Shu-Lin Cong

Control of photodissociation and photoionization of the NaI molecule by dynamic Stark effect

The diabatic photodissociation and photoionization processes of the NaI molecule are studied theoretically using the quantum wave packet method. A pump laser pulse is used to prepare a dissociation wave packet that propagates through both the ionic channel (NaI-->Na(+)+I(-)) and the covalent channel (NaI-->Na+I). A Stark pulse is used to control the diabatic dissociation dynamics and a probe pulse is employed to ionize the products from the two channels. Based on the first order nonresonant nonperturbative dynamic Stark effect, the dissociation probabilities and the branching ratio of the products from the two channels can be controlled. Moreover the final photoelectron kinetic energy distribution can also be affected by the Stark pulse. The influences of the delay time, intensity, frequency, and carrier-envelope phase of the Stark pulse on the dissociation and ionization dynamics of the NaI molecule are discussed in detail.

Field-free molecular orientation with terahertz few-cycle pulses

We demonstrate theoretically an efficient field-free orientation in LiH and LiCl driven by available terahertz few-cycle pulses (TFCPs). Exact results by numerically solving the time-dependent Schrodinger equation including the vibrational and rotational degrees of freedom are compared to the rigid-rotor approximation (RRA) as well as to the impulsive approximation (IA), and the effect of rotational-vibrational coupling on the both RRA and IA is examined in detail. We find that the current available TFCPs may overcome the technical limitation of terahertz half-cycle pulse for enhancing the field-free molecular orientation.

Femtosecond stimulated Raman scattering for polyatomics with harmonic potentials: application to rhodamine 6G.

The perturbation theory of stimulated Raman scattering (SRS), with Raman pump on minus pump off and heterodyne detection along the probe direction, is reviewed. It has four third-order polarization terms, labeled as SRS or inverse Raman scattering (IRS): SRS(I), SRS(II), IRS(I), and IRS(II). These four polarizations have a wave packet interpretation. The polarizations, with homogenous and inhomogeneous broadening included, can be written as integrals over four-time correlation functions, and analytic formulas are derived for the latter for multidimensional harmonic potential surfaces with Franck-Condon displacements in the modes which facilitates the calculation of the SRS cross sections. The theory is applied to understand recent experimental results on the femtosecond SRS (FSRS) of a fluorescent dye, rhodamine 6G (R6G), where the Raman pump pulse is about 1 ps long, and the probe pulse is about 10 fs. The calculations compared very well with the R6G experimental results for off-resonance and resonance FSRS spectra spanning both Stokes and anti-Stokes bands, and for negative and positive pump-probe delay times on resonance.

Photodissociation dynamics of the S2 state of CH3ONO: State distributions and alignment effects of the NO (X2Π) photofragment

The photodissociation dynamics of methyl nitrite (CH3ONO) was studied using 266 nm laser photolysis, and NO photofragments X 2∏ ν″=0,1,2,3) were probed by single photon laser-induced fluorescence spectroscopy. The ground vibrational state of the NO was found to be most populated, and the rotational distributions of each vibrational level were quite hot. The alignment A0(2) between the electronic transition dipole moment involved in the absorption of the parent molecule and the rotational angular momentum J of the photofragment NO (v″=0) was measured. Polarization experiments showed that the rotational angular momentum of NO was aligned perpendicularly to the transition moment of the parent molecule. The negative A0(2) values and a preferential population of the ∏− doublet state revealed that the dissociation process has some characteristics of in-plane dissociation.

Steering dissociation of Br2 molecules with two femtosecond pulses via wave packet interference

The dissociation dynamics of Br2 molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from X 1Sigma g+ to A 3Pi 1u+ and 1Pi 1u+ and parallel transition from X 1Sigma g+ to B 3Pi 0u+, involving two product channels Br (2P3/2)+Br (2P3/2) and Br (2P3/2)+Br* (2P1/2), respectively, are taken into account. Two pump pulses create dissociating wave packets interfering with each other. By varying laser parameters, the interference of dissociating wave packets can be controlled, and the dissociation probabilities of Br2 molecules on the three excited states can be changed to different degrees. The branching ratio of Br*/(Br+Br*) is calculated as a function of pulse delay time and phase difference.

Optimal control of charge transfer for slow H+ + D collisions with shaped laser pulses.

We show that optimally shaped laser pulses can beneficially influence charge transfer in slow H(+)+D collisions. Time-dependent wave packet optimal control simulations are performed based on a two-state adiabatic Hamiltonian. Optimal control is performed using either an adaptive or a fixed target to obtain the desired laser control field. In the adaptive target scheme, the target state is updated according to the renormalized fragmentary yield in the exit channel throughout the optimization process. In the fixed target scheme, the target state in the exit channel is a normalized outgoing Gaussian wave packet located at a large internuclear separation. Both approaches produced excellent optimal outcomes, far exceeding that achieved in the field-free collisional charge transfer. The adaptive target scheme proves to be more efficient, and often with complex final wave packet. In contrast, the fixed target scheme, although more slowly convergent, is found to produce high fidelity for the desired target wave packet. The control mechanism in both cases utilizes bound vibrational states of the transient HD(+) complex.

Collisional quantum interference effect on rotational energy transfer in an atom-diatom system

Abstract The theoretical model of collisional quantum interference (CQI) in intramolecular rotational energy transfer is described in an atom–diatom system, based on the first Born approximation of time-dependent perturbation theory and considering a long-range interaction potential. The relation between differential and integral interference angles is obtained. For the CO A 1 Π(v=0)/ e 3 Σ − (v=1) –He collision system, the calculated integral interference angles are consistent with the experimental values. The physical significance of interference angle and the essential factors it depends on as well as the influence of the short-range interaction on CQI are discussed.

Selecting ionization path by dynamic stark shift with strong laser pulse

Abstract Multiphoton ionization of NO via intermediate Rydberg states with ultra-short laser pulses is investigated with time-resolved photoelectron spectroscopy in combination with femtosecond pump–probe technology. The Rydberg states of NO, which are characterized by obvious ac-Stark shift in ultra-strong laser field, can be tuned in resonance to ionize NO molecule at one’s will with identical laser pulses, i.e., one can ‘select’ resonance path to ionization. The results shown in this Letter demonstrate that the states holding notable dynamic Stark shift provide us another dimension to chemical control with strong laser field.

The carrier-envelope phase dependence of above threshold dissociation for HD+ driven by the modulated laser field

The dependence of above threshold dissociation (ATD) dynamics of HD+ on the carrier-envelope phase (CEP) of the modulated laser pulse is studied theoretically by numerically solving the time-dependent Schrodinger equation including the molecular vibrational and rotational degrees of freedom. The energy-dependent spectra of dissociated fragments, resulting from the ATD, are calculated by using an asymptotic-flow expression in the momentum space. We find that the ATD spectra are dependent on the CEP , indicating that the ATD dynamics can be efficiently controlled by varying the CEP . Moreover, we examine the ATD dynamics for different intensities and CEPs of the modulated laser pulse. The ATD dynamics process is interpreted in terms of the light-dressed potential.

Resonance-enhanced above-threshold ionization of polar molecules induced by ultrashort laser pulses

The dynamics of resonance-enhanced above-threshold ionization (REATI) is theoretically investigated for polar molecules exposed to ultrashort laser pulses. Numerical calculations are performed for a three-state NaK molecule including two bound states and an ionization continuum. It is found that, due to the effect of a permanent dipole moment, an enhancement of ATI can be achieved. The visible Autler–Townes (AT) splitting of the ATI spectrum induced by a sufficiently rapid Rabi oscillation is observed. Moreover, the effect of other states on the REATI process is discussed based on a four-state REATI model including three bound states and an ionization continuum.