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We present an experimental study of the dynamics following the photoexcitation and subsequent photoionization of single Cs atoms on the surface of helium nanodroplets. The dynamics of excited Cs atom desorption and readsorption as well as CsHe exciplex formation are measured by using femtosecond pump-probe velocity map imaging spectroscopy and ion time-of-flight spectrometry. The time scales for the desorption of excited Cs atoms off helium nanodroplets as well as the time scales for CsHe exciplex formation are experimentally determined for the 6p states of Cs. For the 6p 2 Π 1/2 state, our results confirm that the excited Cs atoms only desorb from the nanodroplet when the excitation wavenumber is blue-shifted from the 6p 2 Π 1/2 ← 6s 2 Σ 1/2 resonance. Our results suggest that the dynamics following excitation to the 6p 2 Π 3/2 state can be described by an evaporation-like desorption mechanism, whereas the dynamics arising from excitation to the 6p 2 Σ 1/2 state is indicative for a more impulsive desorption process. Furthermore, our results suggest a helium-induced spin-orbit relaxation from the 6p 2 Σ 1/2 state to the 6p 2 Π 1/2 state. Our findings largely agree with the results of time-dependent 4 He density functional theory (DFT) simulations published earlier [ Eur. Phys. J. D 2019 , 73 , 94].

The real-time dynamics of excited alkali metal atoms (Rb) attached to quantum fluid He nanodroplets is investigated using femtosecond imaging spectroscopy and time-dependent density functional theory. We disentangle the competing dynamics of desorption of excited Rb atoms off the He droplet surface and solvation inside the droplet interior as the Rb atom is ionized. For Rb excited to the 5p and 6p states, desorption occurs on starkly differing time scales (∼100 versus ∼1 ps, respectively). The comparison between theory and experiment indicates that desorption proceeds either impulsively (6p) or in a transition regime between impulsive dissociation and complex desorption (5p).

Dissociative photoionization of the He···Li2 van der Waals complex to the ground electronic state of the He···Li2+ ion is investigated theoretically. The photoionization cross section is computed using existing interaction potentials. Resonances are found on top of a structured continuum. They are assigned to vibrational predissociation of the ion by comparison with Fermi Golden Rule calculations. Because of the differences in potential energy surfaces between the neutral and ionic complexes, only the resonances corresponding to quasibound states with the highest excitation in the van der Waals modes are visible. The other quasi bound states obtained in the Fermi Golden Rule calculations can give information on vibrational energy relaxation rates in other collisional processes involving the lithium dimer ion and a helium atom.

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.

Valence electronic excitation spectra are calculated for the H(2)O···Br(2) complex using highly correlated ab initio potentials for both the ground and the valence electronic excited states and a 2-D approximation for vibrational motion. Due to the strong interaction between the O-Br and the Br-Br stretching motions, inclusion of these vibrations is the minimum necessary for the spectrum calculation. A basis set calculation is performed to determine the vibrational wave functions for the ground electronic state and a wave packet simulation is conducted for the nuclear dynamics on the excited state surfaces. The effects of both the spin-orbit interaction and temperature on the spectra are explored. The interaction of Br(2) with a single water molecule induces nearly as large a shift in the spectrum as is observed for an aqueous solution. In contrast, complex formation has a remarkably small effect on the T = 0 K width of the valence bands due to the fast dissociation of the dihalogen bond upon excitation. We therefore conclude that the widths of the spectra in aqueous solution are mostly due to inhomogeneous broadening., (© 2011 American Chemical Society)

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.

Valence electronic excitation spectra are calculated for the H(2)O...Cl(2) dimer using state-of-the art ab initio potentials for both the ground and the valence excited states, a basis set calculation of the ground state nuclear wave function, and a wave packet analysis to simulate the dynamics on the excited state surface. The peak of the H(2)O...Cl(2) dimer spectrum is blue-shifted by 1250 cm(-1) from that of the free Cl(2) molecule. This is less than the value previously estimated from vertical excitation energies but still significantly more than the blue shift in aqueous solution and clathrate-hydrate solid. Seventy percent of the blue shift is attributed to ground state stabilization, the rest to excited state repulsion. Spin-orbit effects are found to be small for this dimer. Homogeneous broadening is found to be slightly smaller for the dimer than for the free Cl(2). The reflection principle and spectator model approximations were tested and found to be quite satisfactory. This is promising for an eventual simulation of the condensed phase spectra.

All electron ab initio calculations for the interaction of H2O with Cl2 and Br2 are reported for the ground state and the lowest triplet and singlet Pi excited states as a function of both the X-X and O-X bond lengths (X = Cl or Br). For the ground state and lowest triplet state, the calculations are performed with the coupled cluster singles, doubles, and perturbative triple excitation level of correlation using an augmented triple-zeta basis set. For the 1Pi state the multireference average quadratic coupled cluster technique was employed. For several points on the potential, the calculations were repeated with the augmented quadruple-zeta basis set. The ground-state well depths were found to be 917 and 1,183 cm-1 for Cl2 and Br2, respectively, with the triple-zeta basis set, and they increased to 982 and 1,273 cm-1 for the quadruple-zeta basis set. At the geometry of the ground-state minimum, the lowest energy state corresponding to the unperturbed 1Pi states of the halogens increases in energy by 637 and 733 cm-1, respectively, relative to the ground-state dissociation limit of the H2O-X2 complex. Adding the attractive ground-state interaction energy to that of the repulsive excited state predicts a blue-shift, relative to that of the free halogen molecules, of approximately 1,600 cm-1 for H2O-Cl2 and approximately 2,000 cm-1 for H2O-Br2. These vertical blue-shifts for the dimers are greater than the shift of the band maximum upon solvation of either halogen in liquid water.