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We have measured the rovibronic densities of four symmetric (C2v) and two asymmetric (Cs) isotopologues of nitrogen dioxide just below their photodissociation threshold. At dissociation threshold and under jet conditions the laser-induced fluorescence abruptly disappears because the dissociation into NO(2Π1/2)+O(3P2) is much faster than the radiative decay. As a consequence, in a narrow energy range below D0, the highest bound rovibronic energy levels of J=1/2 and J=3/2 can be observed and sorted. A statistical analysis of the corresponding rovibronic density, energy spacing, and rovibronic transition intensities has been made. The observed intensity distributions are in agreement with the Porter-Thomas distribution. This distribution allows one to estimate the number of missing levels, and therefore to determine and compare the rovibronic and the vibronic densities. The four symmetric NO2 isotopologues, 16O14N16O, 18O14N18O, 16O15N16O, and 18O15N18O, have, respectively, a sum of J=1/2 and J=3/2 rovibronic densities of 18±0.8, 18.3±1.4, 18.4±2.7, and 19.8±3.5/cm-1, while for the two asymmetric isotopologues, 18O14N16O and 18O15N16O, the corresponding densities are 20.9±4.5 and 23.6±5.6/cm-1. The corresponding vibronic densities are in agreement only if we include both the merging of symmetry species (from those of C2v to those of Cs) and the contribution of the long-range tail(s) of the potential-energy surface along the dissociation coordinate. The effects of isotopic substitution on dissociation rates and the possible relation to mass-independent isotopic fractionation are discussed. [ABSTRACT FROM AUTHOR]

We have measured the rotationless photodissociation threshold of six isotopologues of NO2 containing 14N, 15N, 16O, and 18O isotopes using laser induced fluorescence detection and jet cooled NO2 (to avoid rotational congestion). For each isotopologue, the spectrum is very dense below the dissociation energy while fluorescence disappears abruptly above it. The six dissociation energies ranged from 25 128.56 cm-1 for 14N16O2 to 25 171.80 cm-1 for 15N18O2. The zero point energy for the NO2 isotopologues was determined from experimental vibrational energies, application of the Dunham expansion, and from canonical perturbation theory using several potential energy surfaces. Using the experimentally determined dissociation energies and the calculated zero point energies of the parent NO2 isotopologue and of the NO product(s) we determined that there is a common De=26 051.17±0.70 cm-1 using the Born-Oppenheimer approximation. The canonical perturbation theory was then used to calculate the zero point energy of all stable isotopologues of SO2, CO2, and O3, which are compared with previous determinations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose an efficient method for calculating the eigenstates and adjusting the parameters of an effective Hamiltonian, which reproduces the experimentally observed energy levels of NO[sub 2] up to 11 800 cm-1 above the quantum mechanical ground state, that is a few thousands of cm-1 above the X [sup 2]A[sub 1]–A [sup 2]B[sub 2] conical intersection, with a rms error less than 4 cm-1. This method principally relies on the determination, through first-order perturbation theory, of an optimal basis for each surface, which takes into account the nonresonant energy shifts experienced by the states of this surface. As a result, the size of the matrix, which one has to build and diagonalize to converge the spectrum up to 11 800 cm-1, is of the order of 500–1000 instead of several tens of thousands. Thank to this Hamiltonian, the analysis of the experimental spectrum up to 11 800 cm-1 could be completed. A detailed description of all states located above 9500 cm-1 is proposed, those lying below 9500 cm-1 being already known and tabulated. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using intracavity laser absorption spectroscopy combined with a supersonic slit jet, we have observed and analyzed seven vibronic cold bands of NO[SUB2] between 10 800 and 11 680 cm[SUP-1]. The vibronic energies, relative intensities, A, B, and C rotational constants, and the spin-rotation constants have been determined. The rotational constants play a crucial role in the vibronic assignments. The seven observed states are vibronically mixed. However, three of them have a dominant à [SUP2]B[SUB2] electronic character while the four others have a dominant &Xtilde; [SUP2]A[SUB1] character. The vibrational assignments and energies of the three à [SUP2]B[SUB2]) levels are :(1, 0, 0) at 10 999.42 cm[SUP-1], (0,2,) at 11 210.50 cm[SUP-1], and (0, 0, 2) at 11 283.15 cm[SUP-1]. The frequencies of the symmetric stretch, (ω[SUB1] ≃ 1265 cm[SUP-1]), and bending, (ω[SUB2] ≃ 738 cm[SUP-1]) modes derived from the (1, 0, 0) and (0, 2, 0) levels are in agreement with ab initio calculations. In contrast, the frequency of the antisymmetric stretch, ω[SUB3] ≃ 775 cm[SUP-1], derived from the observed (0, 0, 2) level, agrees only with the ab initio value obtained by Kaldor while other ab initio values ranging from 390 to 1750 cm[SUP-1] were reported. The four levels with a dominant &Xtilde;[SUP2] A[SUB1] character have been tentatively vibrationally assigned. The present experimental data allows for a first experimental determination of the two Ã[SUP2] B[SUB2] stretch vibrational frequencies which are necessary to model the &Xtilde; [SUP2]A[SUB1] - à [SUP2]B[SUB2] vibronic interactions, i.e., the conical intersection between &Xtilde; [SUP2]A[SUB1] and à {SUP2]B[SUB2] potential energy surfaces. [ABSTRACT FROM AUTHOR]

Defines the spectroscopy and intramolecular vibrational energy redistribution dynamics of hydrochloric acid (HOCl) molecules probed by infrared-visible double resonance overtone excitation. Promotion of HOCl molecules to single rotational level of high OH stretching states; Monitoring of double-resonance spectra by laser induced fluorescence.