Collisionally Induced Bottleneck Weakening in IR Multiphoton Dissociation

Abstract

An extension of the method of McRae et al. is proposed to quantify the contribution of the distinct collisional processes to the total yield obtained in infrared multiphoton dissociation experiments. The extension consists of the introduction of a substrate pressure dependence in the parameter representing one homogeneous collision. This allows the application of the method at a pressure where the collision time is of the order of the temporal width of the laser pulse and the discrimination between the different processes associated in a given collisional sequence. We have tested the reliability of this extended method by applying it to the multiphoton dissociation of CF 2 HCl, where the dependence of the dissociation yield on initial pressure and laser fluence has been investigated. We have shown that the homogeneous collisions occurring during the laser pulse may play a fundamental role in the weakening of the bottleneck absorption effects. It has been also suggested that fluence is the main excitation parameter for fixing the pressure interval along which dissociation can be regarded as collisionless.

One- and Two-Color Infrared Multiphoton Dissociation of C3F6

“…Under similar experimental conditions, a change in the laser fluence from 2.6 to 5.9 J cm -2 gives rise to only a 4-fold increase in h 10 in the MPD of CF 2 HCl. 6 This difference supports the idea that the anharmonic barrier can be circumvented more easily in C 3 F 6 than in CF 2 HCl.…”

“…Similar experiments carried out in CFHCl 2 with cells of different diameter support this interpretation. 24 Together with this thermoacoustic wave, there may exist another wave associated with the V-V′ intermolecular transfer. This vibrothermal wave is reflected on the cell walls and returns to the central core, resulting in an overexcitation of the molecules therein.…”

“…On the other hand, it is not necessary to include the parameter h 20 a in the fits, confirming in this way that collisions between C 3 F 6 molecules excited in the quasicontinuum do not contribute to the MPD process. In addition, a study of the MPD of C 3 F 6 carried out in the presence of argon 22 strongly suggests that the nature of the anharmonic barrier in C 3 F 6 is essentially vibrational.…”

“…In these experiments, it can be considered that a change in the effective size of the molecule in relation to the MPD process is induced; that is, the strength of the anharmonic barrier is modified. We have defined the anharmonic barrier as the ensemble of all quantum characteristics of a molecule that inhibit the multiphoton absorption process. It consists of two main components: (a) the vibrational barrier, originated by the progressive anharmonic detuning of the absorption of the infrared photons, initially resonant with the first vibrational transition; (b) the rotational barrier, originated by the existence of the rotational selection rules, which prevents a large proportion of molecules from interacting with the radiation field.…”

“…Recently, we have extended the method previously developed by McRae et al − for the analysis of the contribution of the different collisional processes to IR multiphoton dissociation. Our extension allows the distinction, in the collisional sequence corresponding to one homogeneous collision, of those processes that affect the dissociation yield and that need the presence of IR photons to be effective (e.g., overcoming of the anharmonic bottleneck) from those that have not this requirement (e.g., vibrational energy pooling).…”