In this paper Stark spectroscopy is performed for the Rydberg states (n≥16) of molecular hydrogen, and the deflection of Rydberg H₂ molecules by an inhomogeneous electric field is demonstrated. The Rydberg states are excited by two-photon, two-color (VUV+UV) excitation via a selected rotational level of the B¹Σ_u⁺ intermediate state. At zero field, the Rydberg states accessed are the ns and nd series converging to the v⁺=0, N⁺=0 and 2 vib-rotational level of the H₂⁺ ion. In the region of 200 cm^-1 below the ionization threshold the only Rydberg states surviving after a 3.7 μs delay are the N⁺=2, nd, J=1 series (hereafter designated as (nd2)₁), and all the other states are rapidly predissociated. The Stark map showing how the energy levels vary with electric field is determined by recording Rydberg excitation spectra for a range of fields F=0-1000 V/cm and then be used to predict the force exerted on the molecule in an inhomogeneous field.
The Rydberg states around n=17-18 are excited in the presence of an inhomogeneous electric dipole field. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H₂ molecules in the field. The trajectories of the molecules are monitored using ion-imaging and time-of-flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k. The Rydberg states excited in the field are found to be long-lived; they survive for over 100 μs after the dipole field is switched off before being ionized at the detector and the time of flight is measured. This long lifetime is caused by the adiabatic transition to the high-l Rydberg states from the initially populated Stark eigenstates when the field is abruptly switched off.
Simulations for the Stark spectra are presented based on the matrix diagonalization scheme. The observed intensity distribution of Stark manifolds are fairly well reproduced only if the transition probability to the (nd2)₁, M_J=0 states is assumed to be non-zero and to the other states zero. The (nd2)₁ state is observed above the N⁺=0 ionization threshold even though rotational autoionization is energetically possible. However, the (nd2)₁ series cannot autoionize because there is no J=1 continuum channel of gerade symmetry associated with the N⁺=0 threshold, and in the absence of a field the gerade Rydberg stetes cannot autoionize according to the ΔJ=0 selection rule.