Controlling molecular rotational population by wave-packet interference

Orientation and Alignment dynamics of polar molecule driven by shaped laser pulses

We review the theoretical status of intense laser induced orientation and alignment-a field of study which lies at the interface of intense laser physics and chemical dynamics and having potential applications such as high harmonic generation, nano-scale processing and control of chemical reactions. The evolution of the rotational wave packet and its dynamics leading to orientation and alignment is the topic of the present discussion. The major part of this article primarily presents an overview of recent theoretical progress in controlling the orientation and alignment dynamics of a molecule by means of shaped laser pulses. The various theoretical approaches that lead to orientation and alignment such as static electrostatic field in combination with laser field(s), combination of orienting and aligning field, combination of aligning fields, combination of orienting fields, application of train of pulses etc. are discussed. It is observed that the train of pulses is quite an efficient tool for increasing the orientation or alignment of a molecule without causing the molecule to ionize. The orientation and alignment both can occur in adiabatic and non-adiabatic conditions with the rotational period of the molecule taken under consideration. The discussion is mostly limited to non-adiabatic rotational excitation (NAREX) i.e. cases in which the pulse duration is shorter than the rotational period of the molecule. We have emphasised on the so called half-cycle pulse (HCP) and square pulse (SQP). The effect of ramped pulses and of collision on the various laser parameters is also studied. We summarize the current discussion by presenting a consistent theoretical approach for describing the action of such pulses on movement of molecules. The impact of a particular pulse shape on the post-pulse dynamics is also calculated and analysed. In addition to this, the roles played by various laser parameters including the laser frequency, the pulse duration and the system temperature etc. are illustrated and discussed. The concept of alignment is extended from one-dimensional alignment to three-dimensional alignment with the proper choice of molecule and the polarised light. We conclude the article by discussing the potential applications of intense laser orientation and alignment.

Tracing the Coherent Manipulation of Rotational Dynamics by Shaped Femtosecond Pulse-Induced Two-Dimensional Rotational Coherent Spectrum

The temporal delayed orthogonal pulse pairs generated by the phase shaping technique are used to study the coherent control of the rotational wave packet dynamics in air. By continuously changing the intrapulse delay of the pump pulse, we measured the corresponding revival signals and obtained a two-dimensional rotational coherent spectrum (2D RCS). An additive property of the rotational dynamics is observed from the revival signals. Moreover, combining with the coherent control model, we find that the 2D RCS can be used to demonstrate the control over the underlying Raman rotational excitation. A beat frequency-dependent oscillation of each rotational transition is obtained. The transition process is revealed from the Fourier transformation about the pump delay. The scheme of this work can be used for further control and detection of the rotational wave packet and can be extended to other molecular dynamic researches.

Reactive scattering dynamics of rotational wavepackets: a case study using the model H+H2 and F+H2 reactions with aligned and anti-aligned H2

We propose a method to steer the outcome of reactive atom-diatom scattering, using rotational wavepackets excited by strong non-resonant laser pulses. Full close-coupled quantum mechanical scattering calculations of the D+H2 and F+H2 reactions are presented, where the H2 molecule exists as a coherent superposition of rotational states. The nuclear spin selective control over the molecular bond axis alignment afforded by the creation of rotational wavepackets is applied to reactive scattering systems, enabling a nuclear spin selective influence to be exerted over the reactive dynamics. The extension of the conventional eigenstate-to-eigenstate scattering problem to the case in which the initial state is composed of a coherent superposition of rotational states is detailed, and a selection of example calculations are discussed, along with their mechanistic implications. The feasibility of the corresponding experiments is considered, and a suitable simple two pulse laser scheme is shown to strongly differentiate the reactivities of o-H2 and p-H2.

Commensurate two-quantum coherences induced by time-delayed THz fields

The interaction of carbonyl sulfide dipolar gas molecules with two time-delayed, single-cycle THz pulses is shown both experimentally and theoretically to induce two-quantum rotational coherences that are significantly enhanced with respect to those induced by one THz pulse, depending on the relative delay of the pulses. The underlying phenomenon is quite general in that it can occur even after a single THz pulse if more than one molecular species is present, since the free induction decay emitted by one species (demonstrated here by atmospheric water vapor) can provide the second field interaction for the other.

CONTROLLING ABOVE-THRESHOLD DISSOCIATION BRANCHING RATIOS OF HD+ WITH FEMTOSECOND LASER PULSE TRAIN

Above-threshold dissociation (ATD) process of the molecular ions HD+ steered by a femtosecond laser pulse train (LPT) is investigated theoretically using the time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by using an asymptotic-flow expression in the momentum space. It is found that fragment kinetic energy spectra shift to low energy region with increasing pulse number of LPT. The photofragment branching ratio between the 1sσg and 2pσu dissociation channels is sensitive to the pulse number of LPT. The momentum distribution of the ATD fragments is discussed in detail.

Rotational coherence imaging and control for CN molecules through time-frequency resolved coherent anti-Stokes Raman scattering

Numerical wave packet simulations are performed for studying coherent anti-Stokes Raman scattering (CARS) for CN radicals. Electronic coherence is created by femtosecond laser pulses between the X(2)Σ and B(2)Σ states. Due to the large energy separation of vibrational states, the wave packets are superpositions of rotational states only. This allows for a specially detailed inspection of the second- and third-order coherences by a two-dimensional imaging approach. We present the time-frequency domain images to illustrate the intra- and intermolecular interferences, and discuss the procedure to rationally control and experimentally detect the interferograms in solid Xe environment.

Alignment structures of rotational wavepacket created by two strong femtosecond laser pulses

We experimentally and theoretically study the alignment structures of the rotational wavepacket created by linear molecules and two strong femtosecond laser pulses. In the experiment, we observe that the alignment structures depend on the time delay between the two laser pulses. In the theory, we find that the alignment structures are composed of the self-coupling term and the cross-coupling term. The contributions of these two terms are separately calculated. Their coherent superposition reproduces the alignment structures observed in the experiment.

Molecular rotational excitation by strong femtosecond laser pulses

We study the rotational wave packet created by nonadiabatic rotational excitation of molecules with strong femtosecond laser pulses. The applicable condition of the Delta-Kick method is obtained by comparing the laser intensity and pulse duration dependences of the wave packet calculated with different methods. The wave packet evolution is traced analytically with the Delta-Kick method. The calculations demonstrate that the rotational populations can be controlled for the rotational wave packet created by two femtosecond laser pulses. The evolution of the rotational wave packet with controlled populations produces interference patterns with exotic spatial symmetries. These calculations are validated by comparing the theoretical calculations with our experimental measurements for the rotational wave packet created by thermal ensemble CO(2) and two strong femtosecond laser pulses. Potential applications in molecular science are also discussed for the rotational wave packet with controlled populations and spatial symmetries.