Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm−1

Abstract

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO2 spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, X̃2A1 and à 2B2, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO2 absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO2 spectrum down to 11200 cm−1 with a new Ti:sapphire ICLAS spectrometer. As a result 249 2B2 vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm−1 energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N ⩽ 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the X̃ 2A1 and à 2B2 electronic states, we observed not only the a1 vibrational levels of the à 2B2 state but also the b2 vibrational levels of the X̃ 2A1 state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of 2B2 vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm−1, corresponds to the bending motion of the à 2B2 state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10−12 s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.