Diode laser based studies of the UV photolysis of molecular iodine.

The photolysis of molecular iodine at 193 and 248 nm has been studied by diode laser based frequency-modulated (FM) absorption spectroscopy with detection of the nascent iodine photofragment via the I((2)P(1/2)-(2)P(3/2)) transition at 1.315 microm. Use of narrow band radiation enables nascent measurements with sufficient speed resolution to allow both the character of the initial electronic transition and speed of the fragments to be determined. The time dependence of the integrated area of the measured Doppler profiles has been used to determine both the I* quantum yield and the collisional electronic quenching rate constant of I* (I* = (2)P(1/2)) by I(2). These values are also determined using the diode laser gain versus absorption technique. In the 248 nm case an I* quantum yield of 0.45 +/- 0.04 and 0.42 +/- 0.04 is found by each method, respectively, and an electronic quenching rate constant of (3.6 +/- 0.5) x 10(-11) cm(3) molecule(-1) s(-1), consistent with literature, is determined. The form of the nascent Doppler profile indicates that excitation to a Omega = 1 state dominates, with subsequent dissociation to I((2)P(1/2)) + I((2)P(3/2)), in keeping with assignment of the upper state as 1441 (3)Sigma(+)(1(u)). The deviation from Phi(I*) = 0.5 can be attributed to a contribution from the 1441 (3)Sigma(+)(0(u)(-)) state which dissociates to two ground state iodine atoms. 193 nm excitation exhibits more complicated dynamics and kinetics, including a pressure dependent I* quantum yield. At the low pressures,