Two possible outcomes were initially entertained during investigations of how diatomic molecular charges are distributed over the fragment ions. Firstly, that both fragment ions have the same charge (or differ by one charge unit, in the case of an oddly charged transient ion) which is referred to as symmetric fragmentation. Secondly, that the charge states of the ions differ by ^2 , which is referred to as
asymmetric fragmentation.
The first molecule studied for the purpose of investigating the way in which molecules fragment in the intense laser field was nitrogen (Boyer et al, 1989) using 248 nm light of peak laser intensity 10^® W/cm^. They found that asymmetric channels appeared to be dominant. Analysis of their one dimensional time of flight (1 D-TOF) spectra led them to conclude that they had detected (1,1), (0,2), (1,2) and
(1,3) channels but not the (2,2) channel, where is denoted by (q, p). They
proposed that charge asymmetric fragmentation was a natural consequence of the large induced dipole moment arising from the collective motion of the electrons along the laser electric field and that this displacement of charge leads automatically to a corresponding charge asymmetry in the dissociating products. However, Frasinski etal, 1989, also investigating the nitrogen molecule but using a 600 nm wavelength, 0.6 ps pulse duration and a peak laser intensity of 3 x 10^® W/cm^ found that symmetric fragmentation channels dominated the spectrum. Using covariance mapping they determined that the main channels were (1,1), (1,2), (2,2) and (3,3). The technique allowed the fragmentation channels and also the strength of the channels to be determined thereby eliminating some of the ambiguity of the 1 D-TOF analysis. Covariance mapping also allowed the determination of the total kinetic energy release for a fragmentation channel. Normand et al, 1991, found that for wavelengths of 305 nm and 610 nm the charge
fragmentation patterns for CO, N2 and O2 were asymmetric for the former
wavelength and symmetric for the latter wavelength. However, when the Reading group took their covariance mapping equipment to Saclay and performed the same experiment (Codling et al, 1991 ) on the French equipment they found that in all cases the fragmentation pattern was symmetric.
Electron xjtential energy .50^ (eV) -100- b 0 4 -4 0 4
Axial distance (A)
Figure 1.5: Thomas-Fermi-Dlrac model of field ionization where all three graphs are for the same laser intensity but with increasing internuclear separation of (a) 1.6 Â, (b) 2.3 Â (dissociation point) and (c) 3.0 Â (Brewczyk et al, 1991)
Brewczyk et al, 1991, presented the Thomas-Fermi-Dirac (TDF) model which was used to calculate charge states us the Fermi gas distributions. It treated the electrons as a continuous cloud distribution whose total charge need not be a multiple of the elementary charge. The external field perturbs the electron potentials and the electronic charge cloud spills over the potential barrier and the molecule becomes ionized, as shown in Figure 1.5. Branching ratios for N2 were plotted and
the theory predicted that symmetric fragmentation channels should dominate the spectrum.
Strickland et al, 1992, disagreed with the Reading group’s position on symmetric charge fragmentation channels and suggested an explanation as to why they appeared to detect only symmetric fragmentation channels. Strickland et al, 1992, pointed out that using the covariance mapping technique nascent ions, produced at the beginning of the laser pulse, cannot be distinguished from ions produced much later in the pulse as secondary products from ions or atoms which have separated from the other constituents of the molecule during a primary dissociative ionization process. These ions produced later in the pulse are known as post-dissociative ionization (PDI) products. It was suggested that asymmetric fragmentation is the norm for ions produced in the initial interaction and but that at a later stage in the laser pulse the ions are further ionized so that the complete process produces products which can be erroneously interpreted as a single interaction producing a
symmetric fragmentation pattern. To investigate this theory Strickland et al, 1992, used molecular iodine as a target gas which has a long ground state vibrational period (155 fs) in comparison to the short laser pulse (30 fs). Thus the molecule was effectively vibrationally frozen and inertially confined for the entire duration of the pulse, eliminating the possibility of PDI. As predicted, the group detected both symmetric and asymmetric fragmentation patterns.
Stankiewicz et ai, 1993, reanalyzed the data from Cornaggia et al, 1990 and 1992 and Lavancier et al, 1991, and was able to re-interpret the data in terms of asymmetric primary processes masked by PDI. The authors proposed that all of the data, interpreted as symmetric fragmentation channels, were actually predominantly asymmetric after the initial dissociative ionization process. However, Hatherly et al, 1994, asked that if asymmetric fragmentation was such a dominant process why was not even a trace of asymmetric fragmentation channels found using covariance mapping.
It became obvious that in order to determine the initial processes PDI had to be distinguished from them. Schmidt et al, 1994, investigated the dissociative ionization of CI2 for two different pulse lengths 2 ps (610 nm) and 130 fs (395, 610
and 790 nm) and found no evidence of PDI. They determined the various channels produced and the kinetic energy releases of these channels by analyzing 1 D-TOF spectra. The channel energies were compared to each other to see if any were of the same magnitude as those found for lower ionization channels in order to try and deduce if PDI processes were occurring. PDI creates no further kinetic energy for the ion since it involves only removal of the electron in the flat part of the Coulomb curve. Thus, channel (2,1) could have the same energy as a channel (2,2) involving PDI of one of the ions. Schmidt et al, 1994, concluded that since they had found no evidence of PDI the observed charge symmetric channels were not due to an experimental artifact arising from PDI.