Multiphoton Ionization of Molecular Clusters

Mixed clusters of the form (NO)_m⁺ (Ar)_n (m ≤ 4 ≤ 22) were produced in a supersonic expansion and ionized by nonresonant two-photon absorption of 30 ps, 266 nm photons. The cluster distribution exhibited intensity anomalies for NO⁺ (Ar)_n (n = 12, 18, and 22) and (NO)₂⁺ (Ar)_n (n = 17, 21). These “magic numbers”, at m + n = 13, 19, 23 were compared with those observed in rare gas and other M⁺ Ar_n clusters and were assigned to symmetric and stable icosahedral structures. Four-photon ionization with 30 ps, 532 nm light yielded a cluster distribution in which only the NO⁺ (Ar)_n cluster series was observed. The known photodissociation of (NO)₂⁺ and (NO)₃⁺ and the cluster distributions obtained at 266, 355, and 532 nm light suggested that the dissociation of (NO)_m⁺ (Ar)_n (m > 1) was taking place in the ion.

A variety of binary heteroclusters of the form (NO)_m(Y)_n, where Y = N₂O, CO₂, NO₂, H₂O, CS₂, SO₂ and SF₆ were investigated by coexpanding nitric oxide, argon and the complexing agent. Cluster distributions of the (NO)_m⁺ exhibited an odd-even intensity alternation which was explained by electron pairing effects and differences in the relative stabilities of the cluster ions which are enhanced by the kinetic effects in the evaporation process. The observation of the exceptionally stable (NO)₃⁺ (Y)_n cluster series is of particular interest and possible structures for these species are discussed.

The vibronic spectroscopy of NO, ArNO, CH₄NO and CD₄NO was investigated using mass resolved REMPI. Spectra for NO are presented for transitions associated with the A²∑⁺ state (v = 2 and 3). Multiphoton ionization spectra are also presented for various van der Waals molecules, namely ArNO, CH₄NO, and CD₄NO. The spectra were obtained via a two-photon electronic transition associated with the C²∏ (v = 0) state of nitric oxide. Dissociation energies of the ground and excited states of the vdW molecules are discussed.

Laser induced polymerization was investigated in clusters of carbon disulfide. The observation of various ionic photoproducts such as (S)_m⁺, (CS)_m⁺, S_m⁺, (CS₂)_n, (CS₂)_m⁺ and CS_m⁺ (CS₂)_n is attributed to a cycle of ion-molecule reactions occurring entirely within an isolated cluster of carbon disulfide.