Vibrational-state-to-state collision-induced intramolecular energy transfer N2(A 3Σu+, v″→B 3Πg, v′)

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Abstract
Absolute cross sections for collision-induced intramolecular energy transfer from the metastable A 3Σu+ state into the radiating B 3Πg state of N2 have been measured for the first time under single-collision conditions, using a thermal energy molecular beam of N2(A). The collision partners studied were the five rare gases, H2, N2, NO, and O2. The product vibrational levels (B, v′=4–10) were separated using spectrally resolved detection by means of filters as in our earlier related work [R. Bachmann, X. Li, Ch. Ottinger, and A. F. Vilesov, J. Chem. Phys. 96, 5151 (1992)]. In addition, in the present study the contributing reactant state vibrational levels (A,v″) were labeled, using optical pumping by a specially developed broad-band (∼1 nm) pulsed tunable dye laser. A depletion of up to 30% of a given v″ level could be achieved, about one-half of the theoretical maximum, at a pump pulse energy of 4 mJ. This quantity was also measured directly using a second synchronized probe laser. Pumping on a particular A,v″ level reduces the emission from the collisionally coupled B,v′ level by an amount which is a measure of the state-to-state cross section. Quasiresonant energy transfer was found to be strongly preferred, the cross section decreasing exponentially with an increasing energy gap. Absolute cross sections were obtained from a simultaneous measurement of the intensity of the fluorescence induced by the laser pumping of the selected A,v″ level, with corrections for predissociation of the excited upper state. Cross sections on the order of 0.1 Å2 for He to 15 Å2 for Xe were found for closely resonant (ΔE≊100 cm−1) processes. These results as well as our earlier, absolute measurements of the analogous intramolecular N2(W→B) transfer, are discussed in terms of interaction potential models.