Photodissociation of alkyl iodides in helium nanodroplets. III. Recombination

Collision of rare-gas atoms on helium nanodroplets: Theoretical evidence for an efficient coagulation of heavy rare-gas atoms

The coagulation of rare-gas atoms (RG = Ne, Ar, Kr, Xe, and Rn) in helium nanodroplets (HNDs) composed of 1000 atoms is investigated by zero-point averaged dynamics where a He-He pseudopotential is used to make the droplet liquid with proper energies. This method reproduces the qualitative abundances of embedded Arn+1 structures obtained by Time-Dependent Density Functional Theory and Ring Polymer Molecular Dynamics for Ar + ArnHe1000 collisions at realistic projectile speeds and impact parameters. More generally, coagulation is found to be much more efficient for heavy rare-gases (Xe and Rn) than for light ones (Ne and Ar), a behavior mainly attributed to a slower energy dissipation of the projectile in the HND. When coagulation does not occur, the projectile maintains a speed of 10-30 m s-1 within the HND, but its velocity vector is rarely oriented toward the dopant, and the projectile roams in a limited region of the droplet. The structure of embedded RGn+1 clusters does not systematically match their gas-phase global minimum structure, and more than 30% of RGn-RG unbound structures are due to one He atom located in between the projectile and a dopant atom.

Quantum dynamics of the Br2 (B-excited state) photodissociation in superfluid helium nanodroplets: importance of the recombination process

We have studied the Br₂ photodissociation dynamics (B ← X electronic transition) of Br₂(v = 0, X)@(⁴He)_N doped nanodroplets (T = 0.37 K) at zero angular momentum, with N in the 100-1000 interval. To do this, we have used a quantum mechanical hybrid strategy proposed by us and, as far as we know, this is the second quantum dynamics study available on the photodissociation of molecules in superfluid helium nanodroplets. While the results obtained for some properties are qualitatively similar to those reported previously by us for the Cl₂(B ← X) related case (in particular, the oscillating Br final velocity distribution which also arises from quantum interferences), large differences are evident in three key properties: the photodissociation mechanism and probability and the time scale of the process. This can be interpreted on the basis of the significantly lower excitation energy achieved by the Br₂(B ← X) transition and the higher reduced mass of Br-Br in comparison to the chlorine case. The Br₂(B) photodissociation dynamics is significantly more complex than that of Cl₂(B) and leads to the fragmentation of the initial wave packet. Thus, the probability of non-dissociation is equal to 17, 18, 51, 85 and 100% for N = 100, 200, 300, 500 and 1000, respectively, while for chlorine this probability is equal to zero. In spite of the very large experimental difficulties that exist for obtaining nanodroplets with a well defined size, we hope that these results will encourage experimentalists to investigate these interesting systems.

Fragmentation dynamics of Ar4He1000 upon electron impact ionization: Competition between ion ejection and trapping

The fragmentation upon electron impact ionization of Ar₄He₁₀₀₀ is investigated by means of mixed quantum-classical dynamics simulations. The Ar₄ ⁺ dopant dynamics is described by a surface hopping method coupled with a diatomics-in-molecules model to properly take into account the multiple Ar₄ ⁺ electronic surfaces and possible transitions between them. Helium atoms are treated individually using zero-point averaged dynamics, a method based on the building of an effective He-He potential. Fast electronic relaxation is observed from less than 2 ps to ∼30 ps, depending on initial conditions. The main fragments observed are Ar₂ ⁺He_q and Ar₃ ⁺He_q (q ≤ 1000), with a strong contribution of the bare Ar₂ ⁺ ion, and neither Ar⁺ nor Ar⁺He_q fragments are found. The smaller fragments (q ≤ 50) are found to mostly come from ion ejection, whereas larger fragments (q > 500) originate from long-term ion trapping. Although the structure of the trapped Ar₂ ⁺ ions is the same as in the gas phase, trapped Ar₃ ⁺ and Ar₄ ⁺ are rather slightly bound Ar₂ ⁺⋯Ar and Ar₂ ⁺⋯Ar⋯Ar structures (i.e., an Ar₂ ⁺ core with one or two argon atoms roaming within the droplet). These loose structures can undergo geminate recombination and release Ar₃ ⁺He_q or Ar₄ ⁺He_q (q ≤ 50) in the gas phase and/or induce strong helium droplet evaporation. Finally, the translational energy of the fragment center of mass was found to be suitable to provide a clear signature of the broad variety of processes at play in our simulations.

Heterogeneous Clusters of Phthalocyanine and Water Prepared and Probed in Superfluid Helium Nanodroplets

Superfluid helium nanodroplets comprised of thousands to millions of helium atoms can serve as a reactor for the synthesis of heterogeneous molecular clusters at cryogenic conditions. The cluster synthesis occurs via consecutive pick-up of the cluster building blocks by the helium droplet and their subsequent coalescence within the droplet. The effective collision cross section of the building blocks is determined by the helium droplet size and thus exceeds by orders of magnitude that of a reactive collision in the gas phase. Moreover, the cryogenic helium environment (at 0.38 K) as a host promotes the formation of metastable cluster configurations. The question arises as to the extent of the actual involvement of the helium environment in the cluster formation. The present study deals with clusters of single phthalocyanine (Pc) molecules with single water molecules. A large fluorophore such as Pc offers several sites where the water molecule can attach. The resulting isomeric variants of the Pc-H₂O complex can be selectively identified by electronic spectroscopy. We compare the experimental electronic spectra of the Pc-H₂O complex generated in superfluid helium nanodroplets with the results of quantum-chemical calculations on the same cluster but under gas-phase conditions. The number of isomeric variants observed in the helium droplet experiment comes out the same as that obtained from our gas-phase calculations.

Quantum-classical approach to the reaction dynamics in a superfluid helium nanodroplet. The Ne2 dimer and Ne-Ne adduct formation reaction Ne + Ne-doped nanodroplet

The dynamics of the Ne₂ dimer and Ne-Ne adduct formation in a superfluid helium nanodroplet [(⁴He)_N; T = 0.37 K], Ne + Ne@(⁴He)_N→ Ne₂@(⁴He)_N'/Ne-Ne@(⁴He)_N' + (N-N')⁴He with N = 500, has been investigated using a hybrid approach (quantum and classical mechanics (QM-CM) descriptions for helium and the Ne atoms, respectively) and taking into account the angular momentum of the attacking Ne atom, Ne₍₁₎. Comparison with zero angular momentum QM results of our own shows that the present results are similar to the quantum ones for the initial Ne₍₁₎ velocities (v₀) of 500 and 800 m s^-1 (the former one being the most probable velocity of Ne at 300 K), in all cases leading to the Ne₂ dimer (r_e = 3.09 Å). However, significant differences appear below v₀ = 500 m s^-1, because in the QM-CM dynamics, instead of the dimer, a Ne-Ne adduct is formed (r₀ = 5.45 Å). The formation of this adduct will probably dominate as the contribution to reactivity of angular momenta larger than zero is the leading one and angular momentum strongly acts against the Ne₂ production. Angular momentum adds further difficulties in producing the dimer, since it makes it more difficult to remove the helium density between both Ne atoms to lead, subsequently, to the Ne₂ molecule. Hence, the formation of the neon-neon adduct, Ne-Ne@(⁴He)_N', clearly dominates the reactivity of the system, which results in the formation of a "quantum gel"/"quantum foam", because the two Ne atoms essentially maintain their identity inside the nanodroplet. Large enough Ne₍₁₎ initial angular momentum values can induce the formation of vortex lines by the collapse of superficial excitations (ripplons), but they occur with greater difficulty than in the case of the capture of the Ne atom by a non doped helium nanodroplet, due to the wave interferences induced by the Ne induced by the solvation layers of the Ne atom originally placed inside the nanodroplet. We hope that this work will encourage other researchers to investigate the reaction dynamics in helium nanodroplets, an interesting topic on which there are few studies available.

Conservation of Hot Thermal Spin-Orbit Population of 2P Atoms in a Cold Quantum Fluid Environment

The 0.4 K internal temperature of superfluid helium nanodroplets is believed to guarantee a corresponding ground-state population of dopant atoms and molecules inside this cryogenic matrix. We have recorded 6s ← 5p excitation spectra of indium atoms in helium droplets and found two absorption bands separated by about 2000 cm^-1, a value close to the spin-orbit (SO) splitting of the In ²P ground state. The intensities of the bands agree with a thermal population of the ²P_1/2 and ²P_3/2 states at 870 K, the temperature of the In pick-up cell. Applying femtosecond pump-probe spectroscopy, we found the same dynamical response of the helium solvation shell after the photoexcitation of the two bands. He-density functional theory simulations of the excitation spectra are in agreement with the bimodal structure. Our findings show that the population of SO levels of hot dopants is conserved after pick-up inside the superfluid droplet. Implications for the interpretation of experiments on molecular aggregates are discussed.

Quantum-classical dynamics of the capture of neon atoms by superfluid helium nanodroplets

The capture dynamics of Ne by a HeND was studied theoretically in a detailed manner (energy and angular momentum transfer and vortex formation).

Mass effects in the photodissociation of homonuclear diatomic molecules in helium nanodroplets: inelastic collision and viscous flow energy exchange regimes

The photodissociation of a diatomic molecule X₂ in a superfluid helium nanodroplet presents two sequential dynamic regimes: the starting perfectly inelastic collision followed by the viscous flow. This mechanism has probably general character.

Reaction dynamics inside superfluid helium nanodroplets: the formation of the Ne2 molecule from Ne + Ne@(4He)N

A hybrid TDDFT approach was proposed to consider bimolecular reactive processes in superfluid helium nanodroplets. The Ne + Ne@(⁴He)_N reaction was considered as the first application example. The formation of Ne₂ is a complex process related to the nature of the helium density waves and their reflection from the nanodroplet surface.

Quantum dynamics of the pick up process of atoms by superfluid helium nanodroplets: the Ne + (4He)1000 system

The quantum dynamics of neon atom capture by a superfluid helium-4 nanodroplet has been theoretically investigated using a hybrid method.

Photoinduced molecular dissociation and photoinduced recombination mediated by superfluid helium nanodroplets

Photoinduced predissociation of Cr₂ in helium nanodroplets causes stable, quantum state specific spatial separation followed by geminate recombination upon photoionization.

Quantum interferences in the photodissociation of Cl2(B) in superfluid helium nanodroplets (4He)N

The origin of quantum interferences theoretically found in the photodissociation of chlorine in superfluid ⁴He nanodroplets was investigated in detail.

Fragmentation of ionized doped helium nanodroplets: theoretical evidence for a dopant ejection mechanism

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 Ne(n) (+) (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 Ne(n) (+) 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 Ne(2) (+) and He(q)Ne(2) (+) 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 He(q)Ne(2) (+) species to Ne(2) (+) 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 He(q)Ne(+) fragments are produced only from the highest electronic state, whereas He(q)Ne(2) (+) 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.