André D. Bandrauk

Molecules in laser fields

Current laser technology is capable of generating laser fields from the IR to visible wavelength regions in the form of well-tailored sequences of pulses with controllable phase and envelopes (pulse shape) [1–2]. Such pulses can be used for the efficient preparation of ensembles of atoms or molecules in specific states. This is of considerable interest not only in spectroscopy but also in studies of chemical dynamics [2–4]. Furthermore short pulses allow one to attain electric field strengths e (V cm-1) or equivalently laser intensities I (W/cm2) = ce 2 /8π (c = velocity of light) which are comparable or greater than atomic fields. An important consequence of the progress in this area is that one can greatly enhance radiative transition rates and even ionize molecules. Clearly strong field science and short pulse science are two fields which will become increasingly intertwined. Efficient rapid excitation requires increasingly higher intensities as can be seen from the simple example of a resonantly driven two-level system [6–7].

Exponential split operator methods for solving coupled time‐dependent Schrödinger equations

Coherent excitation of molecules with laser pulses are usually described by coupled time‐dependent linear parabolic partial differential equations, i.e., Schrodinger equations. Numerical solutions of these equations based on splitting (factorization) of the exponential form of the evolution operator or time‐dependent propagator are examined for accuracy of amplitude and phase as a function of various unitary splitting schemes.

Measurement and laser control of attosecond charge migration in ionized iodoacetylene.

Electronic movement flashing into view Numerous chemical processes begin with ionization: the ejection of an electron from a molecule. What happens in the immediate aftermath of that event? Kraus et al. explored this question in iodoacetylene by detecting and analyzing the spectrum of emitted high harmonics (see the Perspective by Ueda). They traced the migration of the residual positively charged hole along the molecular axis on a time scale faster than a quadrillionth of a second. They thereby characterized the capacity of a laser field to steer the holes motion in appropriately oriented molecules. Science, this issue p. 790; see also p. 740 High harmonics reveal fine details of electronic rearrangement in a molecule in the first instants after ionization. [Also see Perspective by Ueda] The ultrafast motion of electrons and holes after light-matter interaction is fundamental to a broad range of chemical and biophysical processes. We advanced high-harmonic spectroscopy to resolve spatially and temporally the migration of an electron hole immediately after ionization of iodoacetylene while simultaneously demonstrating extensive control over the process. A multidimensional approach, based on the measurement and accurate theoretical description of both even and odd harmonic orders, enabled us to reconstruct both quantum amplitudes and phases of the electronic states with a resolution of ~100 attoseconds. We separately reconstructed quasi–field-free and laser-controlled charge migration as a function of the spatial orientation of the molecule and determined the shape of the hole created by ionization. Our technique opens the prospect of laser control over electronic primary processes.

Photodissociation in intense laser fields: Predissociation analogy

Using the dressed molecule picture, it is shown that photodissociation in intense laser fields can be treated as a predissociation induced by the radiative coupling betweem the initial bound state and final dissociative state. Ar2+ is used as an example as it has a large electronic transition moment typical of molecular ions. It is found that at intensities of 1014 W/m2, one can no longer speak of Franck–Condon factors and that one must introduce into the description new adiabatic states created by the intense fields. It is further suggested how these new laser induced electronic states might be detected experimentally.

Improved exponential split operator method for solving the time-dependent Schrödinger equation

Abstract A new method of splitting exponential operators is proposed in the exponential form of the operator solution of the time-dependent Schrodinger equation. The method is shown to be third-order accurate in the time increment. In particular the phase of the wavefunction is shown to be exceptionally accurate for time-independent potentials. The new method is shown to be more efficient than the standard second-order evolution operator algorithms for both time-independent and time-dependent potentials.

Control of molecular vibrational excitation and dissociation by chirped intense infrared laser pulses. Rotational effects

We extend our previous studies on control of dissociation and vibrational excitation of a diatomic molecule using chirped, intense, infrared laser pulses [Phys. Rev. Lett. 65, 2355 (1990)]. The present model includes molecular rotations and a realistic molecular dipole function. The results obtained from numerical integration of the time‐dependent Schrodinger equation show a considerable sensitivity of dissociation probabilities to the initial rotational quantum number. Although rotational effects generally decrease the excitation efficiency compared to previous nonrotating molecule results, the dissociation probability induced by chirped pulses is still four to eight orders of magnitudes greater than that for monochromatic pulse dissociation.

Control of vibrational excitation and dissociation of small molecules by chirped intense inflared laser pulses

Abstract The time-dependent Schrodinger equation describing the interaction of an HCN molecule with intense, ultrashort, chirped infrared laser pulses is solved numerically. The molecule is represented as two coupled Morse oscillators and the chirped laser pulse frequency ω( t ) is adapted to the CH-bond anharmonicity in such a way that the pulse is nearly resonant with the vibration of this bond during the whole excitation process. It is shown that by controlling the chirping rate and the area of the pulse, one can selectively and efficiently excite and dissociate one particular bond and control the excitation of the other bond in a triatomic molecule. Such pulses should become important tools in photochemistry since one can thus prepare non-statistical quantum vibrational states and control the reactivity of a molecule by varying the laser phase.

Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses

We describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H+2 molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrodinger equation (TDSE). In intense bichromatic circularly and linearly or circularly polarized laser pulses with intensity I0 and angular frequencies ω0 and 2ω0, it is found that maximum molecular high-order harmonic generation (MHOHG) energies are functions of the molecular internuclear distance. Based on a classical model of laser-induced electron collisions with neighbouring ions, the optimal values of the pulse relative carrier envelope phase , the molecular internuclear distance R and the angle of molecular alignment to the laser polarization axis are obtained for efficiently producing MHOHG spectra with the maximum harmonic energy Ip + 13.5Up, where Ip is the ionization potential of the molecule and Up = I0/4meω20 is the ponderomotive energy of the continuum electron at intensity I0 and frequency ω0 of the laser pulse. The results have been confirmed from corresponding TDSE nonperturbative numerical simulations. The polarization property of the generated harmonics is also presented. The mechanism of MHOHG is further characterized with a Gabor time frequency analysis. It is confirmed that a single collision trajectory of the continuum electron with neighbouring ions dominates in the MHOHG processes. The high efficiency of the proposed MHOHG scheme provides a possible source for production of elliptically and/or circularly polarized attosecond extreme ultraviolet pulses. Circularly polarized attosecond pulses can also be generated by using intense ultrashort circularly polarized laser pulses in combination with static electric fields of comparable intensity for H+2 at equilibrium. A time frequency analysis also confirms the role of single recollisions as the dominant mechanism of the generation of circularly polarized harmonics.

Raman Chirped Adiabatic Passage: a New Method for Selective Excitation of High Vibrational States

It is demonstrated that efficient and high vibrational excitation can be achieved using non-resonant stimulated Raman transitions for subsequent step by step climbing of vibrational levels. The pump laser (or both the pump and Stokes laser) frequency are to be swept in such a way that the frequency difference sweeping allows molecular anharmonicities of the final states to be matched. It is shown that amplitudes of successive Raman transitions can be quantitatively described with the help of the effective Raman two-level systems. This selective scheme of vibrational excitation, called Raman chirped adiabatic passage (RCAP), should be useful in controlling excited-state populations and chemical reactions. The limitations of another well known adiabatic scheme of population transfer, stimulated Raman adiabatic passage (STIRAP), are outlined and it is shown that RCAP is a complementary method to STIRAP. RCAP should be useful for selective excitation of highly polarizable symmetric bonds such as metal–metal bonds.

Effect of absolute laser phase on reaction paths in laser-induced chemical reactions

Potential surfaces, dipole moments, and polarizabilities are calculated by ab initio methods [unrestricted MP2(full)/6-311++G(2d,2p)] along the reaction paths of the F+CH4 and Cl+CH4 reaction systems. It is found that in general dipole moments and polarizabilities exhibit peaks near the transition state. In the case of X=F these peaks are on the products side and in the case of X=Cl they are on the reactants side indicating an early transition state in the case of fluorine and a late transition state in the case of chlorine. An analysis of the geometric changes along the reaction paths reveals a one-to-one correspondence between the peaks in the electric properties and peaks in the rate of change of certain internal geometric coordinates along the reaction path. Interaction with short infrared intense laser fields pulses leads to the possibility of interferences between the dipole and polarizability laser-molecule interactions as a function of laser phase. The larger dipole moment in the Cl+CH4 reaction can lead to the creation of deep wells (instead of energy barriers) and new strongly bound states in the transition state region. This suggests possible coherent control of the reaction path as a function of the absolute phase of the incident field, by significant modification of the potential surfaces along the reaction path and, in particular, in the transition state region.