Abstract
The competition between multiphoton ionization and fragmentation in the diatomic molecule hydrogen chloride is reviewed. Emphasis is laid on recent experimental results employing chemical imaging methods in order to obtain kinetic energy distributions and angular distributions of photoproducts. The energy range considered is 15 to 20 eV, equivalent to the absorption of three or four photons in the ultraviolet wavelength range. The role of Rydberg states as resonantly excited intermediate states in the ionization/fragmentation processes is assessed. Mixing among Σ1+ states gives rise to peculiarly shaped double minimum potential energy curves which allow for the production of hydrogen and chlorine atomic and ionic fragments via several competing pathways, in addition to the production of molecular HCl+ ions. States with different electronic properties show a qualitatively different behaviour from Σ+ states. Accidental resonances between states of differing orbital angular momentum or multiplicity serve to override these differences and cause subtle as well as significant deviations from the unperturbed behaviour.
Highlights
Hydrogen halides are molecules of fundamental interest in chemistry and physics and have extensively been studied theoretically as well as experimentally. They can be regarded as prototypes of heteronuclear diatomic molecules
The ion-pair path (II) in particular is subject to subtle details in the topology of the involved potential energy curves
The photoionization of hydrogen chloride, a prototype of a heteronuclear diatomic molecule, has been intensively studied for decades, recently novel sophisticated experimental techniques have revealed several details associated with the competition of multiphoton ionization with fragmentation of HCl
Summary
Hydrogen halides are molecules of fundamental interest in chemistry and physics and have extensively been studied theoretically as well as experimentally. Photoion imaging allows one to simultaneously determine kinetic energy as well as state-selected spatial distributions of ionized photoproducts These techniques include the two-dimensional (2D) imaging technique [6], its two-dimensional velocity mapping counterpart (2D-VMI) [7, 8], and the corresponding threedimensional (3D) techniques [9,10,11]. Photoproduct spatial anisotropies and kinetic energy distributions were analyzed by Loock and coworkers [1, 3, 19] and by Gericke and co-workers [20, 21] using velocity map imaging techniques Both groups investigated the ionization/fragmentation pathways through various vibrational bands of the B1Σ+(0+) state. Renewed interest in HCl excited potential energy surfaces has been raised by the new, intriguing experimental results, and high quality calculations are under way [40]
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