An overview of the linear vibrational action spectrum of Re(CO)3(CH3CN)3 + •N2 in the carbonyl stretching region is presented in Fig. . The N2 tag-loss action response was monitored as a function of the delay time τ between an IR pulse pair (Fig. ). The tag-loss interferogram is shown in Fig. , which was undersampled using a 1700 cm -1 rotating frame (i.e., the phase difference between the two pulses, Δϕ12, was incremented by ω1700τ). Rotating frame sampling greatly reduced the number of points and, therefore, time required to collect each interferogram. The interferogram shows a clear beating pattern expected from absorption at two frequencies, and the Fourier transform (Fig. ) indeed reveals two sharp features near 2000 cm -1 and 2080 cm -1 (300 cm -1 and 380 cm -1 in the rotating frame, respectively). Also shown in Fig. is the linear action spectrum collected in the traditional manner by frequency-scanning a tunable, few-cm -1 resolution IR source (light red trace). The two methods yield essentially identical vibrational action spectra, as expected. The observation of two carbonyl stretch features is consistent with the fac isomer of Re(CO)3(CH3CN)3 + shown in Fig. . The lower-energy peak at 2000 cm -1 derives from a doubly degenerate pair of asymmetric stretch normal modes while the higher-energy 2080 cm -1 peak results from the lone symmetric stretch normal mode. The mass spectrum of Re(CO)3(CH3CN)3 + •(N2)n is presented in Fig. while harmonic calculations for the fac and mer isomers are provided in Fig. .