Infrared photofragmentation of “hot” and “cold” nitric oxide cluster ions

Abstract

A line-tunable CO laser has been used to record infrared absorption profiles for (NO)n+ and (NO)n+.Ar clusters for n in the range 2–19 and at wavelengths between 1600 and 1900 cm−1. The presence of an argon atom is shown to have the effect of reducing the internal energy content of the ions, and for the case of (NO)2+. Ar is the only circumstance under which infrared absorption can be detected for the dimer ion. The trimer ion, (NO)3+, is found to have a very strong absorption feature at ∼1700 cm−1, which remains both static and comparatively narrow in subsequent cluster ions containing up to 19 nitric oxide molecules. It is concluded that (NO)3+ forms a stable core in larger clusters and that there is very little additional charge delocalization.

Infrared photodissociation of (NO)n+·X cluster ions (n ≤ 5)

“…Under these circumstances, photodissociation will only be observed if the presence of an active vibrational mode is accompanied by one or both of the following conditions: (i) the ions have an internal energy within 1900 cm À1 of the dissociation energy; (ii) the binding energy of X is r1900 cm À1 . Support for one or more of these suggestions comes from the fact that photodissociation is observed when Ar, Kr, Xe, and CF 4 are attached to the NO dimer ion (as has already been seen, the attachment of NO to (NO) 1 2 also leads to IR photodissociation, 27 but this will be discussed as a separate case below). In the case of (NO) 1 2 Á Ar, the principal fragmentation pathway is the loss of Ar.…”

“…1, but for (NO) 1 n CO 2 cluster ions. had noted that bare (NO) 2 1 ions showed no evidence of photodissociation at any of the accessible infrared wavelengths; 27 this is not too unexpected, the binding energy of the dimer ion is ca. 0.6 eV, 1,2,13 and so the energy of a single photon is insufficient to break the ion apart.…”

“…In this work, the ionization potentials of the NO clusters studied were smaller than the host atom/molecule attached. This new series of experiments builds on an earlier study of the infrared photodissociation of pure (NO) n 1 cluster ions, 27 where it was observed that the trimer ion, (NO) 3 1 , possessed a comparatively larger infrared absorption cross at ca. 1760 cm À1 .…”

“…15 To complement the above experiments, it has recently been observed that (NO) n 1 cluster ions exhibit a very strong infrared transition within the wavelength range of a CO laser. 14,27 Smaller members of the pure (NO) n 1 series are quite strongly bound, 1,2,13 therefore their formation by electron impact from neutral clusters results in a significant degree of internal excitation (re 0 , the minimum binding energy). By the time ions reach the point of laser excitation, they are ca.…”

Electron ionization study of ammonia micro-clusters

“…The adiabatic gas expansion method is today the most commonly used technique to produce neutral gaseous clusters. These clusters are vibrationally and rotationally cooled in the process of adiabatic (free jet or pulsed supersonic) expansion 1–17. The amount of condensation is determined by the structure and energetic properties of molecules, the gas stagnation pressure p o and its temperature T o , the diameter and the shape of the expansion nozzle, and the temperature T of the expanded beam 18,.…”