Molecular dump processes induced by chirped laser pulses (original) (raw)

Femtosecond pulses and dynamics of molecular photoexcitation: RbCs example

Physical Review A, 2012

We investigate the dynamics of molecular photoexcitation by unchirped femtosecond laser pulses using RbCs as a model system. This study is motivated by a goal of optimizing a two-color scheme of transferring vibrationally-excited ultracold molecules to their absolute ground state. In this scheme the molecules are initially produced by photoassociation or magnetoassociation in bound vibrational levels close to the first dissociation threshold. We analyze here the first step of the two-color path as a function of pulse intensity from the low-field to the high-field regime. We use two different approaches, a global one, the 'Wavepacket' method, and a restricted one, the 'Level by Level' method where the number of vibrational levels is limited to a small subset. The comparison between the results of the two approaches allows one to gain qualitative insights into the complex dynamics of the high-field regime. In particular, we emphasize the non-trivial and important role of far-from-resonance levels which are adiabatically excited through 'vertical' transitions with a large Franck-Condon factor. We also point out spectacular excitation blockade due to the presence of a quasi-degenerate level in the lower electronic state. We conclude that selective transfer with femtosecond pulses is possible in the low-field regime only. Finally, we extend our single-pulse analysis and examine population transfer induced by coherent trains of low-intensity femtosecond pulses.

Control of Molecular Fragmentation Using Shaped Femtosecond Pulses

The Journal of Physical Chemistry A, 2008

The possibility that chemical reactions may be controlled by tailored femtosecond laser pulses has inspired recent studies that take advantage of their short pulse duration, comparable to intramolecular dynamics, and high peak intensity to fragment and ionize molecules. In this article, we present an experimental quest to control the chemical reactions that take place when isolated molecules interact with shaped near-infrared laser pulses with peak intensities ranging from 10 13 to 10 16 W/cm 2 . Through the exhaustive evaluation of hundreds of thousands of experiments, we methodically evaluated the molecular response of 16 compounds, including isomers, to the tailored light fields, as monitored by time-of-flight mass spectrometry. Analysis of the experimental data, taking into account its statistical significance, leads us to uncover important trends regarding the interaction of isolated molecules with an intense laser field. Despite the energetics involved in fragmentation and ionization, the integrated second-harmonic generation of a given laser pulse (I SHG ), which was recorded as an independent diagnostic parameter, was found to be linearly proportional to the total ion yield (I MS ) generated by that pulse in all of our pulse shaping measurements. Order of magnitude laser control over the relative yields of different fragment ions was observed for most of the molecules studied; the fragmentation yields were found to vary monotonically with I MS and/or I SHG . When the extensive changes in fragmentation yields as a function of I MS were compared for different phase functions, we found essentially identical results. This observation implies that fragmentation depends on a parameter that is responsible for I MS and independent from the particular time-frequency structure of the shaped laser pulse. With additional experiments, we found that individual ion yields depend only on the average pulse duration, implying that coherence does not play a role in the observed changes in yield as a function of pulse shaping. These findings were consistently observed for all molecules studied (p-, m-, o-nitrotoluene, 2,4-dinitrotoluene, benzene, toluene, naphthalene, azulene, acetone, acetyl chloride, acetophenone, p-chrolobenzonitrile, N,N-dimethylformamide, dimethyl phosphate, 2-chloroethyl ethyl sulfide, and tricarbonyl-[η5-1-methyl-2,4-cyclopentadien-1-yl]manganese). The exception to our conclusion is that the yield of small singly-charged fragments resulting from a multiple ionization process in a subset of molecules, were found to be highly sensitive to the phase structure of the intense pulses. This coherent process plays a minimal role in photofragmentation; therefore, we consider it an exception rather than a rule. Changes in the fragmentation process are dependent on molecular structure, as evidenced in a number of isomers, therefore femtosecond laser fragmentation could provide a practical dimension to analytical chemistry techniques.

Coherent control with shaped femtosecond laser pulses applied to ultracold molecules

Physical Review A, 2006

We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the Rb2 molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photo-induced processes in molecular quantum gases.

Solvation Stokes-Shift Dynamics Studied by Chirped Femtosecond Laser Pulses

The early optical dynamic response, resulting population, and electronic coherence are investigated experimentally and modeled theoretically for IR144 in solution. The fluorescence and stimulated emission response are studied systematically as a function of chirp. The magnitude of the chirp effect on fluorescence and stimulated emission is found to depend quadratically on pulse energy, even where excitation probabilities range from 0.02 to 5%, in the so-called "linear excitation regime". Interestingly, the shape of the chirp dependence on fluorescence and stimulated emission is found to be independent of pulse energy. The chirp dependence reveals dynamics related to solvent rearrangement following excitation and also depends on electronic relaxation of the chromophore. The experimental results are successfully simulated using a four-level model in the presence of inhomogeneous broadening of the electronic transitions.

Pulse trains in molecular dynamics and coherent spectroscopy: a theoretical study

New Journal of Physics, 2009

Pulse trains (PTs) generated by sinusoidal phase masks emerged as a popular tool in the field of coherent control and have been applied successfully in many experiments. Although many attempts were made to throw light on the mechanism of the induced processes, it is not yet fully understood. Based on nonperturbative quantum dynamical calculations in the grid representation, we will elucidate the mechanism of PT excitation between anharmonic bound molecular electronic states. We extract general rules for the prediction of induced dynamics and the resulting spectra. The results allow us to outline perspectives for new applications, especially in the field of nonlinear spectroscopy.

Unstable states in laser assisted and controlled molecular processes

Many aspects of intense-field molecular dynamics rely on the concept of resonances. The chapter gives a thorough review of these aspects, bringing out the specificity of laser-induced resonances, in particular those defined in the Floquet or dressed molecule picture. The role of these resonances in the timeresolved dynamics of molecules subjected to an intense, ultrafast laser pulse is discussed and basic mechanisms of molecular fragmentation and its control are reviewed. We discuss how a thorough interpretation of two-colour XUV + IR pump-probe experiments on the dissociative ionization of H 2 can be made in terms of adiabatic vs. non-adiabatic resonance transports (i.e. laser-induced time evolutions) and in terms of field-induced processes such as Bond-Softening (BS) and Vibrational Trapping (VT), associated with the Floquet representation or the Dynamical Dissociation Quenching (DDQ) effects associated with a time-dependent quasi-static representation. Another application of the concepts of laser-induced resonances, and of their adiabatic evolution, is devoted to laser control strategies based on Zero-Width Resonances (ZWR) and Exceptional Points (EP), the approach of which in laser parameter space corresponds to the coalescence of two laser-induced resonances. We illustrate how the concept of ZWR can be useful for the molecular cooling problem. We then show how advantage can be taken of resonance coalescence at an EP to devise new laser control strategies pertaining to vibrational energy transfer processes.

Probing Molecular Dynamics at Attosecond Resolution with Femtosecond Laser Pulses

Physical Review Letters, 2003

The kinetic energy distribution of D + ions resulting from the interaction of a femtosecond laser pulse with D2 molecules is calculated based on the rescattering model. From analyzing the molecular dynamics, it is shown that the recollision time between the ionized electron and the D + 2 ion can be read from the D + kinetic energy peaks to attosecond accuracy. We further suggest that more precise reading of the clock can be achieved by using shorter fs laser pulses (about 15fs).

Selective excitation of diatomic molecules by chirped laser pulses

The Journal of Chemical Physics, 2000

A new method for the selective excitation of diatomic molecules in single vibrational states on excited electronic potentials by two-photon absorption is proposed. The method implies the use of two chirped strong pulse lasers detuned from the optical transition to an intermediate electronic state. We show under what scenarios the method is successful on the time-energy scale in which the pulses operate. They involved a long-time ͑nanosecond͒ weak-field regime and a short-time ͑picosecond͒ strong-field regime. The adiabatic representation in terms of energy levels or in terms of light-induced potentials is used to interpret the physical mechanism of the excitation. The efficiency and robustness of the scheme are demonstrated by the excitation of the ground vibrational state of the 1 ⌺ g (4s) electronic potential of the Na 2 molecule.

Laser control of electronic transitions of wave packet by using quadratically chirped pulses

The Journal of Chemical Physics, 2005

An effective scheme is proposed for the laser control of wave packet dynamics. It is demonstrated that by using specially designed quadratically chirped pulses, fast and nearly complete excitation of wave packet can be achieved without significant distortion of its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura theory of nonadiabatic transition. If the wave packet is not too narrow or not too broad, then the scheme is expected to be utilizable for multidimensional systems. The scheme is applicable to various processes such as simple electronic excitation, pump-dump, and selective bond breaking, and it is actually numerically demonstrated to work well by taking diatomic and triatomic molecules ͑LiH, NaK, H 2 O͒ as examples.