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AIP conference proceedings

Laser Control of Photoassociation in Reaction O+H→OH

Kiyoshi Nishikawa, Kimikazu Sugimori, Hidemi Nagao, Theodore E. Simos, George Maroulis

Published Jan 1, 2007• 0 citations
Publisher

Abstract

In this work, we simulate the laser‐induced photoassociation of the collision reaction O+H to generate the OH molecule. The photoassociation process involves the continuum‐discrete transition, and the initial wavepacket corresponding to the collision pair is generally given by a superposition of continuum state. By numerically solving the Schrödinger equation with split‐operator method, we can take into account implicitly but completely for the continuum state in the photoassociation process. In our simulation, we find interestingly the above threshold dissociation spectrum due to the continuum‐continuum transition as well as the target bound states by the photoassociation. In order to analyze the continuum state involved, we show the effective method by means of the quasicontinuum state on the Morse potential obtained by numerically diagonalizing the total system.

DOI
10.1063/1.2836224

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In this work, we simulate the laser‐induced photoassociation of the collision reaction O+H to generate the OH molecule. The photoassociation process involves the continuum‐discrete transition, and the initial wavepacket corresponding to the collision pair is generally given by a superposition of continuum state. By numerically solving the Schrödinger equation with split‐operator method, we can take into account implicitly but completely for the continuum state in the photoassociation process. In our simulation, we find interestingly the above threshold dissociation spectrum due to the continuum‐continuum transition as well as the target bound states by the photoassociation. In order to analyze the continuum state involved, we show the effective method by means of the quasicontinuum state on the Morse potential obtained by numerically diagonalizing the total system.
The laser-induced vibrational state-selectivity of product HF in photoassociation reaction H+F→HF is theoretically investigated by using the time-dependent quantum wave packet method. The population transfer process from the continuum state down to the bound vibrational states can be controlled by the driving laser. The effects of laser pulse parameters and the initial momentum of the two collision atoms on the vibrational population of the product HF are discussed in detail. Photodissociation accompanied with the photoassociation process is also described.
Abstract Using the time‐dependent quantum wave packet method, the photoassociation (PA) processes of He + H +→ HeH+ and He + D +→ HeD+, driven by the sin2‐shaped femtosecond laser pulse in the electronic ground state, including multiphoton transitions and dissociations, are investigated for a wide range of initial collision momenta spanning from 1 to 4 a.u. (or for the collision energy roughly in the ranges of 0.009∼0.148 eV and 0.006∼0.089 eV for HeH+ and HeD+ systems, respectively). It is found that, at some collision momenta, multiphoton transitions to deeply bound states are inevitable to occur and can greatly decrease the PA probability of the target state that selected is the vibrational state v = 6. For the dissociation process, the higher‐order (two‐ and three‐photon) dissociations, measured from the target state, tend to be significant at relative high collision energies, which implies that above‐threshold dissociations may also be an important loss mechanism in the PA process. In addition, it is also shown that the higher‐order dissociation is much stronger for HeH+ systems than that for HeD+ systems at a given collision momentum, and could be enhanced by the strong transitions among deeply bound states.
Photoassociation via reverse ladder transition controlled by two and four laser pulses is investigated using the time-dependent quantum wave packet method. The calculated results show that the amplitudes of the pulses have an enormous effect on the target population and total yield of association. For the target state with a high energy level, the population of background states can reduce the state-selectivity. Although, the total yield of association is decreased, the four pulses can induce the population transferring to low vibrational levels, and the state-selectivity of the target state is high.
We analyze photoassociation theoretically by treating the dissociation continuum of a molecule as a quasicontinuum (QC) of discrete states. In this manner, free-bound transitions are reduced to a few-level problem. The correct continuum limit is deduced from an explicit analysis of the density of QC states. Besides giving the usual photoassociation rate, the QC model facilitates the analysis of the steady-state molecular yield. Moreover, complications such as velocity redistribution and line broadening due to atom-atom collisions may be treated basically in the same way as in connection with the customary two-level atom. Next we adapt the standard three-level quantum optics to describe free-bound-bound photoassociation for both continuous-wave and pulsed two-color excitation. For a quantum degenerate gas of atoms, a proper two-photon resonant scheme may transform atoms to ground state molecules with near-unit efficiency even in the presence of spontaneous losses from the intermediate state.
Abstract Using the quantum wave-packet method, we investigated the photoassociation (PA) processes of H + + He → HeH + in the ground electronic state (X 1 Σ g + ). Two cases are considered where the target vibrational state selected is v = 4 and 8, respectively. It is found (for the first time) that above-threshold dissociation (ATD) occurs alongside the PA process (PA-ATD). ATD dissociation can be the major loss mechanism for the PA process. Compared with the direct dissociation from a given vibrational state and continuum–continuum transitions to the same final energy, the overall PA-ATD process is enhanced in comparison to individual contributions due to correlations. The one-photon dissociation from the target vibrational state, the ATD signals from two-photon dissociation and the higher-order (three- and four-photon) dissociations are the results of different mechanisms.
The photoassociation (PA) reactions of H + F → HF and D + F → DF are investigated by using the time-dependent quantum wave packet method. For each system, we consider four cases, where the target state is set to be ∣ ν = 16, 17, 18, 19〉, respectively. The effects of the initial collision momentum and laser parameters, including intensity, pulse duration, and frequency, on the final association probability are calculated and discussed. It is found that the PA probability for H + F → HF (∣ ν = 16, 17, 18〉) is significantly greater than that for D + F → DF (∣ ν = 16, 17, 18〉), while the PA probability to ∣ ν = 19〉 for the latter system is greater than the former one. This isotope effect has been connected with the Franck–Condon overlap integral between the target vibrational state and the collision continuum state.