Photons, Ions, Atoms and Molecules in Collision
This portion of the LBNL Chemical Sciences Division Atomic, Molecular
and Optical Physics program is active in studies of low velocity
collisions between ions, atoms and molecules and photoionization
of simple atoms and molecules. Low velocities means less than
the Bohr velocity (2.18 x 108 cm/sec), where charge transfer is
often the dominating inelastic process. The photo-ionization studies
have concentrated on single photon double ionization of two electron
systems, He, or H2 (D2) utilizing photons
from the LBNL Advanced Light Source and, more recently, on photo
electron emission from the K-shell of selected atoms in simple
diatomic molecules. Recently new studies of the double ionization
of D2 by intense, femto-second laser fields were initiated.
Following are brief descriptions of some of this work.
Auger Anisotropy: A Probe of Doubly Excited Product States
A highly charged ion colliding with an atom at low velocities
may capture one, two or several electrons into highly excited
states which then decay by the emission of photons and/or Auger
electrons. In the case of collisions with He atoms, at most two
electrons may be captured and these populate short lived doubly
excited states on the ion core. We studied double electron capture
to the one electron ions B4+, and C5+ which
were produced by the LBNL Electron Cyclotron Resonance ion source.
The transient Li-like B2+ and C3+ ions formed
by double capture from a He target atom, have both electrons in
n=2 levels. These states decay by the emission of an Auger electron.
Because the magnetic substates of the doubly excited levels are
not equally populated, the Auger emission is not isotropic. We
have measured the anisoptropy in the Auger emission, in coincidence
with measurement of the momentum transferred to the ion, which
yielded the most detailed description of double electron capture
in the low velocity regime. This work formed the thesis project
of Dr. H. Khemliche. See: Physical Review Letters 74, 5013 (1995),
and Nuclear Instruments and Methods in Physics Research B 124,
243 (1997).
Single Photon Double Ionization of Two Electron Systems
In collaboration with groups at U. Frankfurt, Kansas State U,
and Western Michigan U., detailed studies of single photon double
ionization of two electron systems have been conducted utilizing
photons from the LBNL Advanced Light Source. This work initially
focused on the double ionization of He, and utilizes the powerful
Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) technique
developed at U. Frankfurt. Double photo-ionization of He is a
simple example of the extensive, and largely undeveloped study
of many particle dynamics in AMO physics, and is a bridge between
single particle and multiple particle excitation. The COLTRIMS
approach measures, with high efficiency, the vector momenta of
the two electrons and the recoiling He++ ion for each
photo-ionization event, without a priori selection of any particular
geometry or emission angles for the outgoing particles. The He
results include fully differential electron and ion momentum distributions
at photon energies up to 20 eV above the double ionization threshold
(79 eV), and definitive measurements of the ratio He++/He+
produced by single photons over the energy range 85-100 eV. More
recent work has extended single photon double ionization studies
to the D2 molecule. Removing two electrons from D2
yields a four body continuum Coulomb system, with no remaining
molecular degrees of freedom, and the rapid separation of the
D+ ions allows determination of the alignment of the
molecule at the instant of photo-ionization. The first study of
this system measured the vector momenta of both ions and one of
the two electrons in coincidence. The results provided angular
correlation information for one of the two electrons with respect
to the photon polarization and the alignment of the molecular
axis and also between the energy of one electron and the energy
the D+ fragments.
Molecular Alignment Dependence in Slow Charge Transfer Collisions.
The sensitivity of molecular interactions to the relative orientation
of the participants could provide a means to control reactive
processes increasing yields, or forcing particular desired outcomes
in chemical, plasma or material processing environments. In biochemistry,
sensitivity of processes to the shape and approach of large reactant
species is well known and is taken into account in the design
of new pharmaceuticals and labeling compounds. In some astrophysical
regions or planetary atmospheres ion molecule reactions can be
important processes also sensitive to molecular alignment or orientation.
Since electron rearrangement is central to nearly all reactive
processes, basic studies of orientation dependent electron transfer
form a useful window into the correlation between structure and
collision dynamics.
Utilizing modified versions of the Cold Target Recoil Momentum
Spectroscopy technique together with fragment imaging we have
performed unique studies of charge transfer to or from aligned
molecules in collision with He atoms or ions at velocities well
below 1 au. The results have revealed the stereo-dynamics of a
class of simple reactions of molecular ions with an atomic target.
Stereo-dynamics refers to the dependence of a reaction upon the
relative geometric orientation of the participants. In particular
we have studied charge transfer from He to the HeH+
molecular ion forming dissociative HeH levels and more recently
charge transfer channels in collisions of He++ ions
with D2 molecules which yield dissociating D2+or
D2++ "molecules"
The molecular alignment is determined from the position and
time of impact of the molecular fragments in coincidence with
time and position measurement of the impact of the other partner
in the collision. The results provide vector momentum distributions
which yield the energy shared by the fragments, their orientation
with respect to the projectile momentum and the momentum transferred
between the reactants (i.e. with respect to the scattering plane).
To the extent that one can resolve the final internal states,
these measurements can provide all the information one could obtain
on the collision dynamics from the asymptotic momenta of the participants.
The differential momentum transfer cross-sections are convolutions
of the impact parameter dependence of the particle scattering
with the probability for the selected process to occur. In the
case of an aligned diatomic molecule the "impact parameter"
is two dimensional, hence the anisotropy resulting from a two
center scattering is included in the correlation between the momentum
transferred and the probability of exit in a selected channel.
Electron Emission from Core Excited, Fixed in Space CO Molecules
Using the techniques of electron and ion momentum spectroscopy,
we have measured the complete emission patterns of core photoelectrons
emitted from the C and O atoms for any (and all) relative orientations
of the CO molecular axis and the linearly polarization of the
incident soft X-ray photons. This has been done for photon energies
extending to 30 eV above the K-shell thresholds for both the C
and O atoms at beamline 9.3.2 at the LBNL Advanced Light Source.
This energy range covers the region of the s
shape resonance in this system, which occurs at about 10 eV photoelectron
energy. The emission patterns, in the molecular frame, show rich
structures in their angular variation which reflect the propagation
of the photo electron wave in the two center molecular potential.
In terms of a multiple scattering view, the intensity at a given
angle with respect to the molecular axis is the net result of
the addition of a direct wave from the atomic K-shell with waves
scattered one or more times from the other atom in the molecule.
The measurement show that the electron emission patterns can be
accurately described by a set of real amplitudes and relative
phases for excitation of s and p
transitions to electron partial waves with angular momentum £
4, and the analysis provides these amplitudes and phases. The
complete analysis will display dependence of the amplitudes and
phases on the photo electron energies up to ~30 eV above the K
shell threshold. Accurate and complete knowledge of intramolecular
scattering, beyond its intrinsic value as a test of electron wave
propagation in multiple centered potentials, can refine the understanding
of photo absorption and electron emission diagnostics of materials
and surfaces. The various near edge diagnostic methods used in
solid and surface analysis (XAFS; NEXAFS, XANES) are manifestations
of the electron wave propagation in a situation with many more
scatterers than in a diatomic system (e.g. CO ). The observation
of multiple scattering effects in the outgoing propagation of
a core photoelectron from one of the atoms in CO thus represents
a basic and reduced example of a phenomena widely applied to materials
and surface analysis. Theoretical approaches which successfully
deal with the CO system can have application to these important
and more complex environments. This work is collaborative with
groups from U. Frankfurt, Kansas State Univ. and Western Michigan
U.
Double Ionization of D2 in Intense Laser Fields
The behavior of simple molecules exposed to intense femto-second
laser pulses includes the dynamics of electron motion in the combined
molecular and photon fields and the nuclear motion in the molecular
potentials modified ("dressed") by the intense field.
Studies of H2 (or D2) in high intensity
optical fields has revealed interesting phenomena, such as "bond
softening", where breakup of the molecule occurs via avoided
crossings in the manifold of field dressed molecular potentials.
Or the phenomena of enhanced ionization at inter-nuclear separations
much outside the equilibrium values. An intense linearly polarized
field may also orient the molecule along the polarization axis,
yielding highly non-isotropic distributions of ionization products.
Nearly all of the experimental studies of double ionization of
H2 (or D2) molecules in intense laser fields,
have been rationalized in terms of a sequential process wherein
the molecule is first singly ionized, and then ionized again in
a second step as the singly charged molecule, separating on a
dissociative curve, passes through inter-nuclear separations where
enhance ionization occurs. Often concentration is upon the second
step in the process and, although a neutral molecule is the target,
the studies are often labeled as ionization of H2+
(or D2+) ions. This situation contrasts
markedly with studies of the double ionization of He, where a
direct non-sequential process dominates for all except the highest
laser intensities.
Recently we have utilized the technique of coincident momentum
spectroscopy to measure, for the first time, the momentum of both
D+ions following the double ionization of D2
by 790 nm 100-200 fs pulses. The measurements show evidence of
a new channel for the double ionization of D2, where
the two ions have substantially more energy than seen in the previously
observed enhance ionization process. Analysis of these new measurements
is continuing. Participants in this work are: André Staudte
(graduate student, U. Frankfurt), C.L. Cocke (sabbatical leave
from KSU), C. Ray (LBNL CSD), H. HW Chong, T.E. Glover, R. Schoenlein
(LBNL MSD) , C. Ray, A. Belkacem and M. Prior (LBNL CSD).
