Collisionless Multiphoton Dissociation of SF₆: A Statistical Thermodynamic Process

Abstract

The interaction of an SF₆ molecule with strong infrared CO₂ laser pulses has been the subject of intense theoretical and experimental investigation in many laboratories. Several review papers [1,2] have appeared, summarizing the state of the art in 1976, but new results are still being published in abundance [3–8]. In this paper we shall restrict ourselves to results where the effect of collisions is clearly eliminated, i.e., data obtained with molecular beams [3,4], or with very short laser pulses in vessels at sufficiently low pressure [5,6]. The problem, how one isolated SF₆ molecule bathed in a sea of infrared photons, absorbs more than thirty quanta necessary for dissociation, may be discussed with the aid of the schematic diagram of vibrational energy levels shown in Fig.l. Three regions may be distinguished. The molecule is excited from the ground vibrational state in a coherent multiphoton process, in which three to six quanta are involved [9]. This process is frequency — and therefore isotope — selective. The data in Fig.2 show that the dissociation rate, or dissociation probability per pulse, is nearly independent of pulse duration for constant pulse energy fluence. The dissociation process does not exhibit the high power dependence on the peak intensity, as a true multiphoton transition would have. The transitions in region I are therefore not the dissociation rate limiting step.