Recent progress on attosecond science at RIKEN
We have been developing two types of intense attosecond beam lines for investigating attosecond wavepackt dynamics in atoms and molecules. One produces an attosecond pulse train for attosecond Fourier transform spectroscopy by attosecond-pump/attosecond-probe scheme, while the other produces an intense isolated attosecond pulse for attosecond nonlinear optics.
Settling time of a vibrational wavepacket in ionization
The vibrational wavepacket of a diatomic molecular ion at the time of ionization is usually considered to be generated on the basis of the Franck–Condon principle. According to this principle, the amplitude of each vibrational wavefunction in the wavepacket is given by the overlap integral between each vibrational wavefunction and the ground vibrational wavefunction in the neutral molecule, and hence, the amplitude should be a real number, or equivalently, a complex number the phase of which is equal to zero. Here we report the observation of a non-trivial phase modulation of the amplitudes of vibrational wavefunctions in a wavepacket generated in the ground electronic state of a molecular ion at the time of ionization. The phase modulation results in a group delay of the specific vibrational states of order 1 fs, which can be regarded as the settling time required to compose the initial vibrational wavepacket.
Photoionization of molecular hydrogen
A theoretical formulation of diatomic photoionization is given in terms of an entirely body‐frame Hund’s case b description. Within this formalism, the electronic wavefunctions for both the bound initial state and the free final state of photoionization are described with conventional configuration interaction wavefunctions expanded in a Gaussian basis. The Stieltjes imaging procedure is used to recover the continuum oscillator strength from the discrete eigenspectrum. The continuum oscillator strength is combined with variationally determined vibrational wavefunctions to yield model photoelectron spectra, specific v–v ionization cross sections, a photoion kinetic energy spectrum, dissociative ionization spectra, and a total photoionization spectrum. For a series of photon energies near threshold, cross sections from these calculations are compared with previous theoretical results and with recent experimental measurements.