Rovibronic interactions in NO2 around 17 700 cm−1 observed by Zeeman effect and anticrossing experiments

Abstract

We have observed the Zeeman effect on N=1, K=0 rotational levels of seven vibronic levels of NO2 located between 17 438 and 17 842 cm−1. We have used a supersonic jet, (Trot≊4 K) located inside a 5 MW Bitter coil of 100 mm bore which allows magnetic field scans up to 8 T. CW monomode ring dye laser excitation allows a resolution of about 300 MHz limited by the residual Doppler effect. We have observed the evolution of the Zeeman energy levels versus the field. The values of high field Landé factors range from 1.80 to 1.98, significantly lower than the free spin value (2.0023). The standard perturbation theory of Curl [Mol. Phys. 9, 585 (1965)], which relates Landé factor and the spin-splitting constant ε̄, does not fit the observed results. In addition, 54 anticrossings due to rovibronic interactions have been observed. The corresponding matrix elements range from about 50 MHz (limited by field inhomogeneities) up to 15 GHz, (0.5 cm−1). The expected number of anticrossings in the magnetic field range scanned (from the known rovibronic density of state and from first-order ‘‘spin–rotation’’ interaction selection rules), is only 27. We explain the additional anticrossings by higher order interactions. In fact, the distribution of observed matrix elements is smooth, without any gap between first order and higher order matrix elements. In this case, we have assumed that the first order matrix elements are the larger ones. With this assumption, we have determined the average reduced matrix element of first-order spin–rotation interaction: 0.73±0.15 cm−1. These off diagonal spin–rotation interactions are expected to be roughly independent of the N rotational quantum number. This contrasts with the diagonal electronic-spin interactions (spin splittings) which increase linearly with N but which are significantly weaker than off-diagonal interactions at least for the N=1, K=0 levels studied here. We show that these rovibronic interactions (by both first order and higher order) induce the numerous irregularities previously observed in the zero field jet cooled excitation spectrum of NO2. Moreover, the average reduced matrix element of first order spin–rotation interaction observed in the zero field spectrum from 16 500 to 18 500 cm−1 is about 0.76±0.25 cm−1 in agreement with the above-mentioned high field measurement.