Using theory to guide the choice of pulse shape, we have synthesized frequency-chirped laser pulses and used them to control the evolution of vibrational wave packets on the 8 excited state of iodine.A negatively chirped pulse produces a wave packet at the The spread of the target in position and momentum is given by the formulas Ax = Qh/2m' and Ap = /mesh/2, where m is the reduced mass of the iodine molecule and the parameter co (units of frequency) was chosen to correspond to 250 cm '. The target is centered about a chosen position (0.372 nm) and a chosen momentum (corresponding to a kinetic energy of 50 meV).In this case, the momentum is negative, meaning that the iodine atoms are moving toward each other. The target is achieved by using a tailored, ultrashort light field to create an initial wave packet that evolves under the influ- state at a later time. The wave packet evolution is monitored by exciting the wave packet to the higher-lying e state with a second ultrashort pulse which opens up a "window" in internuclear distance (shown schematically by the striped box).The experimental signal is the total laser-induced fluorescence (LIF) from the e state as a function of delay between the pump and probe pulses, allowing detection of the wave packet density in a region of internuclear distance selected by the probe wavelength.ence of the molecular Hamiltonian to yield a wave packet that has maximum overlap with the target distribution at a later time. To reach the target in a reasonable amount of time, the electric field is allowed to be nonzero only during a temporal interval of our choice (550 fs, in the present case). Thus, the task of theory is to find the electric field F(t) that best steers the system to our chosen target at the chosen time. We note that to achieve this target, the optimal field must overcome the wave packet spreading which ordinarily occurs in anharmonic potentials. 33600031-9007/95/74(17)/3360(4)$06. 00

We present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase I2 and for I2 embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly tailored ultrafast light field can create a localized vibrational wavepacket which persists significantly longer than that from a general non-optimal ultrafast light field. The results show that, under suitable conditions, quantum control of vibrational motion is indeed possible in condensed media. Such control of vibrational localization may then provide the basis for controlling the outcome of chemical reactions.

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Recent experiments by Herek, Materny and Zewail [Chem. Phys. Lett. 1994, 228, 15] have demonstrated that the timing between two transform-limited, ultrafast laser pulses can be used to control the branching ratio of Na* (electronically excited atomic sodium) to Na in the photodissociation of NaI. In this work, we theoretically show that, by varying the linear chirp of the first pulse without changing its power spectrum or field strength versus time, the Na* to Na branching ratio can be controlled over a large range with a fixed interpulse delay time and a fixed form of the second pulse. Theory predicts that at 0 K the branching ratio can be varied by a factor of 3, while at high temperatures (1000 K), the factor drops to approximately 1.2 due to the effect of the wide distribution of initial states. Experimental results at 1000 K are presented and are found to be consistent with theory. Several possible experimental methods are discussed to overcome the effects of the thermal distribution of initial states.

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