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