Bo Y. Chang

Transferring vibrational population between electronic states of diatomic molecules via light-induced-potential shaping

We investigate two-photon, selective excitation of diatomic molecules with intense, ultrafast laser pulses. The method involves transfer of a vibrational population between two electronic states by shaping of light-induced potentials (LIPs). Creation and control of the LIPs is accomplished by choosing pairs of transform-limited pulses with proper frequency detunings and time delays. Depending on the sequence of pulses (intuitive or counter-intuitive) and on the sign of the detuning (below or above the first transition) four schemes are possible for population transfer by LIP shaping. We develop a simple analytic model to predict the optimal laser pulses, and to model the adiabatic dynamics in the different schemes. Based on a harmonic, three-state model of the sodium dimer we demonstrate numerically that all four schemes can lead to efficient, selective population transfer. A careful analysis of the underlying physical mechanisms reveals the varying roles played by the adiabatic and diabatic crossings of ...

Selective excitation of diatomic molecules by chirped laser pulses

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 (nanosecond) weak-field regime and a short-time (picosecond) 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 1Σg(4s) electronic potential of the Na2 molecule.

Further aspects on the control of photodissociation in light-induced potentials

In this work we show how to control the photodissociation of a diatomic molecule in the frame of light-induced potentials for different shapes of the transition dipole moments. A sequence of a half-cycle or control pulse and a delayed pump pulse is used for achieving state-selective photodissociation with high yields. The effect of the control is to shift the photodissociation bands to higher frequencies. It is also possible to dissociate the molecule in a superposition of electronic states of the fragments, even when the photodissociation bands corresponding to the different electronic states of the products are largely separated. In this case one needs to engineer the sequence delaying the half-cycle pulse after the pump pulse and additionally turning off rapidly the control pulse. Depending on the shape of the dipole functions the duration of the pulses in the sequence must be constrained to shorter times as well. Finally we show that the control scheme affects the velocity of the fragments. Although broad kinetic energy distributions are always obtained when the half-cycle pulse is short, if the Stark effect implies a blueshifting in the energy of the electronic states, the distribution of the relative speed of the fragments will be redshifted.

Bond breaking in light-induced potentials

We study the photodissociation of ICl(-) under moderately strong (TW/cm(2)) and short (below picosecond) laser pulses. Using a single resonant pump pulse, the photodissociation spectra shows two barely overlapping bands corresponding to Frank-Condon excitation and dissociation in two electronic states. By adding a nonresonant stronger control pulse we show that (1) the photodissociation bands can be blueshifted and (2) the asymptotic state of the fragments depends on the chosen pulse sequence. If the pump pulse precedes the control pulse or the control pulse straddles the pump pulse, the outgoing wave packet has components in the two dissociation channels, whereas if the control pulse precedes the pump pulse, the photodissociation proceeds selectively in a single channel.

Adiabatic squeezing of molecular wave packets by laser pulses

Strong pulse sequences can be used to control the position and width of the molecular wave packet. In this paper we propose a new scheme to maximally compress the wave packet in a quasistatic way by freezing it at a peculiar adiabatic potential shaped by two laser pulses. The dynamic principles of the scheme and the characteristic effect of the different control parameters are presented and analyzed. We use two different molecular models, electronic potentials modeled by harmonic oscillators, with the same force constants, and the Na(2) dimer, to show the typical yield that can be obtained in compressing the initial (minimum width) molecular wave function.

Ultrafast control of the internuclear distance with parabolic chirped pulses.

Recently, control over the bond length of a diatomic molecule with the use of parabolic chirped pulses was predicted on the basis of numerical calculations [Chang; et al. Phys. Rev. A 2010, 82, 063414]. To achieve the required bond elongation, a laser scheme was proposed that implies population inversion and vibrational trapping in a dissociative state. In this work we identify two regimes where the scheme works, called the strong and the weak adiabatic regimes. We define appropriate parameters to identify the thresholds where the different regimes operate. The strong adiabatic regime is characterized by a quasi-static process that requires longer pulses. The molecule is stabilized at a bond distance and at a time directly controlled by the pulse in a time-symmetrical way. In this work we analyze the degree of control over the period and elongation of the bond as a function of the pulse bandwidth. The weak adiabatic regime implies dynamic deformation of the bond, which allows for larger bond stretch and the use of shorter pulses. The dynamics is anharmonic and not time-symmetrical and the final state is a wave packet in the ground potential. We show how the vibrational energy of the wave packet can be controlled by changing the pulse duration.

Stationary molecular wave packets at nonequilibrium nuclear configurations

We study different schemes that allow laser controlled adiabatic manipulation of the bond in diatomic molecules by using sequences of nonresonant time-delayed chirped pulses. The schemes rely on adiabatic passage of the vibrational wave packet by laser-induced potential shaping from the ground electronic state to a laser-stabilized dissociative electronic state by two-photon absorption. The degree of control that is possible over the position (bond length) and width (bond spread) of the vibrational wave packet is compared for the different schemes. The dynamics is analyzed detailing the role of the different control knobs and the conditions that allow or break the adiabatic passage.

Oscillating molecular dipoles require strongly correlated electronic and nuclear motion

To create an oscillating electric dipole in an homonuclear diatomic cation without an oscillating driver one needs (i) to break the symmetry of the system and (ii) to sustain highly correlated electronic and nuclear motion. Based on numerical simulations in H2+ we present results for two schemes. In the first one (i) is achieved by creating a superposition of symmetric and antisymmetric electronic states freely evolving, while (ii) fails. In a second scheme, by preparing the system in a dressed state of a strong static field, both conditions hold. We then analyze the robustness of this scheme with respect to features of the nuclear wave function and its intrinsic sources of decoherence.

Bond lengths of diatomic molecules periodically driven by light: the p-LAMB scheme.

A laser scheme using a periodically changing frequency is used to induce oscillations of the internuclear motion, which are quantum analogs of classical vibrations in diatomic molecules. This is what we call the periodic laser adiabatic manipulation of the bond, or p-LAMB scheme. In p-LAMB, the carrier frequency of the laser must vary periodically from the blue to the red of a photodissociation band and backwards, following for instance a cosine-dependent frequency of period τ(c). In the adiabatic regime the dynamics is fully time-reversible. The amplitude of the internuclear oscillation is controlled by the pulse frequency ω(t), while τ(c) determines the duration (or period) of the bond oscillation. In the presence of efficient dipole coupling, the bandwidth of the pulse is the main constraint to the maximum bond stretch that can be obtained. Before the onset of the adiabatic regime the dynamics are more complex, showing dispersion of the vibrational wave packet and anharmonic deformation of the bond. However, the nonadiabatic effects are mostly canceled and full revivals are observed at certain multiples of τ(c).

Selective photodissociation in diatomic molecules by dynamical Stark-shift control

Selective population transfer in electronic states of dissociative molecular systems is illustrated by adopting a control scheme based on Stark-chirped rapid adiabatic passage (SCRAP). In contrast to the discrete N-level system, dynamical Stark shift is induced in a more complex manner in the molecular electronic states. Wavepacket dynamics on the light-induced potentials, which are determined by the detuning of the pump pulse, can be controlled by additional Stark pulse in the SCRAP scheme. Complete population transfer can be achieved by either lowering the energy barrier along the adiabatic passage or placing the initial wavepacket on a well-defined dressed state suitable for the control. The determination of the pulse sequence is sufficient for controlling population transfer to the target state.