Time-resolved dispersed fluorescence spectroscopy as a method of determining energy transfer rates in very low energy molecular collisions

Abstract

The understanding of energy transfer processes is an essential prerequisite for a thorough understanding of all chemical reaction processes. The physical understanding of the temperature dependence of reaction rates lags very far behind the empirical knowledge that a reaction will progress faster if heated. Bulk reactions are the result of billions of individual encounters between atomic or molecular species with a combined total energy above the threshold energy necessary to react and produce an altered species. Internal vibrational and rotational energies within molecular interactions are independent of the potential energy surface attracting the species together.
This paper investigates the process of vibrational relaxation (or de-excitation) of vibrationally excited molecular iodine (I₂) in its B electronic slate induced by very low energy collisions at low Kelvin temperatures in a supersonic free jet.
The scope of the study involved exploration of vibrational relaxation from I₂ (B ³Π_0u⁺) ν = 21 to 24 in collision with He, Ne, Ar, H₂, D₂ and N₂, and incorporated the design and implementation of an experimental procedure that resolves the laser-excited state selected fluorescence with respect to both spectral frequency and time, i.e. time-resolved dispersed fluorescence spectroscopy. The experimental method developed may be summarised as follows.
• Excitation via laser of a set of rotational states within a particular |B, ν^*〉 transition.
• Measurement of the spectrally resolved fluorescence emanating from the levels |B, ν^*−1〉, |B, ν^*−2〉, |B, ν^*−3〉, |B, ν^*−4〉, subsequently populated by collisions.
• Recording the time evolution of the dispersed fluorescence bands as the raw data from which the rates of the vibrational energy transfer processes may be deduced.
The unimolecular decay rates may be measured in an independent experiment which does not resolve the bands spectrally. These collision-free rate data are required for extraction of state-to-state rates from the spectrally resolved, time varying data. The procedure not only yields the state-to-field overall vibrational deactivation rates for a selected vibrational level, but coupled with a novel deconvolution method for growth curve fitting, yields absolute state-to-state rates for vibrational relaxation involving quantum changes Δν = −1, −2, −3 and −4. The dependence of the relaxation rates on the collision partner, on temperature and on Δν are discussed.