Rotational state distributions from vibrational autoionization of H2

Abstract

Optical–optical double-resonance excitation together with electron spectroscopy was used to measure the H+2 rotational state distributions produced by vibrational autoionization of singlet np Rydberg states of H2 . In the two-color excitation scheme, one laser was used to excite the two-photon transition to the H2 E, F 1∑+g, v′=1, J′=1 state, and a second laser was used to probe single-photon transitions to the vibrationally autoionized np Rydberg series converging to the X 2∑+g, v+=1, N̄+=1 and N̄+=3 levels of the ion. The expected P(1)npσ, Q(1)npπ, R(1)np1, and R(1)np3 Rydberg series converging to v+ =1 were observed and assigned, as were several interlopers converging to higher vibrational levels of the ion. Rotationally resolved photoelectron spectra were determined for all of the autoionizing transitions by using a magnetic bottle electron spectrometer. Under the normal assumptions that p waves are ejected and that spin effects are negligible, vibrational autoionization of the upper levels of the P(1)npσ and Q(1)npπ transitions should produce only v+ =0, N̄+ =1, while vibrational autoionization of the upper levels of the R(1)np1 and R(1)np3 transitions should produce a mixture of v+ =0, N̄+ =1 and v+ =0, N̄+ =3. Significant deviations from these expectations were observed. For example, vibrational autoionization of the upper levels of the Q(1)npπ transitions produced substantial amounts of v+ =0, N̄+ =3, while vibrational autoionization of the upper levels of certain Q(1)npπ, R(1)np1, and interloper transitions produced nonnegligible amounts of v+ =0, N̄+ =5. This indicates that vibrational autoionization of npπ Rydberg states is accompanied by rotational state changes in the H+2 core to an unexpected degree, and that additional mechanisms for exchange of angular momentum within the excited complex must be considered. Possible contributing mechanisms are critically assessed.