Vibrational modes of (NO)2+ and what they reveal about geometry

[physgirl]

Homework Statement

For (NO)2+, experiments show 1 Raman active vibrational mode around 1450/cm; 2 strong IR active modes at 2400/cm and 550/cm; and 1 weak IR active mode around 3600/cm. What does that say about its geometry?

The Attempt at a Solution

Never mind, I think I got it :)

 

[Gokul43201]

Out of curiosity, what kind of mode does the weak IR active line correspond to?

The first 3 are clearly the symmetric stretch, asymmetric stretch and symmetric bend (in that order). That there are 4 modes suggests that the ion is linear, as indeed NO2^+ is. What is the fourth vibrational mode?