We report the first precise measurements of the ionization rate ${\Gamma }_I\mathrm{}\left(F\right)\mathrm{}$ in the tunneling regime for resolved $n=30,40\mathrm{}$ Stark substates of hydrogen. Excellent agreement is obtained with nonrelativistic numerical calculations. No effects of wave function mixing induced by relativistic or other interactions were observed. We consider and give an experimental upper limit for the magnitude of the H-D isotopic shift in ${\Gamma }_I\mathrm{}\left(F\right)\mathrm{}$.

• Received 24 November 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.1275