A HElium NanoDroplet Isolation (HENDI) investigation of the weak hydrogen bonding in the propyne dimer (CH3CCH)2.

A HElium Nanodroplet Isolation (HENDI) experiment was performed to explore the absorption spectra of the propyne monomer (CH3CCH), dimer and (CH3CCH)≥3 multimers in the vicinity of the CH stretch region ν1 of the monomer. Ab initio calculations were performed at the MP2 level to document the potential energy surface of the dimer. This provided the necessary parameters to simulate the absorption spectrum of the dimer and thus facilitate the interpretation of the experiment. The central result was to observe three isomers of the dimer, hence reflecting the complexity of the weak CHπ H-bonding when several H-donors are at play.

Dynamics of acetylene dimers hosted in helium droplets.

The CH antisymmetric stretch of the C2H2 moieties in acetylene dimers was explored over the range 3270-3290 cm-1 using the helium nanodroplet isolation (HENDI) technique. This work is part of a general investigation which addresses the dynamical consequences of coupling the deformation motions of weakly bound complexes with a finite size quantum liquid (the helium droplet). The acetylene dimer is attractive from this point of view because one of its deformation coordinates promotes a tunneling isomerization process. A numerical simulation of the observed spectrum allows deriving a set of effective spectroscopic constants which help understanding the dynamical role played by the droplet on the rotation and deformation of the dimer.

Multipronged mapping to the dynamics of a barium atom deposited on argon clusters.

The dynamics of an electronically excited barium atom deposited at the surface of an Ar≈500 cluster was explored in a multipronged approach which associates information from frequency-resolved nanosecond experiments and information from femtosecond time-resolved experiments. In both types of experiments, the dynamics is monitored by photoelectron and photoion spectroscopy.

Large amplitude motion of the acetylene molecule within acetylene-neon complexes hosted in helium droplets.

Superfluid helium droplets provide an ideal environment for spectroscopic studies with rotational resolution. Nevertheless, the molecular rotation is hindered because the embedded molecules are surrounded by a non-superfluid component. The present work explores the dynamical role of this component in the hindered rotation of C2H2 within the C2H2-Ne complex. A HENDI experiment was built and near-infrared spectroscopy of C2H2-Ne and C2H2 was performed in the spectral region overlapping the ν3/ν2 + ν4 + ν5 Fermi-type resonance of C2H2. The comparison between measured and simulated spectra helped to address the above issue.

Coupled electronic and structural relaxation pathways in the postexcitation dynamics of Rydberg states of BaArN clusters.

We investigate, theoretically, the joint relaxation of orbital and structure in postexcitation dynamics of Rydberg states of cluster BaArN (N=250). Mixed quantum-classical dynamics is used to account for the nonadiabatic transitions among more than 160 electronic states, represented via a diatomics-in-molecules Hamiltonian. The simulation illustrates the complex multistep relaxation processes and provides detailed insight in the mechanisms contributing to the final-time experimental photoelectron spectrum.

Theoretical investigations of the electronic states of NaXe: a comparative study.

The electronic state properties of NaXe are investigated using ab initio methodologies and various pseudopotential approaches for comparison. The spectroscopic terms and dipole moments of the lowest electronic states up to the Na(3d) +Xe dissociation limit are determined. The difference between valence or smaller core pseudopotential on Xe is shown to be negligible and so is the difference between all-electron and valence pseudopotential completed by core-polarization treatments of Na. These calculations are used as references to test the performance of a treatment involving a zero electron pseudopotential description of xenon together with a one-electron pseudopotential description of Na. When compared with the reference calculations, the one-electron model leads to reasonable quantitative results. The potential energy curves and spectroscopic data of all Rydberg excited states of NaXe up the Na(5f)+Xe dissociation limit are determined using this method. Long distance wells and barriers in the range R = 15-40 bohrs are identified for some of the higher states with (2)Σ(+) symmetry.

Fluorescence emission of Ca-atom from photodissociated Ca2 in Ar-doped helium droplets. I. Experimental.

The Ca(2) → Ca(4s4p(1)P) + Ca(4s(2)(1)S) photodissociation was investigated in a He droplet isolation experiment where the droplets are doped by Ar atoms. Fluorescence spectra associated with the Ca(4s4p(1)P → 4s(2)(1)S) 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(1)P → 4s(2)(1)S) fluorescence emission was recorded as a function of the wavelength of the photodissociation laser, hence providing the action spectrum of the Ca(2) → Ca(4s4p(1)P) + Ca(4s(2)(1)S) process. The latter spectrum suggests that in He droplets doped by argon, Ca atoms are attracted inside the droplet where they associate as Ca(2). 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.

Fluorescence emission of Ca-atom from photodissociated Ca2 in Ar doped helium droplets. II. Theoretical.

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 Ca(2) 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 CaAr(M) (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 Ca(2) photodissociation in an argon-doped droplet in Paper I could be Ca∗Ar(M) in a partly evaporated droplet where less than 200 helium atoms remain.

A new Monte Carlo method for getting the density of states of atomic cluster systems.

A novel Monte Carlo flat histogram algorithm is proposed to get the classical density of states in terms of the potential energy, g(E(p)), for systems with continuous variables such as atomic clusters. It aims at avoiding the long iterative process of the Wang-Landau method and controlling carefully the convergence, but keeping the ability to overcome energy barriers. Our algorithm is based on a preliminary mapping in a series of points (called a σ-mapping), obtained by a two-parameter local probing of g(E(p)), and it converges in only two subsequent reweighting iterations on large intervals. The method is illustrated on the model system of a 432 atom cluster bound by a Rydberg type potential. Convergence properties are first examined in detail, particularly in the phase transition zone. We get g(E(p)) varying by a factor 10(3700) over the energy range [0.01 < E(p) < 6000 eV], covered by only eight overlapping intervals. Canonical quantities are derived, such as the internal energy U(T) and the heat capacity C(V)(T). This reveals the solid to liquid phase transition, lying in our conditions at the triple point. This phase transition is further studied by computing a Lindemann-Berry index, the atomic cluster density n(r), and the pressure, demonstrating the progressive surface melting at this triple point. Some limited results are also given for 1224 and 4044 atom clusters.

Dynamics of highly excited barium atoms deposited on large argon clusters. I. General trends.

Ba(Ar)(approximately 750) clusters were generated by associating the supersonic expansion and the pick-up techniques. A femtosecond pump (266.3 nm)-probe (792 or 399.2 nm) experiment was performed to document the dynamics of electronically excited barium within the very multidimensional environment of the argon cluster. Barium was excited in the vicinity of the 6s9p (1)P state and probed by ionization. The velocity imaging technique was used to monitor the energy distribution of photoelectrons and photoions as a function of the delay time between the pump and the probe pulses. A complex dynamics was revealed, which can be interpreted as a sequence/superposition of elementary processes, one of which is the ejection of barium out of the cluster. The latter has an efficiency, which starts increasing 5 ps after the pump pulse, the largest ejection probability being at 10 ps. The ejection process lasts at a very long time, up to 60 ps. A competing process is the partial solvation of barium in low lying electronic states. Both processes are preceded by a complex electronic relaxation, which is not fully unraveled here, the present paper being the first one in a series.

Observation of a barium xenon exciplex within a large argon cluster.

Spectroscopic measurements provide fluorescence and excitation spectra of a single barium atom codeposited with xenon atoms on argon clusters of average size approximately 2000. The spectra are studied as a function of the number of xenon atoms per cluster. The excitation spectrum with approximately 10 xenon atoms per cluster is qualitatively similar to that observed when no xenon atom is present on the cluster. It consists of two bands located on each side of the 6s6p (1)P-6s(2) (1)S resonance line of the free barium. In contrast, the fluorescence spectrum differs qualitatively since a barium-xenon exciplex is observed, which has no counterpart in xenon free clusters. In particular an emission is observed, which is redshifted by 729 cm(-1) with respect to the Ba(6s6p (1)P-6s(2) (1)S) resonance line.

Bidentate ligation of magnesium by 1,2-dimethoxyethane in the gas phase.

The 1:1 Mg...1,2-dimethoxyethane (Mg-DXE) complexes are studied experimentally and theoretically. They are generated by a laser ablation source in a supersonic expansion. They are studied spectroscopically by resonance two-photon ionization. Density functional theory/Becke three-dimensional Lee, Yang, and Parr and ab initio calculations using the MOLPRO quantum chemistry package are performed to document their ground and excited states in a series of geometry ranging from monodentate to bidentate ligation of Mg by the O atoms of DXE. An absorption band is observed in the 27 800-30 500 cm(-1) range, which, thanks to the calculations, is attributed to the bidentate complex. The structure of the band is discussed in terms of the excitation of electronic states of the complex correlating adiabatically to the 3s3p (1)P and 3s4s (1)S states of Mg at large separation between Mg and DXE.

Transition-state spectroscopy of the photoinduced Ca + CH3F reaction. 3. Reaction following the local excitation to Ca(4s3d 1D).

The Ca* + CH3F --> CaF* + CH3 reaction was studied both experimentally and theoretically. The reaction was photoinduced in Ca...CH3F complexes, which were illuminated by a tunable laser in the range 18 000-24 000 cm-1. The absorption band that leads to the reaction extends between 19 000 and 23 000 cm-1. It is formed of three broad overlapping structures corresponding to the excitation of different electronic states of the complex. The two structures of lowest energy were considered in detail. They are associated with two series of respectively 2 and 3 molecular states correlating to Ca(4s3d 1D) + CH3F at infinite separation between Ca and CH3F. The assignment of these structures to specific electronic transitions of the complex stemmed from theoretical calculations where the Ca...CH3F complex is described by a linear Ca-F-C backbone. 2D potential energy surfaces were calculated by associating a pseudopotential description of the [Ca2+] and [F7+] cores, a core polarization operator on calcium, an extensive Gaussian basis, and a treatment of the electronic problem at the CI-MRCI level. All the excited levels correlating to the 4s2 1S, 4s3d 1D, and 4s4p 1P levels of Ca in the Ca + CH3F channel were documented in a calculation that explored the rearrangement channels where either Ca + CH3F or CaF + CH3 are formed. Then, wavepacket calculations on the 2D-PES's allowed one to simulate the absorption spectrum of the complex, in an approximation where the various electronic states of the complex are not coupled together. The assignment above stemmed from this. The second outcome of the calculation was that whatever the excited level of the complex that is considered, the reaction has to proceed through energy barriers. The electronic excitation of the complex on the red side of the absorption band does not seem to deposit enough energy in the system to overcome these barriers (even the lowest one) or to stimulate tunneling reactions. An alternative reaction mechanism involving a transfer to triplet PES's is proposed.

Transition state spectroscopy of the photoinduced Ca + CH3F reaction. 2. Experimental and ab initio studies of the free Ca...FCH3 complex.

The Ca* + CH3F --> CaF + CH3 reaction was photoinduced in 1:1 Ca...CH3F complexes formed in a supersonic expansion. The transition state of the reaction was explored by monitoring the electronically excited product, CaF, while scanning the laser that turns on the reaction. Moreover, the electronic structure of the Ca...FCH3 system was studied using ab initio methods by associating a pseudopotential description of the [Ca2+] and [F7+] cores, a core polarization operator on calcium, an extensive Gaussian basis and a treatment of the electronic problem at the CCSD(T) (ground state) and RSPT2 (excited states) level. In this contribution we present experimental results for the free complex and a comparison with the results of a previous experiment where the Ca...CH3F complexes are deposited at the surface of large argon clusters. The ab initio calculations allowed an interpretation of the experimental data in terms of two reaction mechanisms, one involving a partial charge transfer state, the other involving the excitation of the C-F stretch in the CH3F moiety prior to charge transfer.

Transition state spectroscopy of the photoinduced Ca + CH(3)F reaction. 1. A cluster isolated chemical reaction study.

The "cluster isolated chemical reactions" technique is used to examine the dynamics of the photoinduced reaction producing electronically excited CaF when 1:1 Ca-CH(3)F complexes are deposited at the surface of large argon clusters. This technique ensures quantitatively that 1:1 complexes are actually at the origin of the observed signals. The reaction is monitored by observing the CaF chemiluminescence while scanning the photoexcitation laser. The resulting action spectrum contains information about the absorption bands of the complex, filtered by the dynamics of the reaction. The observations suggest a profound alteration of the calcium electronic structure and a control of the reaction by the CF stretch in CH(3)F.