The rotational structure in the vibrational transitions from vϭ0 to vϭ1, 2, 3, 4 of H 35 Cl and H 37 Cl is studied in Xe, Kr, and Ar matrices with high spectral resolution. A consistent set of rotational constants B v for the vibrational levels vϭ0 to 4 is derived. B 0 decreases with the tightness of the cage from 9.78 cm Ϫ1 in Xe to 8.83 cm Ϫ1 in Ar for H 35 Cl ͑gas phase 10.44 cm Ϫ1 ͒. The values for B 0 to B 4 decrease linearly with v due to the vibration-rotation-coupling constant ␣ which increases from 0.37 cm Ϫ1 in Xe to 0.479 cm Ϫ1 in Ar ͑gas 0.303͒ according to the cage tightness. The splitting of the R(1) transition which originates from the hindering of rotation is analyzed in Xe using the T 2g-T 1u and T 2g-E g transition energies. A comparison with force field calculations yields a dominant contribution of the sixth spherical harmonic Y A 1g 6 of the octahedral matrix potential. The modulation of the potential takes a value of K 6 /Bϭ17 which corresponds to a barrier for the rotation of 160 cm Ϫ1. The splitting increases with the vibrational level v which can be interpreted as a weak admixture of the Y A 1g 4 spherical harmonic. A large isotope effect and a reduction of the T 1u-A 1g transition energy ͓R(0)-transition͔ beyond the crystal field value are attributed to an eccentric rotation with a displacement of the center-of-mass of the order of 0.05 Å. The vibrational energies e show an opposite trend with matrix atom size and decrease with polarizability from 2970 cm Ϫ1 in Ar to 2945.4 cm Ϫ1 in Xe ͑gas 2989.9 cm Ϫ1 ͒ while the anharmonicity e x e of the free molecule lies close to the Kr value and thus between that of Ar and Xe.
Vibrational levels v=1, 2, and 3 of HCl in Kr matrices are populated with tunable IR radiation and the excited molecules are dissociated by UV excitation to the repulsive A 1∏ state. Cl fragments are recorded by laser induced fluorescence of Kr2Cl and dissociation rates are determined from the increase in LIF with UV dose. The enlarged UV Franck–Condon range for overtones allows the study of cage exit of H fragments with small kinetic energy Ekin. A threshold at Ekin=1.4 eV and a steep rise indicate a predominant sudden exit. Monomers, different initial rotational states and transients in the relaxation cascade are preselected with overtone excitation and the feasibility of a discrimination between isotopes, aggregates, and local structures is illustrated.
HCl-doped Xe and Kr films are irradiated with wavelength dispersed synchrotron radiation in the wavelength range from 200 to 130 nm. The growth of H, Cl, Xe2H+, XeH2, HXeCl, Kr2H+, and HKrCl as well as the decomposition of HCl are recorded by a combination of UV, VIS, and IR spectroscopy. A turnover in the formation of Xe2H+ and Kr2H+ by a predominant two-step reaction on neutral surfaces at low energies to a one-step formation on ionic surfaces is determined at 172 and 155 nm in Xe and Kr, respectively. A potential energy diagram for neutral and ionic states is derived that is consistent with a DIIS calculation, with new UV fluorescence bands from Xe+HCl- centers, with the turnover energies and with a deconvolution of the absorption spectra in neutral and ionic contributions. The cage exit of charged as well as of neutral H, the latter via a harpoon reaction, is discussed for the ionic surfaces. The self-limitation of HCl decomposition on the neutral surfaces due to absorption by H and Cl fragments is treated quantatively. Dissociation efficiencies phi(e), together with absolute absorption cross sections sigma(H) and sigma(Cl) of the fragments, are derived. sigma(H) and sigma(Cl) are of the order of 10(-16) cm(2) compared to 10(-18) cm(2) for sigma(HCl). Dissociation is accompanied by many excitation cycles of the fragments, which leads to light-induced migration of H and recombination. phi(e) therefore represents a product of the cage exit probability phi that was treated theoretically and the survival probability concerning geminate and nongeminate recombination.
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