Intensity and wavelength control of a single molecule reaction: Simulation of photodissociation of cold-trapped MgH[sup +] (original) (raw)
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Photo-dissociation of Cold MgH mathsf+\mathsf{^ + }mathsf+ ions
The European Physical Journal D, 2004
Molecular ions sympathetically cooled into a Coulomb crystal by laser cooled atomic ions represent in many ways an ideal target for molecular physics studies due to achievable low translational temperatures (∼10 mK) and strong spatial localization (∼1 µm). In particular, in experiments with focused laser beams, both these features can be extremely useful. Here, we present results from experiments on photo-dissociation of MgH + ions in Coulomb crystals, and discuss possible avenues for using such processes for rotational temperature measurements as well as for manipulating the branching ratio of the dissociation channels Mg+H + and Mg + +H through intensity and frequency control of the laser beams involved.
Two-color coherent control of H+2 photodissociation in intense laser fields
Physical Review Letters, 1993
We study the multiphoton dissociation of H2 + by a coherent superposition of an intense short pulsed laser radiation and one of its harmonies (second or third), in a phase-Iocked regime. We show that the total dissociation probability, the energy distribution, and the direction of ejection of the protons are very sensitive to the relative phase of the two radiations. A high degree of control may th us be achieved for the branching ratio between dissociation via bond-softening and above-threshold dissociation, in the realm of current experimental capabilities. PACS numbers: 33.80.Gj, 33 .80.Wz, 34.50.Rk Molecular dissociation in strong laser fields has been actively investigated in the past few years, both experimentally and theoretically. Several interesting processes have been found : (j) above-threshold dissociation (ATO) with stimulated emission as weil as multiphoton absorption occurring in the dissociation continuum; (ij) bond softening, [2,3] when a potential barrier in the dressed potential curves is sufficiently lowered by the radiative interaction that the initial vibration al level becornes unbound; (jji) suppression of dissociation, or stabilization due to temporary vibrational trapping in upper potential wells in the dressed potential curves [51. The relative importance of these three competing processes depends on the initial vibration al level, the laser intensity and wavelength, and the pulse duration . From the existing studies the A TO process appears to be the "Ioser," at least when starting from a mixture of vibrational levels, because dissociation by bond softening is very rapid as soon as the potential barrier can be overpassed.
The Journal of Chemical Physics, 2013
Two-photon photoassociation of hot magnesium atoms by femtosecond laser pulses is studied from first principles, combining ab initio quantum chemistry and molecular quantum dynamics. This theoretical framework allows for rationalizing the generation of molecular rovibrational coherence from thermally hot atoms [L. Rybak et al., Phys. Rev. Lett. 107 , 273001 (2011), arXiv:1107]. The coupled cluster and multi-reference configuration interaction frameworks are used to calculate the relevant potential energy curves, one-photon and two-photon transition matrix elements, dynamical Stark shifts, as well as spin-orbit couplings and nonadiabatic radial couplings. Random phase thermal wavefunctions are employed to model the thermal ensemble of hot colliding atoms. Comparing three different choices of random phase thermal wavefunctions, the free propagation approach is found to have the fastest initial convergence for the photoassociation yield. When further refinement is required the eigenvalue approach is superior. The interaction of the colliding atoms with two time-delayed femtosecond laser pulses is modeled non-perturbatively to account for strong-field effects observed in the experiment. Good agreement between experimental and theoretical results is obtained. 2 J P 2 J P g J where P g J = Tr[(e −βĤ J g ) 2 ]/Z 2 J , andĤ J
Femtosecond coherent control of thermal photoassociation of magnesium atoms
Faraday Discussions, 2011
We investigate femtosecond photoassociation of thermally hot atoms in the gas phase and its coherent control. In the photoassociation process, formation of a chemical bond is facilitated by light in a free-to-bound optical transition. Here, we study free-to-bound photoassociation of a diatomic molecule induced by femtosecond pulses exciting a pair of scattering atoms interacting via the van-der-Waals-type electronic ground state potential into bound levels of an electronically excited state. The thermal gas of reactants is at temperatures in the range of hundreds of degrees. Despite this incoherent initial state, rotational and vibrational coherences are observed in the probing of the created Mg 2 molecules.
Multiarrangement photodissociation calculations utilizing negative imaginary potentials
The Journal of Chemical Physics, 2001
Over the past three years we have made significant contributions to the ongoing development of the coherent control of atomic and molecular processes. Specifically, we have contributed to (1) bimolecular reaction dynamics and controlled collision phenomena; control of molecular chirality and asymmetric synthesis; (3) theory, and practical considerations, in the control of the photodissociation of real systems; (4) control in large molecular systems; (5) the continued development of semiclassical mechanics specifically for coherent control applications; and (6) control of molecular nanoscale deposition on surfaces.
Quantum dynamics studies of the photodissociation of molecular systems
2013
Bibliography 218 Contents v List of Tables xxiii Glossary xxv the absorption of light. Within the framework of non-relativistic quantum mechanics, such a process may be cast in the following form. An initial state |ψ i is taken to propagate on the ground state potential until time t = 0. Taking |ψ i to be an eigenstate of the ground state potential, the initial wavefunction changes only by a phase factor. At time t = 0, the absorption of a single photon results in the promotion of the state |ψ i to an electronically excited state, which, assuming vertical excitation, is given in Photodissociation processes are typically challenging to model in an accurate manner for two primary reasons. Firstly, unlike the preponderance of ground state processes, the excitation of a molecular system to an electronically excited state denies us, in general, the possibility of treating in a satisfactory manner the electronic and nuclear degrees of freedom as being decoupled. Thus, the evolution of the wavepacket |φ i has be considered to occur over a manifold of vibronically coupled electronic states. Secondly, the πσ * , valence-type characters as the the X-H dissociation coordinate is traversed. Explanation of this change in character is typically made by taking the adiabatic potential in question to correspond to an avoided crossing between two diabatic states: a lower, bound 3s Rydberg state that is vibronically coupled to a higher-lying, purely dissociative πσ * state by the X-H
Collisions of cold magnesium atoms in a weak laser field
Physical Review A, 1999
We use quantum scattering methods to calculate the lightinduced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 30 atomic linewidths to the red of the 1 S0-1 P1 cooling transition. Magnesium has no hyperfine structure to complicate the theoretical studies. We evaluate both the radiative and nonradiative mechanisms of trap loss. The radiative escape mechanism via allowed 1 Σu excitation is dominant for more than about one atomic linewidth detuning. Molecular vibrational structure due to photoassociative transitions to bound states begins to appear beyond about ten linewidths detuning. 34.50.Rk, 34.10.+x, 32.80.Pj Light-induced collisions between cold, neutral alkali atoms have been widely studied experimentally and theoretically in magneto-optical traps . Such collisions cause loss of atoms from the trap due to radiative or nonradiative molecular processes after laser excitation of molecular states of the atom pair at large internuclear separation R. These loss processes for alkali atoms are still not well-understood when the laser detuning ∆ is only a few natural atomic linewidths Γ at to the red of atomic resonance, primarily because of complications due to the molecular hyperfine structure in the alkali species . Molecular vibrational and rotational structure in the spectrum of loss versus ∆ is unresolved due to broadening by fast spontaneous decay and the high density of molecular states. On the other hand, high resolution photoassociation spectra for trap loss at much larger detunings are quite well-understood quantitatively, even for alkali species .
Laser control of reaction paths in ion–molecule reactions
Molecular Physics, 2006
Ion-molecule reactions are characterized by long-range dipole moments which can be used to control and manipulate their reaction path in the presence of intense laser fields. We present here an analytic model for the control of such reaction paths, including the effect of an intense laser field on the scattering and tunneling probability. We illustrate such effects for the Li þ þCH 4 and reverse reaction using ab initio methods to calculate the reaction path and the permanent dipole moment of this system.
Local control theory applied to molecular photoassociation
The Journal of chemical physics, 2007
Local control theory ͑LCT͒ is employed to achieve molecular photoassociation with shaped laser pulses. Within LCT, the control fields are constructed from the response of the system to the perturbation which makes them accessible to a straightforward interpretation. This is shown regarding the ground-state collision of H + F and H + I atoms. Different objectives are defined, which aim at the formation of vibrational cold or hot associated molecules, respectively. Results are presented for s-wave scattering, where the rotational degree of freedom is ignored and also for full scale calculations including rotations, in order to describe more realistic conditions. Physics 127, 084115-1 084115-3 Local control theory J. Chem. Phys. 127, 084115 ͑2007͒ 084115-4 P. Marquetand and V. Engel J. Chem. Phys. 127, 084115 ͑2007͒ 084115-6 P. Marquetand and V. Engel J. Chem. Phys. 127, 084115 ͑2007͒
Photodissociation of H_{2}^{+} and HD^{+} in an intense laser field
Physical Review A, 2002
The photodissociation of H 2 ϩ and HD ϩ by an intense laser pulse is investigated by solving the closecoupling equations without discretization. For the case of H 2 ϩ the photodissociation spectra are calculated under the condition mimicking the experimental one, and a fairly good agreement with the experiment is obtained. The uncertainty in the relative phases of initial states is found to lead to somewhat smoothing of the spectra, depending on the pulse length. It is also found that Raman-type transitions via intermediate dissociation continuum play an important role in determining the photodissociation spectra. This leads to a population increase of lower vibrational states and deforms the spectral profile. Dissociation from the lower vibrational states due to the bond softening is not strong enough. Photodissociation spectra and angular distribution are calculated also for HD ϩ under the same conditions as in the H 2 ϩ case. The dipole transitions lead to additional structures in the energy spectra and angular distribution. There is a noticeable difference in the peak positions of dissociation spectrum for particles dissociated by the direct electronic-dipole transition and by the transitions via intermediate bound states. The photodissociation dynamics is further clarified by using the threedimensional plots of the spectra as a function of the field intensity and frequency.