Détection de complexes d’intérêt atmosphérique en phase gazeuse
6.2 Méthodes utilisées et principaux résultats obtenus
Le travail a été réalisé en utilisant plusieurs méthodes ab initio : dans un premier temps, la méthode Hartee-Fock-Rootan pour les couches fermées, et la méthode restreinte de Hartee-Fock, spécialisée pour les systèmes qui présentent des couches ouvertes pour leurs différents états électroniques. On a utilisé également des approches prenant en compte la corrélation électronique : méthode d’interaction directe de configuration (CI) et méthode de multiconfiguration (MC SCF). L’étude de la structure des complexes moléculaires est fondée sur l’obtention des points stationnaires de la surface de l’énergie potentielle (SEP), compte tenu de la corrélation électronique et de l’erreur liée à la superposition des bases. Pour déterminer les structures géométriques optimales, on a utilisé des méthodes numériques de recherche de points stationnaires : méthodes du gradient, ou encore méthodes fondées sur le calcul des dérivées secondes de l’énergie potentielle.
On définit les configurations d’équilibre pour [(H2O)n(HF)m] avec n=m=2,3, et pour [(H2O)n(HCl)m] avec n :m = 1 :2, 2 :2, 3 :3, à la suite de quoi on calcule les fréquences vibrationnelles et on définit le
électronique de basse énergie est localisée sur les liaisons O-H d’une des molécules d’eau, ce qui assure la préservation du caractère dissociatif de la bande d’absorption comme dans la molécule d’eau isolée, avec cependant un déplacement vers le domaine des ondes courtes. Les sections de l’absorption des spectres de photodissociation des molécules liées par une faible liaison hydrogène, telles que (H2O)2 et H2O...HCl, pour une excitation électronique modérée (énergie de vibration de la molécule 6 15 % de l’énergie de dissociation D0), sont bien décrites par un modèle fondé sur l’utilisation des fonctions d’Airy (qui représentent la solution de l’équation de Schrö dinger dans un champ homogène)
Ψv,E(x) = Nv,E
Z ∞
0 cosz3/3 + ˜νβ−2/3z − 2β1/3(x/ρ) zdz (6.1) où β = ρ3mF~2/4 et ˜ν = (E(R0)−E)/~Ω, et où N˜ν,Eest une constante de normalisation pour le potentiel homogène dont les paramètres sont définis par le calcul ab initio.
Les modèles développés sont en bon accord avec les données expérimentales disponibles. Les résultats numériques coïncident également avec ceux qui ont été obtenus par d’autres auteurs pour les complexes examinés. Il faut retenir du travail les résultats originaux suivants :
– on a montré l’applicabilité du potentiel homogène et des fonctions d’Airy pour la description des spectres de photodissociation des dimères ;
– on a révélé les caractéristiques principales des spectres IR et UV des complexes formés de molécules d’eau et des molécules chimiquement actives HF et HCl, ainsi que leurs configurations optimales.
6.3 Perspectives pour le complexe H
2O − HF
Des études concernant la surface de potentiel du dimère H2O-HF et l’interprétation des spectres IR expérimentaux du mélange H2O et HF sont en cours. Pour ce complexe :
1. une série de calculs ab initio CCSD(T)/aug-cc-pVTZ ont été effectués avec le programme MOLPRO (avec 9021 points autour de l’équilibre) afin de déterminer la surface d’énergie potentielle complète H2O-HF dans les coordonnées polysphériques symétrisées compte tenu des permutations des atomes H ;
2. l’analyse des spectres IR expérimentaux du mélange H2O et HF, donc du complexe, sur la base de calculs ab initio du dimère et des complexes (avec un nombre des monomères variant jusqu’au trois) est en cours, de même que le calcul des bandes rovibrationnelles.
Les calculs de la structure électronique du dimère H2O–HF étant réalisés, il reste à caractériser la surface de potentiel, et la structure du spectre d’absorption. La structure du spectre d’absorption de la bande ν1(H-F stretching) (3600 - 3800 cm−1) résulte de la superposition de la bande fondamentale et des bandes chaudes issues des modes de basses fréquences (intermoléculaires) du complexe. Les maxima du spectre d’absorption correspondent aux têtes de branches P des bandes parallèles. Les premiers résultats (Fig.6.1) montrent un bon accord général dans la zone 3300–3900 cm−1, mais il reste des différences entre expérience et simulation aux environs de 3200 cm−1, zone dans laquelle le calcul prévoit une absorption qui n’est pas présente dans l’expérience. L’interprétation doit donc faire appel à de nouvelles hypothèses.
M.A. Buldakov et al. Vol. 8, No. 11 /November 1995/ Atmos. Oceanic Opt. 927
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PHOTODISSOCIATION OF WATER VAPOR BY UV LASER RADIATION
M.A. Buldakov, N.A. Zvereva, I.I. Ippolitov, and A.F. Terpugova V.D. Kuznetsov Siberian Physicotechnical Institute
at the State University, Tomsk Received May 18, 1995
The energy of low electronic states of monomer and dimer water complexes, H2O and (H2O)2, has been calculated. It is shown that KrF–laser–induced photoabsorption of water vapor may be accounted for by the transitions from hot rovibrational levels to quasicontinuum of H2O states, whereas the fluorescence may be explained by the recombination of the products of monomer water complex disintegration.
Water vapor fluorescence induced by Kr–F laser radiation has been studied within 250–280 nm (Ref. 1) and 250–400 nm (Refs.2, 3) spectral ranges. In Refs.2 and 3 it has been concluded that
1) fluorescence is observed under excitation mode, linear relative to the laser radiant exitance up to 107 W/cm2;
2) there are regions in the fluorescence spectrum with essentially different decay time.
To interpret the observed fluorescence properly, it is necessary to obtain the energy states between which the radiative transitions take place connected with light absorption. The absorption spectra of water vapor in 250 – 350 nm spectral range has been studied in Refs. 4–7. As has been found, the maximum of absorption band lies at λ = 270 nm (K = 3ā10–5 cm–1), whereas the bottom corresponds to λ = 320 nm, and the band itself is continual without evident structure. It was concluded that the observed spectra are connected with the novel electronic state of H2O.
To verify this hypothesis, we have calculated the ground and low electronic states of H2O and (H2O)2. For each electronic state the energy has been optimized. The calculations have been made with the MONSTERGAUSS program package.
The key idea in studying the spectroscopic properties of water is the Rydberg character of excited electronic states. The electronic configuration of water ground state can be presented as
(1a1)2 (2a1)2 (1b2)2 (3a1)2 (1b1)2 – X~1A1 .
Ten excited singlet states result from the electron transition from 1b1 orbital to 3s, 3p, 4s, or 3d Rydberg orbitals. Some states result from the transition from 3a1 orbital. In this paper we consider only low electronic states. In calculations we use the expanded basis including 3s and 4s orbitals for oxygen atom O, (5211/311), and hydrogen atom H, (211). The calculations have been made for the following transitions: (1b1 → 3sa1)1B1 and (1b1 → 3pb2)1A2.
The MKSSP method (49 electronic configurations) has been used to calculate the excited states.
To study the low electronic states in detail, the H2O potential surfaces have been calculated with geometry optimization. Due to the transition from ~
X1A1 state to the singlet state A1B1 (1b1 → 3sa1), the H2O molecule becomes linear with ROH = 1.2288Å and α = 180° that corresponds to the intermediate state from which photodissociation into H and OH occurs. This stationary point of the potential surface lies in the region of nonequilibrium configurations, and any displacement from the region brings in the valley of the disintegration products. The energy corresponding to this stationary point is – 75.32147 a.u.
In the transition X~1A1 → 1A2 (1b1 → 3bp2) the molecular geometry is near–linear: α = 179.7939°, ROH1= 3.2722Åand ROH2 = 1.0284Å. The total energy E equals to 75.4037313 a.u. This state corresponds most likely to the complex with hydrogen bond H ... OH. The calculational results for H2O are shown in Fig. 1.
The total energy of the ground state X~1A1 has been found to be –75.5494 a.u., whereas energy of the vertical transitions into state 1B1 and 1A2 is –75.26131 and –75.16918 a.u., respectively that corresponds to 7.8 and 10.3 eV, i.e. there is no energy values less than 7 eV for vertical transitions from zero–point vibrational level.
It is not possible, in the present state of the art of both theoretical and computational methods of quantum chemistry, to confirm the hypothesis about the existence of H2O states intermediate between X~1A1 and A1B1. Interpretation of the experimental data obtained is needed to be made within the framework of the present concept concerning the structure of H2O energy levels. Let us now turn our attention to the diagram presented in Fig.2.
The energy of one quantum of radiation at λ = 248.5 nm (4.99 eV, 40257 cm–1) is not enough for
H2O molecule to dissociate into H + OH, if considered are only transitions from zero–point vibrational level 000. However, this quantum energy is practically sufficient for H2O to dissociate into O+H2 (D0= 5 eV), especially when the finite spectral width of KrF laser radiation (∼100 cm–1) is taken into account. The energy of 5 eV corresponds to the wavelength λ = 247 nm, consequently, the absorption of radiation with λ > 247 nm is connected with the excitation of rovibrational levels of the ground state of quasicontinuum near the dissociation boundary.
FIG. 1. Potential curves for low electronic states of water monomer H2O.
FIG. 2. Diagram of potential curves for H2O and O2.
The energy deficit, needed for dissociation to follow the reaction H + OH, amounts to 968 cm–1. Then the transitions, induced by radiation with λ = 248.5 nm, from, for example, virbrational level 010 (1647.59 cm–1) will lead to formation of OH (X2o ) radicals. With excitation of water vapor by radiation at λ = 248.5 nm and taking into account the Boltzmann distribution over the energy levels, the following processes will occur:
H2O + hν 248.5 nm$→ H2O* H2O + M O + H2 O + OH H2O + hν′ and, consequently, H2O, H2O*, O, and OH can take part in the succeeding fluorescence. It is naturally to suppose that luminescence of excited molecule, H2O*, and recombination luminescence appearing in association O + OH → H2O and O + H2 → H2O are responsible for the fluorescence. This supposition is confirmed by the fact that in pure water vapor the fluorescence can be quenched effectively by molecular oxygen. To explain qualitatively the existence of the fluorescence continuous spectrum recorded,4–7 the following assumption should be made.
Excitation of H2O molecules in the quasicontinuum of states lying below the dissociation threshold is low–efficient due to small values of the Frank–Condon factor. The rate of excitation of continuous states lying above the dissociation threshold may be essentially greater due to borrowing intensities from 1B1 state. Then every radiative transition in absorption will be connected with the continuous spectrum of states, whereas the decay of absorption coefficient in the longwave wing of the band will be due to the Boltzmann distribution of energy levels of the ground electronic state.
It should be noted in conclusion that the absorption and the fluorescence observed in experiment are not connected with water vapor dimer, (H2O)2. The computations we have made in Ref. 8 have shown that only unplane structure (see Fig. 3) has bound ground and S1 and T1 excited states with bond energy of 5.5, 2, and 4.4 kcal/mole, respectively. In this case, as compared to the monomer, a shift in absorption takes place to the blue region by the value within 0.65–0.69 eV.
FIG. 3. Geometry of the dimer (H2O)2 in the equilibrium configuration corresponding to the global energy minimum.
M.A. Buldakov et al. Vol. 8, No. 11 /November 1995/ Atmos. Oceanic Opt. 929
REFERENCES
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