Photodissociation and Control of CH₃I in Strong Pulsed Laser Fields

Quantum symplectic method is a time-dependent propagation method with high precision and high efficiency.Recently,a new algorithm was proposed for the systems with time-dependent Hamiltonian.In this article this algorithm was first testified in a study of the process of photodissociation of CH₃I in strong pulsed laser fields.This process has been well known as a prototype of the photodissociation of small molecules.The CH₃I molecule was modeled as a pseudolinear triatomic in which the three H atoms were treated as a single pseudoatom and then the calculation was done in a diabatic representation which includes the coupling between the two excited potential energy surfaces.The results of photodissociation of CH₃I excited by a single laser pulse was found in good agreement with the results in the literature.Then the possibility of laser control of this process was investigated.Two strong femtosecond laser pulses were used to control the branching ratio of products.The influence of various laser parameters,such as electric strength,pulse width,excitation frequency,delay time between two pulses,were investigated in calculation.It was found that the branching ratio can be mostly manipulated by changing the delay time and the second pulse was the key of the control.In our calculation,a minimum of 12.3% was obtained for the branching ratio compared with the results of ordinary photodissociation,75%.The mechanism of control was analyzed briefly by the superposition of transient wavepackets induced by the laser pulses on the excited states.