Vibrational relaxation of T1 pyrazine: Results from the refined competitive radiationless decay method

Abstract

Gas phase collisional vibrational relaxation of pyrazine in its lowest triplet electronic state has been further investigated using a refined version of the competitive radiationless decay (CRD) method. Improvements to the experimental apparatus now provide primary kinetic data of much greater accuracy and precision, allowing the use of sample pressures low enough to ensure negligible self-relaxation. A major refinement of the data handling procedure permits the sample’s energy-dependent triplet–triplet molar absorptivity to be determined through an iterative analysis. To validate the refined CRD method, average energy loss per collision has been deduced as a function of donor energy under conditions of varying sample pressure, relaxer pressure, excitation beam energy, and excitation wavelength. Consistent results have been obtained for all of these variations, suggesting the absence of major systematic errors. The two main findings of earlier pyrazine triplet relaxation studies are confirmed by the new measurements: the presence of a threshold donor energy above which relaxation becomes much more efficient, and relatively large values of average energy loss per collision. Comparing to results from a recent ground state study [L. A. Miller and J. R. Barker, J. Chem. Phys. 105, 1383 (1996)], it is found that pyrazine containing 5000 cm−1 of vibrational energy is relaxed by a variety of monatomic, diatomic, and polyatomic gases approximately seven times more efficiently when the donor is in its triplet rather than its ground electronic state. The order of relaxer efficiencies toward triplet pyrazine is found to be He<H2<Ne<D2<N2<Kr<Ar<Xe<CH4<CO<CO2<CH3F<H2O. Energy loss efficiencies correlate rather well with relaxer boiling points.

Recoil energy distributions for dissociation of the van der Waals molecule p-difluorobenzene–Ar with 450–3000cm−1 excess energy

“…Weisman's group has studied collision-induced energy transfer involving pyrazine derivatives in the triplet state. [51][52][53][54] They find that the average energy transferred per collision is significantly higher than that for the ground electronic state 55 at the same vibrational excitation. It was proposed that this is due to the lower vibrational frequencies in the triplet state.…”

“…[51][52][53][54] They find that the average energy transferred per collision increases rapidly with energy above ϳ2000 cm −1 . At lower energies the amount of energy transferred drops to almost zero.…”

Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). II. The collisional relaxation of toluene: P(E′,E) and moments of energy transfer for energies up to 50 000 cm−1

“…[20][21][22][23][24] Others have monitored the uptake of energy in the bath medium during the relaxation by various techniques [25][26][27][28][29][30][31][32][33][34] or even identified state-specifically the energy transferred, e.g., to CO 2 colliders in single collisions. [35][36][37][38][39][40][41][42] The vast majority of those data are on ͗⌬E͘, the average amount of energy transferred per collision.…”

Energy transfer of highly vibrationally excited molecules studied by crossed molecular beam/time-sliced velocity map ion imaging

“…Weisman and co-workers studied the energy loss from vibrationally excited triplet state pyrazine using the refined competitive radiationless decay method [124][125]. Their results suggested that the average energy transferred per collision increases rapidly from an apparent threshold that lies between 2000 and 3000 cm À1 .…”

“…The enhancement of the methyl group on the energy transfer was always observed for various colliders, including atomic, diatomic and polyatomic molecules [124,125]. Flynn and co-workers studied the energy gain in CO 2 from collisions with highly vibrationally pyrazine and 2-methylpyrazine using high-resolution transient infrared absorption [50,[126][127].…”