Nuclear Dynamics of Benzene···(Ar) n Clusters (original) (raw)
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Electronic-vibrational excitations of aromatic molecules in large argon clusters
In this paper we report the results of an experimental study of excited-state energetics, line broadening, and vibrational relaxation (VR) dynamics of electronically-vibrationally excited states of anthracene, tetracene, and pentacene embedded in clusters of Ar. Large clusters of Ar, each containing a single aromatic molecule, were synthesized in supersonic jets of seeded Ar, expanded through a 150-fim nozzle in the pressure range p= 3000-14000 torr. We applied the interrogation techniques of laser spectroscopy in supersonic expansions, monitoring the fluorescence excitation spectra, the energy-resolved fluorescence, and the simultaneous time resolved and energy-resolved emission. Information was obtained on the energetics and vibrational structure of the S1 state of anthracene, tetracene, and pentacene in Ar clusters. We have also observed a structureless spectrum of the anthracene dimer embedded in Ar clusters. The red spectral shifts and the line widths of the electronic origin of the So - S1 transition of the anthracene, tetracene, and pentacene molecules increase with increasing stagnation pressure in the range p = 1000-2500 torr and are independent of the stagnation pressure in the range p = 6000-14000 torr. An intrinsic line broadening in the energy-resolved S,(O) - So(0) emission of tetracene in clusters synthesized at p 2 8000 torr was observed, which provides evidence for homogeneous line broadening, originating from electron-phonon coupling, in these clusters. Vibrational relaxation of intramolecular vibrations of tetracene and anthracene in their S1 state was explored by the bservation of hot luminescence and by the easurements of its energy-resolved decay time. The vibrational relaxation process in clusters prepared at p = 6000-14000 torr is surprisingly slow, occurring on the nanosecond time scale. These vibrational relaxation lifetimes decrease with increasing stagnation pressure.
Berichte der Bunsengesellschaft/Physical Chemistry Chemical Physics
The method previously presented for mass-selective ground-state vibrational spectroscopy of aromatic van der Waals complexes by populating ground-state levels via a pumpldump laser pulse sequence, followed by selective resonant two-photon ionization (R2PI) of the vibrationally relaxed complexes [T. Burgi et al., Chem. Phys. Lett., 225, 351 (1994)l is here extended to the detection of the aromatic molecular product from the So state vibrational predissociation (VP) process. Schemes and results are presented which allow (a) extended measurements of intraand intermolecular vibrational frequencies of carbazole.Ar in the So state, (b) detection of vibrationally hot, but rotationally cold carbazole VP product, and (c) vibrationally and rotationally hot carbazole VP products. A much improved determination of the van der Waals dissociation energy was made by combining results from various experiments, as Do(S,) = 530k1.5 cm-'.
Rigid and nonrigid benzene-AT2 van der Waals heteroclusters
The nonrigidity of the one-sided (2 IO) isomer of benzene,Ar2 is manifested by the occurrence of a hierarchy of distinct iso-merization processes and by the marked broadening of the photoionization efficiency curves, which were explored by constant energy molecular dynamics simulations. M.A,, heteroclusters, consisting of an organic aromatic molecule (M) bound to rare-gas atoms (A), provide interesting information regarding the size dependence of energetic, structural, thermodynamic, dynamic and chemical properties of large finite systems [ l-4 1. In this context, various isomerization processes were identified by molecular dynamics (MD) and Monte Carlo computer simulations in carbazoles Ar, [ 4-71, 9,l 0-dichloroanthracene*An (A=Ar, Kr) [8], tetracene.Ar, [9] and ben-zene.Ar,, [ lo,1 1 ] heteroclusters, which involve correlated surface motion (of A atoms on one side of M), surface melting (i.e. uncorrelated motion of A atoms on the microsurface of M), side crossing (of A atoms from one microsurface of A to the other), wetting-nonwetting (or two-dimensional (2D) to three-dimensional (3D)) transition and a rigid-nonrigid transition. These studies [4-91 established the hierarchical occurrence of several isomerization processes for a single heterocluster composition and the sequential occurrence of distinct isomerization processes with increasing the size of the M.A, het-erocluster for a given M. This isomerization pattern is specified by the finite surface density p,=n/(M microsurface area) of the rare-gas atoms [8 1. Ben-zene.Ar,, heteroclusters, which are characterized by a large value ofp,, are expected to exhibit a hierarchy of isomerization processes even for small values of n. In addition, isomerization phenomena are governed by the initial isomer configuration, being distinct for interior and for surface states of M in or on the heterocluster [&lo]. In order to explore the M and n size dependence and the isomer specificity of M,A,, heterocluster isomerization phenomena, we have conducted constant energy MD simulations of the benzene.Ar, isomers, which constitute the smallest heteroclustets of the M.A,, family. The recent spectroscopic studies of Schmidt, Mons and Le Calve? [ 11,12 ] on the spectral shifts and rotational counters of the SO+S, transition and the ionization potentials from So for benzeneeAr heteroclusters, provide compelling evidence for the existence of two structural isomers, i.e. the two-sided (1 I 1) structure and the one-sided (2 IO) structure of this heterocluster. Our MD simulations of the nuclear motion in So of benzene.Arz isomers reveal that: (i) The (111) structure constitutes a rigid molecule over a broad temperature domain, until it converts to the (2 IO) structure at P 50 K. (ii) The (2 IO) structure constitutes a nonrigid molecule over all the relevant temperature domains (T< 70 K), with an onset of isomerization processes at T> 3 K. (iii) The (2 IO) benzene.Ar, heterocluster exhibits a hierarchy of isomerization processes with increasing temperature. In conjunction, we have performed MD simulations of the ionization potentials of the (11 1) and (2 (0) benzene. Arz heteroclusters. The simulated photoionization efficiency curves are confronted with the experimental data of Schmidt et al. [ 121, elucidating the effects of nuclear dynamics and the implications of molecular nonrigidity for the ionization processes of these heteroclusters. The constant energy MD simulations of ben-zene-A+ utilized a fifth-order predictor-corrector
Intramolecular vibrational redistribution in alkylbenzenes. II. Spectroscopy and dynamics
The Journal of Chemical Physics, 1991
We report a study of intramolecular vibrational distribution (IVR) occurring in the electronic ground state of uracil (S 0 ) in the gas phase, following photoexcitation in the lowest energy bright excited state (Sp) and decay through the ethylene-like Sp/S 0 Conical Intersection (CI-0p). To this aim we have performed 20 independent ab initio molecular dynamics simulations starting from CI-0p (ten of them with 1 eV kinetic energy randomly distributed over the different molecular degrees of freedom) and 10 starting from the ground state minimum (Franck-Condon, FC, point), with the excess kinetic energy equal to the energy gap between CI-0p and the FC point. The simulations, exploiting PBE0/6-31G(d) calculations, were performed over an overall period of 10 ps. A thorough statistical analysis of the variation of the geometrical parameters of uracil during the simulation time and of the distribution of the kinetic energy among the different vibrational degrees of freedom provides a consistent picture of the IVR process. In the first 0-200 fs the structural dynamics involve mainly the recovery of the average planarity. In the 200-600 fs time range, a substantial activation of CO and NH degrees of freedom is observed. After 500-600 fs most of the geometrical parameters reach average values similar to those found after 10 ps, though the system cannot be considered to be in equilibrium yet.
Electronic—Vibrational Spectroscopy of Large van der Waals Molecules
In this paper we focus on the electronic—vibrational spectra of tetraccne • Ai (A = Ar, Kr, Xc) heteroclusters, which provide information on the intermolecular nuclear motion of a rare-gas atom on the microsurrace of a large aromatic molecule. Calculations of the energy levels and nuclear Franck—Con-don factors for the excitation of the parallel and perpendicular intermolecular vibrations in the electronic So-4 SI excitation of hetcroclusters were performed. The experimental energetic and intensity data for the perpendicular intermolecu-lar nuclear excitation are well accounted for by our calculations. This study provides some information regarding the potential surfaces of large hetero-clusters in electronically excited states, which are important for the elucidation of the spectroscopy, nuclear motion, and isomerization dynamics of large aromatic molecule. (rare gas), hetcroclusters.
The fast dynamics of benzene in the liquid phase. Part II. A molecular dynamics simulation
Physical Chemistry Chemical Physics, 2001
A molecular dynamics simulation is performed for liquid benzene in the rigid body approximation. The results concerning the structural and dynamical properties of the system provide the basis for the interpretation of recent experimental data. In particular, it is shown that the system is characterised by a well deÐned cage structure, and that the average dynamics of the cages describe the main dynamical features of the bulk liquid. The calculated mean lifetime of the cages is in good agreement with the value of the Kubo correlation time derived from the experiments. In the picture emerging both from experiments and calculations, the fast intermolecular dynamics of liquid benzene is characterised by an inhomogeneously broadened distribution of intermolecular vibrational frequencies, whose dephasing is primarily due to the relaxation of the local structures. In particular, this mechanism is responsible for the dephasing of the low frequency librations giving rise to the intermediate quasi-exponential relaxation observed in the optical Kerr e †ect experiments.
Simulated structure, dynamics, and vibrational spectra of liquid benzene
Chemical Physics, 2000
A classical molecular dynamics simulation of liquid benzene is performed, using a potential model which allows for full molecular flexibility. The short range intermolecular radial distribution function is on average reminiscent of the crystalline structure, although practically no preferential orientation can be found for the molecules in the first coordination shell. The average cage lifetime and its vibrational dynamics are obtained from appropriate time correlation functions. The intramolecular vibrations are investigated by calculating the vibrational density of states and the infrared and Raman spectra, achieving an excellent agreement with the experimental data. Finally, the dephasing of the 1 (A 1g ) ring breathing mode and of the 6 (E 2g ) in-plane bending mode is analyzed on the basis of the Kubo dephasing function. For 1 mode the Kubo correlation time of 516 fs agrees with the experimental value, and is consistent with a relaxation mechanism involving the cage reorganization. In contrast, 6 has a practically pure Lorentzian line shape, with a width of 7.16 cm Ϫ1 in perfect agreement with the experimental value of 7.2 cm Ϫ1 .
Experimental and Theoretical Studies of Intramolecular and Intermolecular Dynamics
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