Rotationally resolved magnetic vibrational circular dichroism of the paramagnetic molecule NO

Abstract

Magnetic vibrational circular dichroism (MVCD) enables the measurement of molecular magnetic moments with modest spectral resolution. Due to its paramagnetism, NO gives a much stronger spectral response, about 3 orders of magnitude more intense, than do typical diamagnetic molecules. The molecule thus provides a convenient test for the experiment and theory of paramagnetic rotors. We have measured and analyzed the MVCD, equivalent to the molecular Zeeman spectra, of NO in co-linear magnetic fields of 0.1, 0.2, 2, 4 and 8 Tesla. Similar MVCD intensities were observed for both the ²Π_1/2 and ²Π_3/2 components of NO, particularly for high J values, which demonstrates a considerable deviation from pure Hund's case (a) for NO. The g_J-values for the ²Π_1/2 components of NO, which can be determined from our experimental spectra by moment analysis, agree well with the predicted values from Radford's theory. For the ²Π_3/2 components, we tested this theory by simulating the MVCD and absorption spectra, and comparing them with our experimental spectra by use of moment analysis to show that they match well in terms of magnetic properties. While 0.2 T experiments easily develop sufficient MVCD for analysis of NO spectra and these low field intensities have a linear field dependence, spectra in the strong fields accessible in our study showed non-linear response due to onset of saturation effects. We also observed a strong field dependence for the absorption intensities for some ²Π_3/2 components that was not encompassed in the theoretical model. Finally, a full coupling scheme provided analytical MVCD and absorption intensities that were in good agreement with the experimental values.