Multiphoton dissociation of a diatomic molecule: Laser intensity, frequency, and pulse shape dependence of the fragment momentum distribution

Abstract

We study by an exact method the infrared multiphoton dissociation of a rotationless diatomic molecule and calculate the fragment relative momentum distribution as a function of the laser intensity and frequency, using either a square or smoothly varying pulse shape. The distribution has peaks due to multiphoton transitions. The nature of the peak structure depends on the laser intensity and whether the laser frequency is comparable to (i.e., within 20%) the ν=0 to ν=1 transition frequency (ω10) of the diatomic: If it is the distribution has bands spaced by the photon energy which contain peaks due to transitions from many bound states; if the laser frequency is not comparable to ω10, the distribution consists of isolated peaks spaced by the photon energy, which result from multiphoton transitions from the ground vibrational state. Changing the pulse shape from smoothly varying to square adds additional structure to the distributions. At sufficiently high intensities (1015 W/cm2) the high momentum peaks increase in intensity and the low momentum peaks are suppressed as the laser intensity is increased (this effect is often referred to as peak switching). At high laser intensities and frequencies comparable to ω10, classical mechanical calculations of the fragment momentum distribution give a smoothed out approximation to the quantum results and display a shifting similar to peak switching. Classical mechanics is unable to reproduce the quantum results at low intensities or at frequencies not comparable to ω10.