Szczepan Chelkowski

Two-step Coulomb explosions of diatoms in intense laser fields

A two-step model of Coulomb explosions of diatoms in intense laser fields is presented. In this model the molecule loses several electrons when the atoms are at the equilibrium internuclear distance and then fast Coulomb explosions occur until the products reach a critical distance Rc approximately=9 Bohr, at which several additional electrons are lost due to a recently discovered maxima of ionization rates occurring at R=Rc. Then the subsequent Coulomb explosion for the higher-charged ions takes place. The total combined Coulomb explosion energy agrees well with experimental results, showing striking regularities. The origin and intensity dependence of unexpectedly high ionization rates of dissociating nuclei at preferential, large internuclear distances R=Rc is also discussed and an analytic expression for Rc is derived.

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.

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.

Observing electron motion in molecules

We study analytically the possibility for monitoring electron motion in a molecule using two ultrashort laser pulses. The first prepares a coherent superposition of two electronic molecular states whereas the second (attosecond pulse) photoionizes the molecule. We show that interesting information about electron dynamics can be obtained from measurement of the photoelectron spectra as a function of the time delay between two pulses. In particular, asymmetries in photoelectron angular distribution provide a simple signature of the electron motion within the initial time-dependent coherently coupled two molecular states. Both asymmetries and electron spectra show very strong two-centre interference patterns. We illustrate these effects using as an example a dissociating hydrogen molecular ion probed by the attosecond pulses.

NUMERICAL SIMULATION OF THE ISOMERIZATION OF HCN BY TWO PERPENDICULAR INTENSE IR LASER PULSES

Isomerization of HCN is studied numerically for a laser excitation configuration of two perpendicular intense IR pulses. This scheme confines the molecule to a plane and promotes proton transfer along the curved reaction path. It is shown that internal rotation of the CN group enhances isomerization when compared to a fixed C≡N orientation model. Isomerization rates with rotation exceed those without rotation of the CN by about a factor of 3. Internal rotation also enhances dissociation and destroys phase control of the isomerization. It is found that at intensities I∼1013 W/cm2, maximum isomerization occurs with negligible dissociation for a 2 ps pulse excitation. Maximum isomerization is also found for one field frequency resonant with the CH bend frequency ωbend and the other perpendicular frequency at 2ωbend.

Saturation of the nonlinear refractive index in atomic gases

Motivated by the ongoing controversy on the origin of the nonlinear index saturation and subsequent intensity clamping in femtosecond filaments, we study the atomic nonlinear polarization induced by a high-intensity and ultrashort laser pulse in hydrogen by numerically solving the time dependent Schrodinger equation. Special emphasis is given to the efficient modeling of the nonlinear polarization at central laser frequency corresponding to 800 nm wavelength. Here, the recently proposed model of the Higher-Order Kerr Effect (HOKE) and two versions of the Standard model for femtosecond filamentation, including either a multi-photon or tunnel ionization rate, are compared. We find that around the clamping intensity the instantaneous HOKE model does not reproduce the temporal structure of the nonlinear response obtained from the quantum mechanical results. In contrast, the non-instantaneous charge contributions included in the Standard models ensure a reasonable quantitative agreement. Therefore, the physical origin for the observed saturation of the overall electron response is confirmed to mainly result from contributions of free or nearly free electrons.

Attosecond photoionization of a coherent superposition of bound and dissociative molecular states: effect of nuclear motion

We study numerically the possibility for monitoring electron motion in a dissociating molecule using an attosecond XUV probe pulse which photoionizes a coherent superposition of two nuclear wave packets. We present the photoelectron spectra and forward?backward asymmetries in these spectra obtained from a numerical solution of the time-dependent Schr?dinger equation forx one electron and two protons both moving in 1D, along the laser polarization vector. In our (non-Born?Oppenheimer) approach the 1D dissociative ionization of the model H+2 with softcore potential is described exactly. We find that in general the nuclear motion in a fast moving light molecule does not wash out the oscillations as function of the time delay between XUV probe and the pump pulses as expected from the model with fixed nuclei.

Effect of Nuclear Motion on Molecular High Order Harmonic Pump Probe Spectroscopy

We study pump-probe schemes for the real time observation of electronic motion on attosecond time scale in the molecular ion H(2)(+) and its heavier isotope T(2)(+) while these molecules dissociate on femtosecond time scale by solving numerically the non-Born-Oppenheimer time-dependent Schrödinger equation. The UV pump laser pulse prepares a coherent superposition of the three lowest lying quantum states and the time-delayed mid-infrared, intense few-femtosecond probe pulse subsequently generates molecular high-order harmonics (MHOHG) from this coherent electron-nuclear wavepacket (CENWP). Varying the pump-probe time delay by a few hundreds of attoseconds, the MHOHG signal intensity is shown to vary by orders of magnitude. Due to nuclear movement, the coherence of these two upper states and the ground state is lost after a few femtoseconds and the MHOHG intensity variations as function of pump-probe delay time are shown to be equal to the period of electron oscillation in the coherent superposition of the two upper dissociative quantum states. Although this electron oscillation period and hence the periodicity of the harmonic spectra are quite constant over a wide range of internuclear distances, a strong signature of nuclear motion is seen in the actual shapes and ways in which these spectra change as a function of pump-probe delay time, which is illustrated by comparison of the MHOHG spectra generated by the two isotopes H(2)(+) and T(2)(+). Two different regimes corresponding roughly to internuclear distances R < 4a(0) and R > 4a(0) are identified: For R < 4a(0), the intensity of a whole range of frequencies in the plateau region is decreased by orders of magnitude when the delay time is changed by a few hundred attoseconds whereas in the cutoff region the peaks in the MHOHG spectra are red-shifted with increasing pump-probe time delay. For R > 4a(0), on the other hand, the peaks both in the cutoff and plateau region are red-shifted with increasing delay times with only slight variations in the peak intensities. A time-frequency analysis shows that in the case of a two-cycle probe pulse the sole contribution of one long and associated short trajectory correlates with the attenuation of a whole range of frequencies in the plateau region for R < 4a(0) whereas the observed red shift for R > 4a(0), even in the plateau region, correlates with a single electron return within one-half laser cycle.

Preparation and alignment of highly vibrationally excited molecules by CARP - chirped adiabatic Raman passage

Abstract Three-dimensional time-dependent Schrodinger equation simulations of Raman excitation of the Cl 2 molecule are used to demonstrate the feasibility of efficient high vibrational excitation ( v ⩾20) of symmetric non-polar bonds using short (ps) chirped frequency laser pulses at intensities below the ionization threshold ( I=2×10 13 W/cm 2 ). The process called chirped adiabatic Raman passage (CARP) involves sequential Raman excitations Δv =+1 during the pulse accompanied by rotational transitions. It is shown that as a result of CARP, considerable laser alignment of the molecule is achieved in high vibrational levels, thus offering a new tool for the study of dynamics and reactivity of aligned excited molecules.