The role of the amplitude of the time delayed field, used to detect the surviving Rydberg states by ionization, is also examined. The observed decay kinetics are as previously reported for cold aromatic molecules: Most of the decay is on the sub-s time scale with a minor ͑ϳ10%͒ longer time component. The decay rate of the faster component increases with the magnitude of the field. Many features in such an experiment, including the absolute time scales, are similar to those found in the classical trajectory computations, suggesting that the Hamiltonian used correctly describes the physics of the faster decay kinetics of the high Rydberg states.
Physical Chemistry Chemical Physics, 2011
The photodissociation dynamics of pyrrole-ammonia clusters (PyHÁ(NH 3 ) n , n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH 4 (NH 3 ) nÀ1 products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (B0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.
Phys. Chem. Chem. Phys., 2014
The real-time dynamics of DABCO-argon clusters is investigated in a femtosecond pump-probe experiment where the pump excites DABCO to the S 1 state within the argon cluster. The probe operates by photoionization and documents the energy and angular distributions of the resulting photoelectrons. The present work complements 1 a former one of our group [Awali et al., Phys. Chem. Chem. Phys., 2014, 16, 516-526] where this dynamics was probed at short time, up to 4 ps after the pump pulse. Here, the dynamics is followed up to 500 ps. A multiscale dynamics is observed. It includes a jump between two solvation sites (timescale 0.27 ps) followed by the relaxation of the solvation cage excess vibrational energy (timescale 14 ps) and then by that of DABCO (timescale >150 ps). Polarization anisotropy, double polarization and angular anisotropy effects are reported also. They are interpreted (quantitatively for the former effect) in terms of decoherence of rotational alignment, driven by the overall rotation of the DABCO-argon clusters. A tomographic view of the DABCO excited orbital, provided by the double anisotropy effect, is discussed on a qualitative basis.