Knowledge Bank

The accuracy of theoretical work on the ground state of $H2$ has improved dramatically in the past few years, considerably surpassing the accuracy of existing measurements. The calculations are now sufficiently good that new experimental measurements can sensitively test the quantum electrodynamic corrections to this simple molecular system, predicted to be $0.378cm-1$ with an uncertainty of a few times $0.01cm-1$. This talk will describe progress in a new program of measurements at Yale, in which both the ionizatiom potential and the dissociation limit are being determined with much improved accuracy. To determine the ionization potential we have extrapolated the Rydberg np series, the same basic approach used in the classic experiment by Herzberg and Jungen1 and the more recent laser experiment by Glab and Hessler,$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$. Our new measurement is a two-part effort: first two-photon intervals from the ground state to the E,F(210) state near $100,000cm-1$ are determined, then transitions from the E, F state to the high Rydberg states are measured. For both measurements, a pulse-amplified cw dye laser is used in conjunction with a collimated molecular beam to provide Doppler-free spectra with linewidths of order 100 MHz. Four rotational branches to Rydberg np states with $n=42-88$ have been studied. One- and two-channel quantum defect theory analyses describe the results within experimental uncertainties, and yield four consistent results for the ionization potential. The result of $124417.524\pm 0.015cm-1$ agrees with the theoretical value of $124417.503cm-1$ and is 7 times more accurate than previous experiments. The dissociation energy can be determined either from direct observation of the dissociative continuum or from extrapolation of the highest bound vibrational levels. We are trying both approaches, using stepwise laser excitation from the E, F state to the region of the second dissociation limit. Our current results indicate that the spectrum just below threshold is unexpectedly complex, so that a careful treatment of non-adiabatic couplings and a modification of existing line assignments may be required. These experiments were supported by the National Science Foundation.