Seung E. Choi

Determination of the bound and quasibound states of Ar–HCl van der Waals complex: Discrete variable representation method

The ArHCl (HCl; ν=0) van der Waals (vdW) molecule has a large number of bound and rotationally predissociative (resonance) states for total angular momentum in the range of 0≤J≤60. Using the Jacobi coordinates and the total angular momentum representation in the body‐fixed reference frame, the Hamiltonian is evaluated in the discrete variable representation (DVR) of the stretch and bend internal vibrational basis and a basis of parity adapted rotation functions. The facile and effective application of the DVR is greatly enhanced by an appropriate choice of the basis set. The sequential diagonalization and truncation of the Hamiltonian permit accurate and efficient determination of eigenstates. Using Hutson’s H6 potential energy surface, the energies and wave functions of all bound and resonance states are computed for selected J’s up to J=60. A novel flux projection technique facilitates semiquantitative evaluation of the lifetimes of all states and, in particular, the simple identification of the resonan...

Use of the discrete variable representation in the quantum dynamics by a wave packet propagation: Predissociation of NaI(1Σ+0) →NaI(0+)→Na(2S)+I(2P)

Using the Gauss–Chebyshev discrete variable representation (DVR), the dissociative quantum dynamics for a wave packet evolving under the influence of the Hamiltonian for two interacting diabatic states of a diatomic molecule is calculated. The split time evolution operator method is used to obtain the solutions to the time‐dependent Schrodinger equation. A specific example of the numerical calculation is shown for the predissociation process of NaI→Na(2S)+I(2P) from its first excited electronic state (0+). The numerical results are compared with the experimental observations from the femtosecond laser photofragmentation, recently reported by Zewail and co‐workers.

Highly excited vibrational eigenstates of nonlinear triatomic molecules. Application to H2O

Highly accurate quantum‐mechanical calculations are presented for highly excited vibrational states of H2O. The vibration Hamiltonian operator Hvib for a nonlinear triatomic molecule is given in Radau coordinates. A direct product basis is chosen, and the Hvib matrix is evaluated in the discrete variable representation (DVR) for the symmetrized Radau coordinates. Vibrational eigenstates are computed from the DVR Hvib via the successive diagonalization/truncation technique. A comparison of the computed eigenvalues with those observed demonstrate the accuracy of our model. Highly excited vibrational states, up to 30 000 cm−1 above the zero‐point energy, are reported for the potential energy surface (PES) given by Jensen [J. Mol. Phys., 133, 438 (1989)]. Using natural orbital expansions, the eigenfunctions of vibrational states are analyzed to understand the origins of the dynamical mixing of the vibrational modes. The local/normal mode transitions, Fermi resonances, Darling–Dennison interactions, and the m...

An inverse method for obtaining smooth multidimensional potential energy surfaces: application to Ar+OH A 2∑+(v=0)

A regularized inverse method is developed to determine an accurate multidimensional potential energy surface directly from spectral or scattering data. The method properly handles the inherent underdeterminancy, instability and nonlinearity of the problem, and extracts a physically acceptable smooth solution. The method is implemented for Ar+OH(A 2∑+).

State‐to‐state measurements of internal rotational predissociation in OH–Ar (A 2Σ+)

Internal rotational predissociative levels of OH–Ar (A 2Σ+) have been identified lying up to 350 cm−1 above the OH A 2Σ+ (v=0, 1)+Ar dissociation limit. The predissociative level energies, lifetimes, and OH A 2Σ+ rotational product distributions have been measured. Complexes prepared in many of these predissociative levels are long lived with lifetimes ≥50 ps. A novel variation of stimulated emission pumping has enabled quantitative OH A 2Σ+ rotational distributions to be obtained following OH–Ar predissociation. The OH product distributions are highly selective. The highest energetically available channel is always populated, yet in many cases, low rotational levels are conspicuously absent. The OH–Ar predissociative levels have been assigned nearly good quantum numbers based on a rotational contour analysis of the predissociative features and/or the OH A 2Σ+ rotational product distributions. A two‐step mechanism involving Coriolis coupling and the potential anisotropy has been proposed to describe the p...

Predissociation dynamics on a highly anisotropic potential : OH-Ar (A 2Σ+)

Metastable levels of OH–Ar which lie significantly above the OH (A 2Σ+)+Ar dissociation limit are identified. The rates of internal rotational predissociation and the resultant OH (A 2Σ+) product rotational distributions are determined. Comparison with theoretical calculations of the predissociative states provides a sensitive test of the short‐range interaction potential.

Application of an inverse method to the determination of a two‐dimensional intermolecular potential energy surface for the Ar–OH(A 2Σ+, v=0) complex from rovibrational spectra

A stable inversion method of determining molecular potentials from a finite number of spectroscopic data is presented. Molecular inverse problems are often underdetermined, unstable, and nonlinear. Specifically, the measured spectra contains only partial information of the sought‐after potential and even a small error in the data may cause a large variation in the inverted solution. Moreover, the underlying potential is a complicated nonlinear functional of the spectral data. The inversion algorithm, based on the Tikhonov regularization method, resolves all of the above predicaments and yields accurate sought‐after potentials with proper analytic properties. The method is applied to extract two‐dimensional Ar–OH(A 2Σ+, v=0) potential energy surfaces from the rotational–vibrational spectral data. Two versions of the recovered potential energy surfaces are obtained using two slightly different sets of rovibrational data. These two potentials are basically equivalent, except in the regions to which the data ...

Rotational predissociation dynamics of OH–Ar (A 2Σ+) using the finite range scattering wave function method

Predissociative resonances of OH–Ar are computed up to 300 cm−1 above the Ar+OH (A 2Σ+, v=0, j=0) asymptote for total (rotational) angular momentum states J≤10. The energies, lifetimes, and OH A 2Σ+ product rotational distributions of the predissociative resonances are calculated using a numerical method based on the ‘‘energy independent integral’’ finite range scattering wave function (FRSW) [J. Chem. Phys. 99, 1057 (1993)]. The FRSW method involves evaluation of the scattering matrix and its energy derivative, both of which are only parametrically dependent on energy. The energy independent matrices are determined from the discrete eigenvectors of the L2 Hamiltonian matrix H, which is obtained in discrete variable representation, and an exact (analytical) eigenfunction of the asymptotic Hamiltonian operator H0. Many long‐lived (≳1 ps) resonances are identified for OH–Ar in J=3 with projections of J onto the intermolecular axis of K=0–3. The resonances are characterized with approximate bend and stretch...

State-to-state dynamics of predissociation in OH–Ar: experiment and theory

Long-lived predissociative levels of OH–Ar which lie as much as 350 cm–1 above the OH (A 2σ+, v= 0, 1)+ Ar dissociation limits have been identified experimentally and computed based on an adjusted semiempirical potential-energy surface for OH (A 2σ+)+ Ar. The rotational state distributions of the OH (A 2σ+) photofragments have been probed using a novel variation of the stimulated emission pumping technique and exhibit a surprising degree of selectivity. The highest energetically available OH (A 2σ+) rotational channel is always accessed, yet low rotational levels are conspicuously absent from the product distributions. Predissociation lifetimes and product state distributions have also been calculated using the finite-range scattering wavefunction method. The theoretical results suggest two main mechanisms for internal rotational predissociation involving Coriolis coupling and the potential anisotropy. Comparison of the energies, lifetimes, and product rotational distributions of the OH–Ar predissociative levels observed experimentally with those obtained from the theoretical calculations provides a sensitive test of the short-range interaction of this highly anisotropic intermolecular potential-energy surface. Product rotational distributions measured following vibrational predissociation of OH–Ar also give new insight into the short-range part of the OH (A 2σ+)+ Ar intermolecular potential.

Rotational resonance states of Ar–HCl(v=0) by finite range scattering wave function method

The low lying rotational resonance states of Ar–HCl van der Waals molecule in the vibrational ground state of HCl are calculated for several total angular momentum states within the model of a rigid rotor‐atom system. The necessary scattering calculations are done by the finite range scattering wave function (FRSW) method [J. Chem. Phys. 99, 1057 (1993)] adopting energy independent auxiliary functions which makes the scattering calculations at many energies much more efficient. Discrete eigenvectors and eigenvalues of Hamiltonian matrix are calculated on a finite range via the successive diagonalization‐truncation scheme combined with the discrete variable representation (DVR). Analytical eigenfunctions of the asymptotic Hamiltonian operator in a body‐fixed frame excluding only the interaction potential, but including all the effective centrifugal potential terms, are used as asymptotic wave functions, which reduces the dynamical range required for the L2 scattering calculations. After a single diagonaliz...