Dephasing Of Wave Packets Research Articles

Criteria for the Classification of System and Bath Variables in Nonadiabatic Dynamics: Application to Pyrazine.

The present work deals with the system-bath separation of the vibrational modes involved in a nonadiabatic system. System modes are the strongly interacting modes which dominate the overall dynamics and, hence, need a near-exact treatment. Bath modes have relatively weaker couplings and can be approximately treated. Thus, the exponential bottleneck involved in computations is controlled by the size of the system-subspace. The aim of this work is to put forward a set of criteria that provides clear guidelines for choosing the system degrees of freedom. The extent of wave packet dephasing caused by repeated crossings across the surface of curve-crossing is the basis for distinction between system and bath modes. The mechanisms of the wave packet dephasing and the criteria are discussed in detail. Numerically converged results presented for the 24-mode pyrazine and 3-mode spin-boson model validate the efficiency of these criteria.

Field-free molecular orientation enhanced by two dual-color laser subpulses

In this paper, we theoretically show that the field-free molecular orientation created by a single dual-color laser pulse can be significantly enhanced by separating it into two time-delayed dual-color subpulses. It is indicated that the maximum enhancement of the molecular orientation created by two time-delayed dual-color subpulses can be achieved with their intensity ratio of about 1:2 and by simultaneously applying the second one at the beginning of the rotational wave packet rephasing or the end of the rotational wave packet dephasing induced by the first one. It is also shown that the enhancement or suppression of the molecular orientation can be coherently manipulated by varying the relative phase between the fundamental field and its second harmonic field of the second dual-color subpulse, and its enhancement is obtained around half rotational period.

Field-free molecular orientation by a multicolor laser field

We theoretically study the field-free molecular orientation by a multicolor laser field with a superposition of the fundamental wave and its harmonics. It is shown that the molecular orientation will be up to a maximum value pumped by the four-color laser field at the same laser intensity, and the molecular orientation direction and its degree can be controlled by varying the carrier-envelope phase of the four-color laser field. It is also indicated that the molecular orientation induced by a four-color laser field can be tremendously enhanced by applying another in-phase or out-phase four-color laser field at the beginning of the rotational wave-packet rephasing or the end of the rotational wave-packet dephasing.

The interplay of wave packet dephasing, optimization efficiency, and target state population in optimally controlled isotope selective photoionization

The mutual dependencies of characteristic quantities for an isotope selective photoionization, namely optimization efficiency, target state population, and wave packet dephasing are presented for the NaK dimer. A pre-optimized pulse shape obtained from the maximization of the isotopomer ratio 23Na 39K/ 23Na 41K for the first excited electronic state serves as an initial guess for the subsequent optimization. For the ionization it provides almost vanishing population of the heavier ionic isotopomer and an ionic isotopomer ratio which is significant higher than the findings obtained from former investigations. The wave packet motions on the first excited state of the neutral molecule are in phase for both isotopomers. The optimization procedure, i.e. the simultaneous maximization of the 23Na 39K + photoionization yield and the minimization of the 23Na 41K + photoionization yield for the electronic ground state is based on optimal control theory and leads to a rise of ionic ground state population for both isotopomers related to a significant wave packet dephasing. Upon optimization the isotopomer ratio of the yields falls due to incidental resonances of vibronic transitions. The interplay of the observables are discussed and compared with the values obtained from an optimization which was started from a pure Gaussian pulse shape.

Suppression of Dephasing due to a Trapping Potential and Atom-Atom Interactions in a Trapped-Condensate Interferometer

We propose and demonstrate a novel trapped-condensate interferometer using optical Bragg diffractions in a harmonic magnetic potential, which can realize a long coherence time with low dephasing. Dephasing of wave packets due to the magnetic potential is canceled by setting the interrogation time equal to the oscillation period of the harmonic potential. The harmonic potential also helps to suppress dephasing due to condensate atom-atom interactions. An interference signal with a fringe contrast of 30% is observed at an interrogation time of 58 ms. For a longer interrogation time about 100 ms, the spatial coherence of the condensate is still maintained with low dephasing, although the interference fringe is washed out by external vibrational noise.

Decoherence and dephasing in strongly driven colliding Bose-Einstein condensates

We report on a series of measurements of decoherence and wave-packet dephasing between two colliding, strongly coupled, identical Bose-Einstein condensates. We measure, in the strong-excitation regime, a suppression of the mean-field shift, compared to the shift which is observed for a weak excitation. This suppression is explained by applying the Gross-Pitaevskii energy functional. By selectively counting only the nondecohered fraction in a time-of-flight image we observe oscillations for which both inhomogeneous and Doppler broadening are strongly suppressed. If no post-selection is used, the decoherence rate due to collisions can be extracted and is in agreement with the local density average calculated rate.

Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag4 clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag4 -/Ag4/Ag4 + system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag4. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm-1 and an anharmonicity χ e2νχ eχ e of 2.65 cm-1 for the active vibrational mode.

Femtosecond interferometric photoacoustic spectroscopy using incoherent light

An interferometric phase-modulated femtosecond wave packet technique has been combined with photoacoustic detection to study rovibrational wave packets, using 30 ps (full-width-at-half-maximum) incoherent light pulses. The phase-modulated technique is particularly suited to be combined with photoacoustic detection, thanks to the adaptability of the modulation frequency of the light pulses to correspond to a resonance frequency of the photoacoustic cell. As an example to demonstrate the principle, the B-state of iodine has been chosen. The average 330 fs wavepacket period of the vibrational levels 12–15 and wave packet dephasing have been observed, identical to those which would be expected if transform-limited, 100 fs fwhm pulses had been used. It is shown that the combination of these detection techniques results into a very sensitive method for performing time-resolved experiments.

Femtosecond time-resolved zero kinetic energy photoelectron and photoionization spectroscopy studies of I2 wavepacket dynamics

We report the application of femtosecond zero kinetic energy (ZEKE) photoelectron spectroscopy and photoionization to the study of bound wavepacket motion in the electronic B state of iodine by a [1 + 2] multiphoton process. The experimental approach is described in detail. Comparisons of ZEKE photoelectron detection with ion detection are made. The pump wavelength was fixed at 580 nm whereas the probe wavelength was tuned from 350 to 300 nm. Wavepacket motion in the B state with the fundamental vibrational period of 340 fs was observed, as well as wavepacket dephasing and rephasing. Differences in the modulation depths and Fourier transform power spectra were observed. The phase shifts (time offset from Δt = 0) and modulation patterns varied as the location of the Condon point for the probe step changed upon tuning the probe wavelength. Higher-order wavepacket fractional revivals were observed in time-windowed Fourier transform power spectra.

Ultrafast quantum dynamics and resonance Raman spectroscopy of photoexcited I2(B) in large argon and xenon clusters

The early quantum dynamics following the B(3Π0u+)←X photoexcitation of I2 in large rare gas clusters is studied and the resonance Raman spectrum of these systems is calculated by a novel time-dependent quantum mechanical simulation approach. The method used is the classically based separable potential (CSP) approximation, in which classical molecular dynamics simulations are used in a first step to determine an effective time-dependent separable potential for each mode, then followed by quantum wavepacket calculations using these potentials. In the simulations for I2(Ar)n and I2(Xe)n, with n=17, 47, all the modes are treated quantum mechanically. The Raman overtone intensities are computed from the multidimensional time-dependent wavepacket for each system, and the results are compared with experimental data on I2 in Ar matrices and in liquid Xe. The main findings include: (i) Due to wavepacket dephasing effects the Raman spectra are determined well before the iodine atoms hit the rare gas ‘‘wall’’ at about 80 fs after photoexcitation. (ii) No recurrencies are found in the correlation functions for I2(Ar)n. A very weak recurrence event is found for I2(Xe)n. (iii) The simulations for I2(Ar)17 (first solvation layer) and for I2(Ar)47 (second solvation shell) show differences corresponding to moderate cluster size effects on the Raman spectra. (iv) It is estimated that coupling to the B″(1Π1u) state or to the a(1g) state have a small effect on the Raman intensities. (v) For I2(Ar)47, the results are in very good quantitative agreement with I2/Ar matrix experiments. The I2(Xe)n results are in qualitative agreement with experiments on I2 in liquid Xe. The reported calculations represent a first modeling of resonance Raman spectra by quantum dynamical simulations that include all degrees of freedom in large systems, and they demonstrate the power of the CSP method in this respect.

Femtosecond wave-packet dynamics studied by time-resolved zero-kinetic energy photoelectron spectroscopy

Femtosecond pump–probe zero-kinetic-energy (ZEKE) photoelectron spectroscopy is studied using the known wave-packet dynamics of I2 (B state). The 340 fs wave-packet period, wave-packet dephasing and rephasing are observed in the ZEKE signal. The effect of various laser and ZEKE parameters on the wave-packet dynamics is discussed.