Excitation of Sodium Atoms Attached to Helium Nanodroplets: The 3p ← 3s Transition Revisited.

The dynamics of Na atoms on the surface of helium nanodroplets following excitation via the 3p ← 3s transition has been investigated using state-specific ion-based detection of the products. Excitation of the system to the 3p (2)Π states is found to lead to the desorption of both bare Na and NaHe exciplexes. The associated speed distributions point to an impulsive desorption process for Na products and a thermally driven process for the NaHe exciplexes. In contrast, excitation of the 3p (2)Σ state leads exclusively to the impulsive desorption of Na atoms. In this case, the desorption is accompanied by a helium-induced relaxation process, as evidenced by the large fraction of detected Na (2)P1/2 atoms. The relaxation process is thought to be related to a crossing between the (2)Π1/2 and (2)Σ potential energy curves at large distance.

Communication: angular momentum alignment and fluorescence polarization of alkali atoms photodetached from helium nanodroplets.

The theory of photofragments angular momentum polarization is applied to the photodetachment of an electronically excited alkali atom from a helium nanocluster (N = 200). The alignment of the electronic angular momentum of the bare excited alkali atoms produced is calculated quantum mechanically by solving the excited states coupled equations with potentials determined by density functional theory (DFT). Pronounced oscillations as a function of excitation energy are predicted for the case of Na@(He)200, in marked contrast with the absorption cross-section and angular distribution of the ejected atoms which are smooth functions of the energy. These oscillations are due to quantum interference between different coherently excited photodetachment pathways. Experimentally, these oscillations should be reflected in the fluorescence polarization and polarization-resolved photoelectron yield of the ejected atoms, which are proportional to the electronic angular momentum alignment. In addition, this result is much more general than the test case of NaHe200 studied here. It should be observable for larger droplets, for higher excited electronic states, and for other alkali as well as for alkali-earth atoms. Detection of these oscillations would show that the widely used pseudo-diatomic model can be valid beyond the prediction of absorption spectra and could help in interpreting parts of the dynamics, as already hinted by some experimental results on angular anisotropy of bare alkali fragments.

Vibrational bound states of the He2Ne+ cation.

The vibrational bound states of the He(2)Ne(+) complex have been determined using a potential energy surface previously published by Seong et al. [J. Chem. Phys. 2004, 120, 7456]. The calculation was performed by sequential diagonalization-truncation techniques in a discrete variable representation using Radau hyperspherical coordinates. There are 52 bound levels. The ground state has an energy of 605.3 cm(-1) above the absolute minimum and lies about half way to dissociation. The evaporation energy of one He atom is equal to 866.1 cm(-1). Only four levels have energies below the classical energy for dissociation, and all the other 48 states are bound by the zero-point energy of the HeNe(+) fragment. The implications of the properties of the eigenvalue spectrum and of the corresponding wave functions on the vibrational relaxation dynamics and infrared spectra of He(N)Ne(+) clusters is discussed.

The H(3) (+) rovibrational spectrum revisited with a global electronic potential energy surface.

In this paper, we have computed the rovibrational spectrum of the H(3) (+) molecule using a new global potential energy surface, invariant under all permutations of the nuclei, that includes the long range electrostatic interactions analytically. The energy levels are obtained by a variational calculation using hyperspherical coordinates. From the comparison with available experimental results for low lying levels, we conclude that our accuracy is of the order of 0.1 cm(-1) for states localized in the vicinity of equilateral triangular configurations of the nuclei, and changes to the order of 1 cm(-1) when the system is distorted away from equilateral configurations. Full rovibrational spectra up to the H(+)+H(2) dissociation energy limit have been computed. The statistical properties of this spectrum (nearest neighbor distribution and spectral rigidity) show the quantum signature of classical chaos and are consistent with random matrix theory. On the other hand, the correlation function, even when convoluted with a smoothing function, exhibits oscillations which are not described by random matrix theory. We discuss a possible similarity between these oscillations and the ones observed experimentally.

On the quantum and quasiclassical angular distributions of photofragments.

Quantum and quasiclassical expressions for the angular distribution of photofragments from an initially polarized precursor molecule are compared under the conditions of a one-photon electric dipole transition to a repulsive state followed by prompt axial recoil into two separating fragments. The treatment is most readily applicable to diatomic molecules, but it is more general than that. It is shown that when the rotational and electronic angular momentum J(i) and its projection along the body-fixed z axis Omega(i) are well defined in the initial state, the quantum and quasiclassical expressions are identical for any initial polarization of the molecule prior to photolysis and for all values of J(i) and Omega(i). For the particular case of an mid R:J(i)Omega(i)M(i) selected state this is in agreement with a previous result [T. Seideman, Chem. Phys. Lett. 253, 279 (1996)]. Moreover, the quasiclassical expression is still a good approximation even when the initial state is a coherent superposition of mid R:J(i),Omega(i),M(i) levels for the same Omega(i). This near identity still pertains even when Omega(i) is not well defined for a parallel transition (DeltaOmega=0) but fails for a perpendicular transition (DeltaOmega=+/-1) if the initial state is in a coherent superposition of Omega(i) states differing by +/-2. These conclusions apply to preparation schemes employing optical excitation, static inhomogeneous and/or homogeneous electric and/or magnetic fields, as well as to molecules physisorbed on solids or clusters. We discuss the importance of these results in the interpretation of photofragment distributions when some other angular momenta are involved, such as electronic angular momentum, with and without nuclear spin, coupled to molecular rotation, asymmetric top rotational angular momentum, or internal vibrational angular momentum in polyatomics.

Combining fixed- and moving-grid methods to study direct dissociation processes involving nonadiabatic transitions.

We present a novel quantum-dynamics approach suitable for computing direct dissociation processes, including electronic transitions. This approach combines quantum trajectories in the Lagrangian reference frame with standard fixed-grid wave packets in order to overcome the limitations and difficulties of both techniques. As a model application, we consider the ultrafast photodissociation of H2 excited by a femtosecond extreme UV laser pulse.