Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. II. Asynchronous concerted elimination of I2 from CH2I2

Abstract

The photoinduced molecular detachment dynamics of CH2I2 have been investigated with femtosecond time resolution. Upon multiphoton excitation of CH2I2 with 312 nm femtosecond pulses, weak fluorescence in the 260–290 nm region was observed in addition to the I2 fluorescence in the 290–345 nm region studied in the previous paper. The weak fluorescence has also been interpreted as due to emission from I2, where I2 was produced from the photodissociation process CH2I2→CH2+I2*. In order to investigate the detailed dynamics of this reaction, femtosecond time-resolved data have been obtained by selective detection of the I2 fluorescence at 272 and 285 nm. From these transients, it has been found that the dissociation process takes place within the temporal width (50 fs) of the laser pulse and that the I2 photofragments exhibit coherent vibrational motion. The 272 nm transients also exhibit clear, fast decaying rotational anisotropy, quantitative analysis of which reveals a distribution of rather high rotational levels of I2. This permits us to conclude that the I2 detachment is an asynchronous concerted process; while breaking of the two C–I bonds and formation of the I–I bond happen in a single kinetic step, one of the C–I bonds breaks faster than the other. In addition, energy partitioning between the CH2 and I2 photofragments has also been explored based on the experimental observations. Since this study involves a multiphoton transition, a theoretical formulation for the time dependent rotational anisotropy is presented for the general case of multiphoton pump and multiphoton probe transitions.