Ion-dip and laser photoexcitation spectroscopy of high Rydberg states in N$_2$

N$_2$ molecules in pulsed supersonic beams have been laser photoexcited from their X $^1$$\sum$$^{+}_{g}$ ground electronic state to selected singlet np and nf Rydberg states using a (2 + 1') two-color three-photon excitation scheme. This required the competition between (2 + 1) resonance-enhanced multiphoton ionization and (2 + 1') Rydberg state photoexcitation to be carefully balanced. This was achieved by performing ion-dip spectroscopy in which the signal from the N$_{2}^{+}$ cations generated by direct photoionization was selectively detected and seen to reduce under conditions in which the predissociative np Rydberg states, or long-lived nf Rydberg states, were populated. The predissocation rates of the np Rydberg states were determined from the n-dependence of the spectral widths of the transitions to them. The long-lived Rydberg states populated by excitation on nf resonances are suitable for deceleration and electrostatically trapping cold samples of N$_2$ using inhomogeneous electric fields.