Crossed beam studies of the dynamics of electronic energy transfer: Quenching of Na(3p 2P3/2) atoms by N2, O2, CO, and NO molecules

Abstract

The dynamics of quenching of electronically excited Na atoms by the diatomic molecules N₂, O₂, CO, and NO has been studied using crossed molecular beam techniques. Distributions in both laboratory scattering angle and recoil velocity provide information on the quenching process and the disposal of energy. For N₂, O₂, and CO, we observe a direct process which occurs in less than one rotational period of the collision system; all the molecules are scattered forward, along their original direction. For N₂ and CO, an average of 30%–40% of the available energy is transferred to internal energy of the diatomic molecules. A comparison with data from infrared absorption studies of the Na*–CO system shows that very little of this internal energy is taken up by rotational modes of the CO molecule. For O₂, a greater fraction of the electronic energy is partitioned into internal modes but the details of this partitioning are further complicated by the accessibility of the low lying O₂ excited electronic states. For NO, quenching proceeds via the formation of a collision complex with a mean lifetime of one rotational period (≳0.5 ps). The angular distribution shows that rotational excitation is again low and therefore, in this case, the recoil velocity distributions can be transformed directly into vibrational state distributions. Of the 2.24 eV of available energy, 30%–40% is partitioned into translation and 60–70% is partitioned into vibration, while less than 5% is partitioned into rotation. Thus, as for the Na*–CO system, the detailed energy partitioning in the Na*–NO system is completely determined. These results are compared with results from other experiments and with predictions from several available theoretical models.