Study Of The Molecular Rotational Coherence Induced By Femtosecond Laser Pulses

Femtosecond laser pulses play a significant role in today's scientific research,especially for the atomic and molecular physics.Femtosecond laser,with the property of ultrashort duration,broad spectral bandwidth and high peak intensity,is suitable for studyig the motion of atom,molecule and even electron.The study varies from observing the ultrafast motion of microscopic system to controlling the system and creating desired state as the laser technique advances.The emerging of the pulse shaping technique,the coherent control and the optimal control theory enable us to conduct more specific operation.As a internal degree of molecular motion,the molecular rotational state has been well studied in the past twenty years.Many achievements have been made in areas such as non-adiabatic alignment,the creation of ultracold molecule and the using of polar molecule in quantum information and quantum computation.This dissertation is about observing and manipulating the macroscopic phenomena of the rotational wave packet and the underlying coherence of the rotational states using the femtosecond laser pulse and pulse shaping technique as the tools.The study of the current dissertation mainly includes three topics:(1)The temporal delayed orthogonal pulse pairs generated by the phase shaping technique are used to study the coherent control of the excitation process of the rotational wave packet in air.We extend the traditional rotational coherent spectrum(RCS)into a two-dimensional RCS with the new dimension of the shaping phase.By continuously changing the intrapulse delay of the pump pulse,we measured the corresponding revival signals and obtained a two-dimensional rotational coherent spectrum.An additive property of the rotational dynamics is observed from the revival signals.This is mainly due to the low intensity of the shaped laser pulses.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.This scheme can be used for further control and detection of the rotational wave packet and can be extended to other molecular dynamic researches.(2)The interference is one of the most important property of quantum mechanics,the observation and reconstruction of quantum system are mainly achieved through the wave packet interferometry.While huge amount of work about wave packet interferometry is focused on atomic Rydberg and molecular vibrational state,the study of the rotational wave pakcet interferomety is rarely.We discuss the rotational wavepacket interferometry in weak field regime.The weak field induced path interference of molecular rotational states is demonstrated in this work.By using the revival character of the rotational wave packet,the change of the population in complex system due to the interference can be reflected from the Fourier spectrum.The phase information of the rotational state can also be achieved from the population oscillation due to the interference.This provides us a new experimental scheme for quantum information processing using molecular rotational eigenstate.(3)Recently,The alignment echo is observed from the study of the nonadiabatic field-free alignment and is used to study the relaxation process in dense environments.Different kinds of rotational echoes,for example,fractional alignment echo,multiple alignment echo,rotated alignment echo,and imaginary alignment echo have also been observed and the AE formation is usually explained in the filamented phase space with respect to the angular velocity and polar angle.We present a theoretical investigation to understand the connection between the formation of different rotational alignment echoes and the underlying interaction pathways induced by temporally delayed laser pulses using the doule sided Feynman diagram.The fractional and multiple imaginary alignment echoes are predicted and demonstrated in the linear polar molecules carbonyl sulfide(OCS).We also use a two-dimensional spectrum obtained from a higher-order alignment echo signal to visualize the underlying quantum coherence processes.It provides a way to gain insight into the relationship between the echo signal and the underlying pathway with potential applications in rotational echo spectroscopy.