Ultrashort Laser Pulse Phenomena, 2nd Edition

Ultrafast Processes in Molecules 543

by a fs photodissociation pulse producing a pair of reagents. Such a technique

has been applied to the reaction [26]

H +OCO →[HOCO]

‡

→ OH + CO. (11.5)

The van der Waals “precursor molecule” was [IH ···OCO] formed in a free-

jet expansion of a mixture of HI and CO

2

in an excess of helium carrier gas.

To clock the reaction, an ultrashort laser pulse photodissociates HI, ejecting an

H atom toward the O atom of the CO

2

. The delayed probe detects the formation

of OH. Such an experiment establishes clearly that the reaction proceeds via an

intermediate state, as shown in Eq. (11.5) and gives values for the lifetime of the

intermediate complex [HOCO]

‡

.

Femtosecond techniques are not limited to the observation of chemical reac-

tions, but can even be exploited to influence the course of the reaction (see, for

instance, Potter et al. [27]). This can open new relaxation channels or increase

the yield of certain reaction products.

11.3.3. Molecules in Solution

Considerable progress has been made toward the microscopic understand-

ing of molecular vibration and chemical reactions in solution. For instance, we

have shown at the beginning of this section techniques to study the wave packet

dynamics of the nuclear motions of iodine in the B state, in the collision-free

limit. These techniques can be applied to solutions of different densities and liq-

uids. The primary effects of the solvent on the fs wave packet are dephasing,

energy relaxation, caging, and recombination [28]. Except for collision-induced

rapid nonradiative transitions in the liquid state, which cause the main fluores-

cence emission to originate from a lower transition, the experimental techniques

are similar to the one used in the gaseous phase.

Femtosecond techniques have also been applied to more complex chemical

problems, such as the study of photodissociation. The influence of the solvent on

the dynamics of photodissociation of ICN can be dramatic [29]. The knowledge

gained of how the solvent influences the decay of photofragment translation and

rotation is useful in understanding the dynamics of thermally activated chemical

reactions [29]. Theoretical simulations have indeed shown that the fluctuations

in reactant and product translational and rotational motions of thermally activated

reactions proceed on the fs scale [30].