Off-resonant vibrational excitation: Orientational dependence and spatial control of photofragments

Abstract

Off-resonant and resonant vibrational excitation with short intense infrared ͑IR͒ laser pulses creates localized oscillating wave packets, but differs by the efficiency of the excitation and surprisingly by the orientational dependence. Orientational selectivity of the vibrational excitation of randomly oriented heteronuclear diatomic molecules can be obtained under simultaneous irradiation by a resonant and an off-resonant intense IR laser pulse: Molecules with one initial orientation will be vibrationally excited, while those with the opposite orientation will be at rest. The orientation-dependent response to the IR fields is due to the anharmonicity of the potential. A subsequent ultraviolet laser pulse in resonance at the outer turning point of the vibrational motion can then dissociate the oscillating molecules, all with the same orientation, leading to spatial control of the photofragment distribution.

Few-cycle laser pulses to obtain spatial separation of OHF− dissociation products

“…In these studies, the molecule was assumed to be perfectly oriented with respect to the laser fields 25,26 or randomly oriented. [19][20][21][22][23][24] In reality neither assumption is realistic, even in a qualitative sense.…”

Breaking the strong and weak bonds of OHF−using few-cycle IR + UV laser pulses

“…Henriksen and coworkers, for example, have demonstrated bond-selective dissociation using a combination of few-cycle IR and UV laser pulses. [5][6][7] In this approach, an intense ultrashort IR pulse excites a superposition of vibrational eigenstates, that is to say it creates a dynamical wavepacket. The UV pulse is fired at the appropriate time, taking advantage of the position of the wavepacket.…”

Separating the photofragments of randomly oriented symmetric reactants by IR + UV laser pulses: Quantum simulations for FHF− → F + FH + e

“…[1][2][3] Extended applications to randomly oriented heteronuclear diatomic reactants show that the resulting different atoms may be dissociated to preferential, opposite directions. 4,5 The present paper essentially combines and extends the virtues of these previous applications, thus achieving spatial separation of the atomic and molecular fragments of an ensemble of symmetric triatomic reactants. The present extension is also based on our previous design of IR + UV laser pulses for dynamical symmetry and selective bond-breaking of This paper is dedicated to Professor Joshua Jortner on the occasion of his 70th birthday, with gratitude and admiration.…”

“…The assumption of collinearity implies that we may set θ = θ 1 = θ 2 and φ = φ 1 = φ 2 , where θ and φ denote the angles that specifiy the orientation of the system relative to the laboratory coordinates, X,Y, Z, and where the definition of X and Y is arbitrary due to the linearity of the system. As a consequence, the kinetic operator is reduced to (5) where L and I denote the total angular momentum and the reduced moment of inertia of the system, respectively. Another consequence of the assumed collinearity of the system is that the "radial" coordinates q 1 and q 2 may be considered as coordinates describing the symmetric and asymmetric stretches, q 1 = q s and q 2 = q as , respectively.…”

“…Most relevant for the present work are the fundamental mechanisms that have been invented by Henriksen and coworkers for selective bond-breaking of oriented asymmetric systems, e.g., HOD, 1,2 or isotopi-cally labeled ozone, 3 and for photodissociations of randomly oriented heteronuclear diatomic molecules with spatial separation of the different atoms, e.g., NaI → Na + I. 4,5 In these approaches, the intense, ultrashort IR laser pulse drives the molecular wave packet that represents the oriented reactant from the ground state equilibrium configuration to a specific different geometry, in other words, an extension of a preselected bond. The ultrashort UV pulse is then applied to dissociate the system to specific products, starting from the selected geometry, e.g., breaking the specific extended bond.…”