Energy Transfer in the Fluorescence of Iodine Excited by the Sodium D Lines

Abstract

Absorption of the sodium D lines excites I₂ molecules predominantly to the vibration—rotation levels v′=14, J′=113; v′=15, J′=44 and 37; and v′=16, J′=106 of the B³II_0+u electronic state. In the absence of foreign gases, approximately 75% of the excited molecules are in v′=15, 10% in v′=14, and 10% in v′=16. Steady‐state fluorescence from these levels and neighboring levels populated by collisions with added inert molecules has been used to obtain collision cross sections for exchange of vibrational energy of excited I₂ with the translational energy of the gases He, Ne, Ar, Kr, Xe, H₂, HD, D₂, N₂, O₂, and NO. Relative emission intensities were measured photoelectrically. Vibrational transfer is highly efficient; cross sections for one quantum transfer average about ⅛ kinetic‐theory cross sections taken from gas viscosities and are similar to those obtained by other workers for transfer from v′=25. Information on multiple quanta transfer could not be obtained directly because of band overlapping. On transfer of a vibrational quantum by He, H₂, HD, and D₂, some persistence of the initial rotational state in I₂ was observed, while with the heavier gases the rotational distribution was spread. In collisions with HD no evidence was found for any resonance exchange between HD rotational energy (91 cm^—1 for the J=0↔1 transition) and I₂ vibrational energy (100 cm^—1 for level separation around v′=15). Quenching of the fluorescence by the above foreign gases and by I₂ itself was found to be proportional to the product of the polarizability of the inert molecules and the reciprocal of the relative velocity of the colliding particles. Absolute radiative transition probabilities for a number of individual rotational lines in the I₂ visible spectrum have also been determined.