Dynamics of I*(2p1/2) production in the UV photodissociationof alkyl iodides

Abstract

The photodissociation dynamics of alkyl iodides in their A?band of absorption (arising from n ? ?* transition) has been of interest for dynamicists for the past three decades, starting from the pioneering work of Kasper and Pimentel in 1964. Moreover, the study of photodissociation of a series of alkyl iodides offers an opportunity to understand the effect of alkyl radical size, structure and substitution on the dynamics. Here we present the results of photodissociation experiments performed on a series of alkyl iodides at four different wavelengths in the A?band. The dynamics of I* production during the dissociation process has been probed by the two?photon laser?induced fluorescence technique. A general introduction for photodissociation along with the various techniques to probe the photofragments is given in the first chapter. At the end of the chapter, the importance of the present work is highlighted. The photodissociation experiments are performed in the gas phase at a very low pressure (a few mTorr) and the nascent fragments are probed. This necessitates a whole set?up consisting of a high vacuum chamber, narrow?band tunable pump–probe lasers and fast signal detection & processing systems which are described in Chapter 2. In Chapter 3, the photodissociation dynamics of n?alkyl iodides is described. We have studied up to n?pentyl iodide and the quantum yield of I* production, i.e., ?* = [I*]/[I* + I] has been measured for the alkyl iodides to understand the effect of alkyl radical size on dynamics. We have compared our values with those of others. Finally, we have made an attempt to trace the potential energy surfaces (PESs) of methyl, ethyl and n?propyl iodides in order to explain the trend in ?* at 222 and 266 nm using a simple Landau–Zener curve?crossing model. The structural effects on the dynamics have been probed by dissociating a series of ?? and ??branched alkyl iodides at various wavelengths. Once again the PESs of the ??branched alkyl iodides are obtained at the 3?21G/CI level and the ?* values are calculated at the crossing points of the surfaces by applying a Landau–Zener model. This is discussed in detail in Chapter 4. To summarize, alkyl iodides on dissociation produce more of I* throughout the A?band except near the red wing. With increase in chain length the ?* value decreases in the case of n?alkyl iodides. The higher ??branched homologs produce mainly I, irrespective of the dissociation wavelength. There is no significant change in ?* in ??branched alkyl iodides compared to the corresponding n?alkyl iodides. However, the ??branching seems to alter the dynamics of I* production drastically, indicating that only high?frequency modes which couple with the C–I bond strongly are important in the time scale of direct dissociation.