Photoelectron angular distributions from strong-field ionization of oriented molecules

The combination of ultrafast light sources with detection of molecular-frame photoelectron angular distributions (MFPADs) is setting new standards for detailed interrogation of molecular dynamics(1-8). However, until recently measurement of MFPADs relied on determining the molecular orientation after ionization(1,9-13), which is limited to species and processes where ionization leads to fragmentation. An alternative is to fix the molecular frame before ionization. The only demonstrations of such spatial orientation involved aligned small linear nonpolar molecules(5,6,14). Here we extend these techniques to the general class of polar molecules. Carbonylsulphide and benzonitrile molecules, fixed in space by combined laser and electrostatic fields, are ionized with intense, circularly polarized 30-fs laser pulses. For carbonylsulphide and benzonitrile oriented in one dimension, the MFPADs exhibit pronounced anisotropies perpendicular to the fixed permanent dipole moment, which are absent for randomly oriented molecules. Furthermore, for benzonitrile oriented in three dimensions, striking suppression of electron emission in the fixed molecular plane appears. Our theoretical analysis, based on tunnelling ionization theory(15,16), shows that the MFPADs reflect permanent dipole moments and polarizabilities of both the neutral molecule and its cation, and nodal planes of occupied electronic orbitals. These results point to future opportunities for time-resolved probing of valence electron dynamics.