ROVIBRONIC INTERACTIONS IN NO2 AROUND 17 700 CM-1 OBSERVED BY ZEEMAN-EFFECT AND ANTICROSSING EXPERIMENTS

We have observed the Zeeman effect on N = 1, K = 0 rotational levels o f seven vibronic levels of NO, located between 17 438 and 17 842 cm-1. We have used a supersonic jet, (T(rot) almost-equal-to 4 K) located i nside a 5 MW Bitter coil of 100 mm bore which allows magnetic field sc ans up to 8 T. CW monomode ring dye laser excitation allows a resoluti on of about 300 MHz limited by the residual Doppler effect. We have ob served the evolution of the Zeeman energy levels versus the field. The values of high field Lande factors range from 1.80 to 1.98, significa ntly lower than the free spin value (2.0023). The standard perturbatio n theory of Curl [Mol. Phys. 9, 585 (1965)], which relates Lande facto r and the spin-splitting constant epsilonBAR, does not fit the observe d results. In addition, 54 anticrossings due to rovibronic interaction s have been observed. The corresponding matrix elements range from abo ut 50 MHz (limited by field inhomogeneities) up to 15 GHz, (0.5 cm 1). The expected number of anticrossings in the magnetic field range scan ned (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, th e 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 rough ly independent of the N rotational quantum number. This contrasts with the diagonal electronic-spin interactions (spin splittings) which inc rease linearly with N but which are significantly weaker than off-diag onal interactions at least for the N = 1, K = 0 levels studied here. W e show that these rovibronic interactions (by both first order and hig her order) induce the numerous irregularities previously observed in t he zero field jet cooled excitation spectrum of NO2. Moreover, the ave rage reduced matrix element of first order spin-rotation interaction o bserved in the zero field spectrum from 16 500 to 18 500 cm is about 0 .76 +/- 0.25 cm-1 in agreement with the above-mentioned high field mea surement.