Ultrafast energy transfer and population buildup among strongly coupled and curve crossing regions of the boron oxide A 2Π and X 2Σ+ states Original Research Article

Boron oxide A 2Π–X 2Σ+ chemiluminescence resulting from the reaction of boron atoms with nitrogen dioxide (NO2) and ozone (O3) has been studied. The reactions are characterized from 10−5 to 10−3 Torr and at Ptotal∼1 Torr in order to assess rapid X→A intramolecular vibrational-to-electronic transfers including BO (X 2Σ+,v″=17)+M→BO (A 2Π1/2,v′=4)+M where M=NO2, O3 or a combination of these oxidants with argon, and the nature of the pressure dependent population buildup due to a possible bottleneck associated with the curve crossing of the BO X 2Σ+ and A 2Π states between v′=8 and 9 A 2Π. At the very lowest pressures, the B+NO2 reaction also yields emission corresponding to the BO2 A 2Πu–X 2Σg+ band system, however, the B–O3 reaction results only in BO* A–X emission and produces neither BO2 A–X nor BO B–X emission. At elevated pressures both the B–NO2 and B–O3 systems yield BO2 A–X where the excited BO2 is formed in a two-step reaction sequence. The effects of ultrafast intramolecular energy transfer, manifest at pressures as low as 2×10−5 Torr for the B–NO2 system, are consistent with expectations based on long-range interactions. For both the B–NO2 and B–O3 metatheses, the BO* A 2Π vibrational distributions are strongly influenced by (1) a combination of population buildup in vibrational levels v′⩾9 in the vicinity of the A 2Π–X 2Σ+ curve crossing region and (2) near resonant energy transfer among nearly isoergic levels of the X 2Σ+ and A 2Π states (both 2Π1/2 and 2Π3/2 components). The effects observed in BO demonstrate the interaction of a highly vibrationally excited ground state molecule in the presence of a collision partner with a cross-section well in excess of that expected for gas kinetic interaction and a rate constant exceeding 1010 Torr s−1.