4 - Selective Photodissociation of Bonds in 18O16O16O Molecule

Summary

Introduction

The easy availability of ultrashort high intensity lasers has fuelled a variety of approaches for the selective control of bond dissociation using appropriate laser pulses[1−22, 24, 25, 27−30, 32−68]. Photodissociation is one of the most fundamental chemical processes. The process involves the distribution of products among the various possible quantum channels and provides unique insights into the energetics and the mechanism of bond breaking[8, 11, 43, 56, 67] which is of fundamental importance in chemistry. Considerable experimental and theoretical effort has been directed over the past few years towards the goal of laser assisted control of molecular dynamics[10, 12, 15, 20, 24, 39−42, 49−51, 58]. The important issue is how to control the branching ratio between chemically distinct channels. To change the branching ratio, several ways have been reported[19, 42]. Infrared mutiphoton excitation is one such process that induces selective bond dissociation. However, in this type of process, the intramolecular vibrational relaxation (IVR) creates a road block to selective control. With a properly designed laser pulse, the effect of IVR can be eliminated or minimized. However, the infrared mutiphoton excitation method requires very high intensity to detect dissociation probability and therefore may lead to molecular ionization.

Another way to investigate photo fragmentation is by using the photons in the ultraviolet part of the electro magnetic spectrum. The limitation of this process is that one of the excited states of molecule of interest must be purely repulsive. Also, with two lasers in the ContinuousWave regime, the dynamics can be explored in accordance with the Franck−Condon principle, which is known as passive control[19, 54]. In this approach, the first laser is used to excite the vibrational eigenstate of themolecule and the second laser to dissociate themolecule via excitation to the repulsive electronic state. The above method is suitable only for a molecule with local mode vibrational eigenstates such as the photodissociation of HOD molecule where the vibrational eigenstates are essentially local modes in the O−H and O−D stretch[6, 16, 22, 60, 61]. The method is inapplicable to those molecules where vibrational eigenstates are of normal mode type[2].

To overcome the limitations of passive control, there is another scheme which has been reported, known as the active control scheme[19, 54].