Rydberg states with the principal quantum number n = 25-32, below the υ + = 0 ionization limit, are excited by double resonance via the υ = 0, N = 0 and υ = 0, N = 2 rovibrational states of the A 2 + state. In the presence of dc electric fields in the range 0-120 V cm −1 , new resonances and hydrogenic manifolds are observed. The experimental spectra are simulated using a matrixdiagonalization approach.

The Stark spectra of Rydberg states of NO below the υ+ = 0 ionization limit, with principal quantum numbers n = 25–30, have been investigated in the presence of dc electric fields in the range 0–150 V cm−1. The Stark states were accessed by two-colour, double-resonance excitation via the υ′ = 0, N′ = 0 rovibrational state of the A2Σ+ state. The N(2D) atoms produced by predissociation were measured by (2 + 1) resonance-enhanced multiphoton ionization, and compared with pulsed-field ionization spectra of the bound Rydberg state population (Patel et al 2007 J. Phys. B: At. Mol. Opt. Phys. 40 1369).

This paper presents state-selective field ionization spectra of highly excited Rydberg states of NO. The competition between electron-nuclear coupling and electron-field coupling is investigated and it is shown that the slew rate of the electric field can be exploited to control the rotational quantum state composition of field-ionized molecules.

Double-resonance spectroscopy has been employed to characterise the autoionising and predissociating Rydberg states of NO converging to the υ + = 0 and υ + = 1 levels of the X 1 + ground state of NO + . Below the lowest ionisation limit, we monitor the formation of the N( 2 D) predissociation product and observe a spectrum dominated by the p(N + = 0) series, with smaller contributions from the p(2) and f(2) series. Many of the lineshapes can be fit to a simple Fano line profile. Upon vibrational excitation, a competing autoionisation channel is opened and we monitor the products of both the dissociation and ionisation channels. The υ + = 1 predissociation spectrum appears much more complex than the υ + = 0 predissociation spectrum, with significant contributions from the p(0), f(2), p(2), s (1) and d(1) series. In contrast, the υ + = 1 ionisation spectrum is dominated by the p(0) and f(2) Rydberg series, with much weaker contributions from s(1), d(1) and p(2) series. The lineshapes in the υ + = 1 predissociation and autoionisation spectra are perturbed and cannot be fit to simple Fano line profiles. In some extreme cases, these perturbations result in the complete disappearance of peaks from either the autoionisation or predissociation spectrum.