J. Alberto Beswick

Three‐dimensional quantum mechanical study of Ne⋅⋅⋅Cl2 vibrational predissociation

Three‐dimensional quantum mechanical calculations for vibrational predissociation of the Ne⋅⋅⋅Cl2 van der Waals complex are presented and compared with experiments. Lifetimes and final rotational state distributions were obtained for the two processes: (i) Ne⋅⋅⋅Cl2(X,v=1) →Ne+Cl2(X,v=0) and (ii) Ne⋅⋅⋅Cl2(B,v=11) →Ne +Cl2(B,v=10,9) where v denotes the vibrational quantum number of Cl2 and X and B specify electronic states of Ne⋅⋅⋅Cl2 which correlate with the X 1∑+0g and B 3∏+0u states of the free Cl2 molecule, respectively. The van der Waals interaction potential was taken to have the same form in the X and B states. At short distances, it is described by a sum of Morse pairwise potentials between the Ne atom and each of the Cl atoms, and between the neon atom and the center of mass of Cl2. At large distances the potential switches to an anisotropic van der Waals interaction with R−6 and R−8 dependence. The parameters were adjusted so that the T‐shaped configuration the potential matched the one determined...

A theoretical study of the Ar2HCl van der Waals cluster

A theoretical method for treating the dynamics of polymeric van der Waals clusters is developed, based on an adiabatic separation of the heavy atom motions. The method is applied to the calculation of spectroscopic parameters for the complex Ar2HCl, and the results are compared with experimental data from high‐resolution microwave studies. Potentials based on pairwise additivity and the known Ar–HCl pair potential are used. Small discrepancies between experiment and theory are observed, and it seems likely that these are attributable to the effects of three‐body forces rather than to deficiencies of the pair potentials used. High‐resolution spectroscopy of van der Waals clusters shows great promise as a tool for investigating nonadditive intermolecular forces.

Dynamics of ozone photoabsorption: A theoretical study of the Chappuis band

The first two excited singlet states of ozone are discussed in light of recent ab initio MRD‐CI calculations. In the asymmetric pathway for O2+O fragmentation, the two 1 1A2 and 1 1B1 states undergo an avoided crossing resulting in the C 1A‘ and D 1A‘ states. The D 1A‘ state has bound vibrational levels in the Franck–Condon region while the C 1A‘ is repulsive (towards O2+O fragmentation) in that area and is found to possess a local minimum away from the Franck–Condon area at small bond angles. A one‐dimensional cut along the dissociation reaction coordinate was extracted from the three‐dimensional calculated potential surfaces. Quantum mechanical calculations of the absorption spectrum based on these one‐dimensional ab initio potentials and of the corresponding ab initio transition moments were undertaken. The present work assigns the distinct features of the Chappuis band to the bound levels of the D 1A‘ state superimposed on the C 1A‘ state continuum as an alternative interpretation of the second absorp...

A theoretical study of the HgAr2(3P1←1S0) vibronic spectrum

A quantum mechanical calculation of the vibronic spectrum of the HgAr2 van der Waals cluster in the region of the Hg(3P1←1S0) electronic transition is presented and compared with experiments. The potential energy surfaces for the ground and excited states are obtained using available empirical Ar–Ar and Ar–Hg potentials. For the ground electronic state, the potential is written as a sum of pairwise Ar–Ar and Ar–Hg(1S0) interactions. On the contrary, for the electronically excited states correlating to Hg(3P1), an axis switching rotation has to be applied to each individual Ar–Hg(3P1; Ωe=0, ±1) interactions in order to define a common quantization axis. This results in a nonpairwise additive diabatic matrix Hamiltonian which after diagonalization provides the adiabatic excited potential energy surfaces. The vibrational wave functions associated to the ground and the first two (A and B+) adiabatic electronically excited states are obtained by variational techniques using basis sets along Jacobi coordinate...

Fine‐structure electronic predissociation in van der Waals molecules. I. Theory

A quantum‐mechanical model for fine‐structure predissociation in van der Waals molecules is presented. The process corresponds to: A(2S+1LJA )⋅⋅⋅M→A(2S+1LJ′A) +M where A is an atom and M an atom or a diatomic molecule A(2S+1LJA) ⋅⋅⋅M represents the van der Waals complex in a state which correlates to the 2S+1LJA electronic manifold of A. The general formalism based on close‐coupling equations is presented first. The limiting case of slow predissociation is then considered and treated by perturbation theory. Finally, the application to the atom–atom and the atom–diatom van der Waals complexes for L=1 and S=1/2 or S=1 is discussed. This simple analysis allows predictions to be made for predissociation rates and lifetimes in cases where the spin–orbit interaction is larger than the van der Waals binding energy.

Decay of vibrationally excited states of the Ne…Cl2 complex

Abstract Three-dimensional golden rule calculations for vibrational predissociation of the Ne…Cl 2 van der Waals molecule are presented for excitation in the region of the Cl 2 B←X 11-0 transition. The lifetime as well as the final rotational state distribution of the Cl 2 fragments are obtained. The initial quasibound state wavefunction of the complex is calculated by direct diagonalization using a product of a van der Waals (harmonic) stretching and a (hindered rotor) bending basis set. The final continuum wavefunctions are obtained by integration of the rotational close coupled Schrodinger equations. A calculation with the rotational infinite-order sudden approximation applied to the final continuum wavefunction is presented and compared to the close-coupling integration.

Time scales for molecular photodissociation

Abstract Direct dissociation dynamics is characterized by two distinct time scales, that is, the decay time of the autocorrelation function of the initial nuclear wavepacket τ d (≈10 −14 −10 −15 s) and the bond rupture time τ b (∼10 −13 s). τ d and τ b are analogous to T 2 -type dephasing and to T 1 -type level depletion processes, respectively.

A test of the rotational infinite order sudden approximation in molecular fragmentation

A quantum mechanical close‐coupling calculation is presented for predissociation of a triatomic molecule and the results are compared with the infinite order sudden approximation for rotational motion (RIOSA). The calculations are performed for a model system which mimics the predissociation of the zero‐point level of N2O+(A). It is shown that for the case treated here the infinite order sudden approximation obtained by setting all the rotational energies equal to zero gives poor results. Good agreement between RIOSA and exact calculations is obtained by setting them equal to the average rotational energy in the fragments. The relationship between RIOSA as applied to full collisions and half‐collisions is also discussed.

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.

A Theoretical Study of Hg⋯Arn (n=1, 2, 3) Clusters Excited in the Hg(3P←1S) Spectral Region

A quantum mechanical theoretical study of Hg...Arn (n = 1, 2, 3) Van der Waals complexes under Hg(3P←1S) electronic excitation is presented. The potential energy surfaces are calculated assuming additivity of the atom-atom pairwise interactions. The Hg...Ar potential is obtained from inversion of electronic spectra using a previously proposed model. For Ar2 the best available empirical potential is used. In the excited electronic manifold, the non-spherical character of the Hg(3P) state is taken into account by appropriate rotations of the wavefunctions in the molecular frame. This allows the determination of electronically excited diabatic potential energy surfaces and couplings. Diagonalization of the most strongly coupled states provides adiabatic potential energy surfaces. Electronic spectral shifts are estimated by computing vertical energy differences.