Antoine Delon

The visible excitation spectrum of jet cooled NO2: The chaotic behavior of a set of 2B2 vibronic levels

We have observed a set of 350 2B2 vibronic levels of NO2 in the 16 000–19 360 cm−1 energy range by the laser induced fluorescence (LIF) technique combined with a supersonic jet. This work extends (i.e., a larger energy range) and improves (i.e., a better detection threshold) our previous study [J. Chem. Phys. 95, 5701 (1991)]. 42 new 2B2 vibronic levels have been detected in this range where 159 vibronic levels were previously observed. In the 16 580–19 360 cm−1 energy range we estimate that the 315 observed levels represent 96% of the existing 2B2 levels. The correlation properties of this large and almost complete set of 315 2B2 vibronic levels have been analyzed. We present the next neighbor distribution, the Σ2(L), and Δ3(L) statistics, the Fourier transform (FT) of the stick spectrum with constant intensities (‖FT‖2), and the intensity distribution. The results of these analyses confirm the chaotic behavior of the 2B2 vibronic levels in this energy range: there are strong level repulsion, long range ...

THE PHOTODISSOCIATION THRESHOLD OF NO2 : PRECISE DETERMINATION OF ITS ENERGY AND DENSITY OF STATES

The NO2 photodissociation threshold has been observed by laser induced fluorescence in a jet at 25 128.57±0.05 cm−1. The unexpected high vibronic level density of 2.7 levels/cm−1 is tentatively attributed to long range interactions between the NO (2Π1/2) and O(3P2) products. The energy spacing and intensity distributions indicate rovibronic chaos.

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

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, X2A1 and A 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 X 2A1 and A 2B2 electronic states, we observed not only the a1 vibrational levels of the A 2B2 state but also the b2 vibrational levels of the X 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 A 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.

The NO2 vibronic levels near the X 2A1–A 2B2 conical intersection observed by laser induced dispersed fluorescence

Following our previous laser induced dispersed fluorescence (LIDFS) study on NO2 [J. Chem. Phys. 95, 5686 (1991)], we observed the vibronic levels up to 13 900 cm−1 by LIDFS. These observations allow one to characterize the effect of the conical intersection between the X 2A1 and A 2B2 electronic potential energy surfaces (PESs). This effect has been investigated by ab initio methods in the same range by Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. Globally we observed 420 vibronic levels of A1 or B2 symmetry up to 13 900 cm−1, while 259 were observed previously up to 12 000 cm−1. Most of these levels belong to the X 2A1 state and only 8 to the A 2B2 state. Below 12 000 cm−1, most of the levels belonging to the X 2A1 state have been vibrationally assigned and only a few are significantly mixed with those of the A 2B2 state. In contrast, each vibrational level of the A 2B2 state is mixed with few nearby high vibrational levels of the X 2A1 state via vibronic interactions. The set of the X 2A1 vibrati...

The visible excitation spectrum of jet cooled NO2: Statistical analysis of rovibronic interactions

We have recorded the high resolution (150 MHz) excitation spectra of NO2 molecules cooled in a supersonic jet in two energy ranges: 16 000–19 362 cm−1 [‘‘yellow’’ range, see R. Georges, A. Delon, and R. Jost, J. Chem. Phys. 103, 1732 (1995), hereafter referred to as paper I] and 23 326–23 945 cm−1 (‘‘blue’’ range). In this paper we are interested mainly in the rovibronic properties of about 1500 rotational levels (N=1, K=0, J=1/2, and J=3/2) observed in these two ranges. Among these levels about 480 are observed via the so‐called extra lines, i.e., the lines which are observable because of rovibronic couplings between bright levels (N=1, K=0, 2B2 vibronic character) and nearby dark levels. These rovibronic couplings result mainly from second order spin–orbit and orbit–rotation interactions which have been evidenced previously by Zeeman effect and anticrossing experiments [A. Delon, P. Dupre, and R. Jost, J. Chem. Phys. 99, 9482 (1993)]. By comparing the average matrix element of rovibronic interactions oc...

Laser induced dispersed fluorescence spectroscopy of 107 vibronic levels of NO2 ranging from 12 000 to 17 600 cm−1

We report the laser induced dispersed fluorescence spectra (LIDFS) of NO2 recorded when exciting 107 vibronic levels lying between 11 961 and 17 627 cm−1. These levels result from the vibronic interactions between the highly excited vibrational levels of the electronic ground state, X 2A1, and the isoenergetic levels of the electronic excited state, A 2B2. One observes that the intensity patterns of almost half of the LIDFS can be interpreted in terms of Franck–Condon factors (FCFs) between a single bright parent level belonging to A 2B2 and the low-lying vibrational levels of X 2A1. This means that the vibronic interactions occurring between the vibrational levels of X 2A1 and A 2B2 are not strong enough to systematically mix together several bright parent states of A 2B2. We have also found that vibronic eigenstates scattering over several hundreds of cm−1 can be assigned to the same bright parent state. However, these features, which are characteristic of individual vibronic levels, must be faced wi...

The NO2 vibronic levels near the X 2A1–A 2B2 conical intersection: Jet cooled laser induced fluorescence between 11 680 and 13 900 cm−1

Following our previous LIF [J. Chem. Phys. 95, 5701, (1991) and 103, 1732 (1995)] and ICLAS [Chem. Phys. 190, 207 (1995)] studies on NO2, we observed by LIF the NO2 vibronic levels between 11 680 and 13 900 cm−1. These observations allow us to characterize the vibronic levels resulting from the conical intersection between the X 2A1 and A 2B2 electronic PESs. Globally, we observed by LIF 78 vibronic levels of B2 vibronic symmetry, among which 61 had previously been observed by ICLAS. Conversely, one vibronic level observed at 13 088 cm−1 by ICLAS has not been observed by LIF. The 17 new levels have been observed thanks to a better sensitivity (about a factor 10) and resolution (0.3 GHz). The LIF intensities range over four orders of magnitude. The 79 (78+1) observed vibronic levels represent 83% of the 95 levels of B2 vibronic symmetry calculated in that range; 85 X 2A1 levels of b2 vibrational symmetry and 10 A 2B2 levels of a1 vibrational symmetry. The missing levels are expected to have a weak A 2B2 el...

Spatial fluorescence cross-correlation spectroscopy

We present an alternative method for diffusion measurements of fluorescent species in solution by use of confocal microscopy and fluorescence correlation spectroscopy techniques. It consists of making a time and spatial dual correlation in which one detects the fluorescence signals from two nearby separate confocal volumes and cross correlates them. To improve the spatial discrimination between the two confocal volumes we propose filtering of fluorescence photocounts by rejecting the fluorescence background, which corresponds to particles located far from the center of the detection volumes.

Absorption cross section of NO2 by the reflection method from ab initio calculations involving the three low lying electronic states

The potential energy surfaces of the three low lying electronic states of NO2, namely the X 2A1, A 2B2, and B 2B1 states, and the transition dipole moment surfaces between the ground state and both excited states have been calculated at two levels of ab initio theory; complete active space self-consistent field (CASSCF) and internally contracted multireference configuration interaction (CMRCI). Only 9 points of these surfaces, located around the equilibrium geometry of X 2A1 and corresponding to C2v geometries, have been found sufficient for calculating the cross section, in the 10000–45000 cm−1 energy range, by means of a 2D version of the reflection method. The agreement between the experimental low resolution data and the ab initio absorption cross section is satisfactory, mainly at the CMRCI level, at which the energy at maximum amplitude, the width, the maximum amplitude and the effective transition dipole moment describing both involved electronic transitions are predicted within 4%, 6%, 20%, and...

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 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 Lande 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 Lande factor and the spin‐splitting constant e, 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 scan...