On isotopically selective collisional energy transfer and infrared multiple photon absorption by vibrationally highly excited molecules

摘要

This study focuses on an interesting and important phenomenon that was employed in a new approach to laser isotope separation that has been recently proposed and developed in our laboratory for highly selective separation of carbon isotopes. This approach consists of pre-exciting CF3H molecules with the desired isotope to a low vibrational overtone of the CH-stretch with a subsequent selective infrared multiple photon dissociation (IRMPD) of only the pre-excited molecules by a CO2-laser pulse. Significant isotopic shifts of the employed CH overtone bands already allow high selectivity at the pre-excitation step. This selectivity, however, can be further greatly increased by increasing the pressure of the sample gas and/or the delay between the two laser pulses; that is, by increasing the number of molecular collisions during the process. At first glance, the observed effect contradicts the general expectations that the isotopic selectivity of such a process should drop with an increase in the number of collisions because of vibrational energy transfer between different isotopic species. In this work we have studied this phenomenon and found its physical origins. We propose the contribution of two different mechanisms to the observed enhancement of isotopic selectivity by collisions. First, the vibrational collisional relaxation itself is isotopically selective, that is vibrationally excited 12CF3H relax on the bath of cold, 12CF3H, molecules faster than the excited 13CF3H do on the same bath. The primary reason for such a selectivity could be a significant isotopic shift of the vibration (CF-stretch, we believe) that mediates the energy transfer. A second mechanism arises from the difference between the IRMPD probabilities of the two isotopic species by the CO2 laser tuned to a particular wavelength that enhances the dissociation yield of the targeted, carbon-13, species. As collisional deactivation of both species increases the number of photons they have to absorb to be dissociated, this difference in dissociation probability increases as well, yielding an additional isotopic selectivity of the process. We perform a set of experiments and numerical simulations to investigate these two mechanisms. Experimentally we find that, indeed, collisional vibrational deactivation of CF3H is isotopically selective. This is, perhaps, the first direct observation of isotopically selective collisional relaxation of highly excited medium-sized molecules. At low pressures and increased time-delay between the lasers both suggested mechanisms contribute equally to the enhancement of isotopic selectivity (with a slight dominance of the IRMPD step). We also perform numerical simulations of vibrational energy transfer (VET) between highly excited and cold CF3H of both isotopic species. The model we employ includes V-V energy transfer due to long-range dipole-dipole interactions and V-V',T,R energy transfer due to head-on collisions between two molecules. The results reproduce well the measured isotopic selectivity in vibrational energy transfer. At the next step, we propose a model for the simulation of the laser isotope separation process. We solve the master equations including vibrational energy transfer and absorption and stimulated emission of IR photons. The main improvement of our calculations with respect to existing models is that we introduce a dependence of the absorption/emission rates on the frequency of the laser, the internal energy of the molecule and its isotopic species. We are able to reproduce our experiments numerically and thus gain information on the laser isotope separation process. In particular, we find that at high sample pressures the mechanism of isotopically selective IRMPD rates prevails over the mechanism of different collisional relaxation rates.