Search

The Ca2 → Ca(4s4p 1P) + Ca(4s2 1S) photodissociation was investigated in a He droplet isolation experiment where the droplets are doped by Ar atoms. Fluorescence spectra associated with the Ca(4s4p 1P → 4s2 1S) emission were recorded as a function of the average number of Ar atoms per droplet. Three contributions were observed depending on whether the emitting Ca atoms are free, bound to helium atoms or bound to argon atoms. Moreover, the full Ca(4s4p 1P → 4s2 1S) fluorescence emission was recorded as a function of the wavelength of the photodissociation laser, hence providing the action spectrum of the Ca2 → Ca(4s4p 1P) + Ca(4s2 1S) process. The latter spectrum suggests that in He droplets doped by argon, Ca atoms are attracted inside the droplet where they associate as Ca2. Full analysis of the spectra indicate that the emission of Ca bound to a single Ar atom is redshifted by 94 cm-1 with respect to the emission of free Ca. [ABSTRACT FROM AUTHOR]

The stability of the ground or excited state calcium atom in an argon-doped helium droplet has been investigated using an extension of the helium density functional method to treat clusters. This work was motivated by the experimental study presented in a companion paper, hereafter called Paper I [A. Masson, M. Briant, J. M. Mestdagh, M. A. Gaveau, A. Hernando, and N. Halberstadt, J. Chem. Phys. 137, 184310 (2012)], which investigated Ca2 photodissociation in an argon-doped helium droplet and the nature of the fluorescent species. It is found that one single argon atom is sufficient to bring the calcium atom inside the droplet, for droplets of over 200 helium atoms. The absorption and emission spectra of CaArM (M = 0-7) clusters have been simulated using the recently developed density sampling method to describe the influence of the helium environment. Absorption spectra exhibit broad, double bands that are significantly blueshifted with respect to the calcium atomic line. The emission spectra are less broad and redshifted with respect to the calcium resonance line. The shifts are found to be additive only for M <= 2, because only the first two argon atoms are located in equivalent positions around the calcium p orbital. This finding gives a justification for the fit presented in the companion paper, which uses the observed shifts in the emission spectra as a function of argon pressure to deduce the shifts as a function of the number of argon atoms present in the cluster. An analysis of this fit is presented here, based on the calculated shifts. It is concluded that the emitting species following Ca2 photodissociation in an argon-doped droplet in Paper I could be Ca*ArM in a partly evaporated droplet where less than 200 helium atoms remain. [ABSTRACT FROM AUTHOR]

A high-dimensional quantum wave packet approach based on Gaussian wave packets in Cartesian coordinates is presented. In this method, the high-dimensional wave packet is expressed as a product of time-dependent complex Gaussian functions, which describe the motion of individual atoms. It is applied to the ultrafast geometrical rearrangement dynamics of NO doped cryogenic Ne matrices after femtosecond laser pulse excitation. The static deformation of the solid due to the impurity as well as the dynamical response after femtosecond excitation are analyzed and compared to reduced dimensionality studies. The advantages and limitations of this method are analyzed in the perspective of future applications to other quantum solids. [ABSTRACT FROM AUTHOR]

Raman spectroscopy of bromine in the liquid phase and in water illustrates uncommon principles and yields insights regarding hydration. In liquid Br2, resonant excitation over the B(3Π0u+) ← X(1Σg+) valence transition at 532 nm produces a weak resonant Raman (RR) progression accompanied by a five-fold stronger non-resonant (NR) scattering. The latter is assigned to pre-resonance with the C-state, which in turn must be strongly mixed with inter-molecular charge transfer states. Despite the electronic resonance, RR of Br2 in water is quenched. At 532 nm, the homogeneously broadened fundamental is observed, as in the NR case at 785 nm. The implications of the quenching of RR scattering are analyzed in a simple, semi-quantitative model, to conclude that the inertial evolution of the Raman packet in aqueous Br2 occurs along multiple equivalent water-Br2 coordinates. In distinct contrast with hydrophilic hydration in small clusters and hydrophobic hydration in clathrates, it is concluded that the hydration shell of bromine in water consists of dynamically equivalent fluxional water molecules. At 405 nm, the RR progression of Br3- is observed, accompanied by difference transitions between the breathing of the hydration shell and the symmetric stretch of the ion. The RR scattering process in this case can be regarded as the coherent photo-induced electron transfer to the solvent and its radiative back-transfer. [ABSTRACT FROM AUTHOR]

We report a theoretical study of the effect induced by a helium nanodroplet environment on the fragmentation dynamics of a dopant. The dopant is an ionized neon cluster Nen+ (n=4-6) surrounded by a helium nanodroplet composed of 100 atoms. A newly designed mixed quantum/classical approach is used to take into account both the large helium cluster zero-point energy due to the light mass of the helium atoms and all the nonadiabatic couplings between the Nen+ potential-energy surfaces. The results reveal that the intermediate ionic dopant can be ejected from the droplet, possibly with some helium atoms still attached, thereby reducing the cooling power of the droplet. Energy relaxation by helium atom evaporation and dissociation, the other mechanism which has been used in most interpretations of doped helium cluster dynamics, also exhibits new features. The kinetic energy distribution of the neutral monomer fragments can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experiments. The results also reveal the predominance of Ne2+ and HeqNe2+ fragments and the absence of bare Ne+ fragments, in agreement with available experimental data (obtained for larger helium nanodroplets). Moreover, the abundance in fragments with a trimeric neon core is found to increase with the increase in dopant size. Most of the fragmentation is achieved within 10 ps and the only subsequent dynamical process is the relaxation of hot intermediate HeqNe2+ species to Ne2+ by helium atom evaporation. The dependence of the ionic fragment distribution on the parent ion electronic state reached by ionization is also investigated. It reveals that HeqNe+ fragments are produced only from the highest electronic state, whereas HeqNe2+ fragments originate from all the electronic states. Surprisingly, the highest electronic states also lead to fragments that still contain the original ionic dopant species. A mechanism is conjectured to explain this fragmentation inhibition. [ABSTRACT FROM AUTHOR]

We present simulation results on the effect of a helium nanodroplet environment on the fragmentation dynamics of embedded molecular systems. The helium atoms are treated explicitly, with zero-point effects taken into account through an effective helium-helium interaction potential. The ionized neon tetramer is used as a model molecular system because, like all the small rare-gas clusters, it fragments extensively upon ionization. All the nonadiabatic effects between electronic states of the ionized neon cluster are taken into account. The results reveal a predominance of Ne2+ and HepNe2+ fragments and the absence of bare Ne+ fragments, in agreement with available experimental data. The neutral monomer fragments exhibit a rather wide kinetic energy distribution that can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experimental results. Purely classical calculations are shown to strongly overestimate the amount of cage effect (cooling), clearly indicating the need to take into account zero-point effects. [ABSTRACT FROM AUTHOR]

Studies the intramolecular vibrational energy redistribution and fragmentation dynamics in I[sub2](B,v=22)...Nen(n=2-6) and I[sub2](B,v=21)...Nen(n=2-5) clusters using hybrid quantum/classical techniques. Use of vibrational version of molecular dynamics with quantum transitions; Representation of potential energy surface as sum of pairwise interactions.

Quantum mechanical calculations have been performed to study the fragmentation dynamics of the Ar–NH3 van der Waals complex excited in the ν2 (‘‘umbrella’’) vibrational level with v2=1. Vibrational predissociation has been investigated for different initial quasibound states, corresponding to excitation in the stretching or bending van der Waals levels or in the tunneling motion of the NH3 umbrella inversion, for both ortho and para varieties of NH3, and for three values of the total angular momentum J and its projection Ω onto the intermolecular axis, (J,Ω)=(0,0), (1,0), and (1,±1). The calculations were performed for two different intermolecular potential energy surfaces, one determined ab initio by Bulski et al. [J. Chem. Phys. 94, 491 (1991)], and the other where the ab initio potential has been scaled to fit spectroscopic data of Ar–NH3, proposed by van Bladel et al. [J. Phys. Chem. 95, 5414 (1991)]. The lifetime obtained from the ab initio potential lies within the limits set by experiment for the lowest ortho Π (i.e., |Ω|=1) level with J=1, while the scaled potential gives too short a lifetime. The results also show that excitation of the van der Waals stretching mode accelerates the predissociation but does not affect the NH3 final state rotational distribution very much. On the other hand, bending or tunneling excitation does lead to a very different final rotational state distribution. Ortho states have very similar final state distributions for Σ(|Ω|=0) and Π(|Ω|=1) states, but this is not true for para states. Finally, comparing the ab initio and scaled potentials, no systematic trend emerges for the amount of even (+) and odd (-) symmetry of the final states, but the scaling of the potential shortens the lifetimes, gives a wider final state rotational distribution, and provides more excitation in the k quantum number (the projection of the NH3 rotational... [ABSTRACT FROM AUTHOR]

Theoretical calculations of vibrational intermolecular states of the aniline–argon van der Waals complex for J=0 are reported. A fully-quantum method (LCHOP) was used in order to describe the van der Waals cluster. Results in the first two electronic states S0 (X 1A1) and S1 (A 1B2) are presented; in the S1 state a comparison with available experimental data is made. We introduce an additive repulsive interaction between N and Ar in the S1 state in order to account for the spectral features observed in larger clusters. Several parametrizations of this term in the potential are discussed with a view to applications to semiclassical simulation of the spectra of the larger An–Arn clusters. [ABSTRACT FROM AUTHOR]

Examines the photodissociation of ICN in an argon matrix. Instrument used in the coupling between electronic states at large I-CN internuclear distances; Reaction path involved in the principal mechanism for geminate recombination; Use of diatomic in molecules treatment of the mixture of spin-orbit states of iodine induced by argon atoms.

Discusses the vibrational predissociation of the ammonia monomer ND [sub 3]-Ar through excitation of the 'umbrella' mode of ND [sub 3]. Effect of a simultaneous excitation of one of the Van der Waals modes on the lifetime of the complex and on the product state distribution; Final state distributions.

Quantum mechanical close coupling calculations on (HF)2 vibrational predissociation are presented. The model considers vibrational excitation of the proton donor monomer. The other momomer is frozen at its equilibrium position. The linewidth (lifetime) and final state rotational distribution of the initially vibrationally excited HF were calculated using the artificial channel method. The potential surface was taken from Cournoyer and Jorgensen. The calculated linewidth is in good agreement with recent experimental measurements. This result is drastically different from what is obtained in the collinear model, merely reflecting the important influence of the rotational–vibrational coupling in the highly anisotropic systems. The final rotational distribution of the initially excited HF monomer is highly inverted. The possibility of a rotational in conjunction with a conventional vibrational HF laser is raised. [ABSTRACT FROM AUTHOR]

The infrared absorption spectrum of the ν4 fundamental band of (NO)2 has been recorded at 0.3 cm-1 resolution in a cell cooled down to 110 K. From the band contour analysis and comparison with a simulated spectrum accurate values of the rotational constants in the ν4 state are derived, showing a reduction of the van der Waals bond on vibrational excitation. Absorption measurements performed using the various lines emitted by an isotopic CO laser suggest an unresolved rotational fine structure. A lower limit of the time for energy redistribution of 50 to 100 ps is proposed. Also, infrared photopredissociation of (NO)2 has been directly observed in a crossed laser-molecular beam experiment using bolometric detection of the beam attenuation. An upper limit of the dissociation lifetime can be set to 10-3 s. [ABSTRACT FROM AUTHOR]

The time-dependent wave packet technique is applied to the Golden Rule treatment of vibrational predissociation. The wave packet at time zero is taken as the product of the quasibound wave function and the coupling inducing predissociation. The rate for vibrational predissociation can then be obtained by Fourier transform into the energy domain of the time-dependent wave packet autocorrelation function. The method has been applied to a model triatomic van der Waals molecule. It is shown that when the bound-state components of the wave packet are projected out, the time-dependent version of the Golden Rule approximation provides an alternative efficient technique to treat intramolecular decay. [ABSTRACT FROM AUTHOR]

Converged three-dimensional quantum mechanical calculations for photofragmentation of the Ne··ICl van der Waals molecule in the energy region of the electronically excited B(3∏0+) state of ICl are presented and compared with experiments. Lifetimes and final state distributions for the ICl fragments were determined for vibrational predissociation from the lowest van der Waals level in the B(v’=2) channel. Good agreement between theory and experiment was achieved using a sum of atom–atom pairwise potentials. This potential energy surface predicts the equilibrium geometry of the complex to be bent at 140° with the Ne atom towards the Cl end of ICl. The diabatic vibrational golden rule (DVGR) approximation, as well as the rotational infinite order sudden approximation (RIOSA), have been tested again the full 3D calculations. Analysis of the quasibound wave function reveals that the highly inverted rotational distribution of the ICl fragments observed in the experiment, is not due to zero-point bending motion. It is more likely to be due to a rotational rainbow effect enhanced by the favorable initial geometry of the complex. The effect of the excitation of the bending van der Waals mode in the complex has also been studied. As compared with the lowest level, a longer lifetime and a different rotational distribution of the fragments is predicted. The results presented in this work not only elucidates many dynamical aspects of vibrational predissociation for the Ne···ICl complex, but also provide benchmark data for the study of other theoretical methods and approximations. [ABSTRACT FROM AUTHOR]