Jianwei Che

Quantum control of I2 in the gas phase and in condensed phase solid Kr matrix

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 ...

Optimal pump-dump control

Abstract A density matrix formulation of quantum control is developed to treat the joint optimization of two ultrafast, phase-unlocked laser fields in the weak response and strong response regimes. For the situation in which the frequency spectra of the two fields do not overlap, a coupled pair of control equations can be derived. These equations are then solved iteratively to determine the pair of pump-dump fields that best drives a material system to a desired objective. A novel method is presented to perform an ensemble average over the random phase between the two control fields. As a numerical example of the formalism, we present calculations on the pump-dump control of vibrational wave packets in the iodine molecule in gas phase. We show that a selected, highly excited vibrational wave packet can be created on the ground electronic state with simple, experimentally achievable laser pulses. Such wave packets might be used as initial states for the subsequent control of chemical processes, or spectroscopic observables.

A classical time-frequency theory of transient absorption spectroscopy

A method of quasiclassical dynamics simulation of transient pump-probe spectra is developed based on an exact time-frequency interference picture for the nonlinear spectroscopy in condensed phases. The present approach allows a proper treatment of the effect of light coherence and interference on material dynamics, and is applicable to fields with arbitrary time scales and profiles. Both the sequential and the coherent contributions to the spectral signal are considered. The proposed numerical approach is suitable to simulate the experimental spectra of molecular systems with many degrees of freedom. A numerical demonstration is presented for the transient laser induced fluorescence (LIF) of I2 molecule at 320 K excited by a pair of pulses.

TIME-FREQUENCY THEORY OF PUMP-PROBE ABSORPTION SPECTROSCOPY

A variation of density matrix formulation based on the nature of field–matter interference in a mixed time–frequency domain is developed to study molecular pump-probe absorption spectra in condensed phases. Considered are both the integrated probe transmitant signals and the frequency-dispersed transient absorption coefficients for molecular systems with either two or three electronic surfaces involved. The present formulation is exact and applicable to any field with arbitrary timescale and shape, and it is valid when the pulses are overlapped as well as separated. The dual temporal-spectral effect of both the excitation and detection fields can be clearly elucidated via a transformation that preserves causality. The resulting field–matter interference picture of transient absorption in the correlated time-frequency domain is conceptually natural and physically transparent. The molecular dynamics, and the field–matter temporal/spectral coherence and interference phenomena can all be clearly demonstrated.

Quantum Control from the Gas to Condensed Phase

Chirped ultrashort light pulses are used to focus vibrational wavepackets in gas and condensed phase systems and to control the yield of a chemical reaction.