  The combination of photoelectron spectroscopy and ultrafast light sources is
on track to set new standards for detailed interrogation of dynamics and
reactivity of molecules. A crucial prerequisite for further progress is the
ability to not only detect the electron kinetic energy, as done in traditional
photoelectron spectroscopy, but also the photoelectron angular distributions
(PADs) in the molecular frame. Here carbonylsulfide (OCS) and benzonitrile
molecules, fixed in space by combined laser and electrostatic fields, are
ionized with intense, circularly polarized, 30 femtosecond laser pulses. For
1-dimensionally oriented OCS the molecular frame PADs exhibit pronounced
anisotropies, perpendicular to the fixed permanent dipole moment, that are
absent in PADs from randomly oriented molecules. For 3-dimensionally oriented
benzonitrile additional striking structures appear due to suppression of
electron emission in nodal planes of the fixed electronic orbitals. Our
theoretical analysis, relying on tunneling ionization theory, shows that the
PADs reflect nodal planes, permanent dipole moments and polarizabilities of
both the neutral molecule and its cation. The calculated results are
exponentially sensitive to changes in these molecular properties thereby
pointing to exciting opportunities for time-resolved probing of valence
electrons dynamics by intense circularly polarized pulses. Molecular frame PADs
from oriented molecules will prove important in other contexts notably in
emerging free-electron-laser studies where localized inner shell electrons are
knocked off by x-ray pulses.
