Alejandro Saenz

Strong field ionization to multiple electronic states in water.

High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H(2)O and D(2)O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by solving the time-dependent Schrödinger equation to simulate strong field ionization and high harmonic generation from the water isotopes. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.

Alignment-dependent ionization of N2, O2, and CO2 in intense laser fields.

The ionization probability of N2, O2, and CO2 in intense laser fields is studied theoretically as a function of the alignment angle by solving the time-dependent Schrödinger equation numerically assuming only the single-active-electron approximation. The results are compared to recent experimental data [D. Pavicić, Phys. Rev. Lett. 98, 243001 (2007)] and good agreement is found for N2 and O2. For CO2 a possible explanation is provided for the failure of simplified single-active-electron models to reproduce the experimentally observed narrow ionization distribution. It is based on a field-induced coherent core-trapping effect.

On the influence of vibrational motion on strong-field ionization rates in molecules

The influence of the vibrational degree of freedom on the strong-field ionization of molecules is investigated. On the basis of numerical simulations it is found that vibrational motion leads to a reduced ionization rate for H2 and O2 compared with the rate of an atom with identical ionization potential. In the case of N2 the vibrational motion has, however, a negligible influence on the ionization rate. While these results are in qualitative agreement with recent experimental findings, the predicted reduction of the ionization rate for H2 and O2 is too small to fully explain the experimental results.

Photoassociative Spectroscopy at Long Range in Ultracold Strontium

We report photoassociative spectroscopy of 88Sr(2) in a magneto-optical trap operating on the 1S0-->3P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600-2400 MHz from the 1S0-->1P1 atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the 1P1 atomic lifetime (tau=5.22+/-0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.

Theoretical two-, three- and four-photon ionization cross sections of helium in the XUV range

We present results of ab initio calculations of generalized cross sections for two-, three- and four-photon ionization of helium by VUV and XUV radiation. These cross sections can be useful in the estimation of the intensity required for observation of these processes. This work has been motivated by recent experimental results using high-order harmonic generation with which we find excellent agreement.

Non-perturbative solution of the time-dependent Schrödinger equation describing H2 in intense short laser pulses

A method for solving the time-dependent Schrodinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

Numerical treatment of diatomic two-electron molecules using a B-spline based CI method

A B-spline based configuration–interaction method for diatomic two-electron molecules has been developed and implemented. The molecular symmetry of the problem is fully accounted for by using the prolate-spheroidal coordinate system. The performance of the method is demonstrated in a number of test applications. This includes the calculation of the energies of the ground and excited states of H2 (even autoionizing doubly-excited states) as well as transition dipole moments. Furthermore, a partial photoionization cross-section for HeH+ was calculated. In all these cases a favourable comparison to the literature values is found. This indicates the broad applicability of the present approach.

Spectroscopic determination of the s-wave scattering lengths of 86Sr and 88Sr.

We report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, 86Sr and 88Sr. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the 1S0-1P1 atomic resonance at 461 nm. A minimum in the transition amplitude for 86Sr at -494 +/- 5 GHz allows us to determine the scattering lengths 610a0 < a86 < 2300a0 for 86Sr and a much smaller value of -1a0 < a88 < 13a0 for 88Sr.

Hydrogen–antihydrogen scattering in the Born–Oppenheimer approximation

We calculate the low-energy scattering between hydrogen and antihydrogen atoms in their ground states using the Born–Oppenheimer approximation. Improved results for rearrangement, direct annihilation and elastic scattering are presented. The elastic cross section agrees well with other calculations. For the rearrangement process we confirm a recent result (Armour E A G and Chamberlain C W 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L489) that rearrangement to the N = 23 state of protonium has a larger cross section than rearrangement to the N = 24 state. For both rearrangement cross sections our results are smaller than those obtained by Armour and Chamberlain. The direct annihilation, and its influence on elastic scattering, is calculated using the scattering length for the strong force between the proton and antiproton. This approach gives strong-force effects qualitatively similar, but smaller than those obtained in another recent work (Voronin A Yu and Carbonell J 2001 Nucl. Phys. A 689 529c).

Tests of Lorentz invariance using hydrogen molecules

We discuss the consequences of Lorentz violation (as expressed within the Lorentz-violating extension of the standard model) for the hydrogen molecule, which represents a generic model of a molecular binding. Lorentz-violating shifts of electronic, vibrational and rotational energy levels, and of the internuclear distance are calculated. This offers the possibility of obtaining improved bounds on Lorentz invariance by experiments using molecules.